Torsion and Combined Loading of Concrete-Filled Fibre-Reinforced Polymer Tubes

Abstract

The concrete-filled fibre-reinforced polymer tube (CFFT) is a unique structural system well suited for applications in harsh environments. The loading response of CFFTs has been widely studied; however, their torsional behaviour has not been evaluated. This thesis presents a study on CFFT response under pure torsion, combined torsion and axial compression, and combined torsion and bending. To facilitate the experimental program, a testing apparatus was developed which could accommodate the complex set of geometric non-linearity effects associated with the deformations arising from multiple degrees of freedom. The pure torsion response was evaluated by testing four CFFTs and two hollow fibre-reinforced polymer (FRP) tubes. The angle-ply CFFTs exhibited a more linear response and greater torque capacity than their near cross-ply CFFT counterparts. The concrete core restrained the buckling of the tubes and greatly increased torque and twist capacity over equivalent hollow tubes. A simplified analytical model based on the superposition of truss and membrane action was developed and used in a parametric study examining the effects of laminate structure and tube thickness. The combined torsion and axial compression response was evaluated by testing five full-sized and three stub column samples. The full-sized samples were tested to failure in torsion under a variety of sustained axial compressive forces. The torque capacity increased with compressive load up to a balance point and reversed thereafter. The torque capacity increased up to 29%, relative to pure torsion, with an axial compressive stress of 0.65f’c. The combined torsion and bending response was evaluated by testing five CFFT samples over a wide range of T/M ratios. Their strengths were minimally affected at low to moderate levels of combined loading and the strength interaction diagram could be reasonably represented by a circle. The twist capacity reduced and the deflections increased with a reduction in T/M. In all CFFTs loaded with torsion, the concrete core was partially confined and failure occurred from rupture of the FRP tube followed by failure of the unrestrained concrete core. Classical lamination theory and Puck’s failure theory were used to evaluate the stress states in the laminate and the contribution of the constituent materials.