Global–global interaction in cold-formed steel plain and lipped channel columns

Abstract

While investigating how accurately the current Direct Strength Method (DSM) global strength curve predicts the major-axis flexural–torsional failures of cold-formed steel columns, the authors (Dinis et al., 2019; 2020) found strong evidence that, for some column lengths and end support conditions, the ultimate strength is eroded by the coupling between major-axis flexural–torsional (FMT - critical) and minor-axis flexural (Fm) bucking (the first two global buckling modes) - i.e, a global–global (G–G) coupling/interaction. This finding is at the root and provided the motivation for the ongoing numerical investigation whose first results are reported in this work. The paper presents and discusses results concerning the behaviour and DSM design of cold-formed steel (CFS) fixed-ended plain and lipped channel columns that buckle in FMT modes and undergo G–G interaction. After a brief review of the DSM-based design approach that was proposed to handle column FMT failures (Dinis et al., 2020), attention is turned to investigating the column (i) buckling behaviour, essential for the selection of column geometries susceptible to G–G interaction, and (ii) FMT non-linear (post-buckling) behaviour — special care is devoted to the identification of the most detrimental initial geometrical imperfection shape. Then, the FMT failure loads of the selected columns are obtained for five yield stresses (to cover wide slenderness ranges) and the gathered failure load data are used (i) to clearly show that the existing DSM-based design approach cannot adequately predict them and (ii) as guidance for its modification, which brings about the Fm-to-FMT buckling load ratio and enables the adequate handling of also G–G interactive failures. The excellent failure load prediction quality exhibited by this modified design approach is very encouraging and makes the authors believe that its domain of application can be extended to CFS columns with other cross-sections and/or end support conditions — research is under way to assess whether this belief is well founded.