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April-May 2004 |
Water on Hot Steel
The heat evolved by a fire can be triaged or considered and treated according to priority needs. In reverse priority, this includes:
Heat leaving the structure. Let it go. Every BTU which leaves is one less to keep the fire going. This is sometimes contradicted by the “Chicken Little” school of fire fighting, which apparently believes that the sky can catch fire and the world will come to an end.
Heat being evolved from contents which are burning. This is of secondary importance. This concept often comes as a surprise to those whose sole objective is to “put the wet stuff on the red stuff.” Contents partially damaged are, in effect, totally destroyed. In fact, much of overhauling consists of throwing out partially damaged contents.
Heat being absorbed by contents or structural elements which will be ignited, or caused to fail. This heat is the most important heat to be removed. In the case of unprotected steel, failure can occur early in a fire and the consequences can be catastrophic.
The fire department’s heat-removal medium is water. It seems apparent that water should be used to remove the destructive heat from the steel. Unfortunately, there is a myth which is fairly widespread in the fire service, and which the author has seen in print, that water should not be thrown on heated steel because of possible dire results. This is simply not true.
If the steel is elongating, the cooling effect of water will draw it back to its original dimensions. Many draw bridges, elongated by the summer sun, refuse to go back into position until the fire department cools the structure.
If the steel is failing, the water will simply freeze it in whatever shape it is in. When I published these statements in the first addition of my book in 1971, they were questioned by several people. I have since checked with the American Iron and Steel Institute, and was assured that I am correct.
IFSTA supports this position and has published the following statement:
“Contrary to widespread belief, heated steel beams do not present a hazard when cooled with streams of water. They will not buckle, but will return to their original shape, or if past the point of strain, will keep their deformed shape.
“The effects of expansion, however, may present some hazards. Steel expands more readily and to a greater extent than most other building materials. There is the possibility that the expansion of a steel beam could crack masonry walls, making them unsafe. Another point to remember is that all the beams used in constructing one building might not be of the same size. Smaller beams will expand or contract quicker than large beams. If two sizes of beams are bolted or riveted together, this difference could result in the bolts or rivets shearing.”
This author observed a Texas fire department fighting a fire in a steel hay shed. All attention was given to throwing water on the hay. No attention was paid to cooling the steel. The shed was severely damaged. The wet, half-burned hay was useless.
Tactical Considerations
It is very likely that the most important use of water in some fires is to remove the heat from the steel so that the structural integrity is maintained. It can be argued that this will create water damage. Note, though, that if sprinklers were installed they would be doing the same thing the hose stream does, although more efficiently. Think of the hose stream as an improvised sprinkler system, necessary because the owner failed to install one. Also consider the argument that there is no water damage at a total loss.
Plans for the cooling of steel should provide for a safe location for personnel. If a safe location is not available, this should be pointed out to the owners of the building so that they may be forewarned to the probable consequences of the fire. They might choose to provide protection for the steel.
It is often difficult, even for technically trained people, to understand how fast steel can be destroyed. Pictures of steel damage might be convincing. The distortion of the steel due to fire may alter the structure of the building so that it will not be able to carry the superimposed loads or unexpected stresses, particularly torsional, that may be created due to buckling. However, structural steel that has been exposed to fire and water will not have significantly different metallurgical properties than steel unaffected by exposure to fire.
The possibility that elongating bar joists may push a block wall out of alignment and when shrunk back may cause the roof to fall off the wall is remote and is guarded against by cooling steel from a safe location.
This article was excerpted from Building Construction for the Fire Service, 3rd Edition, by Francis L. Brannigan. Published by the National Fire Protection Association. Click here to purchase.
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