Document zbGaK4Og5rMVNKaXm6ewoeVZ3

Fic 43--4 --Noncombusbble building with low-hazard, metal-working occupancy Courtesy National Fire Protection Association by masonry walls, heavy timbei columns and beams, and heavy plank floors (Fig 43-3) Although not immune to fire, the gieat mass of the wooden members slows the rate of com bustion Moreover, the chai which forms on wooden surfaces serves as an insulator for the wood within Noncombustible construction includes all types of stiuctuies m which the structure it self (exclusive of turn, interior finish, and contents) is noncombustible but not fire re sistant Exposed steel beams and columns, and masoniv, metal, or asbestos panel walls, are the most common forms (Fig 43-4) Because of the tendency of steel to waip, buckle, and collapse under modeiate file ex posure, noncombustible constiuction is rela tively vulnerable to fire damage It is, theiefore, most suitable for low-hazard occupancies If quantities of combustibles are present, the structure should be protected with automatic spi inklers Ordinary construction consists of masonry exterior beating walls, or bearing portions of exterior walls, that are of noncombustible constiuction Interior framing, floois, and roofs aie made of wood or other combustible material whose "bulk" is less than that re quired foi heavy timber construction If floor and roof construction and their supports have a one-hour fire-resistance rating, and all openings through floors (including stairways) are enclosed with partitions having a one-hour fire-resistance rating, then it is known as "piotected ordinary construction" This type building dominates m congested areas of large cities Its occupancy should be limited to light or moderate hazards, use of highly combustible intenor finishes should be minimized Even when sheathed, ordinary construction (unlike fiie-resistive or noncombusbble con struction) still has combustible materials in concealed wall and ceiling spaces Fire fre quently originates m these concealed spaces 1329