Fire is the result of a series of very rapid chemical reactions between a fuel and oxygen that release heat and light. For combustion to occur, oxygen, heat and a fuel source must all be present; this is the ‘fire triangle’. Flames are the visible manifestation of combustion.
 Flash point
This is the temperature to which a fuel has to be heated for the gases given off to flash when an ignition source is applied.
 Fire point
This is the temperature to which a fuel has to be heated for the vapours given off by the fuel to sustain ignition.
 Spontaneous ignition temperature
This is when these vapours ignite spontaneously without the application of an external flame. It is not the fuel itself that burns but the vapours given off as fuel is heated. Once ignition has begun and the vapours ignited, flames will in turn heat the fuel and increase the rate of production of flammable vapours.
 Fire growth
Fire in an enclosed space such as inside a building, behaves differently from fire burning in the open. A ceiling has the immediate effect of increasing the radiant heat returned to the surface of the fuel. Walls also increase this effect.
Fire passes through a series of stages after ignition:
Growth lasts from the moment of ignition to the time when all combustible materials are alight. A critical stage is reached when the flames reach the ceiling. The radiant heat transferred back to the surface of the fuel is dramatically increased. This usually occurs when the temperature at the ceiling has reached about 550°C.
The remaining combustible materials will then rapidly reach their fire points and ignite within 3-4 seconds.
This sudden transition is called a flashover and represents the start of the stable phase. If there is inadequate ventilation during the growth period, a fire may fail to flashover. It may die out or continue to smoulder. This can be extremely hazardous as a new supply of oxygen may be ‘supplied’, for example, by a door or window being opened.
During the stable phase the flaming occurs throughout an enclosure. This is of greatest significance to designers as it is when the highest temperatures are reached. The fire resistance of elements will have to take into account the maximum temperatures likely to be reached as well as the length of time they are likely to be sustained.
The amount of heat produced in a fire is often regarded as a measure of its severity. An understanding of what affects this is important because it allows an estimate to be made of the potential of a fire to destroy property. The quantity of potential fuel within a building is described as that building's fuel load. This includes the fabric of the building and its contents.
Estimating fuel load can indicate likely heat production and fire severity, but fuel load is difficult to establish due to:
- The multiplicity of different materials in buildings.
- The arrangement of potential fuels.
- The density of the arrangement of potential fuels.
The rate of burning in a compartment is dependent upon the fuel and ventilation available. The same factors determine the heat which will be produced. Both the oxygen supply to feed the fire and the possible removal of heat by air are significant. Ventilation is affected by the size and shape of a building’s windows and other openings.
 Affects of heat on building materials
- Steel will have lost two-thirds of its strength by the time it has been heated to 600°C.
- Timber burns at a constant rate - members can be oversized to provide fire resistance.
- Concrete is more resistant.
- Reinforced concrete must have sufficient insulation to protect reinforcement.
- Bricks provide one of the best fire resistant materials.
Smoke is the general term for the solid and gaseous products of combustion in the rising plume of heated air. It contains both burnt and unburned parts of the fuel, as well as any gases given off by the chemical degradation of the fuel.
The heating of the fuel and the emission of volatile content will cause a plume of heated gases to rise and this will cause air to mix (or 'entrainment') at its base as it rises. Some of this air provides the oxygen necessary to support combustion. The rest will mix with the rising plume and become an inseparable element of the smoke.
Smoke is a complex phenomenon but is best treated as a single problem rather than attempting to design against the constituents of the mixture.
All smoke should be considered dangerous and attempts should be made to limit its production and control its movement. A small percentage of victims die from the indirect effect of heat causing a structure to collapse. The majority of deaths are due to smoke either by the inhalation of toxic gases, or carbon monoxide poisoning.
By far the largest of the constituents of smoke is the air that is entrained in it. Therefore to estimate the rate of smoke production the rate of air entrainment must be assessed. This is complex and is dependent on both the size and intensity of the fire, but very broadly, the larger the fire, the greater the rate of smoke production.
The appearance of smoke reflects its constituents - it varies from a very light colour to deep, sooty black. The density depends on the amount of unburned particles carried in the air, and the denser it is, the more dangerous, because visibility is reduced. Visibility in smoke depends both on smoke density and the psychological condition of the observer.
Fire engineering makes use of scientific and engineering principles to safeguard individuals, property and the environment from the destructive damage that can be caused by fire. This is achieved through the application of established rules and expert judgement together with an in-depth knowledge of the phenomena and effects of fire and the reaction and behaviour of people to fire. Fire protection engineers will identify risks, and design safeguards that help prevent and control the effects of fire.
For more information see Fire protection engineering.
The spread of fire can be restricted by sub-dividing buildings into a number of discrete compartments. These fire compartments are separated from one another by compartment walls and compartment floors made of a fire-resisting construction which hinders the spread of fire.
For more information see Fire compartments.
An escape route as ‘that part of the means of escape from any point in a building to a final exit’ where a final exit is ‘The termination of an escape route from a building giving direct access to a street, passageway, walkway or open space and sited to ensure the rapid dispersal of persons from the vicinity of a building so that they are no longer in danger from fire and/or smoke.’
For more information see Means of escape.
Most fire detection and alarm systems operate on the same basic principles. If a fire is detected, then an alarm is triggered. This warns building managers and occupants that there may be a fire and that evacuation may be necessary. Some systems include remote signalling equipment which can alert the fire brigade or a remote monitoring centre.
For more information see Fire detection and alarm systems.
- Promote fire safety and enforce fire safety regulation.
- Undertake fire fighting.
- Ensure national resilience.
- Offer other special services, such as rescuing people from road traffic accidents.
For more information see Fire and rescue service.
 Related articles on Designing Buildings Wiki
- Approved document B.
- BS 9999.
- Fire and rescue service.
- Fire authority.
- Fire damper.
- Fire detection and alarm system.
- Fire detector.
- Fire inspector.
- Fire prevention on construction sites.
- Fire protection engineering.
- Fire safety design.
- Fire safety officer.
- Ionisation smoke alarm.
- Joint fire code.
- Optical smoke alarm.
- Means of escape.
- Smoke detector.
- The causes of false fire alarms in buildings.
- The Regulatory Reform (Fire Safety) Order 2005.
- The role of codes, standards and approvals in delivering fire safety.
 External references
Featured articles and news
We review a book aiming to unpick the complexities of building physics.
An introduction to the categories, procedures and types of listed buildings.
This Australian robotics firm have developed a bricklaying machine capable of building a house in 3 days.
20bn devices will be online by 2020, generating huge volumes of information. Is society making the most of this rich data?
Built over a period of 632 years, Cologne Cathedral is considered one of the world's finest examples of Gothic architecture.
UandI adds £1.5bn to development pipeline.
Here are 5 things leaders can do to create a truly circular economy.
Find out about the different types of delays on construction projects.
Researchers at Wien university have developed new system to create an inflatable concrete structure.
Take a look at this newly-opened tower in Chicago with a remarkable 20:1 height-to-base ratio.
The principles, practice and formwork of one of the most important components of modern architecture.