Mark Emery discusses why it's time for the contemporary fire service to adapt strategically and tactically. Why It's Time For the Contemporary Fire Service to Adapt Strategically and Tactically Characteristics of the structural fireground began to evolve 50 or 60 years ago. Collectively...
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- Few connections - Structural systems fail at connections. Consider a simple chair. The pieces of the chair are connected by welds or screws. If the chair were to suddenly fail, it is unlikely to be the consequence of one of the legs buckling. It is much more likely that the cause would be failure of a connection.
If you've ever meandered through a heavy-timber building, you were likely impressed by the gigantic structural members. Unless you are a student of building construction, however, you probably did not notice the connections. One reason you may not have noticed is that there are so few connections. A huge heavy-timber girder typically has just two connections, one at each supported end of the girder. Like the chair, it is unlikely that a heavy-timber girder will fail in the middle of the girder; failure of a heavy-timber girder will likely be the consequence of a failed connection or compressive support member. (Failure of a connection will release the girder, failure of the girder will release the floor and so on.) Likewise, an unreinforced masonry floor joist is unlikely to fail in the middle of the joist; it is much more likely that the joist will fail where it inserts into a masonry wall or rests on a ledger or corbel.
In addition, these conventional connections were usually substantial. Often, a timber purlin or girder was sandwiched between metal plates that were anchored with a bolt that extended all the way through the wood and then the bolted assembly is secured with a nut. Because of these rigid connections, many of timber assemblies provide near-monolithic behavior. The heavy-timber "self-releasing floor" is designed to eliminate girder-to-column monolithic behavior.
- Low-Btu fire load - Fire load (the stuff that burns) on the traditional fireground consisted of natural materials such as wood, cotton, wool and paper. If you could afford wall-to-wall carpeting in 1940, it was made of wool. If you had wallpaper, it was, in fact, paper. Sofa cushions were stuffed with cotton batting, sometimes combined with horsehair and other natural materials. The heat release potential of these natural materials averaged around 8,000 Btu per pound of stuff. For example, one-pound of wood has the potential to release around 7,500 Btu per pound. (The denser the wood, the greater the Btu potential.) In addition to relatively low Btu output, the heat was released slowly, cooperating with the gradual incline of the (now traditional) "time temperature curve."
Generally speaking, the traditional fireground featured low Btu and progressive release of heat. This gradual release of heat meant that flashover was less frequent on the traditional fireground because it took more time for a room full of stuff to heat, off-gas, and ignite.
- More time before failure - Because a structural component such as a heavy-timber column or girder contained more material (mass) than was necessary to support intended loading, and because there were fewer and more substantial connections holding all the pieces together, and because of gradual Btu output, fire departments had more time to operate before structural failure. An accidental benefit of these factors was the undesigned fire resistance mentioned earlier. (Fire resistance is a measure of how long a structural component or assembly will resist the effects of fire and continue to perform its intended function - such as resisting gravity.) Bottom line: when exposed to fire, a large-dimension timber will resist the assault of fire longer than an open-web, two-by-four-inch, metal-plate-connected truss.
- Threshold of pain - Now in his mid-80s, my dad was a career firefighter from 1950 to 1977. During a conversation we had around 15 years ago, he recalled that how far firefighters of his day would advance into a fire building was almost entirely dependent on their threshold of pain. In other words, because they lacked the sophisticated encapsulation ensemble of contemporary firefighters, advancing deep into a fire area was not possible because it hurt.
The contemporary fireground (here, now, all around you) features two key strategic factors: lightweight construction and 16,000-Btu fire loads.