Detailed FEM models employing shell elements were developed to evaluate the ultimate strength of square hollow steel columns with varying slenderness ratios and width-to-thickness ratios under elevated temperatures. The analyses considered plasticity, flexural buckling, and local buckling of the columns. It was quantitatively demonstrated that even columns with relatively small width-to-thickness ratios undergo local buckling at high temperatures. Furthermore, by focusing on fiber stresses in the column cross-section, the ultimate axial strength was evaluated using the tangent modulus theory. In addition, the ultimate strength under combined axial force and uniform bending at elevated temperatures was assessed.
Azai et al. (Tue,) studied this question.