Structural performance of short concrete-filled steel tube columns with external and internal stiffening under axial compression

External confinement and internal stiffening have been widely adopted for strengthening concrete-filled steel tube (CFST) columns. This paper presents an attempt to study the possibility of combining the external confinement using reinforcing rings and the internal stiffening using reinforcing bars for strengthening CFST columns. In order to investigate the structural performance of the proposed combination method, a total of 18 specimens were tested under axial compression. These specimens were strengthened using three different methods: confinement in the form of external reinforcing rings (ERRs), stiffening in the form of internal reinforcing bars (IRBs), and a combination of both methods. For all tested specimens, the thickness of steel tube, outer diameter, and tube length were 4.15 mm, 114.3 mm, and 260 mm, respectively. CFST specimens were filled using self-compacting concrete (SCC) of normal compressive strength. The main parameters considered in the WA procedure are the spacing between the ERRs, the number of IRBs, and a combination of the ERRs spacing and the number of IRBs. The experimental results show that the proposed combination method is more effective to improve the structural performance of short CFST columns compared to using only ERRs or IRBs. Further improvement is achieved when the spacing between the ERRs decreases and the number of IRBs increases. The proposed combination method increases the compressive load-carrying capacity up to 27.2% and 57.4% compared to ERRs and IRBs, respectively. It deserves noting that the proposed combination method, ERRs, and IRBs improve the compressive load-carrying capacity up to 120.3%, 93.5%, and 40%, respectively, compared to the control specimen. In addition, specimens strengthened by the proposed combination method show an excellent ductility behavior.