Bending behavior of short steel cylinders under axial force fluctuations

During earthquake excitations, axial forces fluctuate together with bending moment fluctuations, especially in portal frame bridge piers and the arch-ribs of arch bridges. Conventionally, however, the axial force is treated as constant when evaluating the capacity of structures under seismic loading. In this paper, the influence of axial force fluctuations on the maximum strength and ductility of short steel cylinders is considered in an effort to establish practical formulations for the seismic design of steel bridge members with pipe sections. Elastoplastic large-displacement analysis is carried out on parametric models generated by setting the radius–thickness ratio as the main structural parameter. Bending behavior under constant and fluctuating axial force is compared, where the final value of considered axial force fluctuation is the same as the axial force magnitude in the constant axial force case. It is found that the moment and ductile capacity corresponding to the post-peak region of bending behavior are significantly improved when the axial force fluctuation is considered. Design formulae for failure strain taking into account this capacity improvement are proposed for different limit states. The validity of the proposed formulae is demonstrated through numerical analysis.