Mechanical behavior and design applicability of ultra-high-performance concrete-filled circular steel tubular short columns under axial compression

Abstract

This paper presents the results of axial compression tests conducted on 12 short columns made of ultra-high-performance concrete-filled steel tubes (UHPCFST) with different diameter-to-thickness (D/t) ratios and steel fiber contents. The study aimed to investigate the interaction between infilled concrete and steel tubes as well as the confinement effect on the compressive behavior of UHPCFST short columns. Two types of load–displacement curves were observed in the test, which were related to the D/t ratio of the steel tube. Besides, the results showed that columns with smaller D/t ratios exhibited higher strength enhancement and better post-yield performance due to increased concrete confinement. Furthermore, the presence of steel fibers improved the post-yield ductility of the columns but did not significantly contribute to the strength enhancement. Owing to the longer elastic branch and less lateral dilation of UHPC compared to normal-strength concrete (NSC), the establishment of confining effect to UHPC core is relatively later than the yielding of steel tube, leading to a redistribution between axial stress and hoop stress. The test results were compared with current design codes, including Eurocode 4, Chinese code GB 50936, and American code AISC 360. The results showed that Eurocode 4 and GB 50936 overestimated the axial load-bearing capacity when considering the confinement effect of the steel tube, while the calculations by Eurocode 4 without considering the confinement effect and AISC 360 were closer to the test results. Therefore, the consideration of the confinement effect in design is not recommended and a reduction factor of UHPC is suggested to obtain a reliable prediction of the axial load-bearing capacity of UHPCFST short columns as safe as that of the CFST short columns infilled with NSC.