Building Design and Construction Handbook - Merritt - Ventech!

9.116 SECTION NINE
FIGURE 9.48 Design conditions for a rigid mat footing.
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 d p/2 or more inside the critical section, where d p 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 d p/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
d p/2 outside the section and zero at d p/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 P u 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
� �
1 eCr Pur � (1.4D � 1.7L) � n
(9.95)
n 2 C
� r
1
where e � eccentricity of resultant load with respect to neutral axis of pile group,
in.