The Later Deflection Analysis Of Long-span Prestressed Concrete Girder Bridge

Many long-span prestressed concrete girder bridges which are put into operation have continuous deflection at mid-span, far exceeding the scope of the design forecast. This disease has seriously endangerd the continuation of such bridge's service, and caused a widespread concern on it over the world. Now there are many domestic and foreign scholars have done a lot of research on it. However, as a result of the problem's complexity, all the research are at the stage of exploration, an d most of the analyses are qualitative. So far ,this problem has not been completely resolved.The essence of the deflection problem is rigidity problem. Rigidity is changing with load and time, and the change is especially fierce if the beam has a serious dehiscence. Usually the following factors are thought as the cause of the beam deflection's increase: the inaccurate forecast to the effect of concrete's shrinkage and creep deformation, the reduction of the prestressed system's validity, a great quantity of cracking carried out on the beam, and so on.The deficiency of longitudinal prestressing force's validity leads to rigidity's reduction and deflection's increase. The effect to later stage deflection represent through two respects: First, it increase the creep deformation. Second, it leads to plate cracks of beams.As an important fator, Concrete's shrinkage and creep is very different between each kind of forecast model. But to uncracked beam, the difference of the forecast to shrinkage and creep is not enough to create a defacto difference which was confirmed by the project example in the seventh chapter of this article. Before the beam's cracking, the result actually measured is agreed with theory forecast, but if the beam cracked and the deflection increased, the theory forecast will be invalid obviously.The appearances of large amount of cracks lead to rigidity's reduction and deflection's increase. The main causes of the cracks are the shortage of prestressed system's validity and the shortage of calculation methods etc.This article think the development of cracks should be controlled to decrease the later deflection. All the measures which do good to cracks control do good to deflection control.