Experimental study on square steel tubular columns under compressive force with biaxial bending moment

Tubular hollow square steel (HSS) columns are widely used for the steel structural buildings in Japan. The columns will essentially resist horizontal force in arbitrary direction subjected to the spacial moment resisting frame. As a result, biaxial moment and axial force are applied to the columns The Architectural Institute of Japan (AIJ) has a publication that can be used to design the column under compressive axial force and biaxial bending. However, the design rule which is used to design the column in combined loading condition is not shown clearly with the sufficient test data. In this paper, tubular hollow square steel columns are studied by the full-scale testing to verify the ultimate strength and plastic deformation capacity. The parameters selected for testing are the direction of bending moment, the compressive axial force, and the slenderness ratio. From the test results, local buckling at the plate elements were observed in the specimens where the applied axial force was relatively small. However, compared with the results from the uniaxial bending moment where the bending moment was applied to the principal axis, it was found that the column under 45 degrees loading direction was not sensitive to the local buckling due to the stress distribution in the plate elements. Finally, it was found that the formula recommended by AIJ can guarantee sufficient strength and plastic deformation capacity, although the design rule in the recommendation provided overly conservative results.