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
restraint provided by the abutment [30]. For example, active earth pressure
is usually adopted for cantilever abutment because there is possible
occurrence of small relieving movements. However, for abutment founded
on piles, the at-rest earth pressure can be assumed in assessing the earth
pressure as the abutment is considered to be rigidly supported by piles and
is fully restrained against lateral movement.
17. Should at-rest, active or passive soil pressure be used in the
design of abutment? (BA2)
At-rest soil pressure is developed during the construction of bridge
abutment. Active soil pressure are developed when the abutment are
pushed forward by backfilled soils at the back of abutment wall. A state of
equilibrium shall be reached when the at-rest pressure is reduced to active
earth pressure. Hence, at-rest pressure is considered when assessing the
stability of abutment while active pressure is adopted when assessing the
adequacy of structural elements of abutment.
Passive pressure is only considered in integral abutment which
experiences passive pressure when the deck expands under thermal other
effects.
Passive pressures are developed when the abutment wall pushes the soils
at the front of abutment. Given that larger movements is required to
mobilize passive pressure than active pressure and the abutment is
designed not to slide under active pressure, it is normally assumed that
passive pressure does not develop at the front of abutment. Moreover,
there is a possibility that soils may be removed temporarily owing to utility
diversion; it is normally assumed that stability contribution by soils in front
of abutment is ignored.
18. When would torsional stiffness of members be considered in
analyzing a bridge?
If a box-girder type bridge is purposely chosen because of its torsional
strength, then the torsional stiffness and resistance should be considered
in design. However, it is commonly accepted to assume that torsional
stiffness of a beam to be negligible so that it saves the complexity to
provide reinforcement to resist torsion. As such, this would result in higher
bending moments induced in the beam.
If the torsional stiffness has been incorporated in computer model during
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