Renovation Expands Fire Problems

Renovation Expands Fire Problems

Wood trusses are added to masonry wall, providing combustible concealed spaces.

From Maine to California, thousands of structures are being built, renovated, and expanded with that familiar combustible, wood.

Our concern with wood construction in this article centers on commercial occupancies and, more specifically, on exterior cornices and overhangs. These consist mostly of combustibles and they offer little compartmentation or firestopping. They have concealed spaces for fire to travel undetected—many times encircling the entire structure. The result is heavy damage to both the exterior and interior of the structure.

Structures with these exterior fire weaknesses are everywhere, and most often they are fast-food restaurants, professional office complexes, garden apartments or condominiums, singlestory nursing homes, various commercial buildings or light industrial occupancies.

Two construction methods

Exterior cornices are generally made in one of two ways. In wood construction, the first method uses wood framing attached to wood structural components (beams, joists, etc.) to create the framed-out space. In some garden apartment construction, two-story combustible “cornices” extend over the entire face of the building. They are made entirely of combustibles and the have huge voids. The facework is interrupted only by apartment windows.

Wood trusses are made of 2x4 lumber connected by steel gusset plates.

The second method uses the same wood framing, but it is affixed to masonry walls.

Both methods use wood structural members, fashioned into three or foursided trusses. Often the truss members are connected by metal gusset plates (sharp-toothed flat, metal plates that are used as connectors). It is a theory that gusset plates will rapidly absorb fire heat and the wood around the gusset plate teeth will be pyrolized, loosening the grip of the teeth. The result will be truss failure and partial structural collapse.

A variation of the second method is a false mansard roof, which is actually contiguous with the top story, slanting inward. This, too, creates many voids with little fire-stopping. However, it may provide fire department access to an attic or cockloft from the exterior.

Since the cornice or overhang effect is created by attached trusswork, we can expect to find a small lumberyard of combustibles. The probability of fire extension is great in these areas, as conservation of building materials to reduce cost is high, and little firestopping, if any, is used.

Sheathing covers voids

Once the truss work is complete, an exterior sheathing of plywood or particle board is applied to create a packaged effect. One national, fast-food fish chain uses a luminous fiberglass for sheathing. Most sheathing is covered with combustible shingles or shakes. This aesthetic approach is an attempt to streamline the exterior of the building and gives little consideration to the long, combustible voids, where fire can extend.

Hopefully, the fireground commander will have pre-fire plan information on buildings with the problems we have discussed. This information can be acquired during the construction or renovation of a building. Close observation by all fire department members from the time the footings are poured until the doors are opened to the public is highly recommended. Become familiar with the structure—you may meet it in combat on a cold winter morning five years later. Questions should be asked of the designer and the contractor, and if warranted, slides should be taken for training sessions.

Tactical considerations

When a fire occurs in this type of structure, fireground tactical considerations should include:

  1. Anticipating extension. Since fire may be extending to the entire perimeter of the structure, it is imperative to open these horizontal and vertical channels for examination.
  2. Ventilation. Cornices may have to be opened up to prevent mushrooming of fire and combustion products into cocklofts and attic spaces.
  3. Structural collapse. The failure of gusset plate connectors or loss of support from structural members may induce partial or total structural collapse. Masonry walls that support exterior wood framework may be weakened by an interior fire and collapse. Remember, structural collapse is the second leading cause of fire fighter deaths on the fireground.
  4. Aerial equipment. Since cornices and overhangs are normally at least 1 1/2 to 2 stories above the ground, aerial equipment can be invaluable in providing a stable working platform for opening up and fighting fire. Men ventilating wood truss roofs should be supported independently.

These, of course, are not all-inclusive guidelines but only major considerations.

Gusset plate has prongs that are driven into wood. Failure may come with heating of prongs with resultant charring of wood and loss of friction securing prongs.Esthetic design of this building, above, incorporates combustible concealed spaces that lead to heavy fire damage sustained in building of similar style below.

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