Shear Lag Effect Analysis Of Box Girders Considering Influence Of Web Shear Deformation And Cantilever Plate Thickness Variation

The shear lag of box girder has always been a classic subject in the study of bridge theory,domestic and foreign scholars have done a lot of researches on it and obtained many results,but there are still some imperfections.The theoretical analysis and finite element numerical simulation are used to study the shear lag effect of thin-walled box girder in this paper.The main research contents include the following aspects:(1)Combined the relationship between shear force flow,shear deformation and longitudinal displacement of box girder,the warping displacement function considering the influence of shear deformation of box girder web is derived.using energy variational principle to derive the differential equations of box girder section control.The expressions of the longitudinal stress and deflection of the shear lag effect on simply supported box girder and cantilever box girder are deduced by combining with the boundary conditions,the shear lag effect of the straight web cantilever box girder and simply supported box girder model undering the different loads is analyzed,and comparing its theoretical value with finite element value,the results show that the calculated values agree well with the finite element values,which proves the correctness of the theoretical analysis in this paper.(2)The warping displacement function of the whole box girder is obtained by assuming the warping displacement function of the stem,using this warping displacement function,the shear lag effect of the concrete simple box girder model with stems was analyzed by applying the energy variation method.By comparing the theoretical values with the finite element values,the rationality of the shear lag analysis method for box girder considering the influence of stem is proved.After the stem was set,the normal stress of the box girder was reduced,and the maximum reduction of the calculated normal stress on the mid-line of the box girder reached 11.58%.Using ANSYS software to compare and analyze the influence of stems on the upper and lower wing plates and the effect of stem size on the shear lag effect of box girder,the results showed that the effect of stems on the upper wing was significant,and the increase in the horizontal size of the stems will reduce the normal stress of the box girder and make the change more uniform.(3)Using the midline of the cantilever plate to calculate the section characteristic constant for the box girders with variable thickness of cantilever plate that were used commonly in engineering,by assuming the warping displacement function of the variable thickness cantilever plate and combining with the calculation method of the stem,the shear lag analysis method which can visually reflect the thickness variation of the cantilever plate is derived.Shear lag effect of simply supported beams with variable thickness of cantilever plates is analyzed undering concentrated load and uniform load,the theoretical value is compared with the finite element value to prove the correctness of the theoretical analysis in this paper.The finite element software was used to verify the rationality of method that the cantilever plate variable thickness box girder is equivalent to cantilever plate,the results show that the stress error will reach 14.6% at the distal end of the cantilever plate when equivalent to an equal thickness cantilever plate.By defining the thickness ratio of the cantilever plate,the influence of the thickness variation of the cantilever plate on the normal stress and deflection of the box girder is analyzed.With the thickness ratio increases,the stress value on the upper edge of the cantilever gradually decreases,and the deflection of the box girder also decreases slightly.