FIRE LOSS MANAGEMENT
DISASTER MANAGEMENT
Part 10:
METHODS OF EXTENSION
ACCEPTING THE fact that there are many causes of fire, both known and unknown, the preplanner should examine the following: Is it possible for a fire to extend? Is kindling accumulated in an unfavorable location? Is kindling provided by the structure of the building or its trim? Is the fuel for the fire solely that which is necessary and useful in the contents of die building?
There isn’t very much the fire department can do about the contents. Some departments, such as Boston, are actively working in the area of regulating contents as to flammability. After the disastrous BOAC terminal fire at Kennedy Airport in which the fuel was flammable seats, the Port Authority of New York and New Jersey issued regulations respecting the flammability of furniture. The FAA has required airplane seats to have improved flame retardant characteristics. Press reports of the United Flight 232 crash in Sioux City, Iowa in July, 1989 credited the new materials with delaying the spread of the fire in the central cabin area from which almost 200 people escaped. New York State has promulgated regulations respecting die toxicity of materials but not their flammability.
In general, fire departments must accept contents as they are and analyze how the contents can assist the spread of fire through the building. For example, almost half a century’ ago a fourstory dwelling, converted to commercial use and filled with foam rubber, produced an immediate raging fire extending hundreds of feet in the air. A citizen reported the fire to a fire station on the same block. The chief transmitted a fifth alarm as the units left quarters. The fire destroyed several stories of an exposed fire-resistive sprinklered industrial building. More recently we have seen the rapid and total destruction of sprinkled warehouses because of intense, fast-burning contents fires.
METHODS OF HEAT TRANSFER
Every rookie firefighter learns that fires spread by convection, conduction, and radiation. More precisely, these are the methods of heat transfer. There is one additional and important method of fire spread—moving flaming (or hot) material.
Convection is the extension of fire by heated gases that, when generated by the fire, rise because they are lighter than air. The force of gravity pulls the heavier, colder air down, thus forcing the lighter air up. When the rising gases reach an obstacle, they move horizontally. They give up their heat to any cooler objects they find in their path. If these objects are combustible and take up sufficient heat to decompose, they add to the available combustible gases, they reach their ignition temperature, and the fire accelerates due to the fuel contribution of the added gases.
Conduction is the transfer of heat directly from molecule to molecule of the material. Some materials are good conductors, others are not. The experienced cook recognizes this phenomenon and uses wood rather than steel stirring spoons. However, that same experienced cook may think that a steel sheet behind the stove will prevent the ignition of the combustible wall. Heat is transferred by conduction through the steel sheet to the wall. If there is enough heat to decompose and ignite the wall, die fire occurs in a hidden location behind the supposed “fireproofing.”
Many building codes required tin ceilings (actually embossed sheet steel) to “prevent the extension of fire.” Ships are built of steel. Welding or cutting on one side of a steel bulkhead will ignite material on the opposite side. The Navy understands this and appoints a “fire watch.” This is very much the case in many merchant ships in which steel bulkheads have a wooden surface attached with an air space between. The fire spreads laterally along the concealed surface of the wood.
One means of fire extension in combustible single-family and multiple dwellings is the passage of fire through exhaust ducts that are tightly fitted to the combustible structure. Expect this extension in any fire involving an interior bathroom or kitchen. Such rooms must be vented, and the exhaust duct is the method of choice.
Radiation is the transmission of heat by rays, similar to light waves or radio waves, through the air. The radiant heat will warm an object without heating the intervening air. As the convected heat from a fireplace fire goes up the chimney, we are warmed by the radiant heat. A heated cloud of convected gases rolling along a ceiling will radiate heat downward toward the floor. This is often sufficient to ignite objects and floor covering well ahead of the main body of fire. This also is a key factor in the sudden ignition of all available fuel—flashover.
Heat transfer also can result in secondary fires, or fires away from the original point of origin. This can mislead an investigator who is not aware of this phenomenon. Take as an example a chair burning in a living room. As the chair burns, the room begins to fill with hot gases from the ceiling down. There are cloth draperies on windows that extend up close to the ceiling into the hot gas layer. Before the room flashes over, the tops of the draperies ignite, burn free of the drapery rods, and fall in a burning pile onto a sofa. A few minutes later the fire department arrives and extinguishes the fire. In a casual examination of the room, the investigator might conclude that there were two fires independent of each other and might incorrectly suspect arson.
Moving flaming or heated material. Burning shingles, when carried by the updraft of the fire, have long been recognized as a potential method of fire extension. (Recently, Los Angeles City has banned entirely the use of wood shingles—fire-treated or not —in new construction.) The same phenomenon can occur inside a building. Consider an office in which papers are scattered around on the tops of desks, file cabinets, and tables. In the event of a fire in this room, burning papers are likely to be lifted by the hot plume of the fire and deposited in different locations around the room. If the fire department arrives at this time, it may conclude that there were several different fires instead of just one and suspect that the fires were set. It is appropriate, therefore, for the investigator to consider what types of materials were involved in the fire and their likely behavior under fire conditions so he may determine the fire cause and origin.
While fire usually moves upward, it also can fall down through hollow walls and cause confusion as to the point of origin. Many fires have been started by heated metal from cutting torches falling in some cases several stories. Molten aluminum from floor expansion joints extended fire down into the lower level of McCormack Place in Chicago in 1967.
METAL DECK ROOF FIRES
A type of fire in which conduction, convection, radiation, and falling flaming material all play a part in extension of the fire is the combustible metal deck roof fire. This became well-known in insurance fire protection circles after the 1953 General Motors Livonia fire but unfortunately is still unknown to many in fire and building departments.
I have observed the progress of a number of metal deck roof fires:
- The fire starts in the contents of the building.
- The convection gases rise to the metal decking of the roof and heat the metal.
- The heat is transmitted by conduction through the metal to the bituminous material, which is used as a vapor seal to protect the insulation in the roof structure above the metal deck.
- The bituminous material is heated to its vaporization point.
- The vapors cannot escape upward because the roof is sealed, so they escape downward into the building through the spaces between the sheets of metal.
- The heated vapors then ignite because they have reached their autoignition temperatures or are ignited by the direct flame of the fire.
- Since the burning vapors cannot rise, they move horizontally as convection currents, heating more metal, which heats more bituminous material, which generates more gases.
- As the fire moves along underneath the roof, radiant heat is directed downward from the roof level and ignites combustible material in its path.
- Flaming drops of tar fall from the roof and ignite combustibles on the floor.
(Further information is available in Chapter 7 of my book Building Construction for the Fire Service under the heading “Metal Deck Roofs.” See also “The Metal Deck Roof Debate” in the March 1988 issue of Fire Engineering.)
