Fire is a chemical reaction which takes place when a combustible material is heated in the presence of oxygen. The liquid or solid fuel gives of fvapour when treated end burns as flame.
The surface area of a material relative to its volume and density is a major criterium of its ability to burn. Thick, solid material is relatively difficult to ignite and burns only at or near the surface. Thin sheets burn rapidly, while finely divided or pulverized material can become explosive when suspended in air.
Fires can break out in buildings by accident (eg when cooking on open fires, as is common in many developing countries), by self-ignition (eg by the discharge of sparks due to friction between materials in very dry conditions, or by concentration of the sun's rays by the lens effect of some glasses), or by natural hazards (eg lightning, or earthquakes).
The damaging effects of fires in buildings depends on the materials used and the design and construction of the building. Some materials merely shrink and crack, while others may expand, melt or disintegrate causing total destruction. Lives are endangered by burns, collapsing walls and roofs, inhalation of toxic gases and smoke, panic and loss of sensibility and vision.
In hot arid zones, houses are normally built with thick, heavy materials, which do not readily ignite. In warm humid zones, combustible materials are commonly used, but humidity and rainfall can have the same effect. Nevertheless, there is always a fire risk in all climatic zones, and must be taken into consideration in all building designs.
• With regard to planning in warm humid zones, where buildings are generally placed well apart for good cross-ventilation, care must also be taken to maintain a good distance between buildings in the direction of the prevailing winds, to avoid spreading of fire from one house to another.
• Climatically appropriate design in hot dry zones calls for close spacing of buildings, but sufficiently wide escape lanes and access roads for fire-fighting vehicles are essential.
• Combustible building elements should not be used closer than 1 metre to potential sources of fire (stoves, chimneys, etc.); similarly combustible materials stored in and around the house must be shielded from such sources by means of non-combustible materials (eg gypsum, glass, bricks, concrete, metals, stones, mineral wool).
• The design of cavities should take into consideration that they can act as flues, spreading fires rapidly.
• Chemical treatment of timbers and other vegetable products is possible (mainly impregnation with borax compounds), but expensive, and complete resistance is never achieved.
• A fire retardant thatch roof construction has been developed by CBRI, Roorkee in India: a non-erodable bitumen stabilized mud plaster is applied on the upper surface and the drying shrinkage cracks sealed with a slurry of soil and cow dung mixed with a small proportion of bitumen cutback. In this way the dense covering layer stops the passage of air and retards ignition for at least one hour. As an additional advantage, the roof is waterproof.
• As a general precautionary measure, it is advisable to have a water reservoir, hose pipe and pump, and/or hand fire-extinguishers close by.
Combustible and non-combustible materials (from Bibl. 00.14)
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