From past large earthquake, it has been recognized that the highway steel bridge piers in urban areas play a very important role in the social lifeline system. The seismic design specification of steel bridge piers introduced in the current Japan allows independent, longitudinal, and transverse forces. To date the seismic performance of steel bridge piers has been widely studied through static cyclic loading tests, pseudo-dynamic loading tests, and numerical analysis in a single lateral direction under constant axial force. However, the actual seismic waves consist of three-directional components and the seismic response of bridge piers is simultaneously affected by the two horizontal components. It is difficult to properly evaluate the seismic performance of bi-directional horizontal seismic motions through single-directional loading tests because of the complex behavior of local buckling and inelastic behavior caused in the component plates of the pier at the ultimate state. To clarify the seismic performance of partially concrete-filled steel bridge piers subjected to bi-directional seismic loading, the performance of partially concrete-filled steel bridge piers under actual earthquake conditions was investigated using 20 square section specimens through cyclic static loading tests and single-and bi-directional hybrid loading tests in this study. Three acceleration records of two horizontal NS and EW direction components in three different ground types, obtained during the 1995 Kobe Earthquake, were adopted during the dynamic tests. The experimental results clarified that the maximum displacement and residual displacement under actual earthquake conditions cannot be correctly estimated by conventional single-directional loading test results in medium and soft ground types, and the filled-in concrete can effectively improve the seismic resistance performance in sufficiently high concrete filled steel bridge piers. In this study, an analytical model consisting of a concentrated mass and a rigid bar with multiple springs located at the base was developed to simulate the hysteretic behavior of partially concrete-filled steel bridge piers subjected to single- or bi-directional ground motions. In order to describe the complicated nonlinear behavior of each spring element accurately, a series of approximate curves whose parameters were determined by results of single-directional static cyclic loading tests had been adopted. To examine the validity of the proposed model, the results due to the simulation were compared with those of static cyclic tests, single- and bi-directional hybrid tests. By comparison, it is demonstrated that the proposed multiple-spring model can predict well the hysteretic behavior of partially concrete-filled thin-walled steel bridge piers with square cross-section
