Buckling Analysis Of Cold-formed Plain Columns

Cold-formed thin-walled columns have a series of advantages such as light weight,flexible molding method,good bearing performance,high bearing capacity,overall rigidity and good seismic performance.Therefore,they have been widely used in civil engineering,mechanical engineering,aerospace engineering and other fields.For cold-formed thin-walled plain channel columns in compression,the common failure modes are local buckling,global buckling and interactive buckling between local and global buckling.In order to predict accurately the ultimate loads and failure modes of plain channel steel members,the theoretical,the experimental and numerical values are obtained in this work and the main contents are as follows:(1)Based on the nonlinear theory of open thin-walled members,the nonlinear equilibrium equations of a single symmetric cross section are derived.Based on the axial force of the plain channel columns,nonlinear terms are conserved until order 3,and the coupling and shortening effects of the bending and torsion displacement are considered.The non-linear equilibrium equation of four boundary conditions with two fixed ends,fixed and hinged end,two hinged ends and fixed and free end are obtained.Moreover,the ultimate loads of the plain channel columns with large deformation are obtained.(2)The elastic buckling analysis model of cold-formed thin-walled plain channel steel members is established.The eigenvalue algorithm method(EAM)and the arc-length method(ALM)are numerically analyzed.The initial imperfections are introduced in the nonlinear buckling analysis.The initial imperfections are mainly based on the effective length in order to accurately predict the ultimate loads of the elastic buckling member.Moreover,the effects on critical load and ultimate load of constraint application and loading methods are studied,and the post-buckling path of thin-walled plain channel steel is obtained.In addition,compared with the critical force and ultimate loads based on the large deformation elastic theory,the suitability of these two numerical calculation methods for elastic overall buckling thin-walled members with different lengths is further studied.(3)In order to study the elastic-plastic ultimate loads and failure modes of cold-formed thin-walled members,based on the elastic-plastic buckling test of channel steel with axial compression load,the effects of different lengths,flange and web ratios on failure modes and ultimate loads were investigated.The results indicate that when the ratio of the flange to the web is small,plain columns are likely to have local and overall interaction buckling;when the ratio of the flange to the web is large,and the local buckling is more likely to occur.In the case of local buckling,as the ratio of the flange to the web increases,the ultimate loads of the cold-formed thin-walled plain columns under axial compression increases.For columns with elastic-plastic buckling failure modes,the effect of length on their failure modes needs to be considered.This means that as the length of the test piece increases,local buckling will occur near the fixed end rather than the mid-span position.For local and overall interaction of the buckling members,the effect of the length on the direction of flexible needs to be considered.(4)Based on the elastic-plastic test of thin-walled channel steel under axial compression,the nonlinear finite element buckling analysis of channel steel is carried out,and geometric imperfections and initial rotation angles are introduced in the process of nonlinear buckling analysis to analyze the imperfections effect on ultimate loads and failure modes.The results show that for the columns with overall and local interaction buckling failure modes,the initial imperfections of length always play a leading role from the first order to the third order buckling mode,and the initial imperfections of thickness have a greater influence on these columns;For the local buckling plain channel columns,the first-order and second-order initial imperfections have little effect on the ultimate loads,and the third-order mode has a great influence on the ultimate loads of the longer member.In addition,the imperfection can take 40%t of the columns,which is close to the test values.The ratio of the axial displacement of the member to the initial rotation angle affects the ultimate loads and failure modes of the plain channel steel members.As the ratio of the axial displacement to the initial rotation angle increases,the failure modes become more pronounced at the end.(5)The ultimate loads are calculated by classical theoretical analysis methods,the specification of China's cold-formed thin-walled steel structure,the direct strength method of North America and European Eurocode 3.The difference between the calculated ultimate loads of the cold-formed thin-walled plain channel columns in the test and the conventional engineering design method is analyzed.In addition,the applicability of the theoretical calculation and engineering design method to the elastic-plastic buckling members is studied.