Building loads can be transferred to piles by a thick reinforced-concrete slab, called a pile-cap footing. The piles are usually embedded in the pile cap 4 to 6 in. They should be cut to required elevation after driving and prior to casting the footing.
Reinforcement should be placed a minimum of 3 in. clear above the top of the piles. The pile cap is required by the ACI 318 Building Code to have a minimum thickness of 12 in. above the reinforcement. (See also Art. 9.74.)
Piles should be located so that the centroid of the pile cluster coincides with the center of gravity of the column load. As a practical matter, piles cannot be driven exactly to the theoretical design location. A construction survey should be made to determine if the actual locations require modification of the original pile-cap design.
Pile-cap footings are designed like spread footings (Art. 9.76), but for concentrated pile loads. Critical sections for shear and moment are the same. Reaction from any pile with center dp /2 or more inside the critical section, where dp is the pile diameter at footing base, should be assumed to produce no shear on the section.
The ACI 318 Building Code requires that the portion of the reaction of a pile with center within dp /2 of the section be assumed as producing shear on the section based on a straightline interpolation between full value for center of piles located dp /2 outside the section and zero at dp /2 inside the section. For design of pile caps for high-capacity piles, see ‘‘CRSI Design Handbook,’’ Concrete Reinforcing Steel Institute.
For pile clusters without moment, the pile load Pu for strength design of the footing is obtained by dividing the factored column load by the number of piles n.
The factored load equals 1.4D + 1.7L, where D is the dead load, including the weight of the pile cap, and L is the live load. For pile clusters with moment, the factored load on the rth pile is