Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.107
FIGURE 9.41 Common types of foundations for buildings.
combined footing (Fig. 9.41e) orastrap footing (Fig.. 9.41ƒ) can be used to
obtain a nearly uniform distribution of soil pressure or pile loads. The strap footing
becomes more economical than a combined footing when the spacing between the
columns becomes larger, causing large bending moments in the combined footing.
For small soil pressures or where loads are heavy relative to the soil capacity,
a reinforced-concrete mat, or raft foundation (Fig. 9.41g) may prove economical.
A mat consists of a two-way slab under the entire structure. Concrete cross walls
or inverted beams can be utilized with a mat to obtain greater stiffness and economy.
Where sufficient soil strength is available only at lower levels, pile foundations
(Fig. 9.41h) ordrilled-pier foundations (Fig. 9.41i) can be used.
9.74 GENERAL DESIGN PRINCIPLES
FOR FOUNDATIONS
The area of spread footings, the number of piles, or the number of drilled piers are
selected by a designer to support actual unfactored building loads without exceeding
settlement limitations, a safe soil pressure q a, or a safe pile or drilled-pier load. A
factor of safety from 2 to 3, based on the ultimate strength of the soil and its
settlement characteristics, is usually used to determine the safe soil pressure or safe
pile or drilled-pier load. See Art. 6.8.
Soil Pressures. After the area of the spread footing or the number and spacing
of piles or drilled piers has been determined, the spread footing, pile-cap footing,
or drilled pier can be designed. The strength-design method of the ACI 318 Build-