Steel Buildings in Europe

Part 6: Fire Engineering 6 - 39 Heated beam Figure 5.3 Slab bridging by tensile membrane action This forms a self-equilibrating mechanism and supports the slab loading. If the temperature continues to increase, the structure may collapse due to either the failure of the edge support of the slab, or the fracture of the slab at its edges, or within its middle region. 5.1.2 Real fires compared with standard fire In addition to the difference between the behaviour of isolated members and the members in a whole structure, real fires are also different from the standard fire that is used in the simple calculation models. A real fire in a building comprises three phases – initial growth, full development and post-peak decay. These phases are shown in Figure 5.4. The temperature increases most rapidly when all organic materials in the compartment combust spontaneously. This point in time is known as flashover point. The growth and decay phases of a real fire depend on the quantity and type of fuel available, as well as on the ventilation conditions of the compartment. Flashover Standard fire Natural fire Start fire Time Temperature (°C) Figure 5.4 Real natural fire with the standard fire curve The fire resistance time for a member, which is based on standard fire tests and simple calculation models, does not reflect the actual fire performance of the member as part of the whole building in a real fire. It does not indicate the actual time for which the member will survive in a building in fire. 5.2 FRACOF fire tests To demonstrate the actual behaviour and load bearing resistance of composite floors under standard fire exposure, full scale fire tests were carried out in Maizienes-les-Metz (France) in 2008 as part of the FRACOF project, which was funded by ArcelorMittal and the Steel Alliance. Figure 5.5 shows the composite floor, with an occupying area of more than 60 m² and the bottom surface exposed. All structural members of the composite