Page 192 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
191
FOOTINGS, FOUNDATION WALLS, BASEMENTS, AND SLABS

may require a concrete footing that is wider than the wall itself and
capable of distributing the weight of the structure over a larger area.

6.2.1 Concrete Footings
Concrete footings are used to support building walls, freestanding gar-
den walls, and retaining walls for many types of construction. Foot-
ings that are wider than the walls they support are typically called
spread footings. The Code requires that footings be:

■ A minimum of 6 in. thick
■ Supported on undisturbed natural soil or on engineered fill
■ Set below the frost line unless otherwise protected against frost
heave
■ A minimum of 12 in. below grade regardless of frost depth

The required footing width (W) is based on the bearing capacity of
the soil as indicated in Figure 6-4. Footing projections (P) on either
side of the foundation wall must be a minimum of 2 in., but not more
than the footing thickness. For a soil with moderate bearing capacity of
3,000 psf, in a conventionally framed 2-story house, the minimum
required footing width is only 10 in. Soil with a relatively low bearing
capacity of 2,000 psf, supporting a 2-story home of brick veneer over
wood frame construction would require a footing 19 in. wide. The
lower the soil-bearing capacity, the wider the footing required to
spread the building’s weight over a larger soil area. The footing widths
shown in the tables are minimum dimensions. The wider the footing,
the more stable it will be against overturning, rocking, or uneven set-
tlement in any soil. Many industry professionals recommend using a
rule of thumb which says that the footing thickness should be the same
as the width of the foundation wall it supports, and the footing width
should be a minimum of two times the thickness of the foundation
wall it supports. For an 8-in. concrete block wall, this would mean a
16-in.-wide footing, 8 in. thick. The soil-bearing capacity may require
a minimum footing width greater than or less than the rule of thumb,
so the actual width should always be the larger of the two (Figure 6-5).
In soils with high bearing capacity where the minimum required foot-
ing width is 8 in. or less, the foundation wall can be safely and eco-
nomically constructed to bear directly on the subsoil without a spread
footing. Once the width exceeds 8 in., it is usually more economical to
build a spread footing than to unnecessarily increase the thickness of

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