Axial compressive behavior of GFRP tube-reinforced concrete-steel double skin tubular columns

Hybrid FRP-reinforced concrete-steel double-skin tubular columns (DSTCs) exhibit the benefits of high bearing capacity, sufficient ductility and excellent durability. However, there are fewer studies on reinforced DSTCs under axial compression compared to unreinforced DSTCs. Eleven circular cross-section columns were tested to study the compressive behavior of GFRP-reinforced concrete-DSTCs. The main parameters considered were the thicknesses of GFRP tubes, longitudinal reinforcement ratio, hollow ratio and concrete strength. The test results showed that reinforced DSTCs had higher bearing capacity and better ductility than traditional DSTCs, and their bearing capacity could be increased by 36%-48%. The bearing capacity and initial stiffness increased with the increase of longitudinal reinforcement ratio, thicknesses of GFRP tubes and concrete strength, while they decreased with the increase of hollow ratio. When the load was within the range of 55%Pu, the restriction of the GFRP tube to the concrete was not obvious. Furthermore, an analytical model was established to predict the full load-strain curves of reinforced DSTCs, which considered the restraining effect of stirrups and the bi-axial stress state of FRP tube. The established model was in good agreement with most of the test results.