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
In some design guidelines, maximum spacing of piles is specified to limit
the length between adjacent piles so that the assumption of rigid pile cap
can be justified.
6. What is the difference between point of virtual fixity and critical
length of lateral loading for piles? (FD6)
Some engineers may get confused about the difference between the two
terms i.e. point of virtual fixity and critical length used for piles for resisting
lateral loads. For critical length of lateral loading for piles, it refers to a
certain depth from the ground level where the piles behave as if it were
infinitely long. As such, beyond the critical length, the change in lateral
response of piles with increase in pile length will be negligible [26].
Point of virtual fixity refers to a certain dept below ground surface where
the piles are fixed without movement under loads. The depth to the point of
fixity is useful in assessing the buckling loads of piles. It is obvious that the
depth to the point of virtual fixity should be smaller than the critical length of
piles.
7. In modeling a nonrigid mat foundation by using elastic springs,
should a uniform modulus of subgrade reaction be used along the
whole base of mat? (FD7)
By using a bed of springs to simulate the flexible behaviour of mat subject
to loads, care should be taken in selection of the modulus of subgrade
reaction. In fact, the modulus of subgrade reaction depends on many
factors like the width of the mat, the shape of the mat, the depth of founding
level of the mat etc. In particular, the modulus of subgrade reaction is
smaller at the center while it is larger near the mat’s edges. If a constant
modulus of subgrade reaction is adopted throughout the width of the mat,
then a more or less uniform settlement will result when subject to a uniform
load. However, the actual behaviour is that settlement in the center is
higher than that at side edges. Consequently, it leads to an
underestimation of bending moment by 18% to 25% as suggested by
Donald P. Coduto (1994).
In general, a constant value of modulus of subgrade reaction is normally
applied for structure with a rigid superstructure and the rigid foundation.
However, a variable modulus of subgrade reaction is adopted instead for
non-rigid superstructure and non-dominance of foundation rigidity to
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