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[Dr. N. Subramanian]

A note I wrote about pile and pile cap design I wrote today in response to a question in a group:
Pile design is simple as it is designed as a short column, unless there is a poor soil surrounding it. In some old books, the piles are designed as long columns, which is expensive.

Pile cap is designed as a flexural member in many books, which again is not actually represent the behaviour of pile cap. As the usual depth of pile cap will be large, the bending theory is not applicable and hence it has to be designed by strut-and-tie method, which was made popular by the late Prof. Schlaich of the University of Stuttgart, Germany in the 1990s. ACI 318-19 has provisions for this method. There is a classical paper by Prof. Schlaich and his associates in the PCI journal.

All the above is with respect to structural design. Piles also should be checked as per soil design.

Another area which is still not clear is the group effect. We consider that the piles in a group share the load equally, which may not be correct.

One more thing regarding detailing of pile caps. In flexural design, the rebars are placed uniformly in the pile cap. If you design as per strut-and-tie method, only the strips connecting the piles are reinforced for the designed tension. Other areas may be provided with nominal reinforcement.  

In addition to the designed reinforcement, some bursting reinforcement is recommended by codes.