Lateral capacity of square GFRP-reinforced concrete columns with closed-type winding ties: A comparative study

Abstract

Closed-type winding (CW) ties have emerged to overcome limitations of conventional Glass Fiber Reinforced Polymer (GFRP) ties, particularly overlap slippage and low confinement efficiency. Although their effectiveness under axial loading has been widely reported, their lateral performance under varying conditions remains insufficiently understood. This study evaluates the lateral behaviour of GFRP-reinforced concrete columns with CW ties, compares their effectiveness with conventional ties, and proposes design recommendations to satisfy both strength and deformation requirements. A numerical model was developed and validated against previous experimental results. Subsequently, a parametric study was conducted considering shear span-to-depth ratio, axial load, concrete strength, tie size, tie spacing, and tie configuration. The results demonstrate that CW ties significantly enhance lateral performance compared with conventional ties. The reduction in deformation capacity caused by low shear span-to-depth ratios and high axial loads was effectively mitigated using CW ties. In addition, CW-GFRP ties improved confinement efficiency and reduced the adverse effect of high-strength concrete on deformability. Columns confined with CW ties also showed higher capacity improvement with increasing concrete strength, achieving performance comparable to spiral-confined columns. Tie size and spacing had limited influence under low axial loads but became more critical at higher loads, improving post-peak stability and enabling a second peak load. The most favourable performance was achieved with a concrete compressive strength of 50 MPa, a tie volumetric ratio exceeding 1.7%, tie spacing equal to 0.25 of the column width, and a hybrid confinement system combining outer square and inner circular ties.