Research On Stress Amplification Coefficient Of Box Girder Bridge Under Eccentric Load

As the bridge span increases and the bridge deck becomes wider,The influence of constrained torsion and distortion effects caused by eccentric load on the cross-sectional stress of the box girder is becoming more and more significant.In current bridge designs,the effect of constrained torsional effects is usually considered with an empirical coefficient of 1.15,and a large number of literature shows that it is not reasonable to use this coefficient for both normal and shear stress.For the influence of box girder distortion,due to the existence of transverse partitions in the actual construction of box girder,the occurrence of box girder distortion can be effectively suppressed,so its influence on the spatial effect of box girder has not been specifically considered for the time being.Therefore,this thesis takes continuous box girder bridge with variable section as the research subject to examine its restrained torsional and distortion effects under eccentric lane load by using finite beam segment method.The work done and content of this thesis mainly includes the following points.(1)For the asymmetric cross-section box girder with single cross slope structure,this paper derives the practical calculation formulas of the torsion center position,main fan coordinate and main fan inertia moment of its cross section.Based on the practical formulas,the effects of torsional geometrical properties between the inclined and horizontal top plate were compared,and the influences of the variations of the cross slope,cell height to width ratio and cantilever plate width on torsional geometrical properties were analyzed.The research results show that the results in this paper are in good agreement with those computed by the finite element software ANSYS.When the cross slope is not considered,the lateral position of the twist center and the principal sector coordinate at the intersection of the higher web and bottom plate will be significantly affected.When the cell height to width ratio is less than 0.65,its change has a significant effect on the lateral position of the twist center.The principal sector moment of inertia decreases with the increase of the cross slope,and increases with the increase of the cantilever plate width and cell height to width ratio.(2)From the perspective of the distortion center,the basic unknown of distortion associated with the distortion center is selected,and the differential equation of the distortion control of the box beam is derived by the energy variational method,so that the process and method of analyzing the distortion effect of the box beam can be unified with the method of analyzing constrained torsion.Referring to the analysis idea of the finite element method of rod structure,the box girder element with 2 nodes and 4 degrees of freedom is introduced,and the element stiffness matrix and equivalent nodal load are derived by constraining the torsional differential equation and the initial parameter solution of the distortion differential equation.The FORTRAN language is used to calculate the internal forces and displacements of the section of the variable section continuous box girder bridge when constrained torsion and distortion occur,and the reliability of the program written in this paper is verified.(3)Taking a variable-section three-span continuous box girder bridge with a span arrangement of(48+80+48)m as the background project,the internal force and displacement of the cross-section of the girder bridge under different working conditions were calculated by using the constrained torsion calculation program RT.Through the analysis of internal forces,it is found that the peaks of torsional double moment and warpage positive stress both appear at the fulcrum or concentrated load action section,and both have the characteristics of rapid decay from the peak point to both sides.Through the analysis of the cross-sectional displacement results calculated by the program,it is found that the changes of torsion angle and generalized warpage displacement are mainly distributed in the bridge span with external load.The distortion calculation program DI was used to calculate the cross-sectional internal force under different working conditions.Through the analysis of the internal forces,it is found that the distortion double moment and distortion moment will increase sharply near the load action cross-section,and almost 0 away from the load action point.(4)The positive stress amplification coefficient and shear stress amplification coefficient are introduced to characterize the influence of constrained torsional effect and distortion effect on the cross-section stress of box girder.The positive stress amplification coefficient and the shear stress amplification coefficient are larger at the fulcrum section and the concentrated load action point.The influence of constrained torsion effect and distortion effect on the positive stress of the cross-section is consistent,and the shear stress amplification coefficient is mainly determined by the constrained torsion effect,and the distortion effect has little influence on it.(5)For the bridge in this paper,the empirical coefficient of 1.15 is small for both normal and shear stresses.When the two-lane load is distributed,the maximum positive stress amplification coefficient of this bridge can reach 1.420,and the maximum shear stress amplification coefficient caused by torsion effect can reach 3.56;the shear stress amplification coefficient caused by distortion effect is mainly distributed between 0.96～1.14,but this value increases sharply at the concentrated load action point.