Behaviour of CFST slender columns strengthened with steel tube and sandwiched concrete jackets under axial loading

A strengthening method for slender concrete-filled steel tube (CFST) columns using a steel tube and sandwiched concrete jackets was investigated. The strengthening decreases the member slenderness owing to the enlarged cross section, which is beneficial for avoiding instability problems. The effect of the variation in slenderness ratio on the different strengthening parameters was evaluated, and the optimal strengthening strategies for stub, slender and long columns were suggested. The experimental parameters included the length-to-diameter ratio, diameter-to thickness ratio, and sandwiched concrete strength. All specimens tested in this study were intermediate-length columns. The results indicate that the limiting value of the length-to-diameter ratio between intermediate-length and long columns is slightly larger than 10.0. For intermediate length columns, thicker outer steel tubes are more effective than high-grade sandwiched concrete. A finite-element model was then developed using ABAQUS, considering both intermediate length and long retrofitted columns. The differences between the two types of columns were analysed based on the loads on each component. Additionally, a parametric analysis was conducted to determine the effects of the strengthening parameters, including the outer steel tube strength, and damages to the inner column on the capacity after strengthening. The relationship between the reduction factor related to the length and confinement index was then presented. Finally, a model for calculating the load-bearing capacity of slender strengthened columns was proposed.