Theoretic Analysis And Experimental Research About Shear Lag Effect Of Thin Wall Box Girders

Shear lag problems were studied by some scholars long before, where only the shear lag problems of the aviation structures were studied at that time. Until in the 1970s-1980s, some country scholars paid attention to shear lag problems due to neglecting shear lag effect which caused several bridges damaged. Some research outcomes about shear lag problems were attained and part of project problems were solved during^ the twenty years. With the development of communication, some new problems about shear lag effect appear continuously. These problems need farther research. In this dissertation, the shear lag effect is studied about the constant depth box girders, the variable depth box girders, the composite box girders and the main box girders of the cable-stayed bridges. The main contents are the followings.A general potential energy equation is derived according to the principle of minimum potential energy and a set of differential equations with different boundary conditions are derived according to differential method considering the deformation properties and the longitudinal displacement functions of the constant depth box girders under a load. 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 according to the differential equations of the constant depth box girder. Those methods can be applied to solve the shear lag effect of the continuous box girders with the principle of superposition. The negative shear 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. According to the results of experiments and calculations, the main factors which have effects on the shear lag effect of the box girders are the cross-sectional forms, the width/span ratio, boundary conditions and load forms, especially the width/span ratio.The shear lag effect of the steel-concrete composite (T) girders is related to time due to the shrinkage & creep of concrete, and the slippage between the steel beam and the concrete slab which are joined by the part-rigidity shear connectors. The longitudinal displacement patterns of the composite girder are put forward by author according to its structural properties and hypothetic conditions. A set of difFerential equations and the corresponding boundary conditions are derived according to virtual work principle and the longitudinal displacement patterns. It is concluded that the rigidity of the shear connectors and the surrounding relativehumidity have effects on the shear lag effect of the steel-concrete composite girders according to the results of examples. The concrete slab stress of the pressed flange decreases with the increment of time. But the diminished stress value decreases with the increment of the shear connector rigidity. The surrounding relative humidity has the same effect on it as the shear connector rigidity. This is to say that the diminished value of the concrete slab stress decreases with the increment of the surrounding relative humidity.There are bending moments and axial forces at the girder sections of the cable-stayed bridges under loads. Due to the existence of the axial forces, the method of analyzing the shear lag effect of the cable-stayed bridges is different from that of analyzing the general box girders. The bending moments and the axial forces are separately considered according to little deformation theory and the principle of superposition. Energy differential method is adopted to solve the shear lag effect of the cable-stayed bridges caused by the bending moments, where the beam-column effect is considered. According to examples, the shear lag coefficient has relation to the applied axial force/u...