Local buckling behavior and evaluation method for structural performance of square hollow section members under bending shear force

Square hollow section members such as square steel tube members and box section members are commonly used as columns in structural steel buildings. Their structural performance during earthquakes significantly affects the seismic capacities of the buildings. The square hollow section members used as columns are subjected to bending shear force during earthquakes. Local buckling determines their large deformation behavior, except for fracture. Therefore, it is necessary to investigate the large deformation behavior of the members under bending shear force governed by local buckling for an accurate evaluation of the seismic capacities of the buildings. Although numerous studies have been conducted on the local buckling of the members under bending shear force, the local buckling determinant is not sufficiently understood. Various factors affect the large deformation behavior of the members. The effect of member shapes and loading conditions and the effect of material and geometric imperfections on the large deformation behavior under bending shear force have not been sufficiently investigated. Consequently, the ultimate strength and plastic deformation capacity of the members under bending shear force have not been systematically evaluated as a function of member shapes and loading conditions. Given this background, this study aimed to investigate the large deformation behavior of the square hollow section members under bending shear force governed by local buckling and evaluate the ultimate strength and plastic deformation capacity as a function of member shapes and loading conditions. First, for the evaluation of the ultimate strength and plastic deformation capacity of the members, the elastic local buckling strength under bending shear force was shown. A new normalized width-thickness ratio SH based on the elastic local buckling strength and full plastic strength was proposed. Then, after understanding the elastic local buckling behavior purely determined by member shapes and loading conditions, the large deformation behavior of the members under monotonic bending shear force was examined using numerical analysis and structural testing. The effect of initial imperfections and the effect of loading conditions on the large deformation behavior were shown. The ultimate strength and plastic deformation capacity were evaluated using the normalized width-thickness ratio SH based on the elastic local buckling strength. The evaluation expressions derived from this study enable the reasonable prediction of the ultimate strength and plastic deformation capacity under bending shear force as a function of member shapes and loading conditions. Finally, after examining the large deformation behavior under monotonic bending shear force, the large deformation behavior of the members under cyclic bending shear force was examined using structural testing. Cyclic loading decreases the plastic deformation capacity under bending shear force. However, most of the ultimate strength and plastic deformation capacity under cyclic bending shear force can be evaluated using the evaluation expressions derived from this study. The expressions were derived via the conservative analysis where initial deflections by buckling mode were used as the most unfavorable initial imperfections.