Experimental assessment and theoretical evaluation of axial behavior of short and slender CFFT columns reinforced with steel and CFRP bars

Comparisons between the experimental test results and the theoretical design equations are performed in terms of ultimate load carrying capacity. The design equation is modified to accurately predict the ultimate load capacities of CFFT columns reinforced with FRP bars. Furthermore, the experimental results showed that the axial compressive strength of CFRP-reinforced CFFT columns was reduced by 22% with increasing the slenderness ratio from 8 to 20. In general, the use of the FRP tube induces a confinement effect for concrete columns which enhances the strength and ductility of such column's section. This enhancement resulted in a slender section which increased the possibility of buckling instability of the CFFT columns. Thus, a theoretical model was conducted to develop a simplified formula for critical slenderness limit of FRP-reinforced CFFT columns to control the buckling instability mode of failure. Results indicated that the slenderness limit of 14 was suggested as a safe value for the design purposes. A parametric study was conducted which showed the significant effect of the concrete compressive strength and hoop stiffness of FRP tube on the critical slenderness ratio of FRP-reinforced CFFT columns.