A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
account for the effect of pressure bulbs.
8. Which type of pile cap transfers loads equally to piles, flexible pile
cap or rigid pile cap? (FD8)
Loads from columns transferring to pile cap induce tensile forces at the
bottom of the cap. For instance, by using truss analogy to analyze a pile
cap sitting on two piles with a column at the centre of the pile cap, the
tensile force at the bottom is proportional to the pile spacing and is
inversely proportional to depth of pile cap. The bottom reinforcement is
designed to resist the tensile stressed generated from loads in columns.
Sometimes, reinforcement may be designed at the top of pile caps which
serve as compression reinforcement. This type of reinforcement is required
in case there is a limitation on the depth of pile caps. Similarly shear
reinforcement is introduced to the pile caps in case there is a restriction to
the depth of pile caps.
Consider that loads are applied at the centre of a pile cap.
For the case of rigid pile cap, owing to the effect of interaction of individual
piles, the central piles tend to settle more than the edge piles when the pile
cap is under loading condition. For the pile cap to be rigid, the local
deformation of central piles would not occur. Instead, the stiff pile cap
would transfer the loads from the central piles and redistribute them to the
outer piles. Therefore, piles at the edge take up a higher fraction of the total
loads and are subjected to higher axial and bending loads in case the pile
cap is stiff. In the extreme case, the side piles may take up as much as
about two to three times the loads in the central piles and this may lead to
the failure of these edge piles.
For flexible pile cap, load taken up by individual piles are different because
the deformation of pile cap enhances non-uniform distribution of loads
among piles. The piles closer to the load tend to share more loads when
compared with those which are located far away from the loads. The
difference of loads induced in piles increase with the flexibility of pile cap.
9. In some pile design, the settlement of piles are not checked. Is it
correct?
The performance of piles mainly consists of the two elements, namely
ultimate bearing capacity and settlement. The local practice of pile design
is place emphasis on checking if the bearing capacity of piles would be
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