Hysteretic Modeling of Thin-Walled Circular Steel Columns under Biaxial Bending

To ensure the safety of elevated highway bridges under major earthquakes, it is important to predict the ultimate behavior of columns used as bridge piers. After the Kobe earthquake, many analytical and experimental studies have been conducted to examine the ultimate seismic behavior of thin-walled steel columns. However, these studies are mostly restricted to the in-plane behavior of the columns. As observed in the Kobe earthquake, the damages to the columns are usually influenced by the two-dimensional horizontal seismic excitations. Therefore, it is more desirable to consider the effect of biaxial bending at the ultimate stage of earthquake response in the design of steel columns. Herein, we propose a hysteretic model of thin-walled circular steel columns under biaxial bending to predict the ultimate seismic behavior of cantilever-type thin-walled circular steel columns with constant axial load. The proposed column model consists of a concentrated mass and a rigid column with multiple nonlinear springs located at the column base. As the hysteretic constitutive relation for each spring, we adopt the Dafalias and Popov’s bounding-line model by modifying the original model in order to take into account the local buckling behavior. The material constants of the modified bounding-line model can be determined, based on the in-plane hysteretic behavior of the column obtained either by FEM shell analyses or by experiments. The validity of the proposed model is examined by comparing it with results of the two-dimensional horizontal earthquake-response analyses, using the FEM shell model.