Foundations

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FOUNDATIONS A foundation system has two functions. First, it sup pons the building striicmrally by keeping it level minimizing settling, preventing uplift from th e fortes of frost o i expansive soils and resisting horizontal forces such as winds and earthquakes. Second, a foundation system keeps the wooden parts o f the building above the ground and away from the or­ ganisms and moisture in tire soil that both cat wood

Many slab-on-grade systems allow the concrete foot­ ing, foundation and suhfloor to be poured at the

and cause it to decay The foundation is the part o f a building that is most likely to determine its longevity If the foundatitm does not support the building adequately, cracks and openings will occui over time, even in the most linely aalted structure. No amount ol repair on the sirucnire above th e loundation will compensate lor an inadequate foundation; once a foundation starts to move significantly, it will continue to move. We now have developed the knowledge to design and construct durable foundations, so there is no reason to invest in 3 m odem building that is not fully sup­ ported on a foundation that ^¡11 endure for th e life of

CRAWL SPACES

Crawl spaces are found in all climates but predomi­ nate in temperate regions. In this system, the insu­ lated wooden ground floor is supported above grade on a foundation wall made o f concrete or concrete block. The resulting uawl space introduces an accessi­ ble zone for durtworlc, plumbing and other utilities, and allows for simple remodeling.

the struaure. In the United States, there are three toiiiinon founda­ tion types. Each perfonns in different ways, but all rely on a perimeter foundation, i e., a continuous sup­ port around the outside edge o f the building.

BASEMENTS

SLAB-ON'GRADE FOUNDATIONS Slab-on-grade systems are used mostly in warm cli­ mates, where living is close to the ground and die frost line is close to the surface. The looting is usually shallow, and the ground floor is a concrete slab.

Basemeiib are th e dominant foundation system in the coldest parts of th e country, where frost lines mandate deep footings in any case. Like crawl spaces, basements are accessible, and in addition they pro­ vide a large habitable space. Basement foundation systems are usually constructed o f concrete or Luiicrete-block foundation walls. Drainage and water­ proofing are paiticulady rritical with basement systems

fOUNDATIONS CHOOSING A FOUNDATION ia c h foundation system has many variations, and it is iiiipurtaiu to s d e tl the one best suited to the climate, the soil type, the site, and the building program. With all toundations, you should investigate the local soil type. Soil types, along with ih cir bearing capacities, are often described in local soil profiles based on information from the U S. Geological Survey (USGS). If there IS any question about matching a foundation system to the soil or to the topography o f the site, consult a soil or structural engineer before construclioii begins. This small investment may save thou­ sands o f dollars in future repair bills.

DESIGN CHECKLIST Because the foundation is so im portant to the longevity o f the building and because it is so difficult to repair, it is wise to be conservative in its design and cuiisiruaioii. Make the fuundatiuii a little stronger than yon think you need to. A.s a minimum, even if not required by code, it is recoiimieiided dial you fol­ low this rule-of-thumb checklist: 1- Place the bottom o f the fooling below the frost line on solid, undisturbed soil that is free o f organic iiiaterial. (Local codes will prescribe frost-line depth.) 2. Use continuous horizontal rebar in the footing and at the top o f foundation walls (join t reinforcing may be allowable in con aeic-block walls). Tic the footing and wall together with vertical rebar. 3. Use pressure-treated or other decay-resistant wood in contact with concrete. Use a moisture barrier between all concrete and untreated wood.

4. Tie wood members to the foundation with bolts or straps embedded in the foundation, Anchoiing requirements in hurricane and severe earthquake zones are shown in the following chapters, but specific requirements should be verified with local rndes 5. Provide adequate drainage around the foundation. Slope backlill away Iroin the building and keep soil 6 in. below all wood. Many codes and many site conditions require mca.siires beyond these minimum specifications. In addi­ tion. tlieiv die several other considerations important to a permanent foundation system, and these are dis­ cussed in this chapter, They include suppoi t ul loads that do not fall at the perimeter wall, such as footings for point loads within the structure and at porches and decks; insulation and moisture barriers; water­ proofing and drainage; protection against termites, other insects and wood-decaying organisms; and pre­ cautions against radon gas.

ABOUT THE DRAWINGS ihe sizes o f building elements indicated in the draw­ ings in this section are for the purposes o f illustrating principles and reminding the designer and the builder to consider their use carefully. These drawings should therefore he used only for reference.

Footings art tilt paj'l o( a tourdation that transfers the building's loads— its weight in materials, comeitis, uotupaiUs, and snow, and possibly wind and earth­ quake loads—directly to the ground. Cousequeiuly.. the size and type o f footing should he matched care­ fully to the ground upon which it bears. Soil typ e— Concrete footings should be placed on firm, undisturbed soil that is free from organic mate­ rial. Soil cjq)es are tested and rated as to their ability to .support loads {bearing capacity).

Soil type

Bearing capacity fpsf)

Soft clay or silt

do not build

Medium day or silt

1,500-2,200

Stiff day or silt

2,200-2,500

Loose sand

1,800-2,000

Dense sand

2,000 3,000

(travel

2,500.3.000

liedrock

4,000 and up

Com panion of soil may be required before footings die placed. Consult a soil engineer if the stability of the soil at a building she is unknown.

R ein fo rc ing—Most codes require steel leinforcing rods (called rebar) in footings Rebar is a sound investment even if it is tiol requited, because it gives tensile strength to the footing thereby minimizing cracking and differential settling. Rebar is also the most common way to connect the footing to the (oundation wall. For rebar rules of tbnmb, ier .SB. S iz e —Footing size depends m ainly on soil type and the building's weight The i.ltail below shows footing sizes for soils with bearing capacities of 2.000 pounds per square fool (psl). No. of stories W

A rule of thum b for estimating the .size o f standard footings is that a footing sliuuld be 8 in. wider than the foundation wall and twice as wide as high. F ro st line—The base o f the footing must be below the frost line ID prevent the building from heaving as the ground swells during freezing. Frost lines range liom 0 It to 5 ft- in the continental United States. Check local building depariiiieiiis lot Irost-lme requirements.

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