Ultimate Strength Of Closed Section Steel Members Subjected To Combined Compression,Bending And Torsion

Steel members with the closed-section are widely used in structural engineering and bridge engineering for its outstanding bearing capacity.And those members are usually subjected to combined compression,bending and torsion.However,there is still no method to obtain the ultimate strength for those members when the out-of-plane buckling is prevented in the design codes.Hence,an intensive research of the ultimate strength of members subjected to combined bending,torsion and compression is needed.The prediction equation is significant.In this paper,the ultimate strengths of closed-section steel members subjected to combined bending and torsion and combined bending,torsion and compression with lateral supports are studied.And the influence of shear is discussed.Both the finite element method and theoretical analysis are used in this paper.Steel members with rectangular and circular sections are simulated by ABAQUS,the effect of initial geometric imperfection and residual stresses are taken into account when modeling and the effect of geometric nonlinearity and material nonlinearity are taken into account when analyzing.For cold fabricated rectangular box beams,the distribution of residual stresses is complicated and analyzed by a new presented method.The influence of residual stresses and initial geometric imperfection are studied based on the FEA results.And the prediction equation for steel members subjected to combined bending and torsion is obtained based on the existing equations considering the influence of the aspect ratio and width to thickness ratio.For steel members subjected to combined bending,torsion and compression,the prediction equation is obtained by improving the existing design method by changing the yield rules.Besides,the influence of load path to the ultimate strength is studied.The main conclusions are as follows:1)The effect of initial geometric imperfection on the ultimate strength is larger than that of residual stress.The influence of initial geometric imperfection can be expressed by a linear equation.When one or two kinds of loads of bending(B),torsion(T)and compression(C)are applied,the influence of initial geometric imperfection can be listed as C,BC,TC,T,B and BT from great to small.It means that when there is compression the influence shall be greater.And the effect of load path to the ultimate strength can be ignored.2)The proposed predictor equation based on local buckling theory can be used to predict the ultimate strength of box beams with different aspect ratios,width to thickness ratios and web thickness to flange thickness,and the error between the predictor equation and FEA results can be less than 6.8%.Besides,the effect of shear on the distribution of bending and shear stress is taken into account when obtaining the modified equation,and the error is less than 12.1%.3)Based on the design method of steel members subjected to combined bending and compression and Umansky's torsion theory,a new design method for steel members subjected to combined bending,free torsion and compress is obtained.It is proved that this design method is also suitable to closed section members with the restrained torsion.