Modelling of concrete-filled FRP tubes considering nonlinear biaxial tube behavior

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) are an attractive form of hybrid members. The FRP tube in CFFTs is typically manufactured by filament-winding, with the fibers suitably oriented for the desired mechanical properties. Such members have many advantages over traditional columns, including their excellent corrosion resistance and ductility. As a result, a significant number of experimental/theoretical studies have been conducted on the behavior of CFFTs under axial compression. In the existing theoretical work on concrete-filled FRP tubes under axial compression, the FRP tube has commonly been assumed to have linear-elastic behavior, and has often been taken to be under a uniaxial stress state (i.e., hoop tension), especially when the fibers are oriented close to the hoop direction. However, in reality, FRP tubes in CFFTs are subjected to a state of biaxial stresses and may exhibit significantly nonlinear behavior. This paper presents an improved model for the axial compressive behavior of CFFTs with the nonlinear behavior of the FRP tube under biaxial stresses (hoop tension in combination with axial compression) duly accounted for. Comparison between the proposed model and test results indicates that the proposed model leads to much more accurate predictions than when the biaxial nonlinearity of the FRP tube is ignored for certain cases although the improvement in accuracy is much smaller for other cases.