Axial and lateral stress-strain model for concrete-filled steel tubes with FRP jackets

In a concrete-filled steel tube under axial compression, since the steel tube has a larger Poisson's ratio than the concrete, delamination at the steel-concrete interface occurs and the steel tube is ineffective in providing confinement at the elastic stage. For resolving this problem and enhancing the concrete confinement, the provision of an external FRP (fiber reinforced polymer) jacket to restrict the lateral expansion of the steel tube is an effective means. Herein, to investigate the effectiveness of FRP-jacketed steel tube confinement, a theoretical model for evaluating the lateral strain, confining stress and axial stress in a concrete-filled steel tube with FRP jacket at various stages of loading is developed. The theoretical model is first applied to analyze specimens tested by other researches to verify its accuracy and then used to work out the FRP confining stiffness required to eliminate steel-concrete delamination and the FRP confining stiffness required to achieve Level I ductility (no strain softening at the inelastic stage until failure).