Strength of Cold Formed Sections Subjected to Axial Compressive Force and Bending Moments

Cold formed sections are usually made of thin plates that make them susceptible to fail in three basic modes which are local, distortional, and global. Moreover, due to the redistribution of stresses after sectional (local/distortional) buckling they can carry additional loads up to failure. These additional capacities depend on the interaction of the plate slenderness ratios and the overall member slenderness ratios. Consequently, they are behaving differently than sections that do not exhibit sectional (local/distortional) buckling modes. Therefore, the objective of this paper is to make an in-depth investigation for the capacity of steel cold formed section columns subjected to axial compressive force in-addition to either major or minor axis bending moments. For this purpose, a nonlinear finite element model has been constructed using the commercial software, ANSYS. Thin-shell elements with four nodes and six degrees of freedom at each node were used to model the columns. Both large deflection analysis and elasto-plastic material response have been incorporated in the model. Newton-Raphson iterations were used in solving the nonlinear system of equations. Pin-ended support conditions with overall geometric imperfections of l/1000 are adopted in this study. Both lipped channel and sigma sections are considered. Further, a group of sections varied in their dimensions (web, flange, lip, thickness) are selected. In-addition, wide range of column slenderness are examined to explore the transition between the different modes of failure (local, distortional, and global). Results reflect that for the two studied sections, the axial-bending interaction diagrams are not linear, and concave upward, providing significant additional capacity to that predicted by the linear interaction equation that is adopted in the North American specification, AISI-2017.

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