Study About Shear Lag Characteristic In The Self-Balancing Construction Of Large Span Prestressed Concrete Box Girder Bridges

The structural system of prestressed concrete continuous (rigid frame) box girder bridges has the advantages of high stiffness, driving comfortably, fewer expansion joints, lower maintenance cost, and for a variety of span etc. Box girder sections are of great web spaces and wide flanges, but the shear lag effect is obvious. Therefore, it is of significant importance to make the research of prestressed concrete box girder bridges on the shear lag effect to prevent the top and bottom plate from transversal cracks. At present no matter design or research about the shear lag effect is mostly based on the whole bridge under formed state, although the study on the shear lag effect is mostly based on the model of the cantilever beam, it does not take into account the load, stiffness, constraint vary of system conversion, the cumulative effect of the shear lag in the construction phase and the impact of the curved prestressing tendons. This paper studies the shear lag characteristic in the self-balancing construction of large span prestressed concrete box girder bridges. The research and results are as follows:1. The differential equation and boundary conditions of the shear lag are established based on quartic parabolic longitudinal displacement along the flange width. The finite difference method is applied to solve the shear lag effect in the self-balancing construction of the Jian River Bridge in Xia Rong Expressway considering the load vary and stiffness vary in various stages. The shear lag characteristics of concrete weight is studied according to the construction calculation, and compared with the shear lag characteristic of cumulative weight in the longest cantilever state.2. The finite difference method is used to solve the shear lag effect in the self-balancing construction. The hanging basket and wet concrete weight are considered as the concentrated force acting on the front construction beam. The results show that there is the positive shear lag in the moving of hanging basket and the construction of pouring concrete.3. The effect of the prestress cable in the top plate is simplified to the equivalent concentrated force in the beam-end. The prestress cable in the web is simplified to the concentrated force in the junction and the beam-end by the broken line model. The results illustrate that there's only negative shear lag in the effect of some beam prestress in the top plate of the Jian River Bridge. With the increasing of the beam there's only positive shear lag in the effect of prestress in the top plate. There only exists the positive shear lag in the effect of prestress in the web and under the action of prestress in the top and web.4. It demonstrates that the smaller the curve power of the variable height box girder is, the more obvious is the positive shear lag. The shear lag effect of cumulative concrete weight which is calculated by the construction process is more obvious than that of cumulative concrete weight in the maximum cantilever state.5. The Jian River Bridge in Xia Rong Expressway is adopted as an engineering example to make a test about the shear lag characteristic in the self-balancing construction of large span prestressed concrete box girder bridges. The strain in the flange-web intersections is larger than the one in the center and end of flange. It is accord with the distribution of the positive shear lag. The coefficient of shear lag measured is consistent with the theoretical result.