This article concludes the two-part summary of the 54-page Chapter 3, "Wood Construction," of the 667-page third edition of my book, Building Construction For The Fire Service . Part 1 of "The Building Is Your Enemy" was published in Firehouse® February 1996, part 2 in August 1996, part 3 in...
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:
This article concludes the two-part summary of the 54-page Chapter 3, "Wood Construction," of the 667-page third edition of my book, Building Construction For The Fire Service. Part 1 of "The Building Is Your Enemy" was published in Firehouse® February 1996, part 2 in August 1996, part 3 in January 1997 and part 4 in February 1998.
Facts about structures are printed in regular type. Firefighting implications are printed in italics. Page references are to Building Construction For The Fire Service, third edition.
Beware of code definitions. Publications on building construction divide buildings into five nice, neat groups, which makes a good exam question. From the firefighter's point of view, however, these groups do not necessarily define the hazards or the fire characteristics of the building.
Heavy Timber Construction
This type of construction will be discussed further in future articles when we discuss ordinary construction, since the great majority of heavy timber buildings have masonry exterior bearing walls.
Illustration by Christopher Brannigan
This Maryland building appears to be of brick construction. The removal of the adjacent building, however, exposes the original log walls. Note the wood-stud wall above the logs. Know your buildings!
There are five general types of buildings; heavy timber is one. It is usually described in favorable terms, such as slow burning. Unfortunately, the generalized description of favorable characteristics does not apply to a great many such buildings. They have serious and life threatening hazards. Unless a building FULLY meets the description of mill construction on pages 204-207, it can be very dangerous to us.
After the Seattle tragedy in which four firefighters died in what was widely reported as a heavy timber building, I was asked many times, "How could this happen in a heavy timber building?" The answer was that the portion which collapsed was supported on a wood stud wall. Get the report on this fire from the U.S. Fire Administration (see U.S. Fire Administration Report No. 77, prepared by Gordon Routley of TriData Corp. Copies of the report are available without charge from the U.S. Fire Administration, 16825 S. Seton Ave., Emmitsburg, MD 21727). Read and heed it.
"Non-combustible" buildings, by code definition, may have substantial wood components, typically roofs and balconies. Fire-resistive buildings may have substantial areas of wood trim in restaurants and executive offices. Do not base pre-fire plans simply on the code, classification or a general description of the building.
Pre-fire planners should actively look for exceptions to the general characteristics. All should be alert for alterations. Analyze their possible effect on your life.
For a fuller description of the hazard of applying very general type descriptions to specific buildings see The NFPA Handbook, 18th edition, Section 10, Chapter 11, "Building Construction Concerns Of Fire Departments," by this author.
A serious problem in combustible construction is the combustible voids, usually interconnected, in which fire, like a cancer, can spread, involve the entire structure and often burst out violently, causing injury and death to firefighters.
Unfortunately, this deadly hazard is not taught in live fire training. In such training the fire is visible and the focus is to "put the wet stuff on the red stuff." The fire service should give better training on this deadly hazard.
The use of truss floors has greatly increased this hazard, since the trusses provide a void or "trussloft" in every floor. In order to get laws passed to provide sprinkler protection against flashover, partial sprinkler standards have been introduced for residential structures. On balance this is a good concept. However, such sprinklers are not effective against fire which starts in or penetrates into the void space. If the piping is plastic it may burn through and waste the water.
Bear in mind that all the wood in the interior of the building has been drying since the day it was installed. Sprinklers which are not hitting fire may pour scalding water on firefighters and otherwise interfere with operations. It is a real unanswered question as to whether such sprinkler systems should be allowed to continue operating when fire is in the voids. If any reader has experience to relate, I would like to hear of it.
The building code answer to the void space problem is firestopping and its lesser cousin draftstopping. To the best of my knowledge, no firestopping method for combustible buildings has ever been tested.
One difficulty with the concept is that firestopping must be perfect. Fire gases are under pressure and the slightest opening can become a "nozzle" to send a jet of flaming gas past the barrier. It is difficult to obtain a perfect barrier, and even good barriers are often violated for passage of utilities.
Firestopping and its lesser cousin draftstopping are not the same. Firestopping has consisted generally of a two-inch thickness of lumber and is used in relatively small spaces. Draftstopping is used in larger voids and generally is made of gypsum board, plywood or chipboard. Even if installed correctly under the eye of a vigilant knowledgeable inspector, they are both subject to penetration for any reason.
Tightly fitted heavy wood firestopping is the best. In one case it was removed and not replaced to accommodate a fire main, thus fire in the truss voids will follow the fire main throughout the building.
By my definition there are two types of firestopping:
Inherent: This grows out of a necessity of construction. A wet masonry panel wall built on a concrete floor automatically is fire-stopped.
Legal: This is firestopping which (to the builder) serves no purpose but to meet the code. This is more likely to be deficient.
Firestopping and draftstopping cannot be relied on. "Undress the building," by which I mean "see" the interconnected hidden voids and assign units to deal with the problem. Because the voids may be complicated and stream obstructed, as by cross pieces of trusses, it appears that the better tactic might be to locate hot spots by use of a thermal detecting device and penetrate the wall or ceiling sheath with a piercing nozzle. Opening the void gives the fire the oxygen it needs, and the fire may be beyond the reach of hose streams. In addition all hose streams, but particularly fog streams, carry air to the fire.
Wood is subject to insect infestation, wet and dry rot, and other forms of decay, which may cause weakening without being apparent to the casual observer.
Deterioration of this nature in structural members should be carefully noted on pre-fire plans.
Protecting Wood From Ignition
Wood burns, and much effort has gone into overcoming this characteristic. There are coatings which when heated intumesce (swell up like marshmallow) and delay ignition. These coatings must not be applied like paint, but each gallon to a specified area. If the wood is furred (spaced) off from the wall, the reverse side is not covered.
Wood can be impregnated with mineral salts. This will retard surface ignition. Some materials used can leach out and corrode metal connectors.This FRT (fire retardant treated) wood is harder to cut and its strength is decreased. In some codes it is "accepted as non-combustible." Wood cannot be made non-combustible, but its ignition can be delayed.
Do not confuse this wood with wood treated to resist rot and insects. As noted earlier, such wood gives off toxic smoke.
Plywood Roofing Problems
Some years ago, to avoid parapetting fire walls through the roof, builders adopted a practice of placing FRT plywood, one sheet wide, on the roof, on both sides of a firewall, which does not penetrate the roof. At the time, I set forth potential deficiencies of this method of stopping the extension of the fire over the top of the firewall since the plywood delaminates (layers separate) and fire passes over the top of the wall.
Recently, it also has been found that plywood treated with certain chemicals decays from heat and is subject to failure if walked on. The problem is compounded if treated plywood is used not only along the line of the fire wall, but randomly throughout many roof areas. In some cases, the entire roof is treated plywood. There have been huge lawsuits.
Where this condition exists, firefighters should not stand on roofs, but should actually be on an aerial apparatus.
In an attempt to provide some sort of barrier to the spread of fire over an unparapetted fire wall, some jurisdictions require a sheet of gypsum board to be attached to the underside of the roof, on each side of the fire wall. Presently, there is no evaluation of the adequacy of this concept, but 19th century attempts to protect wood with plaster caused the wood to rot. Wood must "breathe."
In the trade the term engineered wood refers to heavy members made of wood in various thicknesses laminated (glued) together. Beams, arches and catenary curves (beams curved downward, the opposite of an arch) are made to order.
In my work I consider any wood which is not in its natural state to be engineered wood. Some examples:
- Plywood is made of several thin layers of wood each layer at right angles to the next,glued together under pressure. Plywood delaminates (layers come apart) in a fire. This increases the RHR (Rate Of Heat Release). The fire is hotter and faster.
- Wooden I Beams and Trusses will be discussed later in this series. In the meantime, they are covered fully in Chapter 12.
- Flitch plate girders are discussed in the next section.
Steel In Wooden Buildings
In past years, a wooden building was entirely of wood. Today, steel is used in many otherwise wooden buildings, thus its fire protection deficiencies are significant.
Flitch plate girders are used when a long span is needed or a particularly heavy load must be carried. A steel plate is bolted between two wooden members. The resultant composite has the strength of steel, but can be handled by the carpenter like any other wood member. Such girders should be noted on pre-plans. If the wood burns away, the steel will buckle and fail. I have seen thick plywood used in place of the steel.
Unprotected steel girders are now commonly used to carry heavy loads. The collapse of such a girder cost a firefighter his life. Often, the floor beams are just sitting on the flange of an I beam. If the beam is subjected to 1,000 degrees Fahrenheit, it will elongate; if restrained, it will overturn and drop the floor beams. We will discuss the negative fire characteristics of steel later in this series. In the meantime, you can read Chapter 7, pages 255-257.
Many churches and similar buildings are built of wood arches. Arches thrust their load outward. This must be resisted by a mass of masonry or by tying the legs of the arch together.The ties are steel rods like concrete reinforcing rods. If the building is on a concrete slab, the ties are buried in the concrete.
If there is a basement, the ties will be unprotected and located in the basement ceiling. A fire could cause elongation of the rods; collapse might result. The status of ties in all wood arched structures should be determined and recorded.
Long heavy timbers are much scarcer than they were years ago.
Many long timbers are spliced with steel connectors. Such connectors have burned out and caused collapse.
Sheathing is the material laid on the studs. Several materials are used. Diagonal wood sheathing was used on high-quality older buildings. It was expensive, but kept the building from listing from constant pushing by the wind. Parallel wood sheathing was also used.
More recently, a low-density combustible wood or sugar cane fiber (bagasse) material (one brand name is Celotex) was widely used.
This fiberboard has often been ignited by plumbers' torches. Fire inside the void may escape the water splashed on the fire by the plumber. The fire breaks out possibly hours later.
Weather-resistant gypsum board is used for sheathing. Combined with brick veneer it has been used where fire exposure protection was required. Foamed plastics have become widely used because of their insulation qualities.
Foamed plastics are basically combustible. Even if chemically inhibited to reduce combustion, they will melt and degrade.
Siding is the exterior weather covering of the building. It may be solid wood, plywood, shingles, asphalt- based imitation stone or brick, aluminum, vinyl, stucco, brick or stone, asbestos-concrete shingles. Whatever the siding, the building is a wooden building.
Except for metal and masonry, all siding is easily ignited by an exterior fire or by auto-extension of fire from floor to floor.
All wooden buildings are liable to ignition from exposure fires because of the windows.
Fire departments with significant exposure potentials must have standard operating procedures (SOPs) that bypass the usual initial attack with small lines to mount a heavy-caliber defensive attack. These plans may even include sacrificing the original heavily involved building to devote all resources to cutting off a possible conflagration.
Illustration by Christopher Brannigan
Heavy timber structures have a "good press," but beware. Fire-vulnerable connections like this steel connector are one of the principal weaknesses. (See Building Construction For The Fire Service, third edition, page 129.) Know your buildings!
In one case of a fast-spreading fire in wooden apartment houses, it was reported that shorthanded units had difficulty getting heavy-stream appliances into place on the ground because of their location high up on the apparatus. Units, especially those undermanned, should drill on getting heavy-stream appliances into operation as rapidly as possible. Time standards should be set and slower units should drill regularly on this evolution.
Firefighters operating handlines should not get into narrow spaces between the fire building and the exposure, firefighters have died when the fire building collapsed. Plan to use heavy-caliber stream appliances for such exposures.
Brick veneer siding is very common and can be dangerous. The word brick seems to connote strength and durability, e.g. "built like a brick outhouse." Brick masonry, usually a composite with concrete block, structurally supports the building. Brick veneer is not structural. It is just another siding, but a potentially dangerous one. A one-brick-thick wall is very unstable. The brick is tied to the basic structural wall, usually wood but occasionally concrete block, cast concrete or even steel, with steel ties. When finished, it looks like any structural wall.
In older buildings about every seventh course (layer of brick) was turned so the only the headers (the ends of the brick) showed. This tied the wall together. Brick veneer was laid all stretchers (the long side of the brick showing), thus you could tell one from the other.
Many times, the header bricks would crack. Wire trusses were developed. They are laid in some of the mortar joints between courses and eliminate the headers. You can no longer tell brick veneer from brick masonry.
Some buildings have bearing walls of solid masonry and non-bearing walls of cheaper brick veneer. Others may have a first story of load-bearing masonry and upper floors of brick veneer.
Don't get people injured or killed in the collapse of a brick (or stone) veneer wall. If the basic wall is steel, movement of the steel, may throw down the brick. If the wall is wood, the burning of the wooden wall deprives the brick veneer of support and it may likely collapse. This is not a structural collapse, but the veneer bricks are as heavy and hard as structural brick. Your new standard helmet may not help you. If you get hit with a mass of bricks, your helmet frontpiece may become your belt buckle!
An old wooden house in New York City had been "rehabilitated" by the addition of a masonry veneer wall. It was heavily damaged by fire. During overhaul, an interior collapse knocked down the brick wall. A lieutenant was killed while pushing an injured probationary firefighter out of harm's way.
Wood shingles or shakes are split pieces of wood used for roofing or siding. Shakes are larger than shingles. Some of the greatest fire disasters in history have been due to the spread of fire by wood-shingle roofs.
There are many areas now where the majority of houses have wood-shingled roofs. In some areas, they are permitted wherever frame buildings are permitted. Fast fire department response, one-story buildings, wider spacing between buildings than in bygone years, and the fact that we have no extensive amount of shingled, closely spaced 2 1/2-story buildings such as existed 60 years ago have combined to keep the conflagration rate low. But given the coincidence of hot dry weather, brushfires that engage a large portion of available firefighting forces, high winds, and a hot fire in a wood-shingled structure, the threat of conflagration is still very great.
On the afternoon of July 31, 1979, the Houston City Council was considering an ordinance with minor restrictions on the use of wood shingles. The ordinance was tabled. The day was hot and dry. At about the same time, the fire department responded to an alarm for a fire at the Woodway Square Apartments. By that evening, 30 apartment buildings were destroyed, hundreds of people were left homeless, and an estimated $44 million in damage was incurred. The fire spread because of the wood-shingle roofs.
In wood-shingle areas prepare for a blitz attack to knock down the original fire quickly. Call additional units for downwind patrol immediately. Don't wait for confirmed reports of extension.
Row Frame Buildings
In older parts of many cities frame buildings were often erected in rows. These structures are contiguous and often have a common attic or cockloft, and may even have party walls which provide support to both buildings.
Rowhouses have a new life as townhouses.
Unless the units are divided by unpierced masonry fire walls, parapetted through the roof, the entire group should be considered as one fire building. The fire knows nothing of titles or mortgages.
Considering the group as one building may make the incident commander more likely to call immediately for the resources necessary to contain a large fire rather than thinking of the fire as one relatively small unit.
Wooden Cooling Towers
Cooling towers are used to disperse the heat from air-conditioning systems. When the tower is functioning, cascades of water pour down the inside. Despite this, much of the wood in the tower remains dry. If the tower is not operating, all the wood is dry. Electrical shortcircuits, fan-bearing friction or even birds carrying lighted cigarettes to their nests in the towers are few of the possible fire causes. Such a fire can be spectacular and disastrous. Embers from a tower atop a high-rise spread fire over a wide area.
Be aware of cooling towers in your area. Are they wood or non-combustible? Are they sprinklered? Don't be surprised by a huge wooden cooling tower recessed several stories below the roof line of a "fire-resistant" building. KNOW YOUR BUILDINGS!
Wooden buildings represent a major exposure hazard to nearby structures even sprinklered fire resistive buildings. The largest amount of resources ever devoted to a structural fire in the City of Los Angeles was for a three-story wooden apartment building that exposed a 14-story fire-resistive high-rise occupied by senior citizens. All floors of the exposure were involved on arrival. In addition, flying embers ignited 14 other serious fires.
Pre-fire plans should take such exposure problems into account. Units should be prepared to go to heavy-caliber streams immediately. Nothing is more ridiculous than videotapes showing a firefighter directing the piddling stream from a small line into an inferno. Drill on getting heavy-duty lines into action rapidly.
There is really no effective fire protection for large wooden (or wooden-interior) buildings, other than FULL automatic sprinkler protection, with all voids covered, an adequate water supply and proper maintenance.
A sprinkler system is like having the first-due engine company on the scene, in the building, with a charged line hitting the fire. Under this premise, fire departments should be much more proactive in the maintenance of sprinkler systems in service. Tight administrative systems should be in place to insure that the fire department is aware of sprinkler impairment. It is not just a matter between the owner and the insurance company or just a matter for the "Fire Prevention Division." Study Chapter 13, "Automatic Sprinklers."
Francis L. (Frank) Brannigan, SPFE, a Firehouse® contributing editor, is a 56- year veteran of all phases of the fire service. Thirty years ago he began concentrating on the hazards of buildings to firefighters. He is the acknowledged fire service resource in this area and his work has been credited with saving lives.
Building Construction For The Fire Service is published by National Fire Protection Association (NFPA), 800-344-3555. For autographed copies at a special reader's discount or for information on his slide tapes, call or write Frank Brannigan at 2041 Daylily Road, Port Republic, MD 20676-2646 (301-855-1982).