This　study　investigated　the　seismic　performance　of　combination　piers　using　steel　tubes　(STs)　encased　with　ultra-high-performance　concrete　(UHPC)　as　a　demolition-free　formwork　with　normal　concrete　(NO　cast　inside.　With　the　engineering　background　of　Luoyu　Bridge　as　the　motivation　for　the　study,　a　structural　design　of　UHPC-ST-NC　combined　piers　was　carried　out.Three　combined　piers　with　different　design　parameters　(labeled　as　URST-1,　UHST-1,　UHST-2)　and　one　monolithic　cast-in-place　reinforced　concrete　(RC)　pier　(labeled　as　ZT-1)　were　fabricated.　A　static　test　of　the　bridge　pier　specimens　was　conducted,　and　the　damage　mode　of　each　specimen　was　analyzed.　Subsequently,　the　hysteresis　curve,　skeleton　curve,　ductility　performance,　energy　dissipation　capacity,　stiffness　degradation,　and　other　performance　indexes　of　each　specimen　were　compared,　and　the　effects　of　the　diameter-thickness　ratios　of　UHPC　and　steel　pipes　on　the　seismic　performance　of　the　combined　bridge　pier　were　analyzed　using　the　Abaqus　finite　element　software.　The　results　show　that　the　use　of　the　UHPC-ST　formwork　instead　of　a　steel　formwork　can　not　only　improve　the　corrosion　resistance　and　scour　resistance　of　bridge　piers　but　also　save　the　time　required　for　demolding　and　improve　the　construction　efficiency.　Furthermore,　the　damage　mode　of　the　UHPC-ST-NC　combined　piers　under　seismic　action　is　similar　to　that　of　the　overall　cast-in-place　piers.　Both　pier　types　show　compression　and　bending　damage,　but　the　damage　degree　of　the　combined　piers　is　significantly　lower　than　that　of　the　overall　cast-in-place　piers.　Compared　with　ZT-1,　UHST-1,　UHST-1,　and　UHST-2　exhibit　the　same　damage　mode　as　that　of　the　overall　cast-in-place　piers.　The　peak　load　of　URST-1,　UHST-1,　and　UHST-2　is　56.　9%,　68.　0%,　and　69.　6%　higher　than　that　of　ZT-1,　respectively,　which　results　in　a　higher　horizontal　load　carrying　capacity.　Meanwhile,　the　peak　displacements　are　11.　8%,　24.　2%,　and　68.　0%　higher,　respectively,　which　result　in　a　smaller　residual　displacement　and　a　higher　energy-consuming　capacity.　In　addition,　the　ductility　coefficients,　ultimate　displacements,　and　energy-consuming　capacities　of　the　UHPC-ST-NC　combined　piers　with　structural　hoops　are　higher　than　those　of　UHPC-ST-NC　combined　piers　without　structural　hoops.　According　to　the　results　of　finite　element　parameter　analysis,　it　is　suggested　that　the　diameter-thickness　ratio　of　the　UHPC　section　of　the　UHPC-ST-NC　combined　pier　should　be　0.　2-0.　3,　that　of　the　steel　pipe　should　be　0.　01-0.　015.　The　test　results　show　that　the　seismic　performance　of　the　UHPC-ST-NC　combined　pier　is　better　than　that　of　the　whole　cast-in-situ　pier,　and　that　the　welding　of　the　horizontal　circular　hoop　reinforcement　at　the　outer　side　of　the　steel　pipe　can　improve　the　seismic　performance　of　the　bridge　pier.　©　2023　Xi＇an　Highway　University.　All　rights　reserved.