It should be the objective of every engineer and architect, while planning and designing the buildings, that the structures offer sufficient resistance against fire so as to afford protection to the occupants in the event of fire. This objective is achieved by adequate planning, use of fire-resisting materials and construction techniques and by providing quick and safe means of escape in the building.
The building should be so planned or oriented that the elements of construction or building components can withstand the fire for a given time depending upon the size and use of building and the various compartments should be isolated so as to minimize the spread of fire. Suitable separation is necessary to prevent fire, gases and smoke from spreading rapidly through corridors, staircases, lift shafts, etc. Adequate separation from adjacent buildings should also be planned.
All the structural elements, such as floors, walls, columns and beams, should be made of fire-resisting materials so that life, goods or contents and activities within the building can be protected.
The construction of structural elements, namely walls, floors, columns, lintels, arches, etc., should be made in such a way that they should function at least for the time, which may be sufficient for the occupants to escape safely in times of fire. Escape elements like stairways and staircases, corridors, lobbies and entrances should also be constructed out of fire-resistant materials and be well separated from the rest of the building.
Adequate means of escape are provided for the occupants, to leave the building quickly and safely, in times of outbreak of fire. This objective is attained by providing an exit from within a building by way of definite escape ways, corridors and stairs to a street or an open space or roof of an adjoining building from where access to escape may be found. The desired degree of fire resistance largely depends upon the use of the buildings. In India, the types of building construction and fire zones in a city are classified on the basis of fire-resistance and fire-hazard characteristics, respectively.
24.2.1 Fire-resisting materials
24.2.1.1 Timber
Timber, though itself a combustible material, offers sufficient resistance to fire when used in adequate sizes. Timber also possesses the properties of self-insulation and slow burning. Timber on exposure to fire first gets charred; this charring provides a protective coating to the inner portions of the timber and prevents it from rapid combustion, even if subjected to a temperature up to 500°C. At still higher temperatures, under continued exposure, it is dehydrated giving rise to combustible volatile gases, which readily catch fire. Additional fire resistance is achieved by impregnating timber with large quantities of fire-retarding chemicals, like ammonium phosphate and sulphate, borax and boric acid and zinc chloride, because these chemicals retard the rate of temperature rise during fire. To make a timber structure more fire resistant the following points should be given due consideration:
- Instead of using a number of smaller sections for joists and floor beams, thicker sections at a wider spacing should be used.
- The number of corners and the area of exposed surface should be reduced to a minimum. All sharp edges should be rounded.
- Timber should not be treated with oil paints or varnishes, which are liable to catch fire. Instead of this, timber ceilings and partitions should be treated with asbestos or ferrous oxide paints if needed.
- In a multi-floor timber structure, there should be a minimum number of floor openings and no through opening in multi-floor levels should be provided. A through opening spreads the fire in the vertical direction and behaves like a chimney and induces draught.
- Adequate fire steps or barriers should be provided in the floors and walls.
24.2.1.2 Stone
The use of stone in a fire-resisting construction should be restricted to a minimum as this material cannot resist the effects of sudden cooling. After becoming hot, if it is cooled, it breaks into pieces. Granite, when subjected to excessive heat, crumbles to sand or cracks and turns to pieces with a series of explosions and disintegration. The use of limestone is not at all desirable as it gets crumbled and ruined (turns to quick lime) under the effects of fire. The compact sandstone has better resistance against fire than limestone as it can stand the exposure to moderate fire without serious cracks.
24.2.1.3 Bricks
First-class bricks are practically fire proof as they can withstand the exposure of fire for a considerable length of time. Being poor conductors of heat the bricks can withstand high temperatures up to 1300°C without causing serious effects. Firebricks are best for use in fire-resisting construction. The degree of fire resistance of bricks depends upon factors like size of bricks, composition of brick clay and method of construction. Though brick has its own structural limitations for use in buildings, brick masonry has been proved to be most suitable for safeguarding the structure against fire hazards.
24.2.1.4 Terra-cotta
Like bricks, it is also a clay product which possesses better fire-resisting qualities than bricks. Being costlier, its use is restricted in the construction of fire-resisting floors.
24.2.1.5 Steel
Steel, although an incombustible material, has a very low fire-resistance value. With increase in temperature, it gets softened and, hence, there is reduction in resistance to the effects of tension and compression. At about 600°C, its yield stress is reduced to only one-third of its value at normal temperatures. When the members made of steel come in contact with water used for extinguishing the fire, they tend to contract, twist or distort and thus the stability of the entire structure is endangered. It has been observed in practise that unprotected steel beams sag and steel columns buckle, resulting in collapse of the structure. It is, therefore, necessary in the fire-resisting characteristics of a structure to protect all the structural steel members with some covering of insulating material. This can be achieved by covering the steel members completely with materials like bricks, burnt clay blocks, terra-cotta or concrete.
24.2.1.6 Wrought iron and cast iron
Wrought iron behaves almost in a similar way as steel when subjected to fire except that it has lesser elasticity and lower strength in compression and tension as compared to steel. Cast iron is rarely used from the fire-resisting point of view in construction, as on sudden cooling it gets contracted and breaks into pieces or fragments. For using cast iron in fire-resistive construction, it should be protected by a suitable covering of bricks, concrete, etc.
24.2.1.7 Aluminium
In some advanced countries, aluminium is being used for reinforcement purposes in multi-storeyed structures because of its light weight and anti-corrosion properties. However, it has a very poor performance as a fire-resisting material and its use (as alloy) should be restricted to those structures which have low fire risks. It is a good conductor of heat and possesses enough tensile strength.
24.2.1.8 Concrete
In general, it is a bad conductor of heat and possesses good fire-resisting characteristics. The actual degree of fire resistance of this material depends upon the nature of aggregates used and its density. In the case of RCC and prestressed construction, it also depends upon the position of steel in concrete. It is found that ordinary concrete, when exposed to fire, gets dehydrated and results in shrinkage cracks. (This happens because on heating aggregates in concrete expand whereas cement shrinks and these two opposite actions lead to the development of cracks.) Coarse aggregates like foamed slag, blast furnace slag, crushed brick, crushed lime-stone and cinder are best suited for concrete from the viewpoint of fire resistance. Aggregates like flint, gravel and granite possess poor fire-resisting characteristics. It has been observed that in the event of average fire the concrete surface gets disintegrated for a depth of about 25 cm because of the fact that the mortar in concrete gets dehydrated by the fire. Hence, in the case of reinforced concrete fire-resistive construction, a cover of sufficient thickness should be provided (cracks generally originate from the reinforcement). RCC structures are considered superior to steel-framed structures since less steel is used and that too are well protected by the mass concrete.
24.2.1.9 Glass
Because of its low thermal conductivity, this material undergoes very small volume changes during expansion or contraction and, hence, is considered as a good fire-resisting material. However, sudden and extreme changes in temperature result in fracture or cracks. When glass is reinforced with steel wire netting, e.g., in wire glass, its fire resistance is considerably increased and its tendency to fracture with sudden changes in temperature gets minimized. Reinforced glass has a higher melting point and, hence, is commonly used for making fire-resisting doors, skylight, windows, etc. in construction work.
24.2.1.10 Asbestos cement
This material, which is formed by combining fibrous mineral with Portland cement, has a great fire-resistive value. Asbestos cement products are largely used for the construction of fire-resistive partitions, roofs, etc. Being poor conductors of heat and incombustible material, the structural members blended with asbestos cement offer great resistance to cracking, swelling or disintegration when subjected to fire.
24.2.2 Causes of fires and their prevention
Fire may not occur under any one of the following conditions:
- Absence of a component necessary for combustion.
- Improper ratio of combustible material to oxygen for the formation of a combustion system.
- Heat source available is insufficient to ignite a combustion system.
- Heat source is not available for a sufficient time to ignite a combustion system
It is possible to establish the different causes of fire which occur as a result of various optimum combinations of combustion systems and heat sources necessary for a fire to start. Fire prevention and limitation of fire spread are achieved by taking action with respect to the combustion systems and heat sources so as to avoid conditions under which fires can originate. Proper design and planning of buildings are important in fire safety.
24.2.3 Fire protection of buildings
All the structural components of a building should be constructed in such a way and of such materials that they withstand, as an integral member of the structure, for the period desired according to the type of construction, in the event of fire (i.e., 1–4 hours for type 4 buildings to type 1 buildings).
The load-bearing walls or columns of masonry should be thicker in section to resist fire for a longer time. Bricks are more preferred to stones and all walls should be plastered with fire-resistive mortar. All steel members should be embedded in dense concrete or some other fire-resistant material. There should be sufficient cover for all the embedded steel material (a minimum of 5 cm). RCC floors and jack arch floor with steel joists embedded in concrete are more preferred. The wall openings provided which act as escape passages should be properly protected, otherwise they help in the on spread of fire. So, all doors and windows are to be provided with fire-proof shutters and frames. Thick wooden members and steel shutters offer resistance to a certain extent. In addition, to the internal stair, fire escapes should be provided which in the form of external stairs. These should be straight flight type with a minimum width of 75 cm and 20 cm rise and 15 cm tread. Non-combustible hand rails should be provided for a minimum height of 100 cm. Ramps with gradient not more than 1 in 10 can also be provided. Alarm systems and fire extinguishing systems are also to be provided. Generally, one fire hydrant per 4,000–10,000 m2 area is provided depending on the density of population and the importance of the region.