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　bi-directional　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.　©　2013　Elsevier　Ltd.　All　rights　reserved.