(Appeared in print at "Information Overload")
The fire service never will be able to take out all the risk out of structural firefighting. Instead, the fire service can expect smaller, more localized failures to continue during rescue and fire attack that will result in near misses, as well as line-of-duty injuries and deaths.
But while it is difficult to determine hazardous features and conditions at night and in smoke, competent firefighters and officers can look at the buildings that exist in their first-due areas before a fire occurs.
Building construction knowledge begins with knowing that all buildings carry and transmit loads (weights or forces) to the ground through load paths. Recall that load transfer redistributes loads and forces to various supports that increases load stress. Collapse occurs when the load capacity is exceeded and structural support can no longer carry the load for which it was designed.
The acronym FEWLOADPATHS+3Us describes several of the more typical hazardous features and conditions that create bad building behaviors at structure fires.
F: Facades and false mansards. Facades or false fronts are the non-structural coverings on the face of a building. Brick, stone and concrete are typically placed on structural frames that may be dislodged by hose streams/master streams and fail. Facades, cornices, canopies and overhangs have failed and injured firefighters, often later during fireground operations. Identify these hazardous features, communicate them, establish collapse zones and observe for failure clues like bending, bowing, sagging or pulling away from the wall.
False mansards are a roof-like structure typically attached with lightweight fasteners (nails, gusset plates) to the face of buildings for better appearance. They often are used to face strip malls and fast-food restaurants. False mansards create horizontal concealed void spaces that let heat, smoke and fire travel through and then fail. Consider penetrating nozzles and working off aerial and/or ground ladders to access those spaces.
E: Engineered wood components and extension. Engineered wood components refer to prefabricated lightweight wood components used for floor- and roof-support systems, such as I-joists and parallel chord trusses. Other engineered wood products utilizing laminated veneer lumber are also used for beams (headers), columns, studs and rim boards.
I-joists, commonly referred to as wood I-beams or TJIs, are used for floor and roof joists. While able to support greater loads under non-fire conditions, I-joists do not have the mass of solid-sawn lumber. Top and bottom flanges are joined by a veneered material for the web. Proper knock-out holes exist for utilities and are located in the middle third of the web. I-joists are easily weakened by improper hole size and location, notching and by damage to the bottom chord. It is important to note that injuries and LODDs have occurred in roof and floor failures of both lightweight wood I-joists as well as solid sawn lumber. Be cautious if fire has reached the bays of floor or roof support systems. Maintain time and situational awareness in all combustible framed structures.
Parallel chord wood trusses are comprised of chords, webs and connector plates and are designed to work together. There are several different kinds of parallel chord wood trusses based on the arrangement of its webs. In short, the top chord acts in compression and the bottom chord in tension. The web members control the shear and will be either in compression or tension. Connector plates are typically 16- to 20-gauge toothed, hot galvanized steel that is hydraulically embedded in the wood of the truss. In residential construction a floor truss most often bears on the underside of its bottom chord. The most common spacing for residential construction is 24 inches and typical truss depths are 14 inches and 16 inches.
These are increasingly used in place of solid sawn lumber as they have the ability to span greater distances, are lighter, more dimensionally stable and able to be installed quickly. Expect engineered wood components in most new construction as well as in renovations and alterations of older buildings. Don’t be surprised to find I-joists in balloon-framed Victorians when a new foundation or floor system is being installed. Unprotected engineered wood components have been noted to fail earlier than dimensional lumber under actual fire conditions in the as-built structure. Identification of buildings with lightweight, engineered wood components may be difficult at a fire. That is why prefire analysis and building walkthroughs and documentation are essential. The fire service can anticipate earlier failure of combustible, prefabricated, unprotected, lightweight engineered beams like parallel chord trusses and I-joists.
Extension refers to the progression of fire and the products of combustion into uninvolved areas. Professional firefighters anticipate fire extension into combustible concealed spaces and voids, particularly into large, undivided attic spaces common in large structures like apartment complexes.
W: Warping and wracking. Warping refers to a variation from straightness often relating to lumber. A warped horizontal structural element like a wood or steel beam indicates a change in load path and subsequent load transfer, which may result in significant stress increases in certain supports. Wracking is when a component, such as a wall, is forced out of plumb. Altered load paths and improper load transfers may result in failure.