Flexural design of cold-formed steel built-up sections failing by local buckling: development of generalised direct strength method

This research paper presents a new flexural design approach for cold-formed steel (CFS) built-up sections failing in local buckling, developed following an extensive numerical investigation involving two novel closed built-up sections. At first, a finite element (FE) model using the widely adopted ABAQUS software was constructed and carefully validated against pertinent experimental data available in the literature. A comprehensive validation process which included comparing flexural strengths, deformed shapes, moment-curvature, and moment-displacement curves was performed. Afterwards, the validated FE model was extended to a numerical parametric investigation comprising of 108 simulations involving both three-point and four-point bending cases. Key parameters, including cross-sectional shapes, slenderness, and screw spacing, were varied primarily to facilitate the formulation of the novel design method. The outcomes of this investigation revealed that the current direct strength method (DSM) available in the North American Specifications (NAS) underestimated the bending strength, particularly for the ultra-thin sections. Considering this observation which is consistent with the findings of past research on similar cross-sections, a Generalised Direct Strength Method (DSM-G) was developed by introducing a new mathematical model, which differs from the traditional slenderness limit equations, instead relying on a conservativeness degree-based approach used for modifying the original DSM equations. The DSM-G method demonstrated a better accuracy in predicting the flexural strengths of different built-up sections (including the ones investigated by other researchers), all failing by local buckling. Moreover, the reliability of DSM-G equations was assessed, satisfying the prescribed threshold index limit suggested in NAS. A comprehensive set of guidelines, along with a design example for implementing the DSM-G method, has been presented to facilitate practical application.