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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I guarantee that computer-aided precision engineering did not contribute to this structural evolution. The San Diego Fire Museum today stands not as designed and not as originally built; from a strategic perspective, it exists "as is." As built, the building was unreinforced masonry conventional; because of the slender steel rods; today, this building exists as an unreinforced masonry lightweight structure (strategically). If you have an "as is" building like this in your community, it should be deemed a "no-go, red-light" building. Exposed to the heat generated by a contents fire, the performance of the steel rods will be much different than the performance of the original load-bearing masonry wall or the performance of the timber columns.
Visualize the fireground progression: During a fire, heat will rise to the ceiling, the unprotected steel rods are at the ceiling; the steel rods will fail when heated to 800 degrees Fahrenheit; should the steel rods fail, gravity will send the upper floor, furniture, masonry bearing wall and roof into the first-floor apparatus bay as compression. (Note: The steel rods will fail when the rods themselves are heated to 800°F, not when the ambient temperature reaches 800°F.)
Firefighters should not be allowed in, on or near this building if there is evidence of heat generated by a contents fire. Remember, what you see through the windshield does not count as size-up; even if the smoke looks incipient and appears to not be a threat to firefighters, the no-value back-side of the fire-growth curve can look and feel exactly like the early front-side of the fire-growth curve, where there can be value. (See "Grading the Fireground on a Curve," Firehouse®, September 2008.) A master craftsman fire officer is not seduced into an offensive position by light smoke showing on arrival; a master craftsman fire officer is not seduced by zero visibility and low heat. Before offensive entry, a master craftsman fire officer determines which side of the fire-growth curve is being considered and whether there is value.
There is another REALLY BIG red flag to look for within conventional, unreinforced masonry (URM) buildings — in particular those buildings erected 100 or more years ago, the classic "taxpayers." During a pre-plan visit, one of the first things to look for is the replacement of an unpenetrated load-bearing masonry wall with unprotected steel columns. The load-bearing walls of an unreinforced masonry taxpayer always run front to back — from the street to the alley (from side A to side C). It was common for the original ground-floor commercial occupancy to expand left and/or right (toward side B and toward side D). The gravity-resistance challenge was what to do about the unpenetrated, load-bearing masonry walls that were in the way; remove this bearing wall and the roof and floors above end up at street level (or in the basement).
To prevent catastrophic failure when the bearing walls are removed, they were replaced with a timber or steel girder and the whole thing supported by unprotected steel or timber columns. During a significant fire, the performance of these unprotected columns will be much different than the performance of the original, unpenetrated, load-bearing masonry wall. I've seen buildings where the failure of a single column would cause the collapse of the entire building!
To exacerbate the problem, further modification of older unreinforced masonry buildings was intended to remove the columns. Column replacement was achieved by transferring the load of the roof and floors above with an inverted king post made of cold-drawn steel rods (as in the San Diego Fire Museum).
Type V Wood Frame
At the bottom of the strategic classification food-chain is Type V, Wood Frame. Of the five types of building construction, Wood Frame is the only type that allows all load-bearing structural members — studs, joists, rafters, purlins, girders — to contribute fuel to a fire. Building codes (unreassuredly) refer to Wood Frame as "unprotected combustible." In other words, all load-bearing structural members can burn. That's the bad news. The good news is gypsum Sheetrock. (Note: Because Type III and Type IV buildings also feature "unprotected combustible" load-bearing structural components, I prefer to call a Type V building "Wood Frame.") "Type X" fire-rated Sheetrock contains gypsum and is used to protect all this combustible load-bearing fireload and prevent the passage of fire to adjacent areas. (Note: Sheetrock protects the combustible load-bearing system from one direction — from the occupied side).