Steel Buildings in Europe

Part 7: Fire Engineering 7 - 2 2 FIRE RISKS IN SINGLE-STOREY BUILDINGS 2.1 Fire safety objectives The primary objective of most fire safety regulations is to ensure the protection of life (building occupants and fire fighters), environment and to some extent property (building contents and building itself). Through a lot of measures including a combination of active and passive fire protection systems, the objectives are:  To reduce and prevent the incidence of fire by controlling fire hazards in the building.  To provide safe escape routes for evacuation of building occupants.  To prevent fire spread from the fire compartment to others parts of the building and to neighbouring buildings.  To ensure that the building remains structurally stable for a period of time sufficient to evacuate the occupants and for the fire-fighters to rescue occupants, if necessary. 2.2 Fire risk analysis Single-storey buildings used as factories, warehouses or commercial centres constitute a very common type of steel construction today. In the specific case of warehouses, according to the storage arrangement (including free standing storage, palletised rack storage, post-pallet storage or storage with solid or slatted shelves) and the combustibility of materials being stored, fire may develop very quickly and then might endanger occupants long enough before the structural collapse of the building . Indeed, fire growth may be extremely important, as the upward flame propagation is usually very rapid. Vertical and horizontal shafts formed between adjacent pallets and racking behave as chimneys, which increase the spread of flames up to the roof. The smoke quickly forms a hot layer under the roof and then descends progressively with fire development. Obviously, the rate at which this occurs varies according to the combustible contents and the building arrangement. In unventilated conditions, single-storey buildings can become smoke-logged in few minutes. Although the smoke is largely made up of ‘entrained’ air, it contains enough toxic substances and asphyxiates to incapacitate or kill within minutes people exposed to them. Moreover, the hot smoke layer will also radiate high heat flux to people escaping from fire area. A hot gas layer at 500°C leads to a heat flux of about 20 kW/m² (corresponding to the radiant energy emitted by a blackbody at the temperature of 500°C) and, under such thermal conditions, skin burn will occur after only a few seconds 4 . Generally, it is agreed that the tenability threshold is 2.5 kW/m 2 , which is much lower than heat flux needed to lead to the failure of structural members. Consequently, buildings will survive longer than occupants and the structural collapse of steel structures of single-storey buildings generally does not provide additional threat to people escaping from the fire area.