Numerical study on the behavior of eccentrically loaded square spiral-confined high-strength concrete-filled steel tube columns

Square spiral-confined concrete-filled steel tube (SCCFST) column with high-strength concrete (HSC) is a new type of composite column, which can give full play to high-strength concrete carrying capacity advantages and has good deformation capacity. A refined finite element model was developed to comprehensively investigate the compression behavior of SCCFST columns with HSC, and parametric analysis was carried out. The results demonstrated that the spiral stress developed slowly in the early loading stage but increased rapidly when the load was about 85% of the peak load. As a result, the compressive strength of concrete in the spiral-confined region was improved by about 14% when the compressive strength fc' = 111 MPa. However, the decrease in the axial load of the concrete outside the spiral-confined region offset this improvement. Thus adding the spiral had little effect (within 6%) on the load-carrying capacity of the columns while significantly improving the postpeak deformation performance. This improvement increased with the spiral confinement index increase. For a given spiral confinement index, the effect of decreasing the spiral spacing on improving the ductility of the column was the most significant. When the peak load was reached, except for the region near the neutral axis, the steel tube stress in most of the region reached the yield stress. Design expressions were derived using the plastic stress distribution method and assuming that the concrete strength in the compression zone is 0.8fc' to determine the axial load and flexural capacity of the SCCFST or CFST columns with HSC.