Earthquake Protection of Building Equipment and Their Systems: Bridging the Implementation Gap 9780784411520, 0784411522

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Earthquake Protection of Building Equipment and Systems

Other Titles of Interest

Earthquakes and Engineers: An International History, by Robert K. Reitherman (ASCE Press, 2012). Provides the first comprehensive treatment of the engineering techniques that have developed in various countries to address their particular seismic problems. (ISBN 978-07844-1071-4) Guidelines for Seismic Evaluation and Design of Petrochemical Facilities, Task Committee on Seismic Evaluation and Design, ASCE Energy Division (ASCE Technical Report, 2011). Offers practical recommendations regarding the design and safety of petrochemical facilities during and following an earthquake. (ISBN 978-0-7844-1140-7) Recommendations for Seismic Design of Hybrid Coupled Wall Systems, edited by Sherif El-Tawil, Patrick Fortney, Kent Harries, Bahram Sharooz, Yahya Kurama, Mohammad Hassan, and Xiandoing Tong (ASCE Technical Report, 2010). Synthesizes the existing information into helpful recommendations for engineers and practitioners involved with seismic analysis and design of hybrid coupled wall systems. (ISBN 978-0-7844-1060-8) Seismic Loads: Guide to the Seismic Load Provisions of ASCE 7-05, by Finley A. Charney (ASCE Press, 2010). Illustrates key concepts and guides structural engineers in applying the most current thinking in this rapidly changing discipline to the design of new structures. (ISBN 978-0-7844-1076-9)

Earthquake Protection of Building Equipment and Systems Bridging the Implementation Gap

Jeffrey A. Gatscher Gary L. McGavin, AIA Philip J. Caldwell

Library of Congress Cataloging-in-Publication Data Gatscher, Jeffrey A. Earthquake protection of building equipment and systems : bridging the implementation gap / Jeffrey A. Gatscher, Gary L. McGavin, Philip J. Caldwell. p. cm. Includes bibliographical references and index. ISBN 978-0-7844-1152-0 (pbk. : alk. paper) — ISBN 978-0-7844-7643-7 (ebook) 1. Buildings—Earthquake effects. 2. Buildings—Mechanical equipment. I. McGavin, Gary L., 1948– II. Caldwell, Philip J. III. American Society of Civil Engineers. IV. Title. TH1095.G38 2012 693.8’52—dc23 2011042505 Published by American Society of Civil Engineers 1801 Alexander Bell Drive Reston, Virginia 20191 www.asce.org/pubs Any statements expressed in these materials are those of the individual authors and do not necessarily represent the views of ASCE, which takes no responsibility for any statement made herein. No reference made in this publication to any specific method, product, process, or service constitutes or implies an endorsement, recommendation, or warranty thereof by ASCE. The materials are for general information only and do not represent a standard of ASCE, nor are they intended as a reference in purchase specifications, contracts, regulations, statutes, or any other legal document. ASCE makes no representation or warranty of any kind, whether express or implied, concerning the accuracy, completeness, suitability, or utility of any information, apparatus, product, or process discussed in this publication, and assumes no liability therefor. This information should not be used without first securing competent advice with respect to its suitability for any general or specific application. Anyone utilizing this information assumes all liability arising from such use, including but not limited to infringement of any patent or patents. ASCE and American Society of Civil Engineers—Registered in U.S. Patent and Trademark Office. Photocopies and permissions. Permission to photocopy or reproduce material from ASCE publications can be obtained by sending an e-mail to [email protected] or by locating a title in ASCE’s online database (http://cedb.asce.org) and using the “Permission to Reuse” link. Copyright © 2012 by the American Society of Civil Engineers. All Rights Reserved. ISBN 978-0-7844-1152-0 (paper) ISBN 978-0-7844-7643-7 (e-book) Manufactured in the United States of America. 18 17 16 15 14 13 12 11

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This book is dedicated to the many people who have suffered hardship, affliction, or bereavement caused by earthquake disasters. May their losses not soon be forgotten.

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Contents

Foreword by James R. Harris ix Preface xi Acknowledgments xvii

Part 1: Earthquake Demand Chapter 1

Introduction to Demand Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Nonstructural Building Systems 4 1.2 Systems Design Primer 11 1.3 Modern Nonstructural Design Philosophy 25 References 40

Chapter 2

Stakeholder Commentary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.1 The Stakeholders 42 2.2 Code Regulation and Enforcement 43 2.3 Building Design and Construction 51 2.4 Product Design and Manufacture 63 References 69

Chapter 3

Geotechnical Primer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 3.1 Plate Tectonics 72 3.2 Stress on Rocks 78 3.3 Damage Potential Based on Earthquake-Facility Variables 79 3.4 Faults 83 3.5 Seismic Waves 89 3.6 Lateral Loads versus Three-Axis Ground Motion 93 3.7 Seismic Early Warning Systems 93 3.8 Earthquake Scales 98 3.9 Foreshocks and Aftershocks 104 3.10 Deriving Seismic Hazard Maps from Seismology 106 References 114

Chapter 4

Building Code Seismic Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 4.1 Basic Elements of Model Building Codes 118 4.2 Elements of IBC and ASCE/SEI 7 Seismic Provisions 120 4.3 Seismic Requirements Summary 146 References 153 vii

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Part 2: Nonstructural Capacity Chapter 5

Introduction to Seismic Qualification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 5.1 5.2 5.3 5.4

Chapter 6

Qualification Ownership 158 OEM Qualification Strategy 174 Nonstructural Compliance Metrics 181 Seismic Qualification Summary 196 References 198

Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 6.1 Applied Seismic Analysis 202 6.2 Advanced Seismic Analysis 268 References 279

Chapter 7

Dynamic Test Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 7.1 Specification of the Test Environment 281 7.2 Seismic Test Machines and Technology 299 7.3 Test Preparation and Execution 307 7.4 Experimental Modal Analysis 312 References 320

Chapter 8

Comparative Experience Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 8.1 SQUG Qualification by Earthquake Experience 324 8.2 GERS Qualification by Testing Experience 331 8.3 Experienced-Based Methods Summary 332 References 333

Chapter 9

Combined Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 9.1 Options for Large-Class Qualification 337 9.2 Large-Class Qualification Summary 358 References 359

Chapter 10

Trends in Earthquake Protection of Nonstructural Systems . . . . . . . . . . . 361 10.1 Performance-Based Design 362 10.2 Nonstructural Research Needs 364 References 374

Appendix. Test Facility Selection: Points to Consider 375 Glossary 379 Notation 389 Index 391 About the Authors 403

Foreword

Earthquake engineering for buildings has seen tremendous change over the past several decades. Earthquake after earthquake has demonstrated the insufficiency of our knowledge and ability to protect people in buildings, let alone our ability to provide buildings in which key systems are functional following the earthquake. As new lessons have been learned in the aftermath of damaging earthquakes and in the laboratory, substantial efforts have been made to improve our standards of practice and model building codes. The majority of people who work on these standards are structural engineers, although several seismologists and geotechnical engineers, plus a few architects and mechanical and electrical engineers, work with those structural engineers. Interdisciplinary communication is not easy, and the authors of this book are among those few who have attempted to bridge the gaps and improve the treatment of nonstructural portions of buildings. For that alone Gatscher, McGavin, and Caldwell deserve commendation. Their work in this book is yet another excellent contribution. This book fills a serious void in the earthquake engineering literature: it illustrates the importance of a true systems analysis approach in providing equipment that will perform as desired in buildings, especially equipment in those systems expected to be in working condition immediately following the earthquake. The book goes far beyond traditional concerns with anchorage and bracing of switchgear, pumps, fans, conduits, pipes, ducts, and the like. It provides the guidance that has been so sorely missing for engineers involved in product design; in fact, it highlights the importance of including seismic resistance concepts in the initial planning and design of equipment before bringing it to market. After defining the kinds of nonstructural components and systems included in buildings and describing common problems with performance in past earthquakes, the authors go back to the beginning with an admirable briefing on the mechanics of earthquakes. They offer a concise but well-detailed description of the seismic demands on nonstructural systems and components found in the most current building codes. In describing the building code provisions, the authors make serious attempts to explain the why behind the what. This content is especially valuable because the competent professional needs solid understanding of the intent in order to adapt terse code rules to real-life situations. The second half of the book offers excellent guidance for equipment designers, code ix

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officials, and building design professionals on state-of-the-art methods for assessing the capacity of equipment and components to meet those provisions. Earthquake Protection of Building Equipment and Systems is designed to be useful for professionals with differing levels of technical expertise, and I am confident that it will be a valued addition to the library of a great many. James Robert Harris Denver, Colorado

Preface

Historically, earthquake protection of active equipment and building distribution systems (i.e., nonstructural category) has been treated as an afterthought. Products designated as nonstructural were designed by manufacturers without specific regard to seismic requirements. Seismic bracing was sometimes added during installation. Stated simply, earthquake protective measures were typically addressed outside of the product development process for most essential building systems. This situation left the equipment supplier unaware of the requirements and detached from the changing evolution in earthquake protection. The nonstructural component was treated as a “black box”—the primary concern was meeting building code anchorage requirements. For decades this was the status quo for nonstructural protection. The basis for this book is that nonstructural systems are no longer treated by the building code as black-box components. Postearthquake active operation is required for designated seismic systems. This functionality mandate cannot be satisfied with simple anchorage calculations. A fundamental shift in code philosophy requires new perspectives from the stakeholders involved in nonstructural earthquake protection. Implementation of protective measures is a critical risk mitigation issue. The supplier can no longer ignore seismic requirements, and the building professional can no longer treat nonstructural items as black-box components. Nonstructural systems are often referred to as secondary systems, with the primary system being the building structure. While the label “secondary” is appropriate in this context, nonstructural systems are far from secondary in importance. The value of nonstructural systems expressed as a percentage of the total cost of a building, excluding the price of the land, has dramatically increased over the past several decades. In acute healthcare facilities and high-value industrial buildings, nonstructural systems can easily comprise 85% or more of the total cost (Charleson 2008). Protecting this investment from earthquake damage is a reasonable goal and is likely a high priority for the building owner. The literature on earthquake engineering lacks a concise summary of practical design methods that provides insight into earthquake protection for products designated as nonstructural. A how-to book for implementation of nonstructural protective measures is sorely needed. This book materialized in the absence of design-oriented material covering xi

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seismic design and qualification principles. We examine design perspectives of nonstructural protection with the objective of pushing protective measures much farther upstream in the product development process. The goal is to influence product development of equipment and distribution systems by including earthquake protection requirements as design drivers during the development cycle. This is not another manual on seismic bracing details for equipment. Rather, it is about shedding new light on nonstructural protection from a systems design perspective. The nonstructural system is a continuous chain of products, distributed throughout a building, that link together to perform an intended building function. Examples include fire suppression, emergency power, and many other essential systems. The chain includes subassemblies located within so-called black box equipment platforms, and the operational attachments responsible for connecting multiple equipment items to achieve a desired function. It is the system that requires earthquake protection, because an earthquake will find the weakest link in the nonstructural chain. The primary objective in earthquake risk mitigation is to identify the system’s weakest links before an earthquake does, such that seismic withstand improvements can be implemented. Systems analysis provides a logical method to allocate seismic requirements to the entire chain of products that comprise a nonstructural system. The reality is that most nonstructural building systems are manufactured products that are pre-engineered to address global applications. Seismic qualification of active building systems, to satisfy increasingly complex requirements, poses significant challenges to the stakeholders responsible for implementation. A systems approach clarifies the problem while establishing the division of responsibility among the stakeholders. Implementation of nonstructural protective measures has lagged behind earthquake engineering research. This book endeavors to close the implementation gap in nonstructural protection, such that protective measures are aligned with present-day building code expectations. Significant changes have been enacted in the codes regarding nonstructural seismic requirements. It is now time for implementation. One final note regarding earthquake protective measures: The earthquake hazard is not influenced by geopolitical considerations and will strike with impunity across political boundaries. Globally, all nations can benefit from improvements in nonstructural earthquake protection to minimize life-threatening risks while protecting capital investment.

Who Will Benefit from This Book The thought occurred to us that this book would be beneficial to anyone who has ever stepped inside a building of any kind. That claim is a bit overstated. There are three distinct groups of professionals who will find the material useful. These groups form a necessary triad supporting three different aspects of nonstructural earthquake protection as shown in Fig. P-1. Overall risk mitigation effectiveness is dependent on all three groups executing their respective missions.

Preface

Product Design and Manufacture

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Building Design and Construction

Nonstructural Earthquake Protection

Code Regulation and Enforcement

Figure P-1. Triad grouping of professionals involved in nonstructural earthquake protection.

Product Design and Manufacture The product design aspect of nonstructural protection has been conspicuously missing from the literature. After all, everything that can be classified as nonstructural is actually a product that is designed and engineered by manufacturing companies. To counterbalance the existing void in useful design-oriented information, a significant portion of this book addresses the needs of professionals involved in product development activities (products designated as nonstructural). Part 2 of this book will be particularly useful to product development engineers responsible for implementing seismic qualification programs.

Building Design and Construction Building design professionals approach nonstructural protection on a project-specific basis. Each building project poses different challenges. Most often, nonstructural protection is considered as secondary, with design of the building structure being the primary focus. When resources are spread too thin, building professionals may simply overlook nonstructural protection or not give it adequate attention. The benefit this book offers to building professionals is to highlight the increased importance of nonstructural systems while clearly defining the relevant building code’s requirements, performance objectives,

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and compliance expectations. The book sections covering qualification ownership will also clarify responsibilities regarding implementation.

Code Regulation and Enforcement Building code regulations and their enforcement become necessary motivators that strongly influence how product and building professionals must approach their missions. Not wanting to sound crass, but without proper “motivation” there would be no implementation of nonstructural earthquake protective measures. The professionals involved in writing seismic code regulations, or those indirectly involved (e.g., researchers, geoscientists, risk managers), will benefit from this book by understanding what is involved at the implementation end. Code regulations must be rooted in practical awareness of the implementation aspect. Code regulations without adequate enforcement makes for a dysfunctional environment in which product and building professionals are not playing on a level playing field. Code enforcement by the authority having jurisdiction (AHJ) is the necessary glue that binds together the stakeholders responsible for nonstructural earthquake protection. Without adequate code enforcement, the well-intentioned goals and expectations of modern-day building codes may not be realized.

How This Book Is Organized This book consists of 10 chapters organized into two parts. The first part starts with an overview of earthquake demand issues and the second part presents detailed discussion on nonstructural capacity topics. Earthquake demand is a term used to describe the environmental loading conditions associated with seismic events, while the term capacity refers to the nonstructural system’s ability to withstand seismic loading. If nonstructural systems have enough capacity to resist earth shaking demands, then all is good. However, this simple concept is fraught with many layers of complexities. To manage the complexities, we utilize systems analysis throughout the chapters. The writing style is informal, and some repetition has been intentionally built into the format of the book to provide a complete reference for all intended readers. This format separates the detailed technical information from the overview information by using explanatory boxes. The boxes are intended to be optional reading, allowing the reader to explore the details or skip them completely; they are a mechanism to provide a detailed treatise on the topic at hand.

Part 1: Earthquake Demand (Chapters 1–4) Chapter 1 provides an introduction to nonstructural demand allocation by employing systems analysis. The systems design philosophy is described with a generalized framework created to facilitate the process of assigning seismic demands to the various elements that

Preface

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comprise nonstructural systems. The framework establishes a generic naming convention used to break down the system into functional elements (i.e., links in the chain). This naming convention is used throughout the book. Systems design is an effective way to unravel the complexities and transform the nonstructural chain into manageable pieces. Chapter 2 is commentary from the various actors involved in nonstructural protection. The three groups of professionals shown in Fig. P-1 have quite different perspectives and motivations. An attempt is made to capture the diverse perspectives that represent the collective stakeholder community. One of the barriers preventing effective seismic risk mitigation is diffusion of responsibility within the stakeholder community. Understanding stakeholder perspectives helps clarify the roles and responsibilities. Common misconceptions regarding implementation of nonstructural protective measures are discussed. Chapter 3 is a primer on the geosciences that govern earthquake hazard characterization. The earthquake is the starting point for implementing nonstructural protection. Understanding the basics of earth science is a necessary prerequisite. Chapter 4 is an overview of building code seismic requirements. Building codes are the primary vehicle for prescribing seismic demands to the built world. Specific attention is given to address the nonstructural seismic requirements contained in the model building code currently enforced in most of the United States, namely, the 2012 International Building Code (ICC 2011), which references the ASCE/SEI 7-10 standard (ASCE/SEI 2010) for its seismic provisions.

Part 2: Nonstructural Capacity (Chapters 5–10) Chapter 5 introduces the concept of seismic qualification. Nonstructural qualification is a validation process, ensuring that the seismic withstand capacity of the system can successfully exceed the building code demand. Clarification is provided regarding qualification ownership among stakeholders, determining which stakeholders are responsible for which part of the nonstructural system. Chapter 5 also establishes a transparent process for compliance assessment using a capacity rating metric that is common across the qualification methods (i.e., analysis, testing, and comparative experience). Chapter 6 covers the various analytical methods used to determine nonstructural capacity and support nonstructural testing strategies. Static and dynamic analysis methods are introduced to provide the design professional with a seismic analysis tool kit to draw upon. Analytical methods enable the achievement of nonstructural qualification. Chapter 7 describes nonstructural testing methods, from definition of the seismic test environment to performing the qualification test. Seismic simulation testing is the most effective means for achieving compliance to satisfy modern-day building code seismic requirements. Knowing how to conduct a seismic test is paramount. Chapter 8 describes how to use earthquake experience methods to infer qualification based on comparison with previous earthquake events. Chapter 9 describes the process of combining analytical and testing methods for qualification of large-class nonstructural systems that are physically too massive or too expensive for practical testing consideration. Large-class systems are the most difficult and challenging systems to seismically qualify.

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Chapter 10 concludes the book by looking into the future and evaluating what is on the horizon for nonstructural earthquake protection. Research needs are identified to help advance nonstructural protection. The time has come to bring nonstructural research out from the shadow of structural protection and into the forefront.

References ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10. ASCE, Reston, VA. Charleson, Andrew. (2008). “Non-structural elements: Those likely to cause structural damage.” Seismic design for architects: Outwitting the quake. Elsevier, Burlington, MA, 157–171. International Code Council (ICC). (2011). 2012 International building code, Country Club Hills, IL.

Acknowledgments

About 13 years ago I was newly hired by Square D Company/Schneider Electric, Nashville, Tennessee. My first assignment was to figure out how to qualify equipment to satisfy the 1997 Uniform Building Code’s lateral force demands. It seemed like a reasonable task at the time but it turned into a 6-year project. Early in that effort I was lucky enough to cross paths with Bob Bachman (who chaired the Structural Engineers Association of California’s Seismology Committee during development of 1997 UBC) and asked him how to dynamically test equipment with static force demands. Bob was curious and agreed to meet with me and coauthor Phil Caldwell. That meeting started the ball rolling. Now I can give proper thanks to Bob, who placed such trust in a couple of equipment guys from Square D. This book is the culmination of that activity, but there was something missing. Gary McGavin brought the missing ingredients. Gary’s geology and architecture expertise and his passion for nonstructural protection made the writing complete. My thanks to Gary. I also want to thank my manager at Schneider Electric, Gary Pollard, who fully supported me during manuscript development; Baltimore Aircoil Company, Baltimore, Maryland, for providing illustrative material on nonstructural cooling towers, and Panos Papavizas for providing valuable case study comments; Dave Jensen of Varian Medical Systems, Inc., Palo Alto, California, for providing illustrative material on nonstructural medical equipment; Denise Loria of APC/Schneider Electric, Costa Mesa, California, for providing UPS product data and models; and Beau Williams of Williams International Consulting, Redwood City, California, for providing building illustration material. Lastly, I need to thank a true rocket scientist, Jim Suhre, whom I had the good fortune of working with in my years with Raytheon, Bristol, Tennessee. Jim schooled me on the dynamics of structures and introduced me to systems analysis concepts. And for that I am grateful. —Jeff Gatscher I have had many mentors throughout my life, all of whom have provided me with the tools that I needed for my nonstructural seismic journey. My geology professors at University of California–Riverside, including Michael A. Murphy, Lew Cohen, and Shawn Biehler, all xvii

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provided me with a sound understanding of basic science and an unending joy and desire to learn, and they supported me as a future architect who wanted to understand geology rather than be a geologist. I still owe much to Charles W. Roberts and Wyle Laboratories for introducing me to nonstructural seismic issues and encouraging me to write my first book on the subject. As an architect, I owe a tremendous gratitude to the late Herman O. Ruhnau, FAIA, who taught me what I know about the profession of architecture. Jeff Gatscher, Phil Caldwell, and I appreciate the assistance of Bob Kain, AIA, of HMC Architects in Ontario, California, and Pat Reyes-Capelli and Eazlene Freeman of Kaiser Permanente Healthcare in Ontario, California, for allowing us to use one of their surgery centers as a case study in our preparation of this text. Scott Nebenzahl and Michael Price of Sesimic Warning System, Inc., of Scotts Valley, California, assisted in getting the technical aspects of Earthquake Early Warning Systems correct, which I sincerely appreciate. Thanks are also deserved to Jeff Gatscher for keeping me on track and actually writing the bulk of this text—especially most of the math parts, which always frighten an architect. Finally, I would like to thank Phil Caldwell for talking to me early on about collaboration on this book at an ATC-58 meeting. Thanks. —Gary L. McGavin, AIA In the 1970s, advances in earthquake engineering practice were mostly developed to meet the needs of the commercial nuclear power industry. In the latter part of the twentieth century, earthquakes in urban areas around the world emphasized the need to improve the seismic design provisions of building codes. These were necessary to both save more lives and to improve the chances of postevent functionality for the critical facilities necessary for recovery and sustaining essential public health and welfare services. In 2000, the U.S. building codes underwent their biggest change ever by being consolidated from three regional model codes into one national model code. While incremental improvements have historically always been made to regional model codes, the 2000 International Building Code (IBC) adopted more fully developed, project-specific concepts embodied in the 1997 National Earthquake Hazards Reduction Program (NEHRP) provisions. I preface my acknowledgments with this overview because this is the “bus” that was about to hit me back in 1998, when I was tasked by my company Square D Company/ Schneider Electric, Seneca, South Carolina, to help review seismic qualification for building code applications. As an electrical engineer, my goal was to keep this assignment brief by getting a new colleague of mine, Jeff Gatscher, to take the lead. Jeff had a more appropriate background in advanced structural analysis and, by getting him working full time on this, I would be able to get back to my day job as soon as possible. My plans quickly fell apart; despite our best efforts, both Jeff and I kept getting hit by this bus—it is a very big bus. To survive, Jeff and I decided that a better strategy was to learn how to get on the bus. By the time the 2000 IBC was first published we were quickly learning how to ride the bus. Our ticket turned out to be finding and forming relationships with the key people in the global earthquake engineering community. This community was eager to work with industry and share insights that would bridge a vast knowledge gap in seismic mitigation. My thanks to our first bus driver, Bob Bachman, for picking Jeff and me up and getting us on the bus.

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During our early efforts to get our arms around the intent of the prescriptive rules of the IBC, Jeff kept asking the question “Where is the book?” to translate the paradigm of structural engineering into the domain of equipment engineering. Despite hundreds of hours of searching I could never find “the book” until I came across Gary McGavin’s 1981 book Earthquake Protection of Essential Building Equipment. My thanks to Jeff for championing the need for a book on building code-oriented equipment seismic qualification and to Gary for joining this project to update his excellent earlier work. A former chairman of the California Seismic Safety Committee, Gary brings the perspective of a practicing architect, academic, shake-table qualification expert, and, as a bonus, he has formal training in geology. Gary’s cross-functional background is rare and gives him the credibility to speak with authority on the need to approach seismic mitigation from a systems perspective. I also want to thank my former Director of Engineering at Square D Company/Schneider Electric, Nashville, Tennessee, Wes Worley, for putting the pressure on me to improve the company’s technical competency in equipment seismic qualification and then providing the resources to make it happen. This not-so-short-term assignment has provided me with the opportunity to form professional relationships with the definitive experts in earthquake engineering and earth sciences around the world. I owe hundreds of them a deep expression of gratitude for listening to my questions and taking the time to teach me. This book is one part of many efforts to give back by carrying their knowledge forward to the end-users in the building design and construction profession. Most importantly, I want to thank my wife, Jackie. Her constant support over the years has made it possible to dedicate significant personal time to attend numerous meetings on weekends in the far corners of the world and spend evenings in university libraries to conduct research. Thanks. —Philip J. Caldwell

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Part I

Earthquake Demand

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Chapter 1

Introduction to Demand Allocation

Earthquake demand is a term to describe the environmental loading conditions associated with earthquake ground shaking. Demand allocation involves proper assignment of earthquake-induced environmental loading to the various building systems and their subsystems that require earthquake protection. This chapter introduces the concept of earthquake demand allocation using a systems strategy that is compatible with the earthquake protection philosophy embedded in modern-day building codes. The target application is nonstructural building systems in essential buildings. The primary focus examines the building code’s seismic requirements for designated seismic systems. The techniques presented are equally applicable for nonessential building applications as well. Systems design and systems analysis are closely related conceptual frameworks used to build processes for defining and analyzing the architecture, components, modules, interfaces, and data for a system to satisfy specified requirements. We combine these constructs to formalize a methodology that can be applied to nonstructural building systems. In this context, systems design is a process for defining the architecture that governs hardware design (e.g., modules, subassemblies, and structural elements), hardware interfaces, and all associated data for a nonstructural system to satisfy functional requirements. The architecture we are interested in is the one that governs nonstructural building systems. The requirements we address are the seismic requirements defined in the ASCE/SEI 7-10 standard (ASCE/SEI 2010). The model building code used in the United States, and elsewhere, that is linked to this standard is the 2012 International Building Code (IBC) (ICC 2011). 3

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Earthquake Protection of Building Equipment and Systems

Nonstructural code provisions have adopted sweeping changes over the last 10 to 20 years. A primary motivation for radical code evolution is to support more advanced treatment of nonstructural systems beyond mere anchorage calculations. Stated simply, the code’s compliance expectations have expanded to include active operation performance at design-level earthquake demands. This requires the suppliers and manufacturers of products designated as nonstructural to treat compliance validation as a product development activity that involves implementation of advanced analytical and testing techniques (as discussed in the second part of this book). Stakeholders new to nonstructural earthquake protection may perceive modern-day code provisions as complex. The underlying theme of this writing is to transform perceived complexities into tangible implementation practices compatible with new code expectations. The code’s philosophy has evolved to support both advanced and basic design techniques. The success of this book will hinge on how well perceived complexities can be resolved into standard operating procedures. The balance of this chapter presents systems design principles and philosophy relevant to nonstructural earthquake protection.

1.1 Nonstructural Building Systems The nonstructural category includes everything contained within the building skeleton that makes a building function. The building skeleton includes the beams, columns, walls, lateral bracing, and floor slabs. Nonstructural elements are the architectural components, equipment, and distribution systems that are not part of the building structure (i.e., nonstructural), whether inside or outside, above or below grade. Nonstructural systems transform an empty skeleton into a functioning building suitable for human occupancy. The nonstructural category covers a plethora of possible building applications. Figures 1-1 through 1-11 present various perspective views of nonstructural systems, from the building macroscopic level down to the equipment subassembly level. The diverse types of nonstructural systems can be grouped into four general categories: • Architectural Components, such as parapets, partitions, façades, soffits, cladding, glazing, ceiling systems, etc. • Mechanical and Electrical Equipment (also called Components), including building service equipment (such as pumps, generators, air handlers, compressors, transformers, switchgear, and power supplies), and building tenant equipment (such as medicalrelated technology and equipment, emergency communication equipment, and other specialized process equipment) that is permanently anchored during installation. • Mechanical and Electrical Distribution Systems, such as process piping, fire sprinkler systems, HVAC ductwork, lighting systems, electrical busway, conduit, cable trays, etc. • Building Occupancy Contents, including bookcases, shelving, office equipment, and everything else a building contains that is not permanently anchored to the building structure. It is worth noting that current codes do not address this category, and thus, there will continue to be problems associated with occupant contents following earthquakes.

Introduction to Demand Allocation

5

Figure 1-1. Exterior of the case study surgery center.

Roof Mounted Nonstructural Installations

Figure 1-2. Macroscopic building view showing several nonstructural items installed on the roof; the nonstructural category includes architectural systems, mechanical and electrical equipment, distribution systems, and building occupancy contents. Source: Illustration by Beau Williams.

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Earthquake Protection of Building Equipment and Systems

Figure 1-3. Macroscopic building overview with the roof section removed, exposing the complex “rat’s nest” of nonstructural building systems; nonstructural systems transform an empty skeleton into a functioning building suitable for human occupancy. Source: Illustration by Beau Williams.

Figure 1-4. Macroscopic building overview with the roof section removed and sunlight added; the shaded areas represent shadows when the roof is removed. Source: Illustration by Beau Williams.

Introduction to Demand Allocation

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Distribution Elements

Equipment Elements

Mechanical Room

Electrical Room

Figure 1-5. Building cut-away view exposing typical electrical and mechanical rooms needed to support nonstructural active systems; active building systems include a chain of nonstructural equipment- and distribution-type elements. Source: Illustration by Beau Williams.

Figure 1-6. Typical mechanical equipment installed in a building mechanical room shown in the center room of Figure 1-5.

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Earthquake Protection of Building Equipment and Systems

Figure 1-7. Typical mechanical distribution piping systems installed in a building mechanical room.

Electrical Distribution

Electrical Equipment

Figure 1-8. Room-level view exploring a typical electrical room needed to support nonstructural electrical active systems. Source: Illustration by Beau Williams.

Introduction to Demand Allocation

9

Figure 1-9. Typical electrical equipment installed in a building electrical room shown in the room on the right of Figure 1-5.

Figure 1-10. Typical electrical distribution conduit systems installed in a building electrical room shown in the room on the right of Figure 1-5.

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Earthquake Protection of Building Equipment and Systems

Device-level View

Equipment With Covers

Covers Removed

Figure 1-11. Device-level view of typical electrical equipment with the covers removed to expose the internal workings (functional devices) of modern-day nonstructural systems; with the covers on, the equipment looks like a “black box” building component; with the covers removed, nonstructural system elements are revealed. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, Rhode Island. The focus of this book is on mechanical and electrical equipment and their distribution systems. The term nonstructural is used throughout this book to describe both equipment and distribution system categories. In the literature, the acronym MEP is often used to describe mechanical, electrical, and plumbing equipment. We decided to use the generic term nonstructural to describe all classes of equipment and distribution systems, which include specialized owner-supplied technology and equipment found in most essential buildings (e.g., hospitals and emergency communication centers). On occasion we have also chosen to discuss other architectural systems for illustration purposes. A fundamental question to ask is, What advantage can be gained by treating nonstructural components as functioning building systems? The answer to this question is paramount when the goal is to mitigate earthquake damage such that essential buildings remain operational in the aftermath of earthquake recovery. This point is best illustrated by considering a single system from the rat’s nest of nonstructural systems depicted in Fig. 1-3. For example, consider the emergency power system. There are literally multiple “equipment items” (i.e., black boxes) connected together via power distribution elements, like conduit and electrical busway, in order to deliver critical emergency power throughout a facility (Fig. 1-8). Within each equipment box there are literally multiple “electrical devices” that need to remain operational in order for the equipment to operate (Fig. 1-11). If one or more of the internal electrical devices fail or if one or more of the distribution artery lines fail, the emergency power system will not deliver the needed emergency power.

Introduction to Demand Allocation

11

The nonstructural system is a continuous chain of elements (i.e., products) that link together to perform an intended building function. The chain includes functional devices located deep within equipment platforms and the operational attachments responsible for connecting the elements to achieve a desired building function. It is the system that requires earthquake protection. The earthquake will find the weakest link in the nonstructural chain. The primary objective in earthquake risk mitigation is to identify the system’s weakest links before an earthquake does, such that seismic design improvements can be implemented. Systems design is the appropriate design tool to help identify a nonstructural system’s weakest links. Once identified, design enhancements can be incorporated into the various system elements to increase the system’s overall seismic resistance. In a nutshell, the identification of system elements and allocation of the seismic demand to each element is systems design in the context used for this book.

1.2 Systems Design Primer Earthquake protection of nonstructural systems has to be approached from multiple perspectives or domains (as shown in Fig. P-1). There is the building domain, which encompasses system installation and placement and routing of nonstructural system elements within the building, and accounts for potential adverse interaction between systems. There is a nonstructural product domain, which includes all design aspects of the various system elements (i.e., products) that make up the nonstructural chain. And lastly is the building code requirements domain, which dictates the earthquake demand placed on both the building and nonstructural systems. All three domains need equal consideration to effectively mitigate the risk of nonstructural earthquake damage. Evolution of modern building codes has indirectly adopted a holistic, systematic approach toward earthquake protection. The nonstructural qualification mandates for essential facilities and designated seismic systems call for three basic requirements (see Chapter 4 for details): • Retain structural position. • Avoid consequential damage caused by adverse systems interaction. • Maintain active operation performance following design-level earthquake demands. The challenge is to allocate code requirements to each of the elements that comprise nonstructural systems and do it using generic terms such that all types of nonstructural systems can be addressed. Requirements flow-down is a process to break down top-level code requirements and apply them at lower levels of system assembly. If the flow-down process can be formalized, this creates design traceability between top-level code requirements and subsystem allocation. Systems design is the most effective method to accomplish this task. This necessitates establishing a generalized systems framework to define the allocation traceability between top and bottom levels of system representation.

12

Earthquake Protection of Building Equipment and Systems

1.2.1 Systems Framework From a big-picture standpoint, the easiest way to conceptualize the issues at hand is by viewing things from the earthquake’s vantage. In essence, visualize riding the earthquake shock wave as it travels from the source, along the Earth’s surface, into the building foundation, through the building structure, into the building’s floor slabs, through the nonstructural anchorage, into the nonstructural’s force-resisting skeleton (FRS), and finally into the many devices that make up the nonstructural assembly. The path traveled by the shock wave from the geologic source to a device located within a nonstructural assembly that is located somewhere within a building is obviously influenced by many variables. Deciding where to start and which variables to consider can be daunting. This picture becomes clearer if you think of buildings as appendages embedded into the Earth’s surface, and nonstructural items as appendages attached to buildings. Ground motion must traverse a directional path into buildings and then into nonstructural items. As the ground shakes, the two appendages will shake and respond to the base input in series (i.e., ground shakes the building and the building shakes the nonstructural). Figure 1-12 illustrates this concept.

Device Response Nonstructural System Floor Response Building System

+

Ground Response Geotechnical System

Earthquake Event Figure 1-12. The three primary systems involved in nonstructural earthquake protection: (1) geotechnical, (2) building, and (3) nonstructural.

Introduction to Demand Allocation

13

This perspective is important for several reasons. First, all nonstructural earthquake demands are dependant on both ground motion characteristics and building dynamic effects (e.g., amplification). Second, nonstructural demands are applied through connection points to the building in the form of input motions and are reacted as inertial forces. And lastly, the division lines between the building appendage, and earth and nonstructural appendage and the building, make logical places to break down the problem into smaller, more manageable pieces.

1.2.1.1 Nonstructural Decomposition At the core of systems design is the concept of framing problems in a top-down perspective and then to divide and subdivide until a big problem is transformed into smaller, isolated elements. This process is called system decomposition. The next step is to identify interface relationships between the isolated pieces. For example, Fig. 1-12 can be decomposed into three primary systems: (1) geotechnical, (2) building, and (3) nonstructural. Figure 113(a) shows this systems view representation as a top-level block diagram. This diagram is not very useful as is. Each system element needs further decomposition and the system interfaces need to include functional relationships between the elements. Figure 1-13(b) shows an expanded version of the block diagram with the nonstructural system divided into two subsystems: (1) mechanical-related elements and (2) active operation-related elements. Mechanical-related elements include a force-resisting skeleton (FRS), attachments, and anchorage. The active operation elements include functional devices. To clarify these terms, a generic definition for each subsystem element is provided in Table 1-1. It should be noted that throughout this book the naming convention used to describe nonstructural system elements is different from the partial breakdown and naming convention adopted in ASCE/SEI 7-10 (see Fig. C11-10 of ASCE/SEI 7-10, p. 476). The difference in naming convention results from a deeper level of decomposition used here to represent nonstructural systems. The code uses the term component to describe the bulk of a nonstructural item. The word “component” conjures up an image of a nonstructural item as a black box building component. Nonstructural systems are more than black boxes that are secured to a building structure. To treat nonstructural “systems” as “components” defeats the purpose of employing systems design principles. Table 1-1 presents the naming convention adopted herein to define nonstructural system elements. These generic terms are referenced throughout the text. Now that we have defined the system elements depicted in Fig. 1-13(b), the remaining task is to identify functional interface relationships between the elements. Identification of functional relationships between system elements is an important aspect of systems design. These interface relationships provide the mechanism to extend seismic protection principles beyond the single-component approach. The objective is to consider an entire building system, which may be composed of multiple equipment items that are functionally connected via a nonstructural distribution chain. This is a key concept. The interface relationships relevant to nonstructural protection and seismic qualification include the following: • Structural Transmissibility: Characterizes the seismic loading transmission path between system elements. The transmission path is quantified by using structural

14

Earthquake Protection of Building Equipment and Systems

System Interfaces

Geotechnical System

System Elements

(a)

Geotechnical System

Geotechnical Elements

(b)

Nonstructural System

Building System

Functional Interface

Building System

Buildingg Elements

Functional Interface

Nonstructural System

Mechanical Elements

Functional Interface

Active Operation p Elements

Local Soils

Floor (n+1)

Anchorage

Earth Crust

Structure

Attachment

Functional Device (1)

Epicenter

Floor (n)

Force Resisting Skeleton (FRS)

Functional Device (2)

Focus

Structure

Attachment

Functional Device (n)

Foundation

Anchorage

Earthquake Demand

Figure 1-13. Top-level systems design block diagrams: (a) basic block diagram concept; (b) expanded diagram using subsystem decomposition.

Table 1-1. Nonstructural System Element Definitions. Nonstructural System Element

Mechanical Subsystem

Definition/Description

Force-Resisting Skeleton (FRS)

Attachments

Operational Attachment

Isolation Attachment

Operating elements that connect between the nonstructural FRS and the building structure or could insert between two nonstructural systems in a distribution chain. An operational attachment includes both mechanical and electrical elements that are intended to support active operation functions. These attachments are necessary operational umbilicals that are required for the nonstructural system to function. Examples of operational attachments include piping, ducting, tubing, cabling, conduit, wiring, and grounding. Structural elements that connect between the FRS and the building structure. A bracing attachment is purely structural in nature and is used to provide a structural link between the FRS and the building to resist environmental loads. Examples of bracing attachments include brackets, angles, or other structural shapes that provide structural support and load resistance to the FRS.

(Continues)

15

Mechanical elements (springs and dampers) that insert between the FRS and the building structure to isolate the FRS from the building. Isolation attachments (also called shock and vibration isolators) are mechanical energy absorbers that are intended to attenuate loading between the nonstructural FRS and the building. Examples of isolation attachments include compression springs and dashpots, and compression springs with snubbers.

Introduction to Demand Allocation

Bracing Attachment

Structural members or assemblies of members, including frames, enclosures, struts, rods, panels, etc. FRS assembly members can be joined together using mechanical fasteners or can be weldments. Monocoque construction techniques are also included as FRS types. The nonstructural FRS provides support for subassemblies, modules, and internal devices. The FRS also provides overall structural stability for the nonstructural platform. The FRS should be viewed as the nonstructural systems structural skeleton to resist all environmental and operating loads.

16

Nonstructural System Element

Definition/Description

Mechanical Subsystem

Anchorage

The final connection points securing the nonstructural FRS to the building structure, with or without the use of attachments. Anchorage includes bolts, concrete anchors, welds, or other mechanical fasteners for positively securing the nonstructural FRS to the building without consideration of frictional resistance produced by the effects of gravity. For example, a typical equipment FRS can be anchored to a concrete pad directly with anchor bolts without the use of either bracing or isolation attachments, or an equipment FRS can be placed on isolation attachments and the isolator is anchored to the pad. Anchorage connection points to the building structure are the elements that cross the line between the building and nonstructural systems and react the dynamic loading experienced during the earthquake.

Active Operation Subsystem

Functional Devices

Logical subgroupings of nonstructural active functions (operational) typically organized and arranged as physical devices, modules, components, or subassemblies that mount to the nonstructural FRS. Functional devices can be electrical, mechanical, or electromechanical in nature. Some devices also function as load-bearing members of the FRS in addition to being functional devices. For example, in a piping distribution system the pipe acts as both an FRS and a functional device with containment of pipe contents being the active function.

Earthquake Protection of Building Equipment and Systems

Table 1-1. (Continued)

Introduction to Demand Allocation

17

dynamic transfer functions. Structural transmission paths should include response modifiers that account for inelastic reductions during occurrences of FRS nonlinear dynamic response. Transfer functions adjusted for inelastic response are more representative of earthquake dynamics. • Functional Interaction: Includes several types of interactions. There are mechanical impedance interactions that represent a structural feedback relationship between the forcing system (e.g., FRS) and the system being driven (e.g., functional device). There are connection interactions as well that describe the electrical and mechanical connection interface relationships between system elements. • Seismic Demand: Defines the resultant environmental loading and becomes the input seismic demand requirement for the system elements. Seismic demand includes forces, accelerations, and relative displacements. Seismic demand also represents a required clearance space or clearance envelope between elements, for example, the clearance space between one nonstructural item and an adjacent nonstructural item, or between a nonstructural system and the building. In essence, each connecting arrow between system blocks in Fig. 1-13(b) is replaced with a set of interface elements. Figure 1-14 displays a generalized systems design framework that includes the necessary interface relationships between system elements. This framework depicts the relationships for one nonstructural system containing one active functional device that is installed between building floor elevations n and n + 1. Two attachment options are depicted, considering possible attachment designs that could secure to the floor, walls, or ceiling of a given floor elevation. The framework may be readily extended by adding more branch points. Additional branches could be included to represent multiple equipment items in a distribution chain and/or represent multiple functional devices per equipment item in a given building nonstructural system. These branches may be cascaded or in parallel. Furthermore, the generalized framework may be simplified when typical engineering assumptions are made regarding the various system interactions, such as ignoring mechanical impedance effects and assuming linear FRS response (i.e., linear transfer functions). At first glance, this framework might appear overly abstract and too complex for implementation purposes. Clarification of the framework is best achieved with an example, as follows.

1.2.2 Systems Framework Demonstration The framework provides a tool for allocating seismic demands on individual nonstructural system elements regardless of whether the system represents a single equipment item or a distribution chain of equipment. This approach is applicable for specifying demand requirements on a project-specific basis or based on a wide geographic area approach. The project-specific method endeavors to evaluate demand requirements for a known building site location and for a specific building type. Building design professionals apply this approach. In this case, the exercise is to use geotechnical site data for ground motion in conjunction with using a detailed building model to arrive at project-specific building floor

18

Geotechnical System

Geotechnical Transmissibility

Local Soils

Earthquake Transmission Path from Earthquake Source

Earth Crust

Epicenter

Nonstructural System

Functional Interaction

Seismic Demand

Buildingg Elements

Structural Transmissibility

Functional Interaction

Seismic Demand

Mechanical Elements

Structural Transmissibility

Functional Interaction

Seismic Demand

Soil / Structure Interaction

Building Ground Input Motion

Floor (n+1)

Building Floor (n+1) Transmission Path from Foundation

Building / Anchorage Interaction

Building Floor (n+1) Input Motion

Anchorage

Attachment Transmission Path from Anchorage

Attachment / Anchorage Interaction

Attachment Input Motion from Anchorage

Attachment

FRS Transmission Path from Attachment

FRS / Attachment Interaction

FRS Input Motion From Attachment

Force Resisting Skeleton (FRS)

Functional Device Transmission Path From FRS

Functional Device / FRS Interaction

Functional Device Input Motion From FRS

Attachment

FRS Transmission Path from Attachment

FRS / Attachment Interaction

FRS Input Motion From Attachment

Anchorage

Attachment Transmission Path from Anchorage

Attachment / Anchorage Interaction

Attachment Input Motion from Anchorage

Clearance Clearance Envelope Envelope

Structure Building / Attachment Interaction

Floor (n)

Clearance Clearance Envelope Envelope Building / FRS Interaction

Focus Structure Clearance Clearance Envelope Envelope Building / Attachment Interaction Foundation

Building Floor (n) Transmission Path from Foundation

Building / Anchorage Interaction

Building Floor (n) Input Motion

Earthquake Demand

Figure 1-14. Generalized systems design framework for seismic qualification applications.

Active Operation p Elements

Functional Device (n)

Earthquake Protection of Building Equipment and Systems

Geotechnical Elements

Building System

Introduction to Demand Allocation

19

input demands. Project-specific nonstructural demand requirements are based on unique calculated floor-level inputs and represent building specific requirements. The geographic area approach attempts to allocate nonstructural demand levels that can envelope (“umbrella”) a known geographic region. The region could be a single U.S. state, a portion of a state, or the entire country. The concept is to establish nonstructural input demands using upper-limit code requirements. This approach typically assumes the maximum ground motion requirement for the target region, in conjunction with selecting upper-level building installation heights for maximum amplification effects. Geographic area demand requirements are based on generic floor-level inputs in accordance with code default maximums. This represents worst-case nonstructural requirements for any building in the specified region and for any floor in a building. The geographic area approach is the method used most often by nonstructural suppliers (i.e., original equipment manufacturers, OEMs) that distribute nonstructural product offerings to satisfy a wide range of seismic demands (i.e., cover a wide geographic region). The systems framework can satisfy the needs of both building design and nonstructural product design professionals. Depending on the nonstructural application, the framework can contract to a few elements and interfaces or can expand to include many application features. When applied to simple nonstructural systems using code default maximums, the framework is minimal. When applied to cover complex nonstructural systems using project-specific geotechnical and building input models, the framework can expand accordingly. Because functional devices contain unique system interfaces, seismic qualification of functional devices can be pursued as a separate activity (see Part 2, Chapter 9 for details). The following example presents an overview of the different approaches used to support the needs of building design and construction, and product design and manufacture stakeholder groups. A single building system (e.g., emergency power) is used for illustration purposes. Note that this example highlights nonstructural demand allocation. Determination of nonstructural capacity for this system is discussed in Part 2, Chapter 5 of this book.

1.2.2.1 Building Design and Construction The building design professional works on specific building projects. The geographic location of the building project and site soil classification dictate the nonstructural demand requirements as defined in the relevant code (2012 IBC, ASCE/SEI 7-10). The detail for code-prescribed seismic demands is covered in Chapter 4. The code’s default ground motion is used in conjunction with a unique building model to define building floor-level demand requirements. Figure 1-15 shows a block diagram to represent an equipment installation that is base-isolated for seismic inputs. This diagram reflects the nonstructural installation features that are relevant from the building designer’s perspective. Soil–structure interaction can be included in the building model and the ground motion input is based on the code’s design response spectrum (see Fig. 4-6 in Chapter 4). The building model provides floor-level demand requirements. A unique floor acceleration requirement is used to define the nonstructural seismic demand at any floor elevation within the building (see Eq. 4-12 in Chapter 4). Equipment anchorage demands are

20

Geotechnical System

Geotechnical Transmissibility

Site Soil Classification

ASCE 7-10 Design-level Response Spectrum (Figure 4-6)

Functional Interaction

Seismic Demand

Buildingg Elements

Structural Transmissibility

Soil / Structure Interaction

Building Ground Input Motion

Floor (n+1)

Building Floor (n+1) Transmission Path from Foundation

Nonstructural System

Functional Interaction

Seismic Demand

Mechanical Elements

Building Floor (n+1) Input Motion

Anchorage

Clearance Clearance Envelope Envelope

Structure

Building Site Lat / Long Coordinates

Building / Attachment Interaction

Floor (n)

Attachment

Clearance Clearance Envelope Envelope Building / FRS Interaction

Building Site Demand

Structural Transmissibility

Functional Interaction

Seismic Demand

Active Operation p Elements

Flexible Conduit Entry

FRS Transmission Path from Attachment

FRS / Attachment Interaction

FRS Input Motion From Attachment

FRS Transmission Path from Attachment

FRS / Attachment Interaction

FRS Input Motion From Attachment

Force Resisting Skeleton (FRS)

Structure Clearance Clearance Envelope Envelope Building / Attachment Interaction

Building Model

Foundation

Attachment

Clearance Assessment Building Floor (n) Transmission Path from Foundation

Building Floor (n) Input Motion

Anchorage

Equipment Base Isolation

Earthquake Demand

Figure 1-15. Systems block diagram for base-isolated equipment installation from the perspective of a building professional in support of a project-specific nonstructural application.

Earthquake Protection of Building Equipment and Systems

Geotechnical Elements

Building System

Introduction to Demand Allocation

21

directly defined and are dependent on floor elevation and equipment mass properties (refer to Box 6-1 in Chapter 6 for an example anchorage calculation). Seismic isolation attachments can be used to attenuate the nonstructural input going into the FRS. In this case, the intended isolation interaction between the anchorage and the FRS is to decrease the nonstructural demand requirements. The earthquake demand in this example first becomes modified by considering soil–structure interaction. Next, the input motion is amplified and filtered based on building structural dynamic effects. Finally, the demand can be attenuated by using seismic isolator attachments in the equipment installation design. However, the use of seismic isolators creates large FRS displacements that must be restricted. Isolation snubber elements (e.g., dashpots) need to be incorporated into the equipment isolation design to limit the amount of FRS displacement. The difference between steady-state vibration isolation and a dedicated seismic isolation design is discussed in Chapter 5. Any operational attachments that are secured to the FRS must have the necessary flexibility to accommodate large displacements. In addition, systems interaction avoidance needs to be considered during equipment installation, such that the FRS has enough clearance space to avoid contact with adjacent systems and the building structure. A visual inspection is done to verify that adequate clearance exists between the FRS and adjacent building systems. The power distribution elements for this system are the power lines (i.e., conduit and/or busway) that connect between equipment boxes. Figure 1-16 shows the typical block diagram for distribution elements. Applications include single-elevation distribution to multielevation systems that contain both horizontal and vertical distribution runs. The primary difference between equipment applications and nonstructural distribution systems is the number of input loading points per system. An equipment application will typically have one input at the anchorage whereas a distribution system like an electrical busway (or piping in a mechanical system) will have many input locations spread throughout the building. There are multiple locations along the length of the distribution system in which earthquake loading is applied. At each input location point there is a bracing-type attachment inserted between the busway and building structure. Each bracing attachment is anchored to the building. Thus, earthquake input is applied through the bracing anchorage, into the attachment brace, and then into the busway distribution system. Seismic relative displacements are a primary concern since distribution attachments will cause differential displacement inputs. This is especially true for vertical distribution runs that anchor between floors. Unique floor-level demand requirements are applied to the distribution system, including inertia loading affects from building inputs at the busway attachment supports, along with seismic relative displacements. System interaction avoidance becomes a fundamental design issue such that system interference between a busway run and everything else can be avoided or accommodated in the design. The resulting nonstructural demand requirements are highly singular and only applicable for each specific building and building location and for the specific nonstructural system installation within the building. This type of project-specific demand allocation is more common for designated essential facilities that are located in seismic-prone areas.

22

Geotechnical System

Building System

Geotechnical Transmissibility

Functional Interaction

Seismic Demand

Buildingg Elements

Structural Transmissibility

Soil / Structure Interaction

Building Ground Input Motion

Floor (n+1)

Site Soil Classification

ASCE 7-10 Design-level Response Spectrum (Figure 4-6)

Building Floor (n+1) Transmission Path from Foundation

Functional Interaction

Seismic Demand

Mechanical Elements

Building Floor (n+1) Input Motion

Anchorage

Functional Interaction

Seismic Demand

Active Operation p Elements

Clearance Clearance Envelope Envelope

Structure

Building Site Lat / Long Coordinates

Structural Transmissibility

Building / Attachment Interaction

Floor (n)

Attachment

Clearance Assessment Clearance Clearance Envelope Envelope

Building / FRS Interaction

Building Site Demand

Busway Distribution Elements (FRS)

Structure Clearance Clearance Envelope Envelope Building / Attachment Interaction

Building Model

Attachment

Foundation

Building Floor (n) Transmission Path from Foundation

Building Floor (n) Input Motion

Anchorage

Earthquake Demand

Figure 1-16. Systems block diagram for distribution systems from the perspective of a building professional in support of a projectspecific nonstructural application.

Earthquake Protection of Building Equipment and Systems

Geotechnical Elements

Nonstructural System

Introduction to Demand Allocation

23

1.2.2.2 Product Design and Manufacture Nonstructural systems are composed of products designed and engineered by manufacturing companies. The professionals responsible for nonstructural product design and manufacture handle seismic demand allocation by using a wide geographic area approach. As a reminder, the geographic area approach treats both the geotechnical and building systems as generic entities. This implies that the evaluation is applicable for any site within the target region and for any building type. Figure 1-17 shows an equipment block diagram that is more relevant to the nonstructural product professional. The geotechnical system contracts to a single variable, SDS, the maximum design-level ground response acceleration at short periods. Because we are using the geographic area approach in this example, the ground motion is the worst-case maximum requirement for the specified region as defined by the code’s earthquake hazard maps (see Chapter 4 for details). Soil–structure interaction is ignored because of the generic treatment. Since equipment applications can occur on building roofs, the worst-case building height factor, z/h = 1, is used to allocate demands. Because the base isolation design is most often the responsibility of the building design professional, there are no attachments between the anchorage and the FRS. The equipment is treated by the product engineer as having a direct load path between the anchorage and the FRS. The clearance envelopes are not reviewed by the equipment professional, since the building installation governs this requirement. The overall goal for the equipment engineer is to reduce FRS displacements as best possible by strengthening the structure as described in Part 2 of this book. The main area of concern to the equipment professional is maintaining the active operation—following application of design-level demands—of the various subassembly devices (i.e., functional devices shown in Fig. 1-17). If the functional device transmission paths from the FRS can be determined, the maximum input demand requirements for any device are also known (within the target region). This could be used to qualify the devices using standalone device testing, as discussed in Chapter 9. Qualification of functional devices could be pursued by either the equipment manufacturer or the device manufacturer if devices are manufactured by different companies. Detailed discussion on the topic of seismic qualification is covered in Part 2 of this book. It is obvious that the professionals involved in building design and construction and those involved in nonstructural product design and manufacture approach their respective missions in different ways. However, the end goal is the same: to identify the weakest links in the nonstructural chain such that design improvements can be incorporated to increase the system’s overall seismic withstand resistance. The third group of professionals depicted in Fig. P-1 are those involved in building code regulation and enforcement; they set the bar for defining the minimum expectations regarding nonstructural earthquake protection. In addition, this group can directly and indirectly influence the general philosophy needed to execute “effective” nonstructural protection strategies. In our opinion, both building professionals and nonstructural product professionals are presently catching up with new code expectations. This is the primary motivation for writing this book—to bridge the implementation gap between new code expectations and effective implementation.

24

Geotechnical System

Geotechnical Transmissibility

Worst Case Site Soil Classification

ASCE 7-10 Design-level SDS Response Acceleration (Figure 4-6)

Functional Interaction

U.S. Region State or Partial State

Seismic Demand

Building Ground Input Motion

Buildingg Elements

Floor (n+1) Height Factor - z/ h

Structure

Structural Transmissibility

Nonstructural System

Functional Interaction

Seismic Demand

Mechanical Elements

Building Floor (n+1) Input Motion

Anchorage

Building Height Factors

Structural Transmissibility

Functional Interaction

Seismic Demand

Active Operation p Elements

Device Demands

Attachment

Floor (n) Height Factor - z/ h

Functional Device (n) Transmission Path From FRS

Functional Device (n) / FRS Interaction

Functional Device (n) Input Motion From FRS

Functional Device (n)

Functional Device (n+1) Transmission Path From FRS

Functional Device (n+1) / FRS Interaction

Functional Device (n+1) Input Motion From FRS

Functional Device (n+1)

Force Resisting Skeleton (FRS)

Geographic Area Demand Structure

Attachment

Foundation

Building Floor (n) Input Motion

Anchorage

Earthquake Demand

Figure 1-17. Systems block diagram for equipment product development from the perspective of a product design professional in support of wide geographic area nonstructural applications.

Earthquake Protection of Building Equipment and Systems

Geotechnical Elements

Building System

Introduction to Demand Allocation

25

1.3 Modern Nonstructural Design Philosophy 1.3.1 A Need for Systems Design in Essential Facilities There are numerous building occupancies that need to maintain their integrity following a damaging earthquake. While this part of the discussion concentrates on healthcare facility seismic design, it is not intended to only apply to such facilities, but, rather, to the broader spectrum of essential facility occupancies. Following the 1994 Northridge earthquake, then–California Senator Tom Hayden remarked, “The public expects to be able to go to an acute-care facility and receive treatment for injuries sustained during the earthquake” (T. Hayden, personal communication to G. L. McGavin, 1994). Patients who are housed in acute-care facilities at the time of a damaging earthquake likewise expect to be able to continue to receive treatment for their ailments, which originally caused them to check into the healthcare facility. Unfortunately, the expectations of the general public are often not realized. Experience has shown that, even with moderate earthquakes, modern healthcare facilities close with all too much regularity. Following the 1971 San Fernando earthquake, legislators and code writers recognized the need for increasing design requirements for new and remodeled healthcare facilities. The Uniform Building Code (UBC, the applicable model code in California at the time) began to address earthquake design in greater detail with each 3-year code cycle. By the 1976 code, there was a provision that called for acute-care facilities to remain functional following earthquakes. Unfortunately, there was no clear linkage between code requirements and the implementation means and methods needed for achieving the function provision defined in the code. Over the years building codes have become increasingly more sophisticated with respect to specifying the seismic structural requirements for building structures. Regrettably, there remains work to do in changing the paradigm for design professionals when it comes to understanding nonstructural equipment functions and adequately addressing this issue via code provisions with respect to overall facility function after an earthquake. In other words, the code implementation being carried out in practice is all too often (and unfortunately) simple nonstructural “component” anchorage (i.e., position retention), which does not address active function to the necessary level through systems design. Other design professions outside the commercial building industry have long recognized that there is a need for systems design when function is a necessary requirement. The practice of systems design can be witnessed in aerospace design, naval design, nuclear power design, weapons design, and even race-car design. Few of us would accept rides on modern aircraft if we did not believe that the aircraft had been designed from a systems engineering point of view. Would we find it acceptable for the landing gear to pinch the hydraulic brake lines when the wheels fold into the fuselage? Yet we commonly do exactly this by not considering systems interaction in the design of our essential facilities such as acute-care facilities, police and fire stations, and emergency communication centers. In addition, systems design methods are not typically employed in the design of high-value industrial complexes that contain multimilliondollar process equipment.

26

Earthquake Protection of Building Equipment and Systems

During the 1994 Northridge earthquake, a simple systems interaction failure forced the closure of the new, state-of-the-art Olive View Hospital. The Olive View Hospital in 1994 was a replacement for the previous hospital that had received serious structural damage in the 1971 San Fernando earthquake. The codes and designers had addressed at least some of the systems and their components individually, which in turn forced the new hospital’s closure. These were mainly the fire sprinkler and suspended ceiling systems. Each system was independently designed to the latest code provisions. Unfortunately, the codes called for a component design philosophy in their requirements. The singlecomponent approach does not take into account the interaction of building systems. Differential movements between the sprinkler and the suspended ceiling systems caused conflicts at the penetrations of the sprinkler system through the ceiling, resulting in crossthread bending failure in sprinkler head pipes and consequent flooding of the hospital (Fig. 1-18). A flooded hospital is likely to fail operationally. This was the case in 1994 at the Olive View Hospital. Figure 1-19 shows a series of cross-polarized light model photos that highlight stress concentrations at the sprinkler drop due to “fixing” of the drop by the ceiling. To address this failure, there are two basic recognized remedies: provide a large enough penetration through the ceiling system (the simple, low-tech, low-cost solution), or provide a flexible connection at the sprinkler drop

Figure 1-18. Nonstructural systems interaction forced the closure of the new Olive View Hospital in the 1994 Northridge earthquake in Los Angeles. Source: Courtesy of Robert Reitherman/EERI.

Introduction to Demand Allocation

27

(a)

(b) Figure 1-19. Series of cross-polarized light photographs studying systems interactions: (a) overall section showing main structure, suspended ceiling system, and fire sprinkler system; (b) low stress level is indicated by few and widely spaced stress lines at sprinkler/ceiling penetration; (c) very high stress concentrations (indicated by many closely spaced stress lines) can be seen at the threaded connection between the main sprinkler feed and the sprinkler drop.

28

Earthquake Protection of Building Equipment and Systems

(c) Figure 1-19. (Continued)

from the main line (the higher-tech, more costly solution). The latter solution has been adopted by many in the building design industry. This and other systems failures in healthcare facilities throughout the affected area prompted the California Seismic Safety Commission to sponsor state legislation authored by the late California State Sen. Alfred E. Alquist to address this issue in 1994. The legislation was very clear in its direction to industry and design professionals for maintaining the operability of healthcare facilities following future damaging earthquakes, and it specifically called for nonstructural systems design considerations as it directed healthcare institutions to Identify the most critical nonstructural systems that represent the greatest risk of failure during an earthquake and submit the timetables for upgrading those systems. . . . California Health and Safety Code 130050 (a) (2) as Chaptered from SB 1953 (Hospital Facilities 1994) Without similar considerations of system design, healthcare facilities and other essential facilities in all earthquake areas will continue to have a higher risk of failure in future earthquakes, even when their primary structures remain relatively unscathed. Building owner contents have been omitted from the qualification procedures unless they met certain anchorage criteria based mainly on physical size and weight. Without

Introduction to Demand Allocation

29

consideration of owner-supplied contents, facilities will remain more vulnerable to failure due to earthquake shaking. Without considerations of systems design, component design, and building contents, the functional mandates of legislative bodies cannot be met. The general public should fully expect to come to an acute-care facility following a damaging earthquake and be able to receive care in the facility. This book endeavors to bring the philosophical approach of systems design to the designer, detailer, and installer. True systems design requires a new methodology of thought in the design and installation process. We believe that emerging parametric design tools in the architecture and construction industry (such as BIM, Building Information Modeling) will greatly assist this shift in design philosophy. There were those who did not believe computer-aided design (CAD) would ever replace hand drawings in the late 1970s and early 1980s, but it did. Similarly, BIM will replace current 2-D CAD drafting with 3-D BIM virtual building models in the not-too-distant future. In fact, standard printed plans and specifications may well fade into history due to BIM capabilities. Although building design professionals have been slow on the uptake of BIM technology, contractors and construction managers have been using it to convert 2-D drawings into BIM models prior to bidding for almost half a decade. Universities with building design curricula have all embraced the BIM method and are educating tomorrow’s designers in the use of this tool. The practicing design profession will also soon embrace this new digital technology for more than its simple 3-D rendering capabilities. The basic premise of BIM is to drive collaboration by different stakeholders using a shared knowledge resource to digitally represent the physical and functional characteristics of a building. The BIM virtual representation facilitates systems designers to query the 3-D building model for identification of potential system interaction problems, based on embedded rules using clash detection. For example, a detection alarm will notify the designer if a secondary structural brace run has violated the required clearance envelope of a fresh air intake and is a potential interference problem, as shown in Fig. 1-20. The BIM approach will greatly improve systems design and will support the early discovery of system interaction problems during the design-build process. However, using building design automation technology is not the only way to implement systems design methods. Systems design can be readily implemented using conventional techniques and does not require additional work from the design professional—it only requires working a little differently. We have provided a simplified architectural systems interactions failure (see Box 1-1) based on a case study of a covered walkway soffit in an elementary school in Calexico, California, due to the April 4, 2010, Sierra El Mayor earthquake in northern Baja. The example illustrates the cascading sequence of events that can occur during an earthquake. This started with the failure of a single component, which overstressed similar components until the entire nonstructural soffit system failed. The failure of the soffit then caused the failure of other components during its collapse through adverse systems interaction, which eventually could have prevented the safe exiting from some of the school’s classrooms had school been in session during the earthquake.

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Earthquake Protection of Building Equipment and Systems

Figure 1-20. The often-quoted structural member penetrating an air duct is shown here, in an emergency operations center in southern California; appropriate use of BIM technology by the design team could have prevented this interference; the missed interference forced the contractor to field-fabricate a sleeve inside the air plenum that was airtight and did not cause whistling.

Introduction to Demand Allocation

Box 1-1. A Systems Interaction Failure ASCE/SEI 7-10 provides a clause to safeguard against unwanted systems interaction. The paragraph is titled “Consequential Damage” and says, The functional and physical interrelationship of components, their supports, and their effect on each other shall be considered so that the failure of an essential or nonessential architectural, mechanical, or electrical component shall not cause the failure of an essential architectural, mechanical, or electrical component. (ASCE/SEI 2010, p. 113) While this paragraph does not specifically mention systems analysis, it does embody the philosophy. We refer to this ASCE/SEI 7-10 requirement as systems interaction avoidance. Systems design requires that the designer consider the impact of the failure of any part of the system on the rest of the system and the potential impact on other systems or components. This concept is generally difficult to get across to some design professionals who have been educated on a “component only” approach. The April 4, 2010, Sierra El Mayor earthquake (Mw7.2) produced a nonstructural walkway soffit failure at Jefferson Elementary School in Calexico, California, that clearly illustrates the concept and importance of systems design. This particular school was designed and constructed in the 1960s, to the California Field Act standards. Many mistakenly think that the Field Act is an actual code, but it is only enabling legislation that allows a higher and better standard to be used for the design and construction for public schools in California. The Division of the State Architect (DSA) has higherdesign-level requirements placed in the CBC that are allowed by the Field Act and are incorporated into the regular model building code for both gravity loads and lateral loads. In the 1960s very little consideration was given to the design of nonstructural components beyond gravity loads, and there was nothing required for systems design. The particular failure at this school was a collapsed exterior walkway soffit that exhibits the importance of systems design consideration, even though the failure in itself was not a typical systems failure mode. It was, in fact, more of a “cascading sequence of events” that can be viewed as a systems interaction failure. The building structures on the campus performed appropriately during the earthquake. One building had a failure of about 450 m2 (⬇ 1,500 ft2) of covered walkway soffit (Fig. 1B1-1) along the eastern and northern sides of one of the classroom buildings (pods). Fortunately, school was not in session, because the earthquake occurred on Easter Sunday; had school been in session, the consequences could have been much worse. Had students been going to or coming from recess at the time of the earthquake, one does not need much imagination to visualize students beneath the stucco soffit in Fig. 1B1-1. When the soffit collapsed, portions of it lodged against the walls and classroom doors (Fig. 1B1-2), jamming some of the doors shut. Again, had children been in school at the time of the earthquake, they clearly could have been trapped in their classrooms. This is an elementary school and being trapped in a darkened room for any length of time could have been traumatic for adults, let alone children. This is the first example of consequential damage or “adverse systems interaction” due to the cascading events

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Earthquake Protection of Building Equipment and Systems

Figure 1B1-1. Nonstructural exterior walkway soffit collapse at Jefferson Elementary in Calexico during the 2010 Sierra El Mayor earthquake in northern Baja, California. from the collapsed soffits. The inability to exit classrooms was a clear failure that is not allowed by the life safety portions of both the current code and the code in effect at the time of the design. The next consequential damage was that the soffit sheared off some of the classroom door knobs when it collapsed (Fig. 1B1-3). This would have hindered the ability of rescue personnel to open those doors after the collapsed soffit was cleared away. The doors had security plates (Fig. 1B1-3) which would have made prying of the doors open by the rescue personnel even more difficult. The final consequential damage was a hose bib that was also sheared off when the soffit collapsed (Fig. 1B1-4), causing flooding in the area adjacent to the door with the sheared-off door knob. This nonstructural failure clearly illustrates the unwanted consequences of systems interaction during earth shaking. As designers, we need to move toward a higher level of understanding in our designs and what the ramifications of failures in our designs will be downstream of the component itself.

Introduction to Demand Allocation

Figure 1B1-2. Nonstructural exterior walkway soffit collapse blocking several classroom doors. As previously mentioned, this school was designed in the 1960s when design professionals paid little attention to any nonstructural components beyond gravity conditions. Interestingly, this school had several similar classroom pods (all were designed and constructed at the same time) that had either minimal or no separation of the soffits, as seen in Fig. 1B1-5. We have idealized what we believe was the type of detail used for the soffits at this school (Fig. 1B1-6) based on field observations in the days immediately following the earthquake. Everything shown is based on our field observation except exactly how all of the vertical hanger wires were attached to the structure above. It is normal today to use threaded eye screws and attachment clips that are either riveted or screwed to the metal decking and attach the hanger wires to these clips. Another older method sometimes used (which we have shown in the figure) is to predrill the decking and loop the hanger wires through the metal decking. We have surmised that the hanger wires were attached through the metal deck as shown in Fig. 1B1-6. Figure 1B1-7 shows a detail of the collapsed soffit where the black iron tubes were tack welded to the black iron channels embedded in the stucco system. The particular tube shown here did not pass through the web of the wide flange but, rather, was tack welded to the bottom side of the wide flange. Both conditions were observed in the field. Adjacent to the tube steel (on the right) is one of the vertical suspension wires that appears to have unraveled at its upper end but remains securely attached to the black iron channel. Figure 1B1-8 shows the point from which this section of the soffit dropped, as well as evidence of penetrations through the metal deck for the vertical hanger wires to pass through the deck.

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Earthquake Protection of Building Equipment and Systems

Figure 1B1-3. The collapsed soffit sheared off door knobs and this door had a security plate that made rescue more difficult.

Interestingly, another school in the district, Mains Elementary, has a similar construction type and was constructed about the same time as Jefferson Elementary. Mains Elementary is located about 3.2 km (2 mi) to the northwest of Jefferson. The USGS shakemap for the main event shows increasing accelerations as one moves west of Jefferson toward Mains Elementary School. The soffits at Mains Elementary were similar except for one detail. The soffit suspension wires were wrapped around the main structure rather than passing into the steel deck as shown in Fig. 1B1-9. The soffit at Mains Elementary was quite large in area, as shown in Fig. 1B1-10. The school district cut a hole in the soffit for viewing by the reconnaissance team. There was one location on the west side of this pod where the soffit was separating. When the district cut a hole in this soffit, it was clear that the suspension wire had not been attached during construction, causing a small area of the soffit to sag about 2.5 cm (1 in). It is possible that the soffit had settled prior to the earthquake.

Introduction to Demand Allocation

Figure 1B1-4. The hose bib in the lower left of the photograph was also sheared off when the soffit failed.

Figure 1B1-5. The westernmost classroom pod soffit on the east and west sides of this classroom pod had some separation of the soffit, but there was no full collapse of any of the soffits on this classroom pod or the remaining classroom pods. The soffit detailing in all of the pods on Jefferson Elementary was similar to the detailing on the east pod with the soffit collapse. Figure 1B1-6 shows the detailing of the soffit suspension system on Jefferson Elementary School. Note that because it was designed in the 1960s, there were no provisions to prevent lateral displacement of the soffit, nor were there any compression posts to prevent vertical displacements.

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Earthquake Protection of Building Equipment and Systems

Built-up Roofing

Metal Decking

Edgemetal Drip 2

2

WF

5

Channel

3 1

Classroom Interior

36

Black Iron In Sheet Metal Channel

1

4

Expanded Metal lath

Sheet Metal Facia

Stucco

Reinforced Masonry Wall

1

Most suspension wires had 3 full turns at the lower connections.

2

Wire turns “untwisted” when the soffit fell. It is unknown how many turns they had. None of the suspension wires appear to have broken.

3

Small black iron square tube tack welded to black iron channel frame.

4

Black iron channel frame used for suspension was wire tied to black iron channels embedded in stucco soffit.

5

Some small black iron tubes passed through the web of the wide flange and some were tack welded to the lower flange.

Figure 1B1-6. Architectural detail showing the as-built condition of the soffit in the 1960s based on field observations by the author on April 6, 2010. It is unknown whether anything secured the wires into the metal decking. The wires did not appear to be broken, and there were no portions of the wires left in the metal deck. The wires appeared to have “untwisted.” It is not known how may turns the upper connections had. One hundred percent of the upper connections failed in the collapsed soffits. The bottom connections mostly had three turns, which is the industry standard today. There were no splay wires or compression posts, as are common in modern ceilings. The soffit cantilevered about 3 m (10 ft).

In the case of Jefferson Elementary School, it is apparent that the soffit collapse could have been avoided by better architectural detail requirements and attention to those details during the installation of the soffits. The lack of compression posts and splay wires, as are used today, did not play any role in the collapse, as evidenced by the good soffit performance at Mains Elementary in the same vicinity. The installation

Introduction to Demand Allocation

Figure 1B1-7. The suspension scheme used in the soffit in a collapsed portion of the soffit.

Figure 1B1-8. The structure with some of the black iron tube steel that ripped loose from the soffit; the black iron tube in Figure 1B1-7 was attached to the bottom of the wide flange about 4 ft (1.2 m) closer to the viewer than the arrow.

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Earthquake Protection of Building Equipment and Systems

Figure 1B1-9. Mains Elementary School soffit suspension installation with suspension wires wrapping around the structural members rather than passing into the metal decking above the structural steel.

Figure 1B1-10. Mains Elementary School with a large soffit that suffered no damage.

Introduction to Demand Allocation

39

of the vertical support wires was compromised by not being effectively secured either to the metal deck or to the structure supporting the metal deck. The systems design lesson here is clear. When one component of a system fails, it can cause additional failures within its system and to adjacent components/systems. In the case of Jefferson Elementary, it is apparent that for unknown reasons some of the vertical support wires failed to maintain their attachment to the metal deck. As one wire failed, this transmitted the added load to adjacent wires and the wires began to unzip. As the unzipping progressed down the length of the soffit, the weight of the soffit was transferred to the center black iron channels with the tack-welded tubes that ran through the wide flange web (Fig. 1B1-6), overstressing the light tack welds and likely causing the soffit to continue its collapse in an accelerating manner. As the soffit collapsed, it slid down the exterior wall of the classrooms, severing some door knobs and a hose bib before coming to rest blocking some of the exit doors. The nonstructural soffit collapse compromised the life safety exit system for some of the classrooms at Jefferson Elementary. Fortunately, school was not in session. The key point here is that if component and systems interaction are not considered in design and construction, there is a higher likelihood for facility operational failure.

1.3.2 Systems Mantra We rely on systems design as a guiding principle and frequently have been asked, over the course of writing this book, why we have placed such an emphasis on systems design. Individually and collectively we have learned, through our own mistakes and successes, that not considering system interactions and functional interfaces, at the design level, can be a recipe for failure. This is especially true for nonstructural systems. In addition, the systems framework introduced here can be viewed as a necessary foundation schema that could support performance-based seismic design principles (further discussed in Chapter 10). Code provisions are likely years away from adopting a truly comprehensive, performance-based approach that includes building equipment and their systems. Any such implementation would require employing a systems framework with attributes similar to the framework outlined here. Lastly, the overriding motivation for incorporating a systems design philosophy is the increased importance of nonstructural performance. A critical reason for maximizing nonstructural protection is ensuring that emergency services are operational in the aftermath of damaging earthquakes, for hospitals and other essential facilities. The underlying premise we subscribe to is that the systems design approach to nonstructural protection is the most viable method to effectively minimize the seismic risk to nonstructural systems. This opinion is based on the historical poor performance of nonstructural systems over the last half-century where a systems approach was not adopted. There are two hurdles in the way of effective implementation of nonstructural protective measures based on the systems principles prescribed herein. The first hurdle is communicating the technical ideas in a manner that all stakeholders can understand and

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Earthquake Protection of Building Equipment and Systems

implement. The communication hurdle is believed to be one that can be adequately navigated. The second hurdle is much less tangible. Because nonstructural systems include such a diverse population of applications, there is a noticeable diffusion of responsibility within the stakeholder community. In other words, no single stakeholder can say, “The buck stops here,” regarding responsibility for ensuring nonstructural protection. Most often, the buck never stops, and stakeholder delegation is the governing motivation regarding nonstructural protection. Also, nonstructural protection is often delegated to the point where it falls through the cracks and is simply overlooked. Jumping this hurdle is more difficult. It requires that all stakeholders involved in nonstructural protection understands their roles and responsibility with respect to requirements implementation. Chapter 2 discusses the various stakeholder perspectives and misconceptions regarding codes and stakeholder responsibilities.

References ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, Reston, VA. Hospital Facilities Seismic Safety Act of 1994, SB 1953 (1994). International Code Council (ICC). (2011). 2012 International building code, Country Club Hills, IL.

Chapter 2

Stakeholder Commentary

The nonstructural seismic problem is not as comprehensively controlled as the problem of preventing unacceptable structural earthquake damage (Masek and Ridge 2009; Reitherman 1990). There are several basic reasons why nonstructural seismic protection is less comprehensively treated than the structural aspects of earthquake engineering: • The structural aspect of earthquake protection has simply preceded the evolution of nonstructural protective measures. If society had to choose one or the other, the argument would favor structural protection first and foremost. The logic is simple: without structural protection, there is no need for nonstructural protection. Thus, nonstructural seismic mitigation efforts have historically been treated as secondary and have evolved in the shadow of structural protection. • The scope of effort required for structural protection is less ambiguous than that required for nonstructural. Ask structural engineers to design a building to meet current seismic requirements and, more often than not, there will be consensus on the list of physical components involved: beams, columns, walls, lateral bracing, floor slabs, etc. Now ask the stakeholders involved in nonstructural protection what scope of effort would be required, and there will be no consensus list. In fact, more likely than not, there will be divergent ideas as to what constitutes nonstructural systems and equally divergent ideas on how to invoke protective measures. • The nonstructural problem in general is a moving target with no clear and concise understanding by the stakeholders as to the division of responsibility. The root cause of this problem is the continuous delegation of responsibility to the point

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where, in the end, no one accepts responsibility for or budgets for nonstructural protection (Masek and Ridge 2009). This problem is called diffusion of responsibility. We believe these factors have contributed to various stakeholder misconceptions regarding code interpretation and implementation. The result is implementation gaps, thus preventing a more comprehensive treatment of nonstructural earthquake protection. In order to push the ball forward in advancing seismic risk mitigation practices, a review of how stakeholders perceive nonstructural protection is warranted. Identification of common stakeholder misconceptions is discussed such that greater awareness between the stakeholders can be achieved. In other words, the goal is to bridge the perception gaps.

2.1 The Stakeholders The stakeholders involved in nonstructural earthquake protection include the following: • Building owners and facility managers involved in the long-term seismic mitigation for the life of critical facilities and industrial/commercial complexes (i.e., essential buildings). • Capital risk managers, capital investors, and insurers. These stakeholders all share a common need to minimize financial losses due to an earthquake event. • Governments—including federal, state, and local jurisdictions—responsible for public safety and economic stability. • Original equipment manufacturers (OEMs) and suppliers of mechanical and electrical equipment and distribution systems that qualify manufactured products to satisfy the seismic requirements of model building codes and standards. This includes manufacturers of subassembly devices and modules that are packaged into equipment platforms. • Construction trade professionals who must install and/or maintain nonstructural equipment and systems. • Building design professionals (i.e., architects, structural engineers, and specialty subcontractors) who specify nonstructural equipment and systems for new installations and/or delineate the seismic restraint details for such equipment installations. • Building occupants, including leaseholders, tenants, and employees. • The academic researchers, scientists, and design professionals who influence development of foundation research material that is used to help define new seismic design provisions for model building codes and standards and/or influence changes to existing requirements. • The collective body of professionals directly involved in development and writing of building code provisions and/or standards as related to the seismic requirements for buildings and nonstructural systems, including geotechnical aspects. • Local jurisdictions (each referred to as an AHJ, Authority Having Jurisdiction) are the local, state, or federal authorities charged with enforcing building codes and issuing building permits and certificates of occupancy (or their equivalent).

Stakeholder Commentary

43

• An Inspector of Record (IOR) is generally involved with two types of inspection. First is the basic code enforcement inspection by the AHJ. This type of inspection is generally quite minimal and involves city or county employees inspecting projects at specific milestones during construction. Some more-complex types of construction require full-time and continuous inspection rather than periodic inspection. These commonly include very large projects, schools, acute healthcare facilities, correctional facilities, etc. These IORs are generally employed by the facility owners; they typically report directly to the design professional but are paid by the owners. There are also specialty inspections for welding, shop-fabricated equipment, etc. • Commercial and university test laboratories that provide shake-table testing services for seismic qualification of nonstructural equipment and systems.

2.1.1 Stakeholder Perspectives This list of stakeholders was not cited in any particular order. To better appreciate the typical hierarchy of information flow between stakeholders, Fig. 2-1 organizes the participants into three primary groupings. In this context, the code regulation and enforcement group is associated with front-end activities that support hazard assessment and seismic requirements development and enforcement. The building design and construction group is associated with building design activities compatible with a desired occupancy use over the life of the building. The product design and manufacture group is at the end of the information hierarchy and is associated with product development activities for building products categorized as nonstructural. Stakeholder perspectives are addressed along these three domains.

2.2 Code Regulation and Enforcement The code regulation and enforcement domain is the front end of seismic risk mitigation. The very nature of earthquakes being low-frequency, high-impact disasters makes the process of establishing effective risk mitigation policy a significant challenge to the stakeholders involved (see Fig. 2-1 for participants). Competing interests motivate these actors. Capital risk managers, insurance companies, and capital investors have an obvious financial motivation. The government, including federal, state, and local jurisdictions, is motivated by public safety concerns and economic impacts. Researchers and geotechnical specialists are motivated by the pursuit of advancing the science with a desire to transfer knowledge into mitigation practices. It is the job of code writers and code governing bodies to juggle these competing interests and condense the science into evolving code requirements that are captured via code provisions and enforced during building construction. Figure 2-2 encapsulates the spirit of code development. The term “evolving” is a key descriptor when discussing model building code seismic provisions. The continuously transforming content of seismic design codes reflects the evolution of design practice as it takes place in changing technical and political contexts.

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Earthquake Protection of Building Equipment and Systems

Information Flow

Code Regulation and Enforcement

• Capital Risk • • • • • • • • •

Managers Insurance Companies Capital Investors Governments Researchers Code Writers Seismologists Geotechnical Specialists AHJ - Authority Having Jurisdiction Building Inspectors

Building Design and Construction

• Building Owners • Facility Managers • Architects • Structural Engineers

• Mechanical & Electrical Engineers

• Building •

Construction Trades Building Occupants

Product Design and Manufacture

• Original •

• •

Equipment Manufacturers Mechanical & Electrical Product Design Engineers Test Laboratories Equipment Installation Trades

Figure 2-1. Participants associated with nonstructural earthquake protection grouped into three domains, showing the typical information flow hierarchy.

These changes are significantly influenced by hard lessons learned from earthquake damage to the built environment. Earthquake protection issues are addressed by the earthquake engineering community through a combination of credible academic research and forensic investigations of what works and does not work during and after simulated or real earthquake events. Once vetted, these protection concepts are implemented in model building codes and design standards, such as the International Building Code (IBC) and ASCE/SEI 7, through their normal update cycles.

Stakeholder Commentary

New Science

Governing Code Body

45

Lessons Learned

Financial Interests

Public Support Political Support

Practitioner Interests

Figure 2-2. The general spirit of code development.

The following discussion is a top-level overview of roles and responsibilities for each of these stakeholder groups, with insights on how they affect code development and implementation. We are taking a historical background approach to provide the reader with a better sense of context of how each of these stakeholders has responded to past challenges. These past events provide precedence as to how each will move forward in the future. It will also become obvious that the lines of responsibility separating the stakeholders are sometimes not clear. A key point needs to be stated: Without adequate local jurisdictional enforcement by the AHJ, the seismic mitigation objectives of the building code may not be realized.

2.2.1 The Earthquake Scientist From the early 1900s through the end of the twentieth century, the role of the scientist has dramatically changed. While many branches of science are involved in the study of earthquakes, it is the earth science field of seismology that is the most important. Initially, the task of the newly formed field of seismology was to simply catalog earthquake events. This evolved over the next 50 years to provision of more complete scientific descriptions of all earthquakes. However, no matter how precise an earthquake description was made, very little was known about the resulting ground motion intensity (earthquake demand) upon which the building design must survive. This was the problem engineers and government regulators struggled with during introduction of the first generation of commercial nuclear power plants in the 1960s.

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Earthquake Protection of Building Equipment and Systems

Without credible assessment of how much earthquake demand a nuclear power plant could experience over its projected 40-year life (which eventually became 60 years for many plants), the ability to establish a basis of design to ensure reactor safety was at risk. In the late 1960s, researchers C. Allin Cornell and Luis Esteva, working with Emilio Rosenbluth at the National University of Mexico, developed the basis of probabilistic seismic hazard analysis (PSHA). This mathematical tool filled an urgent need for a quantitative means of expressing the future likelihood of strong ground motions due to earthquakes that would affect nuclear power plants. The method was adopted by the U.S. Nuclear Regulatory Agency in the 1970s. The development of advanced probabilistic methods to assess earthquake ground motions provided a needed scientific methodology to determine the basis of seismic design for nuclear power plants (Panel 1997). Today, the PSHA methodology is the starting point for the development of IBC and ASCE/SEI 7-10 seismic design maps, which are refined with deterministic procedures in areas of active large faults. These probabilitybased methods are at the core of building code seismic hazard maps globally. They have become an essential requirement for the risk assessment portion of negotiations with large capital investors and reinsurers for owners of capital-intensive facilities.

2.2.1.1 Science Develops Answers for Where and How Big At the turn of the twentieth century, the first mechanical instruments had begun to capture crude records of earthquake-induced ground motions. More advanced mechanical instruments were soon developed and enabled the creation and rapid global implementation of Charles Richter’s logarithmic scale for assessing the size of an earthquake, ML (local magnitude), which he developed in 1935 (in association with Beno Gutenberg) (Richter 1958). The ability to report earthquake size in the context of simple numbers like 6 or 7, independent of the location of the earthquake, greatly enhanced public awareness. By the late 1930s, society had a new level of understanding regarding the deadly and destructive acts of nature called earthquakes. The rising public awareness and the need to be reassured that science could provide some answers resulted in Charlie Richter becoming the first person the public wanted to hear on the radio, and later TV, after a notable earthquake event. Ironically, Dr. Richter’s unique ability to describe earthquakes in laymen’s terms and handle live media interviews—for which he was a natural—was in direct contrast to his shy and reclusive personality. His deep technical knowledge of earthquakes and his ability to explain the basics in easily understandable terms made him one of the most widely recognized public figures in the world. That Richter has become a global icon is significant, speaking volumes about the social and financial impact earthquakes have on modern society—a concept deeply instilled in all stakeholders (Hough 2007). In the 1950s the development of practical electronic devices, such as the “operational amplifier,” enabled a significant leap forward in the capability of ground motion instrumentation. In 1959 the U.S. Department of Defense launched the Vela Uniform research program to improve the capability of detecting, monitoring, and analyzing underground nuclear weapons testing (Scott 1999). Closely following these developments was the ever-

Stakeholder Commentary

47

increasing use of digital computers, which enabled the development of sophisticated models to understand these events. By the late 1970s ground motion instruments themselves were able to provide digital data from the instrument to the first generation of digitally linked seismic networks. Now a seismologist had near-real-time data to validate new theories about earthquake sources and energy propagation through the Earth’s structure. These advances helped form a better understanding of the earthquake hazard in areas with active faults, such as the west coast of the United States. In areas in the United States east of the Rocky Mountains, earthquakes do not occur frequently enough to capture such data, and assessing the earthquake hazard in similar areas around the world is of great concern to seismologists. Most recently, seismologists have responded to public policy and risk management requests for maps that reflect design values associated with the less-frequent earthquakes in the eastern United States. Working with experts from the field of structural reliability, the seismology community has developed ground motion models to provide for a “risk targeted” approach that forms the basis of the seismic design maps in ASCE/SEI 7-10 and the 2012 IBC (ASCE/SEI 2010; ICC 2011; FEMA 2009). This direct link to the building code seismic design maps has brought the seismologist directly into the code creation process. The field of seismology has advanced significantly over the past 100 years, yet it still struggles as a science. By its nature, science should be able to predict an outcome from a series of observations. However, the ability to predict big earthquakes, within a timeframe useful for evacuations (such as with hurricanes), still does not exist and is not likely to exist in the foreseeable future. This fact only emphasizes the extreme complexity of the subject. There obviously remains much to learn regarding earthquakes.

2.2.2 Financial Risk Management The primary stakeholders for financial risk management are governments, capital investors, and insurers. These stakeholders all share a common need to minimize financial losses due to an earthquake. While they have a common interest to minimize risk, their involvements in the code-making process are quite different.

2.2.2.1 The Government Stakeholder Governments are most likely to be involved at the earliest stage of the code-making process. The goal is to advance cost-effective earthquake mitigation strategies that can save lives and reduce direct and indirect recovery costs. For example, the U.S. Congress funds advances in earthquake mitigation through federal funding of • • • •

The National Institute of Standards and Technology (NIST); Federal Emergency Management Agency (FEMA); U.S. Geological Survey (USGS); and Academic research under sponsorship of the (U.S.) National Science Foundation, which is a direct report to the U.S. president.

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Earthquake Protection of Building Equipment and Systems

This collection of federal entities directly influences the seismic design provisions of the U.S. building codes by • Congressional funding of NEHRP: the basis of seismic design provisions for the IBC and ASCE/SEI 7; • USGS: development of national seismic hazard design maps that are directly incorporated in the IBC and ASCE/SEI 7; and • Direct participation in the codes and standards process by participation in the ASCE/SEI 7 seismic provisions update committee. A study conducted by the Multihazard Mitigation Council (MMC) found that, on average, every $1 spent by FEMA on hazard mitigation (actions to reduce disaster losses) resulted in about $4 in future benefits to the U.S. treasury (MMC 2005).

2.2.2.2 The Financial Stakeholder Globally, the impact of financial benefits can be seen in the World Bank’s policy regarding preconditions for financing high-cost capital projects. Having provided about USD $40 billion in emergency and reconstruction loans over the past 20 years, the agency is now paying more attention to hazard mitigation programs. World Bank-funded programs in Colombia, Honduras, and Turkey have implemented methodologies for integrating risk analysis into urban and regional development planning (Ghesquiere et al. 2006).

2.2.2.3 The Insurance Industry Stakeholder For typical losses such as fire, the insurance industry relies on historical statistical data. To ensure that minimum profit margins can be maintained in a given area, assessments are based on anticipated payouts for claims. The concept of using historical data to assess investment risk was first proposed in 1671 in England for pricing annuities. At that time statistics were only associated with the disreputable study of gambling. However, by the end of the eighteenth century statistics had been elevated to a respected branch of mathematics, which insurers could use with confidence to meet profit margins for investors (Vick 2002). For example, in the case of fire insurance, the premise is that for an aggregate collection of insured buildings in a given area a statistical study of the historical fire data would yield an average payout risk on an annual basis. With enough data collected over a sufficient period of time for a given area, the statistical method was quickly proven reliable and became the global basis for underwriting risk. Earthquakes are an entirely different type of financial risk for the underwriter. Risk managers call it “dealing with the rare.” Since earthquakes are destructive but rare for any given location, reliance on historical data for pricing and underwriting decisions is dan-

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gerous, because meaningful data do not exist. Even when data do exist they are not likely to be useful because of the ever-changing types of insured properties. Especially risky is the dramatic impact a large event will have on aggregate losses (summed over all locations) for dense areas of built environment. In areas built to modern building fire codes, a structure fire is highly likely to be contained to mostly the individual structure. In contrast, a large earthquake in a dense urban setting will be more likely to cause widespread damage. This large and unpredictable aggregate loss may not be fully absorbed by the primary and secondary insurers.

2.2.2.4 The Emerging Role of Catastrophe Models Insurance risk assessment methodologies have relied heavily on statistical analysis of historical data since the eighteenth century. Even with two centuries of refinement, this approach is simply not adaptable to large catastrophic risk associated with low probability of occurrence and high-consequence, very rare events such as large earthquakes. Catastrophe models introduced in the mid-1980s have revolutionized how both primary and secondary insurers deal with rare natural hazards. To the global pool of investors, catastrophe bonds (“cat bonds”) are very attractive investments because they are decoupled from swings in the capital markets of national or global economies. Unlike investments in a known company, the investors in catastrophe bonds know little or nothing about their investment; therefore, they rely heavily on the robustness of catastrophe models, which must undergo intensive scrutiny and testing to gain investor confidence. The widespread damage caused by the 1992 Hurricane Andrew in south Florida elevated the awareness of the urgent need for new tools for underwriters to use in dealing with large-scale catastrophic risk. This put catastrophe models in the spotlight, globally, for the insurance industry. Prior to 1992, the largest insured loss from a hurricane was $4.2 billion, as a result of Hurricane Hugo in 1989. Given that historical bar, the loss prediction methods commonly used at that time considered $7 billion to be the ceiling. However, newly evolving catastrophe models were indicating that the classical methods were severely underestimating losses, which could actually run as high as $20 billion. In the end, the Hurricane Andrew insured losses came in at approximately $15.5 billion. Beginning around 2000, catastrophe models introduced physics-based models as a means to reduce uncertainty. This procedure involves replacing some of the statistical components of the model with physics-based simulations, which employ the actual physics of an earthquake. The objective of this approach is to reduce the reliance on expert opinion and subjective assessments to reduce the uncertainty (Porter and Virkud 2002). As risk managers become more sophisticated in how they deal with catastrophic risk, they are demanding both more disclosure from their modelers and more detailed exposure data from their clients. This means that for high-capital-investment projects, owners are increasingly likely to find themselves employing the services of building design and construction professionals with cutting-edge earthquake engineering skills. This is especially true if the reinsurance carrier is underwriting a large risk.

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Earthquake Protection of Building Equipment and Systems

2.2.2.5 Catastrophe Model Development Example: China The explosive economic growth in China has seen property and casualty premiums grow at an estimated rate of 15% per year as of 2010, as reported in a study conducted by the China Insurance Regulatory Commission. China is exposed to multiple natural hazards, for example, floods and earthquakes. The risk to both the primary and secondary insurers cannot be ignored as the market matures and their shares of total insured losses increase significantly. To manage the development of risk management models, collaborations have been undertaken in China, such as with the Beijing Institute of Architectural Design. This collaboration is aimed at the development of damage assessment models for building inventories, which consider location-specific construction practice and quality, year built, as well as typical building structural system and design (Guin and Lai 2007). Key elements of China’s damage assessment model include the following: • • • • • • •

Assessment of all natural hazards for engineering demands Involvement of all stakeholders Catalog of the inventories of the built environment Extensive use of computer models to determine damage to each building type Damage assessment to include both structural and nonstructural Location-specific construction practices and quality Year built, to factor in building code(s)

While the insurance industry does not directly interact with the enforcement of building codes, it does increasingly influence the implementation of earthquake mitigation by the use of models as a basis of underwriting, which are more attractive when such mitigation is employed.

2.2.3 Public Policy Public policy pertaining to seismic mitigation is the most complex and elusive aspect to understand, because it is an amalgam of public perception, profit-centric developers, and government responsibilities. These entities have widely varying positions, which has a randomizing effect on the degree to which verifiable code enforcement is implemented at the AHJ level. When speaking with any government official who is accountable for the adoption and implementation of earthquake codes to protect society, they are likely to describe the following public perception road blocks: • “Earthquake code compliance equals ‘earthquake-proof,’ and I don’t expect damage will occur, even in the largest of events.” • “Earthquakes have never happened in my lifetime, so why should I add cost to my building project?” • “Earthquakes are a California issue.” • “We have earthquakes all the time and nothing ever happens, so my building is better than code.”

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• “Earthquake codes are just a jobs program for engineers.” • “Public money is wasted on seismic mitigation.” • “Our school buildings are built better and would never collapse.” In most countries the developer has little motivation to spend money on seismic mitigation because, without market awareness and demand for it, they cannot recover their investment. In the absence of public demand, seismic compliance does not sell and the financial exposure for the developer only exists for the brief period of time of construction—prior to sale in most locations. An exception to this is Chile, where the developer owns the risk for a period of 10 years after the building is sold. Also unique to Chile is that the project structural engineer is retained directly by the owner and therefore has control over building design concepts. This ensures that visually striking designs are also inherently safe when earthquake loads are applied. As a result, in the 2010 Chile earthquake a field survey team deployed by the American Society of Civil Engineers concluded that building performance met or exceeded the design goals of the 1994 Uniform Building Code (Hooper 2010). The Chile success story is an exception in the world. According to the University of Colorado seismologist Roger Bilham, with the Earth’s population growth projected to demand an additional one billion more dwellings by 2050, an unrecognized weapon of mass destruction exists: residential houses. Such was the case in the 2010 Haiti earthquake, with more than 200,000 deaths as a result of construction that had no tolerance for earthquake demands. In the case of Tehran, Iran, it is estimated that if a quake similar to the one that struck Haiti were to occur (not a matter of “if” but “when”), up to one million people could die (Revkin 2010). Given the highly advanced state of earthquake engineering, with practical, affordable, and proven construction methods ranging from simple school buildings in developing countries to advanced structures in upscale urban developments, the last mile in realworld implementation is public policy. Public policy has a significant impact on the effectiveness of earthquake mitigation, because it is the legal adoption of model codes and their enforcement, which is critical to ensure that the code provisions are implemented and maintained in the built environment. Again, because the earthquake risk is a low-probability-of-occurrence, high-consequences issue, it can be extremely difficult for both the public and governments to prioritize seismic mitigation. Other pressing social issues, such as jobs, traffic, crime, and other urgent community needs, are competing considerations.

2.3 Building Design and Construction The goal of this second group in Fig. 2-1 is to design, build, and commission a specific building to satisfy the owner’s needs and expectations. The primary participants during building design and construction include the owner and the building professional, and the interaction between these two is highly collaborative. The owner picks the geographic location of the building site and defines the building’s primary intended functions. These

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Earthquake Protection of Building Equipment and Systems

choices then drive the regulatory requirements of all downstream activities, including nonstructural systems specification, selection, and final installation. In general, the primary motivation in building development is to design once and build once. Of course, there is often the desire to reuse building design elements across many building applications, when applicable. But for the most part, buildings are highly individualistic and tailored to meet the unique functional characteristics for the specific facility and geographic characteristics of the building site.

2.3.1 Responsibility of Design The 2012 IBC and its reference document ASCE/SEI 7-10 for nonstructural design may at first seem a bit too complex for some members of the design team. Over the years, as nonstructural sections of the code have increased in complexity, there seems to have been a trend to ignore the arithmetic and instead rely on standardized design/installation details that have become “accepted practice.” Much of this acceptance has been reliant on the comfort level of the designer and/or building official. One of the goals in writing this book is to convince the design profession to view ASCE/SEI 7-10 as an opportunity to embrace a much more comprehensive seismic design philosophy and not to simply rely on older standardized details. Who are the participants (stakeholders)? • • • • • • • • • •

Government, by establishing public safety standards through adopting model codes Building owner Architect Structural engineer Specialty engineers, such as for electrical, plumbing, HVAC, and fire protection General contractor and all the specialty subcontractors Building official (building authority—authority having jurisdiction, AHJ) Inspector of Record (IOR) Owner’s insurance company Building occupants

In FEMA-454, “Designing for Earthquakes: A Manual for Architects” (FEMA 2006), one of the authors of this work provided a table that showed specific pieces of equipment and common failure examples if appropriate design and execution during construction are not implemented. Also shown was a table of “responsible” design parties showing primary and secondary responsibilities. We have reproduced the FEMA-454 information in Table 2-1. Clearly, the FEMA-454 “design responsibility” can be expanded to show that there are numerous stakeholders previously not discussed. Each stakeholder has his or her own concerns with the design. It is generally the responsibility of the architect and owner to work out the plan for the performance of the facility within the available time and budget. Most facilities do not need much beyond the basic code provisions to avoid suffering either partial or full collapse and to allow people to safely exit the facility when strong ground motion stops.

Table 2-1. Design Responsibilities for Nonstructural Components. Nonstructual System or Component

Architect Structural Electrical Mechanical Other Design Engineer Engineer Engineer Professionals a

1

2

Doors/Windows

1

Access Floors

1

HVAC Systems

2

1

Plumbing Systems

2

2

Communication Systems

2

1

1

a

Data Systems

2

1

1

a

Elevator Systems

1

2

2

2

2

Emergency Power Supply System

2

2

1

2

2

Fire Protection Systems

2

2

1

1

a

Kitchen Systems Lighting Systems

1 2

2

a

a

Small glazing panes perform better in earthquakes. Avoid window film unless properly applied. Consider how doors will avoid racking in nonstructural walls. Consider in-the-floor ducts rather than raised floors where practical. Systems that require vibration isolation also require snubbing. Vertical plumbing runs are subject to floor-to-floor drift. Some communications systems come as a package. Make sure that they interface with the building appropriately. Consider support systems such as cooling environments. Design some elevators to operate after the earthquake. All systems interfaces need to be considered as their vulnerability can cause an entire facility to become nonoperational. Floor-to-floor piping is subject to story drift.

Stakeholder Commentary

Curtain Wall

Remarks

1 53

(Continues)

Table 2-1. (Continued ) Architect Structural Electrical Mechanical Other Design Engineer Engineer Engineer Professionals

1

2

2

2

Ceiling Systems

1

2

2

2

Unbraced Walls and Parapets Interior Bearing Walls

1

2

1

2

Interior Non-bearing Walls

1

Prefabricated Elements (Architectural Appendages) Chimneys Signs Billboards Storage Racks

1

2

1 1 2 1

2 2 1

Cabinets and Book Stacks

1

Wall Hung Cabinets Tanks and Vessels Electrical Equipment Plumbing Equipment

1 2 2 2

a

Often, the architect needs to provide protection to equipment because it is not included in the code requirements. Avoid drop ceiling elevation changes. Avoid large ceiling cavities.

Consider earthquake effects on doors for egress.

2

2 2

2 1 2 2 2

1

Note: 1, primary responsibility; 2, support responsibility. a Consider a specialty consultant. Source: FEMA (2006).

1

1

a a

Proprietary manufactured racks may or may not include seismic design considerations. Architect needs to provide proper wall backing. Architect needs to provide proper wall backing.

Earthquake Protection of Building Equipment and Systems

Medical Sytems

Remarks

54

Nonstructual System or Component

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These facilities can be designed and constructed using conventional methods, including using standardized details. There are, however, some facility types where this approach is not desirable. For these, the design team sometimes must educate the owner on why it is in his or her best interest to design based on a thorough understanding of dynamic loads and systems/component interactions. While possibly more expensive in the short run, a comprehensive design is less expensive if a damaging earthquake does occur. An astute owner knows the value of comprehensive seismic design. Hospital owners, for example, often provide their own equipment that is not fixed in the building, and this equipment sometimes needs protection. Figures 2-3 and 2-4 illustrate owner-supplied equipment and the means available for minimal seismic protection from rolling or sliding (but not tipping). In many cases, design professionals fail to have this important conversation with their clients, subconsultants, contractors, and the IOR. The building official will seldom balk at

Figure 2-3. Owner-supplied healthcare equipment that typically is not included in the original design documents.

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Earthquake Protection of Building Equipment and Systems

Figure 2-4. Detail of wheel chocks to prevent rolling and reduce sliding but do not protect against overturning. a thorough analysis and design, although even a preliminary meeting can assist the process. This is unfortunately not always the case with the other stakeholders. Convincing the entire team can be an extensive education process the first time they work together. Reviewing the examples of unprotected or inadequately protected equipment and systems, as shown in FEMA-454, Chapter 9, Fig. 9-2 (FEMA 2006) might help with this process. Seeing damage photos illustrates the concern. Explaining the code philosophy is also advantageous. It is generally the architect’s responsibility to coordinate the entire team so that everybody is working toward the same goal and at the same level of heightened awareness of the seismic vulnerability. The thesis of this particular writing is to break away from the historical seismic code approach of component qualification in favor of systems design. Success in this design opportunity will work wonders in the protection of essential-type facilities. Chris Arnold,

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an architect in Palo Alto, California, has long understood the importance of the guidance the architect provides with respect to seismic design. Well over 30 years ago he was advocating that the success or failure in seismic design is largely in the hands of the architect, because it is the architect who decides the building configuration. Building configuration is not driven by the structural engineer. The architect sets the plan shape, the floor-to-floor heights, the building irregularities, and so forth. Many architects mistakenly set these parameters without realizing the importance of their decisions. If the architect says to the structural engineer, “This is my vision, now you make it work,” the facility may or may not be able to be economically solved for the seismic environment. The codes finally recognized that, all too often, architects were ignoring the importance of building configuration; this is why many codes established irregularity tables. Every architect needs to carry Arnold’s philosophy forward into building function with respect to nonstructural system/component design. Architects can orchestrate the likely success or failure of the facility in the seismic environment by setting the stage appropriately, based on their philosophy. How does the architect do this difficult task? In some cases the architect will need to be prodded into setting higher standards by some of the team members who may in fact be more familiar with seismic design requirements than the architect. Over time, architects will hopefully become more familiar with designing for earthquakes and realize their full charge. Architects who read and learn from material, such as FEMA-454 and Andrew Charleson’s work, Seismic Design for Architects: Outwitting the Quake (2008), which are not mathematically driven, will grasp their charge much more easily. There is no reason for architects to be consumed by the math. Architects really need to understand the concept of good seismic design and how their decisions will affect the performance of the facility as a whole in the seismic environment. Design does not end with the architect’s original vision but, rather, is carried on throughout the building process with sound professional judgments that are developed through an understanding of seismic performance during the development of architectural detailing. Detailing demands an understanding of what is being detailed for all of the environments under consideration, including structural stability, not letting water into the building, not letting smoke pass from one space to another, seismic response, and so forth. Smoke passage can serve as an example of interdisciplinary design responsibility. The architect sets the basic functional building space design. In this case, there are some spaces that require fire protection. The architect needs to make certain that the subconsultants, such as the HVAC engineer and plumbing engineer, know of the fire restrictions. The HVAC engineer will need to provide smoke dampers in the ducts to restrict potential smoke flow into fire-rated corridors, for example. The same applies to the plumbing and electrical engineers; the pipe and conduit penetrations must be sealed against smoke and flame spread. Figure 2-5 illustrates such a scenario in the finished condition. The photo is somewhat deceiving, because everything looks so “clean.” The following Fig. 2-6 shows some of the complexity above a corridor ceiling. It should be noted that the space above the ceiling in Fig. 2-6 is not complete. It will be much more complex by the time the building is completed. Figure 2-7 is a detail showing the conduit penetrations with fire and smoke retardant material being used at the penetration

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Earthquake Protection of Building Equipment and Systems

Figure 2-5. A typical fire-rated corridor.

Figure 2-6. A similar ceiling above an exit corridor during construction.

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Figure 2-7. Conduit penetrations during construction with flame and smoke penetration material, as well as soffit diagonal bracing and sprinkler main line bracing.

openings, and Fig. 2-8 shows more clutter when the HVAC ductwork is added. The design team seldom specifically locates all of the ducts, conduit, and pipe work. These items are generally only shown diagrammatically on the contract documents. The design team often leaves the actual location of the required work to the coordination means and methods of the general contractor or subcontractors on multiple prime contracts. Both Building Information Modeling (BIM) and design-build approaches offer the team a method to take care of actual locations during design rather than waiting until construction is in progress. Using BIM, the design team will have the opportunity to solve these issues during design, before the contractor is involved. BIM allows design document “clash control” (see Fig. 120). Although BIM is not yet universally implemented across the design profession, most of the design schools and universities have fully embraced it, and students graduating even prior to the publication of this book are fully conversant in its technology and the capabilities it offers. Under a design-build scenario, the locations of the various ducts, pipes, conduit, braces, and so on. will be determined in conjunction with the contractor and subcontractors during design. BIM should be utilized in design-bid-build as well as in design-build so that everybody is in the loop for coordination. Figure 2-9 shows the congestion in a finished ceiling. This figure is the ceiling space shown above the corridor in Fig. 2-5. If one looks closely at Fig. 2-9 it can be seen that the ceiling system splay wires that wrap around HVAC ducts and pipes, and splay wires that do not approach the suggested 45-degree

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Earthquake Protection of Building Equipment and Systems

Figure 2-8. The same corridor with ductwork added gets quite cluttered; the design professionals seldom specifically locate everything, leaving most actual locations to the contractor.

Figure 2-9. The level of congestion that is commonly found above a ceiling; this facility is a clinic, not an acute-care hospital.

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angle shown on typical details. Imagine the difficulty faced by the contractor and subcontractors attempting to coordinate all of these systems and components when the design team only provides diagrammatic representations rather than specific locations and elevations—much of which is not specifically located in the space provided. In order to attain better seismic performance, the design team needs to provide full design services. This comes at a cost. The owner needs to recognize that the design team cannot provide this service for free. There will need to be additional design fees associated with the level of design we are suggesting here. But there are potential savings, also; the contractor will not as often need to request change orders because “things don’t work” in the space provided. Basic bids will also be lower, because when contractors can bid on specific, detailed designs they will realize they do not need to gamble that there is enough room to make things fit, and they do not need to expend as much time in supervision of the subcontractors. The remaining stakeholders that have not yet been discussed are the inspectors. Inspection generally takes place in two ways. First, the building authority typically has field inspectors who visit the site at particular milestones. These are seldom a single inspector. Most commonly, the building authority employs inspectors for particular trades, including grading (soils), foundation, framing, plumbing, and electrical. Since these inspectors are not on the site full-time, they can easily miss conditions between visits. On complicated projects the design team should work with the owner in selecting an Inspector of Record (IOR) who is on-site full-time. The cost is typically between 1% and 2% of the construction cost for a qualified inspector(s). The IOR is most often an employee of the owner and generally reports to the architect, owner, and building authority simultaneously with written reports. Some owners opt to have the inspector paid by the contractor; however, this practice defeats the purpose of inspection and opens the door for turning a blind eye to inappropriate or unapproved installations. The IOR is the real genius of the 1933 Field Act as it was originally written in California. The Field Act required an IOR on all public K-12 and community college projects during all times when work is being performed on the school (community colleges have recently been given the option of removing themselves from some of the Field Act requirements). On large projects, deputy inspectors and specialty inspectors (masonry and welding are examples) are sometimes warranted. The duties of the IOR are basically not to interpret the design documents but, rather, to make certain that materials specified are used in the construction and that construction detailing required by the approved documents are followed for sizes, materials, and quality. The IOR does not have “stop work authority” similar to the building authority inspector but should issue nonconformance bulletins when there is a deviation in the approved documents so that the owner, designer, contractor, and building authority all know of the construction deficiency before it is too late to remedy the condition. A qualified inspector will understand the importance of seismic design issues and be able to identify issues before they become a problem. An example of design professionals’ past preference to ignore the arithmetic can be seen in the standardized details used for decades for suspended ceiling installations (T-bar grids with lay-in-type ceiling panels). While the partial or complete failure of most suspended ceiling systems is not generally likely to be a life threat, there has been some

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Earthquake Protection of Building Equipment and Systems

reported evidence of potentials for life threats when the failure of these types of ceilings block exit ways. One of our coauthors chaired the Los Angeles City Subcommittee-10 that investigated nonstructural performance following the 1994 Northridge earthquake. One of the subcommittee’s reports to the city was specifically on suspended ceiling systems (G. L. McGavin and Los Angeles City Nonstructural Element Subcommittee-10, Seismic design of suspended ceiling systems and lighting systems, final report 1/26/1995, unpublished report, 1995). While much of the report revolved around anecdotal evidence based on facilities visited during the course of the study, some recommendations for ceiling systems were made to the City of Los Angeles. These recommendations have essentially all been embodied in ASCE/SEI 7-10. The LA City report pointed out failures due to • Partial/full collapse of ceiling system at exit ways; • Numerous items left lying on the grid system over the years due to maintenance (the grid system was not designed for the added loads); • Inadequately tensioned splay wires; • Improper installation (caused a ceiling grid failure at the Landers Elementary School in 1992); • Inadequate or no specific design of the suspended ceiling system; • Excessive distance from ceiling grid to roof structure; • Absence of separation joints at irregular room shapes; • Buckling of excessively long compression struts; • Buckling of T-bar grids caused by compression struts reacting to flexible ceiling systems; • Rigid attachment of suspended ceiling systems to all perimeter walls; • Interaction with sprinkler drops and the suspended ceiling system; • Excessive building displacements (tall buildings) causing excessive ceiling movement; and • Changes in ceiling plane elevations causing eccentric loading. The ceiling system serves as a useful example for our purposes here. It involves numerous participants and should be designed using the systems philosophy in conjunction with ASCE/SEI 7-10. Historically, architects have used half a dozen standardized details rather than thinking through each ceiling system to determine its performance based on the specific design as required by ASCE/SEI 7-10. Until recently there has been little effort put into detailed testing of ceiling systems or calculating the geometry of the details. Testing of suspended ceiling systems has been hindered by small-shake-table size constraints and seismic fixture design. In the early 2000s Masri and Caffrey (USC) constructed a 9 m ↔ 12 m (30 ft ↔ 40 ft) ceiling test fixture at USC. This table was limited to a single axis. It was located in the basement of the civil engineering building, which restricted its motions due to overall building vibrations when it was in operation. Other seismic tests of ceiling systems are limited by sizes in the range of 4 m ↔ 4 m (16 ft ↔ 16 ft), such as the State University of New York–Buffalo and the UCB seismic elevated test frames. These small sizes do not accurately represent actual installed ceiling sizes (Gilani et al. 2010). Manufacturers are interested in qualifying their ceiling products and several have used the 4 m ↔ 4 m seismic tables for their recent test qualifications based on the International Code Council (ICC)’s AC156 test protocol (ICC ES 2010).

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Splay wires have seemed to be reasonable enough to work since the late 1970s, and most architects use them today. Compression posts became a mostly accepted part of the seismic protection of suspended ceiling systems in the late 1980s as a solution that was added to the splay wires. The compression posts were used to reduce potential vertical movements of flexible suspended ceilings in large rooms. There has, however, been little work to confirm under what conditions compression posts provide benefit or when compression posts provide diminishing returns by serving as a potential stress accumulator within the suspended ceiling system. The details for suspension systems, splay wires, and compression posts generally required these items for every 13.4 m2 (144 ft2). The splay wires and compression posts are not supposed to wrap around or otherwise touch the infrastructure that is above the ceiling plane, such as cable trays, HVAC ducts, or pipes. This is often easier said than done. The design drawings commonly are diagrammatic rather than specific, thus leaving the actual installation as “means and methods” of the contractor. As designers move toward BIM 3-D design, producing virtual models of their buildings, this is expected to change if the designers employ the capabilities of the BIM software for true 3-D design. Automated interference clash detection between building structural and nonstructural systems should relieve the contractor of the task of making everything fit, as is now the process for the most part. There is a need to develop testing abilities for larger typical installations with large ceiling drops, variable ceiling drops, and sloped systems. ASCE/SEI 7-10 now generally requires seismic separation joints for ceilings larger than 232 m2 (2,500 ft2). ASCE/SEI 7-10 allows for the integral construction of ceiling systems by design as an alternate to large clearances around the sprinkler system penetrations through the ceiling grid when they are specifically designed to perform as an integral unit.

2.4 Product Design and Manufacture The nonstructural domain from a product development perspective is quite different compared with the perspectives described in the code regulation and building domains. The primary motivation during nonstructural product development is to design for universal application. Stated simply, the OEM’s goal is to pre-engineer nonstructural product offerings to meet as many building application opportunities as possible without restrictions concerning installation variables. The OEM would like to supply the same nonstructural system regardless of whether the application is for an essential or nonessential facility, without having to include special seismic upgrades, and do it all for a reasonable cost. Obviously, a single nonstructural product offering will not be able to address all of the application opportunities that the free market presents. Thus, nonstructural offerings are designed as highly configurable product platforms, such that one of the pre-engineered design variants can be configured to address many market needs. OEMs have become fairly sophisticated when it comes to engineering equipment configuration options that can support mass customization manufacturing strategies. The early twentieth-century approaches regarding manufacturing, as expressed by Henry Ford in saying that, “An

64

Earthquake Protection of Building Equipment and Systems

American can have their Model T in any color as long as it’s black,” are not acceptable practice in the 21st century. Equipment customization to meet a diverse set of customer needs is standard OEM practice today. The major downside of this approach is that the typical nonstructural product platform design is fluid. It is a moving target with continuous upgrades, additions, and deletions being made to the various functional devices and subassemblies. While the basic platform architecture may have evolved over the course of decades, the functioning elements can be modified or replaced every couple of years. The fundamental difference in mindset between the building designer and the nonstructural product designer is defining what is a nonstructural component. To a building designer, a component is the entire nonstructural platform (i.e., the black box), whereas to the nonstructural designer a component is a module (i.e., the widget) located deep within the bowels of the nonstructural assembly. Most nonstructural product platforms are complex pre-engineered systems, with a force-resisting skeleton (FRS) that supports many different mechanical and electrical functional devices. How does incorporation of new devices and widgets affect the qualification status of previously qualified equipment platforms? This question assumes that the OEM fully understands what constitutes seismic qualification in the first place. That is a major assumption that likely applies to few OEMs. The nonstructural qualification problem from the OEM perspective is two-fold: (1) seismic requirements awareness, and (2) effective compliance implementation. These two go hand in hand. One cannot implement nonstructural protective measures without first understanding code requirements. The inherent problem with being at the end of the information hierarchy (as depicted in Fig. 2-1) is that most OEMs have limited knowledge of the drivers for change regarding nonstructural seismic requirements. The OEM’s priorities are to incorporate new devices and widgets into its existing nonstructural offerings and develop new product platforms to satisfy evolving customer needs. Thus, for the most part, OEMs are highly dependent on market forces to shape their knowledge of building code seismic provisions. Put it this way: if the OEM’s customers (i.e., building domain) did not ask for or require nonstructural seismic certification, the OEM would not address it, plain and simple. But OEM customers do ask for compliance verification and are doing this in increasing numbers. However, there is no consistent format to describe either the requirement or the compliance expectation. Thus, there is an ensuing exchange of information between the OEM and customer regarding nonstructural seismic issues. This is the starting point for a common misconception that the normal flow of information in the marketplace will provide a nonstructural supplier with adequate understanding of the issues pertaining to seismic compliance. The OEM’s customer can be far removed down the chain of command from the building professional responsible for the nonstructural application. Specification and procurement of nonstructural systems is most often delegated and redelegated to mechanical and electrical contractors and their subcontractors. By the time the OEM receives a bid specification to supply nonstructural systems for a given application, there will be many layers of middlemen between the OEM and the overarching building architect. At this deep level of delegation, there can be some interesting exchanges on the topic of nonstructural seismic compliance. In some instances, neither the delegated representa-

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tive of the architect nor the OEM has a clue as to what constitutes nonstructural seismic compliance. For essential building applications that specify designated seismic systems, the delegated building professional likely knows more about compliance than does the typical OEM. In most cases, these exchanges lie somewhere between clueless and factbased information flow. But for the most part, many OEMs learn about nonstructural seismic compliance through these dialogues with their customers. The obvious result from this type of learning process is that the OEM is operating in full reactionary mode and is dependent on the marketplace to convey nonstructural seismic issues. This method is ineffective in managing the OEM’s nonstructural product portfolio and can lead the OEM to fall into unforeseen rabbit holes—such as having inadequate seismic compliance to satisfy current code requirements. This unfortunate realization can create another negative aspect of marketplace-based information flow. The OEM might simply revise its seismic compliance story to better fit the language of any new code. We refer to this as the OEM’s compliance “spin,” and it can be dizzying. It is not that OEMs are unconcerned with building code seismic requirements. Rather, they are usually too far removed from the geotechnical and building domain aspects and are unaware of the drivers for code changes. The OEM is primarily concerned with marketing and selling its nonstructural product offerings based on the core functions of their products. Electrical and mechanical functions are the primary design drivers that influence product development decisions. Seismic compliance requirements are treated as secondary at best or not treated at all. Therefore, in the end, it should be no great surprise that many nonstructural OEMs and suppliers do not have a sound understanding of what the seismic requirements actually are for a given application or how best to satisfy them. The three major issues for the OEM regarding seismic compliance of its nonstructural systems are the following: 1. What are the nonstructural seismic requirements for a given target market? 2. How are seismic requirements specified across global geographies or across different target markets? 3. How are nonstructural offerings gauged against seismic requirements for compliance purposes? The first item relates to the awareness issue of seismic requirements for nonstructural systems. It is not just awareness but also an understanding of compliance expectations and how to implement effective qualification programs. The second item is a matter of practicality. If possible, the OEM desires to qualify nonstructural platforms only once and have the qualification be able to satisfy multiple building codes and standards to cover global application opportunities. The final item relates to establishment of a common capacity rating metric such that all seismic ratings use the same yardstick. Nonstructural suppliers and building professionals need to be able to compare apples with apples and oranges with oranges regarding seismic capacity. Without this final item, OEM nonstructural qualification is more spin-based than rooted in accepted engineering principles. Today, nonstructural seismic capacity comparisons are like comparing apples to oranges. There is no consistent method used for rating nonstructural capacity. In fact,

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each nonstructural supplier creates his or her own rating system and it is left to the building professional to sort it all out. Not a desirable situation. The final item also addresses qualification flexibility. The majority of essential nonstructural platforms are highly complex products. Qualification is most often not a clearcut, black-and-white activity but more of an activity that lies in shades of gray. The establishment of a uniform rating system would make life much easier for both OEMs and building professionals. A uniform nonstructural capacity rating would shed light on the gray areas and make the qualification process more transparent.

2.4.1 Requirements Awareness Requirements awareness should be a critical issue for the OEM, especially as building codes have transitioned from older paradigms (e.g., the zone system) to newer approaches based on probabilistic seismic hazard analysis. However, it is common practice for an OEM to not even see a new building code revision until well after the new code has been published. This practice might not have caused disruptive effects 20 years ago when building code revision cycles made only slight changes to the requirements. However, in present times code revisions are making sweeping changes from one code cycle to the next. The common OEM misconception that code changes are made incrementally without major changes between code revisions is flat-out incorrect. Each revision cycle can enact changes that significantly affect an OEM’s ability to satisfy compliance expectations. Thus, requirements awareness is a prerequisite for the OEM. The OEM needs to get plugged into the code development process and stay informed on what is happening in the geotechnical domain with respect to nonstructural seismic requirements. To ignore this issue is a mistake that will affect the OEM’s ability to deliver nonstructural systems that meet evolving marketplace needs. It is preferable that the OEM educates its customers regarding nonstructural seismic topics rather than being educated by its customers. Understanding the requirements is a necessary step beyond mere awareness and translates into conducting effective nonstructural qualification programs. Nonstructural seismic qualification is not free, and there is no point in spending resources to conduct qualification programs that do not yield the maximum amount of flexibility and utility. This means selecting the appropriate ground motion parameters, including vertical earthquake demands; selecting the right nonstructural test samples; converting static requirements into equivalent dynamic requirements; and many other facets of the qualification process that are described in Chapter 5. In this instance, the modern-day code can address complex seismic qualification topics, like dynamic testing and analysis, which enables the achievement of nonstructural qualification. Implementing a model code that can handle the complexities is exactly what is needed, and ASCE/SEI 7-10 fits this requirement.

2.4.2 Global Implications The modern-day trend toward globalization has forced many OEMs to become part of larger multinational corporations that compete for nonstructural application opportunities on a global level. This 21st-century economic reality has complicated the OEMs’ desire to develop nonstructural product offerings that can be globally supplied across many dif-

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ferent code jurisdictions. Thus, addressing the question of how seismic requirements are specified across global geographies is a necessary precursor to fulfill the OEM’s objective of global nonstructural product distribution. At a global level, seismic demand requirements for nonstructural applications are specified by individual countries on a country-by-country basis. Some countries have adopted the newer probabilistic-based approach, while others may use a zone system (or equivalent). Unfortunately, many countries today have very limited seismic requirements definitions and inadequate enforcement practices. However, in the last decade more countries have adopted seismic codes that are based on a technical foundation similar to that which forms the core of the ASCE/SEI 7-10 provisions. These new model building codes, which use spectral ground motion intensity to define seismic demands, are the next-generation codes that have been introduced in North America and elsewhere. As the trend toward a common basis of seismic codes continues, the possibility of a global approach to seismic qualification becomes more realistic and more practical to accomplish. A good example of a global approach is with the Global Seismic Hazard Assessment Program (GSHAP). GSHAP, a demonstration project of the United Nations International Decade for Natural Disaster Reduction, was conducted during 1992–1998 with the goal of improving global standards in seismic hazard assessment (Giardini et al. 1999). The GSHAP Global Seismic Hazard Map was compiled by joining the regional maps produced for different GSHAP regions and test areas (Fig. 2-10). The GSHAP map depicts the global seismic hazard as peak ground acceleration (PGA) with a 10% chance of exceedance in 50 years, corresponding to a return period of 475 years. The importance of a “global rating” should not be underestimated. A global rating system provides the only transparent mechanism to support distribution of nonstructural systems on a worldwide basis. This ensures that a common rating metric is used for compliance purposes. However, reaching consensus agreement on the seismic hazard assessment at the country level has been and is today the primary challenge facing any global approach. Even for a demonstration program such as GSHAP, hazard assessment has proven to be a very political process. Establishing agreement on the hazard intensity levels was never achieved. The GSHAP program did, however, clearly demonstrate the advantages of using a common rating system to measure the global earthquake hazard. In fact, the GSHAP map data can be used today to help determine “unofficial nonstructural capacity” assessments for countries that might not employ a probabilistic-based hazard approach.

2.4.3 Compliance Strategy The last area of concern for the OEM has to do with determining how equipment offerings are gauged against seismic requirements for compliance purposes. This is a key question and is the issue most often incorrectly implemented in industry. Seismic compliance verification is a process that involves comparing the seismic capacity of nonstructural product offerings with the specified demand requirement from a building code to verify that capacity exceeds demand for the building application. While the process of comparing equipment capacity to a specified demand level seems straightforward, it is fraught with opportunities for implementation mistakes. A common OEM misconception is that the process of establishing nonstructural compliance valida-

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GLOBAL SEISMIC HAZARD MAP Produced by the Global Seismic Hazard Assessment Program (GSHAP), a demonstration project of the UN/International Decade of Natural Disaster Reduction, conducted by the International Lithosphere Program.

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tion is clearly defined in code provisions. Unfortunately, it is not. The code does not instruct practitioners on how to implement seismic requirements. This potentially leads to many unanswered questions: • • • • • • • •

What does equipment capacity actually mean? How is capacity determined? How can capacity be related to demand using comparable terms? Is equipment capacity the same as anchorage capacity? How does equipment capacity relate to system capacity? Does equipment capacity consider functional performance aspects? How are lateral force demands correlated with dynamic demands? How can test and analytical methods be combined to achieve qualification of complex systems? • How does an entire product line family become qualified? These questions can trip up even the savviest OEMs. Effectively answering these questions is the focus of the second part of this book (Chapters 5 through 9). Obviously, the OEM is not waiting to receive a request for bid from the building professional prior to developing its nonstructural product offerings. The OEM develops the offerings well before product orders can be taken. The OEM must anticipate the customer needs and develop the offerings based on the OEM’s understanding of the marketplace. This must include a sound understanding of nonstructural seismic requirements. Over the years, many OEMs have designed their nonstructural product offerings to resist seismic demands. However, today compliance validation has become increasingly more demanding. In some respects developing the necessary compliance verification is more difficult than the actual task of designing nonstructural platforms to resist seismic demands. Without clearly demonstrated compliance verification, nonstructural suppliers will face increased scrutiny in state and local jurisdictions that adopt new special enforcement procedures (e.g., California). Requirements awareness—knowing what the current requirements are and knowing what is on the immediate horizon—is a necessary OEM practice. Requirements interpretation— knowing how to interpret in a manner that is aligned with code intent—is also a necessary OEM practice. And finally, implementing an effective OEM product development strategy that considers seismic withstand as a functional design requirement is essential to reduce engineering overhead. The goal is to ensure that seismic withstand resistance is a design driver that gets implemented during early product development.

References ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, ASCE, Reston, VA.

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Charleson, A. (2008). “Non-structural elements: Those likely to cause structural damage.” Seismic design for architects: Outwitting the quake. Elsevier, Burlington, MA. Federal Emergency Management Agency (FEMA). (2006). Designing for earthquakes: A manual for architects. FEMA 454 Risk Management Series, Washington, DC. ———. (2009). “2009 NEHRP recommended seismic provision for new buildings and other structures.” FEMA P-750, Washington, DC. Ghesquiere, F., Jamin, L., and Mahul, O. (2006). “Earthquake vulnerability reduction program in Colombia: A probabilistic cost-benefit analysis.” World Bank Policy Research Working Paper 3939, World Bank, Washington, DC. Giardini, D., Grunthal, G., Shedlock, K., Zhang, P. (1999). “The GSHAP global seismic hazard map.” Ann. Geophys., 42, 1225–1230. Gilani, A. S. J., Takhirovm S., Reinhorn, A., and Mahin, S. A. (2010). “Seismic qualification testing of suspended ceilings: lessons learned and the requirements for a new test standard and qualification procedure.” Proc., ASCE Structures Congress 2010, ASCE, Reston, VA. Guin, J., and Lai, T. (2007). “Understanding the vulnerability of China’s building stock in the face of earthquake risk.” Air Currents, August, 2007, AIR Worldwide Corp., San Francisco. Hooper, J. (2010). “February 2010 Chilean earthquake: Preliminary observations from the SEI/ASCE Chilean Earthquake Assessment Team.” Presentation to the ASCE 7 Seismic Subcommittee, ASCE, Reston, VA. Hough, S. (2007). Richter’s scale: Measure of an earthquake, measure of a man. Princeton University Press, Princeton, NJ. International Code Council (ICC). (2011). 2012 International building code, Country Club Hills, IL. International Code Council Evaluation Service (ICC ES). (2010). “Acceptance criteria for seismic certification by shake-table testing of nonstructural components.” AC156, Country Club Hills, IL. Masek, J., and Ridge, R. (2009). Identification of methods to achieve successful implementation of nonstructural and equipment seismic restraints. Earthquake Engineering Research Institute, Oakland, CA. Multihazard Mitigation Council (MMC). (2005). “Findings, conclusions, and recommendations.” In Natural hazard mitigation saves: An independent study to assess the future savings from mitigation activities, Vol. 1. National Institute of Building Sciences, Washington, DC. Panel on Seismic Hazard Evaluation. (1997). Review of recommendations for probabilistic seismic hazard analysis: Guidance on uncertainty and use of experts, National Academy Press, Washington, DC. Porter, K. B., and Virkud, U. (2002). Catastrophe models: Where they came from and where they’re going. Applied Insurance Research, San Francisco, CA. Reitherman, R. (1990). “Gaps between nonstructural building code and standards provisions, design and construction practices, and safety regulations.” Proc., ATC-29 Seminar on Seismic Design and Performance of Equipment and Nonstructural Elements in Buildings and Industrial Structures, Applied Technology Council, Irvine, CA, 33–38. Revkin, A. C. (2010). “Disaster awaits cities in earthquake zone.” The New York Times, Feb. 25. Richter, C. F. (1958). Elementary seismology, W. H. Freeman and Co., New York, NY. Vick, S. G. (2002). Degrees of belief, subjective probability and engineering judgment. ASCE Press, Reston, VA.

Chapter 3

Geotechnical Primer

Why are insights into seismology and earth mechanics relevant to building code seismic compliance? This chapter has little relevance to a building or architectural project with nonstructural product life cycles of less than a year. But when you are responsible for products with life cycles of 20 years or more, your outlook changes significantly. The reason for these two different perspectives is the 3-year update cycle to the building code for which qualification requirements can and have changed dramatically. Two key technical fields drive these changes: earth science (geophysics, seismology, etc.) and earthquake engineering (geotechnical, structural, etc.). This chapter is an overview to orient the interested reader on the basics of earth science that most significantly influence ground motion demands reflected in model building codes. In 1997 changes in these two fields came together to create the perfect storm that resulted in significant changes to the Uniform Building Code (SEAOC 1998). All of a sudden, commercial test facilities and nonstructural OEMs were faced with seismic qualification demands that were so great there was no clear path forward. On the heels of this change was the obsolescence of the three regional model building codes (the Uniform Building Code, UBC; the National Building Code, NBC; and the Standard Building Code, SBC), and the introduction of two new national model building codes, the International Building Code (IBC) and the National Fire Protection Association’s Building Construction and Safety Code (NFPA 5000). For stakeholders who can be affected by changes to the seismic design provisions of a building code, developing an understanding of new knowledge, which can drive change, becomes important. Advancement of new knowledge in seismology drove most of the increases in seismic design forces in the 1997 UBC.

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Earthquake science began to develop around 1900. By the 1960s this broad field of science had undergone a major revolution with the discovery of plate tectonics, which is the underlying engine powering most earthquakes and which finally explained continental drift. The birth of this science was enabled by the transformation of scientific instrumentation from purely mechanical to electronic, which enabled the acquisition of ground motion data never before possible. This new class of instrumentation provided proof of sea floor spreading and continental rift zones that could only be explained by plate tectonics. Since then, global positioning system (GPS) technology coupled with specialized statistical methods has made it possible to measure the rate and direction of continental movements. The lesson is clear: the field of earth science, which drives significant change to building codes, is fairly young and is changing very rapidly. For those who are responsible for products with long life cycles, it is worth the investment to pay attention to discoveries in this young science, which may influence future code changes. In the fledgling nuclear power industry in the mid to late 1960s and early 1970s there was a sudden concern about understanding how an earthquake could affect the performance of a nuclear facility beyond its structural integrity. Nuclear power facilities had large numbers of nonstructural systems and components that needed to be considered in the seismic environment. This led to the need for a greater understanding by designers as to how earthquakes propagate from their source, through rock, then soil, and finally to building foundations. Accordingly, in those decades there was a great interest in soil–structure interactions. Soil–structure interaction is the effect that the soil below and surrounding a building has on the primary structure of the building due to the earthquake motions imparted through the soil to the building. The first need was a way to predict the response of the building structure to a given earthquake in “regular” buildings, ignoring the soil–structure interaction. The 1976 UBC introduced the “soil–structure interaction” concept to the basic model building code for building structures. The concept has since remained in the codes. But how does the soil–structure interaction originate? It begins with the start of an earthquake and the propagation of seismic waves through the Earth and along the surface of the Earth until the seismic waves reach the facility under consideration. Understanding how an earthquake affects a building’s nonstructural systems and components requires some fundamental knowledge of earthquake geology.

3.1 Plate Tectonics What causes earthquakes? The constitution of the Earth and its dynamic and ever-changing nature results in earthquakes. Primarily volcanism and tectonics, which are often interlinked, are the ultimate causes of earthquakes. An occasional meteor and some manmade events also precipitate them. As rock bodies push on each other due to tectonics or magma movement (molten bodies of rock), stresses build within the rocks. When conditions are correct, the stressed rocks fracture due to their inability to pass any more strain to the stressed system. Imagine a normal window. If you were to apply a load to the glass pane, eventually, if enough force is applied, the glass will fracture. The fracturing of the

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glass is somewhat analogous to an earthquake. Earthquake waves are released from the point of fracture and along the propagating fracture. Tectonics provide the mechanism for the load to be applied to the rocks until they eventually fracture. What is tectonics? It means “builder,” from the ancient Greek and Latin. In earth sciences, it refers to the folding, faulting, and movement of the Earth’s crustal plates. For a long time geologists knew that the continents had moved throughout the history of the Earth. As early as the late sixteenth century, the Dutch cartographer Abraham Ortelius (Kious 2007) noted that the coastlines of North and South America could fit nicely together with those of Europe and Africa. Ortelius postulated that they had been torn asunder by earthquakes. This was about a century before the modern science of geology was born with the writings of James Hutton and Sir Charles Lyell. In 1912 and 1924 Alfred Wegener had postulated that the continents “drifted” (Wegener 1924), but there was no defendable mechanism for their movement. Many geologists could not accept the aimless movement of the continents. Early in the twentieth century Arthur Holmes had pioneered the accepted age of the Earth (Lewis 2002); in the 1920s he postulated that the continents did not simply drift about, but, rather, were slowly driven by thermal convection forces within the Earth’s interior (Frankel 1978). He based his postulation on the heat from radioactivity, rock genesis, and thermodynamic convection within the Earth (Holmes 1945). Holmes’s ideas were met with more than skepticism. His continental movement mechanism was ignored by geologists until the early 1960s, when the concept of “sea floor spreading” was advanced due to repeated magnetic stripe reversals seen on either side of the mid-Atlantic Ridge. The movement of the crustal “plates” quickly gained acceptance in the geologic community. Deep-sea drilling programs in the late 1960s and early 1970s gathered enough data in a very short time to convince even the most skeptical geologists of the viability of the plate tectonic theory. Since that time, it has been assimilated into the basic geologic body of knowledge and is widely accepted as a working model for Earth processes. If we imagine that we could slice the Earth in half, we see that there are three major zones and six subdivided zones that would be seen in the cross section, as follows: 1. Crust • Oceanic crust • Continental crust 2. Mantle • Upper mantle • Lower mantle 3. Core • Outer core • Inner core The core is not well understood with respect to its role in earthquake genesis. It probably plays an indirect rather than a direct role and thus need not be discussed in our context. The outer skin of the Earth is relatively rigid rock and has an overall average thickness of about 30 km (18 mi). There are two distinct types of crustal material. Oceanic crust

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is mostly basalt and is generally about 5–10 km (3–6 mi) thick. Continental crust is also rigid but is made of lighter rocks. The continental crust also varies in thickness and varies from about 30 to 75 km (20–45 mi). Some portions of the crust end up a bit deeper than 75 km, which we will discuss in the following paragraphs on subduction zones. Most of the earthquakes that we experience originate within the crust. However, there are some exceptions, and some earthquakes originate as deep as between 300 and 800 km (200–450 mi). The crustal material is significantly lighter than the underlying mantle material. Therefore, the crust in effect floats on the underlying mantle material, just as slag floats on top of molten iron in a blast furnace caldron. However, the crust does not float aimlessly on the mantle material. It is driven as if on a conveyor belt, and that conveyor belt is a result of thermodynamic convection cells within the mantle. The mantle, like the crust, has two realms. The mantle is divided into an upper mantle and a lower mantle that are composed of much denser rock materials known as anorthosites. Due to high temperatures and pressures within the mantle, some of the rock is molten and some is within its plastic state. Therefore, the mantle materials can flow or, more descriptively, ooze. This oozing, within at least the upper mantle, is caused by convection currents similar to convection currents in the atmosphere. The principle that hot air rises and cold air falls also applies to rock. Hot, plastic rock material tends to rise and cooler rock material tends to fall within the mantle. If one were to put a glass pot of thin oatmeal on the stove (a glass pot is best to be able to see movement in 3-D) and turn the heat on, eventually the oatmeal would begin to move. Hotter oatmeal at the bottom of the pot will become unstable due to the heat and will soon be carried to the top of the pot, forcing the colder oatmeal at the top down to the bottom where it can be heated. The viscous nature of the mantle provides enough friction to engage the crust as the heated mantle material transitions from vertical movement to lateral movement. For decades many geologists thought that the driving mechanism for plate tectonics was basaltic intrusion at spreading centers. This notion has since been replaced by Holmes’s original theory, that the mantle provides a ride for the crustal plates due to the convection cells. Basaltic material is emplaced at spreading centers, but it is now understood that this is not the driving mechanism for plate movement. How fast (or slow) do the plates move? They move at about the same speed as the average human fingernail grows, but their movement varies. Oceanic plates typically move faster than continental plates. The continental plates move more slowly due to mountain roots that cause drag within the mantle. The slowest plates move about 1.3 cm/year (0.5 in./year) while the fastest plates move about 7.6 cm/year (3 in./year). Some plates move a bit faster and some a bit slower, but 2.5 cm (1 in.) per year is a reasonable average. The North Atlantic Ocean is a fairly young ocean and the South Atlantic is even younger. The width of the North Atlantic Ocean is about 3,860 km (2,400 mi) between North America and Europe. By backtracking roughly to the center of the Atlantic where the Mid-Atlantic Ridge marks the spreading center, we can get a rough calculation of the age of the North Atlantic Ocean: • North American coast to Mid-Atlantic Ridge is 3,860 km ÷ 2 = 1,930 km. • 1,930 km × 1,000 m/km × 100 cm/m = 193,000,000 cm.

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• If the plates move at 1.3 cm per year, 193,000,000 cm ÷ 1.3 cm/year ≈ 150 million years. • Sediments along the western North Atlantic are a bit older than this and those on the eastern North Atlantic are about this age (Jurassic). Since the plates are moving, they must therefore “bump” into each other in their travels. They do, and these bumps are where we see most earthquakes. There are four general types of plate boundaries: 1. Divergent plates (plates pulling apart, spreading centers) 2. Convergent boundaries (collision of a continental plate and an oceanic plate or two continental plates) 3. Transform boundaries (lateral sliding of two plates past one another) 4. Plate boundary zones (boundaries between plates that are not well defined) Figure 3-1 shows these types of boundaries. Spreading centers are where new plate material is being added to the crust by the extrusion of basaltic material, generally. An example is the Mid-Atlantic Ridge.

PLATE PLATE ASTHENOSPHERE

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Figure 3-1. Cross section illustrating the main types of plate boundaries; the East African Rift Zone is a good example of a continental rift zone. Source: Data from Vigil (1994).

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An example of a convergent boundary, with two continental plates in collision, is the creation of the Himalaya Mountains as the Indian Plate moves northward in collision with the Eurasian Plate. This collision creates significant and sometimes devastating earthquakes. A convergent boundary where a continental plate and an oceanic plate are in collision produces subduction zones, where the denser oceanic plate is diving (undergoing subduction) below the lighter continental plate. The subduction zones are common around the perimeter of the Pacific Ocean and are known as the “Ring of Fire.” The Ring of Fire is where most of the world’s devastating earthquakes originate, on a sheer numbers basis, as well as a great occurrence of the world’s volcanoes. Hence, the moniker Ring of Fire (Fig. 3-2). The Marianas Trench off the coast of Guam is formed by two oceanic plates in collision. This trench is responsible for very deep-seated earthquakes and is the site of the deepest trench on the planet. The bottom of the Marianas Trench is farther below the surface of the ocean than Mt. Everest is above the surface of the ocean. Mt. Everest is approximately 8,840 m (29,000 ft) above sea level, whereas the Challenger Deep in the Marianas Trench is approximately 10,973 m (36,000 ft) below sea level. When crustal material is subducted below other crustal material, partial melting of the subducted material occurs below the overlying material. Commonly, subduction zones have corresponding mountain chains with active volcanism, as seen in the Andes Mountains just to the east of the subducting South American Trench system. Transform boundaries, where one plate slides past another, can be seen in California with the San Andreas Fault system (Fig. 3-3) and Turkey’s North Anatolian Fault system.

Eurasian Plate

Eurasian Plate North American Plate Cascade Range

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Figure 3-2. Active volcanoes, plate tectonics, and the Ring of Fire. Source: Data from Topinka (1997).

Arabian Plate

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Figure 3-3. San Andreas Fault Zone in the Cajon Pass, California, is not a single fault, but numerous faults that split and come back together within the fault zone; energy can be passed from one end or fault segment to another segment. Both of these systems create notably devastating earthquakes. In fact, the southern leg of the San Andreas has not moved in roughly 300 years and, according to the USGS (Jones et al. 2008), is probably long overdue for a large earthquake. However, there was a smaller event on the southern San Andreas leg in 1812 with an estimated magnitude of about 7.5 and an epicenter near Wrightwood. This earthquake broke from Ft. Tejon to the Cajon Pass. It killed 40 people in San Juan Capistrano in a partial collapse of the Mission San Juan Capistrano (Jacoby et al. 1988). An earthquake along this section of the San Andreas will create a “regional” earthquake. Since 1906, all of the earthquakes in California have been “neighborhood” earthquakes. This is not to diminish the death, injury, and destruction experienced, for instance, in 1971, 1979, 1983, 1989, 1992, and 1994. These earthquakes had sizable magnitudes and did a fair amount of damage, although they affected relatively small areas. From this we use the term neighborhood earthquakes. The expected rupture of the southern segment of the San Andreas Fault will be another story. This break along the San Andreas will likely be between 241 and 322 km (150–200 mi) long. The time required for the rupture from one end of the fault in the south to the termination of movement near Ft. Tejon may take upward of 90 s. The direction of break presented here isn’t the only possibility; it is used as an example. If the break is roughly 241 km, this is a fault propagation rate of approximately 9,656 km/h (6,000 mi/h). It may have offsets between 4.5 and 7.6 m (15–25 ft), and very strong shaking may affect more than 10,000 square kilometers. For comparison,

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strong shaking in the 1994 Northridge earthquake affected about 780 km2 (300 mi2). Strong shaking might last 2 to 3 min in areas with poor ground conditions, such as San Bernardino. It will affect millions of people and will eclipse all previous U.S. disasters if the magnitude is in the range of 7.5 to 8.0. Damage may approach a quarter of a trillion dollars (Jones et al. 2008). As shown in Fig. 3-3, the San Andreas Fault Zone in the Cajon Pass, California, is not a single fault, but numerous faults that split and come back together. Energy can be passed from one end or fault segment to another segment. Potentially damaging earthquakes can also originate within plates, away from their edges (intraplate earthquakes). Volcanism is one such cause, such as in the Hawaiian Islands. The Hawaiian Islands sit atop a “hot spot” in the upper mantle, and the western Pacific plate is moving to the west over the hot spot. The molten material in the hot spot forces its way to the surface through weaknesses in the crust. Older volcanoes stretch for more than 1,600 km (995 mi) west of the main Hawaiian Islands. As the molten material forces its way to the surface, rock can be displaced, resulting in earthquakes. These are not typical tectonic earthquakes but are obviously the result of tectonics due to the moving plates. There are other intraplate earthquakes. The New Madrid Fault Zone in the Mississippi Valley is one such example. Intraplate earthquakes account for a very small amount of energy release overall (on a worldwide basis) for earthquakes, and large events typically have long recurrence intervals. Nonetheless, they do sometimes present a clear and present danger and need to be considered when designing facilities away from plate boundaries. The largest series of intraplate earthquakes in North America occurred as a series of three main events in 1811–1812 on the New Madrid Fault Zone. The USGS Earthquake Hazards program reports these events to have been between Mw7.5 and 7.7. (Magnitude scales will be discussed later in this chapter.) The actual sizes of the events are in question by some, based on the methods and data used in computing their magnitudes. The USGS reports that some researchers place the February 12, 1812, event as higher than 8.0 (NEPEC 2011; Stover and Coffman 1993). Regarding older events prior to modern recording instruments, there is often a divergence of opinion.

3.2 Stress on Rocks Earthquakes result from stress in rocks, but earthquakes do not originate in either soil or at the epicenter. Earthquakes originate in rocks. The depth of the origin of an earthquake has three zones: 1. Shallow: 1 km (0.5 mi) to approximately 70 km (40 mi) 2. Intermediate: approximately 70 km (40 mi) to approximately 300 km (180 mi) 3. Deep: approximately 300 km (180 mi) to slightly greater than 700 km (400 mi) Shallow earthquakes therefore occur within the crust proper. Intermediate and deep earthquakes occur within slabs of crustal material being subducted into the mantle. Some authors call any earthquake deeper than 70 km (40 mi) a deep focus earthquake.

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The single point where an earthquake begins is called its focus. A synonym for focus is hypocenter. Therefore, we have shallow focus earthquakes, intermediate focus earthquakes, and deep focus earthquakes. The epicenter is the point on the surface of the Earth directly above the focus (hypocenter). Rocks need to be able to absorb stress due to tectonic forces and store the energy received. If the rocks cannot store the applied stress, they will continually slip. There are faults that continuously slip, but they are not common. Rocks that are able to store the energy can store it as stress. This stress is a bit different from that generally considered in design. In structural (or nonstructural) design we examine stress as a load divided by area. In rocks, the volume that is accumulating the stress is important. For large earthquakes, that volume can be thousands of cubic kilometers. When enough stress accumulates, the rocks will fail and begin to fracture. They fracture along a plane within the Earth, called the fault plane. Faults are not all a single line; many are made up of numerous smaller faults that allow the transfer of energy from one segment to another during an earthquake. The 1992 Landers (California) earthquake saw such energy transfer along at least three major fault segments. Figure 3-4 shows the transition from one segment to another segment of the faults.

3.3 Damage Potential Based on Earthquake-Facility Variables A common myth is that the most destruction from an earthquake is at or near the epicenter. While the epicentral area may sustain great damage, several variables dictate where the greatest damage potential will be found in any particular earthquake. These variables include the following: • • • • • • •

Where is the human development with respect to strong shaking? What is the quality of construction within the area of strong shaking? What size is the earthquake? What is the depth of focus? What is the distance of focus to the site under consideration? What is the duration of strong shaking? What type of ground exists where the building is located?

Clearly, when there is no construction in an area where strong shaking occurs, there is no damage. But we need to assume development, because we are designing for development. Likewise, we need to assume that we are striving for quality design and construction. Quality in design, plan check, construction, and inspection are all vitally important to successful earthquake-resistant design.

3.3.1 Size of the Earthquake Common sense tells us that the size of the earthquake is important, but size is not allencompassing. For instance, a close magnitude 6 earthquake may very well do more dam-

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Landers Elementary

Figure 3-4. Several fault segments broke during the 1992 Landers earthquake, here showing the proximity of the Landers Elementary School, designed by one of the authors, prior to the 1992 rupture. Source: Data from California Geological Survey (1973).

age to the facility being designed than a distant magnitude 7 earthquake. A personal anecdote following the 1971 San Fernando earthquake might be appropriate. One of the authors was talking with an architect in Riverside after the event. The architect said that not one of his buildings was damaged by the earthquake. The closest building to the area of strong shaking that he had designed was about 90 km (60 mi) distant. The strong shaking had a Modified Mercalli Intensity (MMI) of VIII to XI, as shown in Fig. 3-5. That architect’s buildings did not see anything greater than MMI V. This is a serious disconnect for a design professional to assume that buildings 90 km away from a moderate earthquake “withstood strong ground shaking.”

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38o

120o

118o

81

116o

California II - IV

Nevada 36o

II -IV

II -IV V VIII -XI 34o

VI

Epicenter Riverside

Pa ci

VII

fi c Oc

II -IV

ea n

32o

0

100 Kilometers

United St ates

Arizona

Mexico

Figure 3-5. MMI isoseismal map for the 1971 San Fernando earthquake. Source: Data from Stover and Coffman (1993).

The MMI is an earthquake scale that is rather subjective. It simply reports what the intensity of shaking is at a particular location for a particular earthquake. Intensity can be a very useful parameter for earthquake design, as will be discussed later. Note that in Fig. 3-5 the epicenter lies outside the area of strongest ground shaking and that Riverside, where the above-mentioned architect practiced, lies approximately 90 km from the area of strongest shaking. Other examples of epicenters lying outside the area of strongest shaking include the 1964 Great Alaskan Earthquake and the 1985 Mexico earthquake. The epicenter for the former was approximately 100 km (65 mi) from Anchorage, and for the latter, about 300 km (185 mi) from Mexico City. Clearly, there is something more important

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for damage potential than proximity to the epicenter. A well-designed facility, such as the Landers Elementary School described in Chapter 2, suffered no appreciable damage in 1992 when it was subjected to violent ground shaking and was situated less than 1 km from approximately 3 to 4 m of horizontal offset.

3.3.2 Depth of Focus The depth of the focus can have an impact on damage potential. Guam in the western Pacific Ocean can serve as a good example. Guam sits adjacent to the Marianas Trench, which is a very large and active subduction zone. This is an area of numerous earthquakes that have foci ranging from very shallow to very deep. Large earthquakes with very deep foci typically have less damage potential than similar-size earthquakes with shallower depth. Why? A focal depth of 644 km (400 mi) is clearly a long way away from the surface. In movement through the Earth, some of the released earthquake energy is lost over distance. Per ASCE/SEI 7-10, this results in code acceleration potentials of between 60% and 150% of gravity for Guam (ASCE/SEI 2010) as opposed to southern California of between 100% and 200% of gravity. Guam can have large earthquakes; however, the focal depths are more likely to be greater than in southern California, where focal depths are expected to be much shallower. Deep focal earthquakes can result in attenuated damage potential.

3.3.3 Distance of Focus to the Site under Consideration The distance from the focus to the site under consideration can generally have at least two paths when the site is not at the epicenter. The direct path will have some of the earthquake energy reaching the site quickest in the form of “body waves.” Some of the earthquake energy will take a somewhat longer path. The earthquake waves that radiate outward from the focus in the second path travel upward to the surface, thence reradiating outward from the epicenter as “surface waves.” Body waves and surface waves will be discussed later in this chapter. At this point all we need to know is that there are at least two paths that earthquake energy can take to reach the facility that we need to design. These paths can have a dramatic impact on the damage potential of our facility, which will become a bit more evident as we discuss the various types of waves and how rocks and soil can affect these waves.

3.3.4 Duration of Strong Shaking It should come as no surprise that the duration of shaking significantly affects damage potential. Remembering plots of hysteresis diagrams, which are a function of displacement and time, one can get a vision of earthquake damage potential. The more cycles that are completed, the greater the likelihood of earthquake damage to our facility. Bending a coat hanger is a simple example. Bend it once and it will not break, but bend it 10 or 15 times quickly and most coat hangers will eventually break. Most of the earthquakes that we design for have sustained strong shaking for between 8 and 15 s.

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Consider a facility in San Bernardino and a large-magnitude earthquake, as postulated in the USGS “Shakeout Scenario of 2008” (Jones et al. 2008). Many geologists tell us that strong ground shaking in some areas of the San Bernardino Valley might be between 1 and 3 min due to a regional-type earthquake along the San Andreas Fault. Imagine the damage caused by the 1994 Northridge earthquake to nonstructural systems and components in the short strong-shaking time of 8 to 12 s. Now extrapolate that to about 10 times that duration. Without a full understanding of nonstructural system performance and interaction, our designs might not withstand that duration of strong shaking. Duration of strong ground motion is a vital component in preparing our facilities for survival.

3.3.5 Type of Ground Where the Building Is Located The type and quality of the ground beneath the facilities that we design has a great effect on the performance of these facilities in any given earthquake, and is characterized by geotechnical engineers. Consider the 1989 Loma Prieta earthquake. The epicenter for this earthquake was approximately 100 km (60 mi) from San Francisco and Oakland. The media coverage and heavy damage in San Francisco left the general population with the impression that the epicenter was in San Francisco. Severe damage in both San Francisco and Oakland can be clearly mapped by ground type. The greatest damage in both San Francisco and Oakland was on reclaimed bay muds that shook similarly to a bowl of Jello during the 1989 earthquake. Ironically, much of the reclaimed bay muds where the damage was greatest were formed when debris was dumped into the bay along the shoreline following the 1906 San Francisco earthquake and fire. Unstable or soft ground can increase both the severity and duration of shaking. It also commonly has a longer natural period of vibration. Consequently, “looser” buildings with longer fundamental periods are more likely to have a poorer performance as a result of an earthquake on soft soil. More competent ground has the effect of less severe shaking and shorter durations of ground shaking. It also tends to have shorter natural periods of vibration. More rigid buildings with higher natural frequencies may be exposed to greater damage potential. A simple rule of thumb is that rigid building structures will often perform better on looser soils, and less rigid building structures will likely perform a bit better on stiffer soils. In a similar fashion, consideration must be given to nonstructural systems and components.

3.4 Faults The Earth is approximately 4.6 billion years old. Tectonics as we know it today may have been in operation in some parts of the planet for the past 2.7 billion years or so. The true beginnings of plate tectonics are not well understood, but this topic is receiving considerable attention in the geologic research community. Nonetheless, plate tectonics has probably been working as a model for at least a billion years. Therefore, faulting is not new to the Earth’s tectonic system.

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3.4.1 Are All Mapped Faults Active? Geologic maps show rock units, faults, folds, and so forth. Are all of the faults shown on geologic maps active? No, most faults are in fact quite inactive. The age of tectonism is very long. Much of the tectonic activity that occurred, say, 500 million years ago, is no longer putting stress into the rocks today. Newer tectonic forces and directions of those forces have changed over time. Most of the faults that we see on geologic maps are no longer receiving tectonic forces and are therefore no longer active. Figure 3-3 illustrates the very active San Andreas fault zone in the Cajon Pass of California. Figures 3-6 and 3-7 illustrate a single active fault within the vicinity of the San Jacinto Fault Zone in the Beaumont-San Jacinto area of southern California and an inactive fault in Kingman, Arizona, respectively. There are two basic definitions for active faulting: 1. Legal definition of activity 2. Geologic definition of activity Any state or country has the authority to define fault activity. California, for instance, has defined active faulting in legislation as any fault that breaks Holocene materials (geologically recent or within the last 11,000 years) (California Department of Conservation 1992). It did this with the Alquist-Priolo Earthquake Fault Zoning Act originally passed in 1972 as a result of the 1971 San Fernando earthquake. This may serve well for court cases but it

Figure 3-6. Active faulting near Beaumont, California.

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Figure 3-7. Inactive faulting near Kingman, Arizona.

has limited value for design purposes. It ignores all buried faults (faults with no surface expression) and undersea faults. It also restricts most building structures from being built within only 15 m (50 ft) of active faulting. There is nothing preventing the fault from breaking more than 15 m away from the established fault location in future earthquakes. In geologic distances, 15 m is fairly inconsequential. Most structures within the built environment are not constructed on faults. Clearly it is difficult to design for active fault movement. However, some types of infrastructure must cross active faulting—pipelines and transmission lines, for example—and should be designed accordingly. The Trans-Alaska Pipeline needed to cross the active Denali fault and, therefore, it was designed and constructed in the 1970s for movement on that fault. In 2002, a Mw 7.9 earthquake put this design to the test, and it passed. Most facilities are not designed for ground rupture. Designers need to be much more concerned with the geologic definition of faulting. For design purposes, active faulting may be defined as a body of rock that continues to accumulate stress such that it can ultimately reach its fracture potential with a sudden release of energy. This can include faults that are termed active, capable, and potentially active faults. Faults have recurrence intervals that may range from decades to tens of thousands of years. The recurrence interval leads to probabilistic analyses for the potential of a credible event within a given time frame. ASCE/SEI 7-10 provides contour maps of shortperiod and long-period site acceleration values for all locations in the United States and its territories that have been probabilistically determined based on all known faults in the

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vicinity of a given site. ASCE/SEI 7-10 references a USGS web tool (USGS 2010) that is available for the designer to use to determine mapped values for specific locations. The USGS web tool allows the designer to specify the site based on either an address or by longitude and latitude. The tool then provides the designer with the option of an abbreviated report or a more detailed report. We further discuss this USGS tool in Chapter 4 (see Box 4-1 for a demonstration). Where it is necessary to define a design criterion for a facility, such as an acute-care hospital, site-specific design criteria might be necessary. In the site-specific examination, the various credible active faults within the area that can be expected to create strong ground shaking at the site are examined in detail. We are obviously interested in how any particular fault will affect our designs.

3.4.2 Fault Planes We have previously mentioned fault planes. The fault plane is the rupture surface once fracture begins at the focus and propagates. Rupture can be unidirectional or bidirectional along the fault plane. Movement does not occur along the entire fault plane all at once; it propagates along the fault plane from the point of initial rupture. This movement can be very fast. The 1999 Chi-Chi (Taiwan) earthquake had a reported average rupture velocity of approximately 2.6 km/s (5,800 mi/h) (Zeng and Chen 2001).

3.4.3 Fault Types There are just a few major fault types. Most fault movements incorporate more than one type of movement. The basic types of faults are as follows: 1. 2. 3. 4. 5.

Normal fault Reverse fault Strike slip fault Thrust fault Glide plane fault (generally not germane to this discussion)

3.4.3.1 Normal Faults A normal fault is commonly found in association with linear mountains and valleys. Figure 3-8 shows the configuration and relative movement of a normal fault. The hanging wall has an apparent descent compared to the foot wall. Hanging wall and foot wall are terminology from early mining days.

3.4.3.2 Reverse Faults A reverse fault is the opposite of the normal fault, as shown in Fig. 3-9. The apparent motion of the hanging wall is up compared to the foot wall.

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Foot Wall

Hanging Wall Figure 3-8. In a normal fault, the apparent motion of the hanging wall is down with respect to the foot wall.

Hanging Wall

Foot Wall Figure 3-9. In a reverse fault, the apparent motion of the hanging wall is up with respect to the foot wall.

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3.4.3.3 Strike Slip Faulting: Right Lateral and Left Lateral Strike slip faulting is most common in areas with transform plate boundaries, such as western California. In strike slip faulting, the fault plane is often vertical or nearly vertical. One block of the fault slides past the other block. The sense of the motion is termed either right lateral or left lateral (Fig. 3-10). One determines the sense of motion by standing on one block and facing the opposite block, observing the motion of that block. If the opposite block moves to the right, it is a right lateral fault. If it moves to the left, it is a left lateral fault.

Left Lateral Strike Slip

Right Lateral Strike Slip

Figure 3-10. Strike slip faulting; left lateral apparent movement is shown in the left diagram and right lateral movement is shown in the right diagram.

3.4.3.4 Thrust Faulting Thrust faulting is a subset of reverse faulting. The variation is with the angle of the fault plane. The reverse fault rupture plane is usually considered to be steep, whereas the thrust fault rupture plane is relatively low in its angle with respect to the vertical (Fig. 3-11). The upper block tends to override the lower block. The 1971 San Fernando earthquake, the 1983 Coalinga earthquake, and the 1994 Northridge earthquake were all the result of thrust faults. These thrust faults are the result of geologically young folding in basin sediments. The problem with this type of faulting is that the faults are often not expressed at the surface (see “Buried Faults” following), and they are common in some of the more populated areas such as the Los Angeles Basin and the San Fernando Valley. The faults tend to be shallow and their energy is often very directly focused at the surface of the Earth.

3.4.3.5 Buried Faults Buried faults occur commonly throughout the world. They are defined as faults where the fault plane does not break the surface (Fig. 3-12). Buried faults are sometimes referred to as blind faults but buried fault is the preferred phrase. Any of the basic fault types described previously can be buried faults.

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Figure 3-11. A thrust fault is a reverse fault with a low-angle fault plane.

Notice that the fault plane terminates before it reaches the surface.

Figure 3-12. A buried fault.

3.5 Seismic Waves The storage of stress in rocks results in earthquakes when the rocks storing the energy reach their failure strength. Once rupture begins, it continues by propagating along the fault plane at the same velocity until enough energy has been expended that there is no further rupture. As the rupture propagates along the fault plane, it is releasing energy in the form of earthquake waves. Small earthquakes rupture small areas of the fault plane, and larger earthquakes rupture larger areas of the fault plane. There are typically three types of waves [Fig. 3-13(a,b)].

3.5.1 Primary Waves The primary waves are considered body waves, because they travel through the body of the Earth. The primary waves are compression waves much like sound. Commonly, observers will report that they sense the sound of the earthquake. Compression waves travel through the body of the Earth at speeds up to 6 km/s (22,000 km/h or 13,500 mi/h). The actual speed of the compression waves depends on the rock encountered along the travel path. Compression waves can travel though solid material, plastic or partially molten

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1. Primary Waves (P) Compression Waves 2. Secondary or Shear Waves (S) Shear Waves 3. Surface or Long Waves (L) Love Waves Rayleigh Waves Epicenter

L

Hypocenter (Focus)

S P

(a) (a) P-Wave

compressions

undisturbed medium

dilations (b) S-Wave

wavelength (c) Rayleigh Wave

(d) Love Wave

(b) Figure 3-13. (a) Basic earthquake wave types; (b) the four basic wave types; the top two are body waves and the bottom two are surface waves. Source: Data from USGS Earthquake Wave Types.

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material (mantle), and liquid material (outer core). Compression waves typically will do little to no damage except to the most precariously placed shelved items, for instance.

3.5.2 Secondary Waves The secondary or shear waves travel a bit slower than the compression waves and are also body waves. Within the interior of the Earth their travel speeds are typically in the range of 2 to 4 km/s (roughly 5,000 to 10,000 ft/s). As depth diminishes toward the surface of the Earth from rock to soil, the shear wave velocities decrease significantly and are expressed as the code values in ASCE/SEI 7-10 of less than 183 m/s (600 ft/s) for very soft soils, up to 1,524 m/s (5,000 ft/s) for crystalline rock. These diminished shear wave velocities are for the upper 100 m (330 ft) of soil in site classifications C, D, E, and F below the given structure and should not be confused with the higher-velocity shear waves that are body wave velocities of rock at the depths mentioned previously. Secondary or shear waves can only travel through solid earth material. Hence, based on “shadow zones” on seismographs, geophysicists have determined that the outer core of the Earth is liquid and there are also liquid portions of the mantle. Shear waves themselves typically do not do a lot of damage but they do, in some cases, have the potential to damage some of our structures, especially unprotected nonstructural components such as furniture and equipment.

3.5.3 Long Waves The most damaging waves are the surface or long waves. Surface waves are the slowest traveling of the three types of waves. They form as the body waves impinge the surface of the Earth. In a very small simple model, they basically radiate outward from the epicenter. If the Earth were homogenous, the radiation of the waves would be concentric, like a pebble being thrown into a still pond. But the Earth is not that simple, and surface waves tend to be very complex due to surface and subsurface geologic features as well as the propagation of rupture along the fault plane. Figure 3-14(a) illustrates a pre-earthquake condition with smooth water and ice at the top of the photo. The figure then illustrates Huygens’s Principle using water with ice where a ship’s bow wave begins to strike individual chunks of floating ice. The bow wave represents a spreading seismic wave along the surface of the Earth. Based on Huygens’s Principle, each chunk of ice represents surface geology variations where the spreading earthquake waves form new originator loci at each ice chunk. As the ice reradiates the wave fronts, those wave fronts intersect and mingle, causing complex wave fronts among the chunks of ice, as represented in Fig. 3-14(b). Simple wave fronts can be seen in the middle of the photograph, and very complex wave fronts can be seen in the bottom right of the photograph between the chunks of floating ice. This is similar to the complex surface wave patterns that can be generated by earthquake waves striking basement geological features that are poking through valley sediments. As previously stated, long waves originate when primary waves and secondary waves reach the surface. Some of these waves are reflected back into the Earth and some

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(a)

(b) Figure 3-14. (a) Ice in water illustrating Huygens's Principle; shows smooth water at the top with no wave front and increasingly more complex surface wave patterns as ship bow waves strike individual pieces of ice, similar to P- and S-waves creating complex surface waves; (b) the highly chaotic wave patterns that can be created between geographic features using ice in water as an example.

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re-form themselves into Love waves and Rayleigh waves as they move across the surface of the Earth. Love waves tend to have a side-to-side type of motion, whereas Rayleigh waves tend to have more of a rolling motion. The amplitudes of these waves are large enough to commonly be quite visible when viewing an earthquake with moderate to large magnitude.

3.6 Lateral Loads versus Three-Axis Ground Motion The side-to-side motion combined with the rolling motion of the ground presents a paradox with respect to the codes. The codes historically only considered side-to-side motion. Hence, we usually design for “lateral loads” based on the requirements of the codes. While this may work in most conditions for structural solutions, it is not an option to ignore the vertical contribution of ground shaking when considering nonstructural systems and components within a facility. ASCE/SEI 7-10 addresses this by stipulating a constant 20% modification to the short-period accelerations (SDS for nonstructural considerations, as discussed in Chapter 4). Experience has shown that vertical shaking with respect to free-field ground strong motion seldom yields sustained strong shaking in the vertical axis of more than about two-thirds of the maximum horizontal strong shaking. However, when the earthquake waves enter the building and make their way up through the structure, interesting things can happen. Accelerations can increase as height increases, and the mounting of nonstructural systems and components can have heightened vertical motions depending on their location, geometry, and fixing conditions to the building. Designers cannot ignore the vertical component of shaking with respect to nonstructural systems and components. A vertical component is required when conducting seismic qualification activities, as discussed in Chapter 5.

3.7 Seismic Early Warning Systems Our previous discussion pointed out the speeds of the various seismic waves. In many cases, the speed variances can give a potentially useful window for early warning. The size of the window generally varies from about a second to tens of seconds in many earthquakes. For some earthquakes the warning would be less than a second and for others the warning could be many tens of seconds. If just a few seconds are provided by an early warning system, that is enough time for an audible alert to get occupants to “Drop, Cover, and Hold On” prior to strong motion arrivals, or to initiate automatic machine sequences such as opening fire station truck doors or beginning the start sequence of emergency power supplies. The danger to building occupants is no longer structural collapse. One assumes that modern buildings are not likely to collapse. The danger to occupants in most modern buildings is injury due to getting thrown around, breaking glass, and toppling contents such as unsecured bookshelves. The things that will hurt most occupants are the

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things that the code does not and probably never will cover in most facilities. The danger mostly lies in owner-supplied and unanchored building occupancy contents.

3.7.1 Earthquake Early Warning Systems Are Not Earthquake Predictors Following the March 11, 2011, Great Tohoku ¯ Earthquake (offshore from Sendai, Japan, with the official given name “2011 Off The Pacific Coast of T o¯ hoku Earthquake”), the news media began to discuss at length the Japanese early warning systems. Earthquake early warning is the preferred terminology in the academic community so as not to be confused with military early warning systems for hostilities. The Japanese had multiple early warning systems in place at the time of the earthquake. First was their tsunami alert system that clearly saved many lives by alerting residents to flee to high ground. While they had infrastructure in place for tsunami, such as seawalls, the large magnitude of the earthquake and the size of the tsunami overtopped some of the seawalls. Residents who had more than about 10 minutes of warning were effectively able to escape. Second, Japan—unlike the United States—had begun to invest in effective Pwave early warning systems. The P-wave early warning systems allowed high-speed trains to begin to slow down and other warnings to be issued during the March 2011 T¯ohoku earthquake. U.S. designers should consider incorporating earthquake early warning systems (EEWSs) using P-wave technology into facility design and to discuss this with their clients. EEWS devices are currently available off-the-shelf technology. They take advantage of the speed variation between primary (compression or P-) and secondary (shear or S-) waves. The P-wave travels fast (about 6 km/s in the shallow crust) but is very weak, while the stronger S-wave typically travels less than 4 km/s. Just as the delay between lightning and thunder increases with distance, so the delay between the P- and S-waves increases by about 1 s for every 8 km (5 mi) from the epicenter. In many cases, these waves will both reach the facility before the most damaging surface waves arrive. Body (P- and S-) waves take the short path from the focus to the site through the Earth. Surface waves radiate outward from the epicenter along the surface of the Earth. They travel slower than body waves, but they can also decay much more gradually with distance, so they often cause the strongest shaking at locations farther from the fault. Both P- and S-waves are body waves. The difference in velocities is due to the modes of oscillation. P-waves are compressional waves and S-waves are transverse. A solid material (earth) conveys compressional waves more rapidly than transverse waves. The path for both waves is directly from the hypocenter. When P- and S-waves arrive at the surface, they induce surface waves that radiate from the epicenter. In an ideal Poisson solid, Vp = √3Vs. The Earth is not an ideal solid but is a reasonable approximation, especially in the near field. The body waves actually follow a curved path from the hypocenter to the surface since their velocities increase with depth. A number of variables are involved in the difference between the first compression wave arrivals and the shear wave arrivals. This difference in time can be used to determine how long before the destructive surface waves arrive. Within a fraction of a second to a few seconds following the first arrivals, an audible alert or electronic command can

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be given. Some systems do not wait for the S-wave but use information from just the Pwave to estimate the shaking intensity. Mexico’s SAS EEWS does wait for the S-wave, but that is because authorities are only concerned with protecting Mexico City, which is 300 km (185 mi) from the relevant faults, so they have the luxury of time that much of the United States and Japan do not. The concept of an EEWS is not new. In fact, a proposal was made about 40 years before the Great San Francisco Earthquake of 1906: A very simple mechanical contrivance can be arranged at various points from 10 to 100 miles from San Francisco, by which a wave of the earth high enough to do damage will start an electric current over the wires now radiating from this city and almost instantaneously ring an alarm bell, which should be hung in a high tower near the center of the city. (Cooper 1868) Early warning is needed because the common response of many people when the primary and secondary waves arrive is to sit there and look around, trying to determine why things are wiggling and what other people seem to be doing. If you review surveillance videos of convenience stores that are commonly posted on the Web following an earthquake, you can see shelved contents dancing around while the occupants look around in bewilderment, probably saying something like “What’s going on?” to themselves. It is human nature to not immediately react for some reason. We seem to want to see if it is going to get worse before we begin to take cover. By the time it gets worse, it may be too late. The bookshelf may have toppled on you from behind while you are still sitting in your chair. The EEWS has a chance of preventing this. An EEWS is currently being installed in the Coachella Valley, California, that utilizes a networked system that allows a warning to be provided to all recipients in a region when the nearest sensor to the epicenter detects an earthquake. This can more than double the amount of warning time provided. It will also produce a warning to many locations before the P-wave arrives, so people will not be wondering whether or not to respond. The warning will be first and, if properly designed, should help motivate a response, because there will be no other information available (such as the hard-to-evaluate P-wave shaking). Pwave shaking is nearly imperceptible in all but the largest earthquakes. When people claim their pets went crazy a few seconds before an earthquake, it is probably because they sensed the P-wave that humans could not even feel. Public address systems for announcement must be carefully designed. A “tower near the center of the city,” such as is used for tsunami warnings, would be nearly useless since seismic waves travel so much faster than sound. If people are 1.6 km (1 mi) from a siren tower, more than 5 s will pass before the sound of the siren can reach their ears, by which time the shaking may well have already begun. In addition to audible announcements, the EEWS system can trigger actions, such as switching computers to uninterrupted power supplies (UPSs), opening overhead vehicle doors on fire stations, initiating the start sequence for emergency power supplies in acutecare hospitals, and others. Figure 3-15 displays what can be accomplished by an EEWS.

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No Warning

Financial Data Sent to Off-Site Location

1 sec

Continuity of Operations

60+ sec

8 sec 16 sec 30 sec

Time

Audio alarms could alert people and perhaps give the extra time needed to move to a place of relative safety in the immediate area.

People and Children take Protective Action (”Drop, Cover and Hold On”)

Facilities with high energy or high precision machinery could be shut down before strong motion arrives (e.g., power plants, power distribution centers, refineries, factories, and computer systems)

Doesn’t Matter

Emergency generators could be turned on. Emergency services could be deployed.

Fire station doors could be opened.

Figure 3-15. Showing time of early warning in seconds and what typically might be accomplished with the warning for a variety of facilities. Source: Redrawn with permission from Seismic Warning Systems, Inc.

As a rule of thumb, the amount of early warning can be calculated as follows: • P-wave velocity is commonly about 1.7 times faster than the S-wave velocity. • P-waves travel about 5.5 km/s (3.4 mi/s). In general there is about 1 s of warning that is possible for each 8 km (5 mi) from the hypocenter to the site under consideration. Actual seismic wave velocities depend on local soil and rock conditions, including the amount and pressure of water in the rocks: D1sec =

Vp Vs

(V

p

- Vs

)

(3-1)

where D1sec = distance at which 1 s of early warning is achieved; Vp = velocity of the primary (compression) waves; and Vs = velocity of the secondary (shear) waves. Note: Eq. 31 works, but the units have been lost. The derivation of the equation is Dp = VpTp and Ds = VsTs

(3-2)

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Since we are interested in the distance where the time difference is 1 s: Ts − Tp = 1 sec and Ds = Dp = D1sec

(3-3)

D1sec is the 1-s difference distance. Now substituting: D1 sec D1 sec = 1 sec Vs Vp D1 sec =

Vp Vs

(V

p

- Vs

)

(3-4)

1 sec ( which is in units of km)

Since, for an ideal Poisson solid, Vs = D1 sec = 1 sec

(

Vp

Vp

(3-5)

3

)

3 -1

= 1.37 sec Vp

(3-6)

For the 1-s difference for every 8-km convention, one would expect Vp = 5.8 km/s. For example, consider an earthquake hypocenter that is at a depth of 18 km (11 mi). The Vp is 5.5 km/s and Vs is 3.25 km/s. The facility is 27 km (16.5 mi) from the epicenter. Since Vp and Vs are body waves, they will take the fastest path through the body of the Earth. (Body wave rays are actually curved in the Earth due to Snell’s Law, so they take the minimum-time path rather than the minimum-distance path. However, for short distances the straight-line approximation is sufficient.) The rule of thumb predicts that there should be 4.05 s of early warning based on the facility being 32.45 km (20 mi) from the hypocenter. This was solved by calculating the straight-line distance from the hypocenter to the facility using the Pythagorean formula. Actually running the calculation using Eq. 3-1 yields D1 sec =

(

Vp Vs

Vp - Vs

)

=

5.5 (3.25) 32.45 = 7.94 → = 4.08 se (5.5 - 3.25) 7.94

(3-7)

and provides a close correlation with the 4.05-s rule of thumb (the difference comes in rounding). At the time of this writing, an interagency EEWS is being designed and installed by Seismic Warning Systems, Inc. in the Coachella Valley, located in southern California between the San Andreas and San Jacinto Faults running from the Salton Sea to Palm Springs. The system being installed will cross jurisdictional boundaries when fully completed and will rely on cooperation among cities, police, fire, school districts, and special agencies. The system currently has some components already installed. This system has

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been dubbed CREWS (Coachella Valley Regional Earthquake Warning System) and should be fully operational shortly after this text has been published.

3.8 Earthquake Scales There are basically two methods for measuring earthquakes. One uses earthquake intensity at the point of observation, and the other is earthquake magnitude, which is a method of describing individual earthquakes independent of the point of observation. For any given earthquake there is just one magnitude, but for that same earthquake, there are many different intensities.

3.8.1 Earthquake Intensity Prior to the 1930s the only way to really discuss earthquakes was based on their intensity of shaking at the observer’s location. There was no way for seismologists to independently discuss much about any given earthquake other than intensity. There were several early intensity scales. The one most commonly cited today is the Modified Mercalli Intensity Scale (MMI). It is called “modified” because it was developed by Giuseppe Mercalli in 1902 and was modified in 1931 by Wood and Neumann (1931). This is a closed-end scale that describes the effects on construction (it is essentially a measure of damage to construction). Roman numerals from I to XII are assigned based on the levels of shaking observed. MMI I is the lowest amount of shaking (lowest damage potential) and MMI XII is the highest level of shaking (highest damage potential). The reader can look up specific correlations for the various MMI levels of shaking. Following earthquakes, observers can use reporting mechanisms, such as the USGS website “Did You Feel It?” (USGS 2011a), to record their personal observations. Reporting observations if you experience an earthquake is easy and valuable information for the USGS to be able to report the effects of earthquakes. The USGS site now reports events throughout the world, not just the United States. In 1993 Bruce Bolt published his version of the MMI Scale (Bolt 1993b) with provisions for ground displacements and accelerations shown in Table 3-1. We have eliminated the references to the Rossi-Forel Intensity Scale that Dr. Bolt included. The average person on the street believes that our buildings are designed to specific magnitudes. They actually are not. In reality, most of our buildings are designed to intensity. The MMI type of intensity scale is not of much importance for design purposes. Its value lies in describing the levels of shaking from a specific event. This is the reason why Dr. Bolt included his interpretation of velocity and acceleration values in his abridged MMI Scale. What is more important for facility design is any one of the following intensities: 1. Displacement 2. Velocity 3. Acceleration

Table 3-1. Modified Mercalli Scale. Average Peak Velocity (cm/s)

Intensity Value and Description

I II III

IV

2.0–5.0

V

5.0–8.0

VI

8.0–12.0

VII

20.0–30.0

VIII

0.015– 0.02 g

0.03–0.04 g

0.06–0.07 g 0.10–0.15 g

0.25–0.30 g

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Not felt except by a very few persons under especially favorable circumstances. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing automobiles may rock slightly. Vibration like passing of truck. Duration estimated. During the day, felt indoors by many, outdoors by few. At night some persons are awakened. Dishes, windows, doors disturbed; walls make creaking sound. Sensation like heavy truck striking building. Standing automobiles rock noticeably. Felt by nearly everyone, many awakened. Some dishes, windows, etc. broken; cracked plaster in a few places; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop. Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster and damaged chimneys. Damage slight. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving cars. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving cars disturbed.

Average Peak Acceleration (g is gravity at 9.81 m/s2)

100

Average Peak Velocity (cm/s)

Intensity Value and Description

45.0–55.0

IX

>60.0

X

XI

XII

Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed, slopped over banks. Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. Damage total. Waves seen on ground surface. Lines of sight and level distorted. Objects thrown into the air.

Average Peak Acceleration (g is gravity at 9.81 m/s2)

0.50–0.55 g

>0.60 g

Earthquake Protection of Building Equipment and Systems

Table 3-1. (Continued)

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Acceleration is the typical intensity variable used. Newton described a force as F ⫽ ma, where F is the force, m is the mass, and a is the acceleration. The trick in earthquake design is picking the correct acceleration. Thinking about the complexities of the water waves seen in Fig. 3-14, one can easily see that picking acceleration for an expected earthquake is difficult at best. The codes have evolved over the years from early codes where the earthquake force being designed to was one-tenth of the dead weight of the building for structures in high seismic areas. By the 1960s it was pretty well understood that a value of 0.1 g for the acceleration was much too low. The simple formulas used in model building codes have evolved over the decades from static equivalents to more complex formulas with modifiers that modify other modifiers in an attempt to pick the correct acceleration for an expected event at a specific site. The current IBC code (fully detailed in Chapter 4) has numerous variables that go into making an attempt to select the most accurate acceleration for our designs in static coefficient formulas. The fact is, building codes do a pretty good job of providing the design profession with adequate seismic design guidelines using intensity as a design parameter as long as • • • • • • •

Load paths are understood by the designer; The designer provides for redundancy; Appropriate detailing is provided by the designer; Adequate plan check is performed; Adequate inspection is performed during construction; Projects are constructed per the approved contract documents; and Changes to the design during construction are well thought out.

3.8.2 Earthquake Magnitude Scales In the late 1920s and early 1930s, there was no convenient way for seismologists to independently compare earthquakes independent of their point of observation or occurrence. Charles Richter in association with Beno Gutenberg developed the Richter Magnitude Scale (Richter 1958) for this purpose. This original definition served well for decades. As more was learned about earthquakes, it became clear that other magnitude scales would be more appropriate. The most common scales that have been in use include the following: 1. Local Richter Magnitude (ML), still commonly reported by the media, measuring long waves 2. Surface Wave Magnitude (Ms), measuring Rayleigh waves 3. Body Wave Magnitude (Mb), measuring primary waves 4. Moment Magnitude (Mw), the preferred magnitude scale, measuring rupture area and fault slip. Magnitude scales, unlike intensity scales, are open-ended. There are no theoretical upper or lower bounds. Zero magnitude and negative magnitudes are just as possible as magnitudes greater than 10. However, short of a large space impactor, magnitudes near Mw 10 are pretty unlikely as the crust and upper mantle simply cannot store sufficient energy before the rocks fail.

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3.8.3 Local Richter Magnitude (ML ) and Surface Wave Magnitude (MS ) Dr. Richter defined magnitude using a very specific type of seismograph (a long-period Wood-Anderson seismograph) at 100 km (62 mi) from the epicenter (Richter 1958). This scale was originally conceived for comparing local southern California earthquakes. The fact is that not many earthquakes fit this exact definition, so corrections needed to be made for every earthquake depending on the location of the seismograph and the type of recording instrument used. These corrections led to a slight bit of disagreement between reporting stations. Hence, the media would report several values for the magnitude of an earthquake depending on whom they talked with. Generally, within a short time, seismologists would settle on an average value for the magnitude. Seismologists needed to be able to study earthquakes outside of southern California. Originally, Richter Magnitude was based on the amplitude of the local surface waves or, in some cases, on the secondary waves (shear waves) if they produced larger amplitudes. The Richter Magnitude Scale is related to the energy released by the earthquake. The misconception is how it is related. The media often reports that there is a 10-fold increase (10x) in energy for every full increase in Richter magnitude. This is not true. Magnitude increase is based on the amplitude of the surface waves. There is a log10 increase in the excursion of the recording instrument for every full increase in the recording needle on the seismogram. Simplified, this says that for every full increase in magnitude, the needle on the seismograph moves 10 times. As an example, if the needle moves 2.54 mm (0.1 in.) for an M3 earthquake, then it will move 25.4 mm (1 in.) for an M4 earthquake and 245 cm (10 in.) for an M5 earthquake. Clearly, this is a bit crazy, because one would need a seismograph the size of two or three football fields to record an M8 earthquake. Interestingly, through extrapolating backward from the previous example, we can easily see that M0 earthquakes are possible, as are negative-magnitude earthquakes. Not only are they possible, they exist in reality. Negative-magnitude earthquakes are termed microseisms and M10 or larger events could be caused by space impactors. However, we do not need either the very small or very large magnitude events for earthquake engineering of nonstructural systems and components. Richter Magnitude has an approximate relationship of 31.5 times as much energy release for every full increase on the magnitude scale. The relationship was originally given by Richter and Gutenberg (Richter 1958) as Energy in ergs = 11.4 + 1.5 MS (surface waves due to shollow teleseism)

(3-8)

This formula was refined to Energy in ergs = 11.8 + 1.5 MS

(3-9)

The magnitudes in the formula yield an approximate increase for every full magnitude of 31.5 times. A jump of two full magnitudes therefore yields 31.52 = 1,000 times the energy. This is a reasonable argument to refute those who ask, “Don’t a few M6 earthquakes

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relieve the stress to prevent an M8 earthquake?” No! Based on the foregoing, it would take roughly 1,000 M6 earthquakes to relieve the stress of a single M8 earthquake. Surface wave magnitude uses the Rayleigh waves for the measurement of the size of the earthquake. It was introduced by Richter and Gutenberg (Richter 1958) as a further refinement of the Local Richter Magnitude Scale for more distant earthquakes. It is not in common use today.

3.8.4 Body Wave Magnitude Richter and Gutenberg (Richter 1958) also realized that they needed a scale for distant earthquakes where surface wave magnitudes were not appropriate. They later developed the body wave magnitude (Mb) just for this purpose, although it was not termed Mb originally. Body wave magnitude is similar to surface wave magnitude except that the magnitude is determined using only the primary or compression wave arrivals. Body wave magnitudes are not generally used to develop seismic specifications and therefore do not need further discussion here.

3.8.5 Moment Magnitude Very large earthquakes were somewhat problematic for the early Magnitude scales with respect to surface wave magnitude and body wave magnitude for a variety of reasons. The early magnitude scales did not accurately represent these events, including the 1960 Chilean earthquake (MS8.5) and the 1964 Great Alaskan Earthquake (MS8.4). In the ensuing decades, following the development of the initial magnitude scales, seismologists realized that they needed a less limiting scale. The moment magnitude scale (MW) was developed as a more accurate measurement of earthquake magnitude to better fit the large events. It better describes nearly all earthquakes, not just large events. The subscript “W” designates work. Due to a lack of understanding by the general populace of the newer and more accurate moment magnitude scale, earthquake magnitudes are still commonly reported by seismological stations to the general media today as Richter Magnitude. Technically, all currently used magnitude scales can be traced to Richter in philosophy and, thus, it is not out of the question to refer to any of the scales as “Richter Magnitude.” The moment magnitude for the 1964 Great Alaskan Earthquake was MW9.2 and that of the 1960 Chilean earthquake was MW9.5. The more recent 2004 Sumatra earthquake was MW9.1 according to the USGS, and the March 11, 2011, Great Tohoku ¯ Earthquake in Japan was MW9.0 (USGS 2011b). What is moment magnitude (MW)? First, we need to look at seismic moment (M0), which is defined as M0 = Rock Strength (m) × Fault Rupture Area (A) × Slip Distance (d) = m Ad

(3-10)

Seismic moment refers to torque. Torque in a system is the force required to change the angular momentum of the system and is measured as the force times the distance to the

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center of rotation for the system. Earthquakes are caused by internal torques within earth blocks with forces interacting on opposite sides of a fault plane. The seismic moment can be converted to moment magnitude as defined by Hanks and Kanamori (1979) as ⎛2 ⎞ MW = ⎜ log 10 M0 ⎟ ⫺ 10.7 ⎝3 ⎠

(3-11)

Figure 3-16 shows the relative relationship of earthquake size, fault plane, and moment magnitude. Most seismologists are in favor of using seismic moment in estimating earthquake magnitudes. Ground motion design spectra should originate with estimates of seismic moment yielding estimated moment magnitudes. The moment magnitude information can then be propagated through the crust to the soils at the site in question, arriving at expected ground displacements, velocities, and accelerations at the specific site under consideration for design. The ground shaking intensities at the site can then be extrapolated up through the building to the nonstructural system or component location for specific intensities such as floor response spectra.

10 Mi

300

Large Slip Yields Large Moment Magnitude

Mi

Medium Slip Yields Medium Moment Magnitude

Small Slip Yields Small Moment Magnitude

Figure 3-16. Isometric of fault plane showing three relatively different fault slips yielding three different moment magnitudes along a single 300-mi-long hypothetical fault plane.

3.9 Foreshocks and Aftershocks Fore- and aftershocks can be significant earthquakes in themselves. Foreshocks are defined as events immediately preceding (usually within a few days of the main event) a main shock within the vicinity of the main shock and are smaller in magnitude than the main shock. Not all main shocks exhibit foreshocks. All significant earthquakes produce aftershocks. These are events that may be sizeable but are smaller than the main event. Aftershocks trail off in size with time. The largest aftershocks are most likely to occur within a few days of the main earthquake. When the main earthquake is of large size, smaller aftershocks may occur for several years. Aftershocks must also be within the same vicinity of the main earthquake. Aftershocks are

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essentially adjustments within the Earth following the main earthquake. For a large event, there may be hundreds or thousands of aftershocks. Aftershocks following large earthquakes are of concern to designers. Facilities may experience some degradation of capability of some structural or nonstructural systems where the damage is not immediately evident. An aftershock could exacerbate their condition, leading to failure in the following aftershocks. An example was the Landers Elementary School (Fig. 3-17) that was located less than 1 km (0.4 mi) from approximately 3 to 4 m (8 ft to 12 ft) of horizontal offset. One of the authors of this text (McGavin) was the architect for this school, which had opened approximately a year before the 1992 earthquake. The Landers earthquake occurred shortly before 5 a.m. At about 8 a.m. the Big Bear earthquake occurred a short distance away (40 km west) on a different fault. While the Big Bear earthquake may or may not have been an aftershock of the Landers earthquake, it had a similar effect on Landers Elementary. During the Big Bear earthquake, a below-counter soldered water line broke. The waterline was not broken at the time of the original event. Fortunately, a school custodian was on site at the time of the Big Bear earthquake and was able to shut the water off prior to any flooding damage. Both earthquakes caused only minor nonstructural damage. The school suffered minor suspended ceiling damage during the main event due to an improper installation during construction. Once the damaged ceiling tiles were removed, the California Department of Forestry and the American Red Cross were able to occupy the school site for emergency response purposes in the earthquake aftermath. School functions resumed at Landers Elementary following the summer break.

Figure 3-17. Landers Elementary School, designed by Ruhnau-McGavin-Ruhnau Associates, is located approximately 0.4 mi from the surface expression of the Landers fault.

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Earthquake Protection of Building Equipment and Systems

3.10 Deriving Seismic Hazard Maps from Seismology Effective seismic design can be a very elusive goal and, in some cases, a little capricious. Stories abound where one building is severely damaged and another similar building across the street is left relatively undamaged following a damaging earthquake. The April 4, 2010, Sierra El Mayor earthquake in Baja serves as an example of just such a condition. Jefferson Elementary School, in the Calexico Unified School District, was constructed in the 1960s and had widely varying nonstructural damage within some of the classroom pods (classroom pods are individual buildings with several classrooms in each building). At the Jefferson school site, pods with the same north to south orientation—symmetrical in all aspects and located within approximately 30 m (100 ft) of each other—experienced very different ground motions. There was significant nonstructural damage in one pod (Fig. 3-18) while another pod had very little damage (Fig. 3-19). During that event the USGS reported the MMI intensity at VII and showed peak ground accelerations (PGAs) in the Calexico vicinity of about 27% g (USGS Station 5035, Calexico Fire Station PGA = 27.46% g). The school in question was located about a mile northeast of the recording station. Classrooms near where an exterior walkway soffit collapsed suffered extensive interior nonstructural damage to the suspended ceiling and the classroom contents. Classroom 23 is shown in Fig. 3-18. Notice the severely damaged 1960s-vintage suspended ceiling system in this classroom pod, which did not have the

Figure 3-18. Classroom 23 on the east side of the campus showing extensive ceiling damage and chairs that fell off the desk tops; this building pod had the stucco soffits that collapsed.

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Figure 3-19. Classroom 12 is in a classroom pod immediately west of the pod with Classroom 23; notice that there is significantly less damage in this pod approximately 30 m (100 ft) west of the pod with Classroom 23; the damage included some ceiling tiles and lights that fell and a few chairs that fell off the desk tops.

benefit of either splay wires or compression posts for the ceiling and the lights were not safety-wired to the structure above, as required by modern codes. The suspended ceilings were identical in size and installation in all of the classrooms within this pod. Approximately 30 m (100 ft) west of the classroom 23 building pod, there was very little damage in the classrooms in another building pod. Classroom 12 is shown in Fig. 3-19. Some classrooms adjacent to classroom 12 had more damage than that shown in Fig. 3-19. A little farther away in a different pod, classroom 46 had desktop-type computer monitors that were also left undamaged and did not topple (Fig. 3-20). Very few ceiling tiles fell in this classroom. It is apparent that ground motions on this single small elementary school site varied considerably between adjacent classroom pod buildings. Clearly, there is a need to understand potential ground motions for effective design. One goal of this chapter is to familiarize the reader with a background in basic seismology so as to be able to better utilize the hazard maps and seismic design principles for nonstructural seismic design as contained in model building codes. Readers interested in gaining a more in-depth understanding of the genesis of seismic ground intensity coefficients may want to read more definitive material in either Bruce Bolt’s “Predicting Strong Ground Shaking” in Earthquakes and Geological Discovery (Bolt 1993a, Chapter 7) and/or Egor Popov’s “Development of U.S. Seismic Codes” (Popov 1994). What follows is more

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Figure 3-20. Classroom 46 needed little more than a good sweeping afterward.

of a colloquial discussion than a quantitative scientific discussion. It is our opinion that the following simplified discussion will better serve the intended engineering reader rather than the accomplished earthquake engineer. In the early part of the twentieth century, when seismic design was in its infancy, architects and engineers commonly used assumed lateral loads for both wind and seismic design. Following the 1906 San Francisco earthquake, a value of 1.44 kPa (30 lb/ft2 )was commonly used. Surprisingly, a value of 30 lb/ft2 was not such a bad idea for the time. In areas with nonhurricane winds, this is a substantial value, and it could actually yield a respectable value for earthquake-induced lateral loads. In 1914 the Japanese engineer Riki Sano proposed an equivalent static coefficient method of analysis for structural design (Bozorgnia and Campbell 2004). Then, following the 1925 Santa Barbara earthquake, the basic Newtonian formula for seismic force was employed as a code value using Sano’s static coefficient methodology. Newton’s mass was replaced with the dead load of the building or building component. For acceleration, the early best professional judgment was to use 0.1 g. At the time, there was little evidence to confirm whether this was a valid value. Consequently, following the 1925 Santa Barbara earthquake, a program was initiated to study strong ground motion in the United States. The value of 0.1 g was used in the codes for decades. The most common early strong motion record used in the middle of the twentieth century was the 1940 El Centro record. Peak accelerations for the El Centro event were approximately 0.3 g (Fig. 3-21). The fact that the strong motion instrument was in the base-

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ment of a building (as opposed to a free-field instrument) led to later speculation that higher-frequency accelerations may not have been properly represented. Nevertheless, this event was the gold standard for many years in seismic engineering. In 1966 the Parkfield earthquake in California led to a surprise for engineers. This event contained a peak acceleration in excess of 0.5 g. Conversations—no, disagreements—between early earthquake engineers began with fever as to whether this was an isolated event or whether even higher values were possible. In the 1971 San Fernando earthquake, an acceleration in excess of 1 g was recorded in the vertical direction at the Pacoima Dam site at the northern end of the Los Angeles Basin near Sylmar. This event was often discounted as an anomaly, but 1979 saw a repeat of this at Bonds Corner in the Imperial Valley earthquake, where 1.0 g vertical was observed in ground motions. Early seismic maps used isoseismals (areas of equal shaking shown with contour lines). The default in the United States was for the 1931 Modified Mercalli Intensity Scale. While the MMI scale was subjective, it was nonetheless very convenient for earthquake engineers, because it is assessed in terms of observable varying degrees of building shaking. The subjective isoseismals could easily be converted to somewhat more convenient engineering terms of displacement, velocity, and acceleration. Modern earthquake engineering (including test labs) has mostly settled on response acceleration values at specific frequencies (or periods) and damping since the earthquake demand is dynamic and not static. How are these values obtained? It is as much a “black art” as it is seismology, geotechnical engineering, and structural engineering. Sound scientific judgment is required in the final outcome, along with experimental data and number crunching calculations. While this black art claim may generate more than a few raised eyebrows, it is nonetheless fairly accurate. The black art comes from balancing all of the assumptions that need to be made based on good professional judgment in order to get the earthquake energy from the hypocenter to the building site and then extrapolate that energy into displacement, velocity, or accelerations in the free-field at the site in question. The next difficulty is in getting the free-field energy into the building via the foundation (soil–structure interaction) and propagated up through the building to where in the building the nonstructural systems are located. Figure 1-12 in Chapter 1 illustrates this concept. Following the initial rupture at the hypocenter, the earthquake propagates along the fault plane. Earthquake waves begin to radiate outward from the hypocenter and along the fault geometry as the rupture progresses. The simple description is that the wave fronts are like a pebble being tossed into a still pond. Unfortunately, nature is not that simple; it is elusively nonlinear. The fault plane is not a simple smooth plane. In fact, it is only loosely considered a plane. It can bend and warp. The fault plane is not even necessarily a single entity. Its surface most often has bumps and it commonly can have segments and separate pieces where the rupture must bridge. The bridging complicates the wave fronts. The fault plane might also cross various rock types as the rupture progresses along the fault plane. Before the fault plane has ruptured very far, many variables might be involved in shaping the wave fronts of the transmitted earthquake energy. Once the energy leaves the fault plane, there are many more variables to be considered. Rock and intervening soil types between the fault plane and the specific site pose

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numerous possible variables. Sedimentary rock can, for instance, reflect and refract earthquake energy. It can also focus energy. Earthquake waves can attenuate or amplify. Very poor soils can lengthen the duration of shaking by dozens of seconds and sometimes minutes. Poor soils respond like gelatin to low frequency shaking and can exacerbate ground motion. In some parts of Mexico City in 1985, for example, strong shaking lasted approximately 7 min in some locations due to very poor soils. Mix this with numerous faults in the vicinity that may have a potential impact on any particular site, and the likelihood of selecting the correct fault that will rupture and calculating the correct energy release as well as the probability of rupture, and one can easily see that putting everything together is at best a very difficult task. Add to the already discussed complexities the fact that fault triggers are not well understood and uncertainties about recurrence intervals for many faults are at best educated guesses based on tectonic stresses, rock mechanics, and unknown fault stress relievers, and the task of a truly objective and quantitative fault analysis is simply what we began with—a black art that requires a quantum leap full of sound professional judgment. The best professional judgment is not exercised in a vacuum. It is not based on a lack of knowledge but, rather, on too many variables and the nonlinear math required to feed all of the variables. Therefore, the determination of the likely ground accelerations at a particular site is clearly going to be somewhat of a fuzzy logic process. Yes, math is involved, but so are a good deal of common sense and extensive professional experience. Seismologists, geotechnical engineers, and structural engineers collaborate within their various fields of expertise to “out-guess” any specific earthquake. In the 1970s Algermissen and Perkins (1976) published a seismic risk analysis map of the United States for horizontal bedrock accelerations with what they determined to be a 90% probability of not being exceeded in 50 years. ASCE/SEI 7-10 splits the risk-targeted maximum considered earthquake (MCER) for Site Class B horizontal acceleration maps into a short-period bedrock acceleration (0.2 s) and a long-period soil acceleration (1.0 s) map with a 2% probability of not being exceeded in 50 years. Comparing the Algermissen/Perkins and ASCE/SEI 7-10 maps shows a general conformance of contours, but the contours differ in specific values. Much has been learned in the past 30-plus years with respect to horizontal ground accelerations. The end of this chapter discusses the development of code maps for seismic design. When Algermissen and Perkins published their acceleration map for the contiguous United States in 1976, the model codes were still using seismic zones. These seismic zone maps, which had been in use since the very early model codes, continued in use until the publication of the first International Building Code (IBC) in 2000. Seismic zone maps are finally gone forever for design in the United States. By shifting away from seismic zones, the design engineer is now much more reliant on response spectrum design methods, even for the static equivalent design. To get to spectral design, seismic accelerograms are needed. Accelerograms are recordings of specific earthquakes (displacement, velocity, or acceleration) as a function of time. Every earthquake recorded at any specific site has a unique accelerogram. Accelerograms are commonly recorded in three directions, two horizontal and one vertical. Some older recording stations only record one or two components of motion. Figure 3-21 shows the accelerogram that was recorded during the 1940 El Centro Earthquake (north-south component).

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Figure 3-21. Example accelerogram recorded during the 1940 El Centro Earthquake.

Existing free-field seismic accelerograms are used to reverse engineer the earthquake back through all of the soil and rock to the original earthquake with known magnitude, fault plane characteristics, and others that generated the accelerogram. The reverse engineering of enough accelerograms allows the generation of earthquake ground motion intensity approximations for a geographic area. Many real accelerograms are needed to be able to generate values for future postulated earthquake ground motions. The reverse engineering and then the generation of ground motion intensity predictions is a difficult task. Ground motions from actual earthquakes are random in every sense of the word. Accelerations vary with time during the event, and peak and repeated excursions of the ground are random. Knowing all of this, a response spectrum based on many soil types can be developed (see Figure 7-2 in Chapter 7 for development of response spectrum plots). Although the response spectrum for an actual earthquake is not a smooth curve, smooth response spectrum curves can be developed by calculating numerous responses based on the approximated soil properties associated with geographic regions. Facilities that have heightened performance criteria, such as acute-care hospitals, power-generating facilities, communications centers, or sensitive data storage centers, may well justify or demand a site-specific geotechnical investigation and specific response spectrum generation for that facility at that site rather than relying on the code’s default response spectrum (see Fig. 4-6 in Chapter 4 for the code’s default response spectrum).

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3.10.1 Summary of Seismic Ground Motion Hazard Maps Over 10 years ago the U.S. model building codes replaced seismic zone maps with project-specific ground motion hazard maps (Frankel et al. 2000). However, even today, it is not uncommon to hear the question, “What zone is my building project in?” The change to ground motion maps is the single most significant contribution of seismology to modern building codes and therefore merits a summarized treatment to eliminate any confusion that might exist with stakeholders. The most perplexing shortcoming of the seismic zone maps was that many design professionals did not fully understand exactly what the zone number meant. The first zone map was published in the 1935 Uniform Building Code and divided the western United States into three seismic zones (1 through 3) of equal probability of damage (Beavers 2002). The first seismic zone map of the contiguous 48 states was created by the U.S. Coast and Geodetic Survey in 1948 and divided the country into four zones (0 through 3) of equal probability of damage (Roberts and Ulrich 1950). While there were many reasons for the demise of the seismic zone approach, the most significant was the inability of vaguely sketched lines on a map of uniform zones to convey the design requirement on a project-specific basis. In reality, the earthquake hazard is highly variable, and to treat all building sites that fall into one of the geographic zones equally does not provide enough granularity within each seismic zone. Not all building project sites within a given zone will experience the same demand. By providing the value of dynamic demand at two specific response spectrum ordinates, 0.2-s and 1.0-s periods (at 5% damping), and correcting for site soil conditions, it is possible to define a projectspecific “default response spectrum” to describe the earthquake demand based on site latitude-longitude coordinates (refer to FEMA 450; FEMA 2003). The basis of modern seismic hazard ground motion science has its roots in both the revolutionary science of plate tectonics and probabilistic seismic hazard analysis (PSHA), introduced in the 1960s. The new science of plate tectonics finally established a basis for earth mechanics that powered earthquakes. This new science provided a significant advance in the understanding of the geophysical processes that drive earthquakes, the geological time scale involved, and why some areas are more active than others. Because these processes occur on a geological time scale (the long intervals of hundreds or thousands of years between significant events), there are limited recorded ground motion data to fully understand ground-shaking phenomena, even in very active areas. In the absence of hard data, it is necessary to turn to probability to “deal with the rare” for assessing seismic ground motion demands for building design. This was the conceptual vision of the notable earthquake engineering professor Emilio Rosenblueth working at the National University of Mexico in the 1950s (Newmark and Rosenblueth 1971). This probabilistic approach was translated into the accepted theoretical basis of PSHA by Luis Esteva and C. Allin Cornell in the late 1960s. PSHA has become globally accepted as the starting point to establish seismic ground motion for locations that do not have an extensive library of recorded ground motions and well understood faults. PSHA ground motion maps are established for a specified return period. A return period of approximately 2,500 years is associated with a 2% in 50 years probability of

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exceedance, which means that a given location can be expected to exceed the stated ground motion at least once in a 2,500-year period. For commercial nuclear power plants, a return period of 10,000 years is used. While PSHA is used as a basis, building code design maps are modified to account for other considerations. In locations where extensive ground motion records are available, the PSHA values may be replaced by deterministic values that have been assessed by a seismologist. ASCE 7-10 introduces an additional refinement in ground motion assessment with the introduction of “risk targeted” maps, which take the frequency of damaging events into consideration with a goal of providing for a “uniform risk of collapse.” Measurements of earthquake ground motion at the surface, which are not influenced by buildings or other structures, are called free-field and are specified as peak ground acceleration (PGA). Free-field seismic instrumentation captures the dynamic behavior of the soil column at the surface. Extensive networks of automated seismographs located around the world capture these ground motions for study by seismologists. In active areas these tend to be strong-motion seismographs placed in dense arrays to capture detailed data on how the earthquake energy propagates from the fault and how its behavior is modified by the local geology. Sparsely spaced stations tend to be weak-motion types that are used by seismologists to determine the earthquake source location globally. Starting in the early 1970s data collected from dense arrays of strong-motion seismographs from earthquakes in Mexico City; Landers, California; Kobe, Japan; Northridge, California; and Loma Prieta, California, resulted in significant building code seismic design increases in the 1997 Uniform Building Code. These increases were mainly due to the introduction of the so-called near source design coefficient. Extensive analysis of data from dense strong-motion arrays enabled seismologists to determine within the near-field distance from a shallow fault that the earthquake energy could be greatly increased by the superposition (addition) of the waves radiating from that fault. This newly discovered phenomenon was called directivity to denote how the ground shaking could be much greater in one direction from the fault than in another (SEAOC 1998). A uniform seismic zone map could not convey this location-specific increase in ground shaking, which necessitated the introduction of street-level near-source design maps in the 1998 California edition of the 1997 Uniform Building Code (ICBO 1998). The introduction of project-specific design maps with the 2000 IBC was a superior solution to this limitation of the seismic zone maps.

3.10.2 Review of How Geotechnical Effects Influence Site Requirements While site soil conditions have long been a part of the building codes, they have become more visible in the IBC because of the location-specific approach to deriving the default design response spectrum for the building. This increased visibility merits a brief review of geotechnical effects, because it is the ground motion at the surface that the building must endure. Earthquake energy is transferred to the soil column at the underlying rock-to-soil boundary interface. The looser the soil column, the greater the tendency for the column to become dynamically excited and resonate in a manner that is only dampened by the fric-

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tional resistance of the soil particles moving next to each other. This frictional resistance is linear at low strain rates and nonlinear at high strain rates. Seismic instrumentation placed to capture the bedrock earthquake motion is denoted as rock site. The ratio of the free-field ground motion to the rock motion yields the amount of soil column resonance. Data from soil and co-located rock sites have been used to derive the code’s project-specific geotechnical design requirements. The ability of the soil column to modify the seismic demand for a building site is accounted for in building codes by a design coefficient for the geotechnical site classification or soil type. Since this is a dynamic demand, the values of site class coefficients will be given for at least two spectrum ordinates, typically 0.2-s and 1.0-s periods at 5% damping to establish a simplified “code default” building design response spectrum (see Fig. 46 in Chapter 4). A third spectral ordinate value may also be observed in the site geotechnical report, called TL, but this is a long-period parameter and only applies to structures with long periods (greater than 8 s), such as tall buildings or dams (FEMA 2003). All of these values must also be stated for a specific return period and site class basis. In the end, the model building code is a reflection of current knowledge regarding seismic hazards. The code acts as a funnel for receiving the complexities associated with the seismology, data reduction mathematics, expert opinions, and geopolitical aspects of earthquake engineering. These components are shaken together and simplified into a prescriptive format that can be implemented by design professionals who may not be earthquake engineering specialists. Figure 2-2 in Chapter 2 typifies the code development process. The code needs to be continuously reviewed and updated as ever-broadening knowledge reflects the evolution of design practice in our changing technical and political contexts. In Chapter 4 we get under the hood and take a look at the seismic design requirements as defined by the 2012 IBC, which is based on the ASCE/SEI 7-10 standard.

References Algermissen, S. T., and Perkins, D. M. (1976). “A probabilistic estimate of maximum acceleration in rock in the contiguous United States.” USGS Open File Report 76-416, USGS, Washington, DC. ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, Reston, VA. Beavers, J. E. (2002). “A review of seismic hazard description in U.S. design codes and procedures.” J. Prog. Struct. Eng. and Mat., 4(1), 46–63. Bolt, B. (1993a). “Predicting strong ground shaking.” In Earthquakes and geological discovery, Chapter 7, 152–179. Scientific American Library, New York, NY. Bolt, B. A. (1993b). Abridged modified Mercalli Intensity Scale, earthquakes: Newly revised and expanded, Appendix C. W. H. Freeman and Co., New York, NY. Bozorgnia, Y., and Campbell, K. W., eds. (2004). Earthquake engineering: From engineering seismology to performance-based engineering. CRC Press, Boca Raton, FL. California Department of Conservation. (1992). Special publication 42, California Department of Conservation, Division of Mines and Geology, Sacramento, CA.

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California Geological Survey. (1973). Alquist-Priolo special studies zone map, Landers Quadrangle. California Geological Survey, Sacramento, CA. Cooper, J. D. (1868). Editorial in San Francisco Daily Evening Bulletin, Nov. 3. Federal Emergency Management Agency (FEMA). (2003). “Design response spectrum: NEHRP recommended provisions for seismic regulations for new buildings and other structures.” FEMA 450, Washington, DC. Frankel, A. D., et al. (2000). “USGS national seismic hazard maps, earthquake spectra.” J. Earthquake Eng. Res. Inst., 16(1), 1–19. Frankel, H. (1978). “Arthur Holmes and continental drift.” Br. J. Hist. Sci., 11(2), 130–150. Hanks, T. C., and Kanamori, H. (1979). “Moment magnitude scale.” J. Geophys. Res., 84(B5), 2348–2350. Holmes, A. (1945). “Continental drift: The search for a mechanism.” Principles of physical geology, Chapter 21. Ronald Press Co., New York, NY. International Conference of Building Officials (ICBO). (1998). Maps of known active fault near-source zones in California and adjacent portions of Nevada to be used with the 1997 Uniform Building Code. ICBO, Whittier, CA. Jacoby, G.C., Sheppard, P.R., and Sieh, K.E. (1988). “Irregular recurrence of large earthquakes along the San Andreas fault: Evidence from trees.” IScience, 241(4862), 196–199. Jones, L. M., et al. (2008). “The shakeout scenario.” USGS Open File Report 2008-1150, CGS Preliminary Report 25. USGS, Washington, DC. Kious, J. (2007). “Historical perspective.” USGS: The Dynamic Earth. (Oct. 12, 2011). Lewis, C. L. E. (2002). “Arthur Holmes: An ingenious geoscientist.” GSA Today, March, 16–17. National Earthquake Prediction Evaluation Council (NEPEC). (2011). “Independent expert panel on the New Madrid seismic zone earthquake hazards, April 16, 2011.” (Oct. 12, 2011). Newmark, N. M., and Rosenblueth, E. (1971). Fundamentals of earthquake engineering. Prentice-Hall, Englewood Cliffs, NJ. Popov, E. (1994). “Development of US seismic codes.” J. Const. Steel Res., 29, 191–207. Richter, C. F. (1958). Elementary seismology, W. H. Freeman and Co., New York, NY. Roberts, E. B., and Ulrich, F. P. (1950). “Seismological activities of the U.S. Coast and Geodetic Survey in 1948.” Bull. Seism. Soc. Am., 40, 195–216. Stover, C., and Coffman, J. L. (1993). “Seismicity of the United States, 1568–1989 (revised).” USGS Professional Paper 1527, USGS, Washington, DC. Structural Engineers Association of California (SEAOC). (1998). Blue book, ICBO, Whittier, CA. Topinka, L. (1997). “Active volcanoes, plate tectonics, and the “Ring of Fire.” Online Volcano Maps, USGS/Cascades Volcano Observatory, Vancouver, WA U.S. Geological Survey (USGS). (2011a). “Did you feel it?” (Oct. 14, 2011). ———. (2011b). “Largest and deadliest earthquakes by year.” (Oct. 12, 2011). ———. (2010). (Date accessed, 2011).

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Vigil, J. F. (1994). “This dynamic planet.” Wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory, Washington, DC. Wegener, A. (1924). The origin of continents and oceans. Methuen, London. Wood, H. O., and Neumann, F. (1931). “Modified Mercalli scale of 1931.” Bull. Seism. Soc. Am., 21, 277–283. Zeng, Y. and Chen, C.-H. (2001). “Fault rupture process of the 20 September 1999 Chi-Chi, Taiwan earthquake.” Bull. Seism. Soc. Am., 91(5), 1088–1098.

Chapter 4

Building Code Seismic Requirements

Why do building codes exist? That is too broad a question to answer in this discussion, but to offer some insight we need to go back to an era in the United States when building codes were virtually nonexistent. Imagine a businessman seeking to benefit from the California gold rush of 1849 who sets up shop in rapidly growing San Francisco. From December 1849 through June 1851, he most likely would have rebuilt his building several times because of damaging fires that routinely hampered the city. The good news was that buildings were mostly wooden structures and the city could be rebuilt in a month or two. The problem was that buildings were built close together with no firebreaks or other fireresistant measures, which meant that small fires could spread with voracity. After trial by fire every half-year, the need to implement improved fire control measures became a popular cause, along with growing awareness of the benefits of fire-resistant construction. Due mostly to improved fire protection measures, these changes worked well for the city over the next 50 years, until the Great San Francisco Earthquake of 1906 destroyed water lines essential for fire fighting, and the ensuing fire did more damage than the earthquake (Scawthorn et al. 2006). It did not go unnoticed that, even damaged by fire, a number of concrete and steel structures survived both earthquake and fire well enough to be renovated and are still in use today. This was an important lesson for both fire and earthquake protection. Seismic provisions in U.S. building codes have been evolving for the last half-century and will continue to evolve in the future. The primary intent of seismic requirements is to 117

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ensure a minimum structural resistance to limit building collapse and consequent loss of life, and limit nonstructural life-threatening hazards during an earthquake. However, it is generally acknowledged that seismic code provisions are also intended to control the severity of damage in moderate earthquakes, such that essential buildings can continue to function after the quake. These principles may be durable, but specific details change as knowledge increases, with the most rapid evolution occurring in the aftermath of large earthquakes. It is important to grasp the significance of how the passage of time rapidly obscures the pain of disaster, which drives public awareness and demand for and acceptance of evolving code changes. Even for present-day San Francisco, where major earthquakes occur every 100 years or so, the absence of recent damaging events results in a false perception of the built environment as being earthquake-resistant. This incorrect perception poses a challenge for both public acceptance and enforcement of modern-day building code seismic provisions. To better understand the motivation for changes in modern-day U.S. seismic codes, the FEMA 454 chapter “The Regulation of Seismic Design” offers an excellent summary (FEMA 2006). The goal of this chapter is to introduce the model building code used in the United States—the 2012 International Building Code (ICC 2011) based on the ASCE/SEI 7-10 standard (ASCE/SEI 2010). Our specific interest lies in discussing the seismic requirements imposed on nonstructural systems. The discussion will focus on code intent as best possible, since this perspective is the primary vehicle to transform code language into implementation practices. Building codes, however, will continue to change after the date of this writing, and we can only offer a snapshot of a developing process. Because the complexity of modern-day codes can be a significant barrier to effective nonstructural earthquake protection, we break down the code into its fundamental parts. In fact, these core elements can be found in most modern building codes and standards used around the world. While it is true that nations have evolved their own seismic codes and standards independently, there is similarity when considering the core elements contained within these codes and standards.

4.1 Basic Elements of Model Building Codes The common elements incorporated into most building codes regarding earthquake protection are described next. These are generic definitions applicable for any seismic code or standard. Discussion on the IBC implementation of these code elements follows in the next section. • Construction Type: This is a classification of the various types of construction based on the primary function. This includes differentiation between buildings and everything else that is not a building (e.g., nonstructural). This also includes subcategory differentiation within each of the main construction types. For example, nonstructural could include the subcategories of architectural items, mechanical and electrical service equipment, and building occupancy contents. There can even be differen-

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•

•

•

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tiation within the subcategories, like distinguishing nonstructural anchorage from the force-resisting skeleton (FRS). Construction Importance: This section is used to designate the relative importance of the construction type and the degree of risk to human life associated with earthquake damage. In other words, this differentiates the essential construction types from the less essential. Essential buildings might be classified by the nature of the occupancy (e.g., hospitals, emergency communication centers, fire and police stations) or by the risk associated with unacceptable performance. Essential nonstructural systems are typically those needed for the continued operation of essential buildings (e.g., emergency power systems and service equipment, fire suppression systems). Essential buildings and essential nonstructural systems are given more stringent requirements compared to less essential infrastructure. The importance classification has a direct correlation with construction cost: the higher the importance, the higher the cost of design and construction. Performance Objective: These are functional requirements related to seismic performance and are assigned to each construction type based on the importance classification of the application. For example, a minimum building structure objective related to structural integrity might be progressive collapse prevention. A higher structural integrity objective might be to limit the onset of local collapse. Nonstructural performance objectives are typically more tangible. For example, a minimum nonstructural objective would be positive position retention for anchored components. A higher nonstructural performance objective might be to maintain active operation following an earthquake. Satisfying the performance objectives are key deliverables from stakeholder perspectives. Compliance Expectations: These cover the code-recognized methods that can be used to demonstrate seismic compliance to the identified performance objectives. For example, this could include the use of analytical methods, dynamic testing, or a combination of methods to validate compliance. This section can also describe any necessary documentation to verify that the designers and builders have satisfied the code seismic requirements. For example, inclusion of equipment testing procedures, certifications and laboratory test reports, quality assurance documentation, special inspection procedures, documentation signoff requirements, and other compliance-related procedures are typically identified. This code section can become the legal traceability for the stakeholders involved in earthquake-protection activities. Site Assessment: This is used to provide guidelines on how to classify local soil properties at the building site location. This could include details on minimum geotechnical survey requirements based on building importance. This could also provide guidelines on what is required to conduct a detailed site survey. Default soil properties are typically provided when no detailed site survey is conducted. Earthquake Loads: These define minimum environmental loading requirements (demands) associated with earthquake hazards. This may sound like a rather simple topic, but in reality this subject alone likely contains hundreds of staff-years of research, data reduction, and organized effort by many scientists, engineers, design professionals, and the government. This code element is most often the cornerstone

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of the building code seismic provisions. Effective implementation of nonstructural protective measures requires understanding the intent behind the numbers. • Load Combination: This is used to govern how different types of environmental loads need to be combined to satisfy specific acceptance criteria. Earthquake loads are typically not the only type of load covered in building codes; the codes can also cover wind, snow, ice, and other environmental loads. It would be too conservative to assume that all environmental loads are acting simultaneously. Thus, load combinations are most often dictated by load scale factors that prescribe how the various loads need to be combined. There might be two different sets of load factors depending on whether the analysis is being performed using strength design or stress design approaches. This section essentially defines the acceptance criteria for performing structural analysis. • Load Application: This is used to provide various options and guidelines for applying earthquake loading to the construction types. For example, there may be options for using equivalent static loads or dynamic loads. There might also be procedures for both linear and nonlinear approaches. The dynamic option might allow time-history or response spectrum analysis. Detailed guidelines are usually provided for each load application method. Each construction type could offer different options. For example, the building construction would likely offer both static and dynamic options, and the nonstructural type might only offer the static option. Load application is a complex topic, and most often this section addresses all of the necessary aspects that govern application of engineering mechanics to solve earthquake engineering problems for a given construction type. • Commentary: Not every model code will contain a commentary section directly embedded into the source provisions. However, most codes do have a commentary section that can be associated with the requirements, either directly or indirectly referenced. The commentary section is the behind-the-scenes perspective on the formal code language used in requirements definition. This is the code writers’ attempt to demystify the requirements and explain the basis or origin of the requirement. This section is highly useful and typically contains information regarding code intent that is missing from the actual code language. The basic code elements presented here can be found in most building codes or standards used to define seismic requirements. Different naming and organizing conventions will be instituted from one code to the next. We apply these foundation elements to break down the model building code used in the United States, namely, the 2012 International Building Code, which references the ASCE/SEI 7-10 standard for most of its structural load provisions associated with seismic demands.

4.2 Elements of IBC and ASCE/SEI 7 Seismic Provisions Before we jump into the IBC, there is some history to get through. The incarnation of the IBC as a unified national building code followed a long and winding path. The short ver-

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sion of this story is that prior to adoption of the IBC as a national code, the building code requirements for United States were divided into several regional codes. These regional codes (Uniform Building Code, National Building Code, and Standard Building Code) were developed and administered by independent code organizations. Of these regional codes, the Uniform Building Code (UBC) was the gold standard for seismic provisions in the United States. In parallel with the evolution of these region-based codes, the federal government sponsored development of a National Earthquake Hazards Reduction Program (NEHRP). NEHRP was established by the U.S. Congress in 1977 when it passed the Earthquake Hazards Reduction Act of 1977. With federal government backing (and associated federal money), NEHRP evolved to become the new gold standard in seismic provisions, benefiting largely from extensive earthquake engineering research. However, to become codified, the NEHRP provisions needed a new home and were largely incorporated into the ASCE/SEI 7 standard. The IBC directly references the ASCE/SEI 7 and, thus, when we discuss IBC requirements here we are actually discussing the ASCE/SEI 7-10 provisions, which include most of the code recommendations of the NEHRP provisions. Now that we have made the code’s alphabet soup clear as mud, we can get to the core of our discussion. But seriously, those interested in the fine details of U.S. building code history can refer to the FEMA 454 chapter, “The Regulation of Seismic Design” (FEMA 2006). Our goal for the remainder of this chapter is to walk through the code using the basic elements as previously outlined. Emphasis will be on describing interpretation of code requirements using code intent as the translation filter. To that end, we intentionally avoid making direct reference to code section numbers, table numbers, equation numbers, and other ASCE/SEI 7-10 identifiers. This approach decouples the discussion from formal and prescriptive code language, such that concentration can be better directed to intentbased interpretation. It is worth noting that this discussion is not intended to be a complete regurgitation of code provisions. The code details many application specific requirements and/or exceptions that will not be covered here.

4.2.1 Construction Type The code includes provisions for five types of construction categories: (1) buildings, (2) nonstructural components, (3) nonbuilding structures, (4) seismically isolated structures, and (5) structures with damping systems. Each of these categories has its own set of seismic design provisions. The nonstructural components category is divided into two subcategories: (1) architectural components, and (2) mechanical and electrical components. The mechanical and electrical components category includes both equipment and distribution systems and is the focus of this writing. Nonbuilding structures include all self-supporting structures that carry gravity loads and that may be required to resist the effects of earth shaking. Differentiation between nonstructural and nonbuilding categories is not always straightforward. There are two types of nonbuilding structures. One type has a structural system similar to buildings, and the other type has a structural system that is not similar to buildings. The latter type can be occasionally difficult to distinguish from nonstructural systems. The easiest way to distinguish between nonstructural or nonbuilding is by size. Stated simply,

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Nonstructural Components Access floors Escalator components Air conditioning units Evaporators Air distribution boxes Furnaces Air handlers Generators Nonbuilding Structures Air separators Heat exchangers Not Similar Battery racks Billboards HVAC Boilers to Buildings Bins Inverters Cabinet heaters Chimneys Lighting fixtures Cabinets Amusement structures Cable trays Manufacturing equipment Conveyors Ceilings Hoppers (elevated) Motor control centers Cooling towers Chillers Motors Monuments Signs Communication equipment Panel boards Silos (cast-in-place concrete Stacks Compressors Parapets having walls to foundation) Computers Tanks Penthouse (except where framed by Ductwork Towers an extension of the building frame) Electrical conduit Piping Vessels Elevator Plumbing Engines Process equipment Switch gear Pumps Transformers Walls and wall panels Tubing Water heaters Turbines Veneer Vibration isolated systems

Figure 4-1. Intersection of nonstructural components and nonbuilding structures. Source: Reproduced with permission from Bachman and Dowty (2008).

nonstructural components are typically small and nonbuilding structures are typically large (Bachman and Dowty 2008). Nonstructural systems are typically small enough to fit within a building, something on the order of 3 m (10 ft) tall. There are, of course, exceptions such as very large generators and turbines. Figure 4-1 illustrates the possible intersection of nonstructural with nonbuildings. When in doubt, the designer always has the option of calculating the force demand in accordance with the nonstructural requirement and the nonbuilding structure requirement and using the most conservative design (Bachman and Dowty 2008).

4.2.2 Construction Importance First introduced in the ASCE/SEI 7-10 revision is the concept of a construction risk category. The risk category is a top-level importance classification based on the intended use or occupancy of the construction type. The risk category associates the degree of risk to human life with construction performance under environmental demands (e.g., earthquake). There are four risk categories, from I to IV—the higher the category ranking, the higher the expectation for acceptable performance during and after earth shaking with a lower risk to human life. Table 4-1 summarizes the four risk categories. The primary motivation for including generalized criteria, with regard to occupancy descriptions, is that the acceptable risk for a building is more of an issue of public policy rather than purely a tech-

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Table 4-1. Risk Categories for Buildings and Other Structures. Use or Occupancy of Buildings and Structures

Risk Category

Buildings and other structures that represent a low risk to human life in the event of failure.

I

All buildings and other structures except those listed in Risk Categories I, III, and IV.

II

Buildings and other structures, the failure of which could pose a substantial risk to human life. Buildings and other structures, not included in Risk Category IV, with potential to cause a substantial economic impact and/or mass disruption of day-to-day civilian life in the event of failure. Buildings and other structures not included in Risk Category IV containing toxic or explosive substances where their quantity exceeds a threshold quantity established by the AHJ (authority having jurisdiction) and is sufficient to pose a threat to the public if released.

III

Buildings and other structures designated as essential facilities. Buildings and other structures, the failure of which could pose a substantial hazard to the community. Buildings and other structures containing sufficient quantities of highly toxic substances where the quantity exceeds a threshold quantity established by the AHJ to be dangerous to the public if released and is sufficient to pose a threat to the public if released. Buildings and other structures required to maintain the functionality of other Risk Category IV structures.

IV

Source: ASCE/SEI 7-10 (ASCE/SEI 2010), p. 2.

nical one. The code’s shift toward public policy-based decision making, with the addition of a new risk category, is detailed in the ASCE/SEI 7-10 commentary. The next level of importance ranking is based on the relative magnitude of ground motion at the building site and is defined in conjunction with the risk category. In other words, a high-rank risk category building (i.e., III and IV) that is located in a seismic-prone area is assigned the worst-case requirements compared with other high-rank risk category buildings that are located in less seismically active areas. The next level of code importance ranking is called the seismic design category. There are six possible seismic design categories: A-B-C-D-E-F. Depending on the local ground motion magnitude, the building structure is assigned one of the six rankings, with category F assuming the most stringent requirements (see Table 4-2). Nonstructural systems inherit the same seismic design category as the structure that they occupy or to which they are attached.

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Table 4-2. Seismic Design Category Based on Response Acceleration Parameters. Risk Categorya I or II or III IV

a

Short-Period Response Acceleration Parameter, SDS SDS < 0.167 0.167 ≤ SDS < 0.33 0.33 ≤ SDS < 0.50 0.50 ≤ SDS

A B C D

A C D D

Long-Period Response Acceleration Parameter, SD1 SD1 < 0.067 0.067 ≤ SD1 < 0.133 0.133 ≤ SD1 < 0.20 0.20 ≤ SD1

A B C D

A C D D

Value of S1 0.75 ≤ S1

E

F

Seismic design category is based on worst case of these three assessments.

Source: ASCE/SEI 7-10 (ASCE/SEI 2010), p. 67. (Combines Tables 11.6-1 and 11.6-2)

The final code classification ranking is the seismic importance factor. For building structures, the seismic importance factor, Ie, is inherited from the assigned risk category as shown in Table 4-3. The nonstructural importance factor, Ip, only includes two classification rankings: Ip ⫽ 1.0 and Ip ⫽ 1.5. The nonstructural importance factor must be assigned to each nonstructural component and system (i.e., not inherited from the building). Table 4-3. Building Structure Importance Factors. Risk Category

I II III IV

Seismic Importance Factor, Ie

1.00 1.00 1.25 1.50

Source: ASCE/SEI 7-10 (ASCE/SEI 2010), p. 5.

A nonstructural importance factor of Ip ⫽ 1.5 is assigned if any of the following conditions apply: • The component is required to function for life-safety purposes after an earthquake, including fire protection sprinkler systems and egress stairways.

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• The component conveys, supports, or otherwise contains toxic, highly toxic, or explosive substances exceeding a threshold quantity limit and is sufficient to pose a threat to the public if released. • The component is in or attached to a risk category IV structure and it is needed for continued operation of the facility or its failure could impair the continued operation of the facility. • The component conveys, supports, or otherwise contains hazardous substances and is attached to a structure classified as a hazardous occupancy. All other nonstructural components and systems are assigned a component importance factor of Ip ⫽1.0. There are several exemptions to these rules that are identified in the code. The nonstructural importance factor, Ip, represents the greater of the life-safety risk of the system and the hazard exposure importance of the structure. Figure 4-2 displays a

Table 4-1 Building Risk Category (I, II, III or IV)

Ground Motion Response Acceleration Parameters (SDS , SD1 and S1) See Figure 4-7

Table 4-2 Building Structure Seismic Design Category (A, B, C, D, E or F)

Nonstructural Seismic Design Category Inherited From Building (A, B, C, D, E or F)

Table 4-3 Building Structure Seismic Importance Factor, Ie Inherited from Risk Category (1.0, 1.25 or 1.5)

Nonstructural Importance Factor, Ip Assigned by Application (1.0 or 1.5)

Figure 4-2. Summary of the code’s construction importance hierarchy for building structures and nonstructural systems.

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summary of the code’s construction importance hierarchy for buildings and nonstructural systems.

4.2.3 Performance Objectives The code’s nonstructural performance objectives related to seismic performance are summarized in Table 4-4. The nonstructural importance factor, Ip, is used to differentiate between the objectives for nonstructural applications in designated seismic systems and those applications deemed less essential. The performance objectives for building structures are related to structural integrity and acceptable structural performance under earthquake demands. Suffice it to say, the inherent assumption regarding nonstructural seismic performance is that the building structure has to perform as intended. The obvious manifestation of this concept results from the secondary nature of nonstructural systems. Without a standing structure, there can be no nonstructural seismic performance, or any performance for that matter. Figure 1-12 in Chapter 1 illustrates the dependence relationship that nonstructural systems have on building structures.

4.2.4 Compliance Expectations Table 4-5 summarizes the code’s compliance expectations to validate that nonstructural performance objectives have been satisfied. The process of compliance validation is commonly referred to as seismic qualification. Nonstructural qualification is concerned with establishing seismic capacity levels. The methods used to establish capacity include analysis, testing, earthquake experience data, and comparative assessment using combined methods. Part 2 of this book (Chapters 5–9) covers nonstructural capacity topics. Chapter 5 discusses seismic qualification practices in general and Chapters 6–9 describe the methods used to establish capacity using analysis, testing, comparative experience, and combined methods, respectively. The code requires submittal of appropriate construction documents for nonstructural systems that are designated seismic systems (Ip ⫽ 1.5). The construction documents are prepared by the building design professional for use by the building owner, the authority having jurisdiction (AHJ), and inspectors. A nonstructural quality assurance plan is required as part of the construction documents to document periodic special inspections for installation of designated seismic systems. The acceptance of nonstructural compliance validation is dependent on approval by the AHJ for the project-specific application. Different jurisdictions could have different approval processes and could pose different expectations for compliance. For example, an essential nonstructural application (i.e., a designated seismic system) for a hospital located in the state of California will pose higher compliance expectations compared with similar applications in other U.S. states. The approval process for designated seismic systems in jurisdictions that require special enforcement needs to be clearly understood well in advance. Inadequate compliance documentation can create major problems during the approval process for such nonstructural applications.

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Table 4-4. Nonstructural Seismic Performance Objectives. Nonstructural Importance Factor, Ip

1.0

Performance Objective

Position retention

Systems interaction avoidance

1.5

Position retention

Systems interaction avoidance

Active operation

Design Objective Description

Maintain positive retention of nonstructural position, under design-level earthquake demands, without consideration of frictional resistance produced by the effects of gravity. This includes position retention of nonstructural anchorage, attachments, and the force-resisting skeleton (FRS). Account for unwanted interaction, under design-level earthquake demands, between nonstructural systems and anything else that might be located in the immediate vicinity of the nonstructural installation, so that failure of one system or contact between systems does not cause consequential damage of an essential system. The “anything else” could be building elements (e.g., beams, columns, walls) or other installed nonstructural systems. Maintain positive retention of nonstructural position, under design-level earthquake demands, without consideration of frictional resistance produced by the effects of gravity. This includes position retention of nonstructural anchorage, attachments, and the force-resisting skeleton (FRS). Account for unwanted interaction, under design-level earthquake demands, between nonstructural systems and anything else that might be located in the immediate vicinity of the nonstructural installation, so that failure of one system or contact between systems does not cause consequential damage of an essential system. The “anything else” could be building elements (e.g., beams, columns, walls) or other installed nonstructural systems. Maintain active operation functionality of mechanical and electrical equipment and distribution systems following (i.e., not during) application of design-level earthquake demands.

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Table 4-5. Nonstructural Seismic Compliance Expectations. Compliance Method

Position retention

Analysis

Experience data

Testing

Validation Expectation

Structural analysis can be used to validate that nonstructural anchorage, force-resisting skeleton, and attachments have position retention capacity equal to or greater than the project-specific design-level demand for the application installation location. Both strength design and allowable stress design approaches are accepted. (See Chapter 6 for details.) The use of earthquake experience data, based upon nationally recognized procedures, can be used to establish nonstructural position retention capacity provided that the substantiated seismic capacities equal or exceed the project-specific design-level demand for the application installation location. (See Chapter 8 for details.) The use of seismic simulation testing, based upon a nationally recognized testing standard procedure, such as ICC-ES AC156 (ICC ES 2010), can be used to establish nonstructural position retention capacity provided that the seismic capacities equal or exceed the project-specific design-level demand for the application installation location. (See Chapter 7 for details.)

Systems interaction avoidance

Inspection

Visual inspection of the nonstructural installation is performed to validate that no unwanted system interactions may result under the project-specific design-level earthquake demands.

Active operation

Experience data

The use of earthquake experience data, based upon nationally recognized procedures, can be used to establish nonstructural active operation capacity provided that the substantiated seismic capacities equal or exceed the project-specific design-level demand for the application installation location. (See Chapter 8 for details.) The use of seismic simulation testing, based upon a nationally recognized testing standard procedure, such as ICC-ES AC156 (ICC ES 2010), can be used to establish nonstructural active operation capacity provided that the seismic capacities equal or exceed the project-specific design-level demand for the application installation location. (See Chapter 7 for details.) The use of combined structural analysis and seismic simulation testing can be used to establish nonstructural active operation capacity for physically massive systems (i.e., large-class nonstructural) that are impractical to test as complete systems. The testing aspects need to conform with nationally recognized testing standard procedures, such as ICC-ES AC156 (ICC ES 2010). The established active operation capacity, using combined testing and analysis, is to equal or exceeds the project-specific design-level demand for the application installation location. (See Chapter 9 for details.)

Testing

Combined testing and analysis

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4.2.5 Site Assessment The building site soil properties are classified based on analysis of the upper 30 m (100 ft) of the site profile. The code’s site classification options include Site Class A through F. Table 4-6 summarizes the site classification parameters. In cases where site-specific data are not available to a depth of 30 m, appropriate soil properties are permitted to be estimated by the registered design professional preparing the soil investigation report based on known geologic conditions. Where the soil properties are not known in sufficient detail to determine the site class, Site Class D is used as the default, unless the AHJ or geotechnical data determines that Site Class E or F soils are present at the site. The code provides a section describing the requirements to conduct a site-specific geotechnical survey. Sitespecific surveys are common for risk category III and IV buildings located in seismicprone areas and for significant numbers of risk category II buildings. Table 4-6. Site Soil Classificationa. Site Class

A. B. C. D. E.

Hard rock Rock Very dense soil and soft rock Stiff soil Soft clay soil

F. Soils requiring site response analysis in accordance with the code Site Response Analysis section

– (ft /s) v s

– – N or Nch

–s (psf) u

> 5,000 NA NA 2,500 to 5,000 NA NA 1,200 to 2,500 > 50 > 2,000 600 to 1,200 15 to 50 1,000 to 2,000 < 600 < 15 < 1,000 Any profile with more than 3 m (10 ft) of soil having the following characteristics: –Plasticity index PI > 20, –Moisture content w ⱖ 40%, and –Undrained shear strength –su < 500 psf

See code Site Class F section

For SI: 1 ft/s ⫽ 0.3048 m/s; 1 lb/ft2 ⫽ 0.0479 kN/m2.

a

Source: ASCE/SEI 7-10 (ASCE/SEI 2010), p. 204.

4.2.6 Earthquake Demands Earthquake loads, called demands, for the code’s construction types share common ground motion parameters. The first task is to discuss building structure demands and then shift attention to cover nonstructural demand requirements. Nonstructural earthquake demands cannot be fully understood until the top-level building requirements are reviewed.

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4.2.6.1 Building Structure Demands Building code earthquake demands originate from seismic ground motion hazard maps. The maps associate geographic location with both short-period (i.e., 0.2 s) and long-period (i.e., 1 s) spectral response accelerations—so-called two-factor mapped acceleration parameters. Figure 4-3 is the mapped ground motion, 5% damped, spectral response acceleration parameter at short periods, SS. Figure 4-4 is the mapped ground motion, 5% damped, spectral response acceleration parameter at long periods, S1. Figure 4-5 displays the mapped long-period transition period, TL. All of the code’s construction types utilize the mapped ground motion parameters to define earthquake demands. Note that the hard-copy maps contained in the ASCE/SEI 710 standard are often difficult to work with because of the map scale. A U.S. Geological Survey (USGS) website (USGS 2010) provides digital map tools that access the data behind the maps. These tools are free and quite useful and were intended by the code writers to be typically employed on building projects. The acceleration maps (Figs. 4-3 and 4-4) represent the risk-targeted maximum considered earthquake (MCER), which is defined as the ground motion with a uniform probability of being exceeded at least once in 2,475 years (2% in 50 years adjusted to provide a uniform probability of collapse). Thus, these are often referred to as probabilistic seismic hazard maps. The design maps do not reflect how many earthquakes will occur in the return period or their associated magnitudes—only the ground motion intensity that is likely to be exceeded at least once in the MCER return period. The ground motion intensity is defined in terms of 5% damped, spectral response acceleration for Site Class B. The starting point is defining the two mapped acceleration parameters, SS and S1, for a given building site location. The next step is to use the site class designation, which is assigned from the building site assessment as shown in Table 4-6, in conjunction with the MCER mapped parameters to calculate adjusted MCER acceleration parameters as follows: SMS = Fa SS

(4-1)

SM 1 = Fv S1

(4-2)

where S MS ⫽ MCE R, 5% damped, spectral response acceleration at short periods adjusted for site class effects; SM1 ⫽ MCER, 5% damped, spectral response acceleration at a period of 1 s adjusted for site class effects; Fa ⫽ short-period site coefficient (at 0.2-s period); and Fv ⫽ long-period site coefficient (at 1.0-s period). The two site coefficients, F a and F v, are determined from the site class designation and the MCE R acceleration parameters, SS and S1, as shown in Table 4-7(a) and (b), respectively. Site class adjustment is necessary since the mapped hazard values represent Site Class B data. If the building site happens to be a Site Class B, then the site coefficients, Fa and Fv, are unity and no adjustment results. The code’s design-level acceleration parameters are defined as a straight two-thirds ratio of the adjusted MCER response acceleration parameters. Thus, the design earthquake

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Earthquake Protection of Building Equipment and Systems

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Table 4-7a. Site Coefficient Fa . Mapped MCER Spectral Response Acceleration Parameter at Short Perioda Site Class

A B C D E F

SS ⱕ 0.25

0.8 1.0 1.2 1.6 2.5

SS ⴝ 0.5

SS ⴝ 0.75

SS ⴝ 1.0

0.8 0.8 0.8 1.0 1.0 1.0 1.2 1.1 1.0 1.4 1.2 1.1 1.7 1.2 0.9 Use Site-Specific Ground Motion Procedures

SS ⱖ 1.25

0.8 1.0 1.0 1.0 0.9

a

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Source: ASCE/SEI 7-10 (ASCE/SEI 2010), p. 66.

Table 4-7b. Site Coefficient Fv . Mapped MCER Spectral Response Acceleration Parameter at 1-s Perioda Site Class

A B C D E F

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0.8 1.0 1.7 2.4 3.5

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S1 ⴝ 0.3

S1 ⴝ 0.4

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S1 ⱖ 0.5

0.8 1.0 1.3 1.5 2.4

a

Use straight-line interpolation for intermediate values of S1.

Source: ASCE/SEI 7-10 (ASCE/SEI 2010), p. 66.

spectral response acceleration parameter at short period, SDS, and at 1-s period, SD1, are defined as SDS =

2 SMS 3

(4-3)

SD1 =

2 SM 1 3

(4-4)

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These two response acceleration parameters are used to define design-level earthquake demands for building structures. On a side note, the difference between adjusted MCER response acceleration values and design-level values can be viewed as analogous to the terminology used in the nuclear power industry to differentiate between a safe shutdown earthquake (SSE) and an operating basis earthquake (OBE). The OBE is a smaller earthquake that is likely to be experienced at least once and possibly more than once during the operational life of the facility. The SSE event is a larger earthquake that could probabilistically occur during the life expectancy of the facility but is not likely to occur when the return period is large. In this respect, the code’s MCER earthquake is comparable to an SSE event and the code’s designlevel earthquake is comparable to an OBE event. However, unlike nuclear power requirements, ASCE/SEI 7-10 does not require active operation performance during the earthquake. The performance expectation in ASCE/SEI 7-10 is active operation following design-level earthquake demands (as defined in Table 4-4) for designated seismic systems. The building structure demand requirements are implemented via the code’s design response spectrum, as shown in Fig. 4-6. The two design-level acceleration parameters, SDS and SD1, and the long-period transition point, TL, feed into defining this design spectrum. The long-period transition point, TL, comes from the Fig. 4-5 map, which is also

Figure 4-6. International Building Code (IBC) (based on ASCE/SEI 7-10) design earthquake response spectrum for governing the seismic design requirements of building structures.

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based on the building site location. Thus, by definition of SDS, SD1, and TL, the design response spectrum is fully captured, including all spectrum breakpoints. The design response spectrum is the cornerstone of the code’s seismic demand requirements and is often referred to as the code’s default response spectrum. Note that the code provides an alternative site-specific method to define ground motion and design earthquake demands. The site-specific method is most often used for risk category III and IV buildings that are located in seismic-prone areas. The alternative site-specific method will not be presented for this discussion. There is an interesting aspect of the design spectrum that has direct bearing on nonstructural requirements. At zero period (i.e., T ⫽ 0) the design response acceleration is defined by the equation Sa

T= 0

T⎞ ⎛ = SDS ⎜ 0.4 + 0.6 ⎟ = SDS ⎝ T0 ⎠

0⎞ ⎛ ⎜⎝ 0.4 + 0.6 T ⎟⎠ = 0.4 SDS 0

(4-5)

This acceleration is the zero period acceleration (ZPA) and represents the static ground-level design acceleration (also called PGA—peak ground acceleration). The quantity ( 0.4 SDS ) is directly incorporated into the nonstructural demand requirement. A recap of the necessary steps to follow from building location identification to design earthquake response spectrum development is summarized in Fig. 4-7. As a reminder, the USGS website (USGS 2010) provides a free tool that automatically generates the code’s design earthquake response spectrum based on user input of building site information (i.e., address or latitude/longitude and site soil classification). Further discussion of building structure seismic design requirements is not the focus of this writing. There are many resources that go into the fine details regarding building code structural design. Our interest lies in examining the nonstructural earthquake design requirements.

4.2.6.2 Nonstructural Static Demands Building code nonstructural demands are defined as static design forces and, unlike buildings, are not fully described in terms of response spectrum parameters. The code’s adoption of nonstructural design force equations follows a long history of U.S. building codes that have employed equivalent lateral force to prescribe seismic design requirements. Equivalent static forces are likely perceived by many stakeholders as easier to implement. This perception is quite accurate when nonstructural systems are treated as black-box building components. However, if the intent is to treat nonstructural components as functioning building systems, then the static force approach leaves room for misinterpretation when nonstructural dynamic approaches are needed. This topic is discussed in the next section. The building code’s horizontal seismic design force, Fp, is defined as Fp = ( 0.4 SDS )

( ap ) ⎛1 + 2 z ⎞ W ⎜ ⎟ p ⎛ Rp ⎞ ⎝ h⎠ ⎜ ⎟ ⎝ Ip ⎠

(4-6)

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Figures 4-3 & 4-4 MCER Ground Motion (SS and S1 )

Table 4-6 Site Soil Classification (A, B, C, D, or E)

Figure 4-5 Long Period Transition (TL )

Table 4-7 Site Coefficients (Fa and Fv)

Adjusted MCER Ground Motion (SMS and SM1)

SMS = Fa SS SM1 = Fv S1

Design-Level MCER Ground Motion (SDS and SD1)

2 SDS = SMS 3 2 SD 1 = S M 1 3

Figure 4-6 Design Earthquake Response Spectrum (Sa and T)

Figure 4-7. The code’s default building structure design-level demand definition flow chart applicable for Site Class A–E; Site Class F designation requires site-specific ground motion procedures.

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where Fp ⫽ seismic design force centered at the component’s center of gravity and distributed relative to the component’s mass distribution; SDS ⫽ design earthquake spectral response acceleration at short period; ap ⫽ component amplification factor; Rp ⫽ component response modification factor; Ip ⫽ component importance factor; z ⫽ height in structure at point of attachment of component; h ⫽ average roof height of structure relative to the base elevation; and Wp ⫽ component operating weight. Additional code stipulations are defined for allowable maximum and minimum values for Fp: Fp

MAX

= 1.6 SDS I p Wp

(4-7)

Fp

MIN

= 0.3 SDS I p Wp

(4-8)

The appearance of Eq. 4-6 is slightly different from the force equation as displayed in code provisions. The terms are rearranged and grouped together for isolation purposes. Mathematically, this equation is identical to the code version. The quantity ( 0.4 SDS ) has already been identified as the design earthquake zero period acceleration and represents the static ground-level design acceleration. If we wanted to create the simplest of force equations, starting with the static ground acceleration is a good choice. For example, if our nonstructural component was literally a solid black box made of lead and was anchored to a concrete pad at ground level, the earthquake inertial force experienced during earth shaking would be the ZPA design acceleration times the box weight. This is F = m a in the purest sense. Considering this trivial example we would have Fp = ( 0.4 SDS ) Wp

(4-9)

But if we need to install our solid black box inside a building structure at some floor elevation above grade level, how does that affect our simple force Eq. 4-9? Common sense tells us that the building will most likely amplify the ground acceleration and we need to somehow account for this building amplification effect in our force equation. The quantity [1 ⫹ 2(z/h)] is the needed building height factor to amplify the ground acceleration, accounting for building amplification as we move up in building floor height. Now we can modify our simple force equation to include building amplification effects: z⎞ ⎛ Fp = ( 0.4 SDS ) ⎜ 1 + 2 ⎟ Wp ⎝ h⎠

(4-10)

But what if our solid black box was not solid after all and instead was packaged full of “functioning stuff” that is supported by a box structural system (i.e., a force-resisting skeleton, FRS)? How does that affect our new simple force Eq. 4-10? Common sense again tells us that because the box is not a rigid block of lead, there will be some flexibility in the box FRS, and dynamic response to the amplified building ground acceleration is likely. The flexible response of the box FRS depends on both the dynamic characteristics of the box system and the building structure. When the dynamics of the box and building are closely tuned, vibration resonance occurs in the box system and box FRS dynamic amplification results. Thus, we need to modify our new simple force equation to account for pos-

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sible dynamic tuning between building and box system by inserting a component amplification factor, ap: z⎞ ⎛ Fp = ( 0.4 SDS ) ( ap ) ⎜ 1 + 2 ⎟ Wp ⎝ h⎠

(4-11)

Eq. 4-11 is starting to look similar to the code’s force requirement, but there is something missing. We have recognized that our black box is not a solid lump of dead weight, but a complex dynamic system in its own right, with functional devices packaged into an FRS. With almost all ductile FRS assemblies there is inherent capacity to absorb some of the energy imparted during earth shaking as inelastic response. A structural system, whether it is a building skeleton or a nonstructural FRS, can absorb and dissipate applied loading through the process of inelastic resistance. Buildings could not be designed (at least for a reasonable cost) without accounting for inelastic response. We need to modify Eq. 4-11 to account for the inelastic resistance capacity of the nonstructural FRS by inserting a response modification (i.e., reduction) factor, Rp, into the equation. However, the amount of reduction permitted is limited by the importance rating of the nonstructural system, the logic being that for essential nonstructural systems the amount of response reduction is limited by the ratio 1/Ip. Thus, for essential nonstructural systems (i.e., designated seismic systems) the response modification factor, Rp, is decreased by dividing by 1.5. Now we can modify Eq. 4-11 by inserting the response reduction ratio, Rp/Ip, into the denominator: Fp = ( 0.4 SDS )

(a )

⎛ ⎜1 + 2 ⎛ Rp ⎞ ⎝ ⎜⎝ I ⎟⎠ p

z⎞ ⎟ Wp h⎠

p

Our simple force equation now matches the code’s nonstructural seismic design requirement, Eq. 4-6. Placing the response reduction ratio under the amplification factor is not coincidental. The grouping of these factors together serves a purpose. Both the response reduction ratio, (Rp/Ip), and the amplification factor, (ap), are dependent quantities of the particular nonstructural system. Stated simply, the effects these parameters have on the overall force magnitude are entirely dependent on the type of nonstructural system in question (see Table 4-8 for code-defined ap and Rp coefficients). The other parameter groupings in Eq. 4-6 of (0.4 SDS) and [1 ⫹ 2(z/h)] are independent of the nonstructural system and represent building floor-level demands. In other words, the ground-level static acceleration and the building height amplification factors are applicable regardless of what type of nonstructural is installed. The distinction between nonstructural dependent and independent force parameters may not be important at this juncture, but this distinction is fundamental when there is a need to use dynamic nonstructural procedures. The code options specified for ap and Rp in Table 4-8 cover many different nonstructural types. If a specific type is not listed, the default options can be assumed, which include ap = 2.5 and Rp = 1.5 for flexible systems (i.e., maximum FRS amplification and minimum inelastic reduction). Project-specific ap values may be calculated using dynamic

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Table 4-8. Seismic Coefficients for Mechanical and Electrical Components. (Continues) Nonstructural Component

Mechanical and Electrical Components Air-side HVAC, fans, air handlers, air conditioning units, cabinet heaters, air distribution boxes, and other mechanical components constructed of sheet metal framing. Wet-side HVAC, boilers, furnaces, atmospheric tanks and bins, chillers, water heaters, heat exchangers, evaporators, air separators, manufacturing or process equipment, and other mechanical components constructed of high-deformability materials. Engines, turbines, pumps, compressors, and pressure vessels not supported on skirts and not within the scope of Nonbuilding Structures requirements. Skirt-supported pressure vessels not within the scope of Nonbuilding Structures requirements. Elevator and escalator components. Generators, batteries, inverters, motors, transformers, and other electrical components constructed of high-deformability materials. Motor control centers, panel boards, switchgear, instrumentation cabinets, and other components constructed of sheet metal framing. Communication equipment, computers, instrumentation, and controls. Roof-mounted stacks, cooling and electrical towers laterally braced below their center of mass. Roof-mounted stacks, cooling and electrical towers laterally braced above their center of mass. Lighting fixtures. Other mechanical or electrical components. Vibration Isolated Components and Systemsb Components and systems isolated using neoprene elements and neoprene isolated floors with built-in or separate elastomeric snubbing devices or resilient perimeter stops. Spring isolated components and systems and vibration isolated floors closely restrained using built-in or separate elastomeric snubbing devices or resilient perimeter stops. Internally isolated components and systems. Suspended vibration isolated equipment including in-line duct devices and suspended internally isolated components.

apa

Rp

2.5

6.0

1.0

2.5

1.0

2.5

2.5

2.5

1.0 1.0

2.5 2.5

2.5

6.0

1.0

2.5

2.5

3.0

1.0

2.5

1.0 1.0

1.5 1.5

2.5

2.5

2.5

2.0

2.5 2.5

2.0 2.5

Building Code Seismic Requirements

141

Table 4-8. (Continued) Nonstructural Component

Distribution Systems Piping in accordance with ASME B31, including in-line components with joints made by welding or brazing. Piping in accordance with ASME B31, including in-line components, constructed of high or limited deformability materials, with joints made by threading, bonding, compression couplings, or grooved couplings. Piping and tubing not in accordance with ASME B31, including in-line components, constructed of high-deformability materials, with joints made by welding or brazing. Piping and tubing not in accordance with ASME B31, including in-line components, constructed of high- or limited-deformability materials, with joints made by threading, bonding, compression couplings, or grooved couplings. Piping and tubing constructed of low-deformability materials, such as cast iron, glass, and nonductile plastics. Ductwork, including in-line components, constructed of highdeformability materials, with joints made by welding or brazing. Ductwork, including in-line components, constructed of high- or limited-deformability materials with joints made by means other than welding or brazing. Ductwork, including in-line components, constructed of low-deformability materials, such as cast iron, glass, and nonductile plastics. Electrical conduit and cable trays. Bus ducts. Plumbing. Manufacturing or process conveyors (nonpersonnel).

apa

Rp

2.5

12.0

2.5

6.0

2.5

9.0

2.5

4.5

2.5

3.0

2.5

9.0

2.5

6.0

2.5

3.0

2.5 1.0 1.0 2.5

6.0 2.5 2.5 3.0

a

A lower value for ap is permitted where justified by detailed dynamic analyses. The value for ap shall not be less than 1.0. The value of ap equal to 1.0 is for rigid components and rigidly attached components. The value of ap equal to 2.5 is for flexible components and flexibly attached components. b Components mounted on vibration isolators shall have a bumper restraint or snubber in each horizontal direction. The design force shall be taken as 2Fp if the nominal clearance (air gap) between the equipment support frame and restraint is greater than 0.25 in. If the nominal clearance specified on the construction documents is not greater than 0.25 in., the design force is permitted to be taken as Fp.

Source: ASCE/SEI 7-10 (ASCE/SEI 2010), pp. 120–121.

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Earthquake Protection of Building Equipment and Systems

analysis procedures. However, there are presently no code-specified analytical procedures that can be used to calculate Rp. This is an area that the code needs to reevaluate. Many nonstructural platforms are constructed with ductile FRS members and considerable inelastic reductions are possible. Calculation of a platform-specific Rp value should be a code option. One conclusion that can be drawn from this discussion is that the design force equation is a generic construct to account for building amplification, nonstructural FRS amplification, and inelastic reductions. The design force is independent of building structural properties. This makes the Eq. 4-6 nonstructural design force fully uncoupled from building dynamics. However, the code does provide an option to calculate design force by employing modal analysis procedures used in conjunction with a building-specific structural model. In lieu of forces determined using Eq. 4-6, accelerations at any building elevation can be calculated via modal analysis (with R ⫽ 1.0), and nonstructural design force is calculated as Fp =

ai ap Wp Ax ⎛ Rp ⎞ ⎜ ⎟ ⎝ Ip ⎠

(4-12)

where ai ⫽ acceleration at building height i obtained from modal analysis procedure; and Ax ⫽ torsional amplification factor as defined in the code’s “Amplification of Accidental Torsional Moment” section. Maximum and minimum values for design force (Eqs. 4-7 and 4-8) are still applicable using this alternative design force method. A good example of when this alternative design procedure is useful can be seen with seismically isolated structures. Base isolated buildings are intended to attenuate the seismic input going into the structure, which in turn attenuates the seismic demand for all nonstructural systems that are attached to the structure. In this case, nonstructural demands can be calculated using Eq. 4-12. Code acceptance of alternative procedures for determining seismic design force is a welcome step. This procedure is highly useful when nonstructural static demands are needed to address project- and building-specific application needs. There is, however, a much greater need to address nonstructural dynamic demands. This need is precipitated by the nonstructural performance objective of active operation for designated seismic systems and the exclusion of analytical means for compliance validation. Stated simply, nonstructural active operation compliance requires equivalent dynamic demands in order to test and analyze equipment platforms using dynamic techniques. While static demands are more than adequate for our dead-weight black-box example, when our black box is a functioning building system the static requirements are much less useful.

4.2.6.3 Nonstructural Dynamic Demands The reality of modern-day seismic provisions makes dynamic testing a key enabler for nonstructural compliance. Without the ability to test and analyze using dynamic demands, many essential nonstructural systems (i.e., designated seismic systems) would not be available to populate essential building structures. There is a gap in current code requirements without direct inclusion of nonstructural dynamic requirements alongside static demands.

Building Code Seismic Requirements

143

In the absence of code-specified dynamic demands, stakeholders are forced to interpret dynamic requirements as best understood. Even with existing dynamic interpretation protocols that are already code-sanctioned and readily available for anchored acceleration sensitive equipment (i.e., rigid equipment connections) such as ICC’s AC156 standard (ICC ES 2010), many stakeholders are either unaware of this protocol or simply do not recognize this interpretation as a code dynamic demand requirement. In either case, elimination of all misinterpretations can be readily accomplished with explicit code adoption of a generic nonstructural response spectrum demand. This demand would represent building floor motion spectra requirements independent of building dynamics and independent of nonstructural-type classification. For the time being, the ICC’s AC156 test protocol serves this purpose. The technical merit of AC156 development is discussed in Box 7-1 in Chapter 7 and will not be repeated here. The need for nonstructural dynamic demand provisions will only increase in the future as more systems require active operation compliance. In the end, it will be inevitable for the code to include these needed provisions. Our position is that sooner is better than later to clarify this gray area and thus remove an existing barrier to effective implementation of nonstructural protective measures. Research needs to address this topic are discussed in Chapter 10.

4.2.6.4 Nonstructural Relative Displacement Demands Seismic design force is not the only nonstructural demand requirement; seismic relative displacement requirements are included as well. Relative displacements occur when the nonstructural system is connected to a structure with more than one connection point. This is more common with nonstructural distribution systems, such as piping, ductwork, and electrical busway applications. Maximum relative displacements for design-level earthquake motions need to be considered. Except for glazing applications, no specific acceptance criterion is provided in the code, and there is no requirement to stay within elastic limits. However, the effects relative displacements have on meeting the nonstructural performance objectives should be considered and addressed. The code specifies two types of relative displacement demands: (1) displacements within the same structure A, and (2) displacements between separate structures A and B. The seismic relative displacement, Dp, is defined as Dp = Δ x A − Δ y A Dp = δ x A + δ y B

→

Within Structures

→ Between Structures

(4-13) (4-14)

where Δ xA ⫽ deflection at building level x of structure A, determined by an elastic analysis as defined in the code’s “Story Drift Determination” section; Δ yA ⫽ deflection at building level y of structure A, determined by an elastic analysis as defined in the code’s “Story Drift Determination” section; δxA ⫽ deflection at building level x of structure A, determined by an elastic analysis as defined in the code’s “story drift determination”

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Earthquake Protection of Building Equipment and Systems

section; and δyB ⫽ deflection at building level y of structure B, determined by an elastic analysis as defined in the code’s “Story Drift Determination” section. The worst case of seismic relative displacements from Eqs. 4-13 or 4-14 are multiplied by the seismic importance factor, Ie, for the building to define the nonstructural relative displacement demand requirement: Dpl = Dp I e

(4-15)

The code also specifies alternative methods to calculate relative displacements based on modal analysis procedures and provides a maximum limit on the Dpl requirement. The details for these stipulations are in the code.

4.2.7 Load Combinations The environmental loads covered in code provisions include, flood, wind, snow, ice, and earthquake, to name a few. Obviously, no structure could be designed by assuming simultaneous application of the various loading conditions. This is where the concept of load combinations comes into play. Load combinations are defined to prescribe how the various load types need to be combined or not combined. The load combination formulas apply load scale factors to adjust the relative magnitudes of combined loads. There are two sets of combination formulas, depending on whether stress design or strength design procedures are used. Stress design (also called allowable stress design or working stress design) is defined as a method of proportioning structural members such that elastically computed stresses produced in the members by nominal loads do not exceed specified allowable stresses. Strength design (also called load and resistance factor design, LRFD) is defined as a method of proportioning structural members such that the computed forces (or stresses) produced in the members by the factored loads do not exceed the member design strength. The load combination most applicable for indoor nonstructural applications includes live load, dead load, and earthquake load. It is worth noting that for some outdoor nonstructural applications, wind loads may be a design driver that needs consideration. Nonstructural live loads, L, are those loads associated with normal operation and are dependent on the type of nonstructural system. The nonstructural dead load, D, is simply the operating weight, Wp, due to gravity. The earthquake load includes both vertical and horizontal components. The vertical earthquake load, EV, is defined as EV = ± 0.2 SDS Wp

(4-16)

The horizontal earthquake load, EH, is the seismic design force, Fp, as defined in Eq. 4-6. Vertical demands are a topic of interest within the earthquake engineering community. Box 7-1 in Chapter 7 contains a discussion of vertical demands (p. 293). Nonstructural load combination formulas are not explicitly defined like they are for building structures. Table 4-9 presents a set of load combination formulas that are appropriate to support nonstructural design applications, such as anchorage and FRS capacity calculations. Implementation of these load combinations is demonstrated in Chapter 6.

Building Code Seismic Requirements

145

Table 4-9. Nonstructural Load Combinations. Allowable Stress Design

0.6 D ⫹ 0.7 EV ⫹ 0.7 EH ⫹ 1.0 L

Strength Design

0.9 D ⫹ 1.0 EV ⫹ 1.0 EH ⫹ 1.0 L

D, dead load; EV, vertical earthquake load; EH, horizontal earthquake load; L, live load.

4.2.8 Load Application The code’s design-level demands have now been defined and appropriately proportioned (i.e., combined). The final step is to apply the loads using basic principles of engineering mechanics. The building structure requirements accept three different methods of load application: (1) equivalent static forces, (2) dynamic response spectrum, and (3) dynamic time-history. Each procedure is finely detailed in the code. Which method used is based on the structure’s seismic design category, structural system, dynamic properties, and regularity. Individual exceptions are permitted with the approval of the AHJ. Current code nonstructural requirements technically accept only the equivalent lateral force method of load application although, as previously discussed, dynamic testing based on a nationally recognized testing standard procedure, such as AC156, can be used to establish nonstructural compliance. Note that the AC156 response spectra represent building floor motion spectra requirements independent of building dynamics and independent of nonstructural type classification. This makes the AC156 spectra highly useful for performing nonstructural dynamic analysis using either response spectrum or time-history techniques (see Chapter 6 for examples). Our position on code inclusion of dynamic demands has been clearly stated, so there is no point in beating a dead horse. But a slight clarification is needed regarding application of nonstructural static forces. The code states the horizontal seismic design force, Fp, (Eq. 4-6) shall be applied at the component’s center of gravity (CG) and distributed relative to the component’s mass distribution. To most stakeholders, this is pretty clear-cut and not subject to misinterpretation. However, for some stakeholders using 3-D finite element models (FEMs) to conduct a flexible body static analysis, there is likely no physical model feature that is located at the nonstructural CG location. Thus, there is no FEM node point (or grid point) to which to apply the design force. Believe it or not, we have seen this argument used to justify other nonstructural demand misinterpretations. The nonstructural demand is an inertial force that originates from earthquake ground accelerations. The ground motion inputs are applied to the nonstructural system at the connection points to the structure (i.e., anchorage tie-down points). If the nonstructural installation is above grade level, then the ground motions are amplified and applied at the connection points. Thus, the nonstructural demand, Fp, (Eq. 4-6) is applied to the FEM as an inertial acceleration with the connection points restrained. The operating weight, Wp, is factored out of Eq. 4-6 by dividing both sides by Wp, and the result is the equivalent nonstructural inertial acceleration demand. There is no need to apply a force at the FEM CG

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Earthquake Protection of Building Equipment and Systems

location, although there is a need to ensure the nonstructural mass distribution is adequately represented in the FEM. This approach is demonstrated in Chapter 6.

4.2.9 Commentary The ASCE/SEI 7-10 code provisions directly include a commentary section. This is very convenient and makes all excuses for not reading the commentary pretty weak. The commentary section should be examined before reading the actual code provisions. There is considerable insight gained regarding code intent that is covered in the commentary. It is a must-read for any stakeholder involved in implementation of earthquake protective measures. Seismic commentary from a historical perspective can be gleaned by reading the commentary sections in past NEHRP provisions (FEMA 1997, 2000, 2003). There is also an enhanced seismic commentary for ASCE/SEI 7-05 (Charney 2010).

4.3 Seismic Requirements Summary The continually transforming content of seismic design codes reflects the evolution of design practice as it takes place in changing technical and political contexts (FEMA 2006). The seismic building code provisions for the United States have undergone considerable growing pains over the last couple of decades. The previous region-based codes have been replaced with a unified national code. The old zone system has been abandoned for probabilistic ground motion hazard maps. And if page count is any indicator of code complexity, the seismic provision quadrupled in size from the old Uniform Building Code to the new ASCE/SEI 7-10 standard. The overarching goal for this chapter is to cut through the clutter of code prescriptive language and get to the core of code intent. The code is broken down into basic elements that are needed to form a cohesive whole. Nonstructural performance objectives have been identified and include position retention, systems interaction avoidance, and active operation for designated seismic systems. Compliance expectations regarding the meeting these objectives are delineated by employing analytical, experimental, comparative experience, or combined methods. To better illustrate code implementation techniques regarding seismic demand requirements, Box 4-1 steps through a hypothetical San Francisco building case study. Model code complexity is a perception some stakeholders have that hinders implementation. The code is not complex when viewed as a necessary foundation to support both basic (e.g., lateral force) and advanced (e.g., response spectrum) treatment of nonstructural systems to mitigate the risk of earthquake damage. The nonstructural seismic provisions contained in ASCE/SEI 7-10 become enablers to carry out earthquake protective measures using either basic or advanced techniques. The task of compliance validation to ensure that nonstructural systems have greater seismic capacity than the project-specific earthquake demand is called seismic qualification. Implementation of nonstructural qualification procedures is the focus of the second half of this book.

Building Code Seismic Requirements

Box 4-1. San Francisco Building Case Study

Building Structure Demand Requirements A hypothetical building is planned for construction in San Francisco. The building is an acute healthcare facility. The site address is 50 Beale Street, San Francisco, CA 94105—we did mention this is a hypothetical case study! From this information we can define the following: • Risk category ⫽ IV (essential facility) • Building site latitude ⫽ 37.79142°N • Building site longitude ⫽ 122.3962°W The latitude/longitude coordinates are used to determine the MCER mapped ground motion parameters at short periods, SS, and at long periods, S1, from our hazard maps (Figs. 4-3 and 4-4). An easier method to obtain ground motion requirements for a specific building site is to use the USGS Web tool (USGS 2010). Figure 4B1-1 displays the USGS web tool input page. The end-user must input the Building Code Reference Document, Site Soil Classification, and Site Address (or enter a latitude and longitude instead). The USGS Web tool then automatically generates a detailed report containing all necessary ground motion requirements, including design earthquake response spectrum plots. However, for purposes of this demonstration, the manual method will be used.

Figure 4B1-1. The USGS web tool user interface for determining building site ground motion requirements in accordance with ASCE/SEI 7-10 provisions. Source: USGS (2010).

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Earthquake Protection of Building Equipment and Systems

The MCER mapped ground motion values for this location are SS ⫽ 1.500 S1 ⫽ 0.606 The next step is to perform a site assessment to classify site soil properties using the Table 4-6 site classification matrix. Solely for purposes of this demonstration, the site is classified as Site Class C. Now we can determine the two site coefficients, Fa and Fv, from Table 4-7. Since SS is greater than 1.25 and S1 is greater than 0.5, no interpolation is necessary. For Site Class C we get Fa ⫽ 1.0 Fv ⫽ 1.3 The adjusted MCER acceleration parameters, SMS and SM1, can be calculated directly as SMS = Fa SS = 1.0 ( 1.5 ) = 1.5

(4B1-1)

SM 1 = Fv S1 = 1.3 ( 0.606 ) = 0.788

(4B1-2)

The design-level demand requirements, SDS and SD1, are determined by a twothirds ratio of the adjusted MCER parameters: 2 2 SMS = ( 1.5 ) = 1.0 3 3

(4B1-3)

2 2 SM 1 = ( 0.788 ) = 0.525 3 3

(4B1-4)

SDS = SD1 =

Our final step is to generate the design earthquake response spectrum (Fig. 4-6) using SDS, SD1, and TL (where TL ⫽ 12 s comes from the Fig. 4-5 map). Figure 4B1-2 displays the design earthquake response spectrum requirement for this hypothetical case study.

Nonstructural Demand Requirements

From the design earthquake spectrum we know the ground-level ZPA is ( 0.4 SDS ) or 0.4 g in this example. The nonstructural item is medical equipment. The nonstructural design force requirement is defined as Fp = ( 0.4 SDS )

( ap ) ⎛1 + 2 z ⎞ W = 0.4 1.0 2.5 1 + 2 0 W = 1.0 W ⎜ ⎟ p ( ( )) ( )) p p ⎛ Rp ⎞ ⎝ ⎛ 1.5 ⎞ ( h⎠ ⎜ ⎟ ⎜ ⎟ ⎝ 1.5 ⎠ ⎝ Ip ⎠

(4B1-5)

with ap = 2.5 for maximum flexible FRS amplification; Rp = 1.5 for minimum inelastic z reduction; I p = 1.5 for designated seismic systems in an essential facility; and = 0 for h

Building Code Seismic Requirements

Figure 4B1-2. Building structure design earthquake response spectrum for San Francisco case study using SDS = 1.0, SD1 = 0.525, and TL = 12 s. ground-level installation. We need to check the minimum and maximum design force values using Eqs. 4-7 and 4-8: Fp

MAX

= 1.6 SDS I p Wp = 1.6 ( 1.0 ) ( 1.5 ) Wp = 2.4 Wp

(4B1-6)

Fp

MIN

= 0.3 SDS I p Wp = 0.3 ( 1.0 ) ( 1.5 ) Wp = 0.45 Wp

(4B1-7)

Thus, we use the Eq. 4B1-5 demand, and the nonstructural seismic design force requirement for this example is 1.0 g at ground level. For comparison, if the installation was above grade, such as on the third floor of a four-story structure, then we z substitute = 0.75 into Eq. 4B1-5 and we get 2.5 g. In this case, we would have the h maximum design force from Eq. 4B1-6 govern the requirement and use 2.4 g.

Nonstructural Dynamic Testing Requirements Nonstructural active operation is a performance objective for this example. To achieve active operation validation, seismic simulation testing will be conducted in accordance with AC156 test protocol. The nonstructural dynamic response spectrum requirement uses the design-level ground motion value, ( SDS = 1.0 ) , and the installation height to building height ratio, z/h ⫽ 0, in conjunction with AC156. Figure 4B1-3 displays the generic AC156 floor spectrum.

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Earthquake Protection of Building Equipment and Systems

Figure 4B1-3. Nonstructural response spectrum requirement for dynamic testing or analysis applications based on AC156 formulation.

The parameters that define this response spectrum include (see Box 7-1 in Chapter 7 for derivation of parameters). z⎞ ⎛ ADYN H = SDS ⎜ 1 + 2 ⎟ = 1.0 ( 1) = 1.0 ⎝ h⎠ ADYN H

MAX

= 1.6 SDS = 1.6 ( 1.0 ) = 1.6

∴ ADYN H = 1.0

z⎞ ⎛ AZPAH = 0.4 SDS ⎜ 1 + 2 ⎟ = 0.4 ( 1.0 ) ( 1) = 0.4 ⎝ h⎠

(4B1-9)

(4B1-10)

2 2 SDS = ( 1.0 ) = 0.67 3 3

(4B1-11)

4 4 SDS = ( 1.0) = 0.267 15 15

(4B1-12)

ADYNV = AZPA V =

(4B1-8)

Figure 4B1-4 displays the resulting floor spectrum demand requirements that can be used for seismic qualification purposes (i.e., shake-table testing) to validate nonstructural active operation performance at design earthquake levels.

Building Code Seismic Requirements

1.3 8.3 33.3

1.0 1.0 0.4

0.67 0.67 0.27

Figure 4B1-4. San Francisco case study nonstructural response spectrum requirement for dynamic testing or analysis applications based on AC156 formulation.

The process for developing a project-specific nonstructural dynamic response spectrum requirement is outlined in the flowchart shown in Fig. 4B1-5. This method is fully linked to the code’s lateral force demands. AC156 has been approved by the testing evaluation services affiliates of the code-writing authority responsible for establishing the nonstructural seismic requirements for model building codes used in the United States. One final note: The process to develop a nonstructural dynamic response spectrum requirement that can cover a wide geographic region of the hazard map follows the same logic as shown in Fig. 4B1-5. However, instead of using a project-specific location as the starting point, all of the latitude/longitude coordinates are searched to find the maximum SS value within a given region. The USGS Web tool (USGS 2010) provides the capability to search regions based on U.S. states, counties, or even individual zip codes. Figure 4B1-6 displays a screen capture image for this useful USGS Web tool functionality. Interestingly enough, by scanning the Max SS column in Fig. 4B1-6, there is a significant variation among the few states shown here. Attempting to seismically qualify nonstructural products to the worst-case code maximums for large regions of the hazard map can prove to be a challenging activity. Chapter 5 discusses this in greater detail.

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Earthquake Protection of Building Equipment and Systems

Building Site Geographic Location (Latitude / Longitude)

Nonstructural Installation Height Ratio

( hz

(

Table 4-6 Site Soil Classification (A, B, C, D, or E)

Figure 4-3 MCER Ground Motion (SS )

Table 4-7a Site Coefficient (Fa )

Adjusted MCER Ground Motion (SMS )

SMS = Fa SS Design-Level MCER Ground Motion (SDS )

2 SDS = SMS 3 Figure 4B1-3 AC156 Design Earthquake Response Spectrum

(A

z h

(

(

A DYNH = S DS 1+ 2

DYN H

, A ZPA H , ADYN V , A ZPA V

(

)

A ZPA H = 0.4 S DS 1+ 2

z h

(

152

2 A DYN V = S DS 3

A ZPA V =

4 S 15 DS

A DYNH Max = 1.6 S DS

Figure 4B1-5. Process flowchart for developing a project-specific nonstructural dynamic response spectrum requirement based on ASCE/SEI 7-10 requirements and AC156 test protocol.

Building Code Seismic Requirements

153

Figure 4B1-6. The USGS web tool user interface for determining min/max ground motion values for U.S. regions in accordance with ASCE/SEI 7-10 provisions. Source: USGS (2010).

References ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, Reston, VA. Bachman, R. E., and Dowty, S. M. (2008). “Nonstructural component or nonbuilding structure?” Building Safety J., April-May. Charney, Finley A. (2010). Seismic loads: Guide to the seismic load provisions of ASCE 7-05, ASCE Press, Reston, VA. Federal Emergency Management Agency (FEMA). (1997). “NEHRP recommended provisions for seismic regulations for new buildings and other structures, Part 2: Commentary.” FEMA 303, Washington, DC. ———. (2000). “NEHRP recommended provisions for seismic regulations for new buildings and other structures, Part 2: Commentary.” FEMA 369, Washington, DC. ———. (2003). “Recommended provisions for seismic regulations for new buildings and other structures, Part 2: Commentary.” FEMA 450, Washington, DC. ———. (2006). “The regulation of seismic design.” In FEMA 454 Risk Management Series–Designing for earthquakes: A manual for architects. FEMA, Washington, DC.

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International Code Council (ICC). (2011). 2012 International building code, Country Club Hills, IL. International Code Council Evaluation Service (ICC ES). (2010). “Acceptance criteria for seismic certification by shake-table testing of nonstructural components.” AC156, Country Club Hills, IL. Scawthorn, C., O’Rourke, T. D., and Blackburn, F. T. (2006). “The 1906 San Francisco earthquake and fire: Enduring lessons for fire protection and water supply.” Earthquake Spectra, 22(S2), S135–S158. U.S. Geological Survey (USGS). (2010). (Date accessed, 2011).

Part 2

Nonstructural Capacity

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Chapter 5

Introduction to Seismic Qualification

What constitutes seismic qualification? Ask this question of the stakeholders involved in nonstructural protection and there will not be a consensus answer. In fact, more likely than not, there will be divergent ideas as to what constitutes a nonstructural system and equally divergent ideas on how to establish compliance. Without consensus agreement on the bounds of the problem, there will be stakeholder confusion regarding qualification ownership and implementation responsibilities. This tendency is not due to lack of understanding of qualification principles. After all, qualification, whether it is seismic or otherwise, is a matter of validating that the item in question has enough capacity to exceed the required demand. Thus, seismic qualification is a matter of ensuring that the nonstructural system’s capacity to resist motion and loading exceeds the motion and loading demand placed on it by the earthquake event. On this point all stakeholders would likely agree. The discussion in Part 1 of this book centered on the seismic demand side of the nonstructural problem—exploring the origin of earthquake hazards, allocating seismic demands, and defining associated code requirements. The next few chapters focus on nonstructural capacity issues. Essentially, we answer the question of how to establish a seismic capacity rating for the individual nonstructural system elements using analysis, testing, comparative experience, and combined techniques. However, before delving into

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capacity determination methods, here are three questions that need clarification regarding seismic qualification in general: 1. Qualification Ownership: Who is responsible, from a qualification perspective, regarding the various elements that compose nonstructural systems? 2. OEM Qualification Strategy: What are the top-level activities required to support effective nonstructural qualification practices? 3. Compliance Metrics: How is a common measurement rating used to gauge capacity against seismic demand requirements? Historically, these issues have been problematic for the stakeholders involved in nonstructural protection. This is particularly true for OEMs and suppliers of nonstructural products. Without clear understanding regarding ownership and not having a coherent strategy for execution, confusion can obstruct the qualification process. This effect is compounded when there is no consistent method used to gauge compliance. Where confusion exists, implementation gaps will inevitably surface and create risk mitigation bottlenecks. The goal for this chapter is to dispel the confusion and, hopefully, clear the bottlenecks.

5.1 Qualification Ownership The stakeholders understand qualification principles but perhaps have difficulty in establishing the division of responsibility regarding nonstructural qualification. Chapter 1 introduced the concept of seismic demand allocation via systems analysis. One of the byproducts of a systems approach is isolation of system elements and interfaces. Each nonstructural element is examined individually, and no single stakeholder is responsible for qualification of the entire system. The notion of shared responsibility is a fundamental premise behind systems design and is a central theme behind seismic risk mitigation in particular. By viewing the problem with a systems perspective, the division of responsibility for qualification becomes more evident. At the heart of systems design is the concept of decomposing large, complex problems into smaller, less complex system elements and interfaces. The end result of the decomposition process is transformation of physical nonstructural systems into block diagrams with functional interfaces. These block diagrams are useful to identify the division lines of responsibility and establish qualification ownership (Fig. 5-1). The generic nonstructural system is divided into two subsystems: mechanical-related elements and active operation-related elements. The nonstructural mechanical subsystem includes force-resisting skeleton (FRS), attachments, and anchorage. The nonstructural active operation subsystem includes functional devices. A description of the nonstructural system elements is summarized in Table 5-1. Also identified in the table is stakeholder ownership with regard to seismic qualification and the preferred method used to establish compliance. With these system elements in mind, the task of discussing seismic qualification ownership can be approached using a systems framework.

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Building System

Earthquake Demand

Nonstructural System

Buildingg Elements

Structural Transmissibility

Functional Interaction

Seismic Demand

Mechanical Elements

Structural Transmissibility

Functional Interaction

Seismic Demand

Floor (n+1)

Building Floor (n+1) Transmission Path from Foundation

Building / Anchorage Interaction

Building Floor (n+1) Input Motion

Anchorage

Attachment Transmission Path from Anchorage

Attachment / Anchorage Interaction

Attachment Input Motion from Anchorage

Attachment

FRS Transmission Path from Attachment

FRS / Attachment Interaction

FRS Input Motion From Attachment

Force Resisting Skeleton (FRS)

Functional Device Transmission Path From FRS

Functional Device / FRS Interaction

Functional Device Input Motion From FRS

Attachment

FRS Transmission Path from Attachment

FRS / Attachment Interaction

FRS Input Motion From Attachment

Anchorage

Attachment Transmission Path from Anchorage

Attachment / Anchorage Interaction

Attachment Input Motion from Anchorage

Clearance Clearance Envelope Envelope

Structure Building / Attachment Interaction

Floor (n)

Clearance Clearance Envelope Envelope Building / FRS Interaction

159

Active Operation p Elements

Functional Device (n)

Structure Clearance Clearance Envelope Envelope Building / Attachment Interaction Foundation

(a)

Building Floor (n) Transmission Path from Foundation

Building / Anchorage Interaction

Building Floor (n) Input Motion

Figure 5-1. Systems design framework for seismic qualification applications: (a) generic block diagram with geotechnical system excluded; (b) simplified nonstructural block diagram showing qualification ownership and code requirements. (Continues)

5.1.1 Anchorage This system element is highly important because anchorage is the “mechanical fuse” between the building and nonstructural. Anchorage is defined as the final connection points that secure the nonstructural FRS to the building structure, with or without the use of attachments. For example, a base-anchored equipment platform could include a bracing attachment at the top of FRS. In this case there would be two sets of anchors, one at the base securing the FRS directly to the building, and one set at the top securing the bracing attachment to the building. Dynamic loading experienced by the nonstructural item during earth shaking must be reacted through the anchorage. Anchorage failure not only violates the code’s position retention requirement but also, in most cases, will cause active

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Nonstructural System Mechanical Elements

Functional Interface

Active Operation p Elements

Building Professional

Anchorage

OEM / Building Professional

Attachment

Functional Device (1)

OEM

Force Resisting Skeleton (FRS)

Functional Device (2)

OEM

OEM / Building Professional

Attachment

Functional Device (n)

OEM

Building Professional

Anchorage

OEM

(b)

Position Retention Requirement

Active Operation Requirement

Figure 5-1. (Continued)

operation failure, which violates the code’s expectation for maintaining functionality for designated seismic systems. Many earthquake reconnaissance reports have detailed nonstructural failures due to inadequate anchorage, improperly installed anchorage, and even missing anchorage (Goodno et al. 2011). There are many possible anchorage types available to use. For example, in the concrete anchor category there are wedge, undercut, sleeve, shell, adhesive, and various cast-inplace anchor types, to name a few. Each type has a unique load capacity rating that is dependent on installation variables (e.g., concrete compressive strength, embedment depth, edge distance). These load ratings are most often empirically determined by cyclic pull testing on sample anchors in accordance with American Concrete Institute (ACI)

Table 5-1. Nonstructural System Element Breakdown Showing Qualification Ownership.

Nonstructural System Element

Mechanical Subsystem

Force-Resisting Skeleton (FRS)

Operational Attachment

Preferred Qualification Methoda

Structural members or assemblies of members, including frames, enclosures, struts, rods, panels, etc. FRS assembly members can be joined together using mechanical fasteners or can be weldments. Monocoque construction techniques are also included as FRS types. The nonstructural FRS provides support for subassemblies, modules and internal devices. The FRS also provides overall structural stability for the nonstructural platform. The FRS should be viewed as the nonstructural systems structural skeleton to resist all environmental and operating loads.

OEM

Analysis

Operating elements that connect between the nonstructural FRS and the building structure or could insert between two nonstructural systems in a distribution chain. An operational attachment includes both mechanical and electrical elements that are intended to support active operation functions. These attachments are necessary operational umbilicals that are required in order for the nonstructural system to function. Examples of operational attachments include piping, ducting, tubing, cabling, conduit, wiring, and grounding.

OEM

Analysis

Building Design Professional

Testing

Definition/Description

Testing Experience

Experience

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(Continues)

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Attachments

Qualification Owner

162

Nonstructural System Element

Mechanical Subsystem

Attachments

Bracing Attachment

Isolation Attachment

Definition/Description

Structural elements that connect between the FRS and the building structure. A bracing attachment is purely structural in nature and is used to provide a structural link between the FRS and the building to resist environmental loads. Examples of bracing attachments include brackets, angles, or other structural shapes that provide structural support and load resistance to the FRS. Mechanical elements (springs and dampers) that insert between the FRS and the building structure to isolate the FRS from the building. Isolation attachments (also called shock and vibration isolators) are mechanical energy absorbers that are intended to attenuate input loading between the nonstructural FRS and the building. Examples of isolation attachments include compression springs and dashpots, and compression springs with snubbers.

Qualification Owner

Preferred Qualification Methoda

Building Design Professional

Analysis

Building Design Professional

Analysis

Isolator Supplier

Testing

Testing

Earthquake Protection of Building Equipment and Systems

Table 5-1. (Continued )

Table 5-1. (Continued )

Nonstructural System Element

Mechanical Subsystem

Anchorage

Definition/Description

Building Design Professional

Preferred Qualification Methoda

Analysis Testing

(Continues)

Introduction to Seismic Qualification

The final connection points securing the nonstructural FRS to the building structure, with or without the use of attachments. Anchorage includes bolts, concrete anchors, welds, or other mechanical fasteners for positively securing the nonstructural FRS to the building without consideration of frictional resistance produced by the effects of gravity. For example, a typical equipment FRS can be anchored to a concrete pad directly with anchor bolts without the use of either bracing or isolation attachments, or an equipment FRS can be placed on isolation attachments and the isolator is anchored to the pad. Anchorage connection points to the building structure are the elements that cross the line between the building and nonstructural systems and react the dynamic loading experienced during the earthquake.

Qualification Owner

163

164

Nonstructural System Element

Active Operation Subsystem

a

Functional Devices

Definition/Description

Logical subgroupings of nonstructural active functions (operational) typically organized and arranged as physical devices, modules, components, or subassemblies that mount to the nonstructural FRS. Functional devices can be electrical, mechanical, or electromechanical in nature. Some devices also function as load-bearing members of the FRS in addition to being functional devices. For example, in a piping distribution system the pipe acts as both an FRS and a functional device with containment of pipe contents being the active function.

Qualification Owner

Preferred Qualification Methoda

OEM

Testing

The methods listed are the ones currently accepted by IBC/ASCE/SEI 7-10 code provisions (see Table 4-5).

Experience Combined Test and Analysis

Earthquake Protection of Building Equipment and Systems

Table 5-1. (Continued )

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standards such as ACI 355.2-07 (ACI 2007). Anchor manufacturers and independent evaluation agencies, such as the International Code Council’s Evaluation Services (ICC-ES) organization, typically provide these load ratings. It should be noted that anchorage capacity determination is a complex science in its own right. Review of Appendix D in ACI 318-11 (ACI 2011) is necessary in order to understand the limiting factors that govern anchorage capacity for a given installation design. The standard qualification procedure for concrete anchorage systems employs rigidbody mechanics to calculate the maximum anchor loads (demand). This calculation is based on nonstructural weight, center of gravity (CG), anchorage layout geometry, and the code’s prescribed static force requirement for the installation location (as described in Chapter 4). Next, an appropriately sized anchor (capacity) is selected based on the projectspecific concrete installation variables, such that the anchor capacity exceeds the demand. Box 6-1 in Chapter 6 demonstrates the techniques used to conduct anchor bolt analysis. Box 6-1 also demonstrates a similar process for anchorage system qualification when welded connections are used. The building design professional is responsible for making these calculations. In nonstructural distribution systems (e.g., piping) that are attached at multiple locations to the structure, anchorage capacity needs to consider seismic relative displacement requirements as well (see the discussion related to Eqs. 4-13 and 4-14 in Chapter 4).

5.1.2 Force-Resisting Skeleton (FRS) In essence, the nonstructural FRS functions similar to the building’s structural system. The FRS receives dynamic input from the earthquake via the building system and then filters and responds to the input based on the structural dynamic characteristics of the FRS. The preferred qualification method for the FRS depends on whether meeting active operation requirements is a prerequisite. If the nonstructural component does not need to satisfy active operation, Ip ⫽ 1.0, then analysis can be a cost-effective approach to demonstrate adequate position retention capacity. However, if the nonstructural component has to satisfy active operation concerns, Ip ⫽ 1.5, then testing is the preferred approach. This logic is driven by the fact that present-day building codes do not accept analysis as the sole means for functional device validation. Thus, when nonstructural active operation is required, some form of testing is necessary (assuming valid earthquake experience data are not available). The building code provides latitude in the types of testing that can support active operation validation. The type of testing selected is highly dependent on the complexity of the nonstructural platform with respect to functional devices. Additional consideration is needed regarding the practical size and weight limitations of available shake-tables. Not all nonstructural items (e.g., physically massive equipment) can be tested as complete systems. Chapter 7 covers seismic simulation testing when building code active operation expectations are applicable. Chapter 9 describes the process of combining analysis with testing to qualify largeclass systems that are impractical to test as a complete system. With regard to meeting position retention requirements, anchorage compliance addresses part of the requirement; the other part is the capacity of the nonstructural FRS to withstand seismic loading. It is not useful for the anchorage to remain intact but have the

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nonstructural FRS members break loose (see Fig. 6-6 in Chapter 6 for an example). Meeting position retention implies that both anchorage and the nonstructural FRS can demonstrate compliance. The OEM typically provides compliance validation that the nonstructural FRS can withstand seismic loading. Box 6-2 in Chapter 6 demonstrates an application of flexible-body analysis to satisfy position retention requirements for the nonstructural FRS. The OEM most often performs this calculation as part of the nonstructural platform’s product development process.

5.1.3 Attachments Nonstructural attachments come in three types: (1) operational, (2) bracing, and (3) isolation. An attachment secures to the FRS at one end and at the other end secures to the building or secures to another nonstructural FRS. An operational attachment includes both mechanical and electrical types that are intended to support nonstructural active operation functions. These attachments can be viewed as necessary “operational umbilicals.” Mechanical operational attachments include interfaces, such as piping, ducting, and tubing, that secure to the FRS. Electrical operational attachments include interfaces, such as cabling, wiring, conduit, grounding, or other electrical connections, that secure to the FRS to support electrical functions. Figure 5-2 displays typical operational attachments for mechanical and electrical equipment. If an operational attachment is damaged or fails at the connection interface during earth shaking, the nonstructural system will likely not function after the event. In addition, operational attachment failures could cause significant collateral damage, such as localized flooding or exposing live electrical wires. A bracing attachment is strictly structural in nature and is used to provide lateral or vertical restraint to withstand seismic loads. A good example is adding top bracing to a baseanchored nonstructural installation. Figure 5-3 illustrates this concept. The intent of bracing is to create a structural link between the FRS and the building. Seismic bracing is very effective in eliminating differential displacements due to cantilever mounting schemes (i.e., converting a vertical cantilever beam into a clamped-clamped beam). Seismic bracing is also quite common when nonstructural active operation is required in areas that experience moderate to severe seismic activity. In most cases, reducing nonstructural relative displacements is directly related to increasing the likelihood of maintaining active operation. The FEMA documents listed at the end of this chapter are excellent resources covering the topic of seismic bracing (FEMA 2002, 2004a, 2004b). Isolators (also called vibration isolators) are mechanical attachments intended to attenuate the input loading going between the nonstructural FRS and building. This type of isolation attachment is typically inserted between base anchorage and FRS. Figure 5-4 illustrates this concept. While seismic isolation can be an effective way to reduce dynamic loads experienced during earth shaking, the primary reason isolators are used in practice is to minimize structure-borne noise from platforms that contain rotating devices. This type of steady-state vibration isolation is most often a poor design for attenuation of earthquake inputs. There is a fundamental difference between transient shock isolation for earthquake demands and steady-state vibration isolation for rotating machinery. This means that a good vibration isolator will be a poor shock isolator and vice versa. Isolation systems that

Introduction to Seismic Qualification

(a)

(b) Figure 5-2. Nonstructural equipment items with operational attachments: (a) mechanical attachments; (b) electrical attachments.

167

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Earthquake Protection of Building Equipment and Systems

Seismic bracing attachment added to installation

Figure 5-3. Nonstructural equipment item that is base anchored with top bracing attachment added to the installation design. Source: Illustration courtesy of Square D by Schneider Electric, Palatine, Illinois.

must operate in both environments must be evaluated for both environments to ensure proper performance. Attempting to apply vibration isolation to address the shock isolation need for seismic applications can result in dynamic amplification and not the desired attenuation (Fathali and Filiatrault 2008). Earthquake reconnaissance reports routinely confirm that improper application of isolation attachments can have negative consequences. Unless specifically designed to resist seismic demands, steady-state vibration isolated systems are vulnerable to damage at low levels of earth shaking (Gillengerten 2001). The primary issue with isolation attachments is that large relative displacements need to be accommodated in the installation design. This necessitates that isolation snubbers

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(a) Figure 5-4. Nonstructural platform with isolation attachment added to the anchorage installation design: (a) base-isolated mechanical equipment; (b) coil spring with elastomeric snubbers. (Continues)

are used to limit the amount of FRS free travel and keep the isolators from sliding off their mounts. Viscous snubbers (i.e., dashpots) are more effective than elastomeric bumper designs. The FRS slamming into bumper stops can amplify the loading going into functional devices and into the anchorage. In addition, any operational attachments need to have the necessary interface joint flexibility to accommodate large displacements. With large FRS displacements, system interaction avoidance becomes a primary concern. Qualification of attachments, in general, has been a gray area for many stakeholders, specifically regarding operational and isolation attachments. Operational attachments should be addressed by both the OEM (since the OEM is responsible for nonstructural design) and the building design professional (since the project-specific application needs consideration). However, we speculate that typical OEMs do not treat attachments with the same rigor as they might treat the nonstructural FRS (or functional devices, for that

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Earthquake Protection of Building Equipment and Systems

(b) Figure 5-4. (Continued)

matter). This may be driven by the fact that most OEMs do not test nonstructural systems with either operational or isolation attachments in place. For example, a piece of mechanical equipment might have a lengthy run of ducting or piping attach to it [as shown in Fig. 5-2(a)]. Representing this operational attachment during qualification testing is problematic because of shake-table size constraints. In addition, the attachment configuration design is dependent on project-specific building considerations. There could be literally hundreds—even thousands—of possible configurations to consider. The OEM should concentrate its qualification efforts on the interface joint that secures the operational attachment to the FRS. This can be accomplished using analytical methods. The OEM should also provide alternative design choices to accommodate either rigid or flexible jointed connections. The building design professional should assess the operational attachment from the as-built installation perspective, essentially answering the question of whether there is adequate clearance and flexibility in the attachment installation. Seismic relative displacements concerns are applicable when the operational attachment is fairly rigid, as in a piping connection. In a similar vein, the OEM does not typically test equipment platforms using isolation attachments in place. The reason for this is simple: there are many different types of isolation designs to consider. Some isolators use a compression spring and cup with elastomeric snubbers, others use springs with viscous dampers, and some use coil springs without dampers. The isolation design options are many. There is currently no practical way to test specific nonstructural platforms in conjunction with specific isolation designs. Qualification of bracing and isolation attachments is performed by the building design professional using analysis or by the supplier of the attachment product (e.g., the isolator

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manufacturing company). Both of these attachment types are typically added outside of the OEM’s normal product development process. In other words, these attachments are added as project-specific installation features to address project-specific application needs and are not normally pre-engineered design features offered by the nonstructural OEM.

5.1.4 Functional Devices Analysis is not solely accepted as a qualification method to verify active operation for functional devices. Only testing, combined testing with analysis, and earthquake experience methods are accepted in accordance with building code requirements. In practice, testing and combined methods are preferred. Earthquake experience requires that stakeholders have data to substantiate that a given nonstructural active function (i.e., functional device) has experienced earth shaking in excess of the seismic demand and has demonstrated active operation performance after the earthquake. This makes the experience method significantly more difficult to apply as a qualification tool (see Chapter 8 for details on comparative experience methods). Because the life cycle of most functional devices is relatively short and new devices are continually added to the nonstructural platform, new devices will not likely be based on any earthquake experience. Also, gathering the necessary earthquake experience data for specific nonstructural installations (even if they exist) is very difficult to accomplish. For these reasons, testing-based methods are preferred. Testing of functional devices can be approached in two ways: (1) nonstructural system testing and (2) standalone device testing. Nonstructural system testing implies that the active operation of the various devices is validated as part of the larger system-level test. Stated simply, the functional devices are mounted in the nonstructural system as normally designed and the entire system is tested. Standalone device testing implies that the various functional devices are tested separately from the nonstructural assembly. For example, a functional device is secured to a test fixture that replicates the FRS mounting and the fixture is attached to a test machine (i.e., the test is conducted without the presence of an FRS). The pros and cons of using these two different test methods are summarized in Table 5-2. The prerequisite for conducting standalone device testing is characterization of the nonstructural transmission paths from anchorage to functional device location, as shown in Fig. 5-1(a). The general procedure for conducting standalone device testing is outlined in Chapter 9. It is worth noting that considerable conservatism can result when using the standalone testing method. For example, if the only structural transmission path evaluated is the top of a base-anchored nonstructural platform (i.e., absolute worst case), this location will likely overestimate the functional device test requirements for all other locations within the FRS. In addition, this method typically ignores the mechanical impedance interaction that occurs naturally between the FRS and functional devices, which can be significant (Scharton 1997; NASA 2000). The resulting device test may overtest the functional device by up to a factor of two or three times the demand it would see when tested as a nonstructural system (Neubert 1987). However, for highly configurable nonstructural product platforms that may contain dozens of different types of functional devices, standalone testing is a

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Earthquake Protection of Building Equipment and Systems

Table 5-2. Comparison of Top-Level Nonstructural System Testing and Standalone Device Testing. Type of Test

Pros

Cons

Top-level Nonstructural System Testing

Simulates end-use building application environment What is seen installed is what is tested No ambiguity with transfer functions Mechanical impedance interaction between device and FRS is naturally accounted for No opportunity for overtesting of functional devices

Top-level assembly is functionally complex Top-level assembly is physically large and heavy Top-level assembly test unit is expensive Few test facility options and typically requires the services of large-scale test laboratories where testing cost can be expensive Difficult to simulate building-level functional inputs during testing

Standalone Device Testing

Functional device assembly is functionally less complex Functional device assembly is physically smaller and lighter Functional device assembly test unit is less expensive and the cost of qualification testing can be incorporated into the cost of device product development More test facility options and testing cost is less expensive using small-scale shake-tables Easier to simulate devicelevel functional inputs during testing

Simulates internal nonstructural functional environment, not building-level environment Top-level assembly qualification becomes a composite calculation with uncertainties Internal structural transmission paths (i.e., transfer functions) must be characterized Mechanical impedance interaction between device and FRS is typically ignored The possibility for functional device overtesting is high

practical, cost-effective approach compared with conducting dozens of system-level tests—even with potential overtest conditions. The consideration of mechanical impedance effects during device testing can be accounted for by using dual-control test techniques as discussed in Chapter 9. One final note regarding functional devices: the OEM is responsible for active operation validation. While direct incorporation of seismic compliance requirements may be new

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for some OEMs, most OEMs perform shock and vibration design assurance testing during the product development process. By including seismic requirements during the development cycle of new functional devices, standalone seismic compliance testing can be performed, along with other environmental testing, during the development process. This will streamline seismic compliance for the nonstructural platform in general, thus avoiding endless cycles of retesting when new functional devices are incorporated into the platform. This is a major issue with nonstructural product platforms that contain primarily electrical or electromechanical devices. Many of these types of devices will be updated or replaced every couple of years. The prospect of having to conduct system-level retests with every new device that is incorporated into the platform is a major concern for nonstructural OEMs. Standalone device testing is a viable solution to address this problem.

5.1.5 Clearance Envelope The clearance envelope is not a nonstructural system element per se but is the most important nonstructural/building system demand requirement that is entirely dependent on the project-specific installation. The clearance envelope demand is the existing free space surrounding the nonstructural installation. This demand parameter is not a nonstructural design feature but, rather, a building installation feature to ensure that the nonstructural system does not adversely interact with anything else that might be located in the immediate vicinity during earth shaking. The building code refers to this as consequential damage. Presently, codes do not specify the amount of required clearance space for any nonstructural application. Establishment of the necessary clearance envelope is performed as a visual inspection by the building design professional or inspector of record (IOR) to verify that adequate clearance exists surrounding the installed nonstructural item. Engineering judgment is used to satisfy this demand requirement by answering the question of whether there is adequate clearance between the nonstructural system and adjacent objects, given that the nonstructural FRS will likely deflect (along with everything else) during earth shaking. Clearance assessment needs to consider the likelihood of the nonstructural element banging into other objects, or other objects banging into nonstructural elements, and the potential damage that would result (see Figure 10-1 for example). If it is deemed that the resulting damage would be excessive and could impede the nonstructural active operation requirement, then bracing attachments should be added to the installation design. In many essential nonstructural applications (i.e., designated seismic systems), the easiest solution is to add extra bracing rather than guessing how much clearance is adequate. It should be noted that for many flexible, base-anchored nonstructural platforms, FRS deflections on the order of 75 mm (⬇3 in.) is possible for applications that experience high input demands. The clearance requirements for distribution systems (e.g., piping) are more demanding. A distribution system can sway between bracing points during ground shaking and collide with other systems. Potential impacts between systems should be assessed and avoided. Since distribution systems have multiple input points that can vary throughout a building, differential displacements can cause system-to-system interference contact fail-

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ures. Flexible couplings to accommodate building movements should be provided at structural separations as well as at the penetration points where a distribution system passes between floors. We advocate for more research on this topic. What is the definition of “adequate clearance” to avoid unwanted consequential damage? How can adequate clearance be prescribed in code provisions? Should general clearance guidelines and/or rules be incorporated into the code? These questions need to be addressed, especially since system interaction avoidance is a stated code requirement, but more importantly because many postearthquake essential facility closures can be attributed to unintended nonstructural system interaction problems. Without providing basic guidelines to promote awareness, this requirement will remain vague in the eyes of stakeholders and, most likely, systems interaction avoidance will continue to be overlooked and thus remain the single most important aspect of nonstructural protection that is essentially ignored. Stated simply, nonstructural earthquake protective measures will be ineffectual if systems interaction avoidance is not taken seriously by all stakeholders. Seismic interaction evaluation is further described in Chapter 8 as part of our earthquake experience discussion (p. 329).

5.2 OEM Qualification Strategy In a perfect world the following list of qualification-related support activities would be performed in sequence by the nonstructural OEM or supplier to achieve seismic compliance validation. Reality, however, is far from perfect. Most OEMs will tend to skip a step or two and not follow the steps in sequence. All of these steps are considered paramount in executing an effective and comprehensive qualification strategy for new product development of nonstructural equipment and distribution systems. The degree to which these steps are followed will vary widely in industry. This strategy is applicable to both nonstructural systems and subsystem device qualification over the lifecycle of these products. 1. Requirement Assessment – Marketing: As new nonstructural product offerings are being conceived for development, a marketing assessment is needed to determine appropriate seismic codes and/or standards that are compatible with the target market. The marketing perspective must consider all relevant seismic requirements based on the potential locations for product distribution and different applications (e.g., nuclear power versus building code). If the target market includes international applications, the assessment must address multiple codes and standards representative of regional requirements. This assessment includes both identification of relevant codes and selection of a “minimum threshold” seismic demand level. The minimum demand level is considered the target seismic withstand capacity that the nonstructural platform must achieve in order to satisfy customer needs. 2. Requirement Assessment – Engineering: An engineering assessment is conducted to review the proposed seismic requirements for the nonstructural development project. The engineering perspective must evaluate the selected demand levels to ascer-

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3.

4.

5.

6.

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tain how the proposed demand parameters compare with other previously qualified products. This assessment must also compare different code requirements when the application includes international distribution. The goal of engineering assessment is twofold: (1) a realistic assessment of proposed demand versus anticipated capacity based on product complexity, cost, and previous product experience and (2) consolidation of multiple code requirements into a composite requirement. This step most often requires iteration with the marketing perspective in Step 1 based on the seismic withstand potential of the new product. Some product platforms are simply more “laterally challenged” than others. Starting with unrealistically high withstand requirements is to be avoided. Design Specification: Seismic withstand requirements must be incorporated into the product design specification. Inclusion of a seismic design requirement will force product development teams to address withstand resistance early during the concept phase when design changes have minimal cost impact. This includes allocation of seismic demand, based on systems analysis, for development of new functional devices (as described in Chapter 1). Without direct inclusion of seismic requirements into the design specification, no engineering effort will be dedicated to evaluate withstand resistance during the product development process. Product Development: Analysis is performed to determine the seismic withstand capacity of the product platform. Design changes are made as necessary to satisfy the seismic requirement. Chapter 6 discusses the various analytical techniques used to support product development. Analysis results provide the design team with a good understanding of seismic load path and establishes a ballpark assessment regarding the likelihood of passing qualification testing. Qualification Testing: Seismic simulation testing is conducted to satisfy code requirements. Chapter 7 discusses how to conduct a qualification test. If multiple codes and/or standards are applicable, the seismic testing can be conducted such that all requirements are satisfied with a single test program. Part of test preparation includes product line rationalization. This process is used to select representative product samples for those nonstructural platforms that offer configurable options. The product line rationalization process is discussed in the next section. Test Data Management: Upon completion of a successful qualification testing program, the test data and formal test report need to be properly retained to support ongoing compliance assessment as products are sold and distributed. Automated and semiautomated compliance assessment tools can be used during nonstructural order fulfillment. A description of these assessment tools is discussed later in this chapter. The final activity related to qualification test data is deciding when a retest is necessary to offset any increases made to the code seismic requirements. Potential increases in demand levels defined in code hazard maps may eventually cause older products to not have enough capacity to satisfy the changing customer need. Potential retests will also depend on the extent of design modifications made over a product’s life cycle. Certain types of design changes can have a significant impact on a product’s seismic withstand resistance, such as adding heavier devices and increasing the product’s CG height location.

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The OEM temptation during new product development is to skip past the first three steps and go right to Step 4, Product Development. This is a common OEM mistake. Most often, this results in an after-the-fact requirement for seismic compliance, usually driven by a customer demand for compliance certification. In this situation a testing program is conceived and executed with little engineering ability to improve withstand resistance, because the product platform design has already been committed to production. At this point in the product life cycle, extensive design changes are difficult to justify and expensive to incorporate. There are no shortcuts to effective qualification practices. Unfortunately, this lesson is only appreciated after first experiencing failures the hard way.

5.2.1 Product Line Rationalization Many nonstructural product offerings are pre-engineered to support a wide variety of customer options. These product platforms are typically highly configurable. In this situation, the necessary first step during the qualification planning phase is selecting an appropriate subset of possible design configurations to be used to qualify the entire product platform. This process is called nonstructural product line rationalization. Rationalization is required to narrow down the potential qualification candidates to a few design configurations. The results from this process dictate which nonstructural configurations will be used during qualification, whether using analysis, testing, experience, or various method combinations. This point needs to be emphasized. The prerequisite of any qualification activity is proper selection of nonstructural configurations to be used during the qualification process as representative samples. Moving forward with analysis and testing is impossible without first knowing which nonstructural design configurations need to be evaluated. Box 5-1 describes the process of product line rationalization in greater detail.

Box 5-1. Product Line Rationalization Process Nonstructural qualification is about qualifying specific products, whether they involve equipment or distribution systems. And when we talk about specific products, we are actually talking about specific design configurations of a given product line or product platform. The term platform describes the underlying design and construction architecture in which the nonstructural offering can be varied to meet end-use application needs. A modular product platform can support many different end-use applications and thus will offer many configuration options to satisfy a variety of customer needs (Ericsson and Erixon 1999). The process of selecting the right product configurations for qualification purposes is called product line rationalization (also called type testing). The nonstructural OEM or supplier is responsible for this task. The goal of rationalization is to select the fewest number of configurations that can adequately represent the entire product line family. Thus, by qualifying the rationalized configurations, in effect the entire product line offering is qualified. The difficulty of the rationalization process is directly linked

Introduction to Seismic Qualification

to the complexity of the product line’s configuration offerings. For example, if the nonstructural platform to be qualified has minimal configuration options, then only a handful of possible product variations need consideration. Conversely, if the platform has hundreds or even thousands of possible configuration variants to consider, then the rationalization process becomes significantly more difficult. Present-day nonstructural offerings are highly configurable product platforms that support mass customization manufacturing strategies. In other words, most of the mechanical and electrical nonstructural product offerings available today are user configurable, so that many application opportunities can be serviced by a few highly engineered platforms. The trend of offering more and more design variants to satisfy a growing need for customization has rapidly accelerated over the last half-century (Ericsson and Erixon 1999). This poses a real challenge for the qualification analyst needing to address the product line rationalization task. There is a need to establish objective rationalization guidelines that can be used to assess the product line’s configuration options for seismic qualification purposes. The following general criteria are offered for representing a nonstructural product line by selecting rationalized configuration samples. It is recognized that certain industryspecific product types may present unique rationalization challenges that could deviate from the general rules provided here. An example of a different twist on the rationalization process is described in Chapter 9 for a mechanical large-class platform. In this case, the rationalization process concentrated on identification of mechanical weak links (i.e., structural-related) in the product line. Different nonstructural platform types can present different approaches to the problem, but the end goal is the same—selecting a minimal subset of product configurations to represent the greater platform offerings.

Structural Dynamics Perspective The dynamic response of a product platform is directly influenced by its force-resisting skeleton (FRS), attachments, and anchorage. From a structural dynamics perspective, these combined system elements can behave like a spring-mass oscillator and either amplify the dynamic input motion, attenuate the input, or transmit it as is (neither amplify nor attenuate). In simplistic terms, achieving dynamic attenuation typically requires using shock isolation attachments. Achieving a neutral transmission requires the nonstructural FRS to be rigid enough, forcing the platform to behave as a rigid body over the frequency range of interest (e.g., 1–35 Hz). Dynamic amplification results when the FRS behaves as a flexible body and experiences structural resonance over the frequency range of interest. Most nonstructural platforms remain flexible in the seismic frequency range and will exhibit some amount of dynamic amplification. When evaluating possible configurations during product line rationalization, selecting configurations that amplify input motion the most is the goal. The logic is that if the most dynamically responsive configuration offering can satisfy qualification requirements, then other more rigid offerings will satisfy, too. For example, with a base-anchored nonstructural platform, selecting the smallest footprint available with the greatest mass density and highest

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center of gravity will likely be the configuration candidate that amplifies motion the most (i.e., maximum dynamic response). The objective criteria to use in ranking dynamic responsiveness are natural frequency and structural transmissibility. The natural frequency of interest is the fundamental frequency (i.e., lowest frequency with largest mass participation). Structural transmissibility is quantified by the transmission path transfer function between anchorage and a location on the FRS (see Chapter 9 for an example). In general, the lower the fundamental frequency and the greater the transfer function amplitude (e.g., quality factor – Q), the more susceptible the nonstructural item is to dynamic inputs.

Structural Strength Perspective The assumption made in comparing system configurations as dynamic oscillators is that no matter how flexible or rigid the mechanical subsystem is, the FRS has to react the input loading and maintain structural integrity. When the basic design construction of the FRS is similar across different footprint options, with only width and depth dimensions changing, this assumption is valid. However, if certain footprint options bring with them fundamental changes in the load path of the FRS, then structural strength considerations are needed during rationalization in conjunction with dynamic response. An example of this might be if the largest footprint option within the product line uses a different structural joint scheme to secure vertical members to horizontal members. In this case, even though the larger footprint configuration might display better dynamic characteristics (higher natural frequency and lower transfer function amplitude) compared with the smaller footprint option, because of the different joint design the assumption regarding maintaining adequate structural integrity cannot be justified by dynamic comparison alone. Hence, in this case it might require selecting the smallest footprint configuration based on dynamic considerations and also selecting a larger footprint configuration because of different mechanical joints. The introduction of a unique attachment scheme could cause joint failures due to overloading, even when the dynamics are more favorable for this configuration. Thus, qualification of the larger configuration is required in addition to qualifying the more flexible smaller unit.

Functional Device Perspective It might be surmised that the rationalization problem can be compounding and that explicit guidance is not forthcoming. Unfortunately, this assessment is fairly accurate when consideration is given to the various functional devices that might be offered as configurable options. In this case the rationalization problem grows even more complex. Nonstructural platforms that offer highly configurable functional device options can pose the most difficult rationalization challenge. In addition to considering basic FRS footprints and evaluating competing load path designs, we now have to consider all of the various devices, modules, and operational subassemblies that are potentially packaged into the platform. Since the primary rationalization goal is to select the fewest nonstructural platform configurations that can adequately qualify the entire product family, compromises might be required. First, if active operation is not a requirement for the nonstructural system, then all functional devices are considered dead-weight objects that

Introduction to Seismic Qualification

mass load the FRS. In this case, selecting the heavy devices is the priority. Second, if active operation is required, then devices that are essential to the continued operation of a critical facility or are potentially hazardous when damaged are the priority. Any functional device that can release hazardous material or can release dangerous levels of energy needs to be included in the rationalization process. Those devices that are not essential and do not contain dangerous material may be excluded. The point is that even if the nonstructural system is classified as a designated seismic system, that does not imply that every functional device contained within the platform is essential. If failure of a noncritical device does not impede the functioning of a critical device, then the noncritical devices can be excluded from rationalization. The rationalization process might result in an impractical number of test samples needing consideration because of a high number of functional device options that are packaged into the platform offering. In this situation, the concept of standalone device qualification testing is an attractive solution. The concept of standalone device testing is described in Chapter 9.

Engineering Judgment Obviously, this process is highly dependent on the nonstructural platform and requires review on a case-by-case basis. However, for many types of nonstructural systems the reality is that rationalization requires review of many critical devices, in addition to many combinations of FRS footprints with potentially differing load paths and joint designs. In this case, good judgment is a necessary ingredient to resolve a complex rationalization problem that yields qualification selections that everyone can accept (i.e., cost-effective, meets market need, and complies with code intent). Note that sometimes configuring the absolute worst-case configurations may not be the wisest choice. Selecting the heaviest possible functional devices and placing them in the highest locations within the platform and also selecting the smallest possible base footprint can turn into a qualification activity that is too difficult to successfully pass. Is this a nonstructural configuration that has significant real-world applications? A good rule of thumb is the 80/20 rule. Concentrate on design configurations that can cover 80% of the possible product applications, and handle the remaining 20% by either excluding them altogether or by using special seismic bracing attachments. Product line rationalization is best approached as a collaborative effort with resource help from design engineering, product marketing, and test engineering groups. In other words, someone who knows the design options, plus someone who knows the marketplace needs, plus someone who is familiar with seismic qualification are often required during the rationalization process. And yes, depending on the platform complexity, engineering compromises may be required. The strategy should be to select product line configurations that can cover a high percentage of applications and not become so lost in minutiae that the inherent complexity of the rationalization task paralyzes any qualification activity from advancing. In summary, rationalization is the process for selecting a minimum subset of the available configuration options to represent the entire product line for seismic qualification purposes (or at least represent 80% of the offering for highly configurable

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platforms). Design assessment is needed to consider the following nonstructural platform features from perspectives that include dynamic response, structural strength, and active operation: • Structural Features: An assessment is performed to evaluate the platform’s FRS options. Specifically, the various structural configurations are reviewed to determine which offer the least seismic withstand capacity compared with other options that are available within the product line being qualified. If more than one major FRS is a configurable option, then these other structural configurations should be considered. • Mounting Features: An assessment is performed to evaluate whether the platform’s anchorage configuration is one offering the least seismic withstand capacity compared with other anchorage options that are available within the product line being qualified. If several anchorage configurations are used (e.g., concrete anchors and welded connections), they need to be considered in the rationalization process. However, because of the wide variety of anchor types available, it is impractical and unjustified to evaluate every possible anchorage type when conducting a seismic test of the nonstructural assembly. Thus, seismic testing of nonstructural systems is most often conducted using test fixtures with machine bolts and is not typically conducted using concrete pads with concrete anchor types. • Subassemblies: For designated seismic systems, an assessment is performed to select the platform’s critical functional devices that might be related to continued operation of essential facilities or could release dangerous material or could be life-safety-related. The major subassembly components shall be included in the rationalization process. For nondesignated seismic systems, the heaviest subassemblies should be considered the priority during rationalization. • Mass Distribution: An assessment is performed to verify that the selected platform mass distribution is one contributing to the least seismic capacity compared with other mass distribution options that are available within the product line being qualified. • System Variations: An assessment is performed to verify that the selected platform’s overall variations contribute to the least seismic withstand capacity compared with other variations that are available within the product line being qualified. Other nonstructural variations, such as number of units or sections in production assemblies, indoor and outdoor applications, among others, shall be considered in the nonstructural product line rationalization process.

5.2.2 The Cost of Qualification Nonstructural seismic qualification is not free. This reality has perhaps dissuaded some OEMs from conducting a comprehensive nonstructural qualification program or conducting any qualification at all. One of the primary reasons driving this behavior is that nonstructural seismic compliance has historically not been actively enforced in most jurisdic-

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tions across the United States. The hard reality in the earthquake engineering community is that enforcement of building code nonstructural seismic provisions has been simply inadequate. The reasons behind this are many, but the present-day trend is moving away from an era of little or no enforcement to a new paradigm of “special enforcement.” This new paradigm is being spearheaded by the state of California’s Office of Statewide Health Planning and Development (OSHPD). With the California adoption of IBC and ASCE/SEI 7 requirements in 2008, major changes have occurred with regard to acute healthcare facility regulation in California. OSHPD regulators now enforce the active operation clause in the building code which stipulates that compliance validation requires either earthquake experience-based or testing-based methods. The old paradigm of anchorage calculations being offered up as active operation compliance has ended in California when it comes to healthcare facility applications. The pendulum swing from essentially no enforcement to mandatory enforcement for hospitals and other healthcare facilities makes nonstructural qualification a marketable commodity. While we recognize this statement as being rather callous because nonstructural protection is about reducing capital investment losses and preventing injury to building occupants, there is some truth to it. If the necessary motivation to get OEMs and nonstructural suppliers to take qualification seriously is being caught with inadequate seismic compliance (or no compliance), then so be it. The fact remains that the trend is moving in the direction of special enforcement for essential facilities. California has taken the lead in this effort, and we speculate that many U.S. states and local jurisdictions will follow. Thus, understanding the cost implications regarding seismic qualification is necessary. The cost of nonstructural seismic qualification needs to be viewed as a long-term investment. It is an evolutionary process that requires both financial commitments and human resource development. The expectation that a half-dozen or so different nonstructural platforms can be qualified in a single year’s time without any prior experience in seismic qualification is unrealistic. Conversely, the qualification activity is not rocket science. Most nonstructural OEMs and suppliers have the necessary engineering competencies to adequately address the task. Table 5-3 summarizes the approximate work content required for nonstructural seismic qualification. These estimates address qualification of low-, medium-, and high-complexity nonstructural product lines. Based on these work content estimates, a rough order of magnitude cost estimate can be derived for different complexity systems to be used as a baseline reference point. There is another way to view this. What is the cost of doing nothing? The answer to this question depends on the viewpoint of each stakeholder. The stakeholder relying on nonstructural performance during and after earth shaking might have a much different outlook on this question compared with an OEM that is looking for cost avoidance.

5.3 Nonstructural Compliance Metrics The seismic capacity for a given nonstructural system is composed of the capacity limits established for individual system elements. This implies a least-common-denominator approach to the overall system capacity. The nonstructural system element with the

Table 5-3. Work Content and Hours Estimate for Nonstructural Qualification by Product Platform Complexity. High-Complexity Work Contentc (hours) Internal Externald

Preparation Product line rationalization Requirements assessment Project planning Resource coordination Test unit shipping Postqualification marketing

20 40 30 N/A 2 40

60 40 60 20 4 40

120 40 80 40 6 40

Analysis Model development Model validation Conduct analysis Design modifications Transfer function analysis Posttest data analysis

20 N/A 20 10 N/A 10

40 16 30 20 8 16

120 16 40 30 24 24

Testinge Test plan development Test laboratory coordination Modal survey Conduct test / witness test Test certification Posttest coordination Test report review Test data incorporation

20 4 N/A 40 N/A N/A 4 4

30 4 4 64 N/A 8 4 6

40 16 16 160 N/A 24 16 8

Total hours

264

60 N/A N/A

60

474

8 100 32 4 8 152

860

24 320 80 8 16 448

a Represents “off-the-shelf” nonstructural items. Minimal configurability and least costly. Examples include transfer switch and sprinkler systems. b Represents “standard” nonstructural items. Average design configurability and average cost. Examples include transformers, pumps, switchgear, and compressors. c Represents physically massive “large-class” platforms that offer maximum design configurability and are the most costly. Examples include generator sets, cooling towers, chillers, and uninterruptible power supplies (⬎ 600 Kw). d External hours represent the time spent by contract or external professionls hired to support qualification activities. e The direct cost of supplying test specimens to support seismic simulation testing is not included in this estimate.

Earthquake Protection of Building Equipment and Systems

Medium-Complexity Work Contentb (hours) Internal Externald

182

Qualification Activity

Low-Complexity Work Contenta (hours) Internal Externald

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183

lowest capacity establishes the total system capacity. For example, a given essential nonstructural installation (Ip ⫽ 1.5) could have four different capacity ratings, one for each system element. The anchorage capacity rating is based on anchor type, concrete installation variables, and nonstructural mass properties using anchorage calculations. The FRS capacity could be based on either testing or analysis. The functional device capacity could be based on either testing or earthquake experience. And the attachment capacity could be based on analysis. If all three of the qualification methods (analysis, testing, and experience) used the same seismic parameters to establish a capacity rating, compliance validation would be unambiguous. Each of the nonstructural system elements would have a capacity rating that shares the same base metric used in the qualification process. Unfortunately, this is not the situation when applying IBC nonstructural provisions based on the ASCE/SEI 7-10 standard (ASCE/SEI 2010). Presently, the code’s nonstructural seismic requirement is a static force magnitude (i.e., equivalent lateral force). There is no direct code provision specified to define a dynamic response spectrum requirement to support either testing or dynamic analysis methods. Static analysis methods use the nonstructural force requirements, while testing and dynamic analysis methods must use an interpreted response spectrum as the requirement. This is the starting point for the third problem area, as previously mentioned. Without a common metric in place to compare nonstructural capacity ratings across system elements, regardless of how the capacity was established, the result is the same as comparing apples to oranges. The absence of explicit code requirements governing nonstructural response spectra has forced stakeholders to define their own versions of an interpreted response spectrum in order to conduct qualification testing or perform dynamic analysis. Every nonstructural supplier essentially creates their own capacity rating system. Invariably, this situation creates multiple interpretations for the same requirement. This has resulted in significant variations—all intended to satisfy the same nonstructural requirement (see Fig. 7-4 in Chapter 7 for an example). It is extremely unlikely that code writers ever intended to have multiple interpretations of the same demand requirement. All stakeholders would likely agree on that. The preferred solution to this code deficiency (as recommended in Chapter 4) is direct incorporation of a nonstructural response spectrum requirement into code provisions alongside static force. Both the response spectrum and force demand levels can be linked together using existing ground motion parameters. Stakeholders can apply testing and dynamic analysis methods with a response spectrum requirement, or use static analysis with a force requirement. When both demand requirements are based on the same ground motion parameters, the process is unambiguous. To date, this solution has not been incorporated into current building code provisions. The alternative solution that has been implemented today is to utilize an approved nonstructural testing protocol that provides a response spectrum requirement directly linked to the code’s static force provisions. The International Code Council’s Evaluation Service organization manages the Evaluation Report AC156 intended for this purpose (ICC ES 2010). The nonstructural qualification testing protocol, AC156, has been vetted by code writers, academic researchers, and design professionals. This protocol was first

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adopted in 2000 to address building code nonstructural dynamic requirements (Gatscher et al. 2003). Box 7-1 in Chapter 7 details the technical merits in converting nonstructural force demands into response spectrum dynamic requirements. The assumption made here is that there can be only one interpreted response spectrum that is based on code intent. For the time being, AC156 serves this purpose and is the IBC and ASCE/SEI 7-10 codeapproved nonstructural dynamic requirement, which can be used for either dynamic testing or dynamic analysis purposes. The code’s design earthquake spectral response acceleration parameter at short period, SDS, is the appropriate seismic capacity metric that is common across the compliance methods. Static analysis ratings are based on SDS and dynamic testing ratings are also based on SDS when AC156 is implemented. This design-level parameter is the only appropriate capacity metric such that nonstructural seismic capacity ratings can be compared directly. Box 5-2 demonstrates how individual nonstructural element capacities are combined to arrive at a system-level seismic capacity rating. This approach makes determination of overall nonstructural capacity a transparent process. Multiple stakeholders participate independently as the application need dictates. Without a transparent way to assess nonstructural seismic compliance, the problem creates stakeholder confusion and retards any attempts to lessen the existing implementation gaps surrounding nonstructural protection.

Box 5-2. System-Level Compliance Assessment Nonstructural seismic compliance requires the individual system elements to establish a capacity rating exceeding the required demand for a given nonstructural application. The required demand is defined by the building site geotechnical data, including the code-specified ground motions and the location of the element within the building. The seismic demand can be based on a site survey performed prior to building construction or based on the default building code hazard maps for the location. A geotechnical site survey is common when the building will function as an essential facility that is located in an area prone to seismic activity. However, the building code hazard maps can also be used. Typically, the code’s default hazard maps will envelope the demand requirements obtained with a dedicated geotechnical site survey. For demonstration purposes, the code’s default hazard maps will be used to define the demand requirements.

Project-Specific Demand Requirements A hypothetical essential building facility (risk category IV) is planned for construction in a region that experiences moderate to high seismicity. The task of determining code ground motion requirements, based on building site location, is demonstrated in Box 4-1 in Chapter 4 and will not be repeated here. The design-level MCER ground motion requirement for this location is defined as SDS ⫽ 1.187, spectral response acceleration at short period (g)

Introduction to Seismic Qualification

and the nonstructural system parameters are defined as ap ⫽ 2.5, amplification factor for maximum flexible FRS amplification; Rp ⫽ 1.5, response modification factor for minimum inelastic reduction; z/h ⫽ 0.75, installation height-to-building height ratio for third-floor installation in a four-story building; and Ip ⫽ 1.5, importance factor for designated seismic system. The nonstructural static force requirement is defined by using these parameters directly with the code’s lateral force equation: Fp = ( 0.4 SDS )

(a )

p z⎞ 2.5 ⎛ ⎜ 1 + 2 ⎟⎠ Wp = ( 0.4 ( 1.187 )) 1.5 ( 2.5 ) Wp = 2.97 Wp h ⎛ Rp ⎞ ⎝ ⎛ ⎞ ⎜⎝ ⎟ ⎜⎝ I ⎟⎠ 1.5 ⎠

(5B2-1)

p

The maximum Fp is governed by Fp

MAX

= 1.6 SDS I p Wp = 1.6 ( 1.187 ) ( 1.5 ) Wp = 2.85 Wp

(5B2-2)

Thus, the maximum Fp governs the nonstructural force requirement for this application. Dividing both sides of Eq. 5B2-2 by the operating weight, Wp, results in the design-level static acceleration demand requirement (2.85 g) at 75% building height elevation. The nonstructural floor response spectrum requirement uses the code’s ground motion value, ( SDS = 1.187 ) , and the installation height-to-building height ratio (z/h ⫽ 0.75) in conjunction with AC156 protocol (ICC ES 2010). Figure 5B2-1 displays the nonstructural response spectrum requirement. This response spectrum is the design-level dynamic demand requirement at 75% building height elevation (refer to Box 7-1 in Chapter 7 for spectrum derivation).

Nonstructural System Capacity Ratings The nonstructural system elements are evaluated independently by the appropriate stakeholders (refer to Table 5-1). Each element needs to establish a seismic capacity rating that exceeds the project-specific demand requirements for this application. The nonstructural system for this example is a base-isolated electrical equipment platform that is secured to a concreted pad using seismic isolation attachments. The equipment contains many critical functional devices, two of which will be discussed for this demonstration. There are conduit entries at the top of the equipment platform and the conduit installation uses flexible operational attachments. Figure 5B22(a) displays the equipment installation. Figure 5B2-2(b) highlights the base isolation attachments, and Fig. 5B2-2(c) shows the flexible elbow joints used in the conduit entry design.

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1.3 8.3 33.3

1.90 1.90 1.19

0.79 0.79 0.32

Figure 5B2-1. Nonstructural response spectrum requirement for dynamic testing or analysis applications based on AC156 formulation.

Anchorage The building design professional performs an anchorage calculation (strength design load factors) to determine the maximum anchor bolt loads using the 2.85 g lateral force demand. The nonstructural installation is on uncracked reinforced concrete with known properties (i.e., pad thickness and compressive strength). The building designer selects a specific anchor type and size such that the anchor capacity exceeds the demand in accordance with ACI 318 Appendix D requirements (ACI 2011). For this installation, the maximum anchor loads are calculated as 10.479 kN (2,356 lb) tension and 8.642 kN (1,943 lb) shear. The selected anchor is a 13-mm (0.5-in.) wedge anchor with 80-mm (3.25-in.) minimum embedment. Anchor capacity is rated at 17.467 kN (3,927 lb) tension and 19.945 kN (4,484 lb) shear per anchor based on ACI 318 design requirements. This anchor capacity rating can be converted into an equivalent ground motion rating by reversing the bolt calculation and solving for the maximum static acceleration that can satisfy the tension–shear interaction requirements using 13-mm (0.5-in.) wedge anchor allowable design loads. An equivalent SDS rating can be readily calculated and is determined to be SDS ⫽ 1.43. This SDS capacity rating represents the maximum ground motion that this anchorage installation can satisfy. Stated simply, this is the upper limit for anchorage capacity for this project-specific nonstructural application.

Introduction to Seismic Qualification

Conduit Entry Using Flexible Joints Operational Attachments

Critical Electromechanical Functional Devices

#1 #2

Base Isolation (a) Attachments Figure 5B2-2. Nonstructural electrical equipment installation for assessing overall system capacity: (a) equipment installation that is base-isolated, contains two critical functional devices, and uses flexible conduit attachments at the top; (b) close-up view of base isolation attachment; (c) close-up view of flexible operational attachments. (Continues) Source: Illustrations courtesy of APC by Schneider Electric, West Kingston, RI.

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Isolation Attachments (b) Figure 5B2-2. (Continued )

Introduction to Seismic Qualification

Flexible Operational Attachments

(c) Figure 5B2-2. (Continued)

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Earthquake Protection of Building Equipment and Systems

It should be noted that determination of allowable design loads for concrete anchors is a process that requires satisfying several concrete installation design criteria as defined by ACI 318 Appendix D. It is assumed the appropriate design reductions have been applied in determining the anchorage capacity limit loads. Proper use of ACI 318 Appendix D requires consideration of all concrete installation design variables that govern the project-specific application.

Force-Resisting Skeleton (FRS) The OEM performed dynamic testing to establish FRS position retention capacity for this nonstructural system. A seismic simulation test was conducted using the AC156 protocol. The test was conducted by hard-mounting equipment to a shake-table. No isolation attachments were used during qualification testing. The OEM performed this test well in advance of this specific application. In fact, the OEM used a geographic area approach to select demand levels for testing purposes. IBC and ASCE/SEI 7-10 ground motion parameters were selected to cover (i.e., umbrella) a wide region of the hazard map, while considering the likelihood of passing a dynamic test. The selected ground motion was SDS ⫽ 1.425, spectral response acceleration at short period ( g) and the applicable nonstructural parameters used for testing included z ⫽ 1.0, installation height-to-building height ratio for roof installation; and h Ip ⫽ 1.5, importance factor for designated seismic systems The AC156 response spectrum is used for qualification testing and is called the required response spectrum (RRS) for testing purposes. A seismic simulation test was conducted in accordance with AC156 requirements. The resulting shake-table input motion was recorded and a test response spectrum (TRS) was created from the shaketable control accelerometer in three directions. Figure 5B2-3(a) displays the TRS and the original RRS for each input direction. The RRS is the target requirement and the TRS is the as-tested result. As can be observed, the TRS exceeds the RRS in all three axes, which is indicative of an overtest condition. Since the nonstructural system passed testing at this higher level of input demand, the TRS represents the actual seismic capacity of the equipment’s FRS. The target RRS sets the spectrum shape and is the nonstructural dynamic requirement based on converting force demands into equivalent floor spectrum demands. However, the code’s hazard maps are continuously variable, meaning that the target RRS was created from a specific SDS value and building height ratio, z/h. The maps offer ground motion input options that are either greater than or less than the target RRS we specified. The point is, using a higher value for SDS will force a new RRS to fit nicely under the as-tested TRS. We essentially slide the original RRS target up until a point on

Introduction to Seismic Qualification

(a)

(b) Figure 5B2-3. Nonstructural test data showing RRS and TRS plots: (a) as-tested TRS showing the over-test condition; (b) capacity response spectra created by sliding the RRS up to the as-tested levels.

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the TRS touches the RRS. The corresponding SDS value for the new RRS becomes the astested capacity rating that can be used for nonstructural certification purposes. Figure 5B2-3(b) displays the adjusted RRS levels for each input direction. The adjustment of RRS levels to fit the TRS is performed in each axis. The final capacity for the test unit is the minimum adjusted RRS from the three input directions. In this example, the adjusted horizontal RRS level corresponds with SDS ⫽ 1.6 and the adjusted vertical RRS level yields SDS ⫽ 2.055. Thus, the as-tested nonstructural FRS capacity is SDS ⫽ 1.6, which is the minimum of the three axes. The low-frequency cutoff shown in Fig. 5B2-3(b) is defined in AC156 and is dependent on the minimum FRS natural frequency (75% of the lowest natural frequency). In this example, the cutoff was set at 2.8 Hz. TRS points less than 2.8 Hz are ignored.

Functional Device Functional device #1 [Fig. 5B2-2(a)] was qualified as part of the system-level testing with the same qualification parameters as shown for the FRS capacity. Thus, the capacity rating for device #1 is identical to the FRS capacity (SDS ⫽ 1.6). However, the second functional device is a new product feature that has been recently incorporated into the nonstructural platform. The second functional device was tested as a standalone device and has a different capacity rating. In this case, the transfer function between anchorage and location of device #2 needs to be determined. In other words, the amount of dynamic amplification that device #2 receives from the FRS needs to be accounted for during standalone testing. Then, upon completion of device #2 qualification testing, the device TRS is used to determine a capacity rating while accounting for the system transfer function. The astested capacity of device #2 is finally converted to an equivalent ground motion rating using AC156 formulation. The seismic capacity of device #2 is determined to be SDS ⫽ 1.51 with z/h ⫽ 1.0. The functional device capacity rating in this example is based on two different capacities. The rating for device #1 is based on dynamic testing of the top-level nonstructural assembly, and the rating for device #2 is based on standalone device testing using the response spectrum method (see Chapter 9 for details on conducting standalone device testing).

Attachments and Clearance Envelope The three operational attachments (i.e., conduit) secured at the top of the equipment [Fig. 5B2-2(c)] provide a flexible connection to the FRS, and seismic capacity at the connection points is not a design factor if the necessary clearance is provided during installation. The necessary clearance can be validated by inspection of the nonstructural installation. At least 75 mm (3 in.) of FRS motion should be accommodated. This clearance is considered conservative and is loosely based on typical FRS displacements under seismic testing demands for flexible base-anchored equipment that are between 2 and 3 m (⬇ 7–10 ft) tall. The building design professional performs this visual inspection.

Introduction to Seismic Qualification

The isolation attachment is a spring and elastomeric snubber design [Fig. 5B2-2(b)] that inserts between the FRS and anchorage at eight locations along the base. Force demands calculated from the rigid-body anchorage calculation can be used to determine structural integrity of the base isolation frame, isolators, and welded connection between the FRS and the base isolation frame. For this example, the isolation design is specifically addressing earthquake demands. The intended isolation interaction is to attenuate seismic demands going into the equipment platform. Note that the code states that components mounted on vibration isolators shall have a bumper restraint or snubber in each horizontal direction. The design force shall be taken as 2Fp (Eq. 5B2-2) if the nominal clearance (air gap) between the equipment support frame and restraint is greater than 6.5 mm (0.25 in.). The building design professional is responsible for everything from the welded FRS connection to the base isolation frame itself, including isolators, down to the concrete anchors. The minimum seismic capacity of this base isolation system is determined to be SDS ⫽ 2.1.

Combined System-Level Ratings Table 5B2-1 summarizes the nonstructural capacity ratings for this example. Also shown in the table is the responsible stakeholder. As can be observed, the overall capacity for this nonstructural system (SDS ⫽ 1.43) is based on the lowest capacity rating from the system elements. The anchorage displays the lowest-ranked capacity although overall, for this application, the nonstructural system-level capacity exceeds the project-specific demand (1.43 g ⬎ 1.187 g), resulting in positive compliance. Another way to perceive this is to evaluate the potential limiting factors. This application can be installed in any location where the SDS ground motion is less than 1.43 g (assuming equivalent concrete pad properties and height installation). Building site locations that exceed SDS of 1.43 g will require either an increase in anchor bolt diameter or adding more anchor tie-down points. This will provide additional capacity until reaching SDS demands of 1.51 g and greater. Once the ground motion intensity reaches this magnitude, the only way to show compliance is a retest of device #2 to higher qualification levels. It should be noted that the system capacity ratings are dependent on installation height. The capacity ratings increase if the application requires ground-level installation (z/h ⫽ 0). The point of this example is to highlight the concept of combining individual nonstructural element capacities to determine a system-level capacity and highlight who is responsible from a qualification perspective. One of the benefits from this approach is immediate illumination of the system element that is limiting seismic compliance. It becomes obvious which system element needs improving to increase seismic withstand resistance. Design decisions to improve nonstructural seismic capacity become objective-based actions to improve the system’s weakest link. A secondary point from this example is to illustrate the highly variable nature of present-day qualification. The code’s earthquake hazard maps present continuously variable demands that, in turn, create variable nonstructural capacities to satisfy building applications. Because capacity is a function of ground motion intensity (SDS) and

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Table 5B2-1. Capacity Ratings for Nonstructural System Elements.

Mechanical Subsystem

Validation Method

OEM

1.6

System-level testing performed in accordance with AC156 requirements using: SDS ⫽ 1.425 z/h ⫽ 1.0 Ip ⫽ 1.5

Operational Attachment

Building Professional

N/A

Visual inspection performed at the nonstructural installation location to verify that flexible connection to FRS is adequate.

Bracing Attachment

None

None

None

Isolation Attachment

Building Professional

2.1

Rigid-body analysis performed in accordance with ASCE/SEI 7-10 requirements (snubber air gap ⬍ 0.25 in.) using: SDS ⫽ 1.187 ap ⫽ 2.5 Rp ⫽ 1.5 z/h ⫽ 0.75 Ip ⫽ 1.5

Anchorage

Building Professional

1.43

Rigid-body analysis performed in accordance with ASCE/SEI 7-10 requirements using: SDS ⫽ 1.187 ap ⫽ 2.5 Rp ⫽ 1.5 z/h ⫽ 0.75 Ip ⫽ 1.5

Functional Device #1

OEM

1.6

System-level testing performed in accordance with AC156 requirements using: SDS ⫽ 1.425 z/h ⫽ 1.0 Ip ⫽ 1.5

Functional Device #2

OEM

1.51

Standalone device testing performed by accounting for system transfer function between anchorage and device location.

Force-Resisting Skeleton (FRS) Attachments

Active Operation Subsystem

Seismic Capacitya SDS (g)

A seismic capacity rating SDS ⱖ 1.187g indicates positive compliance for the project-specific nonstructural application.

a

Earthquake Protection of Building Equipment and Systems

Nonstructural System Element

Responsible Stakeholder

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195

installation height ratio (z/h), capacity can vary from application to application. This type of seismic qualification is a fundamental departure from the days when seismic demands were classified as one of four seismic zones. In the past, standard OEM practice was to qualify nonstructural platforms to the maximum Zone IV demand level, and that was it. Today, the absolute worst-case floor motion, derived from IBC and ASCE/SEI 7-10 hazard maps, is too severe for most platforms to realistically meet. Qualification today involves strategically selecting floor motion demand levels that are less than worst-case maximums, based on the relative robustness of the product platform. Analytical methods can be used to help assess a platform’s seismic robustness, as discussed in Chapter 6. Each nonstructural platform will likely have different capacity ratings. This implies that nonstructural systems cannot be supplied without first assessing whether the capacity exceeds the project-specific application demand. This situation places the compliance verification burden directly on the design professional responsible for a given building application (i.e., the engineer of record). It is the nonstructural supplier’s responsibility to clearly identify the equipment capacity rating (i.e., ground motion intensity and installation height ratio), so the engineer of record can evaluate the overall nonstructural system capacity.

5.3.1 Compliance Verification It is apparent that the result from a systems design approach to seismic qualification is multiple capacity ratings for the various nonstructural system elements. After the qualification process is completed, each nonstructural platform will have different seismic capacity ratings based on the least capacity of the individual elements. In fact, each different functional device could have its own capacity rating. This by-product can create significant implementation issues for nonstructural OEMs that need to supply many different nonstructural platforms, with each platform offering multiple design configurations to support various building applications. Keeping track of all of the capacity ratings can be a logistics nightmare. There are a couple of ways to handle this potential problem. One approach is to associate a seismic capacity rating to each nonstructural system element using factory-assigned metadata tags. A metadata tag, in this case, is the SDS capacity magnitude assigned to a factory part number. When a nonstructural product application is fulfilled, the various elements that comprise the nonstructural system will dictate the overall capacity rating. Another way to approach this is to assign a seismic capacity rating tag to the top-level nonstructural part number, based on a worst-case assessment of functional device capacities. In either case, the implementation requires using technology automation tools that can be incorporated into factory order systems. Manually evaluating every nonstructural product application for seismic compliance is highly inefficient and should be discouraged. Automation tools can be developed (i.e., software modules) that will save time, provide consistency across different platforms, and, most importantly, will avoid mistakes during compliance assessment.

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Earthquake Protection of Building Equipment and Systems

Another advantage of incorporating seismic capacity ratings as metadata is the ability to visualize seismic compliance as a function of geography. Figure 5-5 shows the seismic compliance of two nonstructural product platforms as a function of U.S. geography. In the first example, the platform capacity is SDS ⫽ 0.73 g and the second has a capacity set at SDS ⫽ 1.33 g. White areas on the map signify potential installation locations where the nonstructural capacity exceeds the required demand. Dark gray areas signify locations where the nonstructural platform does not meet demand requirements. As can be observed, the first platform [Fig. 5-5(a)] does not have enough capacity to satisfy most of the West Coast demand requirements. The second platform improves on the available locations in which capacity exceeds demand [Fig. 5-5(b)] but still cannot satisfy all of the West Coast demand requirements. Since both plots reflect a capacity rating at grade level (z/h ⫽ 0), applications that require above-grade installation would decrease the available locations even more for satisfying demand requirements (increase in dark gray areas). These graphical maps offer the OEM an immediate assessment of compliance status for given nonstructural offerings while also providing an objective tool to make decisions regarding future design changes. If a particular platform does not cover enough of the target market geography, then platform design modifications can be made to increase the platform’s seismic withstand resistance. If the seismic capacity rating is assigned at the subsystem level, the maps can be used to isolate which of the nonstructural elements needs improving (i.e., which element is limiting the overall nonstructural capacity). In addition, graphical compliance maps provide a tool to assess code changes made to the earthquake hazard maps. Since building codes are typically revised every 3 years (including hazard map revisions), any changes to the hazard maps can be visualized from the perspective of nonstructural compliance impact. Increases in ground motion intensity can potentially reduce the amount of geography that a nonstructural system can satisfy. Nonstructural systems that have lower capacity ratings are more susceptible to increases in the ground motion values prescribed in code hazard maps. Objective decisions can be made regarding nonstructural design changes and qualification retests when ground motion increases have eroded the amount of geography the nonstructural system can successfully meet. This provides the greatest flexibility regarding nonstructural qualification. Ground motion magnitudes will fluctuate from code cycle to code cycle. However, this does not imply that nonstructural requalification is required with every change. Minor fluctuations in ground motion will have minimal impact on nonstructural compliance for most applications.

5.4 Seismic Qualification Summary Nonstructural qualification is a validation process ensuring that the nonstructural system has greater capacity to resist motion and loading than the application demand as specified by building code seismic requirements. Each of the nonstructural system elements could have a different capacity rating based on using an accepted qualification method (i.e.,

Introduction to Seismic Qualification

100

0

100 0

500 Miles

500 Kilometers

(a)

100

0

100 0

500 Miles

500 Kilometers

(b) Figure 5-5. Nonstructural compliance shown as function of U.S. geography; dark gray areas represent locations in which the nonstructural item’s capacity does not meet the required demand: (a) equipment platform with seismic capacity at SDS ⫽ 0.73 g for grade-level installation; (b) equipment platform with seismic capacity at SDS ⫽ 1.33 g for grade-level installation.

197

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Earthquake Protection of Building Equipment and Systems

analysis, testing, comparative experience, or combined methods). Multiple stakeholders are responsible for seismic compliance validation. No single stakeholder is responsible for qualifying the entire nonstructural system. Seismic capacity ratings need to use a metric that is common to code-accepted compliance methods. The IBC and ASCE/SEI 7-10 ground motion spectral acceleration at short period, SDS, is the appropriate common metric to use across the qualification methods. Presently, nonstructural dynamic testing and dynamic analysis must use an interpreted response spectrum that is consistent with code intent. The ICC Evaluation Service’s AC156 is the only code-approved protocol to serve this purpose. We encourage code writers to incorporate a nonstructural response spectrum option directly into code provisions to eliminate the need for interpreted code requirements. This step will greatly minimize the existing implementation gaps with respect to nonstructural dynamic requirements. Nonstructural compliance verification has become increasingly complex over the past decade. This is partially driven by the change in seismic hazard being prescribed by probabilistic ground motion maps and the abandoning of the old zone system. In addition, the expectation of postearthquake active operation for designated seismic systems now requires explicit demonstration of active performance following design-level earthquake demands. These code changes make compliance assessment a much greater challenge for OEMs and nonstructural suppliers. Nonstructural seismic qualification has evolved beyond simple anchor bolt calculations and now requires proactive measures by all stakeholders to get it right. A product’s seismic withstand resistance needs to be viewed as nonstructural design intent. This requires OEMs to adopt a coherent qualification strategy that becomes part of the product development process. The starting point for an effective strategy is requirements awareness—knowing what the nonstructural seismic requirements are for a given target market and how product offerings are gauged against the requirements for compliance purposes. The goal is to ensure that seismic withstand resistance is a design driver that gets implemented during early product development. The cost of nonstructural seismic qualification should be viewed as a long-term investment. Model building codes have significantly evolved and will continue to evolve. The practice of establishing seismic compliance in the 21st century needs to evolve as well. The days of self-serving interpretation of the requirements or essentially ignoring the requirements completely have ended. New special enforcements demand a higher level of compliance with higher expectations for postearthquake performance. The next few chapters provide an updated look at using analysis, testing, comparative experience, and combined methods to establish nonstructural seismic compliance compatible with 21st-century building code expectations.

References American Concrete Institute (ACI). (2007). “Qualification of post-installed mechanical anchors in concrete and commentary.” ACI 355.2-07, Farmington Hills, MI.

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———. (2011). “Building code requirements for structural concrete and commentary.” ACI 318-11, Farmington Hills, MI. ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, Reston, VA. Ericsson, A., and Erixon, G. (1999). Controlling design variants: Modular product platforms. Society of Manufacturing Engineers, Dearborn, MI, 1–15. Fathali, S., and Filiatrault, A. (2008). “Effect of elastomeric snubber properties on seismic response of vibration-isolated mechanical equipment: An experimental study.” Earthquake Spectra, 24(2), 387– 403. Federal Emergency Management Agency (FEMA). (2002). “Installing seismic restraints for mechanical equipment.” FEMA 412, Washington, DC. ———. (2004a). “Installing seismic restraints for electrical equipment.” FEMA 413, Washington, DC. ———. (2004b). “Installing seismic restraints for duct and pipe.” FEMA 414, Washington, DC. Gatscher, J. A., Caldwell, P. J., and Bachman, R. E. (2003). “Nonstructural seismic qualification: Development of a rational shake-table testing protocol based on model building code requirements.” Proc., ATC-29-2 Seminar on Seismic Design, Performance, and Retrofit of Nonstructural Components in Critical Facilities, Applied Technology Council, Newport Beach, CA, 63–75. Gillengerten, J. D. (2001). “Design of nonstructural systems and components.” In The seismic design handbook, 2nd Ed., F. Naeim, ed. Springer Publishing Co., New York, NY. Goodno, B. J., Gould, N. C., Caldwell, P., and Gould, P. L. (2011). “Effects of the January 2010 Haitian earthquake on selected electrical equipment.” Earthquake Spectra, 27(S1), S251–S276. International Code Council Evaluation Service (ICC ES). (2010). “Acceptance criteria for seismic certification by shake-table testing of nonstructural components.” AC156, Country Club Hills, IL. NASA technical handbook. (2000). “Force limited vibration testing.” NASA-HDBK-7004, Jet Propulsion Laboratory, Pasadena, CA. Neubert, V. H. (1987). Mechanical impedance: Modeling/analysis of structures. Jostens Printing and Publishing Co., State College, PA. Scharton, T. D. (1997). “Force limited vibration testing monograph.” NASA Reference Publication RP1403, Jet Propulsion Laboratory, Pasadena, CA.

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Chapter 6

Analytical Methods

The capability of analysis tools available to engineers today is overwhelming. From graphical applications that embed closed formula equations to sophisticated finite element analysis (FEA), many types of analytical software are easily accessible to the modern-day design professional. However, deciding on the right tool and approach for the job at hand is not always a simple task. The explosion of computer processing capability has been a mixed blessing for design engineers. Finite element models (FEMs) are routinely solved that contain hundreds of thousands of degrees of freedom (DOF) and run on modest-sized personal computers. But do bigger models equate with increased accuracy? Does solving more complex models provide any insight into operational performance? Can simple hand calculations provide adequate information for the design professional to make good decisions? Can analysis be used to qualify nonstructural systems? These questions are explored here in the context of performing seismic analysis for nonstructural equipment and distribution systems. In addition, these questions are viewed from a perspective of satisfying building code requirements (IBC and ASCE/SEI 7-10) (ICC 2011; ASCE/SEI 2010) to achieve seismic compliance. The goal of this chapter is to introduce a variety of analysis tools—in essence, to provide the design professional with a seismic analysis tool kit to draw upon. The analyses discussed are certainly not new but, in the context of their seismic application, they should help illuminate the complexities associated with nonstructural qualification. Our assumption here is that analysis application and results interpretation are more important than the specific software being used. There will be no discussion regarding which menu buttons to click for specific software programs. There are many commercially available FEA

201

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Earthquake Protection of Building Equipment and Systems

software programs capable of solving the problems at hand (Adams and Askenazi 1999). Our focus is on sorting out what method to use and what to look for during application.

6.1 Applied Seismic Analysis Seismic analysis covers a broad range of topics, and it is necessary to break down these topics into manageable subgroups such that discussion can focus on identifying which method is best suited for which problem. It is also important to identify the limitations that each method brings. Figure 6-1 displays a classical analysis tree diagram to break down the various options. In this view, the breakdown divides into two categories, depending on whether the analysis performed uses either rigid-body or flexible-body engineering mechanics. As shown, the various analysis options are not aligned with any specific qualification activity. Thus, determination of which method to apply is not intuitively obvious. Figure 6-2 shows a different view of analysis options based on the systems design framework introduced in Chapter 1. By employing the systems framework, the available analysis options can be aligned with both specific qualification requirements and with specific nonstructural elements. This is a more useful view of analysis options from a seismic qualification perspective. In this view the nonstructural system is divided into two subsystems: mechanical-related elements and active operation-related elements. The mechanical elements include force-resisting skeleton (FRS), attachments, and anchorage. The active operation element includes functional devices. Table 5-1 in Chapter 5 describes the nonstructural system elements. Analysis can be used to satisfy the code’s position retention requirement for mechanical elements, but analysis can only be used to help support dynamic testing to satisfy active operation requirements for functional devices. There are several reasons to apply seismic analysis methods on nonstructural systems. One is to somehow demonstrate compliance with respect to the position retention requirement defined in model building codes. The term “somehow” is used because analysis, by its very nature, is theoretical and involves many layers of abstraction to simulate reality. Analytical representations of real nonstructural systems must include simplified abstractions of the physical geometry and mechanical connections that define the assembly. The extent of abstraction used or the type of idealization employed can vary from simple mass-spring models, to beam-like frame models, to complex three-dimensional continuous models, or any combination of these. Figure 6-3 displays three simplified analytical models that can represent nonstructural systems. The analyst must interpret the results from any analysis and then compliance is inferred from the results data. This makes seismic analysis an implicit method for achieving compliance, not an explicit validation like that obtained when using physical testing approaches (i.e., shake-table testing). This is an important distinction and in practice makes analytical methods less useful as a qualification tool compared with physical testing. However, analysis plays a vital role in supporting product development strategies to maximize the seismic withstand resistance of nonstructural product designs.

Analytical Methods

203

Analytical Methods for Nonstructural Systems

Rigid Body Mechanics

Flexible Body Mechanics

Static

Linear

Static

Linear

Dynamic

Non-linear

Transient Full

Linear

Harmonic Response

Normal Modes (Mode Superposition)

Non-linear

Response Spectrum

Linear

Transient Response

Figure 6-1. Classical analysis tree breakdown for available methods to support seismic qualification practices.

The compliance expectations that originate from building code provisions govern what analysis can and cannot do (refer to Table 4-5 in Chapter 4). There are three code requirements that need to be addressed depending on the importance category of the nonstructural system. All nonstructural applications need to satisfy (1) position retention and (2) systems interaction avoidance requirements (excluding code exemptions); and the third requirement is intended for designated seismic systems to maintain active operation

204

Earthquake Protection of Building Equipment and Systems

Nonstructural System Mechanical Elements

Functional Interface

Active Operation p Elements

Rigid Body Statics

Anchorage

Rigid or Flexible Body Statics

Attachment

Functional Device (1)

Flexible Body Dynamics

Force Resisting Skeleton (FRS)

Functional Device (2)

Flexible Body Dynamics

Rigid or Flexible Body Statics

Attachment

Functional Device (n)

Flexible Body Dynamics

Rigid Body Statics

Anchorage

Flexible Body Statics

Position Retention Requirement

Active Operation Requirement

Analysis Used For Compliance

Analysis Used To Support Testing

Figure 6-2. Nonstructural systems block diagram showing analysis options and building code compliance requirements. following earth shaking. Operational compliance cannot be met by using analytical methods alone. Thus, for a select group of nonstructural equipment and distribution systems, analysis is no longer accepted as a sole means of demonstrating compliance. Stated simply, position retention requirements can be met using analysis, but nonstructural active operation requirements cannot. Nonetheless, analysis plays an integral role in satisfying position retention needs. It is routinely used during nonstructural product development and in support of dynamic test-

Analytical Methods

205

m k

c

(a)

(b)

(c)

Figure 6-3. Simplified analytical models to represent nonstructural equipment systems: (a) mass-spring-damper; (b) beam model; (c) shell model.

ing methods; thus, proper application of the techniques is warranted. In addition, combining analysis with testing provides a powerful technique to address qualification of physically massive and costly nonstructural systems (i.e., large-class), as described in Chapter 9. Each of the nonstructural system elements will be addressed by describing the various analytical methods that can support qualification practices. Also addressed is a perspective on failure modes of the nonstructural elements and the effects these failures have on the performance objectives cited in Table 4-4 in Chapter 4. The failure modes presented are generic in nature and not intended to be comprehensive listings. The goal is to highlight potential nonstructural weak links that should be considered during analytical investigations.

6.1.1 Anchorage Anchorage capacity is determined by applying linear rigid-body mechanics (e.g., bolt calculations) and is the most widely used method for the building design professional. It is also the simplest method to apply, since it is based on the assumption that the nonstructural item behaves as a rigid body. This assumption greatly simplifies the analysis requirements, given that the only design information needed about the nonstructural item is its mass properties and anchorage tie-down geometry. The design objective for a rigid-body analysis is to determine the appropriate size and type of anchorage that can satisfy projectspecific installation requirements. Code compliance requires the anchorage system capacity to exceed the seismic demand for each anchorage tie-down point. Figure 6-4 displays three example nonstructural items: (a) a mechanical open-air cooling tower, (b) an electromechanical medical treatment machine, and (c) an electrical unin-

206

Earthquake Protection of Building Equipment and Systems

Height = 3.45 m (11.33 ft) Width = 6.58 m (21.58 ft) Depth = 3.61 m (11.83 ft) Weight = 11,830 kg (26,080 lb)

(a)

Height = 1.83 m (6.0 ft) Width = 2.34 m (7.67 ft) Depth = 1.22 m (4.0 ft) Weight = 2,527 kg (5,570 lb)

(b)

Height = 2.08 m (6.83 ft) Width = 3.07 m (10.08 ft) Depth = 0.99 m (3.25 ft) Weight = 6,405 kg (14,120 lb)

(c) Figure 6-4. Examples of nonstructural items for case study analysis: (a) mechanical cooling tower; (b) medical treatment machine; (c) uninterrupted power supply. Note: Images not shown to scale. Sources: Mechanical cooling tower reprinted courtesy of Baltimore Aircoil Company, Baltimore, MD. Medical treatment machine reprinted courtesy of Varian Medical Systems, Palo Alto, CA. Uninterrupted power supply courtesy of APC by Schneider Electric, West Kingston, RI.

Analytical Methods

207

Anchorage Tie-down Points – Quantity (8) 1/2” Diameter Grade 5 Bolts 0.75 Diameter Clearance Hole

Y

Weight = 4,877 lb

21

2 2

20

12

26

X

Note - Dimensions are inches

Plan View Z

Z

CG

CG

92

92

56

27 30

X 20

Front View

20

13

Y 25

Side View

Figure 6-5. Typical nonstructural design information required for all nonstructural equipment applications.

terrupted power supply (UPS). Figure 6-5 contains the typical design information that is supplied to the building design professional for nonstructural systems. This information is considered the minimum acceptable design detail that should be provided for all nonstructural equipment applications. Given the limited nature of this design data, the only analysis option available to the building professional is to conduct a rigid-body static analysis to satisfy project-specific position retention requirements. Box 6-1 illustrates the techniques used to implement a rigid-body anchorage calculation for the medical treatment machine in accordance with IBC and ASCE/SEI 7-10 nonstructural seismic provisions.

208

Earthquake Protection of Building Equipment and Systems

Box 6-1. Rigid-Body Static Analysis and Anchorage Calculations

Rigid-Body Theory The analysis of rigid bodies is concerned with achieving static equilibrium. The governing equations for rigid-body analysis include

∑

F=0

∑

and

M=0

(6B1-1)

For a rigid body to be in static equilibrium, the vector sum of the external forces should be zero, and the sum of moments of the external forces about an arbitrary point should be zero (Crandall et al. 1972).

Anchorage Load Calculation Practice In practice, the definition of external forces is based on IBC and ASCE/SEI 7-10 nonstructural force provisions. The code considers many different possible sources of applied loading: dead load, earthquake load, snow load, wind load, live load, and several others. The combining of loads is governed by load combination formulas that dictate how the various loads are combined together (see Chapter 4 for details). In many nonstructural applications, the loads that govern anchorage capacity include dead load, earthquake loads, and any nonstructural operating loads (live loads). This is the load combination we will demonstrate. First we need to define the loads. The nonstructural dead load, D, is simply the operating weight, Wp, due to gravity. For this example we will assume that there are no live operating loads to consider. The earthquake load includes both vertical and horizontal components. The vertical earthquake load, EV, is defined as EV = ± 0.2 SDS Wp

(6B1-2)

where SDS ⫽ design earthquake spectral response acceleration at short period; and Wp ⫽ nonstructural operating weight. The horizontal earthquake load, EH, is the code’s lateral force requirement: EH = ± Fp = ( 0.4 SDS )

(a )

z⎞ ⎛ ⎜⎝ 1 + 2 ⎟⎠ Wp h ⎛ Rp ⎞ ⎜⎝ I ⎟⎠ p

(6B1-3)

p

where Fp ⫽ seismic design force centered at the component’s center of gravity and distributed relative to the component’s mass distribution; SDS ⫽ design earthquake spectral response acceleration at short period; ap ⫽ component amplification factor; Rp ⫽ component response modification factor; Ip ⫽ component importance factor; z ⫽ height in structure at point of attachment of component; h ⫽ average roof height of structure relative to the base elevation; and Wp ⫽ component operating weight. Additional code stipulations are defined for allowable maximum and minimum values for Fp: Fp

MAX

= 1.6 SDS I p Wp

(6B1-4)

Analytical Methods

Fp

MIN

= 0.3 SDS I p Wp

(6B1-5)

The load combination formulas provide two basic options or paths to follow: (1) allowable stress design (ASD), and (2) strength design (SD). These options relate to the failure criteria used to gauge acceptance and are applied as load scale factors. We will use both combination options for this demonstration. Figure 6B1-1 displays an irregular-shaped medical equipment item (asymmetric) with a reference coordinate system defined at the base. The applied loads are also shown in this figure, acting through the nonstructural center of gravity (CG). The nonstructural mass properties and the code’s nonstructural seismic parameters being used are also displayed. The worst-case vertical earthquake load, EV, is applied as uplift to oppose the gravity dead load (as shown in Fig. 6B1-1). This is typical for base-anchored equipment. Note that for nonstructural platforms that are anchored vertically to the build-

Mass Properties

Force Parameters

Weight = W p = 5,570 lb

a p = 1.0

X CG = 53.3 in

R p = 2.5

Y CG = 27.6 in

z/h = 1.0

Z CG = 37.2 in

Ip

= 1.5

S DS = 1.481

EV EH CG

EH D

Z Y

X

Figure 6B1-1. Asymmetric nonstructural medical equipment with reference coordinate system at base and applicable loads defined. Source: Illustration courtesy of Varian Medical Systems, Inc., Palo Alto, CA.

209

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Earthquake Protection of Building Equipment and Systems

ing, the worst-case EV load is additive with dead load. The horizontal earthquake load, EH, is applied one axis at a time with the combined vertical earthquake and dead load. Because the horizontal earthquake load requirement is bidirectional, there are several load case combinations to consider. Figure 6B1-2 displays the four load case combinations that need to be considered for this example. Also shown in Fig. 6B1-2 are the ASD and SD load factors that get applied to the four load case combinations, depending on which option is used.

ASD Approach

SD Approach

D = 0.6 (D)

D = 0.9 (D)

E V = 0.7 (E V)

E V = 1.0 (E V)

E H = 0.7 (E H)

E H = 1.0 (E H)

EV

EV

CG

CG

EH D

Z Y

D

Z Y

X

Load Case 1

X

Load Case 2

EV

EV EH

EH

CG

Y

CG

D

Z X

Load Case 3

EH

D

Z Y

X

Load Case 4

Figure 6B1-2. Possible load case combinations for medical equipment. Source: Illustration courtesy of Varian Medical Systems, Inc., Palo Alto, CA.

Analytical Methods

Figure 6B1-3 displays the equipment plan view showing the anchorage layout dimensions with center of resistance (CR, also called center of rigidity) and CG (center of gravity) projection. The anchorage layout displayed is hypothetical to illustrate the procedure using completely asymmetric geometry. Actual anchorage layout for this equipment platform is symmetrical. The horizontal earthquake force, EH, creates an overturning moment that must be reacted by a restoring moment of equal magnitude to satisfy equilibrium. The combined vertical loads (i.e., EV and D) in this example also create an overturning moment that must be restored since the plan view CG projection is not coincident with the geometric centroid of the anchorage tie-down points (CR location). Using classic statics, the external forces (D, EV, and EH) are applied at the CG and transferred to the CR location as equivalent forces and moment couples (FY, FZ, MX, MY, and MZ) as shown in Fig. 6B1-4 for load case #1. To establish static equilibrium, reaction forces must be created at the anchorage tie-down points counteracting the forces and moments at the CR. The anchorage shear loads include a direct component (from FY) shared equally by all anchors, and a twisting component (from MZ) whose action on any anchor is proportional to the distance of the anchor from the CR (d1–d4 as shown in Fig. 6B1-4). The anchorage tensile loads are calculated from the vertical force, FZ, shared equally by all anchors, and from the two restoring moments, MX and MY, whose action on any anchor is proportional to the distance of the anchor from the lines of rotation about the CR.

77.16 9.36

#1

#2

X CG eX CG

46.2

38.3

eY

CR

Y CG

Y X

#4 #3

4.44

84 Note - Dimensions are inches

Figure 6B1-3. Plan view showing hypothetical asymmetric anchorage layout (four anchors, No. 1–No. 4) with CG (center of gravity) and CR (center of resistance) locations defined. Source: Illustration courtesy of Varian Medical Systems, Inc., Palo Alto, CA.

211

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Earthquake Protection of Building Equipment and Systems

#1

#2

FY

d 1

d2

MZ

MX FZ

CR

d4

d3

Y

MY X

Center Lines of Rotation

#4

#3

Figure 6B1-4. Plan view showing resultant loading at the CR (center of resistance) location for Load Case No. 1. Source: Illustration courtesy of Varian Medical Systems, Inc., Palo Alto, CA. In the case of irregular-shaped equipment with asymmetric anchorage, determination of which anchor has the greatest load combination is not always obvious by visual inspection. Thus, each anchor point needs evaluation for all load case combinations. The anchor with maximum combined shear and tensile loads (not compression) establishes the anchor demand requirements for the nonstructural application. For this demonstration, we will assume that the medical component can be anchored using either four concrete anchor bolts or four welded connections. Table 6B1-1 lists the ASD and SD factored load combination results for the four load cases used to determine anchorage capacity.

Anchor Bolt Selection Anchor bolt selection will apply the strength design load combination results [Table 6B1-1(b)]. The project-specific installation calls for concrete wedge-type anchors approved for installation in uncracked concrete. Anchorage is into a reinforced concrete slab of 20-cm (8-in.) minimum thickness with a 3,000 psi minimum compressive strength. Anchor edge distance is not a design factor. Anchorage selection and capacity validation is not a straightforward task. The code requires anchors in concrete to be designed in accordance with Appendix D of ACI 318 (ACI 2011). There are several project-specific anchorage design criteria that need evaluation per ACI 318 requirements, the details of which are not presented here. The assumption for this demonstration is that the selected wedge anchor must have capacity to carry both shear and tension loads and the required ACI 318 design reduction considerations have been adequately addressed: VSD, shear load ⫽ 1,393 lb TSD, tensile load ⫽ 2,767 lb

Table 6B1-1. Rigid-Body Analysis Using Allowable Stress Design Factors and Strength Design Factors. a) Load Results Using Allowable Stress Design Factors Load Case No.

1 2 3 4

Allowable Stress Design Factored Loads (lb) FX FY FZ

0 0 4,158 ⫺4,158

⫺4,158 4,158 0 0

⫺1,692 ⫺1,692 ⫺1,692 ⫺1,692

Anchor 1 V T

965 965 1,238 1,238

1,638 ⫺2,876 264 ⫺1,502

ASD Reaction Loads at Anchorage (lb) Anchor 2 Anchor 3 V T V T

1,100 1,100 1,241 1,241

857 ⫺2,165 ⫺1,571 262

975 975 805 805

⫺2,205 1,981 1,019 ⫺1,243

Anchor 4 V

T

1,108 1,108 808 808

⫺1,982 1,368 ⫺1,405 791

b) Load Results Using Strength Design Factors Load Case No.

0 0 5,939 ⫺5,939

⫺5,939 5,939 0 0

⫺3,363 ⫺3,363 ⫺3,363 ⫺3,363

Anchor 1 V T

1,379 1,379 1,768 1,768

1,994 ⫺4,454 31 ⫺2,491

ASD Reaction Loads at Anchorage (lb) Anchor 2 Anchor 3 V T V T

1,571 1,571 1,773 1,773

Note: Highlighted results in bold indicate worst-case loading conditions.

858 ⫺3,459 ⫺2,610 9

1,393 1,393 1,150 1,150

⫺3,212 2,767 1,394 ⫺1,838

Anchor 4 V

T

1,583 1,583 1,155 1,155

⫺3,003 1,782 ⫺2,179 958

Analytical Methods

1 2 3 4

Strength Design Factored Loads (lb) FX FY FZ

213

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Earthquake Protection of Building Equipment and Systems

We now look for an anchor bolt manufacturer that has wedge anchors approved for installation in uncracked concrete conditions. The ICC’s Evaluation Service organization manages ES Reports that provide concrete anchor design loads in accordance with ACI 318 requirements. By inspection of anchor capacity ratings, a 13-mm (0.5-in.) diameter wedge anchor with 80-mm (3.25-in.) embedment will provide a positive safety margin to carry the combined shear and tensile loads for this application using a trilinear interaction formula: TSD VSD + ≤ 1.2 → Tallowable , SD Vallowable , SD

1393 2767 + 927 4484 39

= 1.02 < 1.2 ∴ Ok

(6B1-6)

Anchor Weld Selection This nonstructural component can also be anchored by welding the four tie-down lugs directly to embedded steel inlays in the floor. For demonstration purposes, anchor weld capacity will use the allowable stress design results [Table 6B1-1(a)]. Figure 6B15 displays the anchor lug details with weld callout. The installation specifies a fillet weld using an arc welding electrode number E60XX or better. Fillet welds fail at the minimum section, which is the throat of the weld. This is true whether the weld has parallel, transverse, or direct loading. The weld shear stress is

τ =

P 0.707 w Lw

(6B1-7)

where w ⫽ fillet weld size; and Lw ⫽ total weld length. The parallel and transverse shear stress components are

τp =

Vp 784 = 1109 psi = 0.707 w Lw 0.707 ( 0.25 ) ( 4 )

(6B1-8)

τt =

Vt 580 = = 820 psi 0.707 w Lw 0.707 ( 0.25 ) ( 4 )

(6B1-9)

Because of the compact lug dimensions, bending due from the tensile reaction load is ignored, and a direct shear stress component is calculated

τd =

1981 T = 2802 psi = 0.707 w Lw 0.707 ( 0.25 ) ( 4 )

(6B1-10)

The yield strength for electrode E60XX is 50 ksi, and the allowable shear stress for ferrous materials is approximated as

τ y ≈ 0.40 Sy = 0.40 ( 50 ) = 20 ksi

(6B1-11)

Thus, the weld safety factor is calculated as SFw =

τ tot 4731 = = 0.24 < 1.0 ∴ Ok τy 20 ( 103

)

(6B1-12)

Note that typical weld analyses use strength design procedures in conjunction with load resistance factor design (LRFD) weld resistance factors. The ASD approach was applied to demonstrate the technique.

Analytical Methods

215

V Y = 964 lb 45 °

T Z = 1981 lb Y

V X = 144 lb

Detail A1

Detail A2 X

¼

2 L 3 x 3 x 5/8 - 3 long

#1

#2

Detail A #4 #3

Figure 6B1-5. Plan view showing anchor lug detail for welding tie-down points to steel inlay and ASD reaction loads at anchor No. 3 resulting from Load Case No. 2. Source: Illustration courtesy of Varian Medical Systems, Inc., Palo Alto, CA.

Rigid-body analysis is applied as a linear static procedure and not used in conjunction with dynamic approaches. Because the nonstructural item is considered rigid, no additional design information is gained from the analysis results other than determination of reaction forces at the anchorage tie-down points.

6.1.1.1 Anchorage: Failure Mode Perspective The concrete anchor is designed to carry both tension and shear loads. Under ideal design conditions the limiting failure mode for concrete anchors is a “cone-out” of the concrete

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Earthquake Protection of Building Equipment and Systems

surrounding the anchor installation (Eligehausen et al. 2006). This failure mode is predictable under controlled test conditions. Anchorage installation variables that can modify design capacity of anchorage systems include concrete compressive strength, embedment depth, edge distance, and joint design, to name a few. Substandard concrete and poor workmanship during installation will reduce the capacity of the anchor. Likewise, a joint design that places the anchor in primary bending may also result in diminished capacity. The building code makes direct reference to ACI 318 Appendix D (ACI 2011) for concrete anchorage system design requirements. During earthquake shaking the effects of anchorage failures can be life-threatening. Injuries are caused by building elements, such as suspended ceilings and lighting fixtures falling, or by building equipment breaking free and toppling over, blocking exit paths. Tanks of hazardous materials may break loose and spill open, or natural gas leaks from failed pipe supports can ignite. Damage scenarios vary from building to building and room to room. Obviously, the failure of nonstructural anchorage systems has serious negative effects, including loss of active operation.

6.1.2 Force-Resisting Skeleton (FRS) The nonstructural FRS is the structural backbone to resist seismic loads and other environmental loads, including all operating loads. In addition, the FRS supports the various enclosed functional devices, provides structural stability, and secures to the anchorage system (with or without the use of attachments). Anchorage compliance addresses part of the question regarding meeting position retention requirements for nonstructural systems. The other part is in the capacity of the FRS to withstand seismic loading. It is not useful for the anchorage to remain intact but have the FRS break apart. Figure 6-6 displays what this might look like. Thus, position retention implies that both anchorage and the FRS can demonstrate compliance. The OEM (original equipment manufacturer, nonstructural supplier, etc.) typically provides compliance validation that the FRS can withstand seismic loading. Using analysis to demonstrate position retention compliance for the FRS is accomplished via application of flexible-body mechanics. Flexible-body analysis requires significantly more design information about the nonstructural platform. In addition to knowing the system mass properties and anchorage layout, the FRS design geometry needs definition and the mass distribution within the FRS. Nonstructural design information of this nature is typically not available for reference to the building professional and is only available to the OEM. Thus, conducting a classic flexible-body analysis is most often limited to manufacturers of nonstructural products, because detailed design information is treated as company proprietary. There are exceptions to this rule, in which case a manufacturer supplies the necessary design details to a building professional on a case-by-case basis. Flexible-body analysis is approached as either a static or dynamic analysis procedure and both methods can be conducted using linear or nonlinear solution routines. The flexiblebody linear static approach is the easiest to implement, and the nonlinear transient dynamic approach is the most difficult and compute-intensive approach. All of the flexible-

Analytical Methods

217

Figure 6-6. Nonstructural FRS position retention failure during seismic simulation shake-table test; base anchors remain intact with FRS separation.

body analysis methods can be applied using finite element-based numerical techniques. However, if a nonstructural system is represented as a single-DOF mass-spring-damper or idealized as a simple beam/frame structure (Fig. 6-3), spreadsheet tools can be used to solve the applicable equations of motion. Flexible-body analysis results yield the most design information about the nonstructural system. Natural frequencies and mode shapes can be determined, and displacements, internal forces, reaction forces, and structural stresses are readily calculated when flexible-body mechanics are applied. The assumption made for this discussion is that FEA is the numerical approach being used. The mathematics involved in applying FEA methods to seismic analysis problems are not trivial and should not be attempted by the non-initiated. Just because the tools are readily available does not mean that everybody knows how to use them. Effective use of FEA methods requires analysis experience and a background in engineering mechanics. For purposes of this chapter, even a cursory review of FEA basics is well beyond the intention of this writing. Thus, treatment and discussion on the basics of FEA or derivation of the applicable equations of motion will not be presented. For those interested in understanding the first principles of FEA, several references are noted at the end of this chapter (Bathe 1982; Cook et al. 1989; Gallagher 1975). It is assumed that the reader has a basic understanding of FEA principles. The objective here is to identify the most commonly applied analysis methods concerning nonstructural systems and to highlight the pros and cons with respect to satisfying building code requirements for seismic qualification.

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Earthquake Protection of Building Equipment and Systems

6.1.2.1 Linear Flexible-Body Static Analysis The first step in conducting a flexible-body analysis is to decide how the nonstructural design geometry will be represented. This task is called model development and, without doubt, model development is the most time-consuming part of the analysis process (at least 60–70% of the total time). There are no absolute rules when it comes to developing analytical models. It essentially comes down to the experience level of the analyst and the specifics of the design geometry. Some nonstructural systems are easier to represent using beam-type models, and others are easier to model using shells (also called plates). Some analysts are more adept at creating simplified representations, while other analysts will tend to reproduce the design geometry in greater detail. Regardless of the model type used, the goal is to assess the structural integrity of the FRS in order to validate that the design can satisfy position retention requirements. However, there are some fundamental tradeoff considerations that need to be mentioned regarding highly detailed models versus highly simplified abstractions. Large FEMs require large computer resources to solve the governing equations of motion. Even with computer resources that are capable of solving large models, the analyst can become overwhelmed with results data and not distinguish the critical results from the superfluous ones. The strong recommendation is to always be aware of model size (number of DOFs). Make a conscious effort to keep the model size to a manageable level. The definition of manageable size is a moving target, since computer resources continually improve. However, a simple rule of thumb for maximum FEM size is being able to solve the model within an 8-h time period. This allows for solving models overnight. Any model that requires more time than that to solve is very likely too large and will soon become unmanageable. This is especially true if dynamic analysis will be performed. While model size is tied to the complexity of the geometry, there is a practical limit. The point is to pay attention to model size before it becomes unmanageable. Conversely, if a model is simplified too much, there is a chance that the behavior of the FRS assembly will not be adequately captured and critical stress results might be missed. Unfortunately, there is no shortcut answer or brief presentation that can substitute for good engineering judgment. Effectual model development is tedious work. It is part science and part art form and is well beyond the constraints of this discussion to cover in any meaningful depth. Several references listed at the end of this chapter provide discussion on FEA model development and FEA best practices (Adams and Askenazi 1999; Steele 1989; Spyrakos 1994). Figure 6-7 displays three analytical models representing the three nonstructural equipment examples shown in Fig. 6-4. The example models are created with proper attention to mass and stiffness distribution, and they adequately capture the governing physics. The mechanical cooling tower is modeled using shell, beam, and link elements; the medical treatment machine is modeled using mostly beams and concentrated masses; and the uninterrupted power supply (UPS) is modeled using mostly shells with beam and mass elements. All three of these FEMs are complex 3-D assembly models composed of many parts that are joined together to form a complete nonstructural assembly. The next step is to apply the code’s force requirements using a static analysis procedure.

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219

(a)

(b) Figure 6-7. Finite element model representations (FEMs) of the nonstructural example items shown in Figure 6-4: (a) mechanical cooling tower; (b) medical treatment machine; (c) uninterrupted power supply. (Continues) Note: Images not shown to scale. Sources: Mechanical cooling tower reprinted courtesy of Baltimore Aircoil Company, Baltimore, MD. Medical treatment machine drawn using data from Varian Medical Systems, Palo Alto, CA. Uninterrupted power supply courtesy of APC by Schneider Electric, West Kingston, RI.

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Earthquake Protection of Building Equipment and Systems

(c) Figure 6-7. (Continued ) The code’s nonstructural seismic requirement is a static force magnitude that is prescribed based on geographic location, local soils, floor elevation within the building, and system classification parameters. Chapter 4 details the building code requirements for nonstructural. The force requirement becomes an inertial acceleration requirement when component weight, Wp, is factored out of the force equation (Eq. 4-6). Thus, a static analysis is conducted by applying an equivalent inertial acceleration and restraining the anchorage points. This is identical to having the building floor accelerate and applying input loading into the nonstructural system via the anchorage tie-down points. Selection of the applied acceleration magnitude is dependent on OEM needs. The typical OEM needs to determine the maximum acceleration limit that can satisfy position retention requirements. The maximum acceleration represents the FRS seismic capacity for meeting position retention requirements. Box 6-2 illustrates application of a linear static analysis (i.e., flexible body) for the electrical UPS platform using the code’s static force requirements. Box 6-2. Static Analysis Demonstration

Static Analysis Theory Linear static analysis determines a structure’s displacement (and derived quantities, such as stresses, strains, and forces) to applied loads that do not vary with time. System damping is ignored. The overall FEA equilibrium equation for linear static analysis is

[ K ] {u}

{ } + {F }

= Fa

r

(6B2-1)

Analytical Methods

where [K] ⫽ stiffness matrix; {u} ⫽ nodal displacement vector; {F a} ⫽ applied load vector; and {Fr} ⫽ reaction load vector. For the case of loading applied as inertial accelerations, the applied load vector is

{ F } = − [ M ] { a} a

(6B2-2)

where [M] ⫽ mass matrix; and {a} ⫽ acceleration vector. The resulting set of simultaneous equations is solved using various matrix solvers (Bathe 1982).

Static Analysis Practice Static analysis is perhaps the easiest and most widespread FEA technique. In the case of nonstructural systems, the user defines the applied acceleration magnitude and direction, and the solver crunches out displacements and other derived quantities. Since the applied loading is an inertial acceleration, material density properties and mass elements need to be properly defined such that the total nonstructural assembly weight is represented and accurately distributed in the FEM. The anchorage tie-down points are constrained. Since the solution is linear, the FEA results are directly proportional to the input acceleration. This is very convenient. The FEA results can be scaled accordingly to determine maximum nonstructural capacity. In practice, there are typically two types of static analyses performed: (1) to satisfy building code position retention requirements and (2) in support dynamic testing. The first type is explicitly defined by code provisions and requires simultaneous application of vertical and horizontal loads. The acceptance criterion for the first type is black and white: positive safety factors are acceptable and negative safety factors are not. The second type of static analysis is more investigative in nature and is used in conjunction with dynamic analysis techniques. The primary goal of investigative analysis is to determine the general likelihood of passing qualification testing. Static analysis in support of testing does not treat safety factors as black-and-white criteria. Engineering judgment is needed to assess design margins and verify that no FRS mechanical joints or structural members reveal grossly negative margins that could contribute to a test failure. The first type of static analysis is detailed here using the uninterrupted power supply (UPS) nonstructural example.

UPS Case Study: Building Code Position Retention The UPS model is base-restrained at the anchorage tie-down points. The applicable loads include dead load and earthquake loads (no live load). This demonstration will apply both allowable stress design and strength design methods to determine which method provides the maximum capacity to satisfy position retention requirements. Figure 6B2-1 displays the UPS with reference coordinate system and building code force parameters identified. It should be noted that the code-defined ap and Rp coefficients (refer to Table 4-8 in Chapter 4) for this UPS application could justify alternative values. The values selected here represent the most conservative option for this equipment type. The earthquake load includes both vertical and horizontal components.

221

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Earthquake Protection of Building Equipment and Systems

Force Parameters a p = 1.0 R p = 1.5 z/h = 1.0 Ip

= 1.5

S DS = 1.0

Y Z

X

Figure 6B2-1. Uninterrupted power supply (UPS) nonstructural equipment item with reference coordinate system at base and the code’s force parameters defined. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI.

The vertical earthquake load, EV, horizontal earthquake load, EH, and nonstructural dead load, D, are defined respectively as EV = ± 0.2 SDS Wp EH = ± Fp = ( 0.4 SDS )

(a )

z⎞ ⎛ ⎜⎝ 1 + 2 ⎟⎠ Wp h ⎛ Rp ⎞ ⎜⎝ I ⎟⎠ p

p

(6B2-3)

(6B2-4)

Analytical Methods

D = − Wp

(6B2-5)

where Fp ⫽ seismic design force centered at the component’s center of gravity and distributed relative to component’s mass distribution; SDS ⫽ design earthquake spectral response acceleration at short period; ap ⫽ component amplification factor per Table 4-8, Chapter 4; Rp ⫽ component response modification factor per Table 4-8, Chapter 4; Ip ⫽ component importance factor; z ⫽ height in structure at point of attachment of component; h ⫽ average roof height of structure relative to the base elevation; and Wp ⫽ component operating weight. Now if we factor out the nonstructural operating weight, Wp, from all three equations and make the appropriate force parameter substitutions in Eq. 6B2-4, we can end up with equivalent nonstructural inertial accelerations (units are g) defined in terms of SDS as AEv = ± 0.2 SDS AEh = ± ( 0.4 )

( 1.0) ⎛ 1.5 ⎞ ⎜⎝ ⎟ 1.5 ⎠

( 3.0) SDS

AD = − 1 g

(6B2-6) = ± 1.2 SDS

(6B2-7)

(6B2-8)

Leaving these accelerations in terms of SDS is useful when our goal is to determine the maximum SDS magnitude that our nonstructural system can satisfy regarding position retention requirements. Unlike the anchorage analysis performed in Box 6-1, the worst-case load combination in the vertical direction could be either opposing or additive with the dead load. This is due to the various orientations of FRS joints, which can include mechanical fasteners oriented along any axis. For purposes of this demonstration, the worst-case load combination in vertical direction is earthquake uplift acceleration opposing the dead load. This combination is applied simultaneously with the horizontal accelerations in each horizontal axis to arrive at four load cases (Fig. 6B2-2) that need consideration. The other four load cases when AEv is additive with AD are not presented here but need to be considered in the static analysis. The position retention acceptance criterion is based on satisfying either ASD or SD approaches. Either approach is accepted per code and in practice is applied as a scale factor acceleration multiplier. Table 6B2-1 lists the scale factors for both ASD and SD approaches. This demonstration evaluates a few key mechanical joints that comprise the UPS FRS. The remaining joints are assumed to be evaluated using similar techniques. Figure 6B2-3 highlights the three mechanical connections under investigation. The four load combinations are applied using either the ASD or SD load factors. However, for this analysis both ASD and SD approaches are applied. The force results, from each load case, are extracted from the FEM at each joint location. Table 6B2-2 summarizes the analysis results. The joint force results are used to calculate safety factors based on the material design allowable per stress design or strength design criterion. In stress design we

223

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Earthquake Protection of Building Equipment and Systems

AEv

AEv

AEh

AEh

AD

AD

Y

Y

X

X

Z

Z

Load Case 1

Load Case 2

AEv

AEv

AEh AD

AEh AD

Y

Y

X

X

Z

Z

Load Case 3

Load Case 4

Figure 6B2-2. Load case combinations for UPS equipment item. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI.

Table 6B2-1. ASD and SD Scale Factors Used in Static Analysis. Load Type

Dead load, AD Earthquake vertical, AEv Earthquake horizontal, AEh

ASD Scale Factor

SD Scale Factor

0.6 0.7 0.7

0.9 1.0 1.0

ASD, allowable stress design; SD, strength design.

Analytical Methods

Joint 1 ASTM A36 Threaded Rod ‡ 0.25 ” Dia With Nut

Joint 2 IFI Grade 30 Steel Rivet ‡ 0.25 ” Dia

Joint 3 SAE Grade 5 Machine Bolt ‡ 0.5 ” Dia With Nut

Figure 6B2-3. Uninterrupted power supply (UPS) nonstructural equipment item with three mechanical joints isolated for evaluation. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI. compare ASD factored working loads to the allowable loads associated with material yield strengths (shear yield and tensile yield). In strength design we compare SD factored working loads to the allowable loads associated with material ultimate strengths (ultimate shear and ultimate tensile). Safety factors are based on satisfying combined shear and tension interaction in accordance with Eq. 6B2-9. Safety factors (SFs) ⱕ 1 are considered acceptable. R 2V + R 2T ≤ 1.0

(6B2-9)

where RV ⫽ ratio of applied load to allowable load in shear; and RT ⫽ ratio of applied load to allowable load in tension.

225

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Earthquake Protection of Building Equipment and Systems

Table 6B2-2. Linear Stress Analysis. Allowable Stress Design - ASD Joint 1 (lb) SDS Max

T

V

SFa

SDS Max

T

V

SFa

SDS Max

Load Case 1

SFa

1 2 3 4 5 6 7 8 9 10

2 ⫺195 ⫺89 60

77 345 395 434

0.01 13.33 200 0.11 2.97 ⫺57 0.15 2.59 ⫺65 0.18 2.35 56 7 ⫺25 ⫺49 32 ⫺223 127

231 134 124 199 177 110 202 79 269 250

0.05 0.01 0.01 0.02 0.01 0.00 0.02 0.00 0.03 0.04

4.49 ⫺2,544 11.63 1,315 12.55 828 7.36 ⫺2,088 8.81 14.15 7.73 17.46 5.80 5.22

1,216 3,977 4,128 2,227

0.01 0.15 0.16 0.05

8.61 2.59 2.52 4.70

Load Case 2

V

1 2 3 4 5 6 7 8 9 10

⫺73 146 84 ⫺149

66 295 370 485

0.00 15.63 ⫺231 0.09 3.34 57 0.13 2.75 66 0.22 2.11 ⫺67 ⫺12 14 54 ⫺52 293 ⫺95

263 141 127 230 196 113 217 56 291 310

0.03 5.94 1,722 0.01 9.78 ⫺2,634 0.01 10.16 ⫺1,669 0.02 6.78 824 0.02 7.97 0.01 13.59 0.02 6.85 0.00 27.72 0.09 3.25 0.04 5.03

4,521 7,663 7,759 5,004

0.20 0.53 0.55 0.23

2.26 1.37 1.35 2.08

Load Case 3

T

Joint 3 (lb)

1 2 3 4 5 6 7 8 9 10

137 ⫺187 176 ⫺247

105 151 163 180

0.02 0.02 0.04 0.03

321 411 200 404 ⫺9 105 54 268 20 170 25 199 ⫺23 124 41 368 ⫺404 137 ⫺320 1,007

0.14 2.67 1,804 0.09 3.25 ⫺664 0.00 14.85 ⫺512 0.03 5.63 ⫺3,233 0.01 9.08 0.02 7.72 0.01 12.61 0.06 4.19 0.01 11.41 0.42 1.55

1,205 2,288 1,712 3,422

0.02 0.05 0.03 0.11

6.56 4.58 6.12 3.06

Load Case 4

Fastener Number

Joint 2 (lb)

1 2 3 4 5 6 7 8 9 `10

⫺208 137 ⫺181 158

120 99 52 188

0.01 8.56 ⫺352 0.02 8.04 ⫺200 0.00 19.87 10 0.04 4.90 ⫺65 ⫺25 ⫺36 27 ⫺61 475 352

0.08 3.54 ⫺2,626 0.07 3.86 ⫺656 0.01 14.12 ⫺329 0.04 5.22 1,969 0.01 8.28 0.02 7.56 0.01 10.66 0.05 4.27 0.17 2.46 0.46 1.48

2,221 1,679 2,269 2,387

0.04 0.03 0.05 0.06

4.72 6.24 4.62 3.96

7.80 6.81 5.33 5.70

440 404 110 299 188 206 142 365 156 950

Note: Bolded results indicate worst case loading conditions for each joint. a ASD safety factor based on combined tension (T) and shear (V) interaction using material yield strength allowables. Less than 1 required. b SD safety factor based on combined tension (T) and shear (V) interaction using material ultimate strength allowables. Less than 1 required.

Analytical Methods

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Strength Design - SD Joint 1 (lb)

Joint 2 (lb)

V

SFb

SDS Max

⫺1 ⫺281 ⫺127 82

111 495 566 618

0.00 0.06 0.08 0.09

18.25 4.09 3.57 3.26

⫺107 206 120 ⫺217

94 418 528 695

0.00 0.05 0.07 0.12

192 ⫺269 251 ⫺357

149 217 238 257

0.01 0.01 0.02 0.02

⫺300 194 ⫺259 222

172 140 69 270

0.01 0.01 0.00 0.02

T

Joint 3 (lb) SDS Max

V

SFb

284 ⫺81 ⫺93 80 10 ⫺36 ⫺70 45 ⫺315 183

328 191 177 283 252 157 287 114 383 355

0.04 0.01 0.01 0.02 0.01 0.00 0.01 0.00 0.03 0.03

4.86 ⫺3,672 12.55 1,817 13.52 1,144 7.97 ⫺3,042 9.52 15.25 8.35 18.73 6.27 5.62

1,584 5,511 5,728 3,058

0.01 0.10 0.11 0.03

11.16 3.11 3.05 5.78

21.58 ⫺331 4.54 82 3.78 94 2.91 ⫺96 ⫺17 19 77 ⫺76 422 ⫺134

377 202 182 330 280 162 310 80 417 447

0.02 0.01 0.01 0.02 0.01 0.00 0.02 0.00 0.08 0.03

6.37 2,421 6,613 10.52 ⫺3,825 11,119 10.94 ⫺2,424 11,254 7.27 1,119 7,280 8.56 14.63 7.36 30.15 3.48 5.37

0.15 0.40 0.41 0.17

2.58 1.59 1.57 2.41

458 286 ⫺13 77 28 35 ⫺32 58 ⫺574 ⫺456

585 577 150 382 242 284 176 527 195 1,441

0.12 0.08 0.00 0.03 0.01 0.01 0.01 0.05 0.01 0.36

2.88 2,539 3.50 ⫺1,010 16.01 ⫺771 6.08 ⫺4,677 9.81 8.33 13.64 4.51 12.34 1.67

1,866 3,434 2,596 4,986

0.02 0.04 0.02 0.08

6.63 5.15 6.81 3.54

11.77 ⫺504 10.04 ⫺285 29.47 14 6.39 ⫺93 ⫺36 ⫺52 39 ⫺88 681 505

630 577 157 428 270 295 204 522 224 1,354

0.07 0.06 0.00 0.03 0.01 0.02 0.01 0.05 0.14 0.39

3.81 ⫺3,790 4.16 ⫺998 15.19 ⫺510 5.60 2,754 8.89 8.13 11.43 4.60 2.63 1.60

3,324 2,557 3,400 3,433

0.04 0.02 0.04 0.05

5.32 6.91 5.20 4.41

9.76 9.31 6.67 7.88

T

V

SFb

SDS Max

T

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Earthquake Protection of Building Equipment and Systems

Our original goal is to determine the maximum position retention capacity of the FRS. Since the results data are based on setting SDS ⫽ 1, we need to proportion the results to find the maximum value of SDS that yields positive design margins (SF ⱕ 1). This can be directly calculated because the analysis is linear. As can be observed from Table 6B2-2, the ASD approach reveals that joint 3 is the limiting design feature. The maximum achievable value of SDS is 1.35 g using ASD. Joint 3 also sets the maximum position retention capacity of the FRS using the SD approach. The SD maximum achievable value of SDS is 1.57 g. In this case, applying strength design would be the preferred option and the maximum position retention capacity of the FRS is SDS ⫽ 1.57 g. This implies that in order to achieve a higher FRS capacity, the design of joint 3 will need a modification (e.g., larger-diameter bolt). However, a more likely scenario to improve FRS position retention capacity is to test the UPS platform. Experience has shown that a dynamic test will always achieve a greater FRS position retention capacity compared to linear static analysis. Elastic analyses are highly conservative and do not account for the inelastic resistance capacity of the FRS. Even though the code’s lateral force requirement (Eq. 6B2-4) includes a response modification parameter, Rp, this value is prescribed in Table 4-8, Chapter 4, which likely does not reflect the actual inelastic resistance capacity of specific nonstructural platforms. Chapter 10 discusses the need for calculating platform-specific Rp values. Flexible-body static analysis is a highly useful technique and becomes an enabler for nonstructural seismic qualification. In practice, this type of analysis is used more often to support dynamic testing strategies versus satisfying building code position retention requirements. However, for large-class nonstructural equipment platforms that are too massive for seismic testing, the static analysis demonstrated here can be used to achieve position retention capacity in accordance with code requirements.

6.1.2.2 FRS: Failure Mode Perspective The concept of the nonstructural FRS is about an assemblage of structural members to form a skeleton capable of resisting applied loading. The dominant failure mode for most assemblies that are intended to provide structural resistance is mechanical joint failure. Mechanical joints are the connection points between members and most often are the weak link when the skeleton is reacting inertial forces from earth shaking demands. Ductile structural members by themselves have considerable strength. However, the jointed connection between members is much more susceptible to failure. Our combined experience points to mechanical fastener failures as the primary failure mode during earthquake simulation testing. Once a key fastener fails during a shake test, the FRS typically “unzips” as other fasteners in the load path fail sequentially due to overloading. This phenomenon is especially pronounced during testing of base-anchored equipment platforms that experience large dynamic amplification effects. The walkway soffit failure discussed in Box 1-1 of Chapter 1 is a real example of the “unzipping” of wire supports during earth shaking. High fiber stress magnitudes in an individual structural member transition from elastic to plastic as load increases and results in plastic deformation—elongation but not

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necessarily separation. Large force loads in mechanical joints is a different story altogether and most often result in joint separation failure. When it comes to potential FRS failures, the focus of our attention should be evaluation of the FRS mechanical joints. Mechanical joint design is highly variable. Each joint type—bolts, rivets, screws, and welds, to name a few—has a unique separation failure mode. For example, with some threaded fasteners, there can be a shear-out failure of the female threads as the governing separation mode. The point is to concentrate effort on the joints and evaluate joint integrity and not be so concerned with fiber stresses in the members. Mechanical joints will typically fail well before a ductile structural member ever reaches the separation point. However, for some equipment types a structural member may form a plastic hinge causing an active operation failure. In this case, high fiber stress concentrations in key structural members should be evaluated in addition to mechanical joints. The effect of FRS joint separation can be catastrophic on meeting position retention requirements (as illustrated in Fig. 6-6). Joint separation can be equally damaging in meeting active operation requirements for functional devices that are contained within the FRS. When various assembly members of the FRS become separated due to joint failure, negative consequences result.

6.1.3 Functional Devices As previously stated, analysis cannot be used to validate nonstructural active operation functions. In other words, seismic qualification of functional devices cannot be accomplished using analysis exclusively. The only qualification methods that can be used to demonstrate active operation performance are dynamic testing and earthquake experience methods, which are covered in Chapters 7 and 8, respectively. The combined methods approach also can be used to qualify functional devices, as described in Chapter 9. However, analysis plays a primary role in supporting dynamic testing strategies to satisfy the code’s active operation requirements for designated seismic systems. No astute OEM would attempt dynamic testing without first performing some level of analysis on the test item. And since the functional devices are physically supported by the FRS, attempting a testing program without having any knowledge of the seismic withstand capacity of the test unit’s FRS system can turn into a very costly activity with little practical benefit. Thus, analysis performed in support of qualification testing is highly recommended. The strategy is to determine the approximate level of ground motion intensity (SDS) that a nonstructural system can realistically meet. The idea is to use analysis to gauge the platform’s seismic withstand resistance such that testing can be conducted at a demand level that is within the system’s design limits. Once this is determined, decisions can be made regarding potential design modifications to improve the platform’s withstand capacity if the ground motion magnitude does not meet a minimum threshold. Two analysis implementations are used to support dynamic testing strategies: (1) analysis performed as an integral design activity during product development and (2) analysis performed as a serial step after product development. We speculate that the majority of nonstructural OEMs implement the second approach, but the modern-day trend is to adopt the first approach as new product development projects are conceived.

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The merits of early analysis are well documented in the literature (Gelgele 2006). By adding seismic qualification as a functional design requirement, engineers can include nonstructural demand requirements as design drivers along with all other applicable environmental and operating loads. The obvious advantage of early analysis is that proposed design enhancements to improve the seismic withstand resistance can be incorporated during the development process. The cost of design changes made early during product development are significantly less (by factor of 10 or more) than making changes after the development process has finished.

6.1.3.1 Linear Flexible-Body Dynamic Analysis The analysis options used to support functional device qualification testing can vary based on platform type. Because the design goal is to improve the overall structural integrity of the platform prior to dynamic testing, the recommendation is to use a combination of both linear static and dynamic approaches. Implementation of the linear static method is demonstrated in Box 6-2. Implementation of linear dynamics requires a multistep process. The assumption made is that dynamic analysis will be performed using mode superposition FEA techniques. Mode superposition is, computationally, a highly efficient method that supports several different dynamic analyses. The method consists of transforming the multi-DOF system of differential equations into a set of independent equations, solving these independent equations, and superimposing the results to obtain the solution for the original system. Mode superposition methods are available in almost all commercial FEA software programs. The downside of mode superposition is the linear nature of the solution. Nonlinear dynamic solutions are not supported using mode superposition techniques. However, we reason that for comparative analysis purposes and to support qualification testing strategies, linear results are adequate. This last statement merits further discussion. Linear analyses are more of a necessity than a preference. The behavior of most structures and nonstructural systems under even moderate earth-shaking demands is nonlinear and, during the seismic shake-table test, nonlinear responses will occur. Thus, to evaluate nonlinear behavior of nonstructural systems, a nonlinear analysis is needed. So why do we employ linear methods to study the predominantly nonlinear behavior demonstrated by nonstructural systems under seismic loading conditions? The answer is efficiency. Linear solutions solve much faster than nonlinear solutions do. The difference is on the order of several magnitudes (10 to 100 times faster), which makes linear solutions the only realistic option when working under the normal time constraints associated with industrial product development. In addition, linear solutions are inherently conservative and are viewed as an upper bound result. So for the time being, linear dynamics is the recommended approach. The first step in dynamic analysis is to calculate the natural frequencies and mode shapes (i.e., eigenvalues and eigenvectors) by running a modal analysis (also called normal modes analysis). Once modal results are available, then a second analysis step can be completed. The second step can be a harmonic-response, response spectrum, or transient analy-

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sis (or all three). Each of these methods yields different results data and, when combined, provides in-depth insight into the dynamic characteristics of the nonstructural design.

6.1.3.1.1 Modal Analysis (Normal Modes) Determination of the fundamental vibration modes is the key result from modal analysis. The fundamental mode is the vibration mode associated with the largest percentage of participating modal mass in each orthogonal axis (Clough and Penzien 1975). The goal in nonstructural product design is to increase the fundamental modes’ natural frequency. Multiple design candidates can be compared to determine which design change increases natural frequency the most: The greater the frequency, the greater the structural stiffness (when total mass distribution is held nearly constant). Modal analysis is a good tool for comparing design changes that can increase the nonstructural seismic withstand resistance. Complex design variations can be objectively evaluated by comparing natural frequency and mode shapes across multiple design candidates. Box 6-3 illustrates application of modal analysis techniques for two design candidates of the UPS example platform. Note that modal FEA solutions are computationally expensive and will consume many hours for large models.

Box 6-3. Modal Analysis Demonstration

Modal Analysis Theory Modal analysis is a technique used to determine a structure’s vibration characteristics: (1) natural frequencies, (2) mode shapes, and (3) mode participation factors (i.e., how much a given mode participates in a given direction). The general equation of motion for a finite element model (FEM) is

[ M ]{u&&(t )} + [C ]{u& (t )} + [ K ]{u (t )} = {F (t )}

(6B3-1)

where [M] ⫽ mass matrix; [C] ⫽ damping matrix; [K] ⫽ stiffness matrix; {u(t)} ⫽ nodal displacement vector as a function of time; and {F(t)} ⫽ vector of applied loads as a function of time. Mathematically, Eq. 6B3-1 represents a series of linear differential equations of second order. If n is the number of active DOFs in the model, then all of the above matrices are of order n. Modal analysis assumes free vibrations (unforced) and ignores system damping. Thus, the general equation of motion reduces to

[ M ]{u&&(t )} + [ K ]{u(t )} = {0}

(6B3-2)

For a linear structure, the displacements due to free vibration are harmonic of the form

{u} = {φ }sin(ω t )

(6B3-3)

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where {␾ } is a vector of order n and ␻ is the natural circular frequency of vibration. Thus, with the assumption of harmonic motion, the equation of motion is transformed into the generalized eigenproblem from which φ and ω must be determined:

([ K ] − ω [ M ]){φ } = {0} 2

(6B3-4)

The eigenproblem in Eq. 6B3-4 can be solved for n eigenvalues and a unique eigenvector for each eigenvalue, yielding n eigensolutions (ω12, ␾1), (ω22, φ2), … (ωn2, φn). The square roots of the eigenvalues are ωn, the structure’s natural circular frequencies (radians/s). Natural frequencies, fn, are then calculated as fn ⫽ ωn /2π (cycles/s). The eigenvectors {φn} represent the mode shapes—the shape assumed by the structure when vibrating at frequency fn. All n modes need not be considered in the eigensolution, because it is very unlikely that all of them will contribute significantly to the response of the system. In most cases, a good approximation of the total response can be obtained by considering only a small fraction of the total number of modes available. The most commonly used approach is to choose the first p modes, where p is significantly less than n. However, deciding on the right number of p modes is not arbitrary and needs to be based on selecting enough modes to satisfy minimum mode participation requirements (90% mass participation). A useful by-product from the eigensolution is the concept of mode participation and effective modal mass. From the results of the modal analysis, a mode participation factor and modal mass can be computed for each eigenvector in each orthogonal axis. The mode participation factor may be thought of as a quantitative measure of the response of a structure at a given natural frequency. In other words, the participation factor represents how much each mode will contribute to the deflections (and derived quantities, stresses, etc.) in a particular axis. When the free vibration mode shapes, {φn}, are normalized to the mass matrix,

{Φ n }

=

{φn } {φn } [ M ] {φn } T

(6B3-5)

the mode participation factor, γn, for the nth mode is defined as

γ n = {Φ n } [ M ] {d}

(6B3-6)

where {d} is a unit vector describing the base input axis direction (Clough and Penzien 1975). The mode participation factors reveal which modes are significant for a given direction. From these participation factors the modal masses, mn, can be calculated directly as γ n2 (6B3-7) mn = T {φn } [ M ] {φn } Modal mass can be interpreted as the percentage of the total mass responding in each mode of vibration for each orthogonal axis. A summation of modal masses in each axis will indicate the total mass retained after the equations of motion are trun-

Analytical Methods

cated from n to p equations. As a general rule of thumb, at least 90% of the total mass should be retained in each axis. If p modes do not yield enough modal mass to cover 90% or more of the total mass, then modal mass can be added to the eigensolution by requesting a larger number of vibration modes (i.e., increasing p).

Modal Analysis Practice Modal analysis is the most fundamental of all the dynamic analysis types and is the first dynamic method performed. In practice, modal results are evaluated graphically using mode shape animation tools after first reviewing the mode participation table. The mode participation table will reveal two things: (1) which modes are the most significant in each axis and (2) the total amount of effective modal mass per axis based on the number of modes selected. If the total modal mass is less than 90% of the overall FEM weight, then the analysis should be rerun by requesting more modes. The downside of extracting more modes is more computer time required for the eigensolution (i.e., more equations to solve). Visualization of the animated mode shapes is a good way to determine whether the FEM is properly constructed. Modeling mistakes become very obvious when reviewing the mode shape animations. This is especially true for complex assembly FEMs that may contain hundreds of mechanical joints that attach discrete structural members together. The animation typically runs in a continuous loop with the loop speed and deflection scale factor set as user-definable parameters. If a mode animation displays assembly members flying apart or assembly members moving as rigid bodies, there are missing joint attachments. If the animation shows members rotating like linkages, there are missing joint attachments. This behavior is called an underconstrained model. Conversely, if the animation displays virtually no motion in locations where relative motion is expected, then the model could be overconstrained. In either case, the animated mode shapes provide visual evidence of modeling errors.

UPS Case Study Two UPS FRS design candidates are compared to determine which design offers better seismic withstand characteristics. Both design candidates are identical except for modifications made to parts of the FRS. Figure 6B3-1 shows the UPS I/O section assembly with the sheet metal covers removed. Figure 6B3-2 displays the two FRS design candidates with the internal subassembly contents removed for clarity. The models are base-restrained at the anchorage tie-down points. Fifteen vibration modes are requested in the modal solution. Table 6B3-1 displays the mode participation results for both candidates. Let us examine this table. The first point to notice, for both design candidates, is the total mass fraction that resulted from extracting only 15 vibration modes. In the side-to-side direction, there is a 75% mass fraction, front-to-back shows 99%, and in the vertical only 39% of the total mass is used when extracting the first 15 modes. This indicates that not enough vibration modes were selected. While the vertical direction is not a major concern for this

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Earthquake Protection of Building Equipment and Systems

Vertical

Front-to-Back

Side-to-Side

Figure 6B3-1. I/O section of UPS nonstructural equipment item with reference coordinate system at base. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI.

design, it is essential to verify that at least 90% of the total mass, in each direction, is accounted for in the modal solution. This is a fundamental requirement when conducting mode superposition dynamic analyses. In this case, it required selecting 52 total modes to achieve that goal. The modal response differences are quite evident between the two designs. In the side-to-side direction, FRS design #2 shows a fundamental mode at 3.92 Hz compared with 2.98 Hz for design #1. That is a 32% increase in natural frequency. In the front-toback direction, for both designs, the fundamental mode originates from the large

Analytical Methods

Design #1

Design #2

Added

Lengthened and Thickened

Figure 6B3-2. UPS section frame showing two design candidates for nonstructural FRS. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI.

transformer assembly that secures to the base of the FRS platform (Fig. 6B3-3). Since the transformer is secured to the FRS base, it vibrates fairly independent of the vertical FRS frame. In both designs the transformer mode is at 3.95 Hz front-to-back and around 13.6 Hz side-to-side. The two design candidates do not influence the transformer modes at all. These modes are easy to identify when viewing the mode shape animations. Essentially, the FRS frame remains mostly stationary and the transformer rocks front-to-back at 3.95 Hz and rocks side-to-side at 13.6 Hz. The first FRS frame modes in the front-to-back direction are at 5.74 Hz for design #1 and 6.15 Hz for design #2. That is a 7% increase in natural frequency. Overall, the modal analysis results objectively point toward FRS design candidate #2 as the better design to resist lateral loading. This type of comparative modal analysis can evaluate other parts of the design as well. For example, design modifications to improve the attachment of the transformer to the FRS base platform could be compared. If the analytical studies are performed during the product development process, design changes to improve seismic withstand resistance can be incorporated with minimal manufacturing cost implications. Making these types of design changes after product development activities are completed will cost considerably more.

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Table 6B3-1. Mode Participation Table. Side-to-Side Direction

Mode No.

Natural Frequency (Hz)

Modal Mass (lb)

Front-to-Back Direction

Vertical Direction

Total Mass Fraction (%)

Modal Mass (lb)

Total Mass Fraction (%)

Modal Mass (lb)

Total Mass Fraction (%)

17.9% 17.9% 17.9% 18.9% 19.1% 19.1% 19.1% 19.4% 60.6% 72.4% 73.2% 73.3% 74.0% 74.7% 74.7%

0.7 3,791.7 923.6 236.0 174.5 0.4 0.6 1,450.2 34.3 24.4 13.5 1.0 74.9 23.5 0.1

0.0% 55.4% 68.9% 72.3% 74.9% 74.9% 74.9% 96.1% 96.6% 96.9% 97.1% 97.1% 98.2% 98.6% 98.6%

0.3 798.7 13.1 82.6 159.1 5.8 12.1 1,064.4 92.0 87.4 146.6 21.0 198.4 8.5 0.3

0.0% 11.7% 11.9% 13.1% 15.4% 15.5% 15.7% 31.2% 32.5% 33.8% 36.0% 36.3% 39.2% 39.3% 39.3%

17.5% 17.8% 17.9% 18.9% 18.9% 18.9% 18.9% 19.2% 61.1% 72.0% 73.3% 73.4% 73.6% 74.5% 74.5%

89.3 3,663.5 1,028.6 178.3 6.9 4.5 943.0 687.2 30.1 24.9 26.6 2.3 46.1 31.5 0.1

1.3% 54.7% 69.7% 72.3% 72.4% 72.5% 86.2% 96.2% 96.7% 97.0% 97.4% 97.4% 98.1% 98.6% 98.6%

20.7 782.4 15.7 87.1 16.3 23.1 722.9 464.4 81.1 77.9 188.9 28.6 157.7 21.8 0.3

0.3% 11.7% 11.9% 13.2% 13.4% 13.8% 24.3% 31.1% 32.3% 33.4% 36.2% 36.6% 38.9% 39.2% 39.2%

FRS Design Candidate #1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

2.98 3.95 5.74 10.86 11.76 11.88 11.89 12.25 13.57 14.20 14.49 15.30 15.35 15.84 16.46

1,226.7 0.1 0.2 64.3 15.6 2.7 0.0 15.5 2,824.6 804.9 59.6 4.6 46.3 47.6 0.0

FRS Design Candidate #2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

3.92 3.95 6.15 11.22 11.89 11.89 12.10 12.57 13.62 14.23 14.55 15.30 15.61 15.92 16.46

1,202.5 19.3 8.6 65.6 1.2 0.0 0.2 18.8 2,875.4 748.1 86.0 6.3 17.8 59.9 0.0

Note: Bolded results are the FRS fundamental vibration modes in the horizontal directions.

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237

Transformer Assembly

Vertical

Front-to-Back

Side-to-Side

Base-mounted (4) Locations

Figure 6B3-3. I/O section of UPS showing large transformer assembly directly bolted to base of FRS. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI. Modal analysis results do not directly reveal structural displacements, internal forces, and stresses. Modal analysis can be thought of as a global indicator of structural stiffness. Finite element models composed of 3-D elements will reveal a fundamental vibration mode contribution in each orthogonal axis. These global modes are dominated by large mass participation. In addition to the global modes, local modes will also be revealed. Local modes are much less significant to the global dynamic behavior but do contribute to localized dynamic responses. An example of a local mode is when an internal subassembly vibrates fairly independent of the overall FRS structural assembly. One of the limitations of modal results data is the inability to assess localized dynamic response. For example, if a certain location within the FRS is designed to enclose a critical functional device, it is often difficult to assess how effective one packaging design candidate is over another when evaluating modal results data. Another key limitation of modal analysis is the inability to assess modal damping characteristics, since system damping is ignored.

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Earthquake Protection of Building Equipment and Systems

A complementary analysis tool to gather localized response data is to perform a harmonic-response analysis. In addition, a harmonic analysis can be used to correlate the FEM to experimental data by tuning the model to match modal properties derived using experimental modal analysis techniques (see Chapter 7 for experimental modal testing).

6.1.3.1.2 Harmonic-Response Analysis Harmonic-response analysis is a technique to determine the steady-state vibration response to sinusoidal (harmonic) loads of known frequency and amplitude. It is very similar to the sine-sweep test method. In essence, the analyst can select specific locations on the FEM to monitor, analogous to placing a response accelerometer on a piece of hardware during a sine-sweep test. The primary result from performing harmonic analysis is the frequency response plot. A frequency response plot is comparable to the transmissibility plot obtained during a sine-sweep test. Figure 6-8 compares the two function plots. Both frequency response and transmissibility plots reveal occurrences of vibration resonance (natural frequency) and associated dynamic amplification characteristics (Q, quality of resonance) across the frequency range of interest for any FEM locations. In this

(a) Figure 6-8. Comparison of frequency response plot from a harmonic-response analysis to a transmissibility plot from a sine-sweep test: (a) analytical frequency response plot; (b) experimental transmissibility plot. (Continues)

Analytical Methods

Sweep Number: 1.00 Sweep Rate: Variable Compression: Variable

Elapsed Time: 000:02:31 Filter Type: Proportional Fundamental: 80.000%, BB RMS: 398. mcyc

239

Remaining Time: 000:00:00 Test Range: 1.000, 33.000 Hz Points Per Sweep: 1000

ACP: 1

(b)

FRONT/BACK RESONANCE SEARCH

Figure 6-8. Comparison of frequency response plot from a harmonic-response analysis to a transmissibility plot from a sine-sweep test: (a) analytical frequency response plot; (b) experimental transmissibility plot. (Continued) regard, harmonic analysis can be thought of as a local indicator of dynamic response. The resonant frequencies used in the harmonic solution are those previously calculated during a modal analysis (i.e., mode superposition). The amount of dynamic amplification (Q factor) at resonance is controlled by the amount of system damping defined. Damping can be defined in several ways. Most often, damping is defined as a constant damping ratio. However, damping can be defined on a modal basis. In essence, each vibration mode can be assigned a unique modal damping ratio. This technique is highly useful when tuning a model to correlate with experimental modal survey data. The topic of model validation is discussed at the end of this chapter along with an example problem using harmonic response to tune an FEM (see Box 6-6). It should be noted that both harmonic analysis and sine-sweep test are pure vibration assessments (not transient shock). The response characteristics reflect maximum response to steady-state vibration inputs. However, if the sole purpose is to compare (i.e., tune) a nonstructural FEM to match sine-sweep test data, the assumption of steady-state harmonic response is valid. In addition, harmonic analysis can be used to compare competing design candidates. Since the harmonic analysis is based on steady-state vibration theory,

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Earthquake Protection of Building Equipment and Systems

the harmonic solution times are negligible compared to the total time in solving the modal analysis problem. This makes the harmonic-response analysis essentially a free analysis from a time/cost perspective. When comparing competing design candidates, multiple locations within the FRS can be evaluated for dynamic response. For example, if two design changes are proposed to package a new functional device into the system, the harmonic analysis results can objectively reveal the better design. The design that exhibits a higher resonant frequency with an equal or lower Q factor, at the same location within the platform, is the better design from a dynamics perspective. In comparative analysis, the combination of evaluating global modal results with the more localized harmonic-response results provides good qualitative understanding of the dynamic behavior for competing designs or for a single nonstructural test item. These two methods should be used in conjunction to eliminate weaker designs when comparing design alternatives to resist seismic loading. The combination of these methods can also be used to correlate FEMs to experimental data (i.e., model validation). However, these methods do not provide much quantitative information when assessing the likelihood of passing qualification testing—in other words, to answer the question, Will this nonstructural platform survive seismic qualification testing? To adequately address this question, we need to introduce two more dynamic analysis approaches, namely, response spectrum and transient analyses.

6.1.3.1.3 Response Spectrum Analysis A response spectrum analysis uses the modal results to arrive at a cumulative dynamic response (by combining modes) based on a given input spectrum. In spectrum analysis the focus is to get the maximum response quickly, and the relative phasing of the vibration modes is lost. The modal responses are combined using algebraic expressions such as square-root-of-sum-of-squares (SRSS). This sums the squares of the individual spectral results and then takes the square root of the result. The mode combination method widely used in structural engineering is the complete-quadratic-combination (CQC) method (Wilson et al. 1981). The CQC method combines the spectral results using system damping and a weighted ratio of relative frequencies and is more accurate when vibration modes are closely spaced together. However, no matter what combination method is used, the fact remains that vibration modes are treated as independent contributors to the total response and are combined in some mathematical manner. The primary limitation of spectrum analysis is the linear combining of modal results. Dynamic testing of equipment is an inherently nonlinear activity. Almost all nonstructural equipment will experience nonlinear response during the qualification shake-table test. Thus, careful evaluation of spectrum analysis results is required. Large stress magnitudes in structural members do not necessarily translate into structural failures during dynamic testing. Sound engineering judgment is needed to look beyond the linear member stresses and to assess the adequacy of the equipment’s structural joints and fasteners. The most critical results from spectral analysis are force loads acting on the mechanical joints that comprise the equipment’s FRS. However, evaluation of joint loads becomes

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241

problematic when using the various mode combination methods (i.e., SRSS or CQC). The act of mathematical combination removes the sign convention of the result, thus leaving only a magnitude for the final answer. The practical result is that each mode combination method reports the magnitude of the force at each mechanical joint but not the sign of the force (e.g., compression versus tension). By treating everything as magnitude only, the resultant joint loads are unrealistically high and most often too conservative for predictive purposes (i.e., quantitative analysis). Box 6-4 demonstrates a response spectrum analysis for a beam representation of the medical treatment machine and highlights potential problem areas to be aware of when using this method.

Box 6-4. Response Spectrum Analysis Demonstration

Response Spectrum Theory Spectrum analysis computes the maximum response of a structure to a given input spectrum at each natural mode of vibration. The vibration modes used are the ones previously calculated by a modal analysis of the same structure. The maximum modal response is computed as a scale factor mode shape. These individual maximum responses are then combined to give a total response of the structure. For each vibration mode of the structure, a mode participation factor is calculated in the direction of applied excitation. The participation factor is a function of the mode shape and the excitation direction. This is a measure of how much a mode will contribute to the deflections (and derived quantities, such as stresses or forces) in the direction of applied loading. Box 6-3 presented the governing equations for free harmonic vibration. The mode participation factor, γn, for the nth mode is defined as

γ n = {Φ n } [ M ] {d}

(6B4-1)

where {⌽n} ⫽ mass normalized mode shapes; [M] ⫽ mass matrix; and {d} ⫽ unit vector describing the base input axis direction. The mode coefficient, An, is the scale factor used to multiply the mode shapes to get the maximum response for each mode. The mode coefficient for the nth mode, computed for base acceleration input, is defined as An =

Sa n γ n ω n2

(6B4-2)

where San ⫽ spectral acceleration for the nth mode obtained from the input acceleration response spectrum at frequency ωn and effective damping ratio ξn ; γn ⫽ mode participation factor for the nth mode; and ωn ⫽ nth natural circular frequency. The scaled mode shapes can now be directly computed from the eigensolution by using the mode coefficient scale factors. Once the maximum response at each mode is known for the given response spectrum, these modes are combined using a variety of methods to get the total response

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Earthquake Protection of Building Equipment and Systems

of the structure. The modal results may be combined using various algebraic algorithms to obtain the total response (e.g., SRSS, CQC, and many other mode combination methods).

Response Spectrum Practice In order to run a spectrum analysis, a response spectrum input curve must be defined. The input should be the required response spectrum (RRS) that will be used during seismic qualification testing. The IBC and ASCE/SEI 7-10 nonstructural dynamic requirement must use an interpreted response spectrum as defined in AC156 (ICC ES 2010). Figure 6B4-1 displays the input spectrum used for this demonstration. As observed, this spectrum is applied in two horizontal and vertical directions (three-axis input). A previous modal analysis is required in order to access the eigensolution (i.e., natural frequencies and mode shapes). The calculation of mode coefficients requires minimal computer time. The mathematical combing of scaled modes will take additional time, especially if the CQC combination method is used. However, total time for the mode combination calculation is relatively small compared with the time required to run the original modal analysis. This is one of the advantages with spectrum analysis—it is computationally efficient.

Figure 6B4-1. Nonstructural required response spectrum (RRS) requirement for dynamic analysis applications based on AC156 formulation.

Analytical Methods

The widely accepted standard procedure for spectrum analysis is to run a modal analysis, input the RRS, specify system damping, select the mode combination method, and review the combined results. In other words, click a few menu buttons, enter some data, and voilà—the output is a nicely shaded contour plot of our structure showing displacements, stresses, or whatever output we are interested in. But what is the physical meaning of this output? There is value in following the standard procedure, but there is more value in understanding how each mode contributes to the total response before combining the modes mathematically. The dynamic behavior of our structure is contained in the individual modes. The process of combining modes can often obscure the system dynamics. Figure 6B4-2 displays the medical treatment machine and a concentrated mass and beam model used to represent it. For demonstration purposes, a hypothetical mechanical joint is inserted that attaches the overhead gantry to the vertical support structure that needs investigation. The goal for this demonstration is to determine the tension and shear loads acting on this joint from a response spectrum input applied at the base. Figure 6B4-3 shows the hypothetical joint design configuration to be evaluated. The spectrum analysis will apply three directions of input (see RRS in Fig. 6B4-1) solved independently: two horizontal (x- and y-axis) and one vertical (z-axis). Table 6B4-1(a) lists the mechanical joint load results for the three input directions for the first 12 vibration modes. Let us review these results and see if the dynamics make sense. First, we can immediately determine which modes are relevant for a given direction. In the horizontal x-axis direction, there are only two modes that participate, modes 1 and 3. The input in this direction forces the gantry to move up and down. Thus, bolts 1 and 2 are in compression, and bolts 3 and 4 are in tension. In the vertical direction (z-axis input), there is a similar result, with up-and-down gantry motion at the same modes. The joint loads are less in vertical because the RRS input is less. In the lateral direction (y-axis input), there are several modes that participate, with modes 2 and 4 being the most significant. This input direction forces the gantry to move sideto-side, placing bolts 1 and 3 in compression and bolts 2 and 4 in tension. Table 6B4-1(b) lists the combined results when the three input spectra are applied simultaneously. This is similar to a triaxial shake test. What can be discerned from the combined results data is that the joint design reacts both lateral and vertical input motion by having two bolts act together and the opposite pair oppose. Now if we look at the SRSS magnitudes shown at the bottom of the table, there is definite conservatism built into the combined results. Both compression and tension resultants are treated equally and simply add into the SRSS calculation. When a conservative answer is needed, the combined SRSS magnitudes can be used as upper limit maximums to size bolt diameters. In this example, the worst-case SRSS maximums (T ⫽ 2,476 lb and V ⫽ 1,689 lb for bolts 3 and 4) would require 0.375-in.-diameter bolts. However, in many situations, these upper limit maximums are too conservative for design purposes. If a transient analysis will not be conducted, then a better option is to apply equivalent static accelerations using the maximum RRS acceleration levels for static analysis inputs. For this example we would apply simultaneous x-y-z static accelerations of 1.9 g, 1.9 g, and 0.79 g, respectively (see Fig. 6B4-1). The worst-case bolt loading using this

243

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Earthquake Protection of Building Equipment and Systems

Overhead Gantry

Height = 72 in Width = 92 in Depth = 48 in Weight = 5,570 lb

Z (a)

Y

X

Hypothetical Joint

Z (b)

Y

X

Figure 6B4-2. Medical treatment nonstructural example: (a) isometric view; (b) concentrated mass and beam finite element model representation. Source: Isometric view reproduced courtesy of Varian Medical Systems Inc., Palo Alto, CA.

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245

Bolt 1

Bolt 2

Bolt 3 Bolt 4

Joint Fasteners Quantity (4) Z Y

X

Figure 6B4-3. Isometric view of mechanical joint to be evaluated using response spectrum analysis; this joint is a hypothetical design solely intended to demonstrate analytical techniques. static approach would be T ⫽ 2,231 lb and V ⫽ 806 lb, which requires a 0.25-in.-diameter bolt. The difference between 0.375- and 0.25-in. hardware is significant, especially for complex electromechanical systems in which available packaging real estate comes at a premium. The equivalent static approach assumes maximum dynamic response and is considered conservative, without the mode combination problem inherent with spectral analysis.

Engineers and analysts should clearly understand that the response spectrum method is an approximate method used to estimate maximum peak values of displacements, forces, and stresses, and it has significant limitations (Wilson 2002). While the method is computationally efficient and widely used in practice, a more reliable method to use for quantitative evaluation is transient analysis.

6.1.3.1.4 Transient Analysis The second dynamic approach that can be used to assess the adequacy of the test item to pass seismic testing is transient analysis. Unlike other dynamic methods that are calculated in the frequency domain, the transient method is calculated in the time domain. Thus, performing a transient analysis requires input of a predefined time-history record [i.e., accelerogram (acceleration versus time)]. The analyst must specify the accelerogram input, and then the time-history response of the FEM is calculated.

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Table 6B4-1. Bolt Loads. a) Joint Load Results for Independent Spectrum Inputs Horizontal Direction, X-Axis Input

Mode No. 1 2 3 4 5 6 7 8 9 10 11 12

Bolt 1 (lb)

Frequency (Hz) 3.3 3.7 6.2 8.2 9.6 14.1 23.2 24.7 28.4 28.9 36.4 41.2 SRSSa

T

Bolt 2 (lb) V

-863 0 664 0 0 2 0 0 11 ⫺1 0 0 1,089

48 0 5 0 0 5 0 0 6 1 0 0 49

T -863 0 664 0 0 2 0 0 11 ⫺1 0 0 1,089

Bolt 3 (lb)

Bolt 4 (lb)

V

T

V

48 0 5 0 0 5 0 0 6 1 0 0 49

874 0 ⫺731 0 0 ⫺14 0 0 ⫺2 ⫺2 0 0 1,139

68 0 8 0 0 7 0 0 8 1 0 0 69

T

V

874 0 ⫺731 0 0 ⫺14 0 0 ⫺2 ⫺2 0 0 1,139

68 0 8 0 0 7 0 0 8 1 0 0 69

b) Joint Load Results for Simultaneous Spectrum Inputs X-Y-Z Tri-axial Input

Mode No. 1 2 3 4 5 6 7 8 9 10 11 12

a

Frequency (Hz) 3.3 3.7 6.2 8.2 9.6 14.1 23.2 24.7 28.4 28.9 36.4 41.2 SRSSa SFb

Bolt 1 (lb) T

Bolt 2 (lb) V

⫺1,341 ⫺2,007 446 609 183 0 ⫺1 85 19 5 ⫺34 ⫺45 2,538

75 1,178 3 500 110 1 0 87 10 4 4 3 1,290 0.31

T

Bolt 3 (lb) V

⫺1,341 2,007 446 ⫺609 ⫺183 0 ⫺1 ⫺85 19 5 34 45 2,538

75 1,178 3 500 110 1 0 87 10 4 4 3 1,290 0.31

T 1,359 ⫺1,915 ⫺491 581 174 ⫺2 0 81 ⫺3 10 ⫺32 ⫺43 2,476 0.36

Bolt 4 (lb) V

T

V

106 1,576 5 578 141 1 0 68 14 5 4 5 1,689

1,359 1,915 ⫺491 ⫺581 ⫺174 ⫺2 0 ⫺81 ⫺3 10 32 43 2,476

106 1,576 5 578 141 1 0 68 14 5 4 5 1,689 0.36

Square root of sum of squares. Strength design safety factor based on combined tension (T) and shear (V) interaction using 0.375-in.-dia. ASTM A307 bolt. Result ⬍1 required. b

Analytical Methods

Horizontal Direction, Y-Axis Input Bolt 1 (lb) T 0 ⫺2,007 0 609 183 0 0 85 0 0 ⫺34 ⫺45 2,108

Bolt 2 (lb)

V

T

0 1,178 0 500 110 0 0 87 0 0 4 3 1,288

0 2,007 0 ⫺609 ⫺183 0 0 ⫺85 0 0 34 45 2,108

V

Bolt 3 (lb) T

0 0 1,178 ⫺1,915 0 0 500 581 110 174 0 0 0 0 87 81 0 0 0 0 4 ⫺32 3 ⫺43 1,288 2,011

Vertical Direction, Z-Axis Input

Bolt 4 (lb) V

0 1,576 0 578 141 0 0 68 0 0 4 5 1,685

T 0 1,915 0 ⫺581 ⫺174 0 0 ⫺81 0 0 32 43 2,011

247

Bolt 1 (lb) V

0 1,576 0 578 141 0 0 68 0 0 4 5 1,685

Bolt 2 (lb)

Bolt 3 (lb)

Bolt 4 (lb)

T

V

T

V

T

V

T

V

⫺479 0 ⫺218 0 0 ⫺2 ⫺1 0 8 6 0 0 526

27 0 2 0 0 4 0 0 4 5 0 0 28

⫺479 0 ⫺218 0 0 ⫺2 ⫺1 0 8 6 0 0 526

27 0 2 0 0 4 0 0 4 5 0 0 28

485 0 240 0 0 12 0 0 ⫺1 12 0 0 541

38 0 3 0 0 6 0 0 6 6 0 0 39

485 0 240 0 0 12 0 0 ⫺1 12 0 0 541

38 0 3 0 0 6 0 0 6 6 0 0 39

At first glance, the transient method seems most similar to the actual qualification test. During a seismic qualification test the test item is subjected to a time-history excitation and the dynamic response is monitored using accelerometers. If the actual accelerogram record that will be used by the test laboratory can be obtained a priori, then transient analysis is an excellent option. A more likely scenario is that the test lab uses a randombased, multifrequency input. This implies there would be infinite possible records to consider. The accelerogram record used for analysis may not match the actual input record used during testing. However, even if the actual test accelerogram does not exactly match the analysis accelerogram, transient results provide good quantitative insight into the range of dynamic response likely to occur over the course of a 30-s shake test. In addition, transient results do not suffer from the mode combination problem as experienced in response spectrum results. Thus, mechanical joint results will be more representative. The major downside of transient analysis is computation time. Transient analysis is by far the most time-consuming analysis option of any linear FEA solution routine. This fact has likely discouraged many analysts from conducting transient solutions on nonstructural systems. A typical 30-s shake test, sampled at 200 Hz, results in 6,000 time steps used to define the accelerogram input. Now add to that a solution time increment of around 0.0015 s (see Box 6-5 for explanation), which results in approximately 20,000 solution steps. That quantity of solution steps takes time to solve and the results data can consume volumes of computer disk space in a hurry for large models.

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However, if only displacement results are calculated (i.e., reduced analysis, no stresses or nodal forces calculated) the transient analysis can be manageable, provided the solution time penalty is acceptable. Attention to model size is very important when performing transient analysis. Keeping the model size to a minimum is the goal. While the overall effort to run a transient is greater than the other FEA methods, the quantitative design information gained is well worth it. Box 6-5 illustrates application of the transient analysis approach for the UPS case study.

Box 6-5. Transient Analysis Demonstration

Transient Analysis Theory Transient analysis is a time integration technique to determine the dynamic response of a nonstructural system to arbitrary time-varying loads such as an earthquake or, more specifically, to a seismic test input motion. The input is an accelerogram (acceleration as a function of time) and the output is time-varying displacements and other derived quantities (such as stresses and strains, forces, etc.). The basic equation of motion being solved in a transient analysis is

[ M ]{u&&(t )} + [C ]{u& (t )} + [ K ]{u (t )} = {F (t )}

(6B5-1)

where [M] ⫽ mass matrix; [C] ⫽ damping matrix; [K] ⫽ stiffness matrix; {u(t)} ⫽ nodal displacement vector as a function of time; and {F(t)} ⫽ vector of applied loads as a function of time. However, we are interested in using the mode superposition approach. The mode superposition method uses the natural frequencies and mode shapes of a linear structure to predict the response to transient forcing functions (Bathe 1982). The basic equation of motion can be transformed by using the matrix of mass-normalized eigenvectors from a previous modal analysis:

{u (t )} = [ Φ ]{y (t )}

(6B5-2)

where [Φ] ⫽ mass-normalized eigenvectors matrix; and {y(t)} ⫽ modal generalized displacement vector. Substituting for {u(t)} in Eq. 6B5-1 and premultiplying by [Φ]T, the equilibrium equation corresponding to modal generalized displacements is

{y&&(t )} +

⎡⎣ 2 ω ξ ⎤⎦{ y& ( t )} + ⎡⎣ Ω2 ⎤⎦{ y ( t )} = [ Φ ]

T

{F(t )}

(6B5-3)

where [Φ]T [K] [Φ] ⫽ [Ω2] ⫽ diagonal eigenvalue matrix; [Φ]T [M] [Φ] ⫽ [I] ⫽ massorthonormal identity matrix; and [Φ]T [C] [Φ] ⫽ [2 ω ξ] ⫽ diagonal modal damping matrix. We have transformed the n-DOF equilibrium equation into a p-DOF modal equation, where p is the number of modes requested during the modal analysis. For the case of loading applied as inertial accelerations, the applied load vector is

{F (t )}

= − [ M ] { a ( t )}

(6B5-4)

Analytical Methods

where {a(t)}⫽ acceleration vector as function of time (accelerogram). The generalized modal displacement Eq. 6B5-3 represents a set of p uncoupled equations for modal coordinates, y, as a function of time. This results in p individual equations of the form y&& ( t ) + 2 ω ξ y& ( t ) + ω 2 y ( t ) = − m a ( t )

(6B5-5)

Several forms of equivalent viscous damping, ξ, can be defined, including proportional mass and stiffness damping, constant damping ratio, and modal damping ratios (or any combination). These uncoupled differential equations are integrated directly by using numerical time integration techniques (e.g., Newmark).

Transient Analysis Practice The quantity of output data that can be produced during a transient analysis is intimidating. At every solution step over the course of the transient, displacements and the other derived outputs can be made available to reference. Deciding on what to look at within this massive amount of results data is absolutely critical. The strategy for this demonstration is to run a reduced transient (i.e., only displacements calculated), evaluate the displacement results at carefully selected model locations, and finally expand the most critical time steps for the full model to extract out the mechanical joint forces. While it is possible to expand the derived outputs (like forces and stresses) for the entire transient, this would consume large volumes of computer disk space and add many hours to the solution time for moderate-sized models. There is no need for that if the selected locations are meaningful. Three issues need consideration to conduct efficient transient analyses: (1) input accelerogram selection, (2) solution time increment, and (3) results data processing.

Input Accelerogram The assumption made is that the input accelerogram will represent the seismic qualification test input as best possible. The test lab works with a required response spectrum (RRS) and develops a shake-table drive signal that becomes the shake-table input accelerogram. The first option is to use the shake-table accelerogram the test lab will use during qualification testing. This can be requested directly from a test laboratory. Once the lab is provided with a qualification RRS, the lab can provide its shake-table accelerogram input. However, the test lab will likely provide the time-history record that is recorded from accelerometers located on the shake-table platform during a shake. This accelerogram may contain high-frequency content originating from table banging, chatter, and other mechanical-induced “table noise.” These raw data need to be filtered to remove any frequency content above 100 Hz. The filtered accelerogram needs to be sampled at a reasonable rate for analysis purposes. A sampling rate of 200 Hz (or every 0.005 s) is more than adequate and provides an accelerogram that contains 6,000 time steps for a 30-s shake-table test duration.

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If obtaining the actual shake-table accelerogram is not possible, the next best approach is to artificially create the accelerogram using mathematical algorithms. By definition, there is an infinite number of accelerograms that can satisfy a given RRS (see Chapter 7 for discussion on this). We need to extract out one or two of the possible accelerograms to use with the transient analysis. There are a number of ways to create an accelerogram that can satisfy a given RRS. Some methods use random multifrequency inputs and others use wavelets or sinusoids. The easiest solution is to use a method that has already been validated as opposed to creating a method from scratch: No reason to reinvent the wheel. Several methods published in the literature can do exactly what we need (Gasparini and Vanmarcke 1976; Unruh 1982; Halldorsson et al. 2002; Irvine 2005). They vary in sophistication. Some methods optimize the accelerogram by performing checks (displacement and velocity) to create signals that are compatible with real-world shake-table machines. Other methods create accelerograms that satisfy the RRS but may not be reproducible using commercial electrohydraulic shake-table machines. The recommended approach is to use two different sets of accelerograms for the transient analysis. Use one accelerogram set provided from a test lab and one set that is artificially created. If a test lab accelerogram set is not available, then use two different artificially created sets. The term set implies three independent accelerograms (phase incoherent), one for each orthogonal axis (two horizontal and one vertical). We will use one test lab accelerogram set and one artificially created accelerogram set for purposes of this demonstration. Figures 6B5-1 through 6B5-4 display the qualification RRS, TRS calculations, and resulting accelerogram sets, respectively. Both the horizontal and vertical RRS are defined with 5% damping and the two accelerogram sets are defined with a 0.005-s time increment for 30-s duration.

Solution Time Increment The mode superposition method greatly reduces the number of differential equations needing integration to solve the equations of motion, but there is an important concept that needs to be explained regarding integration time step size. The integration time step size should be small enough to adequately capture the maximum response frequency of interest in the nonstructural system. A previous modal analysis provided the natural frequencies of the system. Selecting the maximum frequency that has appreciable participating mass is adequate. A more conservative method for selecting the maximum response frequency is to use the RRS zero-period acceleration frequency as the maximum. For this demonstration, the RRS zero-period acceleration is 33 Hz and we will use this frequency as the maximum response frequency. The general rule of thumb for sizing the integration time step when conducting a linear transient is to use 20 time points per cycle at the maximum response frequency 1 (i.e., Δ t = ). For this example, the time step is 0.0015 s, which will capture 20 fn dynamic response out to 33 Hz. Thus, we have our accelerograms with a time increment of 0.005 s and an integration time step of 0.0015 s. Now for the total 30-s transient

Analytical Methods

(a)

(b) Figure 6B5-1. UPS response spectra data set 1: (a) RRS requirement for dynamic analysis based on AC156 formulation; (b) test lab shake-table data showing TRS envelope of RRS target.

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(a)

(b) Figure 6B5-2. UPS accelerograms from test lab data set 1: (a) horizontal x-axis input; (b) vertical y-axis input; (c) horizontal z-axis input. (Continues)

Analytical Methods

(c) Figure 6B5-2. (Continued)

duration we have approximately 20,000 integration time steps to solve for. As previously stated, obviously the transient analysis will take much longer to solve compared to the harmonic and response spectrum solutions. And consequently there are literally 20,000 more data sets to query during postprocessing of results. Model size (number of active DOFs) is of much greater concern when performing transient analysis. Large models will become unmanageable when running a long transient (i.e., 30 s). Model size needs to be minimized when transients are being solved. If the model takes longer than 6 to 8 h to solve for the original modal analysis, then the model is likely too large for practical transient solutions. One of the factors that influence mode superposition transient solve time is the total number of modes requested in the original modal analysis. The more modes requested, the more differential equations to be solved and the more time it takes to complete the transient.

Results Processing The prospect of sorting through 20,000 solution sets is not very inviting, and this is compounded by solving multiple sets of 30-s transients. We have three independent accelerograms to work with, two horizontal and one vertical. Actually, we have two sets of independent accelerograms. We will apply each set as triaxial base input (x-y-z accelerograms applied simultaneously).

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(a)

(b) Figure 6B5-3. UPS response spectra data set 2: (a) RRS requirement for dynamic analysis based on AC156 formulation; (b) artificial data showing TRS envelope of RRS target.

Figure 6B5-5 displays the UPS nonstructural example with axis orientation labels defined. Also shown in this figure are the equipment locations that will be monitored during the transient runs. The only way to process transient results is by evaluating a few select and meaningful locations within the platform. It is virtually impossible to evaluate results data for the entire model across all time steps. Thus, we must select a

Analytical Methods

(a)

(b) Figure 6B5-4. UPS accelerograms from artificial wavelets data set 2: (a) horizontal x-axis input; (b) vertical y-axis input; (c) horizontal z-axis input. (Continues)

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Earthquake Protection of Building Equipment and Systems

(c) Figure 6B5-4. (Continued)

few key locations (e.g., three to six response points). Review the time-history results for these locations. Then make generalizations about the global response based on these results. Obviously, selecting the right locations is critical. Since a prior modal analysis was performed, review of the animated mode shapes should highlight key candidate locations to monitor during the transient. Monitoring the equipment’s FRS is a good choice for one response point. By selecting the top of the FRS for base-anchored equipment, the maximum response motion is captured. Monitoring the locations of critical functional devices and the heaviest devices are also good choices. Again, the analysis goal is to assess FRS mechanical joint integrity; selecting locations that relate maximum displacement with maximum joint stress is the objective. Sound engineering judgment is the guiding principle in selecting meaningful locations to evaluate. The locations of interest within the UPS platform (Fig. 6B5-5) include response of a large transformer, the top corner of the FRS, and one of the internal functional devices.

UPS Case Study Results The accelerograms are applied as time-varying inertial accelerations and the anchorage tie-down points are constrained. This makes the transient response results all relative responses. An alternative method is to use the large mass approach. This method

Analytical Methods

Functional Device CG

FRS at Top Corner

Y Z

X Transformer CG

Figure 6B5-5. Uninterrupted power supply (UPS) nonstructural equipment item showing three locations to monitor transient results. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI. requires inserting a large mass element (≈ 1,000 times greater than the FEM mass) at the equipment base and rigidly connecting this mass to the anchorage tie-down points. Appropriately scaled time-varying forces are applied to the large mass and the transient results represent absolute responses. The large mass approach has the advantage of accessing both absolute and relative responses but introduces zero-frequency vibration modes (rigid-body modes), which directly affect the mode participation factors. Both approaches yield equivalent results, and method selection is a matter of analyst preference. It should be noted that whatever method is used, the original modal solution must be defined in the same manner. Large displacements of the transformer and the top corner of the FRS structure equate with large structural stresses on mechanical joints. Large accelerations of the functional device equate with potential active operation malfunctions. Since the transient is a reduced solution, we can evaluate displacements, velocities, and accelerations for our selected model locations. We could evaluate the displacements one direction at a time. However, since the results are all relative displacements, the easiest way

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to review the transient data is to look at SRSS displacements (square-root-of-sum-ofsquares). Figures 6B5-6 and 6B5-7 display the SRSS displacement results for the three model locations using both input accelerogram sets. Figure 6B5-8 displays the acceleration results for the functional device for the test lab input set. The maximum SRSS displacements for these locations will correspond with maximum joint stresses at the base. We simply need the time at maximum SRSS displacement. This time step will be expanded to get the necessary joint forces. The transient results from the artificially generated input (wavelet data) yield larger response magnitudes compared with the test lab data. There is a 19% increase in SRSS displacement for the transformer location and increases of 0.4% and 8% for the FRS top and device locations, respectively. Table 6B5-1 summarizes the force results from the transient solution runs. The joint load results reveal positive safety margins for most of the evaluated mechanical fasteners. Two of the four transformer bolts display slightly negative safety margins (SF ⬎ 1) for the artificial wavelet input [Table 6B5-1(b)]. Remember that this analysis is not used for building code compliance purposes, but is used to support dynamic testing. The goal is to determine whether there is likelihood of surviving qualification testing at the selected ground motion intensity (SDS) and height ratio (z/h). Slightly negative safety margins are most often good enough, while grossly negative margins (SF Ⰷ 1) need to be flagged for further evaluation. Grossly negative design

(a) Figure 6B5-6. SRSS displacement response from test lab data set 1: (a) transformer CG response; (b) FRS top corner response; (c) device response. (Continues)

Analytical Methods

(b)

(c) Figure 6B5-6. (Continued)

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Earthquake Protection of Building Equipment and Systems

(a)

(b) Figure 6B5-7. SRSS displacement response from artificial wavelets data set 2: (a) transformer CG response; (b) FRS top corner response; (c) device response. (Continues)

Analytical Methods

(c) Figure 6B5-7. (Continued)

(a) Figure 6B5-8. Acceleration response of the UPS functional device from test lab input data set 1: (a) device x-axis response ; (b) device y-axis response ; (c) device z-axis response. (Continues)

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(b)

(c) Figure 6B5-8. (Continued)

Analytical Methods

Table 6B5-1. Joint Load Results from Transient Analysis. (Continues) a) Test Lab Data Set One Input

Time (s)

Fastener Number

17.439

1 2 3 4 5 6 7 8 9 10

17.560

21.391

Device Joint (lb)

FRS Joint (lb)

V

SFa

37 ⫺233 ⫺305 331

98 419 563 628

0.00 0.08 0.14 0.17

1 2 3 4 5 6 7 8 9 10

⫺158 261 111 ⫺87

141 353 376 472

1 2 3 4 5 6 7 8 9 10

393 ⫺656 131 ⫺157

271 654 532 876

T

Transformer Joint (lb)

V

SFb

⫺13 ⫺373 ⫺74 31 ⫺5 ⫺39 ⫺40 33 0 543

250 760 149 59 71 173 141 201 160 1,343

0.01 0.14 0.01 0.00 0.00 0.01 0.00 0.01 0.00 0.40

⫺3,942 6,394 4,152 11,979 2,639 11,987 ⫺2,112 7,035

0.25 0.77 0.74 0.26

0.01 0.06 0.06 0.09

187 328 44 9 15 39 15 ⫺2 ⫺232 ⫺502

343 669 13 137 52 167 49 364 75 1,465

0.03 0.11 0.00 0.00 0.00 0.01 0.00 0.02 0.02 0.45

1,719 ⫺2,995 ⫺1,865 3,214

5,154 8,465 8,568 4,631

0.14 0.38 0.38 0.14

0.06 0.26 0.11 0.30

184 ⫺242 -66 63 9 ⫺6 ⫺48 75 ⫺261 287

231 504 231 190 157 243 182 144 187 600

0.02 0.06 0.01 0.01 0.00 0.01 0.01 0.01 0.03 0.09

907 7,324 3,974 10,978 2,315 11,527 ⫺8,543 6,298

0.27 0.65 0.68 0.40

T

T

V

SFc

a Strength design safety factor based on combined tension (T) and shear (V) interaction using 0.25in.-dia. ASTM A36 threaded rod. Result ⬍ 1 required. b

Strength design safety factor based on combined tension (T) and shear (V) interaction using 0.25in.-dia. IFI Grade 30 Steel Rivet. Result ⬍ 1 required. c Strength design safety factor based on combined tension (T) and shear (V) interaction using 0.5-in.dia. SAE Grade 5 bolt. Result ⬍ 1 required.

Note: Highlighted results indicate safety margins that did not satisfy the load interaction criteria.

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Table 6B5-1. (Continued) b) Artificial Wavelet Data Set Two Input

Time (s)

Fastener Number

16.888

1 2 3 4 5 6 7 8 9 10

22.443

22.446

Device Joint (lb)

FRS Joint (lb)

V

SFa

⫺63 223 182 ⫺154

20 315 511 559

0.00 0.05 0.11 0.12

1 2 3 4 5 6 7 8 9 10

⫺170 483 195 ⫺214

105 481 425 831

1 2 3 4 5 6 7 8 9 10

⫺177 489 187 ⫺201

270 639 440 909

T

Transformer Joint (lb)

V

SFb

T

V

SFc

⫺597 ⫺36 73 ⫺130 ⫺35 ⫺23 77 ⫺113 790 148

694 81 345 531 376 428 339 579 366 809

0.19 0.00 0.02 0.05 0.02 0.03 0.02 0.06 0.21 0.12

3,131 ⫺3,356 ⫺2,110 1,997

5,181 9,649 9,750 5,903

0.16 0.50 0.49 0.19

0.01 0.14 0.08 0.27

⫺470 179 96 ⫺118 ⫺23 2 82 ⫺108 634 ⫺185

535 369 352 438 330 377 338 360 327 259

0.11 0.03 0.02 0.04 0.02 0.02 0.02 0.03 0.14 0.02

3,595 7,890 ⫺4,978 14,209 ⫺3,089 14,311 4,980 7,622

0.35 1.08 1.05 0.36

0.03 0.21 0.08 0.32

⫺482 167 100 ⫺122 ⫺24 4 85 ⫺108 644 ⫺177

548 343 347 451 343 370 351 351 351 238

0.12 0.03 0.02 0.04 0.02 0.02 0.02 0.02 0.14 0.02

3,565 7,801 ⫺4,937 14,114 ⫺3,061 14,234 5,095 7,529

0.34 1.07 1.04 0.36

T

margins are indicative of potential joint failure problems. If this were the case, joint design modifications should be considered prior to qualification testing. Linear analyses are conservative to start with. The transient results should be viewed as ballpark quantitative answers. While the linear transient analysis is as good as it gets for quantitative seismic assessment, good engineering judgment is needed to determine whether or not the evaluated design is in the ballpark.

Analytical Methods

265

In summary, support analysis of any type, whether it is modal, harmonic, spectral, or transient in nature, is just that—support analysis. The final arbitrator in determining how well the nonstructural functional devices can actually function after experiencing earthquake-simulation shaking is the shake-table test results. However, significant progress can be made in developing earthquake-resistant designs by employing analysis as an integral support activity. Our experience clearly points to the fact that passing nonstructural qualification testing is directly related to performing pretest analysis. The greater the support analysis performed, the greater the chance of passing qualification testing, and the greater the likelihood of the nonstructural system maintaining active operation following earth shaking.

6.1.3.1.5 Functional Devices: Failure Mode Perspective Unlike the other nonstructural system elements, functional devices are too diverse a category to isolate a common mode of failure. There are likely many devices that are acceleration sensitive but not all. Each device will have different issues that control failure modes. The effect of any failure mode on a given device is generally related to active operation performance—the greater the failure, the less the likelihood of maintaining active operation.

6.1.4 Attachments Nonstructural attachments are grouped into three categories: operational, isolation, and bracing. The attachment concept, as described here, implies that one end of the attachment secures to the nonstructural FRS, and the other end secures to something else in the building. The “something else” could be another nonstructural system or could secure to the building structure—creating a chain of elements distributed throughout a building. Thus, the attachment design is subjected to differential displacements at the two ends and inertial effects from earth shaking. This concept is mostly applicable for operational- and bracing-type attachments. Isolation attachments are most often compact designs that insert between base anchorage and the FRS (refer to Fig. 5-4 in Chapter 5). The difficulty in performing analysis to support attachment design is not having good definition of design-level demands for differential displacements. For example, when the operational attachment secures to two different equipment platforms, estimating the differential displacement requirement is not easy. The code provides seismic relative displacement requirements (Eqs. 4-13 and 4-14) when a nonstructural system is connected to the building structure at different points or when a nonstructural system is connected to different structures. But that is not the same as being attached to two different nonstructural platforms or when the attachment secures between the FRS and the building. Complicating this matter is the project-specific nature of nonstructural attachments. Both operational and bracing attachments are typically installed per as-built installation considerations. For example, a piping attachment that secures to the FRS will be routed as available space permits at the building site. This makes OEM seismic capacity validation for operational attachments difficult to determine a priori. The following recommenda-

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tions are considered conservative strategies for using analysis to qualify the three attachment types. • Operational: Flexible-body analysis was presented to help establish nonstructural FRS capacity. The approach to establish operational attachment capacity is to add a typical and representative attachment design to the FEM used to establish FRS capacity. For example, a piping run is added to the FEM and secured to the FRS at the most conservative design location (e.g., the top of a base-anchored platform if that location is an option). The pipe run configuration modeled should include the maximum length of unsupported pipe. Then analysis is run that includes the nonstructural FEM (FRS and internal components) and the added pipe run. The pipe end that does not secure to the FRS is restrained. Mechanical joint forces are extracted from the FEA and used to assess joint integrity at the connection points. A static analysis can be used or a transient can be run. The OEM performs this design assessment. • Bracing: The building design professional performs this analysis. In the case of a toprestrained equipment platform, a rigid-body calculation can be made, assuming that the platform is base-anchored and top-restrained using bracing attachments. Bracing analysis is performed to size the bracing members and joint connection to the structure. In the case of distribution systems, bracing attachments are the structural supports. For example, with typical fire sprinkler systems and ductwork, standard industry support details can be used based on the project-specific building demands. With process piping systems, project-specific analysis may be required to determine pipe support capacity. • Isolation: The manufacturer of isolation attachments performs this analysis. Isolation attachments in general are designed to provide a level of dynamic isolation between the building structure and FRS. Most often, equipment platforms are placed on vibration isolators to minimize structure-borne noise during normal operation. In this case, the vibration isolator design is analyzed to attenuate steady-state vibration based on the active operation characteristics of the platform. Historically, this type of analysis does not consider seismic demands. Vibration isolation should consider both steady-state vibration inputs and transient seismic inputs. This will require a compromise in design goals. The steady-state vibration performance will likely suffer from incorporation of shock isolation capacity. This compromise is necessary in order to prevent unwanted isolation failure during seismic events. An isolation attachment design can be achieved that adequately addresses both vibration and seismic demands.

6.1.4.1 Attachment: Failure Mode Perspective Nonstructural attachment failures have definite negative consequences. An operational attachment is the necessary functional umbilical that supports nonstructural active operation. Failure of an operational attachment will cause active operation failure and could cause significant collateral damage. For example, a pipe attachment failure could cause

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the pipe to rupture and flood the area, and an electrical attachment failure could cause short-circuit arcing and exposing live electrical wires. The primary failure mode for operational attachments is joint connection failure where the attachment secures to the nonstructural FRS or the other end where the attachment secures to something else. Isolation attachment failures (i.e., vibration isolators) have occurred in past earthquakes with too much regularity. Code writers have directed their attention on this subject by instituting demand penalties when isolation attachments are implemented. Additional demand penalties are invoked if the amount of free travel that the isolation system can displace is greater than 6.35 mm (0.25 in.). The problem with many isolation attachments is that a steady-state vibration isolation design, intended to limit annoying operation-induced vibration, is not a good design for earthquake shock inputs. The typical failure mode is large displacements that force the FRS platform off the isolators and can cause equipment toppling or cause operational attachment failure and active operation failures. We are not convinced that the annoying operation vibration that placed the platform on vibration isolators in the first place is actually a real issue with modern equipment. The use of vibration isolators to reduce structure-borne noise is more likely based on the mentality that “It’s always done like that” as compared with basing this decision on real science. A hard-mounted equipment platform will perform better against earth shaking compared with a platform that is vibration isolated. However, a platform that is mounted using an isolation design specifically targeting the seismic demand can attenuate the input and perform better than hard mounting. All isolation designs are not created equal when it comes to seismic applications. Bracing attachments are much less susceptible to failure, since the sole purpose of this attachment is to form a rigid link between the FRS and building structure. The primary failure mode of a bracing attachment is at the connection points. The mechanical joint is where a bracing attachment will most likely fail. For equipment platforms, failure of a bracing attachment may not cause additional nonstructural damage, because this attachment could be a secondary additional anchorage point. With distribution platforms, just the opposite is true. For example, failure of a bracing attachment with a piping system (commonly called pipe support or hanger) can cause a pipe break that has serious negative effects. Once again, the joint connection is the failure point.

6.1.5 Clearance Envelope The clearance envelope is a nonstructural installation feature that is dependent on the project-specific as-built condition. The clearance envelope represents the amount of free space between nonstructural systems and all other systems in the immediate vicinity of the nonstructural installation. As previously stated, the code does not currently provide any guidelines concerning nonstructural clearance requirements. The code just indicates that “consequential damage” from the interaction of nonstructural systems shall be considered and avoided. From an analysis perspective, the maximum deflection that the nonstructural system experiences under seismic input is the amount of required clearance space. However, deflection is dependent on input magnitude and also on whether the input is applied as a

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static or dynamic load. A static analysis (like that demonstrated in Box 6-2) will reveal a maximum displacement that is different from the maximum displacement calculated using transient analysis (like that demonstrated in Box 6-5). On a positive note, the fundamental goal in nonstructural product design to resist seismic loading is to increase structural stiffness as best possible. A by-product of this goal is a decrease in nonstructural deflections. For those nonstructural systems that are not baseisolated, the result from design improvements to resist lateral loading is reduced FRS deflections. Gauging whether this presumed decrease in deflections is good enough to avoid consequential damage from the interaction of nonstructural systems is presently an open question.

6.1.5.1 Clearance Envelope: Failure Mode Perspective The obvious consequence of not having adequate clearance between a nonstructural system and anything else is potential contact during earth shaking. Contact can be a general banging between system elements or could be an exerted pressure between elements when there is no initial clearance. Both banging contact and interference contact pressure can have significant negative consequences. For example, contact pressure between the sprinkler head and the suspended ceiling system caused flooding and closure of the Olive View Hospital during the 1994 Northridge earthquake, as described in Chapter 1 (see Fig. 1-18 in Chapter 1).

6.2 Advanced Seismic Analysis The FEA methods presented here are basic engineering approaches that have been used to help design structures and products for more than 30 years. Yet effective implementation of these tools is, even today, not widespread across the nonstructural manufacturing industry. We speculate that perhaps the CAD-centric (computer-aided design) nature of 21st-century product development might be counterproductive to the highly abstract nature of assembly analysis. When assembly models need to be constructed that contain hundreds of discrete parts and each mechanical joint needs proper definition to attach these parts together, creation of the assembly FEM is not a pushbutton process. It is tedious work and does not lend itself well to the CAD-integrated approach that many commercial FEA tools provide. The hope is that an increased number of nonstructural product designers can divorce themselves from the CAD-centric view and begin learning how to make efficient and useful assembly FEMs. Once this point is reached, the techniques presented here will become part of their seismic analysis tool kit during product development activities to achieve better designs for resisting earth shaking. The next sections describe nonlinear analysis and model validation practices that go beyond the basic approaches that have been described. Perhaps the word “advanced” is a misnomer, but these are certainly useful activities nonetheless.

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6.2.1 Material Elastic-Plastic Analysis Up to this point, only linear analysis procedures have been discussed, whether using static analysis to validate position retention requirements or employing dynamic analysis to support qualification testing strategies. Nonlinear approaches can be used equally well, although they require significantly more work to implement and a higher level of user expertise to get it right. One of the areas in which nonlinear analysis can be valuable is addressing the position retention requirement when dynamic testing is not an option and when linear analysis results do not yield adequate capacity. Dynamic testing may not be an option for one of the following reasons: • The nonstructural item is physically too large to fit on a shake-table. • The nonstructural item is too expensive for practical testing purposes (the majority of test specimens cannot be reused after qualification testing and become engineering scrap). • Active operation is not required for the nonstructural item (it was never intended for use as a designated seismic system). When dynamic testing is not going to be performed, the preferred option for meeting position retention requirement for the nonstructural FRS is flexible-body static analysis. Box 6-2 demonstrated the linear static approach. However, basing the position retention capacity on linear results will restrict the capacity level to a level less than maximum. In other words, the assumption of linear stress results to dictate the position retention capacity limit is overly conservative for most nonstructural FRS elements constructed of ductile materials. There is considerable plastic strength (i.e., beyond the elastic limit) in the FRS that is not accounted for in a linear solution. This is especially true for base-anchored nonstructural platforms that are constructed using carbon steel FRS members. Performing a nonlinear elastic-plastic stress analysis will provide the maximum capacity for the nonstructural item to satisfy position retention requirements. As previously stated, there is a significant time penalty associated with nonlinear analysis. Solving a material plasticity problem is no exception to this rule. Solution times could be counted in terms of days, not hours, for large FEMs. Model size is a key factor that needs close consideration when contemplating running an elastic-plastic stress analysis. However, with careful planning a nonlinear stress analysis can be an attractive option when linear results become overly conservative and the nonstructural platform does not yield enough position retention capacity to meet market application needs. Many nonstructural platforms are constructed with ductile FRS members. The current response modification parameters found when using the code’s static force provisions (Table 4-8, Chapter 4) should be expanded. More research is needed to better correlate elastic-plastic behavior of ductile systems with the code’s response modification parameters. Along those same lines, it would be highly useful to apply nonlinear analytical techniques to calculate platform-specific response modification parameters (Rp coefficients). Presently, this option is not included in code provisions unless one uses the alternate means approach, which is difficult to justify.

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6.2.2 Model Validation Analytical models of any form are subject to potential errors. Errors can range from minor deviations attributed to typical modeling issues (for example, overly stiff mechanical joint compliance) to major problems that make the entire model and all associated results complete garbage. As the old saying goes, “Garbage in equals garbage out,” no matter how attractive the results might appear. Model validation is a necessary check to determine whether the model being used exhibits minor deviations or major problems. Even when analysis is used strictly for comparative purposes, models that contain major problems will give the decision maker bad information with which to make decisions. Model validation is the only way to provide the necessary sanity check to guard against using models that bear no similarity to reality. Model validation can be both formal and informal. Informal validation is what experienced analysts do every time they evaluate results data. An experienced analyst can review FEA results and make intuitive engineering assessments regarding the adequacy of the model being used. Bad results data and poorly constructed models can be spotted fairly quickly by an experienced analyst. This is especially true when the analyst reviewing the data did not create the model. Peer review of FEMs and FEA results data is a fundamental practice that will minimize the possibility of using garbage information to make design decisions. Having a few unbiased eyes on the problem at hand is highly recommended. There is no point in spending precious engineering time to build models and run analyses if the process is not being reviewed and monitored by experienced staff. This informal validation is more qualitative in nature and not empirical-based validation. Formal validation is empirical based. Taking test measurements on nonstructural specimens and comparing the test data to analytical results is the most common method of empirical validation. A good example of this is comparing shake-table sine-sweep test data for a given platform to the harmonic-response analysis results of the same platform. Are the resonant frequencies similar? Is the quality of resonance similar (i.e., modal damping ratios)? If not, then what can be modified in the model to better match the real behavior? Another example is comparing pull-test results to static analysis results. In this case the goal is to match pull forces with displacements. Empirical-based model validation attempts to answer these questions and is a science in its own right. The obvious goal of the validation process is to minimize the differences between predicted behavior and analytical models, and real behavior and test specimens. Sometimes this goal can be achieved and at other times this goal is unachievable within the time constraints that engineers must work in. Exhaustive validation studies can be extremely time-consuming. In the nonstructural product manufacturing environment, time is limited for this kind of activity. There is another perspective on this too, which is best summed up by Tony Kordyban’s (1998) quote: “Nobody trusts a computer simulation except the guy who did it, and everybody trusts experimental data except the guy who did it.” Needless to say, experimental results are equally prone to errors, and to blindly associate experimental results with reality can be just as damaging as using bad analytical data. The analyst should have a healthy amount of skepticism for both analytical and experimental results alike. Once again, sound engineering judgment is the proper filter.

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Many times just knowing the model is in the right ballpark is good enough. There are several modal-based testing techniques that can be quickly implemented and will greatly assist the model validation process. A section at the end of Chapter 7 covers experimental modal analysis to support model validation. It describes several techniques that can be used to validate analytical models prior to conducting the seismic qualification test. Of course, the prerequisite for using any of these techniques is having available test specimens ready to evaluate, which can be another problem altogether. Box 6-6 illustrates techniques to modify an FEM to better match empirical data taken using experimental modal analysis. These techniques are geared to quickly get the model in the ballpark so that quantitative analysis results will be more representative of the seismic simulation test environment.

Box 6-6. Finite Element Model Correlation Model correlation is defined as an iterative process of modifying finite element model (FEM) attributes so that analysis results better match empirical test data for a given nonstructural design configuration. The term “iterative” is used because the process requires making multiple FEM changes to determine correlation trends. A single FEM change will not likely result in a correlated model; thus, multiple changes are made until the analytical results better match empirical test data. The goal is to arrive at general agreement between analytical results and empirical data. We need to know that our model is close enough (i.e., in the same ballpark) to the test data and not fixate on achieving an exact match. Exhaustive correlation studies can consume unacceptable amounts of time. The model correlation process needs to be completed over the course of days, not weeks. The correlation process can be divided into three steps: (1) model modification, (2) experimental modal survey, and (3) correlation trends analysis. The assumption made here is that the correlation will be based on steady-state vibration characteristics. While random vibration and impact techniques can be applied equally well, harmonic inputs will be used for this demonstration.

Model Modification We have a fully developed FEM that needs to be modified to facilitate the correlation process. The first point is that the FEM used during correlation must represent the physical nonstructural platform that will be modal tested. This might seem like an obvious point, but many times the analyst building a FEM does not model the exact design configuration being used as a test specimen. There is no way to correlate an FEM that does not represent the actual test specimen. Most often the overall FEM weight will underrepresent the test specimen weight because of simplified abstractions of design geometry. Equivalent weights are needed prior to attempting FE correlation. This example uses the medical equipment beam model shown in Fig. 6-7(b). The assumption is the FEM matches (equivalent weights, mass distribution, FRS design, etc.) a physical test specimen that is available to modal test.

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Finite element models have to simulate the boundary conditions that represent mechanical joints and fasteners. Mechanical joints are the connection points between discrete FRS members and between ground and the FRS (i.e., anchorage connection). A typical modeling procedure to represent mechanical joints in assembly FEMs is to model each fastener as a rigid connection between two FEM node points (or grid points) as shown in Fig. 6B6-1. This modeling technique provides the necessary load path between structural members to transfer translations and rotations. However, this technique does not account for joint compliance. In real joint designs that use mechanical fasteners (e.g., bolts, screws, rivets), there is some flexibility at the connection point. This flexibility is called joint compliance. Modeling mechanical joints as rigid connections between structural members will overly stiffen the joint, which in turn results in overly stiff assembly models. Almost all assembly FEMs that include rigid joint connections have the tendency to overpredict structural stiffness, resulting in natural frequencies that are too high. We are using this modeling artifact to modify critical joints by inserting compliance springs into the joint load path.

Rigid Link Connections at Screw Locations

FE Model

CAD Model

Figure 6B6-1. Nonstructural FRS assembled using mechanical fasteners at the joint locations and represented as rigid connections in the FEM. Source: Illustration courtesy of Square D by Schneider Electric, Palatine, IL.

Analytical Methods

The first set of joints that need to be modified is the anchorage connection points to ground. These joints can be modified by inserting rotation and translation springs between ground and the FRS tie-down points. At each anchorage connection point, three translation springs (ux, uy, uz) and three rotation springs (rotx, roty, rotz) are added. The spring constants are initially defined to be equivalent rigid connections (i.e., large spring constants). Thus, there should be no difference in natural frequency compared to the original FEM results when rigid connections were used. Decreasing the spring constants in an iterative manner forces the joint to gain compliance. The added joint compliance tends to reduce the model’s natural frequencies. Final tuning of joint compliances occurs once experimental data are available to reference. In some cases adding a small amount of joint compliance to the primary anchorage connection points is all that is needed to tune the model’s natural frequencies. However, in most cases this method is not enough. A second set of joints can be modified. For base-anchored platforms these include any critical joints securing vertical FRS members to horizontal FRS members. The process is the same for these joints, inserting translation and rotation springs into the joint connection load path. In this case, there would be two sets of independent spring constants that can be modified: (1) connection between anchorage and ground and (2) connection between key vertical and horizontal FRS members. Mechanical joint compliance can shift (decrease) natural frequencies to better match modal survey test data. This shows up on the harmonic analysis response plot as the frequency associated with a peak response. Assembly FEMs will have multiple peaks with each peak representing FRS natural vibration modes. Figure 6B6-2(a) shows the frequency response for two locations on the medical equipment example. Figure 6B6-2(b) displays the response locations. There are five peak responses out to 50 Hz [identified in Fig. 6B6-2(a) as f1, f4, f5, f7 and f10]. These are the natural frequencies associated with rigid mechanical joint connections by setting the spring constants to high values. The amplitudes of these responses (Q factors) are controlled by the system damping defined for the model. Several different damping effects can be included in the calculation of the effective damping ratio, ξj. The damping effects compatible with mode superposition analytical methods include • Uniform mass damping, specified in terms of a mass matrix multiplier, α; • Uniform stiffness damping, specified in terms of a stiffness matrix multiplier, β; • Frequency-independent damping, specified in terms of a constant damping ratio, ξ; and • Frequency-dependent modal damping, specified in terms of a constant damping ratio for a given mode of vibration, ξmj. Then, for a particular mode j of frequency ωj, the effective damping ratio, ξj, is calculated as follows:

ξj =

β ωj α + + ξ + ξm j 2 ωj 2

(6B6-1)

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(a)

Overhead Gantry Support Structure Z

(b)

Y

X

Figure 6B6-2. Nonstructural medical equipment item: (a) frequency response plot for two locations, y-axis input; (b) identification of equipment response locations. Source: Identification of equipment response locations reproduced courtesy of Varian Medical Systems, Inc., Palo Alto, CA.

To vary the individual response amplitudes to better match empirical test data, the frequency-dependent modal damping ratios, ξmj, are highly useful. Each response peak in the frequency response can be directly controlled by modifying the appropriate modal damping ratios, ξmj, setting the α and β damping to zero, and setting the constant damping, ξ, to a low value (ξ ⫽ 0.01). We will add individual modal damping ratios to the FEM that are associated with the five vibration modes out to 50Hz (i.e., ξ1, ξ4, ξ5, ξ7, and ξ10). Each of these modal

Analytical Methods

damping ratios can be initially set at 4% damping, which equates to 5% total damping since the total damping adds the constant damping with modal damping ratios (per Eq. 6B6-1). Final tuning of the individual modal damping ratios will occur once experimental data are available to reference.

Experimental Modal Survey The techniques used to conduct an experimental modal survey are described in Chapter 7. The in situ forced vibration technique is most appropriate for this example. In situ-based modal measurements can be made at any location where a nonstructural test specimen is available to test. The technique is nondestructive and can be conducted on production platforms. The only stipulation is that the test specimen needs to be anchored to ground using representative in-service anchorage. A small electrodynamic shaker (portable) is used to apply the forced harmonic input to the nonstructural FRS. The input is a low-amplitude sinusoidal sweep across the frequency range of interest (1–50 Hz). The harmonic input is applied independently in both horizontal directions. Response accelerometers can be located at various points on the FRS, such as on the overhead gantry and support structure locations. The result from the sine-sweep survey is a frequency response plot (i.e., transmissibility) at each accelerometer location. The plot is used to identify both natural frequencies and modal damping ratios. Modal damping ratios can be approximated by using the halfpower bandwidth technique as described in Chapter 7.

Correlation Trends Analysis We have modified the FEM to incorporate two sets of translation and rotation springs, at selected mechanical joints in the assembly, to adjust for natural frequency correlation. We incorporated individual modal damping ratios to adjust for response amplitude correlation. The final step is performing the iterative correlation analysis. Correlation can be achieved either automatically or manually. Many FEA software programs provide design optimization tools. These algorithms can be used to automate the process of FE correlation. The user defines the design objectives and the variables to be modified. The software finds the optimal solution. Design optimization is a powerful technique and is highly useful when models are large, since the modal solutions may take many hours to solve. The optimization parameters are set up once and the necessary solutions can run unattended over the course of many hours. In addition, when multiple sets of variables are being modified simultaneously, design optimization algorithms are robust and will typically yield the best solution combination. The recommended approach is to use design optimization for FE correlation purposes. Adjustment of frequency response amplitudes by modifying modal damping ratios is a straightforward correlation process. Increasing a specific modal damping ratio will decrease the corresponding response amplitude. Conversely, decreasing a specific modal damping ratio will increase the corresponding response amplitude. Figure 6B6-3 illustrates this concept. For modes 1 and 4 the damping is increased, for mode 5 damping is decreased, for mode 7 damping is increased, and for mode 10 the damping is decreased. As can be observed, considerable modifications can be made to the frequency response amplitudes by varying modal damping ratios. Even for closely

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Figure 6B6-3. Medical equipment frequency response (y-axis) for overhead gantry location showing results for constant damping and modified modal damping ratio.

spaced modes (like modes 4 and 5), the options to vary response amplitudes are highly flexible. Adjustment of natural frequencies is significantly more complex and less forgiving when attempting to make radical frequency adjustments. Major variations to the spring constants should be avoided. The compliance springs should be viewed as an ability to add a small amount of flexibility to the joint. For example, an analogy can be made to a cantilever beam that is clamped at one end. In real physical systems, there is no such thing as a perfectly rigid clamped end. However, the clamped boundary condition in FEA is perfectly rigid. The more likely boundary condition in the real system is 85–95% clamped, with a little compliance. Avoid adding too much compliance into the joints, as this will radically affect system dynamics. Caution is needed in modifying joint compliance settings. The key is determining the minimum spring constant that can be associated with rigid behavior. Spring constants just less than this value will result in natural frequencies that slightly decrease compared to rigid connections. All spring constants greater than this value yield natural frequencies equivalent with rigid connections. Once this value is determined, the correlation process can be defined to limit the amount the spring constant can be decreased during design optimization. This step requires some analytical experimentation. It will depend on specific joint design characteristics and global FEM properties and is approached on a case-by-case basis.

Analytical Methods

Figure 6B6-4 displays two frequency response plots. Figure 6B6-4(a) highlights a frequency adjustment by adding joint compliance to the anchorage connection points. Added compliance to the anchorage points affected the higher modes (f7 and f10) more significantly than the lower modes. Figure 6B6-4(b) illustrates the effect of adding compliance to FRS joints. In this case, the lower modes (f1 and f4) were affected and not the higher modes. If all modes needed to be adjusted down, then both sets of springs would be given some compliance. A key point needs to be emphasized. The frequency adjustments shown are mostly subtle adjustments and not radical frequency shifts. Subtle frequency adjustments can be readily accommodated using the correlation process as described. However, if the FEM’s frequency response plot bears no similarity to the corresponding modal survey plot, the likely problem is with the FEM. This scenario indicates that the FEM and test data are not in the same ballpark. FE correlation as described should not be attempted until the FEM can be first corrected of gross modeling errors to get it in the general vicinity of the ballpark. In many cases, achieving a frequency match for all modes out to 50 Hz may not be possible with complex nonstructural assemblies. The goal should be to concentrate on

(a) Figure 6B6-4. Medical equipment frequency response (y-axis) for overhead gantry location: (a) frequency adjustment for added anchorage springs; (b) frequency adjustment for added FRS joint springs. (Continues)

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(b) Figure 6B6-4. (Continued) getting a degree of correlation with the first fundamental modes, since these modes are most often associated with large mass participation. Higher-order modes are much less important to the overall dynamic response and thus can be ignored if the correlation trend looks difficult to achieve at higher frequencies. However, response amplitude correlation (Q factors) can be easily achieved for any vibration mode by incorporating modal damping ratios into the FEM solution. A validated FEM offers a high degree of confidence in using numerical simulation data to help drive nonstructural design decisions regarding seismic withstand capacity and qualification compliance.

In some situations formal model validation does not happen until after the nonstructural platform has been tested. While this point in time is obviously too late to influence any pretest design improvements, there is plenty of opportunity to sift through the test data once qualification testing is completed. The experienced analyst will always take a good close look at postqualification test data and compare them to the analysis data that were used prior to testing. This is a fundamental part of the learning process for any analyst. After a few cycles of pretest analysis support combined with posttest data assessment for different nonstructural platforms, the analyst will gain valuable insight regarding how to make models that are close enough to reality to yield good design information for decision making.

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References Adams, V., and Askenazi, A. (1999). Building better products with finite element analysis. OnWorld Press, Santa Fe, NM. American Concrete Institute (ACI). (2011). “Building code requirements for structural concrete and commentary.” ACI 318-11, Farmington Hills, MI. ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, Reston, VA. Bathe, K.-J. (1982). Finite element procedures in engineering analysis. Prentice-Hall, Englewood Cliffs, NJ. Clough, R. W., and Penzien, J. (1975). Dynamics of structures. McGraw-Hill, New York, NY. Cook, R. D., Malkus, D. S., and Plesha, M. E. (1989). Concepts and applications of finite element analysis, 3rd Ed. Wiley, New York, NY. Crandall, S. H., Dahl, N. C., and Lardner, T. J. (1972). An introduction to the mechanics of solids, 2nd Ed. McGraw-Hill, New York, NY, 16. Eligenhausen, R., Mallee, R., and Silva, J. F. (2006). Anchorage in concrete construction. Ernst & Sohn, Berlin, 37–45. Gallagher, R. H. (1975). Finite element analysis fundamentals. Prentice-Hall, Englewood Cliffs, NJ. Gasparini, D. A., and Vanmarcke, E. H. (1976). “Simulated earthquake motions compatible with prescribed response spectra.” Massachusetts Institute of Technology Publ. No R76-4, Cambridge, MA. Gelgele, H. L. (2006). Study of CAD-integrated analysis for complex structures. Springer, Boston, MA. Halldorsson, B., Dong, G., and Papageorgiou, A. S. (2002). “Earthquake motion input and its dissemination via the internet.” J Earthquake Eng. and Eng. Vib., 1(1), 20–26. International Code Council (ICC). (2011). 2012 International building code, Country Club Hills, IL International Code Council Evaluation Service (ICC ES). (2010). “Acceptance criteria for seismic certification by shake-table testing of nonstructural components.” AC156, Country Club Hills, IL. Irvine, T. (2005). “A program to synthesize a time history using wavelets to satisfy a shock response spectrum specification.” (Oct. 10, 2011). Kordyban, T. (1998). Hot air rises and heat sinks: Everything you know about cooling electronics is wrong. ASME Press, New York, NY. Spyrakos, C. C. (1994). Finite element modeling in engineering practice. West Virginia University Press, Morgantown, WV. Steele, J. M. (1989). Applied finite element modeling: Practical problem solving for engineers. Marcel Dekker, New York, NY. Unruh, J. F. (1982). “Digital control of a shaker to a specified shock spectrum.” Shock and Vibration Information Center, The Shock and Vibration Bull., 52(3), 1–9. Wilson, E. L. (2002). Three-dimensional static and dynamic analysis of structures: A physical approach with emphasis on earthquake engineering, 3rd Ed., Computers and Structures, Berkeley, CA, 15–24. Wilson, E. L., Der Kiereghian, A., and Bayo, E. (1981). “A replacement for the SRSS method in seismic analysis.” Earthquake Eng. Struct. Dyn., 9(2), l87–l92.

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Chapter 7

Dynamic Test Methods

Dynamic testing, also called vibration or shock testing, is performed by applying a time-history excitation to the test object and monitoring its dynamic response and the performance of its intended functions. A test machine is used to apply the excitation. The technology of dynamic testing has significantly evolved since the early days of testing in the 1950s, at which time the method was used primarily in aircraft and military testing applications (Curtis et al. 1971). Dynamic testing is used today to validate a wide variety of products across most industries and is considered a routine design assurance activity performed during the product development process. Seismic qualification testing is a specific variation of dynamic testing that is applicable for establishing compliance to the nonstructural seismic requirements for building equipment and distribution systems. Three aspects of seismic testing need consideration: (1) specification of the test environment, (2) test machines and technology, and (3) test preparation and execution. The topic of dynamic testing covers several engineering disciplines and is well beyond the scope of this writing to discuss in detail. The goal of this chapter is to introduce basic concepts on the application of dynamic testing for seismic qualification purposes.

7.1 Specification of the Test Environment The most uncertain part of a seismic testing program is simulation of the seismic environment. The difficultly results from the fact that the probability of predicting the earthquakeinduced floor motion for a given nonstructural application during the design life of the 281

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building is very small. Compounding this problem is the realization that most nonstructural systems are pre-engineered products designed for widespread utilization. The OEM’s ultimate goal (i.e., the nonstructural manufacturer or supplier) is to design for applications in any building and for any given floor location within a building and for any building geographic location across the globe. Stated simply, the OEM does not design a nonstructural product platform to meet a project-specific application need but designs to meet all application opportunities within a given target market. This perspective is a fundamental departure from the project-specific mindset that governs the majority of stakeholders involved in nonstructural protection. Most stakeholders are focused on a single nonstructural application need for a specific building location. Nevertheless, the nonstructural supplier has to think about all application needs within a large geographic area (typically continent-wide). That translates into an unreasonable number of representative floor motions needing consideration for qualification purposes. Obviously, testing nonstructural systems using unlimited numbers of floor motion is neither practical nor feasible. The best that can be done is to formulate conservative estimates for the dynamic nature of floor motions resulting from the strongest earthquakes that can be reasonably expected over the life of buildings. Then decisions can be made regarding the appropriate level of seismic demand to use for qualification purposes for a given nonstructural product platform.

7.1.1 Time Domain versus Frequency Domain Seismic testing machines (i.e., shake-tables or shakers) apply time-history input motions to nonstructural test samples. However, accelerogram records (plots of acceleration versus time) from past earthquake events do not readily reveal the dynamic characteristics of the input motion. A more useful transformation of the accelerogram is obtained by converting the time signal from time domain into frequency domain. Figure 7-1 illustrates this concept. This suggests that frequency domain representation of seismic-induced floor motions provides better insight into their dynamic characteristics than does time-domain representation. The response spectrum transformation satisfies this need. The response spectrum is a plot giving the maximum responses (in terms of displacement or stress or acceleration, for example) of all possible linear 1 degree of freedom (DOF) oscillators that are forced into motion by the same base vibration or shock transient input. The abscissa of the spectrum is the natural frequency (or period) of the oscillator, and the ordinate is the maximum response. Figure 7-2 displays the concept of generating a typical maxi-max response spectrum. In this figure f1 represents a 1 Hz oscillator, f2 represents a 2 Hz oscillator, and so forth out to 100 Hz (with equivalent damping). The response spectrum is then constructed by connecting the maximum responses (absolute value) of the individual oscillators. As the oscillator frequency increases (like at 100 Hz), there is no dynamic response and the oscillator acts as a rigid body and “rides along” with the base input motion. The maximum response of this oscillator is the maximum peak of the input motion. This is called the zero period acceleration (ZPA). The most commonly used response spectrum for nonstructural testing purposes has acceleration as the response ordinate and natural frequency as the abscissa. It is recognized that structural engineers and building professionals might prefer natural period over natural frequency. Both nat-

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0.3

283

Earthquake #1

Acceleration (g)

-0.3

0.3

Earthquake #2

0

-0.3

0.3

Spectral Response Acceleration (g)

0

Earthquake #3

0

-0.3 0

5

10

15

Time (Sec.)

20

25

Frequency (Hz)

Figure 7-1. Time-domain plots versus frequency-domain plots for several earthquakes.

ural period and natural frequency response spectra will be used interchangeably, with natural frequency being the reciprocal of natural period. The seismic ground motion of a building foundation is known as the ground response and is characterized by a ground response spectrum. The ground response spectrum is the starting point for nonstructural requirements and is called the design earthquake response spectrum as defined in model building codes. The dynamic response within the building to which the nonstructural system is attached is known as the floor response (also called instructure response) and is characterized by a floor response spectrum (refer to Fig. 1-12 in Chapter 1). The unique dynamic properties of the building structure govern the relationship between ground motion and resulting floor motion. The nonstructural systems framework (Fig. 1-14) directly captures this relationship. Typically, the building’s structural system filters and amplifies the ground motion. For some building types, significant amplification can result as the nonstructural installation location increases with building height (Drake and Gillengerten 1994).

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Earthquake Protection of Building Equipment and Systems

G peak Frequency, f

m1

m2

m3

f1

f2

f3

m100

f100

Base Acceleration Input

Figure 7-2. The response spectrum transformation and its mathematical development.

Current practice for many building codes and standards used around the world is to define the spectrum shape for the design earthquake response spectrum requirement (ground response), since this governs the building’s structural design. Figure 7-3 displays the IBC and ASCE/SEI 7-10 (ICC 2011; ASCE/SEI 2010) design earthquake response spectrum. The design-level variables that define this spectrum (SDS, SD1, and TL) are obtained from probabilistic hazard maps based on the building’s geographic location and site soil classification. Chapter 4 discusses this in greater detail. However, what is currently not provided in code provisions is a generic definition of a building’s various floor response spectra to represent nonstructural dynamic requirements. What is provided in the code is a static force requirement that varies with building height, geographic location, and several other variables (refer to Eq. 4-6 in Chapter 4). This prescribed static force is called the nonstructural seismic demand. The primary question then becomes how to correctly transform static demand requirements, prescribed in the code, into floor spectra testing requirements. We reason that since the necessary dynamic demand requirements are not defined in code provisions, the absence of a well-defined transformation methodology has created interpretation problems. The result has been industry implementation of different floor spectra interpretations (Caldwell et al. 2007).

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Figure 7-3. International Building Code (based on ASCE/SEI 7-10 standard) design earthquake response spectrum for governing the seismic design requirements of building structures.

Figure 7-4 displays two different code interpretations of the same nonstructural requirement for qualification testing purposes. Clearly, code-writing authorities never intended to have multiple interpretations for the same requirement, especially when the differences are significant. The assumption we have made is that there can be only one interpretation that is consistent with code intent. By using code intent as the guiding principle, a rational transformation methodology can be obtained. The key point is the compelling need to standardize the method of correlating static force demands with dynamic testing requirements such that seismic testing of nonstructural systems can be consistently followed across industry.

7.1.2 Development of Generic Floor Spectra Complete knowledge of the dynamic environment in which the nonstructural system will be operating is not available to the test engineer or to the qualification analyst. The primary reason is that the operating environment is governed by random factors, which are not known ahead of their occurrence. For example, the nonstructural site location could include all potential sites within the building code jurisdiction. The building type could include all possible building types and the earthquake hazard itself is a highly random event. In this sense, the earthquake-induced floor motion is more stochastic in nature than deterministic. Thus, when performing a seismic simulation test, a random-based excitation would seem most appropriate.

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Earthquake Protection of Building Equipment and Systems

Figure 7-4. Different interpretations of the same building code nonstructural requirement for conducting seismic testing. Source: Data provided by Wyle Laboratories.

Looking at this another way might help. If one had the ability to compile seismic records of past earthquakes for every type of building structure ever designed and for all floor elevations within these buildings, and then the resulting floor spectra were plotted onto a single spectrum, one would likely discover that the resulting composite spectrum would look random in nature. The observed randomness would likely cover a limited range of periods. At higher and lower periods there would be noticeable decrease in spectral response. This bandwidth ranges from around 0.1- to 4-s periods, which corresponds to the natural period range for a broad number of building structural systems. This hypothetical discussion on compiling floor responses for buildings of every type was actually the procedure followed to establish the nonstructural building height amplification factor, 1 ⫹ 2(z/h), in the code’s design force equation (Eq. 4-6). In essence, hundreds of seismic floor response records, from various building types and locations, were analyzed and reduced to define an empirically based force requirement that could be generically applied (Drake and Gillengerten 1994; Drake and Bachman 1995; Drake and Bachman 1996; Gillengerten and Bachman 2003). Obviously, not every building type was included in this assessment and not every earthquake event was considered. But what can be concluded from these studies is that the only practical way to develop a generic floor

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test spectrum that can cover all building types and be applicable for all floor elevations is by using a broad-bandwidth spectrum envelope. Spectral enveloping is common practice when the desire is to perform qualificationtype testing. While the code presently does not provide a nonstructural response spectrum alternative to the static force requirement, a broadband floor spectrum test protocol has been developed to conduct nonstructural qualification testing (Gatscher et al. 2003). Box 7-1 details the technical merit of a generic floor spectrum that is fully correlated with the code’s static force demands. The resulting AC156 test spectrum (ICC ES 2010) is used to qualify anchored acceleration sensitive nonstructural equipment and systems (i.e., rigid equipment connections) to IBC and ASCE/SEI 7-10 requirements using dynamic testing techniques. Figure 7-5 displays the AC156 required response spectrum. Essentially, the logic used in AC156 development is to relate the spectrum breakpoints with both the static force demands and with the design earthquake response spectrum (Fig. 7-3). If a nonstructural system is installed at ground level, the nonstructural floor spectrum closely matches the building design earthquake response spectrum. For example, if nonstructural items are installed on a concrete pad at grade level, the spectral test requirement for the nonstructural should be similar to the spectral design requirement for the building structure.

Figure 7-5. AC156 nonstructural seismic qualification required response spectrum (RRS) fully correlated with IBC (ASCE/SEI 7-10) static force provisions.

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Earthquake Protection of Building Equipment and Systems

Box 7-1. AC156 Test Spectrum Development AC156 (“Acceptance Criteria for Seismic Certification by Shake-Table Testing of Nonstructural Components”) was developed as part of a multidisciplinary effort in 1999– 2000 (Gatscher et al. 2003). The goal was to establish a shake-table testing protocol that is fully correlated with IBC and ASCE/SEI 7 lateral force demands. This generic floor spectrum is published by the International Code Counsel’s Evaluation Service organization (ICC ES 2010). AC156 has been approved by the testing evaluation services affiliates of the code-writing authority responsible for establishing the nonstructural seismic requirements for model building codes used in the United States.

Lateral Force Requirement Review Chapter 4 discussed the code’s force requirements in detail. Our goal here is to reintroduce these requirements and frame them into a context that is compatible with dynamic testing. The code’s horizontal seismic design force, Fp, is defined as Fp = ( 0.4 SDS )

( ap )

⎛ z⎞ ⎜1 + 2 ⎟ Wp ⎛ Rp ⎞ ⎝ h⎠ ⎜ ⎟ ⎝ Ip ⎠

(7B1-1)

where Fp ⫽ seismic design force centered at the component’s center of gravity and distributed relative to component’s mass distribution; SDS ⫽ design earthquake spectral response acceleration at short period; ap ⫽ component amplification factor; Rp ⫽ component response modification factor; Ip ⫽ component importance factor; z ⫽ height in structure at point of attachment of component; h ⫽ average roof height of structure relative to the base elevation; and Wp ⫽ component operating weight. Additional code stipulations are defined for allowable maximum and minimum values for Fp: Fp

MAX

= 1.6 SDS I p Wp

(7B1-2)

Fp

MIN

= 0.3 SDS I p Wp

(7B1-3)

The quantity (0.4 SDS) is the design earthquake zero period acceleration (ZPA) and represents the static ground-level design acceleration. The spectral design acceleration factor, SDS, varies per geographic location (taken from design hazard maps) and site classification. The SDS factor ranges from relatively low values for geographic areas that experience little or no seismic activity to high values for those locations that experience the greatest amount of seismic activity. For example, SDS values for locations with little seismic activity range around 0.05–0.15 g and SDS values for locations deemed as having high seismic activity can be upward of 2.5–3.5 g. The quantity [1 ⫹ 2(z/h)] is the building height factor to amplify the ground acceleration, accounting for building amplification as you move up in building elevation. The height factor was empirically determined based on response records from various building types and locations (Drake and Gillengerten 1994; Drake and Bachman 1995;

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Drake and Bachman 1996; Gillengerten and Bachman 2003) and is considered conservative. The height ratio, z/h, ranges from zero at grade level to 1 at roof level. Both the static ground acceleration, (0.4 SDS), and the building height factor, [1 ⫹ 2(z/h)], are independent of a given nonstructural type classification. The quantity (ap ) in Eq. 7B1-1 represents the dynamic amplification of the nonstructural FRS to account for possible dynamic tuning between the building structure and the FRS. Flexible nonstructural systems will dynamically respond to the building input motion (i.e., amplify), and rigid nonstructural systems will not. Rigid systems will simply ride along with the building motion like a lead-filled box. Thus, the component amplification factor, ap, is a force increase factor by accounting for probable amplification of response associated with the inherent flexibility of nonstructural systems, ranging from 1 for rigid components (no amplification) to 2.5 for flexible components (maximum amplification). The last quantity (Rp/Ip) in Eq. 7B1-1 represents the nonstructural FRS capacity to absorb some of the energy imparted during earth shaking as inelastic response (i.e., response reduction). Structural force-resisting systems can dissipate applied loading through the process of inelastic resistance. Thus, the response reduction ratio is inserted into the denominator of Eq. 7B1-1. However, the amount of reduction permitted is limited by the importance rating of the nonstructural system. The logic here is that for essential nonstructural systems the amount of response reduction is limited by the ratio, 1/Ip. Both the response reduction ratio, (Rp/Ip), and the amplification factor, ap, are dependent quantities of a particular nonstructural system. Stated simply, the effects these parameters have on the overall force magnitude are entirely dependent on the type of nonstructural system in question (refer to Table 4-8 in Chapter 4). Hence, the lateral force equation defines the static force design requirement imposed on any nonstructural component or system for any given location within a building structure and for any given building location in the United States and its territories.

Transforming Force into Testing Parameters The force requirement is dependent on nonstructural installation location as a function of building floor elevation. In essence, the force requirement is a building floor requirement for all nonstructural items installed on the floor, walls, or ceiling at a given building floor height. However, in order to determine the nonstructural item’s acceleration at the point of connection to the building structure, the component weight, Wp , is factored out of Eq. 7B1-1, resulting in the following formula for the nonstructural design lateral acceleration requirement: A = ( 0.4 SDS )

( ap ) ⎛1 + 2 z ⎞ ⎜ ⎟ ⎛ Rp ⎞ ⎝ h⎠ ⎜ ⎟ ⎝ Ip ⎠

(7B1-4)

⎛ z⎞ The nonstructural independent variables, (0.4 SDS) and ⎜1 + 2 ⎟ , represent the ⎝ h⎠ building floor motion and are treated independent of the nonstructural items contained

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Earthquake Protection of Building Equipment and Systems

within the building. These two quantities are the earthquake demand at building floor locations. The other two quantities , (Rp/Ip) and (ap) are nonstructural dependent variables that relate specifically to the anticipated dynamic response of the nonstructural item. These variables are associated with specific types of nonstructural and attempt to account for the dynamic effects from earthquake-induced floor motion. The sole purpose of these dependent variables is to directly incorporate nonstructural dynamic effects into the lateral force equation. The key point is that these variables relate to anticipated nonstructural dynamic response and do not influence the actual seismic demand at the building floor location. The shake-table input motion is equivalent to the building floor motion. The response modification ratio (Rp/Ip) is the allowable inelastic energy absorption capacity of the nonstructural force-resisting skeleton. During the seismic shake-table test, the unit under test (UUT) will respond to the excitation in its own manner and inelastic resistance will naturally occur. Therefore, the ratio (Rp/Ip) needs to be set equal to 1. Code intent implies that the importance factor, Ip, should not increase the seismic test input motion, but it does affect the requirement for the UUT to demonstrate both adequate position retention and active operation performance following seismic testing. Likewise, the component amplification factor, ap, has no influence on the building floor motion. During the seismic shake-table test, the UUT will respond to the excitation in its own manner and will either amplify the input motion during the test or act as a rigid body and not amplify the motion. Therefore, the ap factor should also be set equal to 1. The unique dynamic response characteristics of the UUT are not used to increase the shake-table input motion. Each UUT will respond, on its own, to the applied floor motion. Thus, we have isolated the independent lateral acceleration variables that are used to define the shake-table input motion. Equation 7B1-4 can now be rewritten by setting the ratio (Rp/Ip) and ap equal to 1: ⎛ z⎞ A = ( 0.4 SDS ) ⎜1 + 2 ⎟ ⎝ h⎠

(7B1-5)

The fundamental premise is that the shake-table input motions are considered independent of the lateral force variables that govern nonstructural dynamic response. For example, if a nonstructural component is grade-level installed, then the shake-table input motion should be equivalent to the spectral ground motions defined by code provisions for the base of a building. The ground motions are not increased or decreased by the (Rp/Ip) ratio and are not increased by the component response factor designation ap for the UUT. This makes Eq. 7B1-5 an amplified zero-period acceleration that scales with building floor height. For nonstructural applications at grade level, this acceleration correlates exactly with the design earthquake response spectrum value at the zero-period acceleration (see Fig. 7-3 for Sa value at T ⫽ 0). This is no coincidence, since the lateral force requirement was developed from the design earthquake response spectrum. However, our interest is not in static acceleration applications, but in dynamic testing applications. The need is to define a scaled dynamic response acceleration factor that accounts for the strong portion of ground motion in the low- to intermediatefrequency range. The answer lies directly in the design earthquake response spectrum (Fig. 7-3). The strong motion ground spectrum acceleration levels at low to intermedi-

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ate frequencies (between T0 and Ts) are governed by SDS. This code intent can be directly captured into a generic floor spectrum by defining a dynamic response acceleration factor, ADYNH, that scales with building height. The dynamic and ZPA response acceleration factors are defined as ⎛ z⎞ A DYN H = SDS ⎜1 + 2 ⎟ ⎝ h⎠

(7B1-6)

⎛ z⎞ A ZPA H = 0.4 SDS ⎜1 + 2 ⎟ ⎝ h⎠

(7B1-7)

Figure 7B1-1 displays the result. Hence, the nonstructural qualification RRS (generic floor spectrum) is constructed using the two response acceleration variables, ADYNH and AZPAH. This logic could have been made easier by simply starting from the design earthquake ground spectrum (Fig. 7-3) and applying the building height factor, [1 ⫹ 2(z/h)], to amplify spectral acceleration values accounting for above-grade-level installations. However, it is important to identify which design force parameters are associated with floor motion and which are not. The nonstructural response modification ratio, (Rp/Ip), and the component response factor, ap, do not influence building floor motion. The shake-table testing response spectrum is, by definition, equivalent to the building ground spectrum with the added need to amplify for above-grade applications. Thus,

Figure 7B1-1. Qualification RRS spectral shape showing both acceleration and frequency variables.

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Earthquake Protection of Building Equipment and Systems

for a perfectly rigid component (i.e., a lead box) that is grade-level-installed, the shaketable input demand is the building structure input spectrum. And since the component is rigid, the maximum inertial force experienced by the lead box is the shaketable ZPA acceleration times the component weight—which is exactly what is needed. A very important point that needs to be made is that code provisions have specified a maximum cap or limit placed on the nonstructural lateral force value. The maximum limit is defined in Eq. 7B1-2 as 1.6 times SDS. Thus, the same maximum cap is applied when making the conversion to RRS parameters, and A DYN H Max is defined as A DYN H

Max

= 1.6 SDS

(7B1-8)

It is noted again that the Ip factor is not used to increase the maximum value of the shake-table input motion, for the reasons discussed. The maximum cap is only applied to ADYNH and is not applicable for setting AZPAH values. The spectral shape of the seismic qualification RRS has been partially defined by setting the acceleration levels for points B, C, and D using ADYNH and AZPAH variables as shown in Fig. 7B1-1. Setting the acceleration level for point A is not necessary, because this point is the RRS starting acceleration and is much less than point B. This point is also highly dependent on the shake-table’s capacity to deliver very-low-frequency accelerations. This issue is discussed further in the following section. The next step is to determine the frequency values fDYN, f0 , and fZPA to complete definition of the generic floor spectrum break points.

Setting Frequency Points The three frequency points (fDYN, f0, and fZPA) are also directly related to the code’s default design earthquake response spectrum as shown in Fig. 7-3. The f0 value corresponds to the inverse of the design earthquake transition period T0, from increasing acceleration to constant acceleration. Therefore, f0 is defined as f0 =

5 SDS 1 1 = = T0 0.2 TS SD1

In areas that experience high seismicity, the ratio of approximately 0.4 and the ratio of

(7B1-9)

SD1 on Site Class B (soft rock) is SDS

SD1 on Site Class D (moderately stiff soil) is SDS

approximately 0.6. When soil properties are not known, the code requires the site classification to be defined as Site Class D. Using the default options for Site Class D and substituting into Eq. 7B1-9 we obtain f0 =

5 SDS 5 = = 8.3 Hz SD1 0.6

(7B1-10)

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Also in Fig. 7B1-1, the frequency fDYN corresponds approximately to the inverse of the design response spectrum transition period TS, from constant acceleration to SD1 ), constant velocity. It should be noted at periods greater than TS (where TS = SDS the acceleration response of primary structures starts to reduce because the design earthquake spectrum beyond TS reduces by the ratio of TS (where T is the fundaT mental period of the building structure). Since this reduction in the design forces for the structure is accounted for (in the design force equations), it is justifiable to make a similar type of reduction in the design forces used as the floor response spectra of nonstructural components during seismic testing. However, in observing the actual floor response spectra of acceleration recordings measured at the roof levels of buildings with a range of fundamental periods, this reduction in response typically begins at periods about 25% greater than TS. Therefore, ⎛ 1 ⎞ it is assumed the transition period, TDYN ⎜ ⎟ , at the top of the structure at which ⎝ fDYN ⎠ test motion in-structure spectra begin reducing by a ratio of TS has been lengthened T by 25% to account for this observation (Gillengerten and Bachman 2003). At ground level, the test motion in-structure spectra transition period matches the ground spectra. At relative elevations between the roof and ground, linear interpolation is used to ⎛ 1 ⎞ determine the transition period TDYN ⎜ ⎟ . Therefore, fDYN is defined as ⎝ fDYN ⎠ fDYN =

1 SDS = ⎛ ⎛ z⎞ z⎞ TS ⎜1 + 0.25 ⎟ SD1 ⎜1 + 0.25 ⎟ ⎝ ⎝ h⎠ h⎠

(7B1-11)

SD1 Assuming a Site Class D default and substituting into Eq. 7B1-11 for the ratio of , S DS one obtains at the roof of the structure the following for fDYN: fDYN =

1 = 1.3 Hz ( 0.6) ( 1 + 0.25)

(7B1-12)

The fZPA value is defined as the test frequency associated with zero-period acceleration (breakpoint D in Fig. 7B1-1). Code provisions define “rigid components” as having a fundamental frequency greater than 16.7 Hz. A value was selected for fZPA to be at least one full octave greater than 16.7 Hz. Thus, fZPA was set equal to 33.3 Hz. This ZPA cutoff frequency value is consistent with other non-building code-related seismic test standards (IEEE 1987; IEEE 2003; Bellcore 2006).

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One additional point should be noted about the Fig. 7B1-1 seismic qualification envelope. The generic floor spectrum that has been defined is only valid for nonstructural building components and systems with natural fundamental frequencies greater than fDYN (1.3 Hz). For systems such as equipment mounted on isolation attachments (i.e., vibration isolators), with fundamental frequencies less than fDYN (1.3 Hz), projectspecific dynamic analyses are currently needed to develop suitable nonstructural qualification RRS and cannot be tested using the Fig. 7B1-1 RRS shape. This important limitation makes explicit definition of frequency breakpoint A unnecessary, since the seismic qualification RRS is only valid for frequencies greater than 1.3 Hz. A start-up slope around 16 to 18 dB/octave is typical (slope from point A to B).

Defining Vertical RRS Current building codes focus primarily on lateral earthquake loading effects (horizontal loads) and have historically treated vertical loads superficially. Previous revisions of model building codes have stipulated vertical loads for building design purposes as a simple percentage of the base shear load. For example, the 1994 UBC specified the vertical load by scaling the lateral load by a two-thirds factor and did not specify a vertical force requirement for nonstructural. However, the IBC and ASCE/SEI 7-10 (ICC 2011) currently specifies a concurrent vertical earthquake force requirement for nonstructural as E V = ± 0.2 SDS Wp

(7B1-13)

The code’s vertical design requirement is a constant demand and is not dependent on building height. Since both lateral and vertical force components are specified in the code for nonstructural, it is required to include a vertical component in the shake-table testing protocol for nonstructural. The RRS requirement for the vertical direction in AC156 is based on a simple assumption of two-thirds the ground-level acceleration and is considered conservative. Presently, this two-thirds scale factor exceeds the code’s vertical force requirement (Eq. 7B1-13). New research may change the vertical levels as more science becomes incorporated into code provisions. Seismic vertical loading is a topic of interest within the earthquake engineering community. Research results indicate that vertical loads can greatly exceed the requirements for building sites located in the near-field of large earthquakes (Papazoglou and Elnashai 1996). It is recognized that future research efforts are needed to better define the vertical nonstructural seismic requirements. However, for purposes of defining a vertical RRS requirement for dynamic testing, using a two-thirds or greater scale factor of the ground-level requirement is recommended. To clarify the vertical RRS requirement as being two-thirds of the ground response (i.e., not two-thirds of elevated floor response), ADYNV, and AZPAV are defined as A DYN V =

2 SDS 3

(7B1-14)

A ZPA V =

4 SDS 15

(7B1-15)

Dynamic Test Methods

Building Site Geographic Location (Latitude / Longitude)

Nonstructural Installation Height Ratio

Figure 4-3 MCER Ground Motion (SS )

(

( hz Table 4-6 Site Soil Classification (A, B, C, D, or E)

Table 4-7a Site Coefficient (Fa )

Adjusted MCER Ground Motion (SMS )

SMS = Fa SS Design-Level MCER Ground Motion (SDS )

2 SDS = SMS 3 Figure 7-5 AC156 Design Earthquake Response Spectrum

z h

(

(

A DYNH = S DS 1+ 2

DYN H

, A ZPA H , ADYN V , A ZPA V

(

)

A ZPA H = 0.4 S DS 1+ 2

z h

(

(A

2 A DYN V = S DS 3

A ZPA V =

4 S 15 DS

A DYNH Max = 1.6 S DS

Figure 7B1-2. Flow chart depicting the necessary steps to generate the AC156 required response spectrum for seismic qualification testing in accordance with IBC (ASCE/SEI 7-10) requirements with user input of building site information.

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Earthquake Protection of Building Equipment and Systems

The two-thirds assumption for vertical uses the same frequency points as defined for the horizontal RRS. Both horizontal and vertical RRSs are defined using a damping value equal to 5% of critical damping. Figure 7-5 displays the final AC156 nonstructural seismic qualification RRS. This spectrum is fully correlated to both the ASCE/SEI 7-10 lateral force and design earthquake response spectrum requirements. There is no maximum cap limit applicable for setting the vertical response acceleration variables ADYNv and AZPAv. One final note regarding Fig. 7-5 seismic qualification RRS. It should be pointed out, as with all code seismic design criteria, that individual building structures may experience floor spectra that exceed that specified in AC156 in at least some period ranges. The spectra defined in AC156 are not considered to be bounding spectra but a “line in the sand” (Bachman 2008). The goal was to develop a shake-table test standard that is correlated with the code’s force demands, such that OEMs and suppliers can proceed with evaluating and qualifying their nonstructural products and systems in an objective manner. AC156 provides this tool. Figure 7B1-2 presents a flowchart showing the necessary steps required to develop a seismic qualification floor spectrum based on user input of building site information and nonstructural installation height within the building. The preferred method for prescribing nonstructural dynamic demands is explicit code adoption of a generic nonstructural response spectrum demand alternative along with lateral force. This demand would represent building floor motion spectra requirements independent of building dynamics and independent of nonstructural type classification. For the time being, the ICC Evaluation Service’s AC156 test protocol serves this purpose.

7.1.3 Development of Shake-Table Accelerograms to Satisfy Floor Spectra A generic nonstructural floor spectrum has been defined. This spectrum is typically called the required response spectrum (RRS) for qualification testing purposes. The next step is to establish requirements for creating valid time-history accelerograms that can be used to drive shake-table systems. By definition, a response spectrum can be satisfied by an infinite number of different time-history accelerograms. However, not all accelerograms are valid for seismic testing purposes. There are four general characteristics that need to be addressed when developing accelerograms that can be used for seismic qualification: (1) adequate amplitude intensity (including velocity and displacement intensity), (2) adequate frequency content, (3) representative rise and decay rates, and (4) proper phasing interactions. The very nature of any accelerogram that can satisfy a broadband spectrum shape must be composed of multiple frequencies. There are several methods that can be applied to develop multifrequency drive signals that satisfy a broadband RRS shape. The three most common approaches are (1) earthquake time-history, (2) random multifrequency, and (3) combination approaches.

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The earthquake time-history method uses past earthquake records and then modifies the accelerogram to satisfy the RRS shape by employing spectral-raising and spectralsuppressing methods (de Silva 1983). In spectral-raising procedures, a sine wave of required frequency is added to the earthquake record (with appropriate phasing) to improve its capability of excitation at that frequency. Spectral suppressing is achieved by using a narrow-band reject filter for the frequency band that needs to be removed. This procedure requires careful analysis and is somewhat subjective in the need for selecting the correct earthquake record to use as a starting point. The result is creation of a specific—highly tailored—accelerogram that can be reproduced by test labs. An example of this type of predefined accelerogram is the drive signal used to satisfy the Bellcore seismic requirements (Bellcore 2006). This standard requires testing using Bellcore’s Earthquake Synthesized Accelerogram – VERTEQII to drive the shake-table (Fig. 7-6). Test labs can purchase the GR-63 standard and download the VERTEQII accelerograms from the Telcordia website. The random multifrequency approach uses multiple-frequency random excitations, the amplitudes of which are adjusted either manually or automatically based on a narrow bandwidth resolution (Unruh 1982). The drive signal is developed from a weighted sum

Figure 7-6. Example of a predefined (canned) time-history accelerogram when testing to the Bellcore GR-63 standard.

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Earthquake Protection of Building Equipment and Systems

of narrow bandwidth pseudo-random noise signals that have been phase shifted. The relative phasing among the component signals has a uniform random distribution. A buildhold-decay weighting function is imposed on each of the narrowband signals to simulate the nonstationarity of an earthquake event. The composite drive signal is multifrequency, nonstationary random input motion (Fig. 7-7). The combination method uses various combinations of sine beats, decaying sine, continuous sine, or wavelets to create the accelerogram. Drive signals developed from the earthquake time-history method are deterministic in that a known earthquake record is manipulated to satisfy RRS requirements. A drive signal developed using the multifrequency random method is stochastic in nature, and drive signals developed using a combination method could be deterministic or stochastic. All three methods, when properly applied, result in generating multifrequency accelerograms that can satisfy RRS shape requirements. However, the key point is that the resulting accelerogram needs to additionally satisfy the general requirements for ensuring adequate amplitude intensity and frequency content, representative rise and decay rates, and proper phasing interactions. Quantitative measures to objectively determine how well given drive signals meet the four general requirements are not clearly defined in the literature. This topic has been

Figure 7-7. Example of a multifrequency random time-history accelerogram record.

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investigated for qualification testing to satisfy nuclear power applications (Kana and Pomerening 1987). However, a fresh review is needed to determine the applicability of any nuclear power design criteria for more generic building code testing. There is need for research on this topic, such that potential seismic qualification drive signals can be objectively assessed to determine their adequacy in meeting amplitude intensity, frequency content, representative rise and decay rates, and proper phasing interactions consistent with that found in measured building floor motions during earthquakes. For now, the responsibility falls on the test laboratory to create appropriate shake-table drive signals that can satisfy these four general requirements.

7.2 Seismic Test Machines and Technology A properly developed seismic qualification accelerogram must be compatible with and within the constraints and limitations of typical shake-table test machines. The frequency range of interest for the generic floor spectrum lies between 1 and 35 Hz (applicable for anchored acceleration sensitive equipment and systems—rigid equipment connections). The electrohydraulic shaker machine is the primary shaker type that is capable of delivering maximum performance at low frequency. Electrohydraulic shakers can deliver more displacement and velocity and possess a higher maximum force rating compared to other shaker types (e.g., electrodynamic shakers). That makes the electrohydraulic shaker the only realistic option when performing seismic qualification testing. Electrohydraulic shakers come in many different sizes, configurations, and capabilities. Each unique shake-table machine has well defined performance characteristics (maximum stroke, velocity, and force) and thus establishes the physical limitations on meeting a given RRS for a test item. In addition, the shake-table platform size will dictate the maximum footprint size limitation on the test object. To survey a composite list of earthquake shake-tables from around the world, Wikipedia is a good place to start looking (Wikipedia 2011). This global list includes both commercial and research testing laboratories. Note that there is a practical size and weight limit of what can be reasonably tested. A general rule of thumb for maximum weight and footprint size that can be tested using commercially available electrohydraulic shake-tables is up to 5,500 kg (⬇ 12,000 lb) and up to 3.5 ⫻ 3.5 m (⬇ 12 ⫻ 12 ft), respectively. Anything much heavier or physically larger requires using the more-specialized research shake-tables found at several university laboratories. Smaller-sized electrohydraulic shakers, capable of testing standalone functional devices, are not readily available. The current test lab trend is to build large-scale shaketables. Small tables on the order of 1 ⫻ 1 m (⬇ 3 ⫻ 3 ft) are hard to find. We speculate that academic researchers are more interested in studying large-class system behavior or testing scaled building structures than studying much smaller functional devices. There is a need for smaller electrohydraulic shake-tables that can be used to qualify the various functional devices that comprise many types of nonstructural systems. It is not that large tables

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cannot test small items; it is that the cost of testing is generally proportional to the size of the shake-table being used. Small tables typically equate to less-costly testing. In addition, small tables can provide tighter control during the test.

7.2.1 Multiple-Axis Shake-Tables Because the arrival direction of any earthquake event cannot be predicted, the requirement for seismic qualification testing is to test the unit in three orthogonal directions (two horizontal and one vertical). A fundamental issue that influences dynamic testing efficiency arises when multiple-axis testing is performed. Multiple-axis shake-tables perform the shake test in more than one axis simultaneously. Theoretically, there are 6 DOFs that can be applied during a shake test—three translations and three rotations. However, rotational DOFs are difficult to implement and have been generally neglected when performing dynamic testing of any kind. In practice, uniaxial, biaxial, or triaxial shake-table machines can be used to conduct a seismic test. Figure 7-8 displays shake-table configurations that can be used to conduct seismic qualification testing. The demand requirement for testing in the vertical direction is presently a debated topic in the earthquake engineering community. Present-day seismic requirements, found in code provisions, do not define a vertical demand that varies with height elevation within the building, as prescribed for lateral demands. Instead, a simple ratio percentage of the ground-level input is used for qualification testing purposes. At this point in time all that can be said about vertical is that it needs to be included during qualification testing. However, deciding on the appropriate magnitude is a question that will be debated until a consensus decision is reached within the earthquake engineering community. At least for now, using a simple two-thirds or greater ratio of the ground-level input is considered conservative. The generic floor spectra defined in AC156 implement a two-thirds ratio for vertical. Qualification testing using a single-axis shake-table machine requires three separate test sequence steps such that each of the principal axes of the unit under test (UUT) is tested. This also necessitates that either the single-axis machine can be rotated 90 degrees about the horizontal plane, or two separate single-axis machines must be used to accommodate testing in three axes, as shown in Fig. 7-8(a). Testing using a biaxial shake-table (one axis vertical and one axis horizontal) requires two test sequence steps in which the UUT is rotated 90 degrees about the vertical plane to account for testing all three axes [Fig. 7-8(b)]. Testing using a triaxial shake-table machine only requires one test sequence, because all three axes are tested simultaneously [Fig. 7-8(c)]. From our experience, the overwhelming majority of seismic qualification testing is performed using either biaxial or triaxial shake-table configurations. The single-axis shake-table is not practical when testing is required in three distinct steps. In addition, single-axis testing will not excite multiple vibration modes that may have significant cross-coupling effects. For these reasons, single-axis testing should be avoided. In addition, having to perform the test using multiple sequence steps will consume precious lab time in order to detach, rotate, and then reattach the UUT to the shake-table. This should not be underestimated. It requires considerable time to set up, tear down, and re-set up

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+ Vertical Seismic Input

Horizontal E-W Seismic Input

+

Horizontal N-S Seismic Input

(a) Figure 7-8. Possible shake-table configurations for nonstructural qualification testing: (a) uniaxial test; (b) biaxial test; (c) triaxial test. (Continues) Source: Illustrations courtesy of Square D by Schneider Electric, Palatine, IL.

test units. The most expedient and most realistic approach is to use a triaxial shake-table. This is the preferred configuration for conducting seismic qualification testing, whether testing complete nonstructural assemblies or smaller functional devices. When multiple-axis testing is performed, the control of one axis will affect control of the other axes and vice versa. This necessitates that each drive signal used during multipleaxis testing are phase-incoherent with respect to each other. Each unique axis requires a

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+

Horizontal E-W Seismic Input

(b)

Vertical Seismic Input

Vertical Seismic Input

UUT Response Accelerometers

UUT’s

Horizontal N-S Seismic Input

Horizontal E-W Seismic Input Shake-Table Control Accelerometer

(c) Figure 7-8. (Continued)

Horizontal N-S Seismic Input

Vertical Seismic Input

Shake-Table Platform

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drive signal that is independent and out of phase with respect to any other axis. Applying the multifrequency random approach, each axis drive signal will result in a unique phasing relationship and will be phase-incoherent with respect to the others. When using the earthquake time-history method of creating accelerograms, based on modifying earthquake records, careful phasing offsets will need to be applied to each input signal in order to correctly perform a multiaxis test. For example, the Bellcore accelerogram (Fig. 7-6) is applied as uniaxial input (the UUT’s three axes are tested separately) and cannot be applied in a multiaxis test.

7.2.2 Shake-Table Control Electrohydraulic shake-table control systems vary in complexity from state-of-the-art adaptive control strategies found in many university research labs to more simplified open-loop control approaches typically found in commercial testing labs. There is considerable science behind controlling large multiaxis shake-tables. Detailed treatment of shake-table control theory is beyond the scope of this writing. The reader can find several references at the end of the chapter to get more information on various control strategies for electrohydraulic shakers (Gavin and Hoagg 2009; Crewe and Severn 2001; Stoten and Gomez 2001). One of the primary control problems of electrohydraulic shakers is the inherent nonlinear nature of its performance profile. However, the important point regarding any shake-table control system is that reproduction of a desired time-history accelerogram, to satisfy a given RRS floor spectrum, is always a compromise based on the unique performance characteristics of the shake-table and the structural dynamic properties of the test item. The goal for any seismic qualification test is to minimize this compromise. The compromise is quantified by the ability of the shake-table to match the RRS across the frequency range of interest. This is accomplished by comparing the RRS to the test response spectrum (TRS) that is calculated based on actual motion of the shake-table platform during the test. In other words, the input motion of the platform is recorded during the shake test via the shake-table control accelerometer [Fig. 7-8(c)], and a spectrum analysis calculation is performed that becomes the TRS. The TRS represents the as-tested shaketable demand and the RRS represents the target demand. A perfect test is when the TRS closely matches the RRS, as shown in Fig. 7-9(a). This figure is a mathematical creation, not derived from shake-table instrumentation. In a real test situation, perfection is not an option, and the TRS will not ride nicely on the RRS target. In fact, there will be large variations among test labs on how well the TRS envelopes a given RRS. The variables that influence the relative fit of the TRS on a target RRS include nonlinear control methods, poorly maintained table bearings, hydraulic fluid oil column resonance, and shaker platform/UUT structural resonance, to name a few. Figure 7-9(b) displays a test scenario that is more likely to occur using commercial test facilities. In this example there are occurrences of undertesting and overtesting. Let us examine this plot. The overtest condition implies that the actual table motion is greater than the desired RRS target at certain frequencies. Stated simply, the test demand levels exceeded the target

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(a)

(b) Figure 7-9. Graphic showing TRS and RRS results: (a) mathematical example not taken from test data; (b) test data taken from a commercial test laboratory.

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demand levels for some frequencies across the test spectrum (1–35 Hz). If the test unit survived the overtest, then all is good. However, if the overtest caused a failure, the design margin is likely thin for the test unit. An overtest condition can be disconcerting to the engineer in charge of the qualification activity. Nothing can ruin one’s day more than the test lab overtesting the unit. Many test labs, including commercial labs, have implemented newer control technologies and today do a much better job of TRS control compared with a decade ago. If using a new test lab, the engineer should ask the lab to perform a trial run on a dead-weight object approximately the same weight as the test unit. This will provide some evidence as to whether overtesting will be an issue. Undertesting can happen as often as overtesting. Undertesting at low frequencies (i.e., ⬍2.5 Hz) is common, due to displacement and velocity requirements at low frequency. Meeting even moderate spectral acceleration requirements for frequencies less than 2.5 Hz is often difficult to achieve for any shaker that has a stroke length less than 200 mm (⬇ 8 in.) of peak-to-peak displacement. Thus, TRS undertest conditions at frequencies less than 2.5 Hz is quite common for shake-tables in this size category. Of course, shake-tables with longer stroke capability are less affected by this performance constraint. The qualification test standard used for IBC and ASCE/SEI 7-10 compliance defines the frequency range requirements for the TRS envelope of the RRS. A low-frequency cutoff value is defined based on test unit dynamic properties (75% of lowest natural frequency). TRS points less than the cutoff frequency are ignored. Undertesting at frequencies above the cutoff frequency is a test issue that needs to be addressed. The target RRS sets the spectrum shape and is the nonstructural requirement based on converting force demands into equivalent floor spectrum demands using AC156. However, the code’s hazard maps are continuously variable. This implies that the target RRS selected for qualification purposes was created from a specific combination of SDS value and building height ratio z/h (as outlined in Fig. 7B1-2 in Box 7-1). There are floor spectra options that can be greater than or less than the target RRS we specified. The point is using either a higher or lower value for SDS will make our RRS fit nicely under the as-tested TRS. This is illustrated in Fig. 7-10. The spectrum plot in Fig. 7-10(a) shows the target RRS, as-tested TRS, and adjusted RRS for a horizontal test axis. This horizontal direction received an overtest. Thus, by increasing the SDS value from the target RRS we create an adjusted horizontal RRS that just touches the TRS. In this example the building height ratio, z/h, was unchanged. In the vertical direction we do the opposite. Figure 7-10(b) displays the target RRS, as-tested TRS, and adjusted RRS for the vertical test axis. The vertical direction received an undertest between 3 and 6 Hz and negates the requirement for TRS envelope of the target RRS. However, by decreasing the SDS value we can create a new RRS that fits nicely under the TRS. The adjusted RRS curves are used for compliance certification purposes to establish a seismic capacity rating for the test unit. This test compliance flexibility is a direct result from using IBC and ASCE/SEI 7-10 earthquake hazard maps, which offer continuously variable ground motion demands. The adjustment of RRS levels to fit the TRS (either moving up or down) is performed in each axis. The final capacity for the test unit is the minimum adjusted RRS from the three input directions. This concept is demonstrated in Box 5-2 of Chapter 5.

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(a)

(b) Figure 7-10. Graphic showing RRS capacity adjustments using AC156 formulation: (a) horizontal axis increase adjustment due to over test; (b) vertical axis decrease adjustment due to under test.

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In the end, seismic simulation testing is a compromise between what one expects to achieve with a given target RRS and what actually can be achieved with the resulting TRS. Note that smaller shake-tables will have significantly fewer control difficulties compared to large-scale tables and would better approach the elusive goal of a perfect test.

7.3 Test Preparation and Execution There might be a perception with some stakeholders that just showing up at the test lab with the required test samples constitutes preparing for the qualification test. After all, the typical test lab has likely conducted hundreds of qualification tests in the past and it is the lab technician’s job to perform this part of the qualification activity. Unfortunately, the truth is that this kind of stakeholder misperception will almost always yield unsatisfactory test results that provide little practical benefit. Inadequate test preparation is a recipe for failure. This cannot be overstated. Several test preparation tasks need to be investigated well before showing up at the test facility, and the primary requirement is to develop a test plan. The Appendix in this book offers some points to consider regarding test facility selection for nonstructural seismic testing applications. We believe that a test plan is a basic document that describes what is to be accomplished during the test program and, in broad terms, how it is to be carried out. Because the proper format for test plans is often a matter of personal opinion, we offer general guidelines on the basic elements that help define a seismic qualification test plan.

7.3.1 Test Plan Development Product line rationalization was discussed in Box 5-1 of Chapter 5 as a process for selecting test specimens that can represent an entire product line family. This is the necessary starting point in order to develop the qualification test plan. Identification of the test samples (units under test, UUTs) will dictate the general sequence order for qualification testing at the test lab. In addition, the required test fixtures can be designed based on UUT footprints and mounting requirements. Test fixtures can be simple base plates that secure the UUT to the shake-table platform or can be more complex designs that provide both horizontal and vertical mounting options (Fig. 7-11). The fundamental goal of any text fixture is to rigidly attach the UUT to the shaketable. This implies that the fixture does not introduce any structural resonances in the frequency range of interest (between 1 and 35 Hz). This may sound like an easy goal, but in reality designing rigid fixtures that have no structural resonance less than 35 Hz is not trivial when vertical mounting of test units is required. Professional test lab engineers understand this requirement and can design rigid fixtures to accommodate the testing requirements based on the client’s UUTs. One final note regarding test fixture application: the use of concrete anchorage systems during qualification testing is not standard operating procedure. The reasons for this have been previously stated, in that there are hundreds of different types of anchors

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Figure 7-11. Nonstructural test fixture design to attach vertical or horizontal mounted test items for seismic qualification testing. to consider and the addition of concrete pads to the shake-table platform can add anywhere from 1,000 to 3,000 kg (2,200 to 6,600 lb) of dead weight depending on pad dimensions. This amount of extra weight can easily overload many shake-tables. Thus, when conducting seismic qualification testing, the standard method for attaching the UUT to the shake-table is to use machine bolts and rigid test fixtures or welding the UUT directly to the table platform.

7.3.1.1 Elements of a Test Plan The test plan needs to include the following information for each UUT item. This information is provided to the test lab prior to performing seismic qualification testing. • Description of test item: A detailed description of each UUT configuration is provided. This should include a listing of major subassemblies and components (e.g., bills of material) and other applicable product differentiation, including the UUT’s

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unique identification number or serial number. This also includes a general description of the primary function or end-use of the product and physical size and weight characteristics (height, width, depth, and projected UUT weight). • Mounting requirements: Describe how the product platform is typically mounted in building service applications. This should include the quantity, size, grade, and installation torque of mounting fasteners, or weld size and weld type for welded installations. Provide a sketch of the UUT mounting hole pattern or welding details. Next, describe how the UUT needs to be mounted to the shake-table during testing. If a test fixture is required, describe the requirements for the fixture design. Specify who is responsible for test fixture design and fabrication. If bracing attachments are necessary, specify the preferred bracing design. If nonstructural operational attachments are needed, describe the attachment details. Nonstructural connection to the shake-table is highly important, so clearly specify the requirements, including all relevant details. • Instrumentation requirements: This section covers both the minimum necessary instrumentation requirements and any additional instrumentation needs that might be considered optional. The additional instrumentation is what would be nice to have but may be difficult to actually measure with. The test lab will be able to determine if the extra instrumentation requirements can be achieved and the associated cost. Examples of extra instrumentation could be interface force measurements between the UUT and the fixture, strain measurements at specific locations, functional device active operation monitoring measurements, and others. The minimum basic requirements include shake-table control accelerometers for each translation direction of input motion and response accelerometers for each translation direction at OEM-defined locations on the UUT. Defining the most appropriate locations to monitor response motion should be based on prior analytical studies performed on the UUT (as discussed in Chapter 6). Typical locations to monitor include the force-resisting skeleton (FRS) to identify global vibration modes and at interface locations between the FRS and critical functional devices. It is always a good idea to consult with the test lab professionals by describing the instrumentation needs and getting their recommendations for appropriate locations to monitor. Strategic definition of instrumentation needs will facilitate many downstream opportunities to utilize the captured data. Data not captured during the test are gone forever. Plan accordingly. • Seismic test method: The official validation procedure used by the test lab to satisfy IBC and ASCE/SEI 7-10 compliance expectations. The test method defines the qualification acceptance criteria used to judge test results. Nonstructural systems in designated seismic applications are required to demonstrate positive position retention and active operation after being subjected to design-level earthquake demands. The use of a seismic simulation test method, based on a nationally recognized testing standard, such as AC156, can be used to establish nonstructural position retention and active operation capacity. AC156 delineates the testing procedure to be followed by the test lab and is the only approved seismic simulation test protocol referenced in ASCE/SEI 7-10.

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• Qualification test level: Design-level demand requirements for seismic testing are defined as floor response spectra. AC156 provides a generic floor spectrum that is correlated with nonstructural force parameters. Figure 7-5 displays the generic floor spectrum. The recommended approach is to provide the test lab with three to four levels of floor response spectra—a low, medium, and maximum approach. The goal is to start testing at the lowest level and work up from there. • Active operation test requirements: A listing and description is provided of the functional active operation nonstructural requirements (or tests) used to verify pre- and postseismic testing active operation compliance. This listing is dependent on the specific UUT. Pre- and posttest active operation performance verification may require testing at the OEM production factory or authorized service center. • Special requirements: This section communicates any test requirements that are unique and might be considered irregular by the test lab. This could include special power requirements (e.g., line side or load side) and other active operation-related items. Requesting all test data in digital file format can be considered special, depending on lab policy. This is essential and needs to be requested. • Test sequence: The relative order in which UUTs are tested needs to consider variables, such as the likelihood of passing test, physical size of test units, staging requirements of test platforms, mounting requirements, and design importance (i.e., investigative versus production). All of these variables contribute to finding the most appropriate and hopefully most efficient sequence order. The shake-table size dictates how many UUTs can be grouped together during a test run. Each test run typically includes a modal survey (e.g., sine sweep) in each input motion direction, followed by the seismic simulation tests. The seismic testing starts at low levels and increases after each passing test. Large tables can accommodate testing multiple UUTs during a test run, whereas smaller tables require more setup and teardown time between test runs. The test sequence essentially establishes the total number of days needed for the qualification program.

7.3.2 Conducting the Qualification Test The only predictable aspect of qualification testing is that not much is predictable about qualification testing. Most often test reality trumps the finely detailed test plan. Unanticipated events can force a course correction to the test plan. Sometimes the required correction is minor and involves slight deviations from the plan, and other times the test plan becomes scrap paper and major adjustments are needed. There is no better preparation for making real-time course corrections than having a firm understanding on the absolute minimum objectives that need to be achieved during the test program. The test philosophy is simple: achieve the minimum objectives first and then work toward achieving the higher goals. The minimum objective includes attaining some level of compliance from test units using the specified input demands. For example, consider a test plan that identifies three shake-table demand levels, with the lowest level defined as a minimum acceptable demand that will provide marginal compliance capacity to meet marketplace needs. The rationalized test samples might include items that are more laterally challenged compared with the others. First, test the units that are most likely to pass, and then test the more questionable and challenging platforms. The scenario to avoid is spending the first several

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days at the lab trying to troubleshoot a test failure on a marginally designed platform. This will leave little time to test the remaining UUTs that are more likely to survive testing. There is only a finite window of test time allocated, so the time must be managed wisely. Always begin testing at the lowest demand level and then increase to higher levels. If a given platform has successfully passed all of the demands, consider taking it up a little higher until there is a failure. The history of engineering shows that we always learn more from our failures than from our successes. What do we learn from a nonstructural qualification test that passes the designated “Let’s celebrate” test level? Not much. But by taking the platform up to a failure point, we have identified the weak link in the system, which is invaluable design knowledge. Failures are good, and design improvements can be implemented once the weak links are identified (analogous to fragility testing). Do not be afraid to test to failure. However, for platforms that require posttest active operation validation at off-site factory facilities, this strategy is harder to implement. This is where having a spare UUT is useful. Test one unit to the desired qualification level and, if time permits, test the spare to the failure point. Testing to failure brings up another issue at the lab: safety. All test lab environments are potentially dangerous places, and safety is the primary concern. Strict attention to the lab’s safety policy is mandatory. Anyone who is uncertain what the policy is should ask. On occasion, people have been injured because of accidents at a test lab. Do not be one of those accidents. Pay attention to lab personnel direction, and use caution as the overriding motivation. Always consult with lab personnel before doing anything. Everyone’s safety is their primary concern. The last item to discuss is having a thought-out backup plan. Obviously, not everything can be anticipated, but spending some time on creating a backup plan is good test management strategy. The backup plan involves guesstimating what might be needed during testing to facilitate any required course corrections. This includes bringing spare parts, such as empty FRS structures or backup functional devices, extra mounting hardware, bracing, and FRS fasteners, having your own tools and test equipment, and so forth. Murphy’s Law has a way of ruining our well-thought-out backup plans, but many times just the thought process alone can be helpful when test reality rears its ugly head. The test lab has an expectation that the client will actively participate during the testing activity. The best-case scenario is that everything runs smoothly and the original test plan is executed without a hitch. In this case, the testing engineer can end up spending a chunk of time sitting around watching lab personnel do their jobs. The worst-case scenario is that things go badly from the start but something positive can be salvaged from the test activity because of the engineer’s ongoing active involvement and preparation. In our experience the testing activity lies between these two scenarios, and there is a direct correlation between preparation and positive outcomes.

7.3.3 Test Results Documentation A test plan is written by the client (OEM, nonstructural supplier, or designee) and describes what is to be accomplished during the test program and how it is to be carried out. After completion of the test program, a test report is generated by the test laboratory that documents the test results. Each lab will have a unique variation on the format and

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contents included in the test report. The basic elements included in a seismic qualification test report are as follows.

7.3.3.1 Elements of a Test Report • Test program summary: highlights the qualification program objectives. Includes customer information, procedures followed, test units covered, and observed results. • Laboratory equipment: describes the shake-table, UUT connection to the shake-table, instrumentation setup, data acquisition and calibration, active operation test equipment, quality assurance provisions, laboratory certifications, and other items related to the test facility and the equipment used during the test program. • Test procedure: describes the test activity conducted during the test program. This includes descriptions for UUT pre-postinspections, resonance identification tests (e.g., sine sweep, burst random, white noise), seismic simulation tests, pre-postactive operation tests, and other client-requested testing. The applicable test standards, procedures, and test protocols used during the test program are identified. • Test results: presents test data and results from the test program. This includes resonance search test data (e.g., transmissibility and transfer function plots), seismic qualification data (TRS and time-history for control and response accelerometers and other measured data), pre-postactive operation test data, test unit weights, prepost-UUT photographs, qualification acceptance criteria and description of observed UUT performance, identification of test anomalies, and description of any deviations from the test plan. • Approval authority: reviews conducted by proper laboratory authorities and typically includes signature approval by a registered professional engineer. If an independent third-party witnesses the test program, the witness can certify the test report as well. The test report is an official compliance document that is used for IBC and ASCE/SEI 7-10 nonstructural seismic certification purposes. Nonstructural applications that are designated seismic systems may require submittal of the qualification test report to validate compliance. The test report is a legally traceable document for seismic qualification purposes.

7.4 Experimental Modal Analysis There are supplemental test methods that contribute indirectly to nonstructural qualification. These methods are used to support dynamic characterization of nonstructural systems and are nondestructive in their application. Supplemental test data are used primarily to correlate analytical models with physical test specimens and can be used to extend the life of existing qualification data. The test method of interest to support these goals is called experimental modal analysis (also called modal survey). The by-product of experimental modal analysis is measurement

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of a structure’s natural frequencies, modal masses, modal damping ratios, and mode shapes of the unit under test. Of these dynamic properties, modal damping is one of the most important and least understood (Tustin 2005). The effects of damping in general are often misrepresented in dynamic simulations. Nonstructural damping is a primary energy dissipation mechanism that controls dynamic response amplitudes and all associated inertial force magnitudes for both real systems and analytical representations. This makes understanding damping mechanisms and how to measure nonstructural damping characteristics an important topic.

7.4.1 Damping Measurement In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system during and after applied input motion. All nonstructural systems dissipate energy (i.e., damp out) when they vibrate. This is readily observable for anyone who has forced a ruler into motion by holding down one end and flicking the free end. The ruler vibrates for a short time and soon comes to rest. Damping mechanisms are present in this simple example that prevents the ruler from buzzing for a longer time. For example, a steel ruler will vibrate longer and with greater amplitude than a plastic or wood ruler of the same size. Also, how one holds the clamped end affects free vibration. If the ruler is lightly held against a table edge, the vibration will be less compared to tightly clamping the end. The two forms of damping present in this example include friction damping (also called coulomb damping) at the clamped end and material damping (also called hysteretic damping) that occurs within the ruler material. These same damping mechanisms are present in nonstructural systems. Friction damping occurs at every mechanical joint in the assembly, and material damping occurs within the materials that make up the nonstructural assembly. Both of these damping mechanisms are referred to as inherent damping, that is, damping which occurs naturally within the structural assembly. A third form of added damping can be introduced into the system resulting from specifically constructed dampers (e.g., dashpots) that are inserted as isolation attachments between anchorage and the FRS. This form of damping is typically called viscous damping. Almost all of the inherent damping that occurs in nonstructural FRS assemblies arises in the mechanical joints. Approximately 90% or more of the total inherent damping originates in the joints (Beards 1983). Energy dissipation in a mechanical joint is a complex, nonlinear process that is largely influenced by the contact pressure at the joint interface. Describing this damping effect mathematically is difficult and not compatible with the classical techniques used to solve the equations of motion that govern structural systems. Thus, we approximate the inherent damping present in nonstructural systems by using a viscous damping model. The mathematics for incorporating viscous damping are less complex and are compatible with the analytical methods discussed in Chapter 6. Without damping, both our real and analytical systems will become unstable as dynamic input motion is applied. In the real system, a structure without damping would resonate uncontrollably and eventually shake apart. In the analytical world, the absence of damping creates numerical instability and causes matrix solver problems. Obviously, the

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inherent damping in real structures is always present. Thus, the goal is to measure this damping so that our analytical models can incorporate a “viscous representation” of it. The measurement of inherent damping can be accomplished in several ways. There are in situ measurement techniques and damping measurement when the nonstructural test item is placed on a shake-table. In situ-based measurements can be made at any location where a nonstructural product sample is available to test. The shake-table method typically requires shipping the test unit to a test lab. All of the techniques described here are considered nondestructive.

7.4.1.1 In Situ Free Decay The free-decay method is a convenient way to assess the damping in base-anchored nonstructural platforms. The FRS is base-anchored and set into free vibration by applying a shock load. This can be accomplished with a quick-release cable attached to the top of the FRS and tightening the cable until the FRS is displaced and the cable is tight. Once released, the FRS will freely vibrate and the fundamental vibration mode dominates the response. By measuring and recording the decay in the vibration oscillation, the logarithmic decrement, Δ, can be defined as the natural logarithm of the ratio of the amplitudes for any two successive cycles in the same direction of the decaying vibration: ⎛ ⎞ Amplitude of motion Δ = ln ⎜ ⎝ Amplitude of motion one cycle later ⎟⎠

(7-1)

For a viscous damping model, the damping ratio or damping factor, ξ, is defined as the fraction of critical damping for viscous or equivalent viscous damping, and often is stated in terms of percent of critical damping as ⎛ c ⎞ ξ = ⎜ ⎟ ⎝ ccr ⎠

(7-2)

The logarithmic decrement, Δ, is related to the damping ratio, ξ, as Δ =

2π ξ 1 − ξ2

(7-3)

Thus, the equivalent viscous damping ratio can be directly determined from the freedecay method. The viscous damping ratio, ξ, is required input when conducting dynamic analysis. This method has the advantage of not needing vibration test equipment to measure the vibration decay but does require anchoring the platform to ground and using an accelerometer (or equivalent) to measure the decay amplitudes. In addition, this method only measures the fundamental mode of the FRS and not higher-order vibration modes.

7.4.1.2 In Situ Forced Vibration An alternative to the free-decay method for measuring damping is by using forced vibration and measuring the Q factor. The Q factor is typically called quality of resonance or qual-

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ity factor. When a structure is forced into vibration resonance by harmonic excitation, the ratio of the maximum dynamic displacement at steady-state conditions to the static displacement under a similar force is called Q. Determination of the maximum static displacement is most often difficult to measure, but if the vibrating system is considered linear and the damping is relatively small (ξ ⬍ 1), then the Q factor can be approximated by using the half-power points on a frequency response function (Ewins 2000). This is commonly defined as the ratio of resonant frequency, fn, and the difference of two frequencies (f2 ⫺ f1) on either side of resonance that are 3 decibels (dB) down from the peak amplitude: Q =

fn f 2 − f1

(7-4) − 3dB

The half-power points, f1 and f2, are 3 dB down from the peak amplitude, which corresponds to the frequency points at an amplitude of the peak divided by the square root of 2. Figure 7-12 displays the half-power points on the frequency response function for a multiDOF oscillator. The Q factor is related to the damping ratio, ξ, as Q =

1 2ξ

(7-5)

Figure 7-12. Viscous damping ratio approximation measured using the Q factor at half-power points.

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Thus, determination of Q factors and hence the associated equivalent viscous damping ratios can be found from the amplitude frequency measurements taken during a forced harmonic vibration test. A small electrodynamic shaker (portable) is used to apply the forced harmonic input to the nonstructural FRS. The nonstructural item is anchored similar to in-service conditions, and the portable shaker is hung from a support stand and positioned to apply horizontal input excitation to the FRS at several locations. Figure 7-13 shows a typical test setup using this approach. The advantage of this technique over free decay is that higher-order modes can be measured, including internal functional device modes. This technique requires use of portable electrodynamic shakers and associated vibration controller test equipment. With lightly damped equipment platforms, difficulty can be encountered in obtaining accurate measurements for peak amplitudes. In addition, platforms that have closely spaced vibration modes may not provide accurate half-power points and there can be some errors in the calculated Q factors.

7.4.1.3 In Situ Modal Hammer Modal hammer testing involves exciting the structure with a low-magnitude impact shock using a hand-held instrumented modal hammer. The test item needs to be anchored using

Figure 7-13. Typical electrodynamic shaker setup used in conducting experimental modal analysis on nonstructural test items. Source: Photograph courtesy of MB Dynamics, Cleveland, OH.

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in-service mounting conditions. The modal impact hammer technique is the most widely used method because of the extreme portability and minimal setup aspects. The entire modal hammer kit is the size of a briefcase, and a laptop computer is used for data acquisition and processing. Figure 7-14 displays a typical modal hammer kit applicable for testing nonstructural platforms. The modal hammer process consists of tapping on the nonstructural FRS using an instrumented hammer that has a load cell at the hammer tip, and a response accelerometer is attached to the FRS. The hammer impact force and the resultant acceleration responses are measured simultaneously. A spectrum analyzer calculates the resulting frequency response functions. Several measurements are typically made at each impact location and averaged. Sometimes only one fixed accelerometer is used and the hammer taps are

Figure 7-14. Typical modal impact hammer kit for conducting experimental modal analysis on nonstructural test items.

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moved around the structure, or the hammer taps are applied at the same location and the accelerometer is moved around the FRS (using a magnet attachment). This process can determine the nonstructural FRS global vibration characteristics as well as determining local internal characteristics at locations of critical functional devices. Conducting a modal hammer survey is a science and requires trained professionals to properly administer the techniques. Even though the concept of tapping on a structure and having a computer calculate frequency response functions sounds idiot-proof, in reality it is not. Modal hammers come in various sizes and offer different options for the hammer tip (hard plastic, soft plastic, metal, rubber, etc.). Picking the right setup for a given nonstructural platform requires experience. Selecting the right windowing options for spectral analysis and interpreting the frequency response results to obtain the desired modal output should be left to trained professionals. A typical modal hammer survey can be completed in less than eight hours, and there are many companies that specialize in conducting experimental modal analysis.

7.4.1.4 Shake-Table Modal Survey A shake-table modal survey is typically performed during seismic qualification testing. In fact, this test is normally performed right before running the seismic simulation test. A low-amplitude sinusoidal sweep is run in three directions across the frequency range of interest (1–35 Hz). Since the nonstructural test unit is mounted to the shake-table platform, the sweep input is applied as base excitation. This type of forcing function is the most similar to real earth shaking during a quake. In addition to a low-level harmonic sweep, the survey could use low-magnitude burst random or white-noise base excitations as well. The base input forcing function makes the shake-table modal survey results the most realistic from the perspective of modal properties. The major downside of this type of survey is that it is performed during the nonstructural qualification test and is not typically performed as a separate test prior to conducting the qualification. Thus, the modal properties are not available when analytical support activities need them. This point in time is too late for any supporting dynamic analysis that needs to be performed prior to the qualification activity. There are differences in modal properties when tested in situ versus being tested on the shake-table. In situ modal results typically reflect linear properties, and shake-table modal results most often reflect nonlinear properties. The shake-table survey results are more realistic and representative of the dynamic properties that are applicable during earth shaking. The in situ natural frequencies will be slightly higher and the damping ratios will be lower (i.e., less damping). The amount of difference is dependent on the specific nonstructural system. Table 7-1 summarizes the pros and cons of the various techniques for conducting experimental modal analysis. Also included in this table is the resulting modal output that feeds into a nonstructural dynamic analysis.

Table 7-1. Comparison of Modal Testing Techniques. Technique

In Situ Free Decay

Cons

In situ application. Easy setup. Minimal test equipment needed.

Logarithmic decrement (D), Modal damping ratio (␰1)

Modal damping ratio (␰1) can be used as a constant damping ratio.

Modal damping ratios (␰n), Natural frequencies (fn), Mode shapes (␾n)

Modal damping ratios (␰n) can be used as individual damping ratios per mode.

Modal damping ratios (␰n), Natural frequencies (fn), Mode shapes (␾n)

Modal damping ratios (␰n) can be used as individual damping ratios per mode.

Modal damping ratios (␰n), Natural frequencies (fn), Mode shapes (␾n)

Modal damping ratios (␰n) can be used as individual damping ratios per mode.

Modal Hammer

In situ application. Easy setup. Portable equipment. Time-efficient. Can measure modal properties of higherorder modes.

Only measures modal damping ratio for the global FRS vibration mode. Requires vibration testing expertise. Requires portable ED shaker and controller test equipment. Modal results are linear (higher fn and lower ␰n). Requires modal hammer testing expertise. Requires portable modal hammer kit. Modal results are linear (higher fn and lower ␰n).

Test Laboratory Shake-Table

Base excitation most closely represents earthquake excitation. Can measure modal properties of higher-order modes. Modal properties reflect nonlinear measurements and are most accurate.

Requires test laboratory services and appropriately sized electrohydraulic shake-table. Most often occurs at the same time as qualification testing, which is too late for analytical support activity.

Forced Vibration In situ application. Can measure modal properties of higher-order modes.

Measured Data

Dynamic Analysis Input

Dynamic Test Methods

Pros

FRS, force-resisting skeleton; ED, electrodynamic. 319

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References ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, Reston, VA Bachman, R. E. (2008). “Building code requirements for maintaining functionality of nonstructural components in the United States.” Proc., 14th World Conference on Earthquake Engineering, Beijing, China, 1–8, (Oct. 14, 2011). Beards, C. F. (1983). Structural vibration analysis: Modelling, analysis, and damping of vibrating structures. Halsted Press, New York, NY. Bell Communications Research (Bellcore). (2006). “Bellcore Network Equipment-Building System (NEBS) requirements: Physical protection.” GR-63-CORE, Telcordia Technologies, Piscataway, NJ. Caldwell, P. J., Gatscher J. A., and Littler, S. R. (2007). “Essential elements of equipment qualification for building codes by shaker table testing.” Proc., 9th Canadian Conference on Earthquake Engineering, Ottawa, Canada, 1834–1843, (Oct. 14, 2011). Crewe, A. J. and Severn, R. T. (2001). “The European collaborative programme on evaluating the performance of shaking tables.” Phil. Trans. Royal Soc. London, Series A, 359(1786), 1671–1696. Curtis, A. J., Tinling, N. G., and Abstein, H. T. (1971). “Selection and performance of vibration tests.” Shock and vibration monograph series, Vol. 8. The Shock and Vibration Information Center, Washington, DC. de Silva, C. W. (1983). Dynamic testing and seismic qualification practice. Lexington Books, Lexington, MA. Drake, R. M., and Bachman, R. E. (1995). “Interpretation of instrumented building seismic data and implications for building codes.” Proc., 1995 SEAOC Annual Convention, SEAOC, Sacramento, CA. ———. (1996). “NEHRP provisions for 1994 nonstructural components.” J. Arch. Eng., 2(1), 26–31. Drake, R. M., and Gillengerten, J. D. (1994). “Examination of CDMG ground motion data in support of the 1994 NEHRP provisions.” Proc., 5th U.S. National Conference on Earthquake Engineering, Chicago, Earthquake Engineering Research Institute, Oakland, CA. Ewins, D. J. (2000). Modal testing: Theory, practice and application, 2nd Ed. Research Studies Press, Baldock, Hertfordshire, U.K. Gatscher, J. A., Caldwell, P. J., and Bachman, R. E. (2003). “Nonstructural seismic qualification: Development of a rational shake-table testing protocol based on model building code requirements.” Proc. 2003 ATC-29-2 Seminar on Seismic Design, Performance, and Retrofit of Nonstructural Components in Critical Facilities, Newport Beach, CA, 63–75. Gavin, H. P., and Hoagg, J. B. (2009). “Control objectives for seismic simulators.” Proc., 2009 American Control Conference, St. Louis, MO, American Automatic Control Council, Dayton, OH, 3932–3937. Gillengerten, J. D., and Bachman, R. E. (2003). “Background on the development of the NEHRP seismic provisions for non-structural components.” Proc., 2003 ASCE/SEI Structures Congress and Exposition, Seattle, Washington, ASCE, Reston, VA. International Code Council (ICC). (2011). 2012 International building code, Country Club Hills, IL. International Code Council Evaluation Service (ICC ES). (2010). “Acceptance criteria for seismic certification by shake-table testing of nonstructural components.” AC156, Country Club Hills, IL. Institute of Electrical and Electronics Engineers (IEEE). (1987). “IEEE recommended practice for seismic qualification of class 1E equipment for nuclear power generating stations.” Standard 344-1987, Washington, DC.

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———. (2003). “IEEE recommended practice for seismic design of substations.” Standard 693-2003, Washington, DC. Kana, D. D., and Pomerening, D. J. (1987). “A method for correlating severity of different seismic qualification tests.” Trans. ASME, 109, 58–64. Papazoglou, A. J., and Elnashai, A. S. (1996). “Analytical and field evidence of the damaging effect of vertical earthquake ground motion.” Earthquake Eng. Struct. Dyn., 25, 1109–1137. Stoten, D. P., and Gomez, E. G. (2001). “Adaptive control of shaking table using the minimal control synthesis algorithm.” Phil. Trans. Royal Soc. London, 359, 1697–1723. Tustin, W. (2005). A minimal-mathematics introduction to the fundamentals of random vibration and shock testing, measurement, analysis, and calibration. Equipment Reliability Institute, Santa Barbara, CA. Unruh, J. F. (1982). “Digital control of a shaker to a specified shock spectrum.” Shock and Vibration Information Center, Shock and Vib. Bull., 52(3), 1–9. Wikipedia. (2011). “Earthquake shaking table: A world list of shaking tables.” (Oct. 14, 2011).

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Chapter 8

Comparative Experience Methods

Thus far, we have covered qualification methods based on analysis and testing. Given the extreme range of what can be considered nonstructural systems, there may be cases where it is not practical to qualify equipment by these approaches, for a variety of reasons. To provide an alternative method of qualification, the 2000 International Building Code (IBC) introduced the option for qualification by earthquake experience. This method is based on an assessment of actual earthquake performance data for similar nonstructural systems. The concept sounds simple, but the implementation is complex, highly specialized, and requires access to resources that are not openly available. The concept of qualification by earthquake experience has its origins with the nuclear power industry in a program called the Seismic Qualification Utility Group (SQUG). Depending on the qualification application, it may be possible to utilize this option. However, most often implementation requires the services of a qualified consultant experienced in this approach. In addition, the consultant needs access rights to the only viable earthquake experience database for implementation purposes (the SQUG database). A successful qualification will most likely consist of combinations of testing and analysis along with use of the earthquake experience database. Qualification by comparison to a testing database is another means of creating a basis of qualification by experience. Such a process was developed for the nuclear power industry to qualify existing installed equipment beyond the seismic capacity levels established

323

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by SQUG. This program was called Generic Seismic Ruggedness of Power Plant Equipment and was referred to as the GERS program. Due to the highly proprietary nature of the GERS data, its usefulness outside of the nuclear power industry is very unlikely to be a practical consideration. However, some of the GERS concepts could be implemented in development of a proprietary qualification database by a nonstructural OEM or supplier. This option becomes useful when combining methods to address qualification of largeclass systems, as discussed in Chapter 9. This chapter will cover a brief outline of how the nuclear power industry has utilized the qualification by experience method to help the reader evaluate the relevancy to their application. Qualification by experience involves extensive engineering judgment and is not considered common practice in the building construction industry. Therefore, it is recommended that a discussion with the appropriate building officials (i.e., authority having jurisdiction) is conducted to obtain buy-in acceptance prior to pursing this option.

8.1 SQUG Qualification by Earthquake Experience Seismic qualification by earthquake experience for nonstructural components and systems was first formalized as an established practice by the nuclear power industry in the early 1980s. This approach has its roots in the in the 1960s when little or no consideration was given to the seismic performance capabilities of electrical equipment used in nuclear power plant (NPP) safety-related applications. In the late 1970s the Nuclear Regulatory Commission (NRC) elevated its concerns that equipment not tested to IEEE 344-1975, “Recommended Practice for Seismic Qualification of Class IE Equipment for Nuclear Power Generating Stations,” may not have sufficient seismic capacity to handle the demands of a safe shutdown earthquake (safety-related systems must function during and after the earthquake such that the complete plant can be taken off-line and maintained in a safe shutdown configuration). These concerns were validated through field inspections of older plants where equipment installations were observed to have inadequate seismic anchorage (Kennedy et al. 1991). The numerous findings in these reviews led the NRC to issue Information Notice 80-21, “Anchorage and Support of Safety-Related Electrical Equipment” to all operating NPPs in May 1980 (Eder et al. 1990). Eventually the scope was expanded, and the NRC opened “Unresolved Safety Issue A-46” (USI A-46) in December 1980. The objective of this activity was to confirm the seismic adequacy of those systems, subsystems, and components required to maintain a reactor and its systems in a safe shutdown condition. To resolve the structural questions for the NPPs of concern, advanced structural simulation studies and specialized tests were evolved. For equipment evaluations, the options for providing resolution were more complex due to systems considerations. In addition, for many applications the installed equipment was obsolete or the manufacturers were out of business at the time of the assessment, so testing similar equipment was not possible or practical.

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8.1.1 The Foundation A solution for the impasse regarding equipment seismic capacity was proposed by Peter Yanev, CEO of EQE International and SQUG, to the NRC director of the Office of Nuclear Reactor Regulation in mid-1981. The proposal was to develop a qualification by experience procedure to bring closure to USI A-46. The only other option for the NRC was requalification of equipment that was installed in operational NPPs. The removal, decontamination, testing, and reinstallation of this equipment was simply not practical. The NRC found the approach of experience-based qualification promising and encouraged the nuclear power industry to pursue the concept. The foundation for this approach began to form for Yanev as a result of studying how equipment performed in real-world earthquakes starting back in 1971. While reviewing dynamic piping analysis for an eastern NPP in early December 1972, he encountered difficulties in designing pipe supports based on elastic seismic piping analysis. Later that month, during a postearthquake walkdown of the ENALUF power plant in Managua, Nicaragua, Mr. Yanev documented many examples of good and bad performance of equipment and related systems. From the performance of both piping and equipment it became obvious that the piping analysis he had been working with only a few days before was overly conservative. Significant engineering time and resources were being exhausted for the wrong applications. The ENALUF power plant was located on a site that was only 90 m (100 yd) from the fault that had caused the December 23, 1972, M6.2 earthquake. While not a large earthquake, it was a shallow event with the fault slip plane located directly under the city. The close proximity of the earthquake decimated the city of Managua. In 900 photographs taken by Mr. Yanev of the ENALUF event, repetitive examples of what works and does not work for earthquake damage mitigation became obvious. Together with previous similar documentation from the 1971 San Fernando (California) earthquake, the kernel of what later became the EQE Earthquake database was formed. This later became the underlying concept for the SQUG program (Yanev 1990).

8.1.2 Fundamentals behind the SQUG Database SQUG data are collected by a team of subject matter experts by means of on-site surveys around the world where strong motion earthquakes have occurred. The results are used to create a database of how classes of similar equipment have performed during earthquake events. The equipment in the database is similar to that found in NPPs with identified strong motion demands. The purposes of this database are to • Determine the most common causes of seismic damage or adverse effects on equipment and nonstructural components in typical industrial facilities; • Establish thresholds of seismic motion corresponding to various types of seismic damage; • Evaluate the performance of equipment and nonstructural elements during earthquakes independent of seismic demand; and

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• Establish a documented experience history to establish minimum standards in equipment and nonstructural component construction and installation to ensure the ability to withstand anticipated seismic loads.

8.1.3 NRC SQUG Concept Approval Initial NRC approval was granted, and SQUG was formed in January 1980. The goal was to evaluate the performance of common industrial equipment, representative of that also found in NPPs, that had experienced identified strong motion earthquakes. A pilot program for eight classes of equipment was completed by September 1982 and demonstrated its viability. In the late 1980s the NRC closed out the original USI A-46 safety issue regarding the seismic capacity of equipment, and most utilities completed implementation from 1993 to 2000. The overall conclusion of the SQUG field survey findings was that properly anchored equipment, free of certain caveats and seismic interaction concerns, performs well in major earthquakes. In fact, real-world seismic performance was good enough to raise the question of whether detailed analysis and testing was necessary (Yanev et al. 1985). The SQUG database is maintained today by the SQUG and Electric Power Research Institute (EPRI) organizations. In its present form the database is a collection of findings from detailed studies for 20 classes of equipment for a variety of utility and industrial facilities that experience strong motion earthquakes of an identified demand. International utilities have had to address similar A-46-type issues from their regulatory agencies. The SQUG methodology has been a cost-effective alternative for new and replacement equipment for NPPs internationally as well as in the United States (IAEA 2003). In 2003 the Seismic Experience-Based Qualification (SEQUAL) Owners Group became a subset of SQUG. As a result of this combination, the current reference is SQUG/ SEQUAL. SEQUAL was originally formed to implement the experience-based seismic equipment qualification methodology for non-USI A-46 NPPs.

8.1.4 Facilities Surveyed By 1990 the SQUG database included data collection from more than 100 facilities located in the strong motion areas of 42 earthquakes in California, Latin America, Europe, Asia, and the Pacific Rim. The earthquakes ranged from M5.7 to M8.1, with estimated peak ground accelerations from 0.10 to 0.85 g with durations from 5 to 40 s on sites ranging from alluvium to rock. Facilities surveyed were selected based on criteria that included those with substantial inventories of mechanical and electrical equipment, process control and instrumentation, high-reliability power supply systems, and storage tanks. Typical facilities surveyed included the following: • • • •

Fossil-fueled and hydroelectric power plants Electrical distribution substations Oil processing and refining facilities Water treatment and pumping stations

Comparative Experience Methods

• • • • • •

327

Natural gas processing and pumping stations Manufacturing facilities Large commercial facilities Data centers Hospitals Conventional buildings

8.1.5 SQUG Criteria The rules for implementing the use of SQUG experience database were used to develop the formal procedure for implementation called the Generic Implementation Procedure (GIP). GIP has been made public by the NRC but the earthquake experience database remains proprietary to SQUG. If all acceptance rules and caveats are met, the evaluated equipment are deemed to be seismically adequate to perform its function after a safe shutdown earthquake for an existing plant. The use of these data involves inclusion rules that must be satisfied. These rules address physical characteristics, manufacturer’s classification, standards compliance, and findings from testing, analysis, and expert consensus opinion. These inclusion rules are used to define the bounds of each equipment class, the seismic adequacy of which is verified by the earthquake experience data. Four criteria are used to establish seismic qualification by experience: 1. 2. 3. 4.

Seismic capacity versus demand (a comparison of the SQUG bounding spectrum) Earthquake experience database caveats and inclusion rules Anchorage evaluation Seismic interaction evaluation

8.1.5.1 SQUG Bounding Spectrum Bounding spectra are used to define the seismic capacity, based on identified ground shaking, for each equipment class the seismic adequacy of which is verified by successful earthquake performance experience, as shown in Fig. 8-1. The SQUG bounding spectrum places an upper limit on the equipment seismic capacity to ensure that equipment will not be qualified at a higher seismic capacity than there is experience to verify. For equipment located up to 12 m (40 ft) above grade level, seismic demand can be estimated by using a generic building amplification factor of 1.5 times the plant design safe shutdown earthquake spectrum. The SQUG acceptance criteria include detailed application rules for this comparison, provided the building is a stiff reinforced concrete shear wall structure.

8.1.5.2 Inclusion Rules The inclusion rules and caveats were developed to ensure that the equipment to be qualified is represented in the relevant equipment class and is free of known vulnerabilities (caveats). The caveats are used to identify any installation requirements that must be

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Frequency (Hz)

2

2.5 7.5

8

10

12

16

20

28

33

0.8 0.68 0.59 0.53

0.5

1.5 X Bounding Spectrum (g)

0.98 1.2 1.2 1.13 0.9

Bounding Spectrum (g)

0.65 0.8 0.8 0.75 0.6 0.53 0.45 0.39 0.35 0.33

Figure 8-1. NRC Generic Implementation Procedure (GIP) seismic capacity bounding spectrum based on earthquake experience data. satisfied for the qualification to be valid based on known seismic hazards in the database. These inclusion rules address the following: 1. Configuration, size limitations, weight limitation, operating capacity limits, etc. 2. Materials, subcomponents, manufacturer classifications, design and construction standards, etc. 3. Other issues derived from shake-table fragility tests, analyses, expert opinion, etc. An evaluation of equipment for qualification by comparison to similar equipment in the the SQUG database would typically include the following physical characteristics: • Equipment size, mass, and position (vertical, horizontal, etc.) • General quality of construction and age of design

Comparative Experience Methods

• • • • •

329

Location of center of gravity and cantilevered subassemblies Nature of internal moving parts of significant mass Anchorage and supports External operational attachments, substructures, or devices Internal devices sensitive to shock and vibration

8.1.5.3 Anchorage Evaluation Anchorage of equipment in NPPs was evaluated to determine whether it was adequate for effective seismic restraint during an earthquake. This included consideration of other issues that could result in the failure of critical equipment and systems: • Manufacturer’s seismic installation instructions complied with • Review of equipment anchorage calculations of demand versus capacity • Judgment evaluation of the adequacy of the seismic load path from the anchorage to the distributed mass of the equipment • Seismic anchorage has been installed as detailed by a qualified seismic engineer • Correct type of postinstalled mechanical anchor for application • Possible cracks in concrete or other installation variables that may degrade anchorage capacity • Adequate bolts with stiffeners to minimize weak-axis bending at anchorage locations in equipment with structural elements fabricated from sheet metal • Specified embedment depth of anchor maintained • Minimum distance between anchor and edge of housekeeping pad maintained • Minimum distance between anchors maintained • Equipment housekeeping pads properly doweled to floor • Equipment pads with correct strength of concrete and rebar placement • Correct weld details • For equipment mounted on vibration isolators, the isolators include snubbers to absorb horizontal and vertical impacting loads caused by the earthquake

8.1.5.4 Seismic Interaction Evaluation There are generally four considerations for seismic interaction effects that must be considered: 1. 2. 3. 4.

Proximity Structural failure of surrounding equipment, systems, or structural elements Flexibility of operational attached lines and cables Water spray or flooding due to earthquake-induced failures of piping, tanks, or vessels

Proximity items include potential impacts of the equipment with other equipment or walls if the equipment is flexible and will displace at the top during an earthquake. This

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can be negated by the addition of top lateral bracing attachments from the top of the equipment to a load-bearing wall or by tying the equipment together at the top to prevent impacts from relative displacements. Examples of structural failures are unreinforced masonry walls, suspended ceiling system elements, or inadequately restrained architectural attachments that can fail during an earthquake and damage equipment that has been classified as part of a designated seismic system. Other considerations would be the proximity of equipment carts that can roll or slide and cause impact damage. To address potential differential displacements between fixed support points, all incoming and outgoing operational attachments, such as lines, cables, conduits, and plumbing connections must be flexible enough to absorb displacement demands of the equipment. An example would be the exhaust plumbing from a diesel generator to the exterior exhaust stack. This is a rigid system from the engine exhaust manifold to the exterior stack and requires a flexible connection to preclude damage to the engine, which would prevent the generator from being operational without major repairs. Properly anchored electrical equipment has a high probability of surviving the earthquake and being operational after the event. However, tanks, vessels, and pipes that fail and flood the area can result in extensive collateral damage to the equipment, requiring its replacement.

8.1.6 Relevancy of Equipment in SQUG Database Experience data must be used with care because the design and manufacture of nonstructural systems may have changed considerably in the intervening years. The use of this procedure is also limited by the relative rarity of strong motion instrumented records associated with the corresponding equipment experience data.

8.1.7 SQUG Classes of Equipment The SQUG database consists of 20 classes of equipment: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Motor control centers Low-voltage switchgear Medium-voltage switchgear Transformers Horizontal pumps Vertical pumps Fluid-operated valves Motor-operated valves Solenoid-operated valves Fans Chillers Air handlers Air compressors

Comparative Experience Methods

14. 15. 16. 17. 18. 19. 20.

331

Distribution panels Batteries on racks Battery chargers and inverters Engine generators Instruments on racks Temperature sensors Instrumentation control panels and cabinets

8.2 GERS Qualification by Testing Experience In 1984 EPRI initiated a program to collect and evaluate the test data for applications in NPPs, which became known as the Generic Seismic Ruggedness of Power Plant Equipment (abbreviated as GERS). The purpose of the program was to supplement the SQUG effort in resolving NRC USI A-46 by answering questions about the seismic adequacy of NPP equipment, particularly for seismic demand levels exceeding the SQUG bounding spectrum (U.S. Department of Energy 1997). Shake-table data from 300 tests for 15 classes of equipment were collected. The test data were used to construct equipment ruggedness or capacity spectra. The equipment capacity spectra could be used to evaluate existing NPP equipment installations that were qualified at lower demand levels. Use of the GERS generic qualification spectra could determine whether the installed equipment had sufficient reserve capacity to meet the more current assessment of NPP site seismic demands. Associated with GERS were inclusion rules, cautions, and checklists for field screening of in-place equipment for GERS applicability. Because of the proprietary nature of GERS, it is of little or no value outside of NPP applications. It does, however, provide relevant insight on how nonstructural OEMs may utilize their own proprietary test database to support future new product development and qualification activities. Seismic simulation testing has become a key enabler supporting nonstructural qualification. OEMs that conduct qualification testing programs can benefit largely by instituting formal test data retention policies to manage the qualification test data. Test data should be viewed as part of the product’s technical data package that becomes incorporated into a product data management system. This approach is necessary in order to accommodate the automated seismic compliance assessment techniques discussed in the Chapter 5 section “OEM Qualification Strategy.” In addition, formal test data management will facilitate implementation of the combined methods approach to achieve qualification of largeclass systems. An OEM test database supports comparative assessment techniques using a combination of previous test data, new test data, and analytical methods.

8.2.1 DOE Seismic Evaluation Procedure In 1997 DOE adopted both the SQUG and GERS approaches. DOE published its guidelines in DOE/EH-0545 for evaluating installed equipment in DOE facilities that were

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Earthquake Protection of Building Equipment and Systems

never qualified by testing (DOE 1997). This DOE document has been released to the public domain and is available on the Internet. It is mentioned here as a general reference for the reader who is interested in deeper insight into the qualification by experience process. The DOE document is significantly easier to obtain than the SQUG GIP document. It is important to pay special attention to the foreword in this document, because it details the differences between the DOE Seismic Evaluation Procedure and the SQUG GIP. Some of these differences are significant, and care must be used to determine which is most appropriate for the application under consideration. The three major adaptations for non-NPP applications are the following: • The SQUG “40 foot rule” for bounding spectrum is not part of the DOE procedure, since it was developed for NPPs with stiff shear wall structures and are not relevant to DOE facilities with more flexible structures. • The DOE procedure has equipment classes not in SQUG. • The detailed procedure required for relay functionality in NPPs is not included in the DOE procedure. Just like the GIP implementation for NPPs, the DOE Seismic Evaluation Procedure is only to be used in DOE facilities, with appropriate training and judgment, along with peer review.

8.3 Experienced-Based Methods Summary Recognizing that seismic qualification by testing may not always be practical or technically possible, the building code provides for an optional method of qualification by experience. Commentary C13.2.7 in Part 2 of the 2009 NEHRP provisions (FEMA 2009) gives an overview of the code intent for this option. At the core of this approach is access to a credible experience database that verifies equipment performance in actual earthquake events with an identified demand. Presently, the proprietary EPRI SQUG experience database is the only one in existence. Access to this database is limited to authorized users and, due to restrictions imposed by national security concerns, it may not be available for all applications. Access restrictions and the specialized knowledge required to apply the rules for its use will mandate the services of an experienced consultant who has access rights. Lessons learned from the SQUG program consistently point to a lack of confidence in a process that involves a significant amount of specialized “engineering judgment” for experience-based qualification globally (IAEA 2003). Therefore, to minimize the engineering judgment content, nonstructural qualification by experience will likely involve, at a minimum, the use of analysis to confirm the validity of comparing the equipment to be qualified to similar equipment in the database. For active mechanical or electrical equipment, it may also be necessary to shake-table test the active subassemblies (i.e., functional

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devices) to verify that the equipment will be operational after the event. If the manufacturer has implemented a proprietary test data retention program along with a capacity assessment program, similar to the GERS program, reducing or eliminating the need for testing may be possible. In either case, additional analysis or a modal survey may be required to develop the necessary functional device transfer functions to create suitable device-required response spectra. This procedure is detailed in Chapter 9. All things considered, the option for qualification by experience will most likely be more complex, resource-intensive, and expensive for those cases where qualification by shake-table testing is possible. Therefore, qualification by experience is only recommended for nonstructural applications in which shake-table testing of the entire product platform is neither practical nor possible.

References Eder, S. J., Johnson, J. J., and Smith, N. P. (1990). “Developments of the Seismic Qualification Utility Group.” Proc., ATC-29 Seminar and Workshop on Seismic Design and Performance of Equipment and Nonstructural Elements in Buildings and Industrial Structures, October 1990, Applied Technology Council, Redwood City, CA. FEMA. (2009). “NEHRP recommended seismic provisions for new buildings and other structures: Part 2, commentary to ASCE/SEI 7-05.” FEMA P-750, Washington, DC. International Atomic Energy Agency (IAEA). (2003). “Earthquake experience and seismic qualification by indirect methods in nuclear installations.” IAEA-TECDOC-1333, Vienna, Austria. Kennedy, R. P., von Riesemann, W. A., Wyllie, L.A. Jr., Schiff, A. J., and Ibanez, P. (1991). “Part I: Use of seismic experience and test data to show ruggedness of equipment in nuclear power plants.” SAND-92-0140, Rev. 4, Senior Seismic Review and Advisory Panel (SSRAP), Nuclear Regulatory Commission, Washington, DC. U.S. Department of Energy (DOE). (1997). “Seismic evaluation procedure for equipment in U.S. Department of Energy facilities.” DOE/EH-0545, Lawrence Livermore National Laboratory, Livermore, CA. Yanev, P. I. (1990). “The EQE earthquake data base and the performance of equipment and nonstructural components.” Proc., ATC-29 Seminar and Workshop on Seismic Design and Performance of Equipment and Nonstructural Elements in Buildings and Industrial Structures, October 1990, Applied Technology Council, Redwood City, CA. Yanev, P. I., Swan, S. W., Smith, P. D., Smith, N. P., and Thomas, J. E. (1985). “A summary of the Seismic Qualification Utilities Group (SQUG) program.” In Seismic experience data—Nuclear and other plants: Proceedings of a session sponsored by the structural division, Y. N. Anand, ed. ASCE, Reston, VA.

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Chapter 9

Combined Methods

The earthquake experience method described in Chapter 8, while potentially very useful, is difficult to implement in practice. Fortunately, the term “experience” also includes the qualification experience gained from previous analysis and testing of nonstructural systems. The typical nonstructural supplier (i.e., OEM) is aligned within a narrow category of nonstructural products. For example, a mechanical pump OEM is not likely supplying electrical equipment, and vice versa. The pump OEM supplies pumps and more often than not has a rich history in analysis and testing of pumps. This type of platform-specific experience is highly useful when addressing qualification of similar but more challenging nonstructural systems. The concept is to combine analysis and testing with previous qualification experience. In essence, the goal is to employ comparative assessment techniques using a combination of previous test data, new test data, and analytical methods. The building code provides an allowance for comparative assessment-based validation. Thus, combined methods, as described here, implies achieving active operation qualification of nonstructural systems that cannot be qualified or are too difficult to qualify by using a singular method approach. A good example of this scenario is qualification of physically massive nonstructural systems that would require specialized test equipment to perform a seismic simulation test (e.g., requires shake-table capacity not readily available). Another example is when the financial cost of supplying nonstructural test specimens to support de facto destructive qualification testing is too high for practical consideration. The list of nonstructural systems that fall into this special category (herein called large-class nonstructural systems) is

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not inconsequential. In fact, this category constitutes a significant portion of designated seismic systems. Here are a few examples: • • • • • • • •

Generator sets (⬎600 Kw) Uninterrupted power supplies (⬎600 Kw) Cooling towers (⬎1,000 tons) Industrial chillers Semiconductor fabrication equipment Medical radiation treatment equipment Medical imaging equipment (MRI, CT, x-ray) Elevator systems

In the past, qualification of large-class systems was achieved by satisfying position retention requirements. Active operation functionality was implicit and embedded into the requirement for meeting position retention. The logic was that if a nonstructural item remained in position and was stable, the likelihood of the item maintaining operational function following earth shaking would be high. Today this philosophy is no longer valid. Addressing operational requirements for active large-class systems is now a new frontier. We speculate that a high percentage of large-class nonstructural systems have limited compliance data to satisfy the code’s active operation requirements. It is not that suppliers for these systems are intentionally dodging compliance. The reality is that explicit operational validation for large-class systems is a relatively new concept, and no guidance is provided in code provisions regarding meeting code expectations. Stated simply, the suppliers would like to comply but have no guidelines on how to approach the problem. Unfortunately, with position retention analysis (anchorage calculations) no longer tied to active operation performance, the compliance strategy being adopted today for many large-class systems is to blithely ignore the requirements. Obviously, this tactic is unsatisfactory. There is compelling need to illuminate this problem and offer solutions that are based on accepted engineering principles. The simplest, most direct solution is to point large-class nonstructural suppliers to the much larger shake-tables that have been constructed at university research laboratories (University of California–San Diego, University at Buffalo–State University of New York, and University of California–Berkeley, to name a few). These test facilities were designed for large-scale seismic testing and have capability to test many large-class nonstructural items. However, the fact remains that not all large-class nonstructural systems can be tested as complete units. Frequently, the cost factor alone can be incentive for not pursuing full-scale seismic testing. When individual test units can cost more than US$150,000 per item and might require several test units to qualify a platform, full-scale testing becomes less practical; however, when cost is not a primary concern, seismic qualification testing of many large-class systems can be achieved using large-scale university tables. The topic we want to discuss is what to do when cost or physical size is an impediment to qualification based solely on full-scale testing.

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9.1 Options for Large-Class Qualification The available options supporting large-class qualification are analogous to a road map. As with any road map, there are multiple routes that can be taken to arrive at the destination of large-class qualification. The route taken is mostly dependent on the type of large-class nonstructural in question. For example, a predominantly electrical large-class system could follow a different path than a predominantly mechanical system will follow to achieve qualification. Both paths use a combination of analysis and test methods, but the combinations might be different. The following descriptions for two different paths to qualification (i.e., large-class electrical and mechanical systems) are intended to illustrate how a challenging large-class nonstructural problem can be addressed. The reader needs to fully recognize and accept that there is a degree of uncertainty in applying these techniques. Absolute compliance validation is not achievable due to the inherent complexity of real-world large-class systems. This process is not a clear-cut, black-and-white activity but an activity that lies in shades of gray. Nonetheless, we believe that implementation of these and similar methods, in conjunction with exercising good engineering judgment, will provide significant improvement over the present-day mentality of simply ignoring the new requirements. After all, it was not too long ago that active operation compliance was satisfied with a simple anchorage calculation. The methods presented here are well advanced beyond anchor bolt calculations. These methods will not only improve the seismic withstand resistance of the systems, they should also increase the confidence level of those stakeholders depending on the systems to deliver active operation performance following earth shaking.

9.1.1 Electrical Large-Class Nonstructural The electrical system for this example is an industrial-sized uninterrupted power supply (UPS). Figure 9-1 is an illustration of this large-class platform showing overall dimensions and weight. The UPS is an electromechanical apparatus that provides emergency power to building loads when the input power source, typically the utility mains, fails. While the UPS system is not as physically massive as the mechanical cooling tower example discussed next, the cost factor is the primary motivation for not testing the entire system. The functional devices that are critical to the continued operation of this system are either electrical or electromechanical in nature. The active operations for these devices are mainly acceleration-sensitive and less dependent on structural strength aspects. The strategy is to use analysis for qualification of the mechanical subsystems [i.e., anchorage, force-resisting skeleton (FRS), and attachments] to satisfy position retention requirements, and apply standalone device testing to qualify the active operations contained within functional devices using demand levels established from the system transfer functions. Qualification of mechanical subsystems using analysis is discussed in Chapter 6. The focus here is determination of the transfer functions and resulting demands

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Weight = 6,405 kg (14,120 lb)

2.08 (6.8)

3.07 (10.1) Dimensions in meters (feet)

1.02 (3.3)

Figure 9-1. Large-class nonstructural UPS platform used in combined analysis and testing qualification. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI.

between anchorage and the functional device so that functional device test requirements can be developed and used for standalone device qualification testing. Figure 9-2 shows the generalized system diagram to represent the UPS. The system transfer functions can be obtained using empirical testing techniques like a modal survey, or can be obtained analytically if no physical test specimens are available. The preferred method (i.e., most reliable) is to use modal survey test data taken on production units. The test unit needs to be mounted similar to field installations using representative in-service anchorage. Since the modal survey is a nondestructive test, there is no risk of damage when production units are used. The survey can be conducted at a customer site or at the production factory and requires around 4 to 8 h to complete.

Combined Methods

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Clearance Clearance Envelope Envelope Building / FRS Interaction

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Anchorage

Position Retention Requirement Using Analysis for Compliance

Transfer Mechanical Function Impedance Using Effects Analysis or Modal Survey

Device Input

Active Operation Requirement Using Standalone Device Testing for Compliance

Figure 9-2. Systems design framework for a UPS large-class nonstructural system.

Commonly used techniques to conduct in situ modal surveys include forced harmonic excitation using portable electrodynamic shakers or impact excitation using modal hammers. Both the modal hammer and forced vibration approaches yield linear modal properties (i.e., natural frequency and damping). These measurements will be slightly different when compared with the predominantly nonlinear modal properties that result when base excitation is applied. The linear natural frequencies will be greater and the linear modal damping will be less. The amount of difference between in situ and base excitation measurements is dependent on the large-class system, but the transfer function envelopes should account for this effect. Chapter 7 describes the methods used in experimental modal analysis. The transfer function is a measure of the dynamic filtering and amplification effect created by the nonstructural FRS and is analogous to the filtering and amplification created by the building structure as one moves up in building elevation. The filtering part is the

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tendency of the FRS to want to oscillate at its dominant natural frequencies (i.e., global modes). But since the input motion is forced, the FRS is not allowed to freely vibrate and must respond to the forced motion using its natural frequencies as the motion filter. The amplification part is the tendency of the FRS to magnify the input motion and is typically related to the distance from the input source as exhibited in a vertical cantilever or building structure—the higher up, the greater the amplification. Thus, for a base-anchored FRS, the top of the FRS will amplify the horizontal input motion more than FRS locations near the base. In the extreme case of a perfectly rigid FRS, there is neither filtering nor amplification of the input motion. At every location on the FRS, the response motion is exactly the same as the base input. In this case, the transfer function is unity at all FRS locations. In realworld nonstructural applications, there is no such thing as a perfectly rigid FRS. In fact, most large-class FRS designs are nowhere near rigid and will exhibit various levels of flexible dynamic response over the frequency range of interest (1–35 Hz). In this example an analytical model is used to capture the necessary transfer functions between anchorage and functional devices. Figure 9-3 displays the locations for two functional devices that are tested as standalone units. A 30-s input motion excitation is applied to the UPS system model to represent the qualification environment as seen at the system level (i.e., top assembly). The input accelerograms are applied simultaneously in three orthogonal directions (triaxial) using phase-independent signals (multifrequency random). The transient results are used to define the transfer functions and establish the reference benchmark results with which to compare device testing. Our goal is to develop a device qualification test that can apply a seismic demand equivalent to the demand the device would receive from the initial system-level input motion. The term “equivalent” does not mean we will apply the exact time-history captured during the system-level transient. Equivalent means we will apply a similar amount of input energy at similar frequencies during device testing as was seen at the system-level. There are a couple of ways to accomplish this goal, and both options are discussed. The first method is the classical approach using response spectra as the functional device input. A second method is introduced that uses random vibration techniques to apply an acceleration power spectral density—APSD as the input (often referred to as power spectral density). Figure 9-4 outlines the necessary steps to develop a functional device qualification test using either the response spectra or random vibration approaches.

9.1.1.1 Response Spectra Device Test Chapter 7 discussed qualification testing principles. The standard procedure to conduct seismic qualification testing is application of a required response spectrum (RRS). The RRS we need is the response point where the functional device attaches to the large-class FRS—not a point on the device but the attachment point to the FRS. We are looking for the input motion that drives the loading into the device from the FRS. Once we have captured this interface input motion, a device qualification RRS can be constructed and used for standalone testing purposes. This procedure is followed for each response direction (x-, y, and z-axes shown in Fig. 9-3).

Combined Methods

Functional Device 2

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Functional Device 1

Y Z

X

Figure 9-3. UPS nonstructural equipment item with two functional devices identified for standalone qualification testing. Source: Illustration courtesy of APC by Schneider Electric, West Kingston, RI.

Development of RRS test spectra based on FRS transfer functions is a highly subjective process. The primary reason for this is that test RRSs are constructed as smooth spectral envelopes composed of ramp-up and ramp-down portions and flat sections, as shown in Fig. 9-5. The odds are likely high that no two individuals would create the same spectral envelope. This is not optimal for ensuring a degree of consistency when generating spectral envelopes to be used for functional device qualification testing. Spectral enveloping is not new. Many test specifications are developed using smooth up- and down-ramps and flat sections. As a general rule of thumb, 12 to 24 dB per octave is often used as a maximum slope for RRS test envelopes. The restriction on maximum slope is mainly driven by the capability of test machine controllers (specifically analog controllers). For any ramps that are steeper than 24 dB/octave, the test machine may not be able to deliver the requirement. Modern-day digital shake-table control systems may not suffer such restrictions on maximum slopes.

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Define System-level RRS Test Requirements (using AC156 & SDS , z/h)

Step 2 Develop System-level Test Accelerograms for Shake-table Input

or Step 3a

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Develop PSD Transfer Functions Between Anchorage and Functional Device

Develop RRS Transfer Functions Between Anchorage and Functional Device

Step 4a

Step 4b

Develop Functional Device APSD Envelopes used for “Random Vibration” Qualification Testing

Develop Functional Device RRS Envelopes used for “Response Spectrum” Qualification Testing

Figure 9-4. Large-class nonstructural process flow for creating a functional device qualification test requirement used for standalone device testing, supporting either response spectrum or random vibration techniques.

Earthquake Protection of Building Equipment and Systems

Step 1

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Figure 9-5. Functional device RRS showing smooth portions of the spectral envelope.

There are no guidelines that control the length of flat sections, so subjective license is used. Length of the flats is governed by characteristics of the spectral peaks that need to be enveloped. For example, a single peak needs less flat length to envelope compared with a wider double peak. A point to consider is that the transfer function between anchorage and device location is likely taken from a single production unit or is based on an analytical model. There will always be some variability in transfer functions between production units of the same platform. Thus, the RRS envelope should be constructed to account for variability in transfer functions and also account for the linear nature of the transfer function when in situ modal survey data are used. Figure 9-6 displays a revised RRS envelope using 12- to 24-dB/octave ramps, with a flat section that spans a third octave on either side of the peak, and maximum spectral values that include a 10% buffer. At this point we have developed our functional device RRS envelopes in three directions (Fig. 9-7). The next step is to conduct the device qualification test. The test lab must create a time-history accelerogram to satisfy the device RRS input for each direction. Chapter 7 describes the process for this. The device input is applied (via shake-table) to the device mounted on a test fixture that replicates the device mounting to the FRS. A response accelerometer is used to monitor the device response during the device qualification test. Our final check is to evaluate how well the device test input represented the original input the device saw during the system-level transient. Our goal is to create an equivalent

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Figure 9-6. Functional device RRS showing spectral envelope with shape rules to ensure RRS consistency.

demand from an input energy and frequency perspective. Figure 9-8(a) displays the original device response during the system-level transient and Fig. 9-8(b) shows the device response during the standalone device qualification test. The appearance of these two response accelerograms is quite different, although the time domain comparison does reveal similar peak accelerations around 4 g of response motion for each test. Unfortunately, the time domain comparison is of limited utility and a frequency domain comparison is needed. Figure 9-9 shows the response spectral acceleration comparison between the two responses. The frequency domain comparison can now be examined. The device response tested as a standalone unit is dominated by the fundamental natural frequency of the device (i.e., 17 Hz), with a sharp response peak at 17 Hz. Without the presence of the UPS structure, there are no other frequencies that participate in the device response. The device response when tested at the system level displays frequency contributions that originate from the UPS FRS and displays a broader overall response. The UPS structure has a fundamental mode around 6 Hz, and this mode filters into the device response. The peak response of the device when tested at the system level occurs at 13.5 Hz and not at 17 Hz. The 13.5 Hz is a combined peak response that includes both device and FRS mode contributions. Both responses display the 4 g peak acceleration, which shows up as the ZPA (zero period acceleration) on the response spectrum plot.

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(a)

(b) Figure 9-7. Large-class UPS functional device 1 RRS for each input direction: (a) side-to-side xaxis; (b) vertical y-axis; (c) front-to-back z-axis. (Continues)

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(c) Figure 9-7. (Continued)

There is another way to think about this. The low-frequency content that is present at the system level does get captured and is applied during the device test. However, the device has a higher natural frequency (i.e., 17 Hz versus 6 Hz), and the low-frequency input energy does not excite the device during the device test. Thus, the response spectra approach has done a reasonably good job in applying equivalent demands. The variables that influence this relative “goodness” include the device RRS envelope shape and the time-history accelerogram used during the device test. Spectral envelopes that are loose fits with liberal buffers will result in a more conservative device test. However, characteristics of the time signal used to drive the shake-table during device testing have a greater influence on device test results. Chapter 7 discussed this concept and the need to have objective measures in place to determine the drive signal’s adequacy in meeting appropriate amplitude intensity, frequency content, representative rise and decay rates, and proper phasing interactions. Without such measures, there can be considerable differences between two drive signals used to satisfy the same RRS. There can be considerable differences in the test results as well. Research is needed to help define objective quantitative measures used for gauging the adequacy of accelerograms for seismic testing purposes when utilizing the response spectrum approach.

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(a)

(b) Figure 9-8. Large-class UPS device 1 response accelerograms in the front-to-back z direction: (a) from UPS system-level test; (b) from standalone device-level test.

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Figure 9-9. Large-class UPS device 1 response spectra results for the front-to-back z direction, comparing the UPS system-level results to the device-level qualification tests.

9.1.1.2 Random Vibration Device Test Random vibration techniques have been used for decades in product design assurance testing. The techniques were developed to address environmental loading conditions that are difficult to define deterministically (Crandall and Mark 1963). Vibratory loads generated by jet engines, rocket engines, transportation and shipping loads, and loads resulting from earthquakes all exhibit certain random characteristics. In random vibration theory the aim is to predict the probability distribution of the response parameter. The goal is to determine the average energy that can be associated with the mean square value of either an excitation or a response. The mean square value is the time averaged measure of energy and can be related to displacements, forces, stresses, and accelerations, among others. Thus, the mean square acceleration response from our UPS system-level transient is the average of the square of the acceleration over time. In other words, look at the accelerogram of the device-FRS interface, square this time-history, then determine the average value for this squared acceleration over the length of the accelerogram. That is the mean square acceleration. Figure 9-10 displays the APSD of the device–FRS interface that resulted from the system-level UPS test. The APSD plot has units of g2/Hz for the ordinate and frequency in Hz for the abscissa. From the APSD function we can determine two important statistical measures to gauge the overall energy content of the response: (1) root mean square acceleration, GRMS, and (2) effective frequency, fEFF.

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The GRMS term is the square root of the area under the APSD versus frequency curve. If the accelerogram is a stationary Gaussian random time-history, the RMS acceleration (also called the 1 sigma acceleration) is related to the peak acceleration via statistical properties. To put this into perspective, with a pure sinusoidal function the peak acceleration exactly equals 1.414 ⫻ RMS. In random vibration, there is no simple relationship between the peak and the RMS value. The peak value of a random time-history is typically 3 or 4 times the RMS value (or 3–4 sigma). The GRMS value for the Fig. 9-10 APSD is 0.87 g. In this example, the peak value is about 3 times the RMS value (2.47 ⫼ 0.87 ⫽ 2.84), which confirms a random-like response. Effective frequency (or sometimes called statistical average frequency) represents the average number of positive zero crossings per unit time. If it can be assumed that the excitation is normally distributed or Gaussian, then it is possible to determine the overall effective frequency, which is the statistically most significant frequency over the APSD frequency range. The effective frequency for a stationary normal random excitation with zero mean is defined as 2 ∫−• f SR ( f ) df +• ∫−• SR ( f ) df

+•

fEFF =

(9-1)

where SR (f ) is the power spectral density of response variable as a function of frequency (i.e., acceleration spectral density in this example). The fEFF for the device–FRS response taken during the system-level transient is 10.2 Hz.

Figure 9-10. Large-class UPS functional device 1 at the FRS interface acceleration spectral density for front-to-back z axis.

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The approach followed to create a random vibration device test is similar to the response spectra approach in that an envelope APSD must be constructed from the APSD results at the interface of the device and FRS. The same general rules apply for making the APSD spectrum envelope—restriction on maximum slopes, appropriate flat lengths, and spectral magnitude buffers to account for transfer function variability. Figure 9-11 displays our functional device APSD envelopes in three directions. The next step is to conduct the device random vibration qualification test. This time the test lab uses random signal theory to create shake-table drive signals to satisfy the input APSD. Subjective processing of input accelerograms is eliminated with random vibration-based inputs. Similar to the response spectra test, the device APSD input is applied to the device mounted on a test fixture that replicates the device mounting to the FRS. A response accelerometer is used to monitor the device response during the random vibration qualification test. Figure 9-12 displays the APSD results comparing the device response from the UPS system-level test and the device standalone test. The device response (z direction) during the device test is dominated by the 17-Hz device natural frequency. Similar to the response spectrum comparison in Fig. 9-9, the device APSD taken from the UPS system results displays a broader overall response with peaks at 5.9 and 13.7 Hz. These are structural FRS modes from the UPS that have filtered into the device response. Also shown in Fig. 9-12 are the random vibration statistical measures to gauge overall energy content. These statistical comparisons reveal, without doubt, that the device standalone qualification test results exceed the levels that would have been received if the UPS were tested at the system level. In fact, a slight device overtest resulted (29% increase), likely attributed to the APSD spectral envelope that was constructed. One of the major advantages of random vibration-based device testing is that subjective selection of final drive signals used for testing is eliminated. The drive signal used must satisfy random signal theory, which is based on well-established measures. The other advantage is consideration of the GRMS and fEFF statistics from the device test. These measures offer good, objective evidence to compare equivalent demands. The two approaches presented for standalone device testing (i.e., response spectrum and random vibration) both require creation of spectral envelopes at the interface where the device secures to the FRS. The spectral envelope then becomes the input to the device test. There is an often overlooked device testing problem that can occur when test specifications are constructed as described: overtesting of the functional device because of mechanical impedance relationships that occur at the interface connection point. In some situations the potential overtest can be significant (⬎200%) and thus needs to be better understood. This issue only occurs in real testing situations with fully equalized shake-tables and does not manifest when performing analytical investigations as demonstrated here.

9.1.1.3 Mechanical Impedance Effects The standard practice of basing shock and vibration test specifications on an envelope of the device acceleration levels experienced at the interface between the device and FRS often results in excessive levels of overtesting. This is attributed to the large differences

Combined Methods

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(a)

(b) Figure 9-11. Large-class UPS functional device 1 ASD test envelopes for each input direction: (a) side-to-side x-axis; (b) vertical y-axis; (c) front-to-back z-axis. (Continues)

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(c) Figure 9-11. (Continued)

between the mechanical impedance of the device–FRS interface at the system level and that of a fully equalized shake-table used during device testing. The dynamic motion of the FRS can be significantly affected by the interface reactions of the attached functional device. At the device natural frequencies (fixed base), the device may exert large reaction loads that resist the forced FRS input motion. This resistance loading can be enough to drive down the FRS acceleration input at these frequencies. This will show up as a subtle response dip or decrease in acceleration of the device–FRS frequency response. This spectrum dip phenomenon is often called antiresonance. The use of acceleration spectral envelopes for device test requirements will invariably envelope over (i.e., ignore) occurrences of antiresonance. Then, once the device is placed on a fully equalized shake-table for device testing, the device will once again resonate at the device natural frequencies and start to resist (drive down) the shaker input. However, this time the shaker control system will immediately respond to this reduction in input acceleration by simply applying more force to equalize the required acceleration demand. From the perspective of the device, a fully equalized shake-table has an infinite amount of mechanical impedance. A seemingly innocuous spectrum dip can result in significant overtest conditions whether using the response spectrum or random vibration approach. The magnitude of this infinite impedance overtest is dependent on the amount of damping in the system and

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Figure 9-12. Large-class UPS device 1 random vibration ASD results for the front-to-back z direction, comparing the UPS system-level results to the device-level qualification tests.

on the dynamic properties of both the device and the FRS. The assumption that the dynamic properties of the device are negligibly small compared to the dynamic properties of the FRS system is not valid for most cases. Even for cases in which the device mass is 1/100 of the total FRS system mass, there will be some amount of mechanical impedance effects as described here (Crandall and Mark 1963; Neubert 1987). Device testing techniques using dual control of both acceleration and force have been fully incorporated into device tests for high-value aerospace projects (Scharton 1997). This procedure requires insertion of force transducers between the device and the shake-table. During the device test, both acceleration and force levels are monitored to prevent the control system from overtesting the device. As the device resonates at its natural frequencies, the shake-table becomes force-controlled (i.e., force-limited), and at all other frequencies the device test is acceleration-controlled. This prevents overtesting at the critical natural frequencies of the device. The subject of mechanical impedance and the reality of the infinite impedance overtest are not easily recognized by many design professionals. We believe this subject merits discussion such that design professionals are aware of the potential consequences when conducting standalone device qualification tests. There are likely many instances in which a device overtest does not cause problems with active operation performance. For those instances in which the design margin is thin, the dual control device test may be an attractive option.

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9.1.1.4 Inelastic Response Modification A major limitation of subsystem testing, as described here, is the linear nature of the process. The creation of device-level spectral envelopes, based entirely on linear transfer functions, is overly conservative. A linear approach is not representative of system-level response during earthquake excitation or during simulated earthquake excitation. A simple example of this can be observed when comparing transfer functions taken in situ with a modal hammer versus those taken with base input on a shake-table for the same equipment platform. The response amplitudes will be broader with greater damping for base input excitations compared to modal hammer. In situ-based transfer functions are linear with larger peaks, less damping, and at slightly higher natural frequencies. Inelastic response modification is a way to reduce transfer functions that reflect linear modal properties. Nonstructural platforms constructed of ductile structural members (FRS) offer significant inelastic resistance under base excitation. The development of device-level qualification spectra should account for FRS inelastic response. This topic is well suited for academic research. Inelastic reduction is directly embedded into the building code’s lateral force requirement (Eq. 4-6 in Chapter 4). Allowance for inelastic reductions in the development of device-level transfer functions is a topic that needs research investigation. This would affect both Steps 3a and 3b as shown in Fig. 9-4.

9.1.2 Mechanical Large-Class Nonstructural The mechanical system for this example is an open-air, cross-flow cooling tower. Figure 9-13 is an illustration of this large-class platform. This represents a production series of cooling tower offerings that contain many different box sizes and configurations. The larger open-air towers can be upward of 4.3 ⫻ 7.3 ⫻ 8 m (14 ⫻ 24 ⫻ 26 ft) in size and weigh around 25,000 kg (55,000 lb). These larger units are not suitable for testing as complete units due to the size limitations of most test facilities and the general impracticality of testing the units. The primary function of cooling towers is to eject waste heat into the atmosphere by evaporative cooling. This is accomplished through a complex interaction of the various mechanical subsystems within the towers. These subsystems must remain largely intact after an earthquake for cooling towers to perform their intended function (Papavizas 2008). Therefore, the structural integrity of these subsystems is the main concern. In this example, active operation performance is more dependent on structural strength aspects compared with the electrical platform. The qualification strategy for this large-class mechanical platform is to identify, using analysis, the critical subsystem elements (i.e., structural) that are most susceptible to failure during earthquake loading. This process essentially creates a subsystem capacity ranking based on which elements are most likely to fail first (i.e., identification of mechanical subsystem weak links). Next, a smaller-sized platform configuration is selected that contains the critical elements. A dynamic qualification test is conducted on the smaller platform to validate active operation performance. The testing also confirms that no other elements are critical that might have been missed during the analytical investigation. In

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6.3 (20.8)

6.5 (21.5) 3.6 (11.8) Dimensions in meters (feet)

Figure 9-13. Large-class mechanical cooling tower platform used in combined analysis and testing qualification. Source: Illustration courtesy of Baltimore Aircoil Company, Baltimore, MD. addition, the platform test results are used to correlate the predicted behavior from finite element analysis (FEA) with the test behavior. Once a validated finite element model (FEM) is achieved, the rest of the product line is analyzed to extrapolate seismic compliance based on allowable stress design. Figure 9-14 shows a qualification flowchart for this large-class mechanical platform. The final seismic ratings are based on the allowable strength of the weakest structural elements. Figure 9-15 highlights examples of key cooling tower components, including the

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Step 1 Analyze platform offerings (e.g., FEA)

Step 2 Identify structural subsystem critical elements “weak-links”

Step 3 Select test unit(s) based on practical size restrictions

Step 6 Correlate model (FEM) behavior with test unit response

Step 7 Define analysis acceptance criteria based on test/analysis correlation

Step 8 Step 4 Dynamic test unit(s) using system-level response spectra inputs via AC156

Step 5 Monitor and record response of critical elements during dynamic test

Re-analyze platform offerings using defined acceptance criteria (extrapolations)

Step 9 Establish platform seismic capacity ratings based on tests and analytical results

Figure 9-14. Large-class mechanical qualification flow chart used for cooling tower platform. box end tie-rods and tie-rod shear bolts. To validate that these are the only key components, during dynamic testing strain gauges are installed on other possible locations and compared with the stresses of the key components. In the FEA other components are modeled, and the resulting stresses are reviewed. Both analytical and experimental studies confirm that the key structural elements are directly related to the seismic rating. Therefore, the analytical approach is used to derive the ratings for different-sized units (i.e., extrapolation) based on the allowable stress criteria.

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Tie Rods

Tie Rods

Structural Framework

Tie Rod Tie Rod Shear Pin

Shear Pin

Shear Pin Detail

Endwall Detail

Figure 9-15. Large-class mechanical cooling tower platform showing the structural framework and key structural elements used in combined analysis and testing qualification. Source: Illustration courtesy of Baltimore Aircoil Company, Baltimore, MD.

The critical steps in this process include identification of subsystem weak links and correlating the FEM to match qualification test data (Steps 2 and 6 of Fig. 9-14). These steps are not trivial and obviously require platform-specific design experience. When mechanical-type elements (i.e., structural) are the key indicators behind active operation performance, analysis becomes a primary enabler of large-class qualification. Limited system-level qualification tests are conducted on reasonable-sized units to validate active operation performance. Analysis is used to cover the units that are too massive for system-level testing and to assess the impact of product options and accessories.

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9.2 Large-Class Qualification Summary Active operation qualification for large-class systems is very challenging subject matter. We speculate that many suppliers of these systems might have been caught off-guard as the compliance strategy has transitioned away from position retention to active operation validation using either testing or experience methods. This is especially true for state and local jurisdictions that have adopted special enforcement procedures for seismic compliance verification (e.g., California). Most large-class nonstructural systems are expensive, highly complex, and physically massive. These qualities make the task of seismic compliance no easy road to follow. The two examples described here are by no means the only paths to achieve qualification. There are likely as many variations of these paths as there are unique large-class platform types. Each platform type will dictate the most expedient and practical path to follow. The goal of this chapter is to demonstrate several possible solution options to a challenging problem so that large-class product suppliers can move past the ignoring phase and begin initial implementation of the qualification path that best suits their individual needs. This topic is well suited for academic research participation. Partnership relationships between large-class suppliers and researchers could be established that would be mutually beneficial. For example, the standalone device testing method could benefit from comparison investigations between response spectrum and random vibration techniques: How to establish functional (active operation) equivalence at the device level; how to account for inelastic response reductions when using linear transfer functions; how to construct consistent spectral envelopes; how to prevent overtesting and restrict potential undertesting; how to select appropriate shaker drive signals for response spectrum testing; how to adjust procedures for different damping characteristics. These topics are all relevant to effective implementation practices. The intellectual property (IP) barrier between academia and industry needs to be readdressed. This barrier has historically prevented many opportunities for nonstructural suppliers to work with the greater earthquake engineering research community. There is need for a meeting of the minds on IP issues, from both parties, in order to move the nonstructural qualification ball forward. The desired research output is consensus advancement of a set of best practices for implementation of building code seismic requirements specifically targeting large-class nonstructural systems. The expectation that nonstructural suppliers will be willing and able to implement active operation qualification for any large-class system entirely on their own accord is not very likely because of the costs involved and other practical limitations. The expectation from regulators and specifying engineers that large-class suppliers just need to test their platforms using large-scale shake-tables is equally unrealistic, again because of costs and shake-table limits. There is a middle ground using subsystem decomposition and comparative assessment, as described here, that needs to be investigated and requires collaboration between industry and academia. Without a consensus implementation strategy that is rooted in sound engineering principles, engineers end up with a continued approach of blithely ignoring the new code requirements with the hope that no one will notice or care.

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Most large-class electrical and mechanical nonstructural systems have evolved over the last quarter-century to include an increasing number of sophisticated electronics and electromechanical active functions. This trend will only continue in the future and pose new challenges for those involved with meeting the seismic protective measures outlined in model building codes and other standards. Application of combined qualification methods, using subsystem decomposition, will likely prove useful as nonstructural seismic protection evolves to include more performance-based engineering strategies. The future direction of earthquake protection is uncertain. However, it is wise to scan the horizon and investigate what new direction seismic risk mitigation might be taking. In the next chapter we discuss the future trends for earthquake protection and the implications for nonstructural implementation.

References Crandall, S. H., and Mark, W. D. (1963). Random vibration in mechanical systems. Academic Press, New York, NY. Neubert, V. H. (1987). Mechanical impedance: Modeling/analysis of structures. Jostens Printing and Publishing Co., State College, PA. Papavizas, P. G. (2008). “Seismic qualification of cooling towers by shake-table testing.” Cooling Tech. Inst. J., 29(2), 64–72. Scharton, T. D. (1997). “Force limited vibration testing monograph.” NASA Reference Publication RP1403, Jet Propulsion Laboratory, Pasadena, CA.

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Chapter 10

Trends in Earthquake Protection of Nonstructural Systems

Earthquake qualification for nonstructural systems and components made great strides during the 1970s in both philosophy and implementation, mostly due to the nuclear power industry and the Trans-Alaska Pipeline. These megaprojects adopted systems design approaches to achieve seismic qualification. Embedded in their approach was a performance-based philosophy that established levels of performance. Then, in the 1980s awareness of nonstructural issues began to filter into everyday use in the building codes (McGavin 1981). Model building codes gradually evolved to embrace performance-based design principles. Fire codes have historically made significant strides by implementing performance design. ASCE/SEI 7-10 (ASCE/SEI 2010) has set the stage for these future advancements in design for the seismic environment for building equipment and their systems. We support performance-based design; the objective of this chapter is to take a look at the direction that nonstructural protection is heading. We believe that performance-based design and earthquake early warning systems (EEWSs) lie on the horizon as tools for designers. Nonstructural research needs compatible with this new direction are outlined.

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10.1 Performance-Based Design Performance-based design (PBD) as a philosophy has a long historical precedent in the model codes outside of seismic design issues. Originally, the model codes for seismic design were almost completely prescriptive and generally only addressed life safety. PBD will philosophically change the basis for design in the near future, allowing stakeholders more latitude in design methodologies as they embrace the philosophy embodied in ASCE/SEI 7-10. The stakeholders will have new criteria and goals beyond life safety, including continued operation and immediate occupancy following a damaging earthquake, cost ramifications for performance criteria, and life-cycle goals. PBD will also hopefully extend beyond seismic design issues and include multihazards, such as fire following earthquake, flood, and damage caused by terrorism. The original PBD allowances in the legacy model codes sprang from a planning model that wished to restrict designers from designing “dark canyons” in densely packed urban settings, where buildings were commonly designed right to the sidewalks. Building heights were limited by planning ordinances (which are a bit different from building codes) so as to avoid these artificial canyons. Owners who wanted to maximize their usage of building lots with more stories worked with city planners to provide building setbacks as their buildings rose taller and taller, which had a mitigating effect on the canyons. This compromise was a performance-based philosophy that sidestepped the prescriptive nature of the hard-and-fast height limits of the planning ordinances. Another early use of PBD came with increases in allowances for building areas based on front, rear, and side yard setbacks, the use of more fire-resistive materials (no protection, 1-h protection, 2-h protection), and finally the use of fire suppression systems. These mitigating measures provided allowable increases in building area, as well as increases in allowed stories by the building codes. Seismic PBD has been slower on the uptake in the model code arena. The philosophy of performance-based seismic design was in use in the 1970s on projects that were not code-based, which included nuclear power plant design and the Trans-Alaska Pipeline. Nonstructural philosophies were addressed for systems and components within the systems during the design of these megaprojects. Earthquakes were generally defined in two types of categories. These essentially were the following: • Operating Basis Earthquake: A smaller earthquake that was likely to be experienced at least once and possibly more than once during the operational life of the facility, under which the facility would continue to operate throughout the event as well as following the event. • Safe Shutdown Earthquake: A larger earthquake that could statistically occur during the life expectancy of the facility, which could severely affect the facility if operations were to continue throughout the strong motion. In the case of this earthquake, certain systems and components were required to remain functional during the earthquake strong motion so that the facility could shut down in a safe manner without the release of material, such as radioactive coolant or oil.

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The aforementioned complex facilities demanded a systems approach to both the design and qualification procedures. All nonstructural components within the facility were assigned to a nonstructural system. There were a variety of systems ranging from critical for operation to noncritical. An example of a critical system is the emergency power system, which consisted of numerous components and in some cases subsystems. Most nonstructural systems and components, within general-use facilities, cannot be expected or required to operate during strong ground motion. General-use facilities will likely have parameters in PBD for immediate startup following strong motion, or there may be an option for basic life safety. The building owner sets these expectations by selecting a desired level of performance from the code’s PBD criteria. Discussions began in the late 1990s for such an approach (Tang et al. 2008) for building structures with the issuance of FEMA 273/274 (FEMA 1997a, b). In the early 2000s FEMA reissued FEMA 273/274 as FEMA 356 (FEMA 2000b), and then FEMA funded ATC-58 to study the practical applications for PBD for both building structures and nonstructural systems and components. According to Bachman et al. (2004), the nonstructural portion in ATC-58 has one charge, which is stated as a requirement: To develop a basic performance prediction tool that can be used to quantify the performance of nonstructural building components subjected to earthquake hazards. It will do this by 1. Developing a ground motion hazard function for the site; 2. Performing an analysis of the primary structural system for the ground motion hazard; 3. Performing an analysis or test of the nonstructural system using the structure analysis results as the input motion; 4. Determining level(s) of damage by comparing responses of nonstructural systems with fragility functions; 5. Determining direct impacts of damage (cost to repair, downtime, loss of function, personnel safety hazard, etc.); 6. Determining the overall project loss or impact (people killed or injured, overall cost of repair, overall downtime, cost of loss of use, effects associated with loss of function); and 7. Performing all of the above steps on a probabilistic basis with uncertainty bands associated with each determination considered explicitly at each step (Bachman et al. 2004). It is also hoped that the long-term goal of ATC-58 for performance-based nonstructural design will develop and produce the following if long-term funding is realized: 1. Develop a catalog of nonstructural components, systems, and contents that matter and identify scope [Task 3.1.1 in FEMA 349 (FEMA 2000a)];

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2. Identify nonstructural performance measures; 3. Identify input engineering demand parameters (EDPs) for nonstructural components (Task 3.2.1 of FEMA 349); 4. Identify available fragility data for nonstructural components and develop a database to establish comprehensive testing and certification protocols (Task 3.3.1 of FEMA 349); 5. Develop procedures for predicting response EDPs from input EDPs; 6. For selected nonstructural components (Task 3.2.3 of FEMA 349), develop procedures for converting response EDPs to damage measures (or performance measures); 7. Develop standard loss functions for nonstructural components; and 8. Conduct performance evaluation case studies and/or testbed verification checks. Long-term governmental funding is at best tenuous for nonstructural PBD development. It is nonetheless the next logical step for code implementation. Recently, the National Science Foundation (NSF) funded a multihazard grant to study the actual effects of an earthquake on a building to be constructed on the University of California–San Diego seismic shake-table. This particular test is primarily aimed at studying the earthquake effects on nonstructural systems and fire following an earthquake. This test genre is long overdue. It is the first nonproprietary nonstructural systems performance-based test outside of otherwise very ambitious programs such as the nuclear power industry and the Trans-Alaska Pipeline. The latter showed its value in the overall excellent performance sustained by the pipeline and remote pumping stations as a result of the M7.9 Denali earthquake in 2002, approximately 20 years after the pipeline startup. This seismic test, and what will be learned from it, is the first giant step forward in setting the parameters for the implementation of PBD for nonstructural earthquake protection. Hopefully, this study will also lead to many more systems-level test programs in the near future. The maturity of PBD for nonstructural seismic design applications is in the early stages. The time frame for a comprehensive implementation in model building codes is likely years away. There are, however, several areas of nonstructural research that would provide immediate beneficial effects regarding implementation of present-day nonstructural protective measures. These research areas are considered complementary to the new direction toward performance-based seismic design. ASCE/SEI 7-10 contains provisions for PBD in Section 1.3.1.3 for structural and nonstructural applications.

10.2 Nonstructural Research Needs After decades of neglect, nonstructural research has recently become a topic of interest in the earthquake engineering community. This trend is likely attributed to the recognition that nonstructural systems are paramount for the continued operation of essential buildings. In addition to this, as the science and understanding of building structural performance has increased over the past few decades, we have begun to realize that nonstructural

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items may cause the greatest loss to life (and cause the most injuries) as well as causing a great economic loss in otherwise well-prepared and well-designed facilities. Earthquakes over the last quarter-century have proven the nonstructural vulnerability when it comes to maintaining building functions after the event. Building structural designs have made good progress in mitigating earthquake damage, in most part due to extensive research, while nonstructural systems have lagged behind in earthquake protection effectiveness, with limited research interest. Perhaps the time has come for nonstructural research to move to the forefront. A motivating factor for past nonstructural research droughts may be attributed to the issue of intellectual property (IP). The primary barrier has to do with protection of IP rights when industry collaborates with academia in nonstructural research. Most building elements, such as steel columns, reinforced masonry walls, and moment-resisting frames, contain little IP. Conversely, most all architectural, mechanical, and electrical nonstructural platforms as well as owner-supplied building contents contain a significant amount of IP. Thus, manufacturing companies and nonstructural suppliers are, most often, a little nervous when it comes to collaborating with academic researchers because of IP concerns. It is therefore not too surprising that nonproprietary nonstructural research has lagged far behind structural research. The researcher’s goal is to investigate the science behind a specific problem and disclose the findings in open literature. The manufacturer has a diametrically opposed goal of keeping everything regarding product design or product-related manufacturing processes partially if not fully undisclosed. Sometimes these opposing strategies can be worked out between the university and the manufacturer after intensive legal negotiations and, with active management oversight, the collaboration process can reap benefits for both the researcher and manufacturer. However, most often, there is mutual agreement that these competing positions cannot be adequately resolved, and the two parties go their separate ways or produce proprietary reports that cannot be released to the scientific research community at large. While this overly simplistic view skips past the high points of an undoubtedly important and successful relationship responsible for fueling the engine of industrial development in the United States, there is definite truth to it. Tearing down the IP barrier (or at least bypassing it) is necessary in order for nonstructural research to move into the 21st century. How best to ameliorate the IP issue between academia and industry is presently an open question. However, there are several research topics that have absolutely no IP implications and can be immediately pursued. These topics will advance nonstructural earthquake protection and set the stage for eventual PBD procedures.

10.2.1 Earthquake Early Warning Systems Performance-based design will benefit greatly from widespread use of earthquake early warning systems (EEWSs). As previously noted, an EEWS is not earthquake prediction. It is notification that an earthquake has occurred based on the system observation of fasttraveling primary waves and the time delay arrival of the secondary and surface waves. This requires real-time signal analysis of these waves to determine whether the slower,

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more damaging waves are likely to create severe ground shaking such that proactive measures can be taken before they arrive. A comprehensive EEWS system should be capable of the following: • • • • • • • • • •

Real-time P-wave detection Discrimination between seismic and artificial events Distinguish between major and minor seismic events Standalone system or interagency/facility system Redundant sensors Audible and visual alert notification Automated systems response (e.g., begin emergency power transfer sequence) Customization of alerts/alarms Remote monitoring Postevent information, including aftershock evaluation

The future use of EEWSs will be akin to the current widespread use of smoke and fire alarms in use today. Performance-based design will benefit by being able to initiate power transfer sequences, for example, when the early warning is received rather than waiting for the slower and more damaging long (surface) waves to arrive at the site (refer to Fig. 3-15 in Chapter 3).

10.2.2 Nonstructural Dynamic Demands The reality of modern-day seismic provisions makes dynamic testing a key enabler for nonstructural compliance. Without the ability to test and analyze using dynamic demands, many essential nonstructural systems (i.e., designated seismic systems) would not be available to populate essential building structures. There is a gap in current code requirements without direct inclusion of nonstructural dynamic requirements (i.e., response spectrum) alongside static force demands. In the absence of code-specified dynamic demands, stakeholders are forced to interpret dynamic requirements as best understood. Even with existing dynamic interpretation protocols already code-sanctioned and readily available, such as ICC’s AC156 standard (ICC ES 2010), many stakeholders are either unaware of this procedure or simply do not recognize this interpretation as a code dynamic demand requirement. In either case, elimination of all code misinterpretations can be readily accomplished with code adoption of floor-level response spectrum demands for nonstructural equipment and systems. The approach taken in AC156 development was to create a single generic floor motion spectra requirement that is independent of building dynamics and independent of nonstructural type classification. The AC156 floor spectrum is fully correlated with the code’s nonstructural lateral force demands. The starting point for development was translating the code’s static force requirements into dynamic test spectra. The end result is a conservative, broadband response spectrum that essentially umbrellas all building types and all floor elevations. While the AC156 spectrum does implement a maximum cap, it is nonetheless quite conservative.

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The development of more refined generic floor spectra could be based on considering widely used types of building construction. The idea is to create subcategories of generic floor spectra demands that would be more applicable based on the dynamics of typical building classifications. For example, building construction with moment-resisting frames versus construction using shear walls could be defined as subcategory code options. In essence, these alternative floor spectra requirements would be more representative of building dynamics and not suffer from the inherent conservatism built into an “all-inclusive” spectra. Code implementation of this approach could result in three levels of nonstructural building floor spectra options to select from: • Generic Building Spectra: This option would be very similar to the umbrella spectra defined in AC156. A single spectrum intended to umbrella (with a maximum cap) all building types and all floor elevations within the building, independent of building dynamics. • Subcategory Generic Building Spectra: This option would provide response spectra that have been dynamically tailored to address a typical subcategory of building construction. These spectra are still generic in nature but are more representative of (i.e., more dependent on) the various construction types covered. • Project-Specific Building Spectra: This option is not a generic spectra requirement but a fully dependent building-specific floor spectra requirement based on the projectspecific building application. The proposed new floor spectra demands and the code’s existing lateral force demand would all utilize the same ground motion intensity parameters.

10.2.3 Subsystem Qualification Testing Nonstructural OEMs and suppliers are given the majority of work content required to qualify mechanical and electrical equipment and distribution systems to satisfy IBC (ICC 2011) and ASCE/SEI 7-10 seismic provisions. Implementation of modern-day nonstructural protection is no easy endeavor. One of the aspects of nonstructural qualification that likely discourages many OEMs from conducting any qualification is the prospect of having to requalify a product platform because of modifications and/or additions made to the platform’s many functional devices (e.g., subassembly modules). Most nonstructural platform designs are highly fluid. New widgets and devices get incorporated into the platform on a regular basis. Full-scale retesting of platforms every couple of years because of additions or modifications made to functional devices is not a practical option to pursue. A solution to this real-world qualification problem was introduced in Chapter 9. Subsystem qualification testing (i.e., functional device standalone testing) is a viable alternative to address qualification of nonstructural platforms that contain highly fluid functional device subsystems. When qualification testing can be performed at the device level, the overall cost of nonstructural qualification is greatly reduced, making the process more practical to implement.

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The alternative is what we have today. Many suppliers of essential nonstructural systems (i.e., designated seismic systems) simply ignore the active operation requirements and offer anchorage calculations as building code compliance. This is especially true for suppliers of large-class nonstructural systems. When the authority having jurisdiction (AHJ) has too few suppliers able to satisfy active operation stipulations, code waivers are accepted in order to populate essential buildings with the needed nonstructural systems. The premise behind subsystem qualification is that device-level testing, to validate active operation performance, is more reliable than validation based on mere anchorage calculations. In addition, there will be more nonstructural suppliers willing to conduct comprehensive seismic qualification programs knowing that system-level retests are not necessary with every change made to the platform’s devices and widgets. Research participation is needed to arrive at a set of subsystem testing best practices that have consensus agreement on the best implementation strategy. The needed research focus regarding subsystem qualification testing includes the following items (refer to Chapter 9 for discussion on the techniques). These topics are relevant to effective implementation practices: • How to compare device test results between response spectra and random vibration testing techniques • How to establish functional equivalence (for active functions) at the device level • How best to measure system transfer functions • How to adjust transfer functions to account for inelastic response (linear functions are too conservative) • How to construct consistent spectral envelopes from device transfer functions • How to prevent device overtesting and restrict potential undertesting • How to adjust procedures for different damping characteristics A secondary motivation to advance this approach comes from a nonstructural PBD perspective. At the heart of performance-based seismic design is the need to establish fragility functions, where performance is expressed as the probable consequences of earthquake damage. Each damage state is expressed as a probability function, indicating the likelihood of the loss assessment based on the intensity of the earthquake ground shaking exposure. The only realistic method to determine active operation performance is testing. This necessitates using multiple samples of the test item to construct the needed probability functions. The only practical method to conduct fragility-type testing is employing subsystem testing techniques. When testing is conducted at the device level, multiple devices can be appropriated and tested at a reasonable cost. When testing is conducted at the toplevel assembly, full-scale testing of multiple units becomes impractical and too costly for nonstructural suppliers to justify.

10.2.4 Response Spectrum Accelerogram Integrity The response spectrum is a highly convenient method to represent building floor motion demand requirements. The method has a long history in earthquake engineering applica-

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tions. However, the development of shake-table accelerograms (time versus acceleration input) to satisfy a response spectrum requirement is a subjective process. There are no objective measures defined to control the adequacy of accelerograms used for seismic simulation purposes. There are unlimited ways to create accelerograms that can satisfy the general shape of the spectrum requirement. The problem is that not every accelerogram is well suited for seismic testing applications. There is a research need to define a set of objective measures than can be applied to gauge the adequacy of accelerograms to meet response spectrum test requirements. The target application is seismic qualification testing of nonstructural systems. The measures should include criteria that address: (1) adequate amplitude intensity (including velocity and displacement intensity), (2) adequate frequency content, (3) representative rise and decay rates, and (4) proper phasing interactions. The research output would be a set of measurement criteria, which test labs can apply, to determine whether the shake-table drive signal (i.e., accelerogram) used for seismic qualification is acceptable and consistent with that found in measured building floor motions during earthquakes.

10.2.5 Inelastic Response Reduction The nonstructural lateral force equation (see Eq. 4-6) provides a mechanism to account for probable inelastic response of the nonstructural FRS under design-level earthquake demands. The code’s component response modification factor, Rp, is used for this purpose. Response reduction represents the allowable inelastic energy absorption capacity of the nonstructural FRS. The code provides Rp coefficients dependent on the type of nonstructural system (see Table 4-8). These coefficients are generic values based on loose groupings of similar systems. Obviously, not every possible type of nonstructural system can be included in a generic table. In addition, the code-provided Rp coefficients do not represent specific nonstructural FRS designs. There is need for a nonstructural design procedure that can determine actual Rp coefficients for specific nonstructural platforms. This would provide a design option for nonstructural platforms that are presently not listed in Table 4-8 (for example, owner-supplied equipment like medical treatment technology and other specialized process equipment). This would also provide a design option to determine a platform-specific Rp coefficient. Inelastic energy dissipation is an integral aspect of earthquake engineering. The nonstructural FRS is no exception. The various Rp coefficients contained in the code are useful to an extent. While the principle of response reduction is embedded in the code’s nonstructural philosophy, the implementation could use some refinement. The ability to determine platform-specific Rp coefficients is one needed refinement. We believe that inelastic response modification will be an integral aspect of eventual nonstructural performance based assessments. The nonstructural input demand for PBD approaches is based on building structure inelastic response. The nonstructural FRS will also experience inelastic response. Therefore, subsystem testing techniques will require demand levels that are adjusted to account for FRS response reduction due to inelastic behavior.

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Research output would be a design procedure to determine a nonstructural response modification factor using either empirical or analytical techniques. Empirical techniques would likely involve some form of shake-table testing nonstructural systems. Analytical techniques would likely involve nonlinear material elastic-plastic analysis techniques.

10.2.6 Clearance Demand Guidelines The code’s requirement for systems interaction avoidance (i.e., consequential damage) is a necessary first step toward incorporation of nonstructural systems design principles. Implementation of this philosophy requires awareness by building inspectors and design professionals regarding the amount of free clearance space surrounding nonstructural installations. We have coined this phrase as the nonstructural clearance demand. Consequential damage occurs when systems negatively interact under design-level earthquake motions. Contact and collision between system elements occurs when inadequate clearance is established during nonstructural installation. Appropriate clearance is needed between nonstructural systems and between nonstructural systems and the building structure to avoid occurrences of consequential damage during earth shaking. Examples of clearance demand are shown in Figs. 10-1 through 10-5 and in Fig. 1-18 in Chapter 1. Explicit definition of what constitutes appropriate clearance is not a simple task. There are too many variables involved to definitively prescribe clearance requirements. However, there can be general guidelines established regarding nonstructural clearance demands that would provide the needed awareness to the stakeholders responsible for inspection of nonstructural installations. In addition, basic quantitative clearance guidelines could be incorporated into building design technology like BIM (Building Information Modeling). This would provide clearance threshold limits for which BIM clash detection alarms can be set. For example, a detection alarm notifies the designer that a pipe run has violated the required clearance envelope and is a potential system interference problem. Basic clearance limits would establish the BIM clash detection rules. The research need is for a compilation of potential nonstructural system elements that are most prone to create consequential damage when inadequate clearance (or no clearance) exists at the building site. The list should include recommended clearance limits based on earthquake reconnaissance reports and large-scale seismic simulation testing in which building and nonstructural systems are represented. The code’s subtle reference made to consequential damage avoidance is the cornerstone for adoption of the nonstructural systems design philosophy prescribed throughout this book. One of the goals of this book is to be much less subtle regarding communicating the importance of nonstructural systems interaction avoidance. To that end, we believe this goal was achieved. The real test is the influence that our sometimes overt nonstructural philosophy will have on shaping future seismic code provisions. Nonstructural components are only components if one considers the entire building system to which they belong. In fact, even considering nonstructural in the context of

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Figure 10-1. An illustration from the 1994 Northridge earthquake of the concept of “clearance demand” for a water inlet pipe on a water storage tank. The tank assembly was flexible enough for the water inlet pipe to pound the adjacent cement stucco wall, crushing the wall surface; fortunately, the water inlet pipe was not damaged.

building systems, equipment items (i.e., the components) are systems in their own right, once the covers are removed to expose the internal workings. The nonstructural system is a continuous chain of elements that link together to perform an intended building function. The chain includes functional devices located deep within equipment platforms, to the operational attachments responsible for connecting the elements to achieve a desired building function. It is the system that requires earthquake protection. The earthquake will find the weakest link in the nonstructural chain.

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Figure 10-2. This covered walk appears to have plenty of clearance from the main building. Unfortunately, the HVAC ductwork that was added after the building’s initial construction was not able to take advantage of the clearance. The duct was pinned to both the building and the covered walk, which eliminated the separation between the two structures. A flex joint was needed.

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Figure 10-3. These pendant lights had several problems. The original light drops were brittle metal and no bracing was provided. The light drops ran between the exposed joists. During the 1994 Northridge earthquake the lighting began to swing and struck the joists because they were not restrained. Modern codes require safety cables to the structure above as well as bracing to prevent the lighting from being able to strike any adjacent obstructions.

Figure 10-4. The clearance demand is evident in this photograph from the 1992 Joshua Tree earthquake, which caused this suspended ceiling in a Desert Hot Springs, California, school to fail at a column.

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Figure 10-5. These unrestrained lights swung enough to strike and break the adjacent glazing; modern codes require restraints to limit the swing.

References ASCE/SEI. (2010). “Minimum design loads for buildings and other structures.” ASCE/SEI 7-10, Reston, VA. Bachman, R. E., Bonowitz, D., Caldwell, P. J., Filiatrault, A., Kennedy, R. P., McGavin, G., Miranda, E. (2004). “Engineering demand parameters for nonstructural components.” ATC-58 Project Task Report – Phase 2, Task 2.3, Applied Technology Council, Redwood City, CA. Federal Emergency Management Agency (FEMA). (1997a). “NEHRP guidelines for the seismic rehabilitation of buildings.” FEMA 273, Washington, DC. ———. (1997b). “NEHRP commentary on the guidelines for the seismic rehabilitation of buildings.” FEMA 274, Washington, DC. ———. (2000a). “Action plan for performance based seismic design.” FEMA 349, Washington, DC. ———. (2000b). “Prestandard and commentary for the seismic rehabilitation of buildings.” FEMA 356, Washington, DC. International Code Council (ICC). (2011). 2012 International building code, Country Club Hills, IL. International Code Council Evaluation Service (ICC ES). (2010). “Acceptance criteria for seismic certification by shake-table testing of nonstructural components.” AC156, Country Club Hills, IL. McGavin, G. L. (1981). Earthquake protection of essential building equipment: Design, engineering, installation, Wiley, New York, NY. Tang, M., Castro, E., Pedroni, F., Brzozowski, A., and Ettouney, M. (2008). “Performance-based design with application to seismic hazard.” Structure Magazine, (Oct. 14, 2011).

Appendix

Test Facility Selection: Points to Consider

A quick Internet search on the topic “shake-table test labs” will produce a long list of facilities from around the world. For example, a composite list of earthquake shake-tables can be found in Wikipedia (2011). For those who have never planned and executed a shaketable test, the challenge is not developing a list of facilities but, rather, it is the determination of which facility will be the best choice. With no practical experience in earthquake qualification testing to guide one, this can be a daunting task. Building codes around the world rarely go into requirements for mechanical and electrical equipment beyond anchorage requirements. Even in the United States the enforcement of detailed equipment seismic qualification criteria is a new experience for the entire building construction industry. Prior to the introduction of the International Building Code (IBC) in 2000, shake-table testing was mostly conducted for commercial nuclear power plants, the telecommunications industry, or specialized military applications. The low demand for building code qualification and absence of a code-recognized shake-table test protocol resulted in significant variability from one lab to the next. Because there was no consistency in building code testing, it is a “buyer beware” environment when selecting a test facility. For the first-time shopper, initial quoted cost can be a dangerous metric to use in the selection process and can rapidly turn into a significant cost increase when a test has to be repeated. Even if budgets are not limited, finding the technical resources to plan, implement, and execute a test program may not be an option considering new product launch dates. 375

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Following is a general list of points and issues for readers to consider in evaluating which facility can best meet their needs. These points minimize risk that the qualification report will fail to meet the end-user’s compliance requirements. • Physical dimensions of the shake-table platform along with overhang and height limits of the test specimen. • Capacity of cranes and material handling equipment used to move the test specimen from the lab receiving/shipping location to placement and removal on the shake-table. • Dynamic table limits for test specimen weight and center of gravity. The more the table capacity exceeds the qualification test demand, the better. • Table degrees of freedom (DOF). Ideally this would be 3 DOF, or three-axis testing, so that each strong motion test only requires one table run. Single- and two-axis tables can be used by reorienting the test specimen and conducting the necessary number of strong motion runs to accomplish the equivalent of a three-axis test. The need to reposition the test sample and conduct multiple runs can add significant time and possibly require more test samples than for a three-axis table. • The ability of the test facility to control test demand or “shake-table test fidelity.” Because shake-table testing primarily uses a random broadband time-history, the capability of the lab to control the test demand is often challenging. A severe overtest can result in equipment failure and uncertainty about the outcome if the test demand had been just over the qualification requirement. An undertest means that the test motion may not satisfy minimum marketplace demand requirements. Box 5-2 in Chapter 5 discusses the seismic certification implications for overtest and undertest conditions. To confirm the test facility’s ability to control its table, request plots of the table performance at the qualification target demands, with either no payload on the table or with a dummy payload. If the table capacity is much greater than required for the test sample, a bare table run should suffice to give a good indication of the shake-table test fidelity. • Laboratory certifications and accreditations. • Laboratory quality assurance program. • Laboratory instrumentation under a controlled calibration program that is traceable to an appropriate national standards organization. • Ability to provide for specialized data collection such as high-definition video, digitized test data, etc. • Accommodation of customer-provided test instrumentation and data collection equipment, including cameras. • Coordination of specialized contracted resources to move and/or assemble test samples. • Staff experience and training, which must include the oversight of a registered or licensed engineering professional to approve and seal the test report. • Safety program actively enforced. • Security in all aspects to protect proprietary aspects of products from unauthorized third parties or competitors.

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• Confidential treatment of all test data and reports. • Lab history of experience qualifying to the standards required by the customer specifications and/or the local inspection authority. • Ability to verify that the lab is acceptable to local inspection jurisdictions or government regulatory authorities by other means than facility claims. • Sufficient lead time to schedule test dates and cooperatively manage adjustments. Lead times for test slots can range from a few weeks to many months. • Capability to coordinate shipping and storage requirements for test samples. Because of long lead times, rescheduling a test because the test sample did not arrive on time and missed a test date can mean a delay of many months to a qualification program. • Capabilities to design, fabricate, and store special test fixtures. Most test programs will require a special test fixture which, in some cases, requires significant cost and lead time to fabricate. In most cases, such fixtures can be used in future test programs, so the ability of the test facility to store them can be a significant cost savings, which must be factored into the actual cost of testing. In some cases the reuse of test fixtures can dramatically reduce setup time and have an effect on reducing the total cost of testing. • Ability to supply active operation loads such as high kVa and high volume fluid flows.

References Wikipedia. (2011). “Earthquake shaking table: A world list of shaking tables.” (Oct. 14, 2011).

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Glossary

Active Fault—A fault determined to be active by the Authority Having Jurisdiction from properly substantiated data (e.g., most recent mapping of active faults by the U.S. Geological Survey or state geologist). Active Operation—Building code requirement for Designated Seismic Systems to maintain active operation functionality of Mechanical and Electrical Equipment and Mechanical and Electrical Distribution Systems following (i.e., not during) application of design-level earthquake demands. Aftershocks—Quakes that occur following the main event as the tectonic plates readjust to the recent earthquake. Aftershocks from a large event can themselves be quite destructive, especially to a structural or nonstructural system weakened due to the main shock or previous aftershocks. Aftershocks can continue to occur for years following a large event. Anchorage—The final connection points securing the nonstructural Force-Resisting Skeleton (FRS) to the building structure, with or without the use of attachments. Anchorage includes bolts, concrete anchors, welds, or other mechanical fasteners for positively securing the nonstructural FRS to the building without consideration of frictional resistance produced by the effects of gravity. Architectural Elements—Parapets, partitions, façades, cladding, glazing, lighting, ceiling systems, doors, and other building components. ASCE/SEI 7-10—The current source document for the seismic provisions of the 2012 International Building Code. ASD—Allowable stress design. Attachment, Bracing—Structural elements that insert between the nonstructural ForceResisting Skeleton (FRS) and building structure. A bracing attachment is purely structural in nature and is used to provide a structural link between the FRS and the building to resist environmental loads. The application is to establish a Rigid Equipment Connection to the building structure.

379

380

Glossary

Attachment, Isolation—Mechanical elements (springs and dampers) that insert between the nonstructural Force-Resisting Skeleton (FRS) and the building structure to isolate the FRS from building inputs or isolate the building from FRS inputs. Isolation attachments (also called shock and vibration isolators) are mechanical energy absorbers that are intended to attenuate input loading going between the building and nonstructural item. Equipment isolators are used generally where the equipment produces vibrations (e.g., due to reciprocating motions of the equipment). Equipment base isolators commonly have fundamential periods within the earthquake range and require “snubbing” to prevent wild excursions and equipment damage. Attachment, Operational—Operating elements that insert between the nonstructural Force-Resisting Skeleton (FRS) and building structure, or could insert between two nonstructural systems. An operational attachment includes both mechanical and electrical elements that are intended to support active operation functions. These attachments are necessary operational umbilicals that are required in order for the nonstructural system to function. In the case of vibration isolated equipment using base Isolation Attachments, the intent is to maintain Flexible Equipment Connections for the Operational Attachments to accommodate large relative displacements. Authority Having Jurisdiction (AHJ)—The local, state, or federal authority charged with enforcing building codes and issuing building permits and certificates of occupancy (or their equivalent). Base—The level at which the horizontal seismic ground motions are considered to be imparted to the building structure. Base Isolation—Decoupling of the main structure of the building to some extent from the ground motion. The decoupling is not complete and earthquake motions will enter a building with base isolation. Base isolation generally does not eliminate the need to consider vertical earthquake excitations. Body Wave Magnitude (mb or Mb)—Measures the primary waves to determine the earthquake Magnitude. No longer used in a widespread manner. Body Waves—Energy ruptures that travel along fault planes through the body of the Earth. See Primary and Secondary Waves. Building Information Modeling (BIM)—The basic premise of BIM is to drive collaboration by different stakeholders using a shared knowledge resource to digitally represent the physical and functional characteristics of a building. Building Occupancy Contents—See Furniture and Equipment. Building Officials Code Administrators International (BOCA)—The code body responsible for producing one of the three model codes in use prior to 2000 when the International Building Code took effect in the United States. BOCA published the National Building Code. CAD—Computer aided design. Capital Risk Managers (CRMs)—Facility insurers.

Glossary

381

CG—Center of gravity. Characteristic Earthquake—A seismic event assessed for an active fault having a magnitude capable of occurring on the fault, but not less than the largest magnitude that has occurred historically on the fault. Clearance Demand—Meeting the code requirement for systems interaction avoidance. Allowing sufficient movement between components or sufficient restraint between components. (Also nonstructural clearance demand.) Component—A part or element of an architectural, electrical, mechanical, or distribution system. Compressional Waves—See Primary Waves. Consequential Damage—A concept introduced in ASCE/SEI 7-10 wherein the function and physical interrelationship of components, their supports, and their effect on each other shall be considered such that the failure of an essential or nonessential architectural, mechanical, or electrical component shall not cause the failure of an essential architectural, mechanical, or electrical component. Continental Drift—An archaic model for crustal movement, which has been replaced with the Plate Tectonics model. CQC—Complete quadratic combination. CR—Center of resistance (or center of rigidity). Damping—An energy dissipation mechanism that reduces the amplification and broadens the vibratory response in the region of resonance. Expressed as a percentage of critical damping. Designated Seismic Systems—Code for nonstructural components/systems that are assigned an Importance Factor of 1.5 and require seismic certification. This includes active mechanical and electrical equipment that must remain operable following the design earthquake ground motion and components with hazardous substances. Design Earthquake—Seismic event that is most likely considered to occur within the life of the facility. The design earthquake is two-thirds the value of the Risk-Targeted Maximum Considered Earthquake (MCER). Design Earthquake Ground Motions—The earthquake ground motions that are twothirds of the corresponding Risk-Targeted Maximum Considered Earthquake (MCER) ground motions. Division of the State Architect (DSA)—The AHJ for public schools (K–4) in California. DOF—Degrees of freedom. Earthquake—The sudden rupture of rock units at depths resulting in random vibrations of the Earth. Earthquake Demand—The environmental loading conditions associated with seismic ground shaking. Includes forces, accelerations, and relative displacements. Also represents a required clearance space or clearance envelope between nonstructural elements.

382

Glossary

Earthquake Early Warning System (EEWS)—A system that recognizes and analyzes primary earthquake waves to determine the destructive potential for the damaging Surface Waves. When such a potential exists, the EEWS issues a warning or triggers an event sequence, such as emergency power startup and power transfer. Earthquake Intensity Scale—A description (either qualitative or quantitative) of the effects of an earthquake at a specific site. The historic qualitative scale most commonly cited is the Modified Mercalli Intensity Scale (1931), which describes observed effects in lay terms. Today, seismic design revolves around a quantitative intensity scale using displacement, velocity, and/or acceleration. Earthquake Measurement Scales—Two methods for generally measuring earthquakes, magnitude and intensity. Magnitude is a description of the earthquake independent of its location or that of the observer. Intensity measures what happens at a specific site due to an earthquake. Earthquake Engineering Research Institute (EERI)—An interdisciplinary professional organization of individuals interested in furthering knowledge as it relates to earthquakes. The organization was founded on the principle of shared interdisciplinary knowledge and includes a wide range of professions. Equipment, Flexible—Nonstructural equipment, including its attachments, having a fundamental period ⬎0.06 s (⬍16.67 Hz). Equipment, Rigid—Nonstructural equipment, including its attachments, having a fundamental period ⱕ0.06 s (ⱖ16.67 Hz). Essential Components, Equipment, and/or Systems—What is required to perform a function following a damaging earthquake. An essential component can be as simple as a door knob or as complex as an emergency power/power transfer system. Fault Plane—A roughly defined plane, which may or may not be continuous, along which the fault rupture propagates. Not all fault planes are exposed at the surface through surface rupture. FEA—Finite element analysis. FEM—Finite element model. FEMA—Federal Emergency Management Agency. Flexible Equipment Connections—Those connections between equipment components that permit rotational and/or translational movement without degradation of performance. Examples include universal joints, bellows, expansion joints, and flexible metal hose or flexible conduit entries. Focus—See Hypocenter. Force-Resisting Skeleton (FRS)—Structural members or assemblies of members, including frames, enclosures, pallets, casings, struts, rods, panels, etc. FRS assembly members can be joined together using mechanical fasteners or can be weldments. Monocoque construction techniques are also included as FRS types. The nonstructural FRS provides support for subassemblies, modules, and internal devices. The FRS also pro-

Glossary

383

vides overall structural stability for the nonstructural platform. The FRS should be viewed as the nonstructural system’s structural skeleton to resist all environmental and operating loads. Foreshock—Seismic events that occur on the same fault or fault system as the main earthquake; however, foreshocks are smaller than the main event. Functional Device—Logical subgroupings of nonstructural active functions (operational) typically organized and arranged as physical devices, modules, components, or subassemblies that mount to the nonstructural Force-Resisting Skeleton (FRS). Functional devices can be electrical, mechanical, or electromechanical in nature. Some devices also function as load-bearing members of the FRS in addition to being a functional device. For example, with a piping distribution system the pipe acts as both an FRS and a functional device with containment of pipe contents being the active function. Functional Requirement, Equipment or Systems—Components within a building that have an operational requirement following a damaging earthquake (see Active Operation). Furniture and Equipment (F&E)—Items generally not covered by code, are installed in a facility by the owner, and are not permanently anchored to the building structure. GERS—Generic Seismic Ruggedness of Power Plant Equipment Program. GIP—Generic implementation procedure. Go Slow Elevator—One elevator in a bank of elevators in acute healthcare facilities designated to “go slow” following a triggered threshold earthquake whereas the remaining elevators are mandated by legislation to shut down. The go slow elevator allows patients to be relocated by elevator from upper floors. HVAC—Heating, ventilation, and air conditioning. Hypocenter—The single point at depth in rock material of first rupture of an earthquake. Focus is a synonym. International Building Code (IBC)—Produced by the International Code Council (ICC). The IBC began to take effect in the United States as the predominant code in 2000. International Council of Building Officials (ICBO)—The model code body responsible for producing the Uniform Building Code (UBC) up to the 1997 UBC. International Code Council (ICC)—The code body responsible for producing the International Building Code (IBC). International Code Council Evaluation Service (ICC-ES)—The organization responsible for issuing Evaluation Reports and performing technical evaluations for code compliance. Inspector of Record (IOR)—The inspector who is an employee of the AHJ performing periodic inspections during construction or who performs full-time and continuous inspections and is generally employed by the owner. IP—Intellectual property. Local Magnitude—See Richter Magnitude.

384

Glossary

Long Waves—Also known as Surface Waves (not to be confused with S-Waves). Formed through the interaction of the P-Waves and S-Waves impinging with the surface of the Earth. They are complex manifestations of the body waves at the surface of the Earth and are responsible for most of the destruction caused by an earthquake. See also Love Waves and Rayleigh Waves. Love Waves—Surface Waves that tend to have a side-to-side observed motion. Magnitude—A measure of the size of an earthquake independent of its location. There are several magnitude scales. The most commonly referred to by the lay public is the Richter Magnitude, which measures Local Magnitude (ML) based on Surface Waves. Magnitudes for scientific purposes at the present are more commonly referred to as Moment Magnitude (Mw). MCER—Risk-targeted maximum considered earthquake. Mechanical and Electrical Distribution Systems—Process piping, fire sprinkler systems, HVAC ductwork, electrical busway, conduit, cable trays, and other components. Mechanical and Electrical Equipment—Also called components and include building service equipment (such as pumps, generators, air handlers, compressors, transformers, switchgear, power supplies) and building tenant equipment (such as medicalrelated technology, emergency communication equipment, and other specialized process equipment) that are permanently anchored during installation. Mechanically Anchored Tanks or Vessels—Tanks or vessels provided with mechanical anchors to resist overturning moments. MMI—Modified Mercalli intensity. Moment Magnitude (Mw)—The current preferred (and more accurate) scale that measures rupture area and slip rate to determine the size of an earthquake. National Earthquake Hazards Reduction Program (NEHRP)—The source for the sesimic provisions of the ASCE/SEI 7-10 standard. NIST—National Institute of Standards and Technology. Nonstructural—Building systems that do not directly carry overall building loads and only contribute to the dead load or live load of the building. In the context used for this book, nonstructural systems include all Mechanical and Electrical Equipment and Mechanical and Electrical Distribution Systems. Nonstructural Clearance Demand—See Clearance Demand. NPP—Nuclear power plant. OBE—Operating basis earthquake. Octave—The interval between two frequencies that have a frequency ratio of 2. OEM—Original equipment manufacturer. Office of Statewide Health Planning and Development (OSHPD)—The AHJ for critical-care and healthcare facility design and approval in California. Operational Requirement—See Active Operation requirement.

Glossary

385

Partition—A nonstructural interior wall that spans horizontally or vertically from support to support. The supports may be the basic building frame, subsidiary structural members, or other portions of the partition system. Performance-Based Design (PBD)—A design philosophy wherein design critera for performance are established that may exceed the precriptive nature of the code. PGA—Peak ground acceleration. Plate Tectonics—The geologic model where lighter crustal material rides atop the denser mantle material. The mantle material has heat convection cells that act as the driving force for the movement of the crustal plates. Earthquakes usually occur at plate boundaries (although there are exceptions to this rule). Position Retention—Building code requirement to maintain positive retention of nonstructural position, under design-level earthquake demands, without consideration of frictional resistance produced by the effects of gravity. This includes position retention of nonstructural Anchorage, Attachments, and the Force-Resisting Skeleton. Prescriptive Design—The current code design philosophy for life safety based on codeset prescriptive limits or rule-based limits. Primary Waves—Also referred to as P-Waves or Compressional Waves. Energy ruptures that propagate through the body of the Earth by compression and dilatation. These are the fastest-traveling earthquake waves. PSHA—Probabilistic seismic hazard analysis. P-Waves—See Primary Waves. Rayleigh Waves—Surface Waves that tend to have an observed rolling motion. Richter Magnitude—An earthquake scale developed by Charles Richter in the early 1930s to describe an earthquake independent of its location. It is also known as Local Magnitude (ML) and is no longer generally used for scientific or design purposes. The concept introduced by Richter of using logarithm-based scaling to represent earthquake ground motion has been carefully preserved in modern-day scales such as the Moment Magnitude (Mw). Therefore, the term “Richter Magnitude” is still mistakenly used to report earthquakes in the lay press and media. This misnomer by the media is widely viewed by the community of earth scientists as a public recognition of Richter’s important contributions to bringing a deeper understanding of earthquakes to the public. Rigid Equipment Connections—Those connections between equipment and building structure that are rigid and fixed. Examples include base anchorage between equipment base and concrete pad and top restraint between equipment top and building structure. Risk-Targeted Maximum Considered Earthquake (MCER)—The largest seismic event considered likely to occur within the lifetime of the facility for which design is considered. This is based on a uniform risk of structural collapse and adjusted for the orientation that results in the largest ground motion. RRS—Required response spectrum.

386

Glossary

SB-1953—California legislation formulated following the 1994 Northridge earthquake to require the updating of acute-care hospitals to current seismic standards. SD—Strength design. Secondary Waves (S-Waves)—Also referred to as Shear Waves. Energy ruptures that propagate through the body of the Earth by shear action. These travel slower than the Primary Waves and faster than the Long Waves or Surface Waves. They do not propagate through liquid materials such as molten rock. Seismic Demand—The resultant environmental loading that becomes the input requirement for the system element. Seismic demand includes forces, accelerations, and relative displacements. It also represents a required clearance space or clearance envelope (Clearance Demand) between elements, for example, the clearance space between one nonstructural item and an adjacent nonstructural or between nonstructural system and building. Seismic Design Category—A classification assigned to a structure based on its risk category and the severity of the design earthquake ground motion at the site. Seismic Forces—The assumed energy related to the response of the structure to earthquake motions, to be used in the design of the structure and its components. Seismic Hazard Maps—Another name for the building code’s Risk-Targeted Maximum Considered Earthquake (MCER) maps, which define areas of ground motion with a uniform probability of structural collapse. Seismic Safety Commission (SSC)—An independent commission created by legislation, authored by then California Assemblyman Alfred E. Alquist in 1974 following the 1971 San Fernando earthquake, to be an interdisciplinary body that reviews seismic policy and legislation in California. The California SSC reduces earthquake risk by utilizing cost-effective partnerships and supports applied research to meet loss reduction and recovery goals. Some other states have since used California’s SSC as a model to establish their own policy review commissions. Seismic Zone—An archaic method of adjusting acceleration in determining seismic force based on the codes prior to the implementation of the IBC. The seismic zone was determined by zone maps in the code. Self-Anchored Tanks and Vessels—Tanks or vessels that are stable under design overturning moment without the need for mechanical anchors to resist uplift. SEQUAL—Seismic Experience-Based Qualification Owners Group. SF—Safety factor. Shear Waves—See Secondary Waves. Site Class—A classification assigned to a site based on the types of soils present and their engineering properties. Snubbers—Restraining devices used to prevent wild excursions and damage to equipment that is vibration isolated.

Glossary

387

Southern Building Code Congress International (SBCCI)—The code body responsible for producing one of the three model codes in use prior to 2000, when the International Building Code began to take effect in the United States. SBCCI published the Standard Building Code. SRSS—Square root of sum of squares. SSE—Safe shutdown earthquake. SQUG—Seismic Qualification Utilities Group. Storage Racks—Industrial pallet racks, moveable shelf racks, and stacker racks made of cold-formed or hot-rolled steel structural members. Does not include other types of racks, such as drive-in and drive-through racks, cantilever racks, portable racks, or racks made of materials other than steel. Structural—Any building element that is in the direct load-bearing path of the building as a whole. Structural Engineering Institute (SEI)—A part of ASCE dedicated to structural engineers serving professional engineers and academia on structural issues. Structural Engineers Association of California (SEAOC)—An organization of structural engineers who are stakeholders in the development of structural building codes within the state of California. Surface Wave Magnitude (Ms)—Measures the Rayleigh Waves to determine magnitude. No longer commonly used. Surface Waves—See Long Waves. S-Wave—See Secondary Wave. Systems Design—The preparation of an assembly of methods, procedures, or techniques united by regulated interaction to form an organized whole. It is an organization tool or, specifically, a controlled process for defining the functional architecture that governs components, modules, assemblies, interfaces, and data for a system to satisfy specified requirements. The functional and physical interrelationship of components, their supports, and their effect on each other shall be considered so that the failure of an essential or nonessential architectural, mechanical, or electrical component shall not cause the failure of an essential architectural, mechanical, or electrical component. Design with the understanding that one component can rely on the interactions and/or operability of another component and whether the failure of one component will adversely affect the performance of the system as a whole. Systems Interaction Avoidance—Building code requirement to account for unwanted interaction, under design-level earthquake demands, between nonstructural systems and anything else that might be located in the immediate vicinity of the nonstructural installation, so that failure of one system or contact between systems does not cause Consequential Damage of an essential system. The “anything else” could be building elements or other installed nonstructural systems. Tectonics—See Plate Tectonics.

388

Glossary

Test Response Spectrum (TRS)—The acceleration response spectrum that is developed from the actual time-history of the motion of the shake-table test. Test, Biaxial—A dynamic test in which the test specimen is subjected to acceleration in one principal horizontal axis and the vertical axis simultaneously. The horizontal and vertical acceleration components are derived from two different input signals that are phase incoherent. Test, Triaxial—A dynamic test in which the test specimen is subjected to acceleration in two principal horizontal axes and the vertical axis simultaneously. The two horizontal and vertical acceleration components are derived from three different input signals that are phase incoherent. Test, Uniaxial—A dynamic test in which the test specimen is subjected to acceleration in one principal axis. The acceleration components are derived from a single input signal. Uniform Building Code (UBC)—One of the three model codes in use prior to 2000 when the IBC began to take effect in the United States. Uninterrupted Power Supply (UPS)—An electromechanical apparatus (i.e., Mechanical and Electrical Equipment) that provides emergency power to building loads when the input power source, typically the utility mains, fails. USGS—United States Geological Survey. UUT—Unit under test. ZPA—Zero period acceleration.

Notation

ADYN ADYNV APSD Ax AZPAH AZPAV ai ap Cp D Dp Dpl EH EV Fa Fp Fv fEFF fn GRMS g h Ie

Dynamic testing spectral acceleration factor, horizontal direction, as specified in AC156 Dynamic testing spectral acceleration factor, vertical direction, as specified in AC156 Acceleration power spectral density Torsional amplification factor as defined in the code’s Amplification of Accidental Torsional Moment section Dynamic testing spectral acceleration factor, ZPA region, horizontal direction, as specified in AC156 Dynamic testing spectral acceleration factor, ZPA region, vertical direction, as specified in AC156 Acceleration at building height i obtained from modal analysis procedure Component amplification factor Horizontal force factor specified in the 1994 Uniform Building Code Dead load Relative seismic displacement that the component must be designed to accommodate Seismic relative displacement Horizontal earthquake force Vertical earthquake force Short-period site coefficient (at 0.2 s period) Horizontal seismic design force Long-period site coefficient (at 1.0 s period) Effective frequency Natural frequency of vibration Root mean square acceleration Acceleration of gravity, 9.8 m/s2 (386 in./s2) Average roof height of structure with respect to the base Seismic importance factor for buildings 389

390

Ip L MB ML MS MW M0 Q Rp RT RV SD1 SDS SM1 SMS SS S1 T TL TS T0 Wp z ␥n ⌬ ⌬xA ⌬yA ␦yB ␰ ␰m ␻

Notation

Nonstructural importance factor Live load Body wave magnitude Local Richter magnitude Surface wave magnitude Moment magnitude Seismic moment Quality of resonance factor Component response modification factor Ratio of applied load to allowable load in tension Ratio of applied load to allowable load in shear Design earthquake, 5% damped, spectral response acceleration at a period of 1 s Design earthquake, 5% damped, spectral response acceleration at short period MCER, 5% damped, spectral response acceleration at a period of 1 s adjusted for site class effects MCER, 5% damped, spectral response acceleration at short periods adjusted for site class effects Mapped MCER, 5% damped, spectral response acceleration parameter at short periods Mapped MCER, 5% damped, spectral response acceleration parameter at a period of 1 s Fundamental period of the building Long-period transition period SD1 / SDS 0.2 (SD1 / SDS) Component operating weight Height in structure of point of attachment of component with respect to the base. For items at or below the base, z shall be taken as 0. The value of z/h need not exceed 1.0 Mode participation factor for the nth vibration mode Logarithmic decrement Deflection at building Level x of Structure A, determined by an elastic analysis as defined in the code’s Story Drift Determination section Deflection at building Level y of Structure A, determined by an elastic analysis as defined in the code’s Story Drift Determination section Deflection at building Level y of Structure B, determined by an elastic analysis as defined in the code’s Story Drift Determination section Frequency-independent constant damping ratio Frequency-dependent modal damping ratio Natural circular frequency of vibration

Index

A acceleration as intensity variable, 101 peak ground, 113 RMS, 349 variation in, 111 zero-period, 136, 282 acceleration maps examples of, 131f, 132f explanation of, 130 acceleration parameters, 130, 131f–132f, 134t, 135 acceleration spectral envelopes, 352 accelerograms explanation of, 110–111, 297 gauging adequacy of, 346 input, 249–250, 252f–253f, 253–254, 255f–

Alquist, Alfred E., 28 Alquist-Priolo Earthquate Fault Zoning Act (1972), 84–85 anchorage capacity determination for, 205, 205f–207f failure mode perspective, 215–216 of nuclear power plant equipment, 329 test fixture application and, 307–308 anchorage calculations explanation of, 186, 190 rigid-body static analysis and, 208–215, 209f–212f, 213t, 215f anchor bolt selection, 212, 213t, 214 anchor weld selection, 214–215, 215f antiresonance, 352 applied seismic analysis. See seismic analysis architects, role of, 52, 57 architectural components, 4 Arnold, Chris, 56–57 ASCE/SEI 7-10, 2, 3, 13, 31, 62, 63, 67, 86, 93, 114, 123, 130, 133, 146, 195, 207, 208, 288, 294, 305, 367 attachments. See nonstructural attachments

256f

large-class UPS device 1 response, 347f shake-table, 296–299, 297f, 298f, 369 AC156 test protocol (International Code Council), 63, 143, 145, 184, 309, 366 test spectrum development, 287f, 288– 296, 291f, 295f, 306f active faults, 84–85, 84f active operating subsystem, 16t acute-care facilities, systems design for, 25– 26, 26f–28f, 28–29 advanced seismic analysis explanation of, 268 material elastic-plastic analysis as, 269 model validation and, 270–278, 272f, 274f,

B Bellcore seismic requirements, 297 Big Bear earthquake (1992), 105 Bilham, Roger, 51 blind faults, 88, 89f body wave magnitude, 101, 103 Bolt, Bruce, 98, 107 bounding spectra, 327 bracing attachments, 266, 267 building codes basic elements of, 118–120 compliance expectations for, 126, 128t

276f–278f

See also seismic analysis aftershocks, 104–105 allowable stress design, 144 391

392

Index

construction categories in, 121–122 construction importance in, 122–126 development of, 120–121 earthquake protection approach of, 11 evolving nature of, 43–44, 117–118 intensity as design parameter in, 101 regional, 121 building code seismic requirements building structural demands and, 130, 131f–135f, 135–136, 137f commentary in, 120, 146 load application and, 145–146 load combinations and, 144, 145f loads and, 129 nonstructural dynamic demands and, 142–143

nonstructural relative displacement demands and, 143–144 nonstructural seismic performance objectives and, 126, 127t–128t nonstructural static demands and, 136, 138–139, 140t–141t, 142 overview of, 117–118 San Francisco case study, 147–151, 147f, 149f–153f, 153 site assessment and, 126f, 129 summary of, 146 building design/construction requirements for, 19, 21, 22f responsibilities of, 52, 53t–54t, 55–57, 55f, 56f, 58f–60f, 59, 61–63 stakeholders in, 51–52 Building Information Modeling (BIM), function of, 29, 30f, 59 building occupancy contents, 4 building site assessment building code seismic requirements and, 126f, 129 soil properties and, 129, 129f building system, 13, 14f buried faults, 88, 89f C California Seismic Safety Commission, 28

case studies building code seismic requirements in San Francisco, 147–151, 147f, 149f–153f, 153

systems interaction failure following Sierra El Mayor earthquake (2010), 29, 31–39

UPS building code position retention, 220f, 221, 222f, 223, 224f, 225, 225f, 226t–227t, 228, 233–235, 234f, 235f, 236t, 237, 256–258, 257f–262f, 263t–264t catastrophe models, 49, 50 ceiling system, design of, 61–63 center of rigidity (CR), 211 Charleston, Andrew, 57 Chi-Chi (Taiwan) earthquake (1999), 86 Chilean earthquake (1960), 103 Chilean earthquake (2010), 51 China, damage assessment model for, 50 clearance demand, 370–371, 371f–374f clearance envelope compliance assessment and, 192–193 explanation of, 173–174, 267–268 failure mode perspective, 268 complete-quadratic-combination (CGC) method, 240 compliance expectations for, 126, 128t nonstructural earthquake protection and, 67, 69 seismic analysis and, 203 verification of, 195–196 compression posts, use of, 63 compression waves, 89, 91, 94 computer-aided design (CAD), 29 consequential damage, 173 construction importance ranking and, 123–126, 124t, 125f risk categories for, 122–123, 123t type of, 121–122 continental crust, 73, 74 continental plates, 74, 76 convergent boundaries, 75, 75f, 76 core, of Earth, 73

Index

Cornell, C. Allin, 46, 112 correlation trends analysis, 275–278, 276f Coulomb damping, 313 CREWS (Coachella Valley Regional Earthquake Warning System), 98 crust, of Earth, 73–74 D damping effects of, 273–275 explanation of, 313–314 damping measurement comparison of techniques for, 319t explanation of, 313–314 shake-table modal survey, 318 in situ forced vibration, 314–316, 315f, 316f in situ free decay, 314, 319t in situ modal hammer, 316–318, 317f deep earthquakes, 68 deep focus earthquake, 68 demands. See loads Department of Energy (DOE), 331–332 design earthquake response spectrum, 283 design force equation, 142 “Did You Feel It?” (United States Geological Survey), 98 directivity, 113 distribution systems, clearance requirements for, 173–174 divergent plates, 75, 75f dynamic testing, 281, 366. See also seismic qualification testing; seismic test machines/technology; seismic test preparation/execution E earthquake demand, 3 earthquake demand allocation explanation of, 3 nonstructural building systems and, 4, 5f–10f, 11–12 nonstructural design philosophy and, 25– 26, 26f–28f, 28–29, 30f, 31–34, 32f–38f, 39–40

393

systems design elements and, 11–13, 12f, 14f, 15t–16t, 17, 18f, 19, 20f, 21, 22f, 23, 24f

earthquake early warning systems (EEWSs) function of, 94–97, 96f, 365–366 interagency, 97–98 overview of, 93–94 Earthquake Hazards program (USGS), 78 earthquake protection of nonstructural systems, 361–371, 371f– 373f

structural aspect of, 41 systems involved in, 12–13 earthquake risk mitigation, objectives for, 11 earthquakes deep, 68 depth of focus of, 82 distance of focus to site under consideration, 82 foreshocks and aftershocks of, 104–105 intensity of, 98, 99t–100t, 101 intermediate, 68 magnitude of, 101–104 negative-magnitude, 102 operating basis, 362 origin of, 78–79 safe shutdown, 362 scientific developments related to, 46– 47

shallow, 68 size of, 79–82, 81f strong shaking duration of, 82–83 types of, 77–78 earthquake science development in, 72 plate tectonics and, 72–78 earthquake scientists, role of, 45–46 Earthquake Synthesized Accelogram (Bellcore), 297 effective frequency, 348, 349 El Centro earthquake (1940), 108–110, 111f

electrical distribution systems, 4 electrical equipment, 4, 7f–10f

394

Index

electrical large-class qualification, 337–341, 338f, 339f, 341f, 342f, 343–344, 343f– 349f, 346, 348–350, 351f–353f, 352–354 Electric Power Research Institute (EPRI), 326, 331, 332 electrohydraulic shakers, 299–300 ENALUF power plant (Nicaragua), 325 epicenter damage potential and proximity to, 81–83 of earthquake, 79 EQE Earthquake Database, 325 Esteva, Luis, 46, 112 experimental modal analysis damping measurement and, 313–318, 315f–317f, 319t explanation of, 276, 312–313 See also modal analysis F facilities damage potential variables for, 79–83 type of ground of, 83 fault planes, 86, 109 fault propagation rate, 77 faults/faulting active, 84–85, 84f background of, 83 explanation of, 85 inactive, 84, 85f types of, 85–86, 87f–89f, 88–89 Federal Emergency Management Agency (FEMA) designing for earthquakes, 52, 53t–54t earthquake mitigation strategies and, 47, 48

FEMA-454, 21, 52, 56, 57, 118 Field Act (1933), 61 fillet welds, 214 financial risk stakeholders, 47 finite element analysis (FEA), 217, 355, 356 finite element models (FEMs) correlation process and, 271–278, 272f, 274f, 276f–278f explanation of, 145–146, 271

fire codes, 361 flexible-body static analysis explanation of, 216–217 linear, 218, 219f, 220–221, 220f, 222f, 223, 224f, 225, 225f, 226t–227t, 228 floor response, 283 floor response spectrum, 283 focus depth of, 82 distance from, 82 of earthquake, 79 forced vibration method, 314–316, 315f, 316f, 319t force-resisting skeleton (FRS) damping and, 313 explanation of, 15t, 21, 138–139, 216 failure mode perspective, 228–229 flexible-body analysis and, 216–217 mechanical impedance effects and, 350, 352, 353 product platform and, 177–179 testing and, 190, 191f, 192 Ford, Henry, 63–64 foreshocks, 104, 105 free-decay method, 314 free-field seismic instrumentation, 113 friction damping, 313 functional devices analysis and, 229–230 capacity rating for, 192 failure mode perspective for, 265 harmonic-response analysis and, 238– 240, 238f, 239f linear flexible-body dynamic analysis and, 230–231 modal analysis and, 231–238, 234f, 235f, 236t, 237f operation validation for, 172–173 response spectrum analysis and, 240–243, 244f, 245, 245f, 246t–247t testing of, 171–172 transient analysis and, 245, 247–250, 251f, 252–254, 256–258, 260, 263t, 264–265, 264t functional interaction, 17

Index

G generic building spectra, 367 generic floor spectra, 285–287, 287f, 291f, 295f, 367 Generic Implementation Procedure (GIP), 327, 328f Generic Seismic Ruggedness of Power Plant Equipment (GERS program), 324, 331– 332

geotechnical system, 13, 14f GERS qualification by testing experience, 331–332

globalization, original equipment manufacturers and, 66–67 global positioning system (GPS) technology, advances in, 72 global rating system, 67 Global Seismic Hazard Assessment Program (GSHAP), 67, 68f governments, earthquake mitigation strategies advanced by, 47–48 Great Alaskan Earthquake (1964), 81, 103 Great Toˉhoku Earthquake (2011), 94, 103 ground response, 283 ground response spectrum, 283 GSHAP Global Seismic Hazard Map, 67, 68f

Gutenberg, Beno, 46, 101, 103 H Haiti earthquake (2010), 51 harmonic-response analysis, 238–240, 238f, 239f

Hawaiian Islands, 78 Hayden, Tom, 25 Holmes, Arthur, 73 holocene materials, 84 horizontal earthquake load, 208, 210 Hurricane Andrew (1992), 49 Hurricane Hugo (1989), 49 Hutton, James, 73 Huygens’s Principle, 91, 92f hypocenter, of earthquake, 79 hysteretic damping, 313

395

I IEEE 344-1975, 324 inactive faults, 84, 85f inelastic response modification, 354, 369–370 inherent damping, 313, 314 input accelerogram, 249–250 in situ forced vibration, 314–316, 315f, 316f, 319t

in situ free decay, 314, 319t in situ modal hammer, 316–318, 317f, 319t Inspector of Record (IOR), 43, 55, 61 inspectors, 61 instructure response, 283 insurance industry catastrophic models and, 49, 50 as stakeholder, 48–49 intellectual property, 358, 365 intensity scales, 98, 99t–100t, 101 intermediate earthquakes, 68 International Building Code (IBC) 2000, 323 International Building Code (IBC) 2012, 3, 52, 120–121, 309. See also building code seismic requirements International Code Council (ICC), 145 Evaluation Service, 214, 288, 296 See also AC156 test protocol (International Code Council) isolation attachments, 266, 267 isoseismals, 109 J Jefferson Elementary School (Calexico, California), 31–33, 32f, 33f–37f, 39, 106 K Kordyban, Tony, 270 L Landers earthquake (1992), 79, 80f, 105 Landers Elementary School, 105, 105f large-class qualification electrical, 337–341, 338f, 339f, 341f, 342f, 343–344, 343f–349f, 346, 348–350, 351f– 353f, 352–354

396

Index

explanation of, 335–336 mechanical, 354–357, 355f–357f summary of, 358 large mass approach, 256–257 lateral loads, 93 linear flexible-body dynamic analysis explanation of, 230–231 harmonic-response analysis and, 238– 240, 238f, 239f modal analysis and, 231–238, 234f, 235f, 236t, 237f response spectrum analysis and, 240–243, 244f, 245, 245f, 246t–247t transient analysis and, 245, 247–250, 251f, 252–254, 256–258, 260, 263t, 264–265, 264t

linear stress analysis, 226t–227t, 263t load and resistance factor design (LRFD), 144

load application building codes and, 145–146 explanation of, 120 load combination formulas, 144, 145t, 209 load combinations in building codes, 144, 145 explanation of, 120, 144 loads building requirements and, 129 building structure, 130, 131f–133f, 134t, 135–136, 135f, 137f combination, 144, 145t explanation of, 119–120 horizontal and vertical, 208–210 nonstructural dead, 208 nonstructural dynamic, 142–143 nonstructural relative displacement, 143– 144

nonstructural static, 136, 138–139, 140t– 141t, 142 Local Richter Magnitude, 101, 102, 103 Loma Prieta earthquake (1989), 83 long waves, 90f, 91, 92f, 93 lower mantle, 73, 74 Lyell, Charles, 73

M magnitude explanation of, 102 Local Richter and surface wave, 102–103 magnitude scales, 101–102 Mains Elementary School (Calexico, California), 34, 36, 38f–39 mantle, of Earth, 73–74 Marianas Trench, 76 material damping, 313 material elastic-plastic analysis, 269 mechanical distribution systems, 4 mechanical equipment explanation of, 4 illustration of, 7f, 8f mechanical impedance effects, 350, 352– 353, 353f mechanical large-class qualification, 354– 357, 355f–357f mechanical subsystem, 15t–16t Mercalli, Giuseppe, 98 Mexico earthquake (1985), 81 microseisms, 102 modal analysis demonstration of, 231–235, 234f, 235f, 236t, 237, 237f explanation of, 231 use of, 230, 237–238 See also experimental modal analysis modal hammer process, 316–318, 317f, 319t model correlation explanation of, 271 finite element, 271–278, 272f, 274f, 276f– 278f

model validation, 270–278, 272f, 274f, 276f– 278f

mode superposition, 230, 250 Modified Mercalli Intensity (MMI) explanation of, 80, 81, 81f, 98, 99t–100t Sierra El Mayor earthquake and, 106 use of, 109 moment magnitude, 101, 103–104, 104f multifrequency random time-history accelerogram, 298, 298f

Index

Multihazard Mitigation Council (MMC), 48

multiple-axis shake-tables, 300–301, 301f, 302f, 303 N National Earthquake Hazards Reduction Act (1977), 121 National Earthquake Hazards Reduction Program (NEHRP), 121 near source design coefficient, 113 New Madrid Fault Zone, 78 Newtonian seismic force formula, 108 nonstructural attachments explanation of, 265 failure mode perspective, 266–267 types of, 266 nonstructural clearance demand, 370 nonstructural compliance metrics explanation of, 181, 183–184, 198 system-level compliance assessment and, 184–195

nonstructural components design responsibilities for, 52, 53t–54t, 55 explanation of, 10 interaction of nonbuilding structures and, 122, 122f seismic coefficients for, 140t–141t nonstructural dead load, 208 nonstructural decomposition, 13, 14f, 17 nonstructural design force, calculation of, 142

nonstructural dynamic demands, 142–143 nonstructural earthquake protection building design and construction and, 51–52, 53t–54t, 55–57, 55f, 56f, 58f–60f, 59, 61–63, 69 code regulation and enforcement and, 43–51, 44f, 45f compliance strategy and, 67, 69 issues related to, 41–42 product design and manufacture ad, 63– 67, 68f stakeholders in, 42–43

397

nonstructural floor response spectrum requirement, 185 nonstructural performance factor, 124–126, 125f, 127t nonstructural product line rationalization engineering judgment and, 179–180 explanation of, 176 functional device perspective and, 178– 179

process of, 176–177 structural dynamics perspective and, 177–178

structural strength perspective and, 178 test plan development and, 307 nonstructural research in clearance demand guidelines, 370–371, 371f–374f

developments in, 364–365 in earthquake early warning systems, 365–366

in inelastic response reduction, 369–370 in nonstructural dynamic demands, 366– 367

in response spectrum accelerogram integrity, 368–369 in subsystem qualification testing, 367– 368

nonstructural seismic demand, 284 nonstructural static demand, 136, 138–139, 140t–141t, 142 nonstructural static force requirement, 185

nonstructural systems capacity ratings for, 185, 186f–189f categories of, 4, 5f–10f earthquake protection approach and, 11 explanation of, 10, 11, 13, 14f systems framework and, 12–13, 12f, 14f, 15t–16t, 17 testing of, 171, 172t treatment of, 10–11 trends in earthquake protection of, 361– 371, 371f–373f normal faults, 86, 87f

398

Index

normal modes analysis. See modal analysis North Anatolian Fault, 76 Northridge earthquake (1994), 25, 26, 28, 78, 371f Nuclear Regulatory Commission (NRC), 324, 326 O oceanic crust, 73–74 oceanic plates, movement of, 74 Olive View Hospital (Los Angeles), 26, 27f– 29f

1 sigma acceleration, 349 operating basis earthquakes, 362 operational attachments, 266 original equipment manufacturers (OEMs) functional device operation validation by, 172–173 globalization and, 66–67 qualification of attachments and, 169–171 qualification strategy and, 174–181 requirements awareness and, 66 role of, 63, 64, 65, 195 seismic compliance and, 65, 67, 69 Ortelius, Abraham, 73 P peak ground acceleration (PGA), 113, 136 performance-based design (PBD) earthquake early warning systems and, 365, 366 explanation of, 362–364 plate boundary zones, 75, 75f plates, movement of, 74, 75, 75f plate tectonics background of, 72–73 explanation of, 73–78, 112 Popov, Egor, 107 primary waves (P-waves) early warning systems and, 94 explanation of, 89, 90f probabilistic seismic hazard analysis (PSHA), 46, 112–113 product design/manufacture approaches to, 63–65

compliance strategy and, 67, 69 elements of, 23, 24f global issues related to, 66–67, 68f requirements awareness and, 66 seismic compliance and, 65–66 specifications, 175 product development seismic analysis and, 204–205 specifications, 175, 176 product line rationalization. See nonstructural product line rationalization project-specific building spectra, 367 public policy, public perception and, 50–51 Q Q factor, 314–316 qualification by earthquake experience explanation of, 323–324, 335 SQUG, 323–331 qualification by testing experience, GERS, 331–332

qualification of large-class systems. See large-class qualification qualification strategy costs and, 180–181, 182t, 198 elements of, 174–176 product line rationalization and, 176–180 See also seismic qualification testing qualification tests elements of, 310–311 explanation of, 175 subsystem, 367–368 R random vibration device test, 348–350, 349f random vibration theory, 348 relative displacement, 143–144 required response spectrum (RRS) explanation of, 249, 250, 296 shake-table accelerograms and, 296–298 shake-table control and, 303, 304f, 305, 306f, 307 requirement assessment, 174–175 requirements awareness, 66, 69

Index

requirements flow-down, 11 requirements interpretation, 69 research. See nonstructural research response reduction, 369–370 response spectrum accelerogram integrity, 368–369

response spectrum analysis, 240–243, 244f, 245, 245f, 246t–247t

reverse faults, 86, 87f Richter, Charles, 46, 101–103 Richter Magnitude Scale, 101–103 rigid-body static analysis, 208–215, 209f–212f, 213t, 215f anchorage load calculation and, 208–212, 209f–212f

anchor bolt selection and, 212, 213t, 214 anchor weld selection and, 214–215, 215f

equations for, 208 rigid-body theory, 208 Ring of Fire, 76, 76f risk categories, construction performance and, 122–123, 123t risk-targeted maximum considered earthquake (MCER), 130, 131f, 132f

RMS acceleration, 349 root mean square acceleration (GRMS), 348, 349

Rosenblueth, Emilio, 46, 112 Rossi-Forel Intensity Scale, 98 S safe shutdown earthquakes, 362 San Andreas Fault, 76–78, 77f San Fernando earthquake (1971), 25, 80, 81f, 84, 109 San Francisco building case study, 147–151, 147f, 149f–153f, 153 San Francisco earthquake (1906), 83 Sano, Riki, 108 secondary waves (S-waves) early warning systems and, 94 explanation of, 90f, 91 sedimentary rock, 110

399

seismic accelerograms, 110–111, 299, 329– 330. See also accelerograms seismic analysis advanced, 268–278, 272f, 274f, 276f–278f anchorage and, 205, 206f, 207–216, 207f, 209f–212f, 213t, 215f attachments and, 265–267 clearance envelope and, 267–268 explanation of, 202, 203f force-resisting skeleton and, 216–217, 217f, 228–229 functional devices and, 229–265 (see also functional devices) linear flexible-body static analysis and, 218, 219f, 220–225, 220f, 225, 225f, 226t–227t, 228 position retention needs and, 204–205 uses for, 202–203, 204f seismic capacity ratings, 195–196, 198 seismic demand explanation of, 17 soil conditions and, 113–114 seismic design category, 123, 124t Seismic Experience-Based Qualification (SEQUAL) Owners Group, 326 seismic ground intensity coefficients, 107

seismic hazard maps background of, 109 developments in, 110–111 ground motion, 112–113 seismic importance factor, 124 seismic moment, 103–104 seismic qualification nonstructural compliance metrics and, 181, 183–196 OEM strategy and, 174–181 overview of, 157–158 summary of, 196, 198 seismic qualification ownership anchorage and, 159–160, 160f, 161t–164t, 165

attachments and, 166, 167f–170f, 168–171 clearance envelope and, 173–174 explanation of, 158, 159f

400

Index

force-resisting skeleton and, 165–166 functional devices and, 171–173, 172t seismic qualification testing experimental modal analysis and, 312– 318, 315f–317f, 319t explanation of, 281 generic floor spectra and, 285–287, 287f, 291f, 295f shake-table accelerograms to satisfy floor spectra, 296–299, 297f, 298f specification of test environment and, 281–282

time domain vs. frequency domain and, 282–285, 283f–286f Seismic Qualification Utility Group (SQUG), 323, 324. See also SQUG qualification by earthquake experience seismic requirements, 3 seismic simulation testing, 175 seismic test machines/technology function of, 282, 299–300 multiple-axis shake-tables and, 300–301, 301f, 302f, 303 shake-table control and, 303, 304f, 305, 306f, 307 seismic test preparation/execution qualification test and, 310–311 test plan development, 307–310, 308f test results documentation and, 311–312 seismic vertical loading, 294 Seismic Warning Systems, Inc., 97 seismic waves explanation of, 89 types of, 89, 90f, 91, 92f, 93 seismic zone maps, 110, 112, 113 seismology, developments in, 46–47 shake-table accelerograms, 296–299, 297f, 298f, 369 shake-table modal survey, 318 shake-tables control systems and, 303, 304f, 305, 306f, 307 multiple-axis, 300–301, 301f, 302f, 303 single-axis, 300

shake-table test labs, 375 shaking damage potential and strong, 82–83 vertical, 93 shallow earthquakes, 68 shock testing. See seismic qualification testing; seismic test machines/ technology; seismic test preparation/ execution Sierra El Mayor earthquake (2010), 106–107, 106f–108f

case study of systems interaction failure following, 29, 31–39 single-axis testing, 300 site coefficients, 133, 134t smoke passage, 57, 58f, 59, 59f soil conditions, 113–114 soil property classification, 129, 129f spectral enveloping, 287 splay wires, use of, 63 SPSS displacement, 258, 258f–260f square-root-of-sum-of-squares (SRSS), 230

SQUG qualification by earthquake experience background of, 323, 325, 332 database fundamentals and, 325–326 equipment classes and, 330–331 equipment relevancy and, 330 facilities surveyed and, 326–327 NRC approval and, 326 SQUG criteria and, 327–330 stakeholders building design and construction and, 51–52, 53t–54t, 55–57, 55f, 56f, 58f–60f, 59, 61–67, 68f, 69 categories of, 42–43 misconceptions of, 41, 42 perspective of, 43–51 standalone testing, 171–172, 172t standard average frequency. See effective frequency static analysis theory of, 220–221

Index

UPS case study of, 220f, 221, 222f, 223, 224f, 225, 225f, 226t–227t, 228 use of, 221 static equilibrium, 211 strength design, 144 stress design, 144 strike slip faulting, 88, 88f structural transmissibility, 13, 17 subcategory generic building spectra, 367 subduction zones, 76 subsystem qualification testing, 367–368 surface wave magnitude, 101–103 surface waves. See long waves system decomposition, 13 system interaction failure, following Sierra El Mayor earthquake (2010), 29, 31–39 system-level compliance assessment anchorage and, 186, 190 attachments and clearance envelope and, 192–193

capacity ratings for nonstructural system elements and, 194t combined system-level ratings and, 193,

systems framework demonstration and, 17, 19, 20f, 21, 22f, 23, 24f systems interaction avoidance, 31 T tectonics, background of, 72–78 Tehran, Iran, 51 test data management, 175 test facility selection, 375–376 testing. See seismic qualification testing; seismic test machines/technology; seismic test preparation/execution test response spectrum (TRS), 303, 304f, 305, 306f, 307 thrust faulting, 88, 89f torque, 103–104 Trans-Alaska Pipeline, 85, 361 transform boundaries, 75–76, 75f transient analysis, 245, 247–250, 251f, 252– 254, 256–258, 260, 263t, 264–265, 264t two-factor mapped acceleration parameters, 130 type testing. See nonstructural product line rationalization

195

explanation of, 184 force-resisting skeleton and, 190, 191f, 192

functional device and, 192 nonstructural system capacity ratings and, 185–186, 186f–189f project-specific demand requirements and, 184–185 systems analysis, 3 systems design approach to, 39–40 BIM approach to, 29, 30f in essential facilities, 25–26, 26f–28f explanation of, 3, 11 nonstructural decomposition and, 13, 14f, 17

nonstructural system element definitions and, 15t–16t, 18f primary systems and, 12–13, 12f

401

U Uniform Building Code (UBC), seismic design in, 25, 71 uninterrupted power supply (UPS) case study, 220f, 221, 222f, 223, 224f, 225, 225f, 226t–227t, 228, 233–235, 234f, 235f, 236t, 237, 256–258, 257f–262f, 263t–264t

United Nations International Decade for Natural Disaster Reduction, 67 United States Geological Survey (USGS) “Did You Feel It?”, 98 Earthquake Hazards program, 78 unit under test (UUT) explanation of, 300, 302f, 303, 307 test plans and, 308–310 upper mantle, 73, 74 U.S. Nuclear Regulatory Agency, 46 USGS Earthquake Hazards program, 78

402

Index

V Vela Uniform research program (Department of Defense), 47 VERTEQII accelerograms, 297, 297f vertical earthquake load, 208–210 vibration testing. See seismic qualification testing; seismic test machines/ technology; seismic test preparation/ execution viscous damping, 313 volcanism, 72, 78

W Wegener, Alfred, 73 weld safety factor, 214 weld shear stress, 214 World Bank, 48 Y Yanev, Peter, 325 Z zero period acceleration (ZPA), 136, 292

About the Authors

From left to right: Gary McGavin, Jeff Gatscher, and Phil Caldwell. Jeffrey A. Gatscher is currently employed in Schneider Electric’s Technology Innovation department. As a research Fellow Engineer and Edison Expert, he is responsible for engineering competency transfer at the corporate level. His expertise includes the technical disciplines of shock and vibration product testing, structural dynamics, solid mechanics, and systems analysis. Mr. Gatscher has held engineering positions with Raytheon Missile 403

404

About the Authors

Systems and General Dynamics, where he participated in the development and testing of aerospace and military launch vehicles. He started his engineering career working for Bechtel Power Corporation, where he analyzed nuclear power plant piping systems. He has a B.Sc. in mechanical engineering from Michigan Technological University and an M.Sc. in mechanical engineering from the University of Tennessee. Over the last 28 years, Mr. Gatscher has specialized in the qualification of manufactured product designs (commercial and military) to satisfy environmental requirements by employing systems analysis and design assurance testing techniques. As a nationally recognized expert in the field of seismic qualification testing, he has authored or coauthored numerous peer-reviewed papers, conference papers, and invited presentations. Mr. Gatscher is a senior member of the American Society of Mechanical Engineers and his research interests include vibration fatigue, fragility testing, accelerated life cycle testing, and structural dynamics FEA. Jeff Gatscher resides in Nashville, Tennessee, and can be contacted at jgatscher@ yahoo.com. His personal interests include travel, photography, backpacking, and dunebuggy riding with his gracious and loving wife, Alexa. Gary L. McGavin, AIA, is an architect with more than 35 years of experience in the design of public and institutional facilities. He has served for 15 years on California’s Alfred E. Alquist Seismic Safety Commission. Mr. McGavin is also a professor of architecture and teaches structures and lateral loads at California State Polytechnic University–Pomona. He was the Architect of Record for the Landers Elementary School, which was situated less than 0.4 mi from approximately 12 ft of horizontal offset in the 1992 Landers Earthquake. Mr. McGavin is a member of Earthquake Engineering Research Institute and the American Institute of Architects. Mr. McGavin has worked for Wyle Laboratories, 1973–1979; Ruhnau McGavin Ruhnau Associates 1979–1990; and HMC Group, 1990–1995. Since 1995, he has operated his own firm, Gary L. McGavin, AIA in Redlands, California. He is the author of numerous works, including Earthquake Protection of Essential Building Equipment (1981), and coauthored the nonstructural chapter in FEMA 454, Designing for Earthquakes: A Manual for Architects (2006). He has a B.Sc. in geology (University of California–Riverside, 1973) and a master’s degree in architecture (Cal-Poly–Pomona, 1978). Mr. McGavin lives in Redlands, California, with his wife, Carla. Having raced formula cars for decades, he is currently changing venues and is constructing his next racing car for land speed from a 1950s F-80 wing drop tank. Dinosaur bone hunting also remains a lifelong passion that he first enjoyed as a student geologist. Philip J. Caldwell is an Edison Expert with Schneider Electric, where he serves as the company’s codes and standards representative for numerous committees and activities related to earthquake mitigation of nonstructural components. Previously, he held positions with Duke Energy, where he was involved with electrical, instrumentation, and control qualification and issue resolution for commercial nuclear power applications. He received his B.S. in electrical engineering technology from Virginia Tech and has more than 30 years of experience in application, design, qualification, and standards develop-

About the Authors

405

ment of electrical power distribution and control equipment to North American and European standards. He is a committee member of IEEE P693, ASCE 7, ISO TC98 SC3, and NEHRP, which establish earthquake qualification and performance requirements for nonstructural components. He is a member of the Earthquake Engineering Research Institute, Seismological Society of America, ASCE, IEEE, and Applied Technology Council, as well as a charter member of the National Science Foundation–sponsored George E. Brown Jr. Network for Earthquake Engineering Simulation. Along with leading academics and earthquake engineering professionals, Mr. Caldwell has participated in National Science Foundation– sponsored postearthquake reconnaissance surveys to assess the real-world performance of essential building systems in critical facilities. Mr. Caldwell resides in the Clemson area of upstate South Carolina with his wife, Jackie, and trail-loving Labrador retrievers.