Design approach against column local–distortional interactive failures based on the Direct Strength Method

This paper deals with the development and merit assessment of an efficient and rational design approach intended to handle local–distortional (L–D) interactive failures in cold-formed steel columns (CFS) and based on the Direct Strength Method (DSM). Initially, a brief overview of the mechanical aspects underlying local–distortional interaction is provided, for the benefit of readers not familiar with this coupling phenomenon, and its relevance in CFS column design is addressed. Then, the available experimental failure load data, obtained for columns exhibiting various cross-section shapes (plain, web-stiffened and web-flange-stiffened lipped channels, hats and racks) are collected and used as the basis for the development, calibration and validation of this DSM-based design approach, which relies also on the current DSM local and distortional strength curves and existing numerical failure loads. It covers local–distortional​ interactive failures occurring in columns with several ratios between the distortional and local buckling loads, which are grouped in three families, denoted as “true L–D interaction”, “secondary-distortional bifurcation L–D interaction” and “secondary-local bifurcation L–D interaction”. The failure load prediction quality (accuracy and safety) and reliability of the proposed DSM-based design approach are assessed and compared with those exhibited by the existing methodologies to design CFS columns against L–D interactive failures. The reliability assessments follow the procedure prescribed by the current North American Specification for the Design of Cold-Formed Steel Structural Members and indicate that the design approach proposed in this work should be ready for codification in the near future.