Moment capacity of cast-iron beams exposed to fire

Cast-iron girders were used in many nineteenth century structures, especially in fireproof flooring systems (such as jack arches). Many such structures are still in use today and it is important that they fulfil the current requirements on fire resistance when there is a change of use. Cast iron has limited ductility so it is not possible for cast-iron beams to achieve total plasticity as for modern steel beams. Furthermore, cast iron has different mechanical properties in tension and compression and there is often a severe non-uniform temperature distribution in cast-iron beam sections. This paper presents the development of a simplified method to calculate the bending moment capacity of cast-iron beams in a jack arched system exposed to a standard ISO 834 fire. The method is based on an assumed strain distribution in the cross-section. A comparison of the results calculated using the simplified method with those using the fibre method, validated by Abaqus simulation results, indicates that the proposed simplified method is sufficiently accurate as the basis of a design method.