Research On The Shear Lag Effect Of Thin-Walled Box Girder

Shear lag problems were studied by some scholars long before in the shear lagproblems of the aviation structures. Until in the 1970s------1980s,some countryscholars paid attention to shear lag problems of the bridge structure due to neglecting shear lag effect which caused several bridges damaged. Some research outcomes about shear lag problems were attained and part of protect problems were solved during the twenty years, but many problems were not solved by now, which need farther research. In this paper, the shear lag effect is studied about the constant depth box girders, the variable depth box girders. The main contents are the followings. Considering the deformation properties of the constant depth box girders under a load, a general potential energy equation and a set of differential equations with different boundary conditions is given according to the principle of minimum potential energy and energy differential method. As for the variable depth box girders, difference method is applied to solve the differential equations and the boundary conditions of the variable depth box girders, which are derived the constant depth box. Those methods can be applied to solve the shear lag effect of the continuous box girders with the principle of superposition. The negative sheer lag effect of the cantilever and continuous girders appears due to the symbolic change of the additional bending moment caused by shear lag effect under a uniformly-distributed load. And the negative shear lag effect of the cantilever girders does not appear under a concentrated load, this is to say that the negative shear lag effect is related to the constraints and the load forms. However cantilever girders can produce the negative shear lag effect under a uniformly distributed load, a non-free end and non-fixed end concentrated load and a non-free end and non-fixed end concentrated bending moment, so the function form of the load is another determinant factor of the negative shear lag effect. The length of the negative shear lag area is relative to the length of load for the constant depth box girders and the variable depth box girders under a uniformly distributed load. In addition, the shear lag coefficient λ~c increases withrespective to the increment of the span length, whereas the shear lag coefficient ^"decreases with respect to the increment of the span length under dead weight on the cantilever end. Combine the engineering actual, theoretical analysis and experimental investigation are given to analyze the sheer lag effect during the cantilever construction of the cantilever box girder with varying depth. Lastly gives rational suggestion for this structure's construction. Finally the effective breadth of continuous curved RC box girder is introduced in order to provide a reference for design.By the work of this dissertation, safer and more reasonable design , construction and analysis method for box girder have been offered.