On the direct strength method design of columns against global failures

This paper reports an in-depth and extensive numerical investigation aimed at assessing the accuracy of the current Direct Strength Method (DSM) design curve in predicting the failure load of cold-formed steel columns collapsing in global modes, namely flexural or flexural-torsional modes. This study concerns fixed-ended columns with a wide variety of cross-section shapes: plain channels, lipped channels, double-fold (return) lipped channels, web-stiffened lipped channels, web/flange-stiffened lipped channels, lipped zed-sections, hat-sections, rack-sections and I-sections (formed by back-to-back plain channels). The first part of the work is devoted to plain channel columns and starts with a parametric study aimed at gathering failure loads of columns with continuously varying geometries and yield stresses, so that they (i) buckle in either major-axis flexural-torsional or minor-axis flexural modes and (ii) cover a wide slenderness range. These failure loads are then used to assess the quality of their predictions yielded by the codified global DSM design and to propose modifications to improve this quality. The second part of the work employs the modified DSM-based global design curves, as well as the current one, to predict a fairly large number of numerical failure loads of columns exhibiting the remaining eight cross-section shapes. This extensive parametric study shows that the failure load predictions provided by the proposed/modified DSM global strength curves (i) have very high quality and, for the columns considered, (ii) clearly outperform those yielded by the current design curve. In addition, the advantages and disadvantages of the proposed DSM global design curves are also discussed.