Concrete-Filled FRP Tubes under Combined Torsion and Bending

The combined torsion and bending response of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) was investigated in this study. The CFFT samples were produced from the same 166-mm-diameter nearly cross-ply filament wound glass-FRP (GFRP) tubes with a 30 MPa concrete infill. The samples were tested at torque-to-bending moment ratios (T/M) of 2, 1, and 0.67, as well as under pure bending and pure torsion, to cover a wide range of loading cases. The study showed that ultimate torque and bending moment were minimally affected when combined with normalized bending moments (M_u/M_uo) and normalized torques (T_u/T_uo) ≤ 0.5, respectively. A circular normalized ultimate strength interaction could reasonably predict the trend of the experimental data. The postcracking torsional stiffness was not greatly affected by flexure; however, late-stage stiffness was improved at T/M = 2, while it was reduced at T/M = 0.67. The flexural stiffness remained largely unchanged with T/M. Reducing T/M from 2 to 0.67 reduced the twist capacity from 52% to 27% of that at pure torsion but increased the deflection from 41% to 75% of that at pure flexure. Reducing T/M from pure torsion reduces the hoop and shear strains and produces a longitudinal strain gradient over the section height. The strain gradient limits the formation of diagonal cracks in the flexural compression region, reduces the number of cracks, and produces a crack angle variation through the section depth. Failure initiates from rupture of the FRP tube under a complex state of in-plane normal and shear stresses at the flexural tension side of the tube, followed by failure of the concrete core, which experiences partial confinement.