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2.3.2 Subjected to vertical loading and bending moment
2.4 DILEMMAS IN SELECTING RIGID OR FLEXIBLE CAP
Assumption
The design of pile cap forms an important part of the pile foundation design. Prior to the detailed structural design of the pile cap, engineers have to first determine whether the rigid cap assumption or the flexible cap assumption is to be used. Whereas the rigid cap assumption should be used in some special situations, both assumptions are applicable in most circumstances. Hence, engineers often face dilemmas in choosing between these two assumptions, and cost-and-benefit analysis is usually performed in the pile cap design.
Special situations in which the rigid cap assumption should be used include pile groups supporting mega columns. With a size much larger than other columns, mega columns are the skeletal structure of a high-rise building, and support most of the subjected loads of the whole building. In which the vertical load is transferred from the mega columns to the pile foundation through the pile cap, the pile cap is subjected to a great concentrated load from the mega columns. If the flexible cap assumption is used, the allowing of bending causes a large out-of-plane deflection of the pile cap underneath the mega columns, leading to exceptionally large pile loads and possibility of failure of the piles in the highly loaded area. On the contrary, if the rigid cap assumption is used, the concentrated load can be shared among all the piles and the occurrence of pile failure can be prevented, as discussed in Section 2.3. Therefore, the rigid cap assumption should be used in this case.
However, both assumptions can be applied in most cases, leading to engineering dilemmas in foundation design for tall buildings. Generally, the rigid cap assumption is commonly used in civil engineering practice because of its several advantages over the flexible cap assumption discussed below, but the drawbacks of the rigid cap assumption should also be taken into consideration during design.
The advantages of the rigid cap assumption, as illustrated in Section 2.3, include:
( I ) Convenient calculation
With the rigid cap assumption, the pile load of each pile can be conveniently calculated by Equations (2.7) and (2.8),due to the resultant linear pile load distribution shown in Fig. 2.15(a). The time can be saved by omitting the need to construct structural modelling of the pile group for analysis using complicated methods, such as the finite element method or the finite difference method, which are required for the flexible cap assumption due to the non-linear pile load distribution shown in Fig. 2.15(b). This time-saving is more significant when pile groups with great geometry irregularity and structural complexity are dealt with.
( II ) Efficiency of pile group
Since the subjected load is shared by all piles through the force transfer in the rigid pile cap, piles away from the loaded area can also participate in supporting the load, reducing the pile force of the piles in the heavily loaded area. The total pile group acts together and a higher resultant load can be supported.
( III ) Decrease in pile load by increasing the plan area
For the flexible cap assumption, only the piles close to a loaded area are involved in supporting the subjected loads and bending moments. For the rigid cap assumption, however, loads can be transferred to piles away from the loaded area. Therefore, an increase in the plan area allowing more piles can reduce the pile loads of the piles in heavily loaded areas due to vertical loads because the vertical loads are carried to and shared by more piles. In addition, the pile loads due to the subjected bending moments can also be decreased, since piles away from the loaded area can more efficiently resist the moments due to a larger moment arm. This is beneficial to the design of tall buildings which occupy only parts of the site area and a larger plan area for pile foundation is allowed.