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Building FiresConfined fires in building pass through different phases, or stages, of combustion. The rate at which fire will advance from one stage to the next may be quite rapid, as chemical reactions double in speed with temperature rise of burning material. First is the starting phase. During this stage the oxygen content of the atmosphere is normal, about 21%. The fire produces water vapour, carbon dioxide, and a minor amount of carbon monoxide, the amounts of other gases and vapours depending upon the chemical composition of the burning materials. Second is the free – burning stage. The oxygen of the atmosphere is gradually depleted, but a sufficient amount remains to support rapid burning and flame production. Convection currents transfer heat to the upper levels of the buildings heating the combustibles above their ignition temperatures. The heated atmosphere, in addition to the generation of flammable gases and vapours, causes a rapid expansion of pressure in buildings. This is the stage of rapid burning. Entrance to the structure may be difficult because of the intense heat and rapidly increasing smoke and fire gases. Third is the smouldering phase. The fire having completed the oxygen contents of the atmosphere below 16 %, flames are barely visible. The reduction of the oxygen supply has increased the amount of carbon monoxide. The structure may be completely charged with dense smoke and gases if temperature is over 1.200ºF. Carbon monoxide may be heated above its ignition temperature – 1.1280ºF. Firefighting and ventilation operations during this stage of the fire must be carefully planned and executed. The way to fight confined structural fires is to get inside the building, find the source, and get water directly on it. However, when the fire fully developed before the arrival of the firefighters, hose streams may be required to reduce the fire. Ventilation is an important firefighting tactics of extinguishing building fires. As a rule, firefighters are chopping holes in roofs and breaking windows in portions of the building which were not involved in the fire. This tactics is used to ventilate the building and clear it of heavy smoke and heated gases. Ventilation should be done only by experienced fire officers since improper ventilation can spread the fire and increase the damage and can even introduce the risk of explosion. The fire departments may also use electrically driven fans to clear buildings of smoke. The collapse of the structural building elements can be a serious hazard. Although statistically it has not resulted in many deaths or injuries to building occupants, structural collapse is a particular hazard to firefighters. A number of deaths and serious injuries to firefighters occur each year owing to structural failure. The major concerns to occupants are due to smoke and toxic gas and to fast flame spread. In addition, vertical propagation of fire is a serious concern. The ability of the Fire Service to extinguish a fire is significantly diminished in case of vertical fire spread to two or more floors. The fire properties of building materials fall into two categories: combustibility and a fire resistance. Combustibility is further divided into ease of ignition, fire spread, and smoke movement.
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