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. Why is creep a major concern in balanced cantilever method?
(BC3)
In balanced cantilever method, the moment is balanced along the length of
the piers. However, along the extended cantilevers only a part of negative
bending moment is balanced by prestressing bending moment arising from
normal force induced by prestressing. As such, it results in large
deflections induced by concrete creep. These partly unbalanced
permanent loads generate creep which produces unexpected and
unevaluated hyperstatic effect. In fact, the compressive stresses are very
high in lower slab while they are very low in upper slab of bridges.
12. In bridge widening projects, the method of stitching is normally
employed for connecting existing deck to the new deck. What are the
problems associated with this method in terms of shrinkage of
concrete? (SBGB1)
In the method of stitching, it is a normal practice to construct the widening
part of the bridge at first and let it stay undisturbed for several months. After
that, concreting will then be carried out for the stitch between the existing
deck and the new deck. In this way, the dead load of the widened part of
bridge is supported by itself and loads arising from the newly constructed
deck will not be transferred to the existing deck which is not designed to
take up these extra loads.
One of the main concerns is the effect of stress induced by shrinkage of
newly widened part of the bridge on the existing bridge. To address this
problem, the widened part of the bridge is constructed a period of time (say
6-9 months) prior to stitching to the existing bridge so that shrinkage of the
new bridge will take place within this period and the effect of shrinkage
stress exerted on the new bridge is minimized.
Traffic vibration on the existing bridge causes adverse effect to the freshly
placed stitches. To solve this problem, rapid hardening cement is used for
the stitching concrete so as to shorten the time of setting of concrete.
Moreover, the stitching work is designed to be carried out at nights of least
traffic (Saturday night) and the existing bridge may even be closed for
several hours (e.g. 6 hours) to let the stitching works to left undisturbed.
Sometimes, longitudinal joints are used in connecting new bridge
segments to existing bridges. The main problem associated with this
design is the safety concern of vehicles. The change of frictional
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