Restoring force model of high-strength concrete-encased CFST composite columns

To investigate the seismic behavior of high-strength concrete-encased concrete-filled steel tube (HSC-encased CFST) composite column, eight specimens with a high axial compression ratio were tested under lateral cyclic loading and constant axial compressive loading in this paper. Key points on the load-displacement skeleton curve of the composite columns were obtained based on the experimental results. The test parameters include the axial compression ratio, volume stirrup ratio, stirrup form, and the diameter-width ratio. All specimens failed in the flexural mode, with the crushing of compressive concrete and losing of vertical load-carrying capacity. The ductility and deformation capacity decrease with the axial compression ratio increasing, and enhancing the volume stirrup ratio within 1.33-1.63% can provide an effective confinement of composite column. The ultimate strength of specimen with the octagonal stirrup is 23.8% higher than that with well stirrup, and the ductility and load-carrying capacity are enhanced due to the increase of diameter-width ratio. A degrading trilinear restoring force model of HSC-encased CFST composite column with a high compression ratio is proposed. The comparisons made between calculated results and test results show such a high agreement, which indicates the proposed restoring force model is applicable to such HSC-encased CFST composite column. It provides a theoretical basement for further research on the nonlinear dynamic analysis of such CFST composite column.