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
the other end. These frictional losses include the friction induced due to a
change of curvature of tendon duct and also the wobble effect due to
deviation of duct alignment from the centerline. Therefore, the prestress
force in the mid-span or at the other end will be greatly reduced in case the
frictional loss is high. Consequently, prestressing, from both ends for a
single span i.e. prestressing one-half of total tendons at one end and the
remaining half at the other end is carried out to enable a even distribution
and to provide symmetry of prestress force along the structure.
In fact, stressing at one end only has the potential advantage of lower cost
when compared with stressing from both ends. For multiple spans (e.g. two
spans) with unequal span length, jacking is usually carried out at the end of
the longer span so as to provide a higher prestress force at the location of
maximum positive moment. On the contrary, jacking from the end of the
shorter span would be conducted if the negative moment at the
intermediate support controls the prestress force. However, if the total span
length is sufficiently long, jacking from both ends should be considered.
5. What are the three major types of reinforcement used in
prestressing? (PC1)
(i) Spalling reinforcement
Spalling stresses are established behind the loaded area of anchor blocks
and this causes breaking away of surface concrete. These stresses are
induced by strain incompatibility with Poisson’s effects or by the shape of
stress trajectories.
(ii) Equilibrium reinforcement
Equilibrium reinforcement is required where there are several anchorages
in which prestressing loads are applied sequentially.
(iii) Bursting Reinforcement
Tensile stresses are induced during prestressing operation and the
maximum bursting stress occurs where the stress trajectories are concave
towards the line of action of the load. Reinforcement is needed to resist
these lateral tensile forces.
6. Why is spalling reinforcement needed for prestressing works in
anchor blocks? (PC1)
Reinforcement of anchor blocks in prestressing works generally consists of
bursting reinforcement, equilibrium reinforcement and spalling
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