Flexural-Torsional Failure and DSM Design of CFS Lipped-Channel and Rack-Section Columns at Elevated Temperatures

Recently, Dinis et al. (2018, 2019) reported an in-depth numerical investigation providing solid evidence that the current codified Direct Strength Method (DSM) global strength curve underestimates the ultimate strength of cold-formed steel columns buckling and failing in flexural-torsional (FT) modes (particularly in the moderate and high slenderness ranges), thus leading to uneconomic designs. The columns analyzed (i) exhibited various cross-section shapes and several geometries (cross-section dimensions and lengths), (ii) covered wide slenderness ranges and (iii) had fixed and hinged end support conditions. Moreover, these authors used the FT failure load data obtained to propose and validate new DSM column design curves to improve the failure load prediction quality (for columns with moderate and high slenderness). The purpose of this work is to extend the scope of the previous studies, by considering fixed-ended columns under elevated temperatures (up to 800º C) with two cross-section shapes (lipped channels and racks), various cross-section dimensions (five per shape) and different lengths. The results presented and discussed consist of column FT post-buckling equilibrium paths and failure loads, which are obtained through geometrically and materially non-linear ANSYS SFEA. In order to cover a wide FT slenderness range, several room-temperature yield stresses are considered. The model prescribed in EC3-1.2, for cold-formed steel, is adopted to describe the temperature-dependence of the steel material properties. Finally, the numerical failure loads obtained by the authors, together with numerical and experimental ones collected from the literature (e.g., Bandula Heva and Mahendran 2012) are used to propose a modification of the DSM-based FT strength curves developed by Dinis et al. (2019) and assess the merits of the new design curves. It is shown that the proposed modification improves visibly the failure load prediction quality, thus providing encouragement to continue the search for an efficient and reliable DSM-based design approach for columns failing in FT modes at elevated temperatures.

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