Effects of external confinement on structural performance of concrete-filled steel tubes

The provision of external confinement in the forms of either FRP wraps or steel rings to concrete-filled steel tubes (CFST) would alleviate delamination at steel-concrete interface and increase both strength and ductility. To investigate the effectiveness of such external confinement, a parametric study on the effects of external confinement on the structural performance of CFST has been conducted. The axial and lateral stress-strain models of confined concrete previously developed by the authors were used in the study. The parameters studied include the concrete strength, steel tube yield strength and thickness, FRP confining stiffness and rupture strain, and steel ring yield strength and equivalent thickness, while the performance attributes evaluated include the yield strength, ultimate strength and ductility of the externally confined CFST. From the study, an equation for the minimum confining stiffness to avoid delamination is derived, formulas for predicting the yield and ultimate strengths are developed and charts for achieving Level I ductility are presented.