In construction, heavy wood timbers have a relatively high fire resistance, because fire tends to burn very slowly inward from the surface, leaving enough sound timber in the center to prevent collapse. Wood framing can also be impregnated with ammonium phosphate solution or covered with special mastics. Stucco or other incombustible facing also gives a wood frame some protection from fire.
To be classed as fire resistive, buildings must be made of reinforced concrete or protected steel that will stand considerable fire with minor damage; even a building made of unprotected steel may be damaged. While steel retains its strength up to a very high temperature, it fails rapidly at temperatures over 1,000°F (540°C). Structural steel may be protected in a number of ways. It can be faced with brick, concrete, or tile; however, construction with these materials usually adds too much weight to a building. A protective layer of concrete over all surfaces of a beam or over the steel bars in reinforced concrete has to be at least 2.5 in. (6.4 cm) thick to be effective; hollow clay tile used to cover beams and girders has to be at least 4 in. (10 cm) thick. Thus most buildings use lightweight fireproofing such as gypsum, perlite, and vermiculite mixed in plaster, concrete, and mineral fiber; one inch (2.5 cm) of such materials will absorb an equivalent amount of heat as 2.5 in. (6.4 cm) of concrete.
Some recent buildings circulate water inside each column, protecting the structure against meltdown. Asbestos is no longer used, because inhalation of the fibers causes abestosis, a fatal lung disease; fireproof board made from a mixture of asbestos and cement is used only rarely. Concrete is still used, but mostly as a thin slab on floors. In urban areas, buildings must also provide protection against fire in neighboring buildings through fireproof exterior walls—preferably windowless, since windows are fire openings. Standards for fireproofing are set by organizations such as the American Insurance Association and the International Conference of Building Officials.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.