Experimental study on the behavior of circular ultra-high strength concrete-filled steel tube columns subjected to unequal end moments

Abstract

Concrete-filled steel tube (CFT) columns in multi-storey frames may be subjected to unequal end moments, especially during earthquakes. The behavior of ultra-high strength concrete-filled steel tube columns (UHSCFT) with concrete strengths greater than 100 MPa subjected to unequal end moments has not been studied thus far. In this study, 22 UHSCFT columns were designed and tested to investigate the influence of the end moment ratio β, eccentricity ratio e/D, and slenderness ratio L/D on the behavior of UHSCFT columns under different end moments. The results suggested that varying β from 1.0 to 0 resulted in a higher axial load-bearing capacity for the UHSCFT columns with e/D = 0.4, 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, are unable to well predict the resistance to compression and bending of UHSCFT columns under unequal end moments. It was observed that the accuracy of GB 50936 would be improved by assuming K'a ≥ 1.