Last edited 03 Oct 2017

Fire in buildings

Fire.jpg

Contents

[edit] Introduction

Buildings need to be designed to offer an acceptable level of fire safety and minimise the risks from heat and smoke. The primary objective is to reduce to within acceptable limits the potential for death or injury to the occupants of a building and others who may become involved, such as the fire and rescue service, as well as to protect contents and ensure that as much as possible of a building can continue to function after a fire and that it can be repaired. The risk to adjoining properties also needs to be considered, as well as possible environmental pollution.

Fire occurs as a result of a series of very rapid chemical reactions between a fuel and oxygen that releases 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.

The 'flash point' is the temperature to which a fuel has to be heated for the gases given off to flash when an ignition source is applied.

The 'fire point' is the temperature to which a fuel has to be heated for the vapours given off by the fuel to sustain ignition.

The 'spontaneous ignition temperature' is the temperature at which these vapours ignite spontaneously without the application of an external flame. Once ignition has begun and the vapours ignited, flames will in turn heat the fuel and increase the rate of production of flammable vapours.

[edit] Fire growth

Fire passes through a series of stages from the moment of ignition to the time at which all combustible materials are alight.

A critical stage occurs 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. 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.

For more information see: Understanding the factors affecting flashover of a fire in modern buildings.

During the stable phase which follows flashover, flaming occurs throughout the enclosed space. This is when the highest temperatures are reached. The fire resistance of the elements that form the enclosure will have to take account of the maximum temperatures likely to be reached as well as the length of time they are likely to be sustained.

[edit] Heat

The severity of a fire can be measured by the amount of heat produced.

The quantity of potential fuel within a building is described as its 'fuel load'. This includes the fabric of the building and its contents. Estimating fuel load can give an indication of the likely heat production and so 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 and so heat produced in a compartment is dependent upon the fuel and ventilation available. 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.

[edit] Affects of heat on building materials

[edit] Smoke

Smoke is the general term for the solid and gaseous products of combustion in the rising plume of heated air. Smoke may contain 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. Very broadly, the larger the fire, the greater the rate of smoke production.

All smoke should be considered dangerous and attempts should be made to limit its production and control its movement. The majority of deaths in fire are due to smoke either by the inhalation of toxic gases, or carbon monoxide poisoning.

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 very broadly, the denser it is, the more dangerous, because visibility is reduced. Diluting smoke sufficiently to keep escape routes useable can be very difficult, and it is often more straight-forwards to prevent smoke from entering in the first place.

[edit] Legislation

The Building Regulations Part B: Fire Safety, addresses the precautionary measures necessary to provide safety from fires for building occupants, persons in the vicinity of buildings, and firefighters. Requirements cover means of escape, fire detection and warning systems, the fire resistance of structural elements, fire separation, protection, compartmentation and isolation to prevent fire spread, control of flammable materials, and access and facilities for firefighting.

The 'approved documents' provide guidance for how the building regulations can be satisfied in common building situations. Two approved documents are provided:

For more information see: Approved document B.

The Regulatory Reform (Fire Safety) Order 2005 provides the minimum fire safety standards for non-domestic premises. The Order designates a person, usually the employer or the owner as the “Responsible Person”. They are required to carry out certain fire safety duties, including ensuring that general fire precautions are satisfactory and conducting a fire risk assessment.

For more information see: The Regulatory Reform (Fire Safety) Order 2005.

[edit] Fire engineering

Fire engineering makes use of 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 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.

The main design options to ensure fire safety are:

  • Prevention: Controlling ignition and fuel sources so that fires do not start.
  • Communication: If ignition occurs, ensuring occupants are informed and any active fire systems are triggered.
  • Escape: Ensuring that occupants of buildings and surrounding areas are able to move to places of safety.
  • Containment: Fire should be contained to the smallest possible area, limiting the amount of property likely to be damaged and the threat to life safety.
  • Extinguishment: Ensuring that fire can be extinguished quickly and with minimum consequential damage.

For more information see: Fire protection engineering and Fire safety design.

[edit] Fire compartments

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.

[edit] Means of escape

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.

[edit] Fire detection and alarm systems

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.

[edit] Fire and rescue service

The fire and rescue service:

  • 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.

In certain buildings, it can be difficult for the fire and rescue service to safely reach and work close to fires. Under such circumstances additional facilities are required to ensure that there is no delay and to provide a secure operating base. This might include:

For more information see Fire and rescue service.

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