Shear Lag Effect Analysis In Box Girder Of A Continuous Rigid-Frame Bridge With Varying Depth

With the rapid development of transportation engineering constructions, single-chest and single-room box girders which meet the requirements of wide bridge decks are popularly used in continuous girder bridges and continuous rigid-frame bridges. Box girder sections are of great web spaces, wide flanges and thin walls, but shear lag effect is very obvious. This phenomenon would lead stress concentration and transversal cracks in the flange-web intersections, even then result in the insecurity of bridge structures. So it is of significant importance to analyze the shear lag effect in order to understand this phenomenon and guide the optimal design of bridges.Using the energy-variational method based on the principle of minimum potential energy, the governing differential equations for solving the shear lag effect were derived. Assuming that the longitudinal displacement differential function was varying according to the third power parabola and the fourth power parabola respectively, solutions of governing differential equations were obtained. Based on the solutions, the formulas of shear lag coefficient of constant depth cantilever box girder under dead and distributed loads were derived. After analyzing several major parameters, with an improved method by substituting equivalent uniform section for variable section, the shear lag coefficient of box girder with varying depths was solved.The approach of Tianxingzhou Road/Rail Yangtze River Bridge, a prestressed concrete continuous rigid-frame bridge, was taken as an example. This thesis created the plane- member models and spatial models of cantilever box girders under construction phase and whole bridge under formed state. The shear lag effect under several load cases was analyzed, and the results of energy-variational method and finite element method were compared. It is shown that the presented method to solve the shear lag effect of box girder with varying depths by substituting equivalent uniform section for variable section was accurate and effective. Also, these results of adaptive finite element method and conventional finite element method were compared to show that the adaptive finite element method could effectively improve the accuracy and efficiency of computation especially for the edge section with stress concentration.Finally, the effect of several major parameters, which were span-flange width ratio, flange stiffness-section stiffness ratio and forms of longitudinal displacement differential function, on shear lag was investigated for cantilever box girder with varying depth under two types of loads. The influence of prostheses function on shear lag effect was also investigated by using finite element method.