Building a completely fire-resistant building can be expensive and require a lot of expertise. However, a building can be constructed to resist fire up to a certain degree with an acceptable amount of money.
Fire disaster is one of the most common types of disaster that destroys and kills many people compared to all the disasters combined including floods, cyclones, and earthquakes. The fire load in a building should be kept to the minimum possible. The term fire load indicates the amount of heat liberated in kilojoules per square metre (kJ/m2) of the floor area of any compartment by the combustion of the content of the building including its own combustible part. It is determined by multiplying the weights of all combustible materials by their respective calorific values and dividing that by the floor area.
Using Suitable Materials for Fire-Resistant Building
Using suitable materials for the construction of a fire-resistant building can reduce the overall cost of construction and thus minimize the venerability of fire disasters in the future. In this article, we have chosen some of the materials for the discussion of fire-resistant properties.
Characteristics of Fire-Resistant Material
For the construction of a fire-resistant building, the materials used should be fire-resisting materials. The material should have the following characteristics:
- It should not disintegrate under the effect of heat
- It should not expand under heat so as to introduce unnecessary stresses in the building
- The material should not catch fire easily
- It should not lose its strength when subjected to fire.
Fire Resistant Characters of Some of The Commonly Used Building Materials
Stone
Stone is a bad conductor of heat. Sandstones with fire grains can resist fire moderately. Granite disintegrates under fire. Limestone crumbles easily. Most of the stones disintegrate during the cooling period after being heated by fire.
Brick
Brick is a poor conductor of heat and first-class brick which is moulded from good clay can resist heating up to 1200°C when exposed to fire for a longer time. At the time of construction, if good quality mortar is used with better workmanship, fire resistance is extremely good.
Concrete
Concrete has got very good fire-resistant properties. The actual behaviour of concrete in case of fire depends largely upon the quality of cement and aggregates used. In the case of reinforced concrete and prestressed concrete, it also depends upon the position of the steel. The larger the concrete cover, the better the fire resistance of the member.
There is no loss in strength in concrete when it is heated up to 250°C. The reduction in strength starts if the temperature goes beyond 250°C. Normally reinforced concrete structures can resist fire for about one hour at a temperature of 1000°C. It was found that concrete offers much higher resistance to fire than any other building material. Hence cement concrete is ideally used fire-resistant material.
Timber
Any structure made of timber is rapidly destroyed by fire as timber is a good combustible material. Timber enhances the intensity of the fire. The use of heavy sections of timber in buildings is not desirable. To make timber more fire-resistant building materials the surface of the timber is coated with chemicals such as ammonium phosphate and sulphate, boric acid and borax. Sometimes fire resistant paint is applied to timber used in the building.
The following measures are adopted to make timber fire resistant:-
- Use of thicker sections at wider spacing than thinner sections at closer spacing, especially in the case of the floor joist and ceiling framing.
- Reducing the number of corners and area of the exposed surface to a minimum.
- Coating timber surface with chemicals as above mentioned.
- Painting timber surfaces with oil paints. Applying varnish should be avoided since these paints catch fire.
Steel
Though steel is non-combustible, it has very low fire resistance since it is a good conductor of heat. During a fire, it gets heated very soon and its modulus of elasticity reduces. Thus, steel loses its tensile strength rapidly. It is found that the yield strength of mild steel at 600°C is about half of its original value at normal temperature.
Unprotected steel column buckles and steel beam sag, when exposed to fire, which columns and beams to become unsafe during the fire, thus resulting in the collapse of the structure. It becomes essential to protect structure steel members with some coverings of insulating materials such as brick, terracotta, concrete, etc.
During the fire, steel reinforcement weakens the reinforced concrete structures. Reinforcement in concrete is protected by a concrete cover. Steel grills and beams are applied with fire-resistant paints.
Glass
It is a poor conductor of heat. Its thermal expansion is less and expands little during heating. After heating and suddenly cooling, cracks are formed. These cracks can be minimised if the glass is reinforced with steel wire. Thus, reinforced glass is more fire-resistant and can resist variations in temperature without the formation of serious cracks.
Reinforced glass has a higher melting point. Even if it breaks or cracks, fractured glasses pieces are held in their original position. Reinforced glass is therefore commonly used in fire-resistant doors, windows, skylights, etc.
Asbestos Cement
Asbestos cement is formed by combining asbestos with Portland cement and has a low coefficient of expansion. It is known to have the property of incombustibility, thus exhibiting the greater fire-fire resistant property. This product is largely being used for fire-resistant partition walls, roofs, etc. Moreover, it is also being used as a protective covering for other structure members.
Aluminium
Aluminium being one of the most commonly used construction materials is a very good conductor of heat and electricity. However, it shows poor fire-resistant properties. Its use should be restricted to only those structures which have very limited fire risks.
Plaster or Mortar
Cement mortar is non-combustible as well the plaster. Hence it should be used to protect walls and ceilings against fire risk. Cement plaster is better than lime plaster since lime is likely to be calcined during the fire. The fire-resistant ability of plaster can be improved by using it in thicker layers or reinforcing it with metal laths. Gypsum plaster, when used over structure steel members, makes them better fire-resistant.
Precautions Taken for Fire Resistance Building
A building may be made more fire-resistant by minimizing the use of combustible materials, protecting steel with fire-resistant paints and providing stairs at suitable positions and protecting them from fire. Various members of buildings can be made fire-resistant as follows:
Walls: Brick walls with cement plaster is a better fire-resistant building material.
Roof: R.C.C. flat roofs have good fire resistance. Hence they should be preferred.
Ceiling: Ceilings should be made up of cement plaster, asbestos cement board or fibre boards.
Floors: R.C.C. floor is a very good fire-resisting floor.
Stairs: Wood should be avoided in the staircases. To minimize fire hazards, stairs should be centrally placed in the buildings so that people can approach them quickly. More than one staircase is always preferable. The emergency ladder should be provided in the building.
Structural Design: It should be such that under the worst situation, even if part of the structure collapses, it should be localised and alternate routes are available for escape.
Doors and Window Openings: All these openings should be protected against fire by taking the following precautions:
(a) The thickness of shutters should not be less than 40 mm.
(b) Instead of wooden, aluminium or steel shutters should be preferred.
(c) They should be provided with fireproof paints.
Installation of Fire Alarm Systems and Fire Extinguishers
Installation of a fire alarm system and a fire extinguisher is one of the methods of controlling the spread of fire. Fire-Sand is also placed in the open area and near the building.
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