7. Smoke Control Design Criteria

7.1 Use of Design Fires When designing a smoke clearance system strictly in accordance with Building Regulations guidance there shall be no need to consider a design fire. This assumes that the designer has followed the guidance for compartmentation, means of escape and fire fighting access, supported by smoke extract and sprinklers where recommended. This would normally provide an adequate standard of safety. When designing for a smoke control system, a design fire size needs to be specified. 7.2 Design Considerations 7.2.1 Sprinklers Smoke control alone may be enough to aid fire fighting in small loading bays and service yards. In larger compartments where there are large permanent openings to outside, the openings alone may be sufficient for smoke venting without sprinklers. However, in enclosed loading bays, service yards and coach parks sprinklers are likely to be needed to prevent fire spread involving the entire compartment. Due to the weatherproof nature of the vehicles involved, the effect of sprinklers may be limited to a reduction in smoke temperatures, particularly during the early stages of the fire, and to limiting fire spread to adjacent vehicles. Consideration shall be given to the direction of the floor slope in the loading bay as this may influence potential fire spread, particularly when there is a scenario of oil based fuels (or fuels that change state) being present (drainage causing floating fuel to be moved) Consideration shall be given to the effect of sprinkler cooling on the smoke, as sprinkler spray will reduce the smoke temperature. When using mechanical extract ignoring the effect will produce a conservative design. For natural ventilation systems sprinkler cooling may need to be considered as it will reduce the buoyancy of smoke and reduce the effectiveness of natural vents. See also Section 8. 7.2.2 Vehicle Design Fire Size BS 7346-7 gives guidance for the provision of smoke control systems for car parks and provides design fire sizes for sprinklered and unsprinklered car fires. In loading bays, service yards and coach parks, where the main fire source is likely to be vans, lorries and coaches, the potential fire load for design purposes is considerably greater. While there is considerable research in this area for tunnel fires, there is little agreement on what the maximum potential fire size might be. It is recognised in many guidance documents for tunnels for example, that although the potential fire size may be much larger than those specified, once the fire has developed beyond a certain size there is little further benefit that can be achieved by a smoke control system. In that case an appropriate sprinkler system will be the primary means of controlling fire growth. Sprinklers will prevent or at least delay further fire spread. 21 In loading bays and service yards used by lorries, and in coach parks, it is likely that a fire will grow to involve multiple vehicles unless sprinklers are installed. In the absence of sprinklers there is no guidance on a design fire size, and building regulations guidance already provides design criteria for such buildings. Where mechanical smoke ventilation is needed, for example in basements, sprinklers will be required. For the purposes of means of escape it may be appropriate to consider fire growth rates when considering the movement of smoke and its impact on occupants; evacuation may be complete before a vehicle is fully involved, and before fire grows to include more than one vehicle. When considering structural fire resistance or fire fighting, it is recommended that, where sprinklers are installed, a steady state single vehicle fire is considered to be the design fire. The table below gives the fire size and growth rate for a single vehicle fire, whether sprinklers are present or not. It is assumed for the purposes of design that sprinklers will prevent significant fire spread from vehicle to vehicle, but may have little effect on fire growth within a vehicle where there could be shielding of the fire from the sprinkler spray. Vehicle classification Steady State design Fire Size (MW) Van 15 Growth rate Bus/Coach/Lorry/Empty HGV Ultra-fast 20-30 HGV, fully laden Ultra-fast 30-100 Table 1 Design Fires Ultra-fast The design fire sizes and growth rates in the table above are taken from those used in road tunnel design. A range of fire sizes are given which depend on the nature of goods carried. These are taken from the Design Manual for road and bridges; Vol 2; Section 3; part 9; BD 78/99, or derived from more recent research. The growth rates given are the NIST t2 fire growth rates, following BS 7974, which approximate closely to the linear growth rates given in the original research. These may be larger than might be expected in a building, or fire growth may be different. However, in the absence of other guidance the figures given are considered to be reasonably conservative without placing unnecessarily onerous constraints on designers. For further guidance reference shall be made to the relevant research papers (see bibliography). The physical area of the fire will depend on the size of the vehicle involved. Generally growing fires will be appropriate only when designing to protect occupant’s escape. In all other cases a steady state design fire would normally be appropriate. 7.2.3 Fires in stores, storage and other areas Where sprinklers are installed fires in other areas such as storage, refuse and offices will generally be smaller and slower growing than a vehicle fire. Therefore, the use of ultra fast fire growth and the maximum fire size for a vehicle means that the design of systems to deal with a vehicle fire shall cover fires in other areas. 22 If sprinklers are not installed or the other areas form separate compartments then reference shall be made to other guidance for determining design fire size and growth rates. The following will then need to be taken into consideration: • The type of combustible materials stored • The amount and disposition of the fire load • The degree of fire resisting enclosure.