Distortional Moment Capacity Prediction Analysis of Cold-Formed Rack Section Steel Beams Using the Direct Strength Method (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 as the Brazilian, the American Iron and Steel Institute, and the Australia/New Zealand (AS/NZ), have been well accepted for estimating simply and safely the moment capacity of beams subject to distortional buckling. However, more recent studies show that DSM design can lead to unsafe moment capacity for beams with a high slenderness factor of distortional buckling. This study analyzes the results from 64 models developed using the finite element analyses (FEA) with the Abaqus software to determine the distortional moment capacity of CFS rack type beams. 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 nonlinear elastoplastic finite element model was created, including initial imperfections, and a parametric study was developed to investigate the influence of the slenderness factor of distortional buckling on CFS rack beams' moment capacity. The FEA results were compared with DSM results to verify the accuracy of this method to predict distortional moment capacity. It is shown that, for CFS rack beams subject to uniform bending and distortional buckling with slenderness factor of distortional buckling higher than 1.0, the DSM overestimates the moment capacity.