Equation of ultimate N-M interaction curve of circular CFT column

A design equation is proposed to compute the ultimate flexural strength of circular concrete-filled steel tubular (CFT) column. The proposed equation defines the axial load versus ultimate moment interactive curve of a CFT column section and has a very simple mathematical expression. In addition to its’ simplicity, the design equation can take into account inter-confinement effect between the steel tube and the filled concrete on the ultimate moment. Comparison with the experiments available in the literature is also conducted to verify validity of the proposed equation. Good agreement between the experimental results and the theoretical predictions implies that the proposed equation could provide a powerful tool, which enables structural engineers to conduct reasonable ultimate capacity design for the circular CFT column by hand.