05.28: Deformation capacity of steel column under combined loading: Compressive axial force with double curvature bending moment

Horizontal forces subjected to the moment resisting frame will be essentially resisted by the flexural manner of the columns. Horizontal forces are significant under seismic action; combined loading condition have a possibility to destabilize the columns. In ultimate limit state design, formation of the plastic hinge at the columns are allowed; however, it not permitted to lose its resistance. Moreover, sufficient deformation capacity at the plastic hinge should be guaranteed to fulfill the collapse mechanism of the structure. Following failure modes can occur in the column; 1) local buckling, 2) lateral torsional buckling for the open section subjected to strong axis bending, and 3) in-plane second-order effect instability. These failure modes should be prevented to guarantee the ductility of the plastic hinge which is formed in the column. In this paper, column instability due to second-order effect introduced by compressive axial force is studied. Numerical simulation is used to clarify this issue and in-plane behaviour is evaluated. Parameters selected for the analysis are cross-section shape, slenderness ratio, compressive axial force, bending moment distribution, material strength, and initial imperfection. From the results, it was found that material strength is not a significant parameter for column instability which is caused by the second-order effect. Finally, the formula which can guarantee a sufficient deformation capacity to the column was proposed. Proposed formula was composed by the information of cross-section shape, slenderness ratio, bending moment distribution, and force level subjected to the column. © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.