Contrary to popular belief, the likelihood of a building suffering a serious fire is relatively remote. For example, the probability of a fire occurring within any particular retail building is 0.012 each year, where a value of 0 represents an impossibility and 1 represents an absolute certainty.
Expressed as the return period - the time period between two consecutive fires - this equates to more than 83 years. A recent study has shown that the fire service is called to less than 20 per cent of all fires that occur, the majority being dealt with on the spot with a portable fire extinguisher or burning out due to lack of combustible material and air.
Despite this, it is vital to make adequate provisions in case of fire.
The principle adopted in the UK is that the occupants should be able to escape to a place of safety outside the confines of the building, unaided and without relying on rescue by the fire service, preferably by leaving in the opposite direction to the fire. Obviously, this cannot always be achieved, especially in buildings such as prisons, hospitals and nursing homes where occupants may rely on assistance from staff. These are special cases and require careful consideration at the planning stage.
The escape, or egress, considerations within any building include:
the distance to be travelled from any point to a place of safety;
the provision of clear and unambiguous circulation routes;
a system of signs to promote efficient and effective wayfinding; and good visibility to ensure that the egress route chosen is the most appropriate, is available for use and is clear from the effects of the fire.
The maximum recommended travel distances for different purpose groups are detailed in Approved Document B and the British Standards that deal with fire safety. The requirement for emergency signs is fairly straightforward. However, little real attention is given to the problems of wayfinding in spatially complex buildings. The situation is made worse when considering buildings such as shopping malls, hospitals and sports or conference venues, where occupants may be unfamiliar with the internal layout. Even if people recognise the fire alarm siren, will they know what is expected of them and will they be able to find their way from the building without assistance?
The study of human behaviour in fires is a relatively new area of research but has already produced data that can be used to make any given building design function more effectively during an emergency.
By convention, the width of exit routes is determined by dividing the number of people to be evacuated by the number of routes available. This treats individuals as unthinking entities and is based on the study of fluid flow through a system, referred to as a hydraulic model. Experience of human behaviour shows that this approach is an over-simplification.
A better approach acknowledges that people exhibit characteristic behavioural traits. For example, research gathered in the aftermath of large fires indicates that people try to leave the building by the same way they came in, often ignoring exits that would give them a quicker route to safety.
Realistically, therefore, the main access/egress routes need to be 60-70 per cent larger than that calculated by the hydraulic model.
It is also known that people will not undertake unfamiliar actions during an emergency. Deviation only occurs when the conditions dictate that an alternative form of action is necessary. This could be the presence of smoke and/or flames, the ringing of an alarm bell or the presence of fire crews, although film footage from real fires shows that some people ignore all of these warning cues and try to carry on with their normal series of actions. Therefore, if alternative fire exit routes are to be used effectively, they should be integrated into routine use.
In large and complex buildings, the recommended maximum travel distances may impose severe limitations on the design of the interior. Evidence suggests that, while only 10 per cent of buildings need a fire-engineered 'solution', design of any safety strategy would be better based on an objective assessment of actual risk, rather than a set of prescriptive 'rules' from British Standard committees.
One way this can be done is to analyse the maximum time necessary to enable everyone to escape and to compare this with the time taken for the conditions to deteriorate to such an extent that safety will be compromised. In fire engineering terms, the former is known as the required safe escape time (RSET) and the latter, the available safe escape time (ASET). Provided the RSET is less than the ASET, all will be well (see box).
The physical arrangement of the building can influence the time taken to escape, and this is one of the areas in which fire engineering can be of greatest benefit to the architect. By taking each of the components in turn, specific escape arrangements within a building can be designed.
Better recognition time: The time taken for the occupants to realise that something out of the ordinary has occurred, and that an unusual course of action may be called for, can be reduced by the correct selection of fire detection devices. The most common - ionisation and optical smoke detectors - respond to different types of smoke, and the choice of the wrong detector can lead to many false alarms and a resulting loss of confidence in the system.
If a fast response is required, detectors that respond to the radiation from a flame may be preferable, but, these too can, in some circumstances, lead to false alarms. A relatively new development is a detector that responds to the carbon monoxide produced whenever organic, or carbon-containing, materials burn, giving an early warning while the fire is still in its incipient stages. CCTV cameras can, in exceptional circumstances, be adapted to act as discriminating smoke detection devices using a software package based on video motion detection.
Where there is no automatic fire detection system, the occupants themselves act as fire detectors - and generally respond much quicker.
Better response time: The time taken for a body of people to respond to an alarm can be reduced if they are given clear information. A simple bell or siren can often be ignored for as long as 10 minutes while people seek to carry on with their normal actions. Voice alarm systems, on the other hand, giving a verbal instruction to leave the building immediately, can reduce reaction time to less than one minute and are effective in reducing overall evacuation times, especially in public buildings.
Better travel time: When assessing physical movement to a place of safety the designer needs to consider the type of occupants and their needs in the event of an emergency. Factors include:
travel speeds for able-bodied and non-ambulant persons, both on level surfaces and up and down stairs;
wayfinding throughout the escape route, where the location of the nearest exit may not be apparent or where the layout of the building may be confusing;
passage through doorways and other constrictions, where this may lead to queuing;
movement within stair enclosures, where people tend to want to hold a handrail or move against a wall;
the interference of movement by converging flows of people at different landings; and the movement and dispersion of crowds through a final exit and away from the building.
At each stage of the escape journey, it is likely that some people will be exposed to smoke and gases. The calculation of the total egress time can then be assessed in terms of either the tenability throughout the escape route or by an evaluation of the degree of exposure to smoke. Results, in many circumstances, differ markedly from the standard solutions recommended in the prescriptive guidance documents, in which building owners or operators still place such confidence. The 'pay-off ' for using an engineered approach can be extended travel distances, smaller or better-designed escape routes, more complex internal arrangements than would otherwise be acceptable, larger compartments and the omission of other fire safety systems. In some cases, there can be a reduction in the structural fire resistance of many of the building elements.
Steven Cooper is the principal engineer with fire consultant Lawrence Webster Forrest, 9 Woodside Green, London SE25 5EY. tel 020 8655 1605, fax 020 8655 0410, e-mail fire@lwf. co. uk, website www. lwf. co. uk