Past installments of this series introduced the strategic classification of building construction. Ordered strategically, based on perceived fire resistance, the five basic types of building construction were listed as follows: Type I — Fire Resistive Type II — Non-Combustible Type IV...
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How many carpenters and Sheetrock installers know the primary purpose for taping and mudding Type X sheetrock seams? Sure, doing so makes the wall smooth and easy to finish, but the primary reason sheetrock is taped and mudded is to protect the steel. Sheetrock is attached using metal screws or nails. Like all steel, when they are heated, the screws and nails will elongate. When the screws and nails elongate, the ability of the Sheetrock to resist the passage of fire is compromised. The primary purpose of the tape and mud is to protect the steel fasteners that are an integral part of the fire-resistive assembly! Subordinate to that is a smooth finish.
There are two basic methods of wood framing a building: legacy balloon frame or contemporary platform frame. Balloon-frame walls were assembled on the ground and then lifted into place. If you needed a 30-foot two-by-four-inch stud, you simply "scabbed together" (nailed) two or three shorter two-by-four-inch boards. Although conventional vertical balloon framing is no longer permitted, there is an abundance of lightweight horizontal balloon framing. Because the material of a conventional beam has been removed along the I-beam web (or stem), a void is created when two wood I-joists intersect at a 90-degree angle; when solid conventional joists intersect there is no such horizontal void. As designed, this void is not supposed to remain; you will often find them as is. We don't fight fire in an as designed world.
Worse yet is the parallel-chord truss floor system. Parallel-chord truss floors create a large combustible void that did not exist in a conventionally joisted floor. Because oxygenated air is the governing component of fire growth, fire will grow exponentially faster in a trussed floor/ceiling assembly than in a conventionally joisted floor/ceiling assembly. A conventionally joisted floor is compartmentalized (16 inches wide by the length and depth of the floor joists). The open-web truss floor is not compartmentalized; whatever the cubic feet of the trussed floor is how many cubic feet of oxygenated air there is available to support rapid fire growth (once fire penetrates the ceiling and enters the void).
In addition to the large void with plenty of air to support fire growth, the fuel is arranged like kindling for rapid fire growth. Along with plenty of oxygen and plenty of fuel, there are exponential connections where each web member intersects with a top and bottom chord. Whether lightweight or conventional, connections are the weakest point in any structural system.
Platform framing is the assembly of a vertical succession of single-story structures. A single-story building is erected at grade level. To serve as the second floor, another single-story building is erected atop the platform below, and so on; once you've gone as high as you need (or are allowed), a roof is assembled atop the stack of one-story buildings. Thus, we have the term "platform" frame.
There are many contemporary buildings that are strategic hybrids. A good example is the contemporary "taxpayer." During the past 10 or 15 years, there has been a revival of the mixed-use, "Main Street U.S.A." taxpayer configuration. Although similar in occupancy use, the construction is neither traditional nor conventional. The strategic classification of these hybrids is "five-over-two," which means that the first floor is non-combustible concrete and the upper floors are lightweight wood (platform) frame. If the upper floors are framed using steel (studs, etc.), the building would be classified as Type II. Like the legacy taxpayers of 100 years ago, these hybrids feature commercial occupancies at street level with residential occupancies above. Although three floors are common, the vertical limit seems to be four or five floors.
Another hybrid that is becoming more common is the "Two-in-Five" structure. This is often seen in Type V, platform frame, multi-family buildings. The floor joists, typically trusses or I-joists, are supported by a steel I-beam girder and the steel I-beam is usually supported by steel columns. Thus, within the Type V building are Type II components. The purpose of the "Two-in-Five" hybrid is to take advantage of the tremendous strength and dimensional stability of structural steel. This strength allows a long, simple beam span that cannot be achieved with dimensional lumber. Yet another hybrid is the "Five-in-Two" structure, which is found in concrete tilt-up warehouses that feature unprotected steel columns, girders and purlins. The combustible panelized roof deck is comprised of plywood supported by dimensional lumber joists. This building would be strategically classified as Type II, Non-Combustible.