Pc Cantilever Box Girder Of Variable Cross-section Considering Shear Lag Time-varying Effects Analysis

Cantilever construction method is adopted widely in building large-span prestressed concrete (PC) bridges with thin-walled box girder and variable depth. The precision of predicting the deflection stresses in cantilevers during construction is the important foundation to control the outline of bridge meeting with the requirement of the design. Concrete creep will cause time-dependent deflection, while shear lag will increase the deflection, thus, it is important to analyze the time-dependent effect in PC box girders with variable depth. Based on the principle of energy and the conditions of the internal forces'equilibrium, this paper study the creep effect in PC cantilever box girders with varying depth, considering the synthetic effect of shear lag, steel's ratio and relaxation of prestressed steel. The provided train of thought helps to study the time-dependent deflection and stresses of concrete and steels during construction, which bears theoretical significance and engineering value.Firstly, the governing differential equations for shear lag considering the effect of steel are established based on energy variation method, then, the formulae of stresses, deflection and shear lag coefficient of cantilever girders under dead load and prestress are deduced by adopting the finite difference method.Secondly, by introducing the time-dependent relationship between stress and strain of concrete along with the equilibrium of force, compatibility of deformation at any time and assumption of shear lag, a novel approach to analyze time-dependent effct of PC cantilever box girders with varying depth considering the effect of shear lag, shrinkage & creep, relaxation, ratio of steel is presented, and semi-analytical formulae are established. Thus, which provides a approach to the study of the time-dependent behavior of PC cantilever box girders with varying depth.Finally, all the deduced formulae are developed by Matlab Language, which are used to calculate the deflection and stresses of concrete and shear lag coefficients during construction in the DONGJIANG Railway Bridge built by cantilever method. Comparison between the results given by presented method and the results given by software Midas show that the presented approach can be used to analyze the time-dependent effect in PC cantilever box girders with varying depth considering the effect of shear lag, and the deduced formuae can be used to calculate the time-dependent deflection and stresses of concrete taking into account the effect of shear lag, shrinkage & creep, relaxation, ratio of steel.