Steel Buildings in Europe

Part 6: Fire Engineering 6 - 38 framed buildings, even in severe fires, is much better than standard fire tests would demonstrate. As shown in Figure 5.1(b), instead of full collapse, as expected from the standard fire tests, a typical secondary beam in the Cardington fire test maintained its stability even when its temperature reached 954  C. Its vertical displacement reached 428 mm at its peak temperature of 954  C and recovered to a permanent displacement of 296 mm after the cooling phase. This indicates that there are large reserves of fire resistance in steel-framed buildings. The amount of fire protection being applied to steel elements may, in some cases, be excessive and unnecessary. The main reason for the above large reserves of fire resistance of multi-storey framed buildings comes from the tensile membrane action of their steel-composite floors, as shown in Figure 5.2 and explained below. Figure 5.2 Membrane action in a horizontally restrained concrete slab As described in the Section 4, the simple calculation models for fire design deal with each individual member. These simple models assume the floor slab to be a one-way spanning beam, resisting actions through bending and shear. Observations of the Cardington tests, however, show that as the steel beams lose their load bearing capacity, the composite slab utilises its full bending capacity in spanning between the adjacent, cooler members. As its displacement increases, the slab acts as a tensile membrane, carrying loads in tension, as shown in Figure 5.3. If the slab is well supported against its vertical deflection along lines which divide it into reasonably square areas, for example, by the primary and secondary beams on the column grid-lines, then tensile membrane action can be generated as a load bearing mechanism. The slab is then forced into double- curvature and hangs as a tensile membrane in its middle regions. A peripheral compressive ‘ring-beam’ is generated either around its supported periphery or in its edge-beams, as shown in Figure 5.2. Compression Compression zone (‘ring’) zone (‘ring’) Tension zone Tension zone