Buckling Behavior of Thin-Walled Steel

Abstract

Cold-formed steel (CFS) sections are increasingly used as primary and secondary structural members in Iraq. The accurate prediction of elastic buckling (global, distortional, and local) is essential for design methods such as the direct strength method (DSM) and the effective width method (EWM), especially when web perforations are present. This paper compares unconstrained shell finite element eigen-buckling analyses (ABAQUS) with a specialized implementation of the constrained finite element method (cFEM) that enables explicit mode separation and shear-mode control. Three families of perforated pallet-rack columns (C-sections) with rectangular/square web holes were analyzed across ranges of length, thickness, and perforation density. Results show excellent agreement between the same sections analyzed by ABAQUS in the previous study and cFEM for the first (lowest) buckling mode (flexural-torsional), with typical differences within ~0–2% under unconstrained conditions. When constraints and shear-mode components are imposed, discrepancies increase for global and distortional modes but are substantially reduced by using an orthotropic material with ν = 0 to suppress Poisson coupling, thereby isolating pure modes. Sensitivity to mesh refinement around holes and to hole density/arrangement is quantified. The study demonstrates that cFEM can efficiently predict elastic buckling of perforated members while clarifying the role of shear-mode components and Poisson coupling on apparent differences with standard FEM.