On the Accuracy of the Current Direct Strength Method (DSM) Design Curve for Columns Failing in Global Modes

This work presents and discusses the results of an in-depth and extensive numerical investigation aimed at assessing how accurately is the currently codified DSM design/strength curve able to predict the ultimate strength of cold-formed steel columns failing in global modes, namely flexural and flexural-torsional ones. This investigation deals with fixed-ended columns exhibiting a wide variety of cross-section shapes: plain channels, lipped channels, web-stiffened lipped channels, web/flange-stiffened lipped channels, lipped zed-sections, hat-sections, rack-sections and I-sections (formed by two back-to-back plain channels). The first part of the paper, which is devoted exclusively to plain channel columns, begins with a parametric study intended to gather failure loads of columns with continuously varying geometries and yield stresses such that (i) buckling occurs in either major-axis flexural-torsional or minor-axis flexural modes and (ii) a wide slenderness range is covered. These failure load data are then used to assess the quality of their estimates provided by the current global DSM strength curve and to propose two possible modifications that improve that quality. In the second part of the paper, the above two modified DSM-based global design curves, together with the current one, are used to estimate the numerical failure loads of a fairly large number of columns exhibiting the remaining six cross-section shapes: unstiffened, web-stiffened and web/flange-stiffened lipped channels, lipped zed-sections, hat-sections, rack-sections and I-sections. The outcome of this extensive parametric study is that the failure load predictions provided by either of the two proposed/modified DSM global design curves (i) exhibit a very high quality and (ii) clearly outperform those yielded by the currently codified strength curve, for the whole set of columns considered in this work. Moreover, the advantages and disadvantages of the two proposed/modified DSM global design curves are also discussed in some detail.

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