Discussion on the applicability of the M-N interaction curve for the fire resistance design of CFT members

When subjected to combined axial force and bending moments at ambient temperature, concrete filled tubes (CFTs) show a marginal increase in bending resistance at low level of axial compression, but the bending resistance reduces when the compression force is high. To evaluate the buckling resistance of CFT member subject to axial compression (N) and moment (M), a second order analysis approach is proposed by EC 4: EN 1994-1-1 in which the cross section check can be carried out using the N-M interaction curve which is derived based on the plastic design principle. When the CFTs are further subjected to high temperature due to fire, the design method is however not available in EC 4: EN 1994-1-2. This paper proposes a modified M-N interaction curve based on plastic stress distribution as in EC 4: 1994-1-1 for fire resistance design of CFT members. The superiority of the M-N interaction curve is the universality to both axially loaded members and members subjected to combined axial force and bending moments, regardless of the moments being induced by load eccentricities or initial bow imperfections. Backgrounds of the existing simple calculation model in EC 4 using buckling curves and the proposed M-N interaction curve were first discussed. Then the validity of the said M-N interaction curve was established by comparisons with test results. The proposed M-N interaction curve was found to provide better predictions of the fire resistance than the simple calculation model. Hence, it could be safely extended to EC 4: 1994-1-2 for the design of CFT members under fire.