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...
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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.