Flexural behavior of CFRP-CFST Beam connections with hybrid flange-casing joints

The utilization of CFRP-strengthened Concrete-Filled Steel Tube (CFRP-CFST) members in marine environments has garnered progressively increasing attention due to their enhanced load-bearing capacity and corrosion resistance. The connection of CFRP-CFST members poses a fundamental technical challenge due to the inherent non-metallic composition of CFRP. Addressing this issue, the present study introduces an innovative joint system that integrates flanges, casings, high-strength bolts, and epoxy resin adhesive to effectively connect CFRP-CFST members. To evaluate the bending performance efficacy of this novel connection joint, an experimental program involving nine CFRP-CFST beams incorporating the hybrid flange-casing joint configuration was conducted. The main parameters were: concrete strength (C40, C50, C60), steel tube diameter (114 mm, 140 mm, 168 mm), CFRP thickness (2 mm, 3 mm, 4 mm), and the ratio of CFRP thickness to steel tube thickness (0, 0.5, 1). The test results demonstrated that the hybrid flange-casing joint system represented an effective solution for connecting CFRP-CFST beams. The predominant failure mode was characterized by CFRP debonding from the CFST along the longitudinal axis, while both the casing and flange plate remained within their elastic deformation range. The hybrid joint configuration enhanced the flexural capacity of CFRP-CFST beams by 10.1–46.8 % compared to conventional CFST beams. Parametric analysis revealed that the cross-sectional dimensions of CFRP-CFST beams significantly influenced flexural capacity, and CFRP thickness exhibited moderate effects, while the effects of concrete strength were insignificant. Finally, an analytical formula of calculating the ultimate flexural capacity of CFRP-CFST beams with hybrid flange-casing joints was proposed and proved to be accurate through comparison with the experimental results.