MILL CONSTRUCTION
Mills fall into a category of construction known as “heavy timber construction” (a term used in the model building codes). For a building to be considered mill construction, certain minimum dimensions and requirements of structural members must be met. These dimensions and requirements are as follows:
Wood columns—minimum eight inches in any dimension.
Wood beams and girders —minimum six inches in any dimension, minimum 10 inches in depth.
Flooring—three-inch tongue-andgroove or splined planks covered with one-inch planks laid crosswise, or fourinch planks on edge covered with oneinch planks laid crosswise.
Roof deck—two-inch matched or splined planks or three-inch planks on edge: beams and girders supporting roof minimum six-inches in any dimension.
Roof trusses—minimum four-by-six inches framing members of truss.
Interior partitions—one-hour fire resistance.
Rearing walls—two-hour fire resistance, noncombustible.
Nonbearing exterior walls—noncombustible.
Fire-resistance ratings for walls/partitions may be different than those specified above, depending on the model building code being used. It is interesting to note that the model building codes allow for substitution of materials such as concrete and steel for certain building elements provided they have been fire tested and meet a certain minimum fire-resistance rating.
The heavy timber elements are unique in that they have been accepted as having “inherent” fire resistance and thus have not been fire tested and listed by testing laboratories.
An important feature of mill construction is the lack of concealed spaces that can trap and funnel fire as well as hide the fire from view (not to mention the blockage of hose streams and automatic sprinkler spray). This is important to consider when mills are being remodeled into apartment buildings or the like.
The term “slow-burning” often has been associated with mill construction, litis description refers to a mill’s relative resistance to ignition. Starting a fire in a mill’s massive wood members is fairly difficult.
Using the term slow-burning, however. is very misleading. Once a fire has taken hold in a mill, it will spread very rapidly. Extreme radiant heat, flying brands, and falling walls are characteristic fire conditions of a fully involved mill.
Besides the use of massive wooden members, mills may contain other firesafety features. While not only found in mills, the following features may be included in them.
- Stairs and elevators placed outside
- of the main building with fire doors between them. (In my observations of mill complex fires, the enclosed stairways almost always remain intact after complete destruction of the rest of the building.)
- Building divided by fire walls and partitions to contain fire in one area.
- Cutoffs and barriers around openings in floors for power and conveyor belts.
- Use of metal window frames instead of wood.
- Use of brick arches instead of wood or stone arches, which may fail during a fire.
- Metal shutters to protect against exposure fires.
- Girders and beams with a “fire cut” to minimize chances of them acting as a lever and collapsing the wall they rest on.
- Chamfered columns to resist fire form taking hold.
- Scuppers (drains) mounted in the exterior wall at floor-level for draining of accumulated water during a fire.
- Sprinklers and standpipes.
- Solid cornices to avoid trapping and funneling fire from one part of a building to another.
Almost invariably, major mill complex fires can be traced to one single problem—closed valves on an automatic sprinkler system. Fire prevention bureaus must ensure that sprinkler systems remain operating at all times. This includes vacant mills, w’hich must be secured and provided with a dry-pipe sprinkler system if necessary (to protect against freezing). To do otherwise is inviting disaster in the form of a massive conflagration.
