A Closer Look at Prevailing Civil Engineering Practice - European ...

A Closer Look at Prevailing Civil Engineering Practice, What, Why and How
Vincent T. H. CHU
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polyethylene sheathing with external prestressing has lower friction
coefficient than corrugated metal ducts in internal prestressing.
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.
10. What is the optimum size of cable duct for prestressing
The cross sectional area of duct is normally 2.5 times that of the area of
prestressing steel. The size of ducts should be not designed to be too small
because of the followings:
(i) Potential blockage by grout
(ii) Excessive development of friction
(iii) Difficulty in threading prestressing tendon
11. For incremental launching method, the span depth ratio of bridges
is normally low. Why
Bridges constructed by incremental launching method are usually low in
span depth ratio and typical values are 14 to17. With low span depth ratio,
the bridge segments are stiff in bending and torsion which is essential to
cater for the launching process. Such low span depth ratio could tolerate
the discrepancy in vertical alignment on supports over which they slide.
Such differential settlements may occur owing to the shortening of piers
when the superstructure slides over them and the differential deformation
of different piers.
12. Why are split piers sometimes used when piers are built directly
into the deck
When the piers are built directly into deck without bearings, the monolithic
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