Optimal design of external rings for confined CFST columns

Concrete-filled steel tube (CFST) columns are becoming increasingly popular in the construction of tall buildings due to their superior behaviour contributed by composite action. However, this composite action cannot be fully developed owing to the imperfect steel–concrete interface bonding that occurs in the elastic stage. To overcome this problem, external rings were adopted in this study. To further improve the efficiency of external rings, different configurations of steel rings were tested. In this study, 18 CFST columns with different concrete cylinder strengths and configurations of rings were tested under uniaxial load. The experimental results reveal that: rings can improve the strength, elastic stiffness and ductility of CFST columns; denser rings arranged near the mid-height can improve the uniaxial behaviour of CFST columns more effectively; external rings can restrain the lateral dilation of steel tube and concrete in the early elastic stage. Based on the experimental results, an analytical model, taking into account the confinement effect and ring configurations, is proposed to estimate the ultimate strength of CFST columns. The validity of the model was checked by comparing the predicted strengths with experimental strengths from this study and the authors' previous experimental results.