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
14. What is the significance of null point in bridge deck
The null point is the position of zero movement in the bridge deck. When
the bridge deck is pinned at a single pier, it provides the location of null
point with no deck movement. However, when the bridge deck is pinned to
more piers, the position of null point has to be calculated. The
determination of null point is important because it serves to estimate the
forces on the piers by deck length changes and to calculate the sliding
movement of sliding and free bearings.
For symmetrical deck founded on two identical fixed piers, the null point
should be midway between the two fixed piers. However, if one pier is taller
than the other, the null point would be shifted to the shorter stiffer pier.
15. In span-by-span construction, which prestress layout is better (i)
single-span coupled cable or (ii) two-span overlapped cable
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.
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
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