Analysis On Distortion Effect Of Thin-walled Box Girders

With the development of materials and the maturation of prestressing technology in recent years,box girder has gradually developed towards the direction of high strength,thin wall,light weight and no cross-spacer in the span.The distortion effect becomes more and more prominent.The distortion angle of box girders in distortion deformation is taken as the basic unknown quantity in the plate girder-frame method,and the distortion load is obtained by decomposing the eccentric one.The differential equation of distortion for box girders is established by using the energy variation principle.The constrained torsion and distortion of box girders are considered simultaneously by generalized coordinate method.On the basis of reasonable assumption of generalized coordinate and generalized displacement,the sixth order differential equation of torsion with constraints considering distortion is established applying the principle of virtual displacement.In order to better reveal the effect of distortion on the spatial mechanical properties of box girder,the distortion effect of thin-walled box girder is studied in this paper.In order to investigate the influence of double-level cantilever slab on distortion effect of box girders,based on the general definition of distortion angle of box section,an analytic method for distortion effect was presented which was similar to restraint torsion analysis of box girders.The governing differential equation and the corresponding boundary condition were established by applying energy variation calculus based on the principle of minimal potential energy.Numerical example was given to analyze the influences of lower cantilever slab on distortion bi-moment,warping stress and distortion center of the cross section in detail.The result shows that the calculated values of distortion warping stress by using the analytic method are in a good agreement with those by using the shell element in ANSYS and box girder segment element.The distortion bi-moment of the box girder with double-level cantilever slab presents a change law of increasing firstly and then decreasing with the increase of lower cantilever slab length.The distortion bi-moment is maximal when the relative length of lower cantilever slab is 1.2.The warping stress at corner of the box girder with double-level cantilever slab decreases with the increase of the relative length of lower cantilever slab.The distortion center of cross section of the box girder is higher than the section center when the relative length of lower cantilever slab is relatively small.In order to reveal the influence of shear deformation on the distortion effect of box girders,based on the assumptions for distortional warping and tangential displacement functions,the distortional governing differential equations and relevant boundary conditions of box girders with shear deformation were derived by applying the minimum potential energy principle.The influences of shear deformation and geometric parameter variation on distortion effect of box girders were analyzed through a numerical example.It is shown that the values of distortional warping stresses with shear deformation are closer to those obtained by using the shell element in ANSYS.Compared with the results without shear deformation,the decrease degree of distortional warping normal stresses with shear deformation doesn't exceed 5%,and that of distortional warping shear stresses doesn't exceed 10%.Shear deformation has little influence on distortional displacement.With the increase of span to height ratio,the distribution curves of distortional warping normal stresses along span vary from single-peak to twin-peak form and the cross section where the sign of distortional warping shear stresses changes moves towards the ends of the box girder.The distortional warping stresses increase significantly with the decrease of the wall thickness.