Noncombustible materials such as stone brick con

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Noncombustible materials such as stone, brick, con- crete and steel do not burn and therefore are not a source of fuel. Although the physical properties of non- combustible materials may be adversely affected at elevated temperatures, these materials do not con- tribute to either the duration or intensity of a fire. Conversely, combustible materials such as wood, paper and plastic do increase the intensity and/or duration of a fire. Tests conducted by the National Institute for Standards and Technology have indicated that an approximate relationship exists between the amount of available combustible material (fire loading expressed as pounds of wood equivalent per square foot of floor area), and fire severity (expressed as hours of equivalent fire expo- sure based upon the standard ASTM fire test). This rela- tionship is illustrated in Figure 26. Subsequent field sur- veys measured the fire loads typically found in build- ings with different occupancies and are listed in Table 5 ( Fire Protection Through Modern Building Codes ), produced by the American Iron and Steel Institute. For noncombustible framing there is no assigned fire load. However, for conventional wood framing, a rea- sonable estimate of fire load for the structure is 7.5 to 10 psf. For heavy timber construction, the correspon- ding structural fire load might be on the order of 12.5 to 17.5 psf. As a result, building codes generally limit permitted size (allowable height and area) of com- bustible buildings much more than for noncombustible buildings. Fire Resistance of the Structure In addition to regulating buildings according to the combustibility or noncombustibility of the structure, building codes also specify fire resistance requirements Figure 26. NIST graph illustrating the relationship of fire sever- ity to the average weight of combustibles in a building Figure 27. Graph from ASTM E119 test showing relationship of time to fire resistance temperature requirements
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PAGE 59 SYSTEMS according to building size (height and area) and type of occupancy. Generally, fire resistance is defined as the relative ability of construction assemblies (floors, walls, partitions, beams, girders and columns) to prevent the spread of fire to adjacent spaces and/or to continue to perform structurally when exposed to fire. Fire resistance requirements are generally based upon standard tests in accordance with ASTM E119. The ASTM E119 test method specifies a "standard" fire exposure that is used to evaluate the fire resistance of con- struction assemblies (Figure 27). Fire resistance requirements are specified in terms of the time during which an assembly continues to prevent the spread of fire and/or perform structurally when exposed to the "standard" fire. Thus, fire resistance requirements are expressed in periods of time in increments of whole or half hours. The design of the fire resistant buildings is typically accomplished in a very prescriptive fashion by selecting tested con- struction assemblies that meet specific building code requirements. Listings of fire resistance ratings for tested
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  • Structural steel

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