In the paper the linear and non-linear stability analysis of columns and beams made of steel coldformed thin-walled sigma profiles is presented. Both, single (asymmetric) and double (symmetric) sigma sections are considered. Local and global buckling phenomena are studied for a wide range of lengths from short columns and beams to long ones. Particular attention is paid to coupled instability, when local buckling occurs close to global one. Coupled modes can take flexural, torsional or combined flexural-torsional forms. Usually this kind of instability strongly influences the overall post-buckling behaviour. It is well known that these structures are sensitive to initial geometric imperfections  Simplified theories proposed by Timoshenko and developed by Vlasov provide good results only in the case of global buckling. Therefore, in the paper FEM employing shell elements is used to capture the interactive buckling effects. The influence of initial imperfections on buckling and post-buckling behaviour is studied using nonlinear stability analysis with Riks method. The results are compared with results provided by simplified theories.
Buckling and nonlinear post-buckling analyses in compression, in simple and skew bending are performed. These issues were also studied in . Imperfections are usually introduced by perhrbations in the geometly as a linear supelposition of buckling modes, obtained from the linear eigenvalue nroblem. We follow this annroach. however, the patterns and amplitudes of imperfections refer to our own measurements performed “in situ” and processed statistically. These measured imperfections are transferred to FEM model, using discrete Galerkin method for error minimization. Special attention is focussed on modelling the double sigma members, which cross-sections are comnosed of two siema members. connected in discrete noints distributed alone the webs. It was found that buckling loads may strongly differ with those obtained from analysis for continuous web connection . The influence of head plates, stiffness and spacing of connectors and slenderness coefficients on buckling behaviour is studied by the way of examples. The examples are solved using the general pulpose fm:e element program ABAQUS.