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 qa, 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 ing Code (Art. 9.44) uses factored loads of gravity, wind, earthquake, earth pressure, and fluid pressure to determine factored soil pressure qs, and factored pile or pier load. The factored loadings are used in strength design to determine factored moments and shears at critical sections.

For concentrically loaded footings, qs is usually assumed as uniformly distributed over the footing area. This pressure is determined by dividing the concentric wall or column factored load Pu by the area of the footing. The weight of the footing can also be neglected in determining qs because the weight does not induce factored moments and shears. The factored pile load for concentrically loaded pile-cap footings is determined in a similar manner.
When individual or wall spread footings are subjected to overturning moment about one axis, in addition to vertical load, as with a spread footing for a retaining wall, the pressure distribution under the footing is trapezoidal if the eccentricity ex, of the resultant vertical load Pu is within the kern of the footing, or triangular if beyond the kern, as shown in Fig. 9.42. Thus, when ex < L/6, where L is the footing length in the direction of eccentricity ex, the pressure distribution is trapezoidal (Fig. 9.42a) with a maximum

If the bearing stress on the loaded area exceeds Æ’b, reinforcement must be provided by extending the longitudinal column bars into the spread footing, pile cap, or drilled pier or by dowels. If so, the column bars or dowels required must have a minimum area of 0.005 times the loaded area of the column.
Provisions in the ACI 318 Building Code assure that every column will have a minimum tensile capacity. Compression lap splices, which are permitted when the column bars are always in compression for all loading conditions, are considered to have sufficient tensile capacity so that no special requirements are needed. Similarly, the required dowel embedment in the footing for full compression development will provide a minimum tensile capacity.
Required compression-dowel embedment length cannot be reduced by end hooks. Compression dowels can be smaller than column reinforcement. They cannot be larger than No. 11 bars.
If the bearing stress on the loaded area of a column does not exceed , 0.85Æ’c column compression bars or dowels do not need to be extended into the footing, pile cap, or pier, if they can be developed within 3 times the column dimension (pedestal height) above the footing (Art. 9.49.8). It is desirable, however, that a minimum of one No. 5 dowel be provided in each corner of a column.

Footing Thickness. The minimum thickness allowed by the ACI 318 Building Code for footing is 8 in for plain concrete footings on soil, 6 in above the bottom reinforcement for reinforced-concrete footings on soil, and 12 in above the bottom reinforcement for reinforced-concrete footings on piles. Plain-concrete pile-cap footings are not permitted.
Concrete Cover. The minimum concrete cover required by the ACI 318 Building Code for reinforcement cast against and permanently exposed to earth is 3 in.

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