It is reported that in USA fire kills more people each year than all other natural disasters combined including floors, cyclones and earthquake. The fire load in a building should be kept to the minimum possible. The term fire load indicates the amount of heat liberated in kilo joules per square metre (kJ/m2) of 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 with floor area.
A building may be made more fire resistant by
1. Using suitable materials
2. Taking precautions in building construction
3. By providing fire alarm systems and fire extinguishers.
20.10.1 Using Suitable Materials
The fire resisting material is having the following characters:
(a) It should not disintegrate under the effect of heat
(b) It should not expand under heat so as to introduce unnecessary stresses in the building
(c) The material should not catch fire easily
(d) It should not lose its strength when subjected to fire.
Fire resisting characters of some of the commonly used building materials are given below:
Stone: It is a bad conductor of heat. Sand stones with fire grains can resist fire moderately.
Granite disintegrate under fire. Lime stone crumbles easily. Most of the stones disintegrate during cooling period after heated by fire.
Brick: Bricks can resist heat up to 1200°C. At the time of construction, if good quality mortar is used, fire resistance is extremely good.
Timber: Any structure made of timbers is rapidly destroyed in fire. Timber enhances the intensity of fire. Use of heavy sections of timber in buildings is not desirable. To make timber more fire resistant the surface of 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.
Concrete: Concrete has got very good fire resistance. The actual behaviour of concrete in case of fire depends upon the quality of cement and aggregates used. In case of reinforced concrete and prestressed concrete, it also depends upon the position of steel. Larger the concrete cover, better is 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. Hence cement concrete is ideally used fire resistant material.
Steel: It is a good conductor of heat. Steel bars lose tensile strength. Steel yields at 600°C. They melt at 1400°C. Steel columns become unsafe during fire. Steel reinforcement weaken the reinforced concrete structures. Hence steel columns are usually protected with brick works or by encasing in concrete. Reinforcement in concrete are protected by concrete cover. Steel grills and beams are applied
with fire resistant paints.
Glass: It is a poor conductor of heat. It expands little during heating. After heating when it cools,
cracks are formed in glass. Reinforced glass with steel wire is more resistant to fire and during cooling
process, even if it breaks, fractured glasses are in their original position.
Aluminium: It is good conductor of heat. It has got higher resistance to fire.
Asbestos Cement: It is non-combustible material. It posseses high fire resistance.
20.10.2 Fire Protection by Taking Precautions in Building Construction
A building may be made more fire resistant by minimizing use of combustible materials, protecting
steel by 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 gives better fire resistance.
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 very good fire resisting floor.
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 fire proof paints.
Stairs: Wood should be avoided in the stair cases. To minimize fire hazard, stairs should be centrally placed in the buildings so that people can approach them quickly. More than one stair case is always preferable. Emergency ladder should be provided in the building.
Structural Design: It should be such that under worst situation, even if part of the structure collapses, it should be localised and alternate routes are available for escape.
Fire Alarm System and Fire Extinguishers
All important buildings should be provided with fire alarm system. Alarm may be manual or automatic.
Automatic alarm sense the smoke and activate bells.
Fire extinguishers should be provided at all strategic points in the buildings. The common fire extinguishers are as follows:
(a) Manual: Carbon dioxide type portable fire extinguishers are commonly used. Sometimes buckets of water, sand and asbestos blankets are kept ready at all possible places where fire is likely to catch.
(b) Internal Hydrant: The hydrant should be located in and around the buildings so that water is available easily for fire fighting.
(c) Automatic Water Sprinkler: In the buildings vulnerable for fire like textile mills, paper mills automatic water sprinklers are installed. As the fire takes place the sprinkling of water is automatically activated from the piping system containing water under pressure.