Experimental study on concrete-filled steel tube columns with inner distributed seawater and sea sand concrete-filled fiber-reinforced polymer tubes under axial compression.

• A novel concrete-filled steel tube columns with inner distributed seawater and sea sand concrete-filled fiber-reinforced polymer tubes was proposed. • The stress–strain relationship, failure mode and parameters influence of novel structure were studied. • Models based on different theories for ultimate strength were established. To effectively solve the problem of the corrosion of undisturbed seawater and sea sand concrete (SSC) in practical applications, a new structure of concrete-filled steel tube (CFT) columns with inner distributed SSC-filled fiber-reinforced polymer (FRP) tubes was proposed. A total of 40 specimens were tested to investigate the axial compression behavior according to three parameters. The results indicated that the ultimate strength of CFT columns with inner distributed SSC-filled FRP tubes increased with the increase of FRP tube thickness, FRP tube diameter and steel tube thickness. The arrangement of inner FRP tubes effectively improved the axial compressive behavior and ductility of CFT columns. The ultimate strength of the CFT columns improved by 6.23 %–91.14 % due to the inner FRP tube. The failure of the specimens was always accompanied by fracture of one of the inner FRP tubes, while the remaining inner FRP tubes continued providing confinement. Lastly, two models based on different theories were proposed and evaluated to predict the ultimate load of this structure. The evaluation results indicated that both models provided accurate and reasonable predictions for the design of this structure. [ABSTRACT FROM AUTHOR]