Occupancy Hazard Classifications

Building codes classify various occupancies according to fire hazard. These classifications are used to determine the design of sprinkler systems.

Sprinkler Hazard Classification
Figure 45-1 Sprinkler system.

The light hazard classification is used for buildings where it is relatively easy to provide effective fire protection. The quantity and combustibility of the building's contents is considered to be low, and a low rate of heat release is expected from possible fires. Light hazard occupancies include apartments, auditoriums, churches, hospitals, hotels, libraries, museums, nursing homes, office buildings, restaurants, schools, and theaters. A light hazard occupancy is required to have one sprinkler head per 18.6 square meters (200 square ft), with a maximum of 4.6 meters (15 ft) between supply lines and between the heads on each line. The sprinklers don't have to be staggered along their lines.

Ordinary hazard occupancies are considered to have moderate to high quantities of combustible materials, where the level of combustibility is relatively low to high. A moderate to high rate of heat release is expected. The materials may cause rapid fire development. Automotive garages, bakeries, laundries, and machine shops are considered ordinary hazards, as are manufacturing facilities, paper mills, print and publishing establishments, warehouses, and other industrial properties. Ordinary hazard occupancies require one sprinkler per 12 square meters (130 square ft) where there is a non-combustible ceiling, and one sprinkler per 11 square meters (120 square ft) for combustible ceilings. The maximum distance between lines and between sprinkler heads on a line is 4.6 meters (15 ft). Sprinklers are required to be staggered if the distance between heads exceeds 3.7 meters (12 ft).

The quantity and combustibility of materials in extra hazard (severe) occupancies are both very high. Rapid fire development and high heat release rates are expected where volatile flammable materials are processed, stored, mixed, or dispensed. Extra hazard occupancies include aircraft hangers, chemical works, explosive plants, linoleum manufacturing plants, paint shops, and shade cloth manufacturers. One sprinkler head is required every 8.4 square meters (90 square ft) with a noncombustible ceiling, and every 7.4 square meters (80 square ft) with a combustible ceiling. Sprinkler lines and heads must be no more than 3.7 meters (12 ft) apart, and heads must be staggered if they are more than 2.4 meters (8 ft) apart.

Generally, sprinkler systems are required for Factory, Hazardous, and Storage occupancies, or where large groups of people are present, as in Assembly, Institutional, and large Mercantile and Residential occupancies. The requirements are based on the number of occupants, the mobility of the occupants, and the types of hazards present.

Sprinkler systems are also commonly used in basements, windowless buildings, and high-rises. Sprinklers are often found in furnace and boiler rooms, at incinerator, trash, and laundry collection areas, and at the tops of chutes. They are required in kitchen exhaust systems and at spray painting shops or booths. Sprinklers are used in vertical openings, duct systems that exhaust hazardous materials, drying rooms, and atriums.

Residences generally don't have a water supply adequate for a standard sprinkler system. Toxic gases and smoke fill small residential rooms quickly, so a rapid response is essential for life safety. Many codes now require fast-response sprinklers with tested ability to enhance survival in the room where the fire originates in all residential occupancies. Such sprinklers are listed for protection of dwelling units. They are sensitive to both smoldering and rapidly developing fires, and open quickly to fight a fire with one or two heads.

Most codes exempt residential bathrooms under 5.1 square meters (55 square ft), closets with a minimum dimension of less than 91 cm (3 ft), open porches, garages and carports, and uninhabited attics and crawl spaces not used for storage. Entrance foyers that are not a sole means of egress are also exempted.

Residences use a special water distribution pattern, with water sprayed to walls and high enough to prevent the fire from getting above the sprinklers. They cool the gases at the ceiling level, so that fewer sprinklers need to open. The cost of residential sprinkler systems can be recovered through reduced fire insurance rates, but there is a long payback time.

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