Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) are an attractive form of hybrid members, in which the FRP tube is typically manufactured by filament-winding with fibers suitably oriented for desired mechanical properties. A significant number of studies have been conducted on the behavior of CFFTs under axial compression, in which the FRP tube is commonly assumed to have linear-elastic behavior, and is often 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 biaxial stresses (hoop tension in combination with axial compression) and may exhibit significant nonlinear behavior. This paper presents an improved model for the axial compressive behavior of CFFTs with the nonlinear biaxial behavior of the FRP tube duly taken into account. To verify the proposed model, seven circular CFFTs were tested under axial compression. Ancillary tests on bare FRP tubes were also conducted to determine the material properties of the FRP tubes. Comparison between predictions and test results indicates that the proposed model leads to more accurate predictions of the CFFT behavior than the existing ones in which the biaxial nonlinearity of the FRP tube is ignored.
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