The seismic behavior of steel bridge piers partially filled with concrete under actual earthquake conditions was investigated by using 20 square section specimens subjected to static cyclic loading tests and single-directional and bidirectional hybrid loading tests. Acceleration records of two horizontal NS and EW directional components for hard (GT1), medium (GT2), and soft grounds (GT3), obtained during the 1995 Kobe earthquake, were adopted in dynamic tests. Experimental results clearly showed that maximum and residual displacements under actual earthquake conditions cannot be accurately estimated by conventional single-directional loading tests, especially for GT2 and GT3. A modified admissible displacement was proposed on the basis of bidirectional loading test results. The concrete fill can effectively improve the seismic resistance performance if the concrete inside the steel bridge piers is sufficiently high in quantity. Copyright © 2013 John Wiley & Sons, Ltd.