Improving the Direct Strength Method prediction of column flexural-torsional failure loads

Abstract

Abstract This paper reports the latest results of an ongoing investigation on the accuracy of the codified Direct Strength Method (DSM) global strength curve in predicting 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 by the authors [1, 2] 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 normalised 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 codified design curve and improving the DSM-based design approach proposed in [2]. 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: 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). Initially, a parametric study is performed, aimed at gathering failure loads concerning columns (i) with the cross-section shapes considered earlier (plain channels, unstiffened and stiffened lipped channels, return lipped channels, hat-sections and rack-sections), (ii) with 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 codified global DSM strength curve and by the strength curve set developed for fixed-ended columns. It is found that neither of them yields consistently good failure load estimates, which prompts (i) an in-depth comparative study on the elastic post-buckling strength of fixed-ended and pin-ended columns, and (ii) the proposal of modifications that lead to an efficient failure load prediction for the pin-ended columns − although the failure load set obtained in this work is necessarily limited, the fact that their predictions by the proposed DSM global design curves (i) exhibit very high quality and (ii) clearly outperform those yielded by the current design curve provides strong encouragement to search for further experimental and numerical validation.