Fire resistance of composite floors subject to compartment fires

A series of analyses, accounting for the effects of tensile membrane action in the concrete slabs, has been performed on a large generic composite floor under fire conditions, using different patterns of fire protection for the steel beams. These demonstrate that it may often be possible to leave a significant proportion of the beams unprotected. This is because of the beneficial effect of tensile membrane action, which depends largely on the pattern of fire protection adopted for the steel beams, and the slab reinforcement. The analyses also include fire compartments of different sizes and locations, and show that the surrounding cool structure can provide some benefits in increasing the fire resistance of the fire compartment. The behaviour is compared with the critical temperatures according to BS5950 Part 8, which indicates that all beams require fire protection, even for a fire resistance period of 30 min. The results are also compared with the failure temperatures calculated using a recently published design approach, which takes account of tensile membrane action in a simplified manner. Some general guidance is given on making best use of membrane action in structural fire-resistant design.