Editor's note: This article finishes summarizing a small portion of the 79-page Chapter 2, "Principles of Construction," of the 667-page third edition of Building Construction For The Fire Service, by Francis L. Brannigan. Part 1 was published in Firehouse® in February 1996, part 2 in July...
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Any number of foundation problems can affect fire suppression in several ways. Masonry walls above foundations may develop severe cracks which make the wall vulnerable to collapse. Fire doors may not close properly. Openings may develop in fire walls or in floor-wall connections, permitting passage of fire. Dry-pipe sprinkler systems may not drain properly after "going wet." Wooden basement walls are of pressure treated wood which emits toxic fumes (in addition to the normal carbon monoxide) when it burns.
CHARACTERISTICS OF MATERIALS
Each material has its own fire characteristics. They are summarized here and will be developed more fully as the series progresses. At times, the objection is made that this is all negative. The good characteristics of any material can be obtained without cost from the trade organizations. We are interested in those characteristics which are hazardous to firefighters and not always found in industry literature.
Wood is combustible. Fire can spread rapidly over wooden surfaces, particularly when the wood is thin. Wood can be treated to reduce the surface spread by impregnating it with salts that retard ignition. It can be surface treated with flame retardants. These must be applied strictly in accordance with directions and, in the case of wood paneling, may be ineffective if the hidden back surface is untreated. In no way is the wood rendered non-combustible, although some codes use that term.
Wood treated to resist decay and insects emits toxic smoke. When the wood is carrying part of the load of the building, the loss of material can cause collapse.
Steel is non-combustible. It is very strong, thus members of small cross section can carry substantial loads. Mass of material provides inherent fire resistance; thus lightweight steel has very little fire resistance.
Despite its non-combustibility, steel has negative fire characteristics. Steel transmits heat readily by conduction. A metal box provides no protection to vital records.
At relatively low fire temperatures steel elongates significantly. (A rise of 1,000 degrees Fahrenheit will cause a 100-foot steel member to elongate nine inches.) If it can move the supporting wall or column, it will, probably causing collapse; if not, it will buckle. At slightly higher temperatures, depending on the load carried, steel will fail.
Steel which is required to be insulated from fire heat is protected by what is called "fireproofing." This is a poor word, since nothing is fireproof. The better term is fire resistive. Fire resistance is rated by the time in hours an assembly has withstood the Standard Fire Test ASTM E 119. THE HOURLY RATING HAS NO RELATIONSHIP TO REAL-TIME HOURS IN A FIRE (page 248).
In an unprotected steel building, the most important use of water, almost always, is to cool the steel constantly to prevent the structure from failing.
The cooling does not cause failure of the steel if it is elongating, the cooling brings it back to its original dimension; if it is failing, the cooling freezes it in the failed shape. The best use of water is to cool the steel constantly, to prevent the structure from failing and thus reduce the hazard to firefighters and the loss. The burning contents are already a total loss.
STEEL IN NON-STEEL BUILDINGS
The fire characteristics of steel are important in most buildings, because steel is part of the structure of nearly all buildings.
Illustration by Christopher J. Brannigan
An important concern in today's buildings is clear space, without columns. One way is to carry loads up to the roof. The sketch shows a construction in a library. The mezzanine is built on a big beam, supported at one end on the wall while the other end is hung on a rod connected to a ceiling rafter. There is no way to estimate the stability of that connection in a fire. Temperatures up there would generally be much higher than on the floor, and the hazard would be invisible in the smoke. It is too late to detect these potential firefighter killers on the fireground. Know your buildings.
Lightweight wood trusses are held together with steel gusset plates or gang nails. Destruction of the fibers holding the gusset plate by pyrolytic decomposition (burning without flame) releases the plate and the truss fails.
Steel is often intermixed with other combustible construction elements. If the building is not required to be fire resistive, the steel is unprotected. Unprotected steel connections and supporting columns are found with most heavy trusses and laminated beams. If a steel girder supporting floor joists is restrained and cannot elongate, it will overturn and drop its load of wood joists.