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When the superstructure is pushed forward, a temporary nose is usually adopted at the front end of the superstructure to reduce the cantilever moment for which the central prestress is designed. The length of temporary nose is about 60-65% of bridge span.
The bending moment of self-weight for internal spans (equal span) of long bridge is -0.0833WL2
at piers and +0.0417W L2 at mid-span (W = unit weight of deck and L = span length). However, without the use of temporary nose, the bending moment in the leading pier when the deck has to cantilever from one pier to another would be -0.5WL2, which is 6 times higher than normal values at support.
Theoretically speaking, it is possible to reduce the cantilever moment to the value of inner support moment (i.e. -0.0833WL2) with the use of a long nose. However, from economic point of view, it is would better to adopt temporary additional prestressing instead of longer nose. Hence, in actual site practice, the use of temporary nose would not reduce the cantilever moment of superstructure to the value of inner support moment but only to achieve -0.105WL2.
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