In the construction of a two­span bridge (span length = L) by using span­by­span construction, why is a length of about 1.25L bridge segment is constructed in the first phase of construction?

In the construction of a two­span bridge (span length = L) by using span­by­span construction, why is a length of about 1.25L bridge segment is constructed in the first phase of construction?
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1 Answer

Answer :

Basically, there are mainly three reasons for this arrangement:
(i) The permanent structure is a statically indeterminate structure. During construction by using span­by­span construction, if the first phase of construction consists of the first span length L only, then the sagging moment in the mid span of the partially completed bridge is larger than that of completed two­span permanent structure. To avoid such occurrence, 0.25L of bridge segment is extended further from the second pier   which   provides   a   counteracting   moment,   thereby   reducing   the   mid­span moment of the partially completed bridge.
(ii) The position of 1.25 L countering from the first pier is the approximate location of point of contraflexure (assume that the two­span bridge is uniformly loaded) in which   the   bridge   moment   is   about   zero   in   the   event   of   future   loaded   bridge. Therefore, the design of construction joint in this particular location has the least adverse effect on the structural performance of the bridge.
(iii) In case of a prestressed bridge, prestressing work has to be carried out after the construction of first segment of the bridge. If the prestressing work is conducted at the first pier which is heavily reinforced with reinforcement, it is undesirable when compared with the prestressing location at 1.25L from the first pier where there is relatively more space to accommodate prestressing works.
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