ABSTRACT
The main aim of the study is buckling analysis of steel beams made of thin-walled cold-formed sigma profile with respect different numerical descriptions. The analyses are carried out on sigma profile with a height of 140 mm and a span of 2.20 m. The numerical models of the Finite Element Method (FEM), developed in the Abaqus program, include modelling of the so-called boundary conditions of the forks with use of displacement limitations. The beams are modelled using S4R shell finite element with linear or square shape function. Local and global instability behaviour is investigated using linear buckling analysis and are verified by the comparisons with theoretical critical bending moment obtained from analytical close form formulas based on so called Vlasow beam theory dedicated to the thin-walled elements. In addition, the engineering analysis of buckling is carried out for a simple shell (plate) model of the separated cross-section part in form of flange wall using Boundary Element Method (BEM). The discussion concerning geometric simplification in sigma cross-section according theoretical assumptions is performed too. It is worth noting that the value of the critical bending moment calculated on the basis of the Vlasov beam theory does not take into account the loss of local stability or contour deformation. On the other numerical shell FEM models enable multimodal buckling analysis taking into account interactive buckling. In the paper eigenvalue and shape of buckling modes for selected numerical models are calculated for three first buckling modes but the values of critical bending moments are identified basing on the eigenvalue obtained for the first buckling mode.
