Accurate Direct Strength Method (DSM) Prediction of Column Flexural-Torsional Failure Loads

This work presents and discusses the latest results of an ongoing investigation aimed at assessing the accuracy of the currently codified Direct Strength Method (DSM) global strength curve to predict the ultimate strength of cold-formed steel columns failing in flexural-torsional modes. The first part of the paper is devoted exclusively to fixed-ended columns and continues recent work on the improvement of the flexural-torsional failure load estimation, in the moderate or high slenderness ranges -- it is shown that the use of a novel set of strength curves, dependent on a cross-section normalized geometric parameter (involving the area, major and minor moments of inertia, and warping constant), leads to excellent failure load predictions, eliminating the large scatter stemming from the currently codified design curve. The second part of the paper aims at extending the investigation to columns with other support conditions, namely three types of pinned supports, all fixed with respect to torsion and having warping prevented: they correspond to end cross-sections attached to rigid plates resting on spherical or cylindrical hinges (i.e., pinned with respect to major and/or minor-axis flexure). The study begins with a parametric study intended to gather failure loads of columns (i) exhibiting the same cross-section shapes considered earlier (plain channels, lipped channels, return lipped channels, hat-sections and rack-sections), (ii) various geometries (cross-section dimensions and lengths) and (iii) covering a wide slenderness range. These failure load data are then used to assess the quality of their estimates provided by the currently codified global DSM strength curve and by the strength curve set developed in the context of fixedended columns. It is found that neither of them yields consistently good failure load estimates, which prompts the proposal of modifications that lead to an efficient failure load prediction -- although the set of failure loads obtained in this work is necessarily limited, the fact that their predictions by the proposed DSM global design curves (i) exhibit a very high quality and (ii) clearly outperform those yielded by the current design curve provides strong encouragement to search for further validation.

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