Finite Beam Element Analysis On Shear Lag Effect Of Thin-walled Box Girder With Special Support

Special-supported box girders are increasingly used in bridge engineering.This paper mainly analyzes the shear lag effect of thin-walled box girder with special support using finite beam element method.Based on the theory of elementary beam,the shear lag effect of box girder is considered.The additional deflection caused by shear lag effect is adopted as generalized displacement to describe the shear lag deformation state,which is regarded as a kind of independent basic deformation.By introducing 4 initial parameters,obtain the element stiffness matrix of the shear lag in initial parameter method.Considering constrained torsion warping displacement mode under the influence of shear deformation,choose the form of the constrained torsion displacement function of the box girder.Obtain the element stiffness matrix of constrained torsion in initial parameter method.Under the bending and torsion deformation of thin-walled box girder,the above two element stiffness matrix are superposed and obtain element stiffness matrix of thin-walled box girder with orthogonal support.The element stiffness matrix of special-supported box girder is further obtained by introducing rigid arm constraint.According to the principle of finite element analysis,based on the plane finite element program FRAME2,the finite element calculation program MFRAME2 of the specialsupported box girder is worked out.By using one dimensional finite element program MFRAME2,the internal force analysis of a three-span continuous box girder with special support is carried out.The results show as follows: Under concentrated load applied to the middle position of top plate at the midspan,the peak of the bending moment diagram and shear lag generalized moment diagram decrease with the increase of the skew angle.The peaks appear at the position of the concentrated load and the support.At the same special support,the peaks of benging moment and shear lag generalized moment on the support closed to the concentrated load are greater than that stayed away from the concentrated load.The value of bending moment is much larger than that of the shear lag generalized moment in the same section(absolute value).The shape of the bending moment diagram is different from that of the shear lag generalized moment diagram,and both of them reach the peaks in the concentrated load section and the special supported section.After the shear lag generalized moment reaches the peaks,the attenuation is very fast,while the attenuation of bending moment is relatively flat.With the increase of skew angle,the peaks of bi-moment increases and the peaks appear at the position of the special support.At the same special support,the peak of bi-moment on the support closed to the concentrated load is greater than that stayed away from the concentrated load.After bi-moment reaches the peaks,the attenuation is very fast.Under eccentric load,the distribution of torque moment and bi-moment is more complicated.Shear lag warping stress and constrained torsion warping stress accounted for a large proportion on total stress of control section.We should take it seriously in the design.The SHELL63 shell element in the finite element software ANSYS was used to analyze the stress of the three-span continuous box girder with special support.The results show as follows:With the increase of the skew angle,the normal stress of box girder section shows a decreasing trend,but the degree of decrease is rising.The eccentric load has great influence on the section stress of special supported section.With the increase of the skew angle,the shear lag coefficient at the intersection of the web plate and the top plate and the intersection of the web plate and the bottom plate increases,but the shear lag coefficient at the middle position of the top plate and the middle position of the bottom plate decrease.So the skew angle is larger,the shear lag effect is more obvious.The skew angle from 0 degrees to 45 degrees has little influence on the shear lag coefficient of mid-section in the middle span.When the skew angle is greater than 45 degrees,its change has great influence on the shear lag coefficient of mid-section in the middle span.