DSM Design of Cold-Formed Steel Columns Failing Distortionally Exposed to Fire: How Relevant is the Temperature Dependence of the Material Behavior?

This paper presents a numerical (shell finite element) investigation aimed at assessing the performance of the current Direct Strength Method (DSM) provisions against distortional failure to estimate the ultimate strength of fixed-ended cold-formed steel lipped channel and rack-section columns (i) subjected to various uniform temperature distributions caused by fire conditions and (ii) exhibiting different room-temperature yield stresses, covering a wide distortional slenderness range. In particular, the work addresses how does the temperature-dependence of the steel material behavior, which is felt through both the (reduced) Young’s modulus and nominal yield stress values, influences the quality (accuracy and safety) of the column ultimate strength predictions provided by the DSM distortional strength curve. Three different temperature-dependent steel constitutive laws are considered, namely (i) a model prescribed in part 1.2 of Eurocode 3 (EC3) and (ii) two experimentally-based analytical expressions recently reported in the literature. The DSM column ultimate strength estimates are compared with numerical distortional failure loads obtained through geometrically and physically non-linear ANSYS shell finite element analyses that incorporate critical-mode initial imperfections with small amplitudes.

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