Axial compressive performance of concrete-filled steel tube stub columns with high-strength spiral confined concrete core

Concrete-filled steel tubes (CFSTs) are widely used as hybrid columns. However, rectangular/square CFST columns are less ductile and have lower strength than circular CFST columns. To solve this problem, a novel hybrid column, namely, a high-strength steel (HSS) spiral confined CFST (HSSS-CCFST or CCFST) column, was recently proposed and investigated in this study. The spiral confinement can enhance the axial compressive performance of concrete, including load capacity, deformability and especially secondary stiffness. Monotonic axial compressive tests were conducted on 12 CCFFT columns, a steel concrete FRP concrete (SCFC) column and a CFST column. The failure modes, failure processes and mechanical behaviour of the CCFFT columns were compared with those of the SCFC and CFST columns. The relationships between the states of the columns and the materials were analysed. In addition, 3D laser scanning was performed to obtain the deformation modes. Finite element (FE) analysis of the CCFST column was conducted using commercial finite element software (Abaqus). To improve the accuracy of the simulation, the constitutive models of spirals with different strength grades were modified using the section strip method and considering the residual stress from manufacturing; the constitutive model of concrete was also improved. The results of the FE analysis agreed well with the failure modes of the components in the CCFST and the stress-strain behaviours. A parametric analysis was conducted to determine the influence of the strength and dimension parameters, and finally, the design method of the axial load capacity of the CCFST was obtained by fitting the results from the experiments and the parametric analysis.