Distortional buckling of cold-formed steel rack section under uniform and non-uniform bending-Numerical analysis and DSM

Due to the wide variety of cross-sections and their good mass/strength ratio, cold-formed steel (CFS) components are gaining prominence among steel structures, although this material is more susceptible to local, distortional, and global buckling. The design procedure based on the direct strength method (DSM) presented in some codes such as the Brazilian (NBR), the American Iron and Steel Institute (AISI), and the Australia/New Zealand (AS/NZS), has been well accepted for estimating simply and safely the ultimate strength of beams subject to distortional buckling for many situations. However, more recent studies show that DSM design can lead to unsafe ultimate strength for beams with a high slenderness factor of distortional buckling. This study analyzes the behavior from 768 models developed using the finite element analyses (FEA) with the ABAQUS software. Beams were analyzed with 2 support conditions (SCA and SCB), under uniform major axis bending alone. The selection of the specimens in which the distortional buckling mode is predominant (modal participation analysis) was performed through a linear stability analysis using the GBTUL software. The finite element models used in nonlinear elastoplastic analyses included geometrical initial imperfections and material nonlinearity. A parametric study was developed to investigate the influence of the slenderness factor of distortional buckling (lambda(d)) on CFS rack beams' ultimate strength. The FEA results were compared with DSM results to verify the accuracy of this method to predict distortional ultimate strength. It is shown that, for CFS rack beams subject to uniform and non-uniform bending and distortional buckling with lambda(d) > 1.0-1.5, the DSM overestimates the ultimate strength. Modifications were proposed in the DSM to improve the accuracy in predicting the ultimate strength of the analyzed conditions.