BRANNIGAN: COOL IT
(Photo by Francis L. Brannigan)
(Photo by John Mittendorfl)
(Photo John Mittendorf)
A few years ago, the taxpayers suffered a $185 million loss at an Air Force facility in Tinker Field, Okla. Roofers were working on a building that had a typical combustible metal deck roof when it ignited.
The problem of combustible metal deck roofs has been with us for half a century. This writer documented the problem and a solution after two fires in a warehouse in the Marine Corps Depot of Supplies at Norfolk, Va., in the late 1940s. However, Navy fire protection engineers dismissed the problem because the roof was, to use their word, “approved.”
The dangers continued to be ignored until 1953, when the $32 million loss at the General Motors transmission plant in Livonia, Mich., was attributed to an “approved” combustible metal deck roof. GM’s fire protection engineers then realized that the Underwriters’ Laboratories standard for which the roof was approved specified only that the roof was resistant to fire propagated by flying brands and would not itself generate such brands if on fire. The standard had nothing to do with a fire under the roof.
Not recognizing the dangers, officials won’t know the solution, either. The steel decking must be cooled so the tar solidifies and stops producing flammable gases. The old Atomic Energy Commission, now a part of the U.S. Department of Energy, spent millions of dollars doing this 30 years ago. It installed sprinklers for the sole purpose of cooling the roof— which, in many cases, is the only combustible building element in the federal government’s nuclear plants.
If there’s a ceiling, it’s not enough to install the sprinklers below it; the Tinker Air Force Base building had them there. There have to be sprinklers in the void space between the ceiling and the roof. (See Photo 1, upper left.)
Ladder pipe streams to the roof are a waste. The fire showing on a combustible metal deck roof is from burning tar and insulation and is inconsequential. The fire below it will be as much as 30 to 50 feet farther advanced because of the gas from the tar. Water on the roof can’t cool the steel decking, which is vital to success. If the fire is venting itself, ladder pipe streams will drive flames and smoke down onto firefighters below.
Conventional fire service roof venting is useless, because the tar generates an almost unbelievable amount of smoke. I have movies of a series of tests Factory Mutual Insurance Co. did in 1959, showing that in a building just 20 by 100 feet, nothing smaller than 64square-foot vents would cut off the progress of the fire and improve visibility. A 25,000-square-foot building would require 1,500 feet of free venting area.
Structural steel collapse has been documented as starting nine minutes after ignition of a fire of any consequence within a structure of this type. Rather than venting, resources would be much better employed in the only other demonstrably effective firefighting method; exposing the roof from below and directing hose streams to cool the steel underside of the roof covering.
This must be done even if the steel can’t be seen in the incredibly dense clouds of smoke this kind of fire produces. If there’s a ceiling, it must be removed, or the building is likely to be lost.
At a mall in Winter Park, Fla., a decade ago, a metal deck roof fire in the one-story wings was elongating the steel, which in turn moved concrete columns of the high-ceilinged, interior section. This caused concrete T beams supporting the roof to fall, narrowly missing several firefighters. The command officer diagnosed the problem. Ffe had a ceiling pulled in the wings to expose the extending fire. Then he had a deluge gun stream directed up into the void to cool the steel and extinguish the fire.
(The lightweight, suspended ceiling was no problem to pull. But a steel lath and plaster ceiling might take so long to remove that the building would be lost; the fire moves with incredible speed.)
The effect of cooling the steel is almost miraculous. Suddenly the fire is out, leaving “icicles” where molten tar has solidified under the stream of water. (See Photo 2, upper right.)
(photo by Francis L. Brannigan)
(Photo by John Mittendorf)
(Photo by John Mittendorf)
