Behavior of square and rectangular concrete-filled steel tube (CFST) columns with horizontal reinforcing bars under eccentric compression

Abstract

A series of eccentric compression tests on square CFST columns with horizontal reinforcing bars were conducted, focusing on the effects of eccentricity ratio, different types of horizontal reinforcing bars (binding bars and inclined tension bars), and bar spacing. In addition, the results of a previous study on rectangular CFST columns with binding bars were also included for comparison, aiming to investigate the strengthening performance of installing horizontal reinforcing bars for both rectangular and square CFST columns. It is found that the restraining effect provided by the reinforcing bars could not only delay and restrain steel tube buckling, but also enhance the ultimate strength and lead to a more ductile post-peak descending branch for the load-axial strain, load-lateral strain, and moment–curvature curves. For square CFST columns under eccentric compression, installing binding bars which led to a 30%–40% enhancement in ultimate strength was more efficient than installing inclined tension bars of the same reinforcement ratio which only had less than 20% enhancement. Meanwhile, the ductility of the columns was significantly improved by reinforcing bars. Furthermore, the columns with binding bars had better ductility than those with inclined tension bars. Generally, binding bars can more efficiently improve the eccentric load behavior of CFST columns than inclined tension bars. For rectangular CFST columns under eccentric compression, reducing the spacing of binding bars can slightly increase the ultimate strength by around 7% while the ductility of the columns can be increased significantly by reducing the bar spacing. In addition, increasing the cross-sectional aspect ratio of the rectangular CFST columns would offset the restraining effect of the binding bars. In the end, a discussion was provided for a design code assessment against the test results.