Torsional Behavior of Circular Concrete-Filled FRP Tubes

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been studied extensively under almost all loading conditions, except torsion. In this pioneering study, six 168-mm-diameter samples were tested in pure torsion, consisting of two hollow glass-FRP tubes and four CFFTs. Two tubes with distinctly different laminate structures were tested and compared, namely, near-cross-ply and angle-ply tubes. A specially designed Z-shape setup with unique features intended to avoid second-order effects and facilitate gripping of the circular section was developed and used to apply pure torsion. The results showed that the postcracking response was heavily dependent on the FRP tube laminate structure. The tube confined the concrete core diagonal struts, as evident by a state of combined stresses in the tube, which ultimately fractured. The concrete core prevented buckling of the tubes and resulted in a 250% increase in ultimate torques over the hollow tubes. It also increased twist angles by 14% and 45% for CFFTs with near-cross-ply and angle-ply tubes, respectively. The angle-ply CFFT achieved a 118% increase in ultimate torque but was 48% lower in ultimate twist, compared to the near-cross-ply CFFT, with the latter showing a significant nonlinear response. A simplified analytical model based on the superposition of the truss action and membrane shear action of the tube was developed and verified. It was then used in a parametric study to examine the effect of the laminate structure and tube thickness on torsional strength, ranging from the lower bound based on the [0 degrees/90 degrees](s) cross-ply tube to the upper bound based on the [+/- 45 degrees](s) angle-ply tube.

Automated summary

Concrete-filled fiber-reinforced polymer tubes were tested in pure torsion, with results showing that the postcracking response heavily depended on the FRP tube laminate structure. The study also found that the concrete core prevented buckling of the tubes and significantly increased ultimate torques. Comparing near-cross-ply and angle-ply tubes, angle-ply CFFT achieved a higher ultimate torque but lower ultimate twist, with both types demonstrating nonlinear responses. This pioneering study also developed a simplified analytical model to assess the effect of laminate structure and tube thickness on torsional strength.