DSM Design of Cold-Formed Steel Columns Failing in Distortional Modes at Elevated Temperatures

Recently, the authors employed numerical failure loads and temperatures, obtained by means of ANSYS shell finite element analyses (SFEA), to establish guidelines for the design of cold-formed steel fixed-ended lipped channel and rack-section columns exhibiting distortional failures under fire conditions. The design approach was based on the Direct Strength Method (DSM) and had been previously adopted by other researchers − the currently codified design/strength equations/curves, developed for ambient temperature, were modified to account for the appropriate Young’s modulus and yield stress erosion caused by the elevated temperature. Although the modified DSM design curves were shown to predict fairly well the vast majority of available numerical and experimental (only a few) column distortional failure loads under elevated temperatures, it may be argued that the above validation had a limited scope, since it involved exclusively fixed-ended columns with two cross-section shapes and acted by temperatures not exceeding 600 ºC. This work extends the scope of the previous investigations, by presenting and discussing numerical results concerning columns with two end support conditions (fixed and pinned end supports − the latter associated with free warping), four cross-section shapes (lipped channels, hats, zeds and racks), various cross-section dimensions (six per shape) and subjected to eight uniform temperatures (up to 800ºC). Such results consist of column distortional post-buckling equilibrium paths and failure loads, which were determined by means geometrically and materially non-linear ANSYS SFEA. In order to cover a wide distortional slenderness range, several room-temperature yield stresses are considered. The temperature dependence of the steel material properties is simulated using the model prescribed in part 1.2 of Eurocode 3 (EC3-1.2 − CEN 2005) for cold-formed steel. Finally, the numerical failure loads obtained in this work are used to assess the merits of modified DSM distortional strength curves proposed by the authors. Is it shown that these curves improve visibly the failure load prediction quality, which provides motivation and encouragement to continue the ongoing search for an efficient (safe and reliable) DSM-based design approach for columns failing in distortional modes under elevated temperatures.

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