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
Level Two (Advanced FAQs)
Part I: Bridge Structure
1. What are the advantages of assigning the central pier and the
abutment as fixed piers?
(i) For abutment pier to be assigned as fixed pier while the bridge is quite
long, the longitudinal loads due to earthquake are quite large. As the
earthquake loads are resisted by fixed piers, the size of fixed piers will
be large and massive. In this connection, for better aesthetic
appearance, the selection of abutment as fixed piers could
accommodate the large size and massiveness of piers. Normally
abutments are relatively short in height and for the same horizontal
force, the bending moment induced is smaller.
(ii) For the central pier to be selected as the fixed pier, the bridge deck is
allowed to move starting from the central pier to the end of the bridge.
However, if the fixed pier is located at the abutment, the amount of
movement to be incorporated in each bearing due to temperature
variation, shrinkage, etc. is more than that when the fixed pier is
located at central pier. Therefore, the size of movement joints can be
reduced significantly.
2. What are the functions of diaphragms in bridges? (BS1)
Diaphragm is a member that resists lateral forces and transfers loads to
support. Some of the diaphragms are post-tensioned and some contain
normal reinforcement. It is needed for lateral stability during erection and
for resisting and transferring earthquake loads. Based on past research,
diaphragms are ineffective in controlling deflections and reducing member
stresses. Moreover, it is commonly accepted that diaphragms aided in the
overall distribution of live loads in bridges.
The main function of diaphragms is to provide stiffening effect to deck slab
in case bridge webs are not situated directly on top of bearings. Therefore,
diaphragms may not be necessary in case bridge bearings are placed
directly under the webs because loads in bridge decks can be directly
transferred to the bearings [56]. On the other hand, diaphragms also help
to improve the load-sharing characteristics of bridges. In fact, diaphragms
also contribute to the provision of torsional restraint to the bridge deck.
41