Matching relationship study between steel ratio and core concrete strength in UHSCFST stub columns

To investigate the matching relationship between steel ratio and core concrete strength in the ultra-high strength concrete-filled steel tubular (UHSCFST) columns, a total of 40 stub column specimens, considering eight different steel ratios (ranging from 5.97% to 38.72%) and three grades of high-strength concrete (C60, C80, and C100), were tested under axial compression. The results revealed that under axial compression loads, as the steel ratio of the cross-section increased from low to high, the local buckling characteristics on the surface of UHSCFST stub columns gradually diminished, transitioning from shear failure to local buckling and bulging failure. With the improvement of the strength grade of the core concrete, the steel ratio that ensures the full confining effect of the steel tube should also be increased significantly. For the specimens filled with C60, C80, and C100 concrete, the threshold value of the steel ratio that can ensure the axial compressive strength of the specimen does not decrease after reaching the peak load was 13.87%, 20.24%, and 27.16%, respectively. By comparing with the experimental data, it is found that the CFST composite strength formula recommended by the current Chinese code did not apply to UHSCFST members. Based on the regression fitting of experimental data, the formula for calculating the composite strength of UHSCFST members and the ultimate strength of UHSCFST stub columns were proposed.