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It’s ironic that, although fire is used in the manufacture of most materials and can provide the thermal conditions required in a building, it can also be highly destructive to a building and its occupants. Death or injury by fire is particularly horrifying and incidents naturally receive special concern in the minds of most building occupiers. Account must also be taken of the financial loss of a building and its contents when considering the full effects of devastation by fire. Losses, in the US, can be in excess of $120 billion/year when taking account of the current inflationary trends in the cost of material and labor. The design and construction method employed for a building must therefore safeguard occupiers from death or injury and also minimize the amount of destruction. These goals can be achieved through an understanding of the nature of fire and its effects on materials and construction used in a building; methods of containing a fire and limiting its spread; methods of ensuring the occupants of a building being attacked by fire can escape to safety; and methods of controlling a fire once it has started. Combustibility Fire is a chemical action resulting in heat, light and flame (a glowing mass of gas), accompanied by the emission of sound. To start, a fire needs a combustible substance, oxygen, and a source of heat such as from a flame, friction, sparks, glowing embers or concentrated solar rays. Once a fire has started, heat is usually produced faster than it’s dissipated to its surroundings, therefore the temperature rises with time. The fire will burn out once the fuel is removed, become smothered if oxygen is not available, or die if the heat is removed by water, For example. Before this happens, however, a fire is likely to ignite nearby material, causing the fire to spread through a building, and perhaps to adjoining buildings by processes which include radiation. In the context of fire protection, materials used in a building fall into two broad categories: combustible and non-combustible. To determine the extent of combustibility or non-combustibility of a material or application: Fire tests on building materials and structures, should be consulted. It’s produced in several parts to cover the extensive and varied testing procedures for fire propagation, ignitability of materials, resistance to fire, etc. It will generally be found that inorganic materials are non combustible, e.g. stone, brick, concrete or steel, whereas organic materials are combustible, e.g. timber and its by-products (fiberboard, plywood, particle board, etc.) as well as petrochemical products, including plastics. Fire resistance In situations where it’s imperative that a building must not contribute fuel to a fire, only non-combustible materials should be employed. However, even if practical, the sole use of non-combustible materials won’t necessarily avoid the spread of a fire generated by the burning con tents of a building. Avoidance of this requires the parts of the building -- materials and construction used for elements (walls, floors, etc.)- - to have fire resistance. This is the term used to describe the ability of an element of building construction to fulfill its assigned function in the event of a fire without permitting the transfer of the fire from one area to another. Federal Code and 22: Methods for determination of the fire resistance of load-bearing and non-load-bearing elements of construction, respectively, establish a time period during which elements can be expected to perform this function. A sample must be subjected to a simulated building fire which, research has indicated, rises in temperature according to duration; see the time-temperature curve. The tested element is then rated according to the time (0.5, 1, 2, 3, 4, 5 or 6 hours) it’s able to fulfill three performance criteria. These criteria approximate to those necessary to ensure the reasonable safety of occupants and contents in a building, including time to discover the fire and make a safe escape. The performance criteria (-) applied in the tests are as follows: Resistance to collapse (previously referred to as stability) A load-bearing element of construction must support its full load during a fire for a specified period. This minimum period varies from 15 minutes up to 2 hours, depending on which part of a building it relates to and the purpose grouping or type of building it applies to. Integrity: Structural resistance to the passage of flames and hot gases. Failure occurs when cracks or other openings form, through which flame or hot gases can pass; this would cause combustion on the side of the element remote from the fire. Insulation: The ability of the construction to resist fire transmitted heat. Failure occurs when the temperature on the side of the element remote from the fire is increased generally by more than 140°C, or at any point by more than 180°C above the initial temperature. The period of fire resistance suitable for particular elements of a building depends on the functions to be accommodated and the volume, height and floor areas of the spaces involved. In addition to the Building Regulations, more stringent requirements are likely to be determined by the combustibility cannot be equated with fire resistance: one is a characteristic of a material, the other relates to the performance of an element as a whole. For example, an external wall of fiber cement profiled sheeting (non combustible) on a mild steel frame (non-combustible) has no notional period of fire resistance because a construction of this nature would rapidly permit fire to spread by heat transfer, i.e. it would not meet the insulation requirement of fire resistance. Conversely, an external wall of timber cladding (combustible) on timber studs (combustible) with a plasterboard internal lining (combustible because of paper sheathing) will provide full fire resistance for a period of half an hour. However, there are cases where legislation requires both fire resistance and non-combustibility, e.g. external walls of a building located in close proximity to a boundary. ---- Minimum fire resistance for structural elements of construction in buildings of various purpose groups Spread of fire The spread of fire within a building and from one building to another can most effectively be restricted by the identification and isolation of potential hazards. Therefore, the first defense involves siting. Having established that potential fire risks exist by the nature of the functions accommodated, it’s necessary to select the appropriate siting for a building relative to safety of nearby properties. An extreme example might be that of a building accommodating particularly dangerous fire hazards (e.g. manufacture and/or storage of highly flammable chemicals). This should be located in a remote part of the countryside, away from properties likely to be damaged by heat radiation caused as a result of the fire. Building legislation exists (Building Regulations, Fire safety) to restrict fire spread by stipulating periods of fire resistance and construction methods appropriate to the function of a building, as discussed earlier. It also limits the use of certain construction methods according to their distance from other properties and/or boundaries. For example, only a nominal amount (0.1 m^2) of unprotected combustible material (such as timber cladding) is permitted on an external wall of a small residential building if this is within 1m of its boundary. The unprotected area can increase relatively between distances of 1m and 6m; over 6m the amount is unrestricted. Windows, doors and other openings in external walls are also carefully controlled since, unless special forms are used, they do little in stopping the spread of fire from inside or into a building. Regulations restrict the positioning, size and amount of these openings according to function (fire risk of a building as well as location of the external wall relative to other properties and/or boundaries). Whereas the general approach on walls and floors is to 'contain' the fire, there is no legislative requirement for the external surfaces of a roof to have fire resistance. Instead reference is made to test procedures, which grades the suitability of specific roof coverings according to their ability to resist external penetration by fire and their resistance to surface spread of flame. The spread of fire within a building can similarly be restricted by the special segregation of particular fire hazards. Also, fire-resisting compartments or 'cells' can be used to limit the spread of a fire through a building. The precise location, enclosed volume and period of fire resistance required again depends on the nature of the activities accommodated. The height of a building and the ease with which its occupants can escape, and/or fire-fighting can be successfully carried out, are also deciding factors. Structural organization is also important in this respect. Whereas continuous masonry wall construction with reinforced concrete floors uses materials capable of providing a high standard of fire resistance, the 'infill' type of constructions needed for framed buildings must be carefully selected to provide adequate fire resistance as well as fulfilling other performance requirements, including compatibility and continuity with the support system. --- Using compartments to limit the spread of fire in a building --- Methods of providing fire resistance to steel columns. --- Effect of fire on structural timber sections. Once the structural size has been established, extra thickness can provide insulation. The thickness required for a specific period of fire protection for a particular type of timber can be derived from its charring rate. One aspect of the fire-resisting requirements is that the structural organization of a building should not collapse or deform before the occupants can escape safely. A collapse will be of major importance when considering the spread of fire and also fire-fighting, and framed structural organization may need special consideration relative to the materials employed. As long as sufficient insulating cover of concrete is provided to the steel bars of reinforced concrete columns and beams, adequate defense against the untimely collapse or deformation is reason ably easy to achieve. However, steel columns and beams can present bigger problems since, depending on the precise physical composition, their ultimate strength is about 50% when subjected to a temperature of 550 °C. This can be achieved within 15 minutes in a test fire. Unless special design techniques are employed which keep temperature below this level for at least the required fire resisting period of a building, structural steel sections must be insulated with a solid or a hollow casing. This protection must also be compatible with the prevailing environ mental and functional conditions. External casings should be weather and impact resistant, durable and of acceptable appearance to maintain their effective life in a building. Similarly, internal fire-insulating casings may be required to withstand knocks from trolleys, to provide fixings for fitments and space for service pipes or cables, as well as to be aesthetically suitable. Interestingly, timber columns and beams can provide greater fire resistance than unprotected steel. --- indicates the use of sacrificial timber, which provides effective insulation to structural sections subjected to a fire. Apart from normal fire-resisting requirements (including doors, etc.), care must be taken in the selection of finishes which won’t contribute too much to the spread or growth of fire. To insist on the use of non-combustible materials in all circumstances would be too onerous and restrictive. For this reason, the surface spread of flame test set out was developed to classify the relative risk of various combustible materials, and these can be used in certain positions, though the area may be limited. Building legislation has introduced a further surface spread of flame classification that covers not only non combustible materials but also materials with a surface finish giving low fire-propagation properties: Federal Code, which therefore contribute little to the growth of a fire. --- Fire-resisting walls and doors assist escape from a burning building. Means of escape The occupants of a building must be provided with clearly defined and safe escape routes in the event of a fire. These routes must be kept clear of obstruction, be easy to man oeuvre and, very importantly, should be free of the effects of flames, heat and smoke. For this reason escape routes need special consideration regarding safety from fire; access points must be shielded by lobbies and fire-resisting (and smoke-resisting) doors; walls and floors should have sufficient fire resistance to allow escape and subsequent access for fire-fighters to tackle the fire. Some times it may be necessary to provide positive air pressure in the escape route to ensure smoke is forced back into the body of the building, making exit easier, or an extractor ventilation unit can be provided to remove smoke. Internal surface finishes must not contribute to the fire or provide hazard to people escaping, and escape areas must be adequately lit, sometimes by emergency lights powered from a separate generator. Tragedies can occur unless means of escape from a building are correctly designed and sited. In the event of a fire it should be possible to evacuate a building in reason able time (2 1/2 minutes is considered normal for everyone to reach a place of safety, except for special premises such as hospitals). Specific fire safety requirements for hospitals are contained in Federal Fire Code: Fire precautions in new hospitals (Fire code). This document satisfies the objectives set out in the Federal Building Regulations. The width, the size of treads and risers, and heights of handrails for staircases used for escape purposes must also conform to similar safety requirements. Escape-route planning should there fore be related to the use of the building, the number of occupants, the risks involved, and to the heights of floors above ground and the shapes and dimensions of floors. The risk of persons being trapped or overcome by the effects of a fire is greatest in multi-storey buildings, and access routes and standing positions for fire-brigade appliances are critical factors in the design of escape routes. For an appreciation of the minimum dimensions and location of escape routes in a variety of building types, refer to U.S. Federal Fire Code: Fire precautions in design, construction and use of buildings. Fire alarms Dwellings should be provided with an automatic fire detection and alarm system to the recommendations: Fire detection and alarm systems for buildings, i.e.: Code of practice for system design, installation, commissioning and maintenance, or: Code of practice for the design, installation and maintenance of fire detection and fire alarm systems in dwellings. An alternative is a mains-electricity-operated smoke alarm, conforming: Fire detection and fire alarm devices for dwellings. Specifically:
Note that these mains-operated detectors should also have a battery back-up secondary power supply. A large house, i.e. premises having a floor area greater than 200 m^2 in any one storey, and having more than three storeys (a storey includes ground floor and basement floors), should be fitted with a Federal Code system. A large house of no more than three storeys may be fitted with a system. Smoke alarms should be ceiling mounted at least 300 mm from light fittings, with provision of at least one per storey. Specific requirements for loft conversions include an alarm linked to operate the signal in other detectors. Other preferred locations include:
For buildings other than dwellings, consideration must be given to the type of occupancy and purpose of the building. Whilst the basic requirements of alarm systems will suit some buildings, others such as large shopping units may be best cleared by trained staff, rather than an alarm system which could cause panic. Consultation on specific requirements for non-domestic buildings should be undertaken with the local building control authority and the area fire authority, to determine optimum warning systems and procedures in event of a fire. See also:
--- Fire door and frame Fire (-resistant) doors Fire doors have a performance expectation in excess of normal means of access requirements. They are required to have sufficient integrity in a fire, for a predetermined period which complements the element of construction in which they are located. Federal Code: Methods for determination of the fire resistance of non load-bearing elements of construction categorizes door integrity for a period of minutes. For example, a Fire Door classification indicates a Fire Door which can resist fire penetration for 30 minutes. If an ‘S’ is added to the classification this indicates that the door has the added facility to resist smoke leakage. Reference to the Building Regulations provides a listing of fire door locations. These are primarily in compartment walls, escape routes and enclosures. All fire doors should be fitted with an automatic closing device. Some exception is permitted if self-closing would be considered a hindrance. In these circumstances the door can have a fusible link to hold the door open (unless the doorway is an escape route), an automatic release mechanism actuated by a fire detection system, or a door closure delay device. Hardware and ironmongery should satisfy the requirements of:
A color-coded plastic plug is fitted into the door hanging style to indicate the fire resistance, e.g. a blue core with a white background indicates with no intumescent strip in the frame, or approved door with an intumescent strip in the frame. An approved door with two intumescent strips in the frame has a color-coded plug with a red core and blue background. Refer to Fed. Code of practice for fire door assemblies with non-metallic leaves. An intumescent strip can be fitted into a recess in the frame. At temperatures of about 150 °C the seal expands to prevent the passage of fire and smoke, but it does not prevent the door being moved physically. An alternative for smoke sealing only is a brush strip seal which also functions as a draught seal. Fire-fighting The techniques so far described for fire protection can be classified as passive measures as they are an inbuilt feature of a building. Active measures of fire protection incorporate fire-fighting techniques which can be inbuilt (sprinklers, dry/wet risers, alarm systems, fusible links to doors, shutters, fire-fighters' lifts, etc.) or be brought to a building in distress (fire-fighters and appliances). The siting of a building must allow easy access for fire-fighters and their appliances in an emergency. Hard-standing areas must be included in the landscaping and located at prescribed distances from a building to facilitate fire-fighting activities. Fire insurance The influences associated with the insurance of a building against damage or loss through the occurrence of a fire are important aspects of initial design considerations. The U.S. Standards Code of practice for fire safety in the design, construction and use of buildings, provides overall guidance. However, fire insurance companies may require higher or different standards of fire protection, even to that presented in the clauses of fire legislation documents or the recommendations of the fire authority. For this purpose, the fire insurance industry often prefers to apply the Loss Prevention Standards). These Loss Prevention Standards complement the Approved Documents and US/Canada/UK European Standards supporting the Building Regulations and generally require higher performance standards. Failure to consider these publications and any other specific insurance requirement at an early stage of design, or during the selection processes for appropriate construction methods, may result in high premiums for fire insurance or the adoption of costly modifications. Previous: Thermal
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