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...
To access the remainder of this piece of premium content, you must be registered with Firehouse. Already have an account? Login
Register in seconds by connecting with your preferred Social Network.
Complete the registration form.
Cold drawn steel cables lose their tensile strength at about 800 degrees F (lower than the temperature of a self-cleaning oven). Such cables are used to tension concrete, to hold back excavation bracing, to tie failing buildings together, and for elevators.
Concrete is non-combustible but not necessarily fire resistive. Steel is made fire resistive by applied "fireproofing." Concrete is made fire resistive, when required, by specified improvements in the mix. So-called reinforced concrete is actually a composite in which steel provides the tensile strength and concrete the compressive strength.
Many years ago, the Concrete Institute urged me to emphasize the following: Any failure of the bond between concrete and steel means that the structure is failing to some degree.>
Different types of concrete and concrete structures have different fire problems. Cement is a constituent of concrete. Do not use the word cement when you mean concrete. Fires in concrete buildings under construction may pose very serious hazards to firefighters, including total collapse of all floors (pages 342-359).
Plastics describes a wide range of materials. In general, they are easily ignited, produce greater amounts of heat than other materials and emit combustion products which may be more toxic than ordinary smoke. Some become pools of flaming liquid. Some erroneously consider rigid reinforced plastics (e.g., fiberglass) to be non-combustible. The glass is not combustible but the plastic will burn and leave a mat of fibers which, if disturbed, will be an itch hazard.
Some plastics are inhibited with chemicals to limit ignition but may melt or deform. If carrying a load, such as heavy tile on a foam plastic roof, the load can drop.
Fire load is the potential fuel for a fire. For combustible buildings it includes the building itself.
Fire load is measured traditionally in BTU (British thermal units). Fire load was first stated in pounds per square foot, since most fire load was wood and paper, which are taken as being 8,000 BTU per pound. The development of plastics and liquid fuels with BTU emissions of up to 23,000 BTU/pound made problems.
Since it is the heat, not the weight, that concerns us, it is better to speak of BTU per square foot. As a very rough rule of thumb, an estimated fire load of 80,000 BTU per square foot is the equivalent of the standard test fire at the one-hour fire resistance level.
RATE OF HEAT RELEASE
A more recently developed term is RHR (rate of heat release). This estimates how fast the material will burn. A pound of solid wood burns much more slowly than a pound of excelsior (wood wool).
Major sprinkler systems are hydraulically designed to deliver a desired flow of water to each square foot of an area. This is calculated to suppress fire in a certain fire load with a certain RHR. Huge losses have occurred in giant warehouses where the fire load has exceeded the design estimates. My statement of this situation is simply, "The BTU outnumbered the H2O."
Pre-planning should include consideration of the fire load and RHR. This might prevent the futile automatic stretching of small attack lines when big lines or heavy stream appliances should be used on the initial attack.
This concludes the abstracts from Chapter 2, "Construction Principles," of Building Construction For The Fire Service, third edition. Next we will take up the "Hazards of Wood Construction" with abstracts from Chapter 3. The 667-page third edition of Building Construction For The Fire Service, by Francis L. Brannigan, is published by NFPA, 800-344-3555; for autographed copies at a special reader's discount call 301-855-1982.
2 Firefighters Die In Chesapeake, VA, Roof Collapse
The National Fire Protection Association (NFPA) reports that two deaths resulted from the collapse of a 50-foot clear span wood truss roof on an auto parts store. The NFPA noted that the firefighters apparently had no knowledge of the type of roof. I will provide further information when the NFPA investigation is completed.