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
(vii) Improvement of concrete placing in bridge webs owing to the
absence of ducts.
The major distinction between internal prestressing and external
prestressing lies in the variation in cable eccentricity. The deflected shape
of external tendons is not exactly the same as beams because the
displacement of external tendons is controlled by deviators. This is a
second order effect at working load and it is very important at ultimate load.
Based on past research, for small span with shallow cross section (i.e. less
than 3m deep), the use of internal prestressing requires less steel
reinforcement. However, for deeper bridge cross section, the employment
of external prestressing results in smaller amount of steel reinforcement.
3. Under what situation shall engineers use jacking at one end only
and from both ends in prestressing work?
During prestressing operation at one end, frictional losses will occur and
the prestressing force decreases along the length of tendon until reaching
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.
4. Which one is better, one-way prestressing or two-way prestressing?
(PT2)
During prestressing operation at one end, frictional losses will occur and
the prestressing force decreases along the length of tendon until reaching
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