During　the　past　decades,　amounts　of　research　efforts　have　been　concentrated　to　investigate　basic　characteristics　of　the　seismic　response　of　steel　bridge　piers　by　cyclic　bi-directional　loading　tests　or　finite　element　analysis.　However,　dynamic　test　results　on　partially　concrete-filled　steel　tube　(PCFST)　bridge　piers　under　coupled　ground　motions　in　two　horizontal　directions　are　insufficient.　To　investigate　the　behavior　of　PCFST　bridge　piers　under　bidirectional　seismic　excitations,　a　series　of　cyclic　static　loading　tests　and　single-　and　bi-directional　hybrid　(pseudo-dynamic)　loading　tests　for　circular-section　test　specimens　with　three　different　concrete-filled　ratios　were　performed　in　this　study.　The　failure　patterns　were　observed　after　the　tests.　The　experimental　results　showed　that　the　restoring　force　of　PCFST　bridge　piers　obtained　in　bi-directional　loading　tests　was　almost　the　same　as　that　obtained　in　single-directional　loading　tests.　However,　deformation　capacity　deteriorated　considerably　when　subjected　to　bi-directional　dynamic　loadings.　This　study　also　shows　that　the　seismic　behavior　of　PCFST　bridge　piers　can　be　effectively　improved　if　the　concrete　fill　height　is　significantly　increased.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.