Uncontrolled fire in a building is a uniquely deadly and destructive occurrence:
• A building supplies a concentration of fuel for an accidental fire. A wooden building is itself a source of fuel, but even a concrete or steel building usually contains furniture, paper, carpets, and combustible building materials such as wood paneling and plastic insulating materials. Oil, gas, gasoline, paints, rubber, chemicals, and other highly flammable materials are often present in buildings.
• A building supplies many potential sources of ignition for accidental fires. Defective furnaces, spark-throwing fireplaces, leaky chimneys, unattended stoves, loose electrical connections, overloaded electrical wiring, and carelessly handled matches and cigarettes are but a few of the means by which a building or its contents may be set on fire.
• A building, stovelike, contains a fire and encourages its growth by concentrating its heat and flammable combustion gases. Where vertical passages through the building are open to the fire, strong convective drafts fan the flames. Hot gases rise and set new fires in the upper reaches of the structure.
• A building holds dense concentrations of people, subjects them to the heat and gases generated by a fire, and restricts their escape (fig 1). If a campfire gets out of control in a wilderness, few people are likely to be present, and there are a multitude of directions in which to escape. But if a fire of similar magnitude starts in a school, a theater, a department store, or an office building, thousands of people are endangered, and only a few escape routes are available.
• A building serves as a barrier to firefighters. Whereas a wilder ness fire can be fought from all sides and even from the air, a fire in a tall building may be 40 stories above the street, accessible only by stairway. Low, very broad buildings can put an interior fire beyond the reach of fire hoses. Building fires expose firefighters to excessive heat, poisonous gases, explosions, dangerous heights, toppling walls, and collapsing floors and roofs.
ill. 1
These diverse interactions of buildings and fire take a terrible toll. Twelve thousand American lives are lost in building fires each year, and 300,000 people are injured, often very seriously and painfully. Property losses due to building fires in the United States are measured in billions of dollars.
Fire begins when a supply of fuel and a supply of oxygen are brought together at a sufficiently high temperature to initiate combustion. As the fire burns, it consumes the fuel and oxygen, and gives off various gases, particulate emissions, and large quantities of heat. Depending on the available fuel, the combustion gases ma include carbon dioxide, carbon monoxide, hydrogen cyanide, hydrogen sulfide, and sulfur dioxide. Any of these is toxic if inhaled in sufficient concentrations.
People may be injured by fire in several ways. They may be burned, particularly in the lungs and respiratory passages, by exposure to hot air or on the skin by severe thermal radiation. They may be suffocated by oxygen-depleted air or poisoned by toxic combustion gases. Panic often contributes to loss of life in building fires people may make irrational decisions with regard to personal safety (such as running back into a burning building to save personal effects), and they may injure one another by pushing, crowding, or trampling as they rush to escape. The most prevalent cause of death in building fires is suffocation or carbon monoxide poisoning that overtakes the victim after he or she has failed to find a means of escape because of a dense accumulation of smoke.
In designing against fire in buildings, our first aim is to reduce the risk of human injury or death to as low a level as possible. Simultaneously, we wish to minimize fire damage to the building and its contents and to prevent the fire from spreading to neighboring buildings. We would like, of course, to eliminate all risk of fire, but contrary to popular myth, there can never be such a thing as a “fireproof” building. Steel obviously does not burn, but it does lose most of its structural strength and sag or collapse at a temperature that is well below its own melting point and the sustained temperatures frequently reached by ordinary building fires (ill. 2). Concrete is more resistant to fire than steel is, but the crystalline structure of its cement binder progressively disintegrates when exposed to fire, and if the fire lasts long enough, serious structural damage will result. Brick and tile, which are products of intense heat in the kiln, are not themselves weakened by fire, but their mortar joints are subject to disintegration, thereby weakening the entire masonry construction. For reasons such as these, our buildings can't be made perfectly resistant to fire. Nevertheless, we have developed a rather effective and rapidly improving arsenal of weapons to protect life and property against building fires.
ill. 2: Average tensile strength of structural steel
at various temperatures.
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