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What is the difference between span-by-span construction and progressive placement?

Balanced cantilever construction simply cantilevers segments from a pier in a balanced manner on each side until the mid-span is reached and a closure is made with a previous half span cantilever from the preceding
pier.

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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?

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.

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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.

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What is the significance of spacing of split piers?

Live loads on one span tend to cause uplift of outer column of the split piers (twin leaf piers). When the two split piers are designed too close, the uplift may be greater than the dead load reaction of the outer pier so that tension is induced in the outer pier.

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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 construction creates a portal structure which modifies the bending moment envelope in the deck when compared with bridges with bearings. For instance, hogging moments are increased in supports with the decrease in sagging moments in mid-span of bridge deck. On the other hand, the shear stiffness of piers is a major concern because it tends to resist length changes of bridge deck which could not expand and contract readily.

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