Ultimate bearing capacity of UHPC-encased CFST medium-long columns under axial compression

To study the effect of slenderness ratio on the ultimate bearing capacity of UHPC-encased concrete-filled steel tubular (UC-CFST) columns, a total of nine specimens under axial compression were tested. A finite element (FE) model, considering the effects of material nonlinearity and interaction between concrete and steel tube, was developed. The full-range analysis was carried out on the typical load-deformation response of the UC-CFST medium-long columns. The experimental and numerical analysis results showed that with the increase in slenderness ratio, the deformation capacity of the UC-CFST columns increased, the ultimate bearing capacity decreased, and the failure mode changed from exceedance of compressive strength to loss of stability. The internal forces and stress distributions at the mid-height section of the columns at different stages were also investigated. Finally, the applicability of various methods for calculating the ultimate bearing capacity of the ordinary concrete-encased CFST (OC-CFST) columns to the UC-CFST columns was discussed, including the methods recommended by Chinese standard T/CECS 188-2019, European standard Eurocode4, Australian/New Zealand joint standard AS/NZS 5100.6:2017, and American standard ANSI/AISC 360-16. It was found that the method recommended in the AS/NZS 5100.6:2017 was suitable for calculating the ultimate bearing capacity of the UC-CFST medium-long columns.