Ultimate State of Thin-Walled Circular Steel Columns under Bidirectional Seismic Accelerations

The effects of coupling of two horizontal seismic acceleration components are investigated for the ultimate states of thin-walled circular steel columns used as bridge piers. Ultimate states of these columns are mainly governed by the instability caused by the local buckling at the lower part of the columns. First, an accurate bidirectional pseudodynamic experiment is carried out to examine their ultimate seismic behaviors as well as to confirm the accuracy of our nonlinear dynamic FEM shell analysis. The experiment showed that the columns under bidirectional seismic excitations suffer larger local buckling deformations than those under unidirectional excitations. Furthermore, the experimental results agree well with those obtained by the FEM analysis. Based on the so-called pushover analysis under monotonically increasing unidirectional horizontal displacement, two types of circular ultimate interaction curves for columns are derived in terms of two horizontal restoring force components and two sway displacement components. The validity of these two interaction curves is examined by the nonlinear dynamic shell analyses under various bidirectional seismic excitations. In this analysis, the ultimate states of columns are identified by the elastic-plastic stability theory. The numerical analysis shows that the ultimate interaction curves expressed by the horizontal restoring force components give a good prediction of the ultimate states of columns. For the sake of practical applications, a formula is derived to provide the ultimate interaction curves for arbitrary thin-walled circular columns.