A testing research on the local-biaxial flexural interactive buckling of high strength steel Q460C, Q550D and Q690D welded box slender section beam-columns subjected to combined compression force and biaxial bending is reported. A total of thirteen median long steel columns, of which four manufactured from Q460C steel, five from Q550D steel and four from Q690D steel, were loaded until failure under biaxial eccentric compression forces. The local-biaxial flexural interaction buckling behavior is investigated by analyzing the test results. As an addition to the test data, the finite element parameter analyses are performed. The ultimate bearing capacities obtained from the test in this study and the numerical results presented in this study or in the existent literatures are compared with the calculation results in accordance with the design methods recommended in the American Specification ANSI/AISC 360-16, Eurocodes BS EN 1993-1-1 and BS EN 1993-1-5, and the newly proposed Effective Yield Strength Method (EYSM) and Effective Section Method (ESM) to check whether the current technical provisions and the new methods are suitable to the high strength steel beam-columns examined in this paper. The experimental and numerical results show that after taking the resistance factor into account, the ANSI/AISC 360-16, EN 1993-1-1 and BS EN 1993-1-5 are very conservative for the high strength steel beam-columns with greater member slenderness ratios (lambda = 80, 90, 100) or with smaller plate width-to-thickness ra-tios (square box sections with b/t = 40, or rectangular box sections with h/t = 40 or b/t = 17); the new proposed EYSM provides an excellent prediction of the ultimate bearing capacities of the beam-columns and thus suitable for the high strength steels; but the new proposed ESM gives a very conservative estimate, over the range of steel nominal yield stress fy = 460 MPa similar to 690 MPa, plate width-to-thickness ratio b/t = 40-80, member slenderness ratio lambda = 40-100.
