Stress-strain behavior of concrete in hybrid frp-concrete-steel double-skin tubular columns

Hybrid fiber-reinforced polymer (FRP) concrete-steel double-skin tubular columns are a new form of hybrid columns recently proposed by the second writer. The new column consists of an inner steel tube and an outer FRP tube, with the space between them filled with concrete. The new column possesses several advantages over existing columns including excellent ductility and corrosion resistance. This paper first examines the behavior of the confined concrete between the two tubes using a finite-element model to understand the effects of the key parameters including the stiffness of the FRP tube, the stiffness of the steel tube, and the size of the inner void. Based on the available experimental observations and the results from the finite-element model, a simple stress-strain model for the confined concrete is proposed for use in practical design. Comparisons with test results show that the proposed model provides reasonably accurate and conservative predictions.