Combined torsion and axial compression loading of concrete-filled FRP tubes

Abstract

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFT) were studied under combined torsion (T) and axial compression (P). Tests were performed on 166 mm diameter CFFTs with 30 MPa unconfined concrete strength (f’c) and a nearly cross-ply FRP tube. Three stub column samples were tested in axial compression and five samples were tested in torsion while subjected to varying levels of effective axial compressive stresses, namely zero, 0.22, 0.45, 0.65, and 0.85f’c. The study showed that the P-T interaction curve was characterized by an increasing trend of T with small-to-moderate levels of P, up to a ‘balanced’ point where concrete damage mode changes, followed by a reduction in T with P thereafter. Effective axial stresses of 0.45f’c and 0.65f’c increased maximum T by 21% and 29%, respectively, while at 0.85f’c the maximum T reduced to a value similar to pure T. The maximum increase in T occurs at the balanced point where P restrains diagonal tension crack opening but does not cause concrete shear–compression damage and sliding at cracks. Failures occurred from sudden rupture of the FRP tube near a concrete crack location at high levels of twist, with a pseudo-ductile response from partial concrete confinement and FRP non-linearity. The torque contribution of the concrete core (Tc) and FRP tube (Tfrp) were determined using experimental data and Classical Lamination Theory (CLT). The total torque was predominantly from Tc (66%–76%); however, this significant concrete contribution was only enabled by the FRP tube.