Compression performance of FRP-steel composite tube-confined ultrahigh-performance concrete (UHPC) columns.

• Axial compression tests on the ultrahigh-performance concrete (UHPC)-filled FRP-steel composite tube (UHPCFSCT) columns were conducted. • The influences of the FRP layers, FRP types, FRP hybrid and loading mode were examined. • The stress–strain curves of UHPCFSCTs can be divided into weak, medium and strong restraint curves. • Existing bearing capacity models under different loading modes are evaluated. • A new strength model suitable for steel tube-confined UHPC, FRP-confined UHPC and UHPCFSCTs was proposed. A new ultrahigh-performance concrete (UHPC)-filled fibre-reinforced polymer (FRP)-steel composite tube column (UHPCFSCT) was designed. This structure uses not only UHPC with high strength and durability to reduce the section size and improve the stiffness of the column but also FRP–steel composite tubes as confinement tubes to reduce the brittleness of the UHPC. Thirty-two UHPCFSCTs were subjected to monotonic axial compression tests. The key parameters include the number of FRP layers, FRP type, FRP hybrid condition and loading mode. The full-section-loaded specimens mainly exhibit shear failure, while the core-loaded specimens mainly exhibit middle expansion deformation. The stress–strain curves of UHPCFSCTs represent three types of columns with different numbers of FRP layers and FRP types: columns with weak, medium and strong confinements. The weak and medium confinement curves exhibit a softening section after the elastic–plastic stage, and the strong confinement curve shows a bilinear response. With the increase in the number of FRP layers, compared with those of the unconfined UHPC column, the ratios of the increases in peak stress, ultimate stress, and ultimate strain are 1.436∼2.794, 2.046∼4.822 and 0.778∼2.315, respectively. The bearing capacity of the CFRP-confined specimens is stronger, while the deformation capacity of the BFRP-confined specimens is stronger. The ultimate stresses of the core-loaded specimens are 19.6%∼30.6% higher than those of the full-section-loaded specimens. Existing strength models were used to evaluate the test data of UHPCFSCTs, and the Hu et al. model was used to predict the strength of UHPCFSCTs under core loading. Finally, through database regression analysis, a strength model that can be used to predict steel tube-confined UHPC, FRP-confined UHPC and UHPCFSCT strength under full-section loading was proposed. [ABSTRACT FROM AUTHOR]