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
11. What are the differences between pinned bases and fixed bases?
(FD9)
When structures like portal frames are connected to the base foundation,
engineers have to decide the degree of fixity for the connection. In general,
the two common design options are pinned bases and fixed bases. Pinned
bases have the advantage that the design of foundation is made simple so
that some cost savings may result. However, fixed bases design provides
additional rigidity and stiffening to the structures and the stability of the
structures can be enhanced. Therefore, the use of fixed bases helps to
improve the structural performance of the structures [41].
12. Do edge piles take up same loadings as central piles in rigid cap?
(FD10)
Due 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 a uniform load.
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, raking 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 raking edge piles.
There are several choices regarding the design to tackle the uneven
distribution of loads. The first one involves the lengthening of side piles to
stabilize the piles under high loads. However, the increased length of outer
piles tends to attract more loads and this seems not to be a good solution.
The other way out is to lengthen the central piles aiming at getting more
loads and this evens out the load distribution among the piles [26].
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Fig. Stress bulb effect on
load distribution in piles