caltrans Bridge Design Practice
PRESTRESSED CONCRETE ANALYSIS AND DESIGN
Fundamentals Second Edition
Antoine E. Naaman
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PCI Bride Design Manual 3rd Edition
8.7 CAMBER AND DEFLECTION
8.7.1 Multiplier Method
Perhaps the most used method for predicting time-dependent camber of precast, prestressed members is the set of multipliers given in Table 8.7.1-1 (Martin, 1977). This method is fairly straightforward. First, elastic deflections caused by the effects of prestressing, beam self-weight, and other dead loads are calculated using conventional elastic analysis techniques. These are multiplied by the appropriate factors selected from Table 8.7.1-1 to determine the deflections that occur as a result of time-dependent behavior.
This method gives reasonable estimates for cambers at the time of erection. The method does not, however, properly account for the significant effects of a large cast-in-place deck. The presence of a deck, once cured, drastically changes the stiffness of a typical bridge member. This has the effect of restraining the beam creep strains that are the result of prestressing, member self weight, and the dead load of the deck itself. Also, differential creep and shrinkage between the precast beam and the cast-in-place concrete can produce changes in member deformation. The multipliers for long-term deflection suggested by this method, therefore, should not be used for bridge beams with structurally composite cast-in-place decks.
In addition, it is not recommended that prestressing levels be increased in order to reduce or eliminate long-term downward deflection that might be predicted if the multipliers in Table 8.7.1-1 are used.
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