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In span-by-span construction, which prestress layout is better (i) single-span coupled cable or (ii) two-span overlapped cable?

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For single-span coupled cable, the length of cable is one span and they are coupled at the construction joint which is located at 0.25 of span. The use of single-span coupled cable in span-by-span construction suffers the following drawbacks:

(i) Stressing all tendons in one span is time consuming. Moreover, the construction team has to wait until the concrete has gained enough strength before all tendons in the span to be stressed.

(ii) Extra time is required for coupling of tendons.

(iii) The accommodation of coupler requires the lowering of designed tendon profile. Moreover, the coupler occupies large space in bridge web which is the region of high shear forces. To avoid generating a weak point in web, the web has to be locally thickened to maintain sufficient thickness of concrete.

(iv) Couplers have a higher risk of failure when compared with normal anchorages. The success of such prestress layout is highly dependent on the quality of coupler and workmanship because coupling of all prestressing tendons is carried out at the same point.

(v) The tendon length is only one span long which is economically undesirable.

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For two-span overlapped cable, the cable is two-span long. At each construction phase in span-by-span construction, only 50% of tendons are stressed. In most cases, 50% tendons stressing would be sufficient to carry its self weight upon removal of falsework. As such, it allows the use of more economically longer cable with a reduction in construction time.
Arrangement of single-span coupled cable and two-span overlapped cables

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

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