Experimental behaviour and design model of FRP-UHPC-steel tubular columns under monotonic axial compression

•FRP-UHPC-steel tubular columns (FUSTs) exhibited ductile behaviour with remarkable strain and strength improvement.•The steel fibers in UHPC had marginal influences on the ultimate conditions of FUSTs.•A larger inner void had a general effect to lead to more localized damage for the FRP tube.•The FRP thickness was the predominant influencing factor on the ultimate conditions of FUSTs. By integrating structural steel with Fiber-Reinforced Polymer (FRP) and Ultra-High Performance Concrete (UHPC), FRP-UHPC-steel tubular columns (FUSTs) emerge as innovative composite members that offer exceptional corrosion resistance and lightweight properties. FUSTs hold significant potential for use as thin-walled tubular columns working in harsh environments, such as wind turbines and high-voltage transmission towers. To obtain in-depth understanding of key parameters including the steel fiber ratio of UHPC, the specimen void ratio and the FRP thickness, this paper tested 24 specimens to evaluate their compressive behaviour, including 18 FUSTs and 6 UHPC-filled FRP tubes (UCFFTs). Experimental results showed that: (1) FUSTs demonstrated ductile behavior with significant strain enhancement and notable strength improvement; (2) the steel fibers in UHPC had marginal influences on the ultimate condition of FUSTs; (3) a larger inner void had a general effect to lead to more localized rupture for the FRP tube; (4) the FRP thickness was the predominant influencing factor on both the general shape and the ultimate point of the normalized axial stress–strain curves. Finally, a design model was proposed, which was able to capture the general shape of the axial load–strain curves, and could generate reasonably accurate predictions for the peak load.