Experimental study on inelastic lateral torsional buckling strength of H-shaped steel beam-columns under high axial force

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There is a case in which the energy absorption capacity of H-shaped beam-columns subject to axial forces and double curvature bending moment during severe earthquakes is limited by lateral torsional buckling. Most of past studies concerning the lateral torsional buckling of beam-columns in plastic range have been carried out only for cases where monotonic bending is loaded. However, beam-columns of multi-story frames are subjected to both fluctuating axial forces and cyclic bending moment during earthquakes. The fluctuating axial forces are caused by overturning moment and vertical vibrations. In this study, the maximum strength as well as the energy absorption capacity of beam-columns which is determined by lateral torsional buckling was studied by carrying out experiments under various loading conditions with consideration to the states in which columns are subjected to loads during earthquakes.The loading conditions in the experiments are as follows: (1) Monotonic horizontal loading … More under constant axial force, (2) Cyclic horizontal loading under constant axial force, (3) Cyclic horizontal loading under fluctuating axial force, (4) Random cyclic horizontal loading under random fluctuating axial force.When comparing the skeleton curve obtained from the relationship between the load and the deformation for the specimen subject to the cyclic horizontal load under constant axial force to the load-deformation relationship for the specimen subjected to monotonic horizontal loading under constant axial force, it is discernible that the maximum strength and the deformation at the time of maximum strength of the skeleton curve are smaller than those seen for the monotonic loading. In cases where the ultimate strength is determined by local buckling, it is generally known that the skeleton curve obtained from cyclic experiments nearly coincides with the monotonic loading curve. It was found out that the maximum strength is affected by plastic hysteresis when the strength is limited by lateral torsional buckling unlike the case in which the strength is determined by local buckling. This is so because in the case of the plastic lateral torsional buckling the buckling deformation progresses not only due to the increase in internal force as well as the extension of plastic regions but also due to the accumulation of plastic deformation caused by the cyclic bending about a strong axis. The effects of the fluctuating axial force and the plastic bending hysteresis upon the buckling strength and energy absorption capacity were clarified. Less