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THE BASICS OF FIRE PROTECTION

Safety Criteria
In order to evaluate fire safety of a structure, building authorities consider many factors, including flame spread and fire resistance ratings.

Flame Spread in general means spread of fire within a room - and is measured by the performance of the materials used for interior finish, such as walls, ceilings, partitions, paint and wall paper (not, in code considerations, such non structural materials as drapes and furnishings, though these may often be primary fuel sources).

Flame Spread is a property of the surface material once fire has started, not the structure.

The best known flame spread test is the tunnel test, American Society for Testing Materials (ASTM) Test Method E-84 or the Underwriters Laboratories (UL723) or the National Fire Protecion Agency (NFPA 255). In this test, a sample of the material, usually 20 inches wide and 25 feet long, is installed as ceiling of a test chamber, and exposed to a gas flame at one end. The rate at which flame spreads across the specimen is compared to and calibrated on a scale of 0 for inorganic reinforced cement board, flame spread 0, and red oak flame spread 100.

Another property measured in the ASTM E-84, UL 723 test is the opacity of the smoke generated by the burning material. This measurement provides an indication of the amount of smoke released.

Model building code interior finish classifications are summarized in Table 1. Materials with the lowest rate of flame spread (0 to 25) are classed by all building codes as Class I (or A), and are permitted for areas where fire hazard is most severe, such as vertical exitways of unsprinklered buildings for public assembly.

Materials with rating from 26 to 75 are Class II (or B) and are permitted in areas of intermediate severity: for example, corridors providing exit way access in business and industrial buildings.

Materials rated from 76 to 200 are Class III (or C). APA trademarked panels such as plywood, oriented strand board (OSB) and composite panels fall generally in this class and are permitted in rooms of most occupancies. (Exceptions: hospitals, or institutions where occupants are restrained.) For exitways and for most interiors where Class I or Class II flame spread performance is required, fire-retardant-treated plywood (which falls in Class I) is permitted.

Table 2 shows typical flame spread requirements, as called for under the National Building Code.

Fire Resistance. Though codes are concerned with how fast fire can spread on a room’s surface, they are even more specific about fire resistance: the measure of containment of fire within a room or building. It is defined as protection against fire penetrating a wall, floor or roof, either directly or through a high rate of heat transfer that might cause combustible materials to be ignited on the side of the wall or floor away from the actual fire. Thus, it is a property of an assembly of several materials, including fastenings, and of the workmanship.

A fire-resistive construction gives time to discover a fire, to suppress it before it spreads, and to evacuate the building if need be.

The standard test for measuring fire resistance is ASTM Test Method E-119. Ratings of assemblies are determined by test procedure somewhat simulating actual fire conditions. Floor-ceilings and roof-ceilings are tested flat, while loaded to their full allowable stress. Walls are tested vertically, either as bearing wall, under limited axial load, or as nonbearing wall, under no load. The resistance rating is expressed in hours or minutes that the construction withstands the test. So it approximates the time the assembly would be expected to withstand actual fire conditions.

A one-hour rating, for example, is taken to mean that an assembly similar to that tested will not collapse, nor transmit flame or a high temperature, while supporting its full load, for at least one hour after fire commences.

Table 2

(a) Requirements for rooms or enclosed spaces are based upon spaces enclosed in partitions of the building or structure, and where fire-resistance rating is required for the structural elements, the enclosing partitions shall extend from the floor to the ceiling. Partitions which do not comply with this shall be considered as enclosing spaces and the rooms or spaces on both sides thereof shall be counted as one. In determining the applicable requirements for rooms or enclosed spaces, the specific use or occupancy thereof shall be the governing factor, regardless of the use group classification of the building or structure. Where an automatic sprinkler system is installed (in accordance with code provisions) throughout a building, Class II or III interior finish shall be permitted where Class I or II materials, respectively, are required in the table.

(b) Class III interior finish materials are permitted in places of assembly with a capacity of 300 persons or less.

©) Class III interior finish materials are permitted for wainscoting or paneling for not more than 1000 square feet of applied surface area in the grade lobby where applied directly to a noncombustible base or over furring strips applied to a noncombustible base and fire-blocked (in accordance with code provisions).

(d) Class III interior finish materials are permitted in mercantile occupancies of 3,000 square feet or less gross area occupied for sales purposes on the street floor only (balcony permitted).

(e) Lobby areas shall be not less than Class II.

(f) Where building height is over two stories, Class II shall be required.

(h) Walls and ceilings shall be a minimum of Class II materials in individual rooms or not more that four persons capacity. Where a building is equipped throughout with an automatic sprinkler system (installed in accordance with code provisions), the minimum requirement for interior finish shall be Class II

(I) In Use Groups A, 1-2 and 1-3, Class II interior wall finish shall be permitted as wainscoting extending not more than 48" above the floor in corridors providing exit access.