Civil Engineering Reference
In-Depth Information
that soil pressures are unevenly distributed due to variations in soil properties, footing
rigidity, and other factors. A uniform-pressure assumption, however, usually provides a
conservative design since the calculated shears and moments are usually larger than those
that actually occur.
As an example of the variation of soil pressures, footings on sand and clay soils are
considered. When footings are supported by sandy soils, the pressures are larger under the
center of the footing and smaller near the edge [Figure 12.2(b)]. The sand at the edges of
the footing does not have a great deal of lateral support and tends to move from under-
neath the footing edges, with the result that more of the load is carried near the center of
the footing. Should the bottom of a footing be located at some distance from the ground
surface, a sandy soil will provide fairly uniform support because it is restrained from lat-
eral movement.
Just the opposite situation is true for footings supported by clayey soils. The clay
under the edges of the footing sticks to or has cohesion with the surrounding clay soil. As
a result, more of the load is carried at the edge of the footing than near the middle. [See
Figure 12.2(c).]
The designer should clearly understand that the assumption of uniform soil pressure
underneath footings is made for reasons of simplifying calculations and may very well
have to be revised for some soil conditions.
Should the load be eccentrically applied to a footing with respect to the center of
gravity of the footing, the soil pressure is assumed to vary uniformly in proportion to the
moment, as illustrated in Section 12.12 and Figure 12.23.
12.4
ALLOWABLE SOIL PRESSURES
The allowable soil pressures to be used for designing the footings for a particular structure
are desirably obtained by using the services of a geotechnical engineer. He or she will de-
termine safe values from the principles of soil mechanics on the basis of test borings, load
tests, and other experimental investigations.
Because such investigations often may not be feasible, most building codes provide
certain approximate allowable bearing pressures that can be used for the types of soils and
soil conditions occurring in that locality. Table 12.1 shows a set of allowable values that
are typical of such building codes. It is thought that these values usually provide factors of
safety of approximately 3 against severe settlements.
Section 15.2.2 of the ACI Code states that the required area of a footing is to be de-
termined by dividing the anticipated total load, including the footing weight, by a permis-
sible soil pressure or permissible pile capacity determined using the principles of soil
mechanics. It will be noted that this total load is the unfactored load, and yet the design of
footings described in this chapter is based on strength design, where the loads are multi-
plied by the appropriate load factors. It is obvious that an ultimate load cannot be divided
by an allowable soil pressure to determine the bearing area required.
The designer can handle this problem in two ways. He or she can determine the bear-
ing area required by summing up the actual or unfactored dead and live loads and dividing
them by the allowable soil pressure. Once this area is determined and the dimensions are
selected, an ultimate soil pressure can be computed by dividing the factored or ultimate
load by the area provided. The remainder of the footing can then be designed by the
