Study On Deflection Of Long-Span Prestressed Concrete Girder Bridges

With the development of science technology and traffic enterprise the long-span continuous girder (rigid frame) bridge has been growing up rapidly in the recent years with it's unique predominance of handy construction, economical cost, reasonable interal force and comfortable traveling. But the theories and experience in it are not very perfect because of it's short appearance. There are still several questions in the construction. In our country, main problems of large span prestressed concrete girder bridges are excessive midspan lag and cracks in box girder. So, it is quite necessary to study how to control long-term deflection of large span prestressed concrete girder bridges.For prestressed concrete structures, it is important to know how much the prestress loss is occurred and calutate the concrete creep exactly for the deflection of the structures. For multi-span continuous rigid frame bridge, the closure order to bridge internal forces and structural deformation of late should not be overlooked. Firstly, this thesis starts from the point of concrete creep analyses the creep mechanism and the factors affecting the creep, creep theory and the calculation method, discusses their impact on the deformation of girder. Secondly, starting from the prestress loss disicusses its influence to the deflection and derives the corresponding conclusions. Thirdly, comparison the re-construction of a different order of closure, and analysis its main beam deformation of late.Then study the current control of long-span prestressed concrete girder bridge deflection of a common method,including short-term and long-term.The traditional way that control the long-term deformation is to set up pre-camber. Based on analysis the different methods of pre-camber settings, to provide accurate methods for bridge. Short-term deflection relying on control the construction line to adjust. At present the theory of long-span concrete bridge in the construction control methods are more abstract, which of the process are difficult to operate in practice. This paper will explore more useful of the linear control methods, that combine the practical experience of the Pingzhai bridge monitoring. For example, using regression analysis to determine basket deformation of the true value of deflection,and use of the relative height difference law to avoid the effects of working-load and temperature-load .Last, using temporary cables together with designing prestress according to integral cast construction to control long-term midspan deflection is studied. Longitudinal prestress tendons are designed according to internal force of integral cast construction, sum of prestress bending moment, selfweight bending moment and bridge deck pavement bending moment is approximately zero. Then to optimize the prejacking force to improve the structure of the internal forces and the main beam late deformation. During construction stage, with the help of temporary cables, box girder will not deflect, so that displacement during construction is very little. After the construction, long-term shrinkage and creep displacement will be very little as well, as a result, long-term midspan deflection is controlled.