Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
Lawrence Webster Forrest
Tel: +44 (0)20 8668 8663 Fax: +44 (0)20 8668 8583
According to Home Office statistics, there were 92,800 road vehicle fires in the
The largest, and most widely quoted, example of a car park fire occurred in a multi-storey car park above
As automobile manufacturing technology has evolved, together with the use of more advanced materials of construction, the risk of a conflagration in a car has fallen significantly. But the fire load in vehicles has actually risen – fuel tanks are now often designed to contain larger amounts of petrol than in the past. Car bodies that used to be all-metal are now a mixture of metal and plastic. Designers have improved passenger comfort using foam or leather; but these materials present flammable characteristics if not protected by flame retardant measures.
The amount of electrical equipment has also greatly increased. Where old cars were simply equipped with a battery, an alternator and a radio; new cars have electrically movable wing mirrors and lamps, as well as many more electronically controlled pieces of equipment. The extra quantity of cable involved increases the fire load significantly.
Experiments on heat release rates carried out by the Fire Research Station and Building Research Establishment on various sized cars, where a fire began as a result of a fault in the engine, indicate heat releases rates of approximately 2MW rising to brief peaks of 4MW.
Types of car park
The type of car park involved in a fire has a significant impact on the development and spread of fire. Car parks vary from open-sided to totally enclosed, if they are underground. Open-sided car parks provide all the necessary conditions for a ventilated fire, as fresh air is always available. The fire can burn freely as long as combustible materials are available. Totally enclosed car parks are more likely to see under-ventilated fires due to the lack of fresh air. These conditions lead to incomplete combustion and the release of more toxic gases in the car park area, endangering persons who may be present.
Fire spread between cars
The layout and arrangement of car park premises mean that fire-spread from one car to another is a distinct hazard. Cars are generally parked very close to each other, facilitating the spread of fire by conduction and radiation. Car park premises are designed with a low roof-height, allowing hot smoke and heat to build up just above the cars. This will also help the fire to spread by means of convection and radiation.
All this would appear to indicate that fire spread between adjacently parked vehicles is almost inevitable, and could only be prevented by the use of sprinklers. In fact, however, past experience has shown that the number of car park fires involving a large number of cars is actually quite limited.
The Building Regulation requirements
The main intention of The Building Regulations Approved Document B (ADB) 2000 (Amended 2002) is to protect occupants evacuating by avoiding the build-up of smoke and toxic gases in a car park; also to prevent the spread of fire from one storey to another, and to assist the fire brigade in their operation. So, current legislation states that car park premises must be provided with a means of ventilating the area, and be separated from other parts of the premises with fire resisting construction. ADB recommends three different methods:
· open-sided (high level of natural ventilation)
· natural ventilation (other than open-sided)
· mechanical ventilation
Open-sided ventilation relies on the geometry of the car park itself, together with its capacity to naturally ventilate smoke and hot gases. Each storey should be naturally ventilated by permanent openings at each car park level, with an aggregate vent area not less than 1/20th of the floor area at that level, of which at least half (1/40th) should be equally provided between two opposing walls.
Natural ventilation other than open-sided can be used to extract smoke and hot gases where more limited natural openings exist. Each storey should be ventilated by permanent openings at each car parking level, having an aggregate vent area not less than 1/40th of the floor area at that level, of which at least half (1/80th) should be equally provided between two opposing walls.
Should smoke extraction not be possible using natural means, a mechanical ventilation system must be installed. This should be independent of any other ventilation system and be designed to operate at 10 air changes per hour in the event of a fire. All components should comply with BS 7346 part 2: 1990 – Components for smoke and heat control systems, Specification for powered smoke and smoke exhaust ventilators. Approved Document B 2000 (Amended 2002) states that ‘a system of mechanical extraction may be provided as an alternative to natural venting to remove smoke and heat from basements, provided that the basement storey(s) are fitted with a sprinkler system’. The air extraction system should be capable of handling gas temperatures of 300oC (fan) up to 800 oC (ductwork and fixings) for not less than one hour.
Importance of compartmentation
Compartmentation will prevent the spread of fire and smoke throughout the building and adjacent areas. For this reason, both internal and external fire spread possibilities are considered.
Where a building contains a car park, separation between the car park and the rest of the building must ensure a certain level of fire resistance. The level of resistance depends on the type of smoke-ventilation system used in the car park. ADB table A2 allows for a reduction in fire resistance when open-sided ventilation is used, compared to mechanical ventilation and natural ventilation other than open-sided, where the level of fire resistance is higher.
Fire precaution systems
The types of fire precaution systems likely to be installed in car parks will vary between premises. Some such buildings are fitted with smoke ventilation and/or a sprinkler system and/or an automatic fire detection system.
Studies of existing situations, and experiments carried out by different countries, have shown that sprinkler systems in buildings greatly help to save lives and reduce fire damage. Car park premises are designed with a low roof-height, allowing a fast sprinkler response. Since cars are designed to prevent water getting inside, activating a sprinkler system will have no effect on a fire inside a car, assuming the car has a hard top, as most have. Sprinklers work by creating a cooling effect. Operating a sprinkler system during a car park fire will improve the temperature conditions around the fire and limit its spread. Sprinklers will, however, not remove smoke and toxic gases generated by the fire.
One or more means of ventilating smoke and heat are required in all car park premises. The level of ventilation deemed to be sufficient by ADB has been proved by research to vastly reduce the likelihood of fire spread between adjacent cars, irrespective of fire brigade intervention. As an additional measure of protection, however, automatic fire detection is usually provided to the area to enable a rapid response to a fire incident.
The ‘impulse fan’ is a recent innovation. These systems, installed at strategic points, will drive smoke and toxic gases towards the extraction points. Tests carried out by various manufacturers have demonstrated the effectiveness of this system. LWF’s previous experience in car park design has shown that the approving authority may allow the absence of a sprinkler system in a basement car park, as long as it is equipped with impulse fans. Impulse fans are not stated in any prescriptive codes, and up until now have been used as an engineered solution.
This article began with the risk of car park fires. The possibilities and reasons for fire spread between cars and the remainder of the building was also discussed. It also described the different types of car parks available and what would happen to them in a fire. Building regulations requirements are then outlined, as well as the fire precaution systems available to architects to ensure the safety of the building and its occupants.
The best solution currently available to designers and architects is not set down; it all depends on the specific characteristics of the car park. As all buildings are different, it is essential that fire protection solutions are decided on an individual basis.
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