The study, co-led by Associate Prof. Siqi Lin from Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology and Prof. Yan-Gang Zhao, was published in the international journal Engineering Structures.

Concrete-filled steel tube (CFT) columns may be subjected to unequal end moments, especially during earthquakes. With the development of concrete technology and availability of various high-performance materials, ultra-high strength concrete (UHSC) can be now produced. So far, there are very limited studies to guide the design of UHSC-filled steel tube columns (UHSCFT) subjected to unequal end moments, especially those with concrete strengths greater than 100 MPa.

In this study, 22 UHSCFT columns were designed and tested to investigate the behavior of UHSCFT columns under different end moments, and assess the accuracy of current design codes. The results suggested that varying the end moment ratio β from 1.0 to 0 resulted in a higher axial load-bearing capacity for the UHSCFT columns with eccentricity ratio e/D = 0.4, and slenderness ratio L/D = 7.5, whereas for β ≤ 0, the effect of the end moment ratio was marginal. Moreover, the axial load-bearing capacity generally decreased with increasing L/D and/or e/D ratios. Smaller β, smaller e/D ratios, and/or larger L/D ratios generally led to more brittle behavior in UHSCFT columns. The second order effect of UHSCFT columns with equal end moments was significant, whereas those of specimens with unequal end moments (L/D ≤ 12, e/D ≤ 0.4) could generally be neglected.

Current design codes, such as EC4, AISC 360 and GB 50936, predicted unsatisfied results of the resistance to compression and bending of UHSCFT columns under unequal end moments. Especially, GB 50936 may significantly overestimate the capacity of UHSCFT members under unequal end moments, which could lead to unsafe design. This study suggested that the K'_a factor in GB 50936 should be not less than unity. If mandatorily assuming K'_a ≥ 1, the accuracy of GB 50936 would be improved and can be adopted for the design of UHSCFT members under unequal end moments with reasonable accuracy.