Shear Lag Effect Analysis Of Box Girder Based On Additional Deflection

Thin-walled box girder has been widely used in bridge engineering because of its good mechanical properties. However, box girder causes the shear lag phenomenon under the influence of loading. At a very early time, many researchers at home and abroad have explored and studied on shear lag effect of box girder, some achievements were gained, which have been used in the construction processes of the bridge; However, there are still some relevant problems to be solved. In this paper, the problems of shear lag effect are studied.Through selecting the addition deflection caused by shear lag effect and take it as research object. New generalized force and generalized warping displacement function are also defined and take the deformation caused by shear lag effect as an independent deformation. By using energy variation principle, the control differential equation about additional deflection is established. Take generalized shear, generalized moment, additional deflection and its change ratio as the four initial parameter. According to the boundary conditions of the box girder, the initial parameter' answers about the differential equations are given.Calculation formula of additional deflection and generalized moment would be derived under the influence of simply supported box girder, cantilever box beams and two span continuous beams on average load and concentrated load. Addition deflection and section stress would be analyzed separately under the influence of loading. The results would be compared with another result obtained by using traditional force method. And comparing with the stress values obtained from the theory of elementary beam, it is concluded that the effect of shear lag effect on the simply supported box girder and the cantilever beam is relatively small. However, it has a big effect on two span continuous beams and its effect cannot be ignored. By using the finite element ANSYS to build a model of simply supported box girder,cantilever box beams and two span continuous beams under the average load and concentrated model. And compared with the calculation values of derivation formula in order to verify the correctness of the theory and methods used in this paper.Using the method of additional deflection to calculate the simple box girder, analyze the change law of shear lag coefficient with the wide span ratio under the influence of concentrated load in the different locations and its loading effect on the cross section's roof and stern's connection. Under the average load, the shear lag coefficient with the wide span ratio on different cross sections would change. It is calculated that the closer to pedestal, the bigger of shear lag coefficient and it increases with the increase of the wide span ratio.According to the method of additional deflection to calculate the shear lag coefficient of fixed end section of cantilever box girder when concentrated loads lying on different positionsand also analyze the change law of wide span ratio. It is concluded that the closer to fixed end of loading, the bigger of shear lag coefficients of fixed end. According to the same theory,shear lag coefficient of different sections are calculated under the influence of the cantilever box girder in average loading. It is concluded that when the cross section is at a certain distance from the fixed end, the negative shear lag phenomenon occurred. However, no matter under the concentrated load or average load, the shear lag coefficient of the cross section is increased with the increase of the wide and span ratio.Under the influence of concentrated load and average load, by using the method of additional deflection to calculate the shear lag coefficient of two span continuous beams of different cross sections with the change of wide span ratio, it is concluded that the bigger of wide span ratio, the bigger of shear lag coefficient. Therefore, no matter it is a simply supported box girder, or cantilever beam or two span continuous beam, its shear lag coefficient would increase with the increase of width span ratio.