Research On The Compression And Bending Performance Of Cold-formed Thin-walled Rectangular Steel Tube Short Columns Considering Interactive Buckling Of Plate Group

Cold-formed thin-walled rectangular steel tube columns exhibit good compression and bending performance due to their reasonable cross-sectional form.Therefore,this type of steel member is widely used in cold-formed thin-walled multi-layer frame structures as a lateral force-resistant member.However,cold-formed thin-walled rectangular steel pipe columns generally have a large plate width-to-thickness ratio,which is prone to local buckling and affects the ultimate bearing capacity of the member.At present,the domestic steel structure design code divides the section of steel components into different aspect ratio grades according to the section bearing capacity and plastic deformation capacity,and adopts the corresponding ultimate bearing capacity design method.However,the domestic thin-wall specifications not yet have a clear classification of cold-formed thin-walled sections.The effective width method is adopted for the design of cold-formed thin-walled rectangular compression-bending members.This method follows the section edge yield criterion and does not consider the buckling of thin-walled members.Related studies had found the significant plastic behavior,so the domestic thin-walled code may be too conservative in estimating the ultimate flexural bearing capacity of this type of member,and the design process based on this code,in which uses the effective width method to calculate the ultimate bearing capacity of the membe,is complicated and inconvenient.In addition,many related studies have shown that the plate-related effects after buckling of the members have a greater impact on the ultimate bearing capacity of the members.In view of the limitations in the design of cold-formed thin-walled rectangular compression-bending components from the perspective of section classification,the domestic thin-walled codes have not yet been designed.According to the domestic steel structure design specifications,this thesis selects cold-formed thin-walled rectangular steel pipe short columns with section plate width-thickness ratios belonging to S3,S4 and S5,considering the relevant buckling effect of the plates,based on the effective plastic width method,and the directly related parameters,such as axial compression ratio n,flange width-thickness ratio r_fand web width-thickness ratio r_w,which are defined as independent variables to establish the calculation formula of the ultimate bearing capacity for the compression-bending members.To study the compression-bending load-bearing performance of the cold-formed thin-walled rectangular steel tube short columns,1105 cold-formed thin-walled rectangular steel pipes with different axial compression ratios,flange width-thickness ratios and web width-thickness ratios were analyzed under the constant axial pressure and variable horizontal forces.Finite element nonlinear analysis was carried out on short columns of bent thin-walled rectangular steel pipes.The main contents and results of this research are introduced as follows:(1)Considering the material nonlinearity and geometric nonlinearity,the nonlinear finite element model of the cold-formed thin-walled rectangular steel tube short column was established and verified.The above-mentioned model was used to study the cross-section failure form of the component bearing limit state.The results show that there are mainly two failure modes in the bending process of the short cold-formed thin-walled rectangular steel tube selected in this paper:full-section yield and compression flange and web buckling.when the ratio of the ultimate bending moment to the plastic bending moment considering the axial pressure M_u/M_pc?1,the member will yield at full section;and when M_u/M_pc<1,both the compression flange and web of the member will buckle.(2)The influence of the three parameters of flange width-thickness ratio r_f,web width-thickness ratio r_wand axial compression ratio n on the ultimate bearing performance of members was analyzed.The plate-related effects of components with different parameter combinations in the failure process had been concluded.The results show that n and r_fhave a greater impact on the ultimate flexural bearing performance of compression-bending members.As r_fincreases,M_u/M_pcdecreases significantly;as n increases from 0 to 0.4,M_u/M_pcdecreases in the beginning and then increase.Thus,the influence of r_won the ultimate flexural bearing capacity of members can not be ignored.M_u/M_pcdecreases with the increase of r_w,and r_whas a greater influence on the ultimate bearing capacity degradation process of compression-bending members.The greater r_wis,the stiffness of the member is degraded faster.In addition,the relevant buckling of the member flange and the web is affected by their respective width-thickness ratios and axial compression ratios.(3)According to the stress distribution characteristics of the members selected in this thesis in the limit state,the calculation formula of the ultimate flexural bearing capacity considering the relevant buckling effect of the plates is fitted based on the effective plastic width method,and the calculation results are verified accurately compared with the finite element values.In addition,calculation results are compared with the limit values calculated by the effective width method of the relevant domestic thin-walled specifications,and it is found that the specification design is conservative.Therefore,the effective plastic width method used in this article can provide a reference for the bearing capacity design of thin flexible members in specification,and the calculation formula is not limited to thin flexible bending members that do not develop plasticity due to local buckling,but also applicable to partially developed plastic or full-section plastic bending members.