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Many renovated buildings replace wood and masonry structural elements with steel. The replacement of heavy wooden roof and floor joists with the lighter steel open-web bar-truss joists is a common renovation change. A lightweight steel open-web bar-truss joist, similar to the floors of the World Trade Center and the roof of the Charleston, SC, sofa store, is a long steel bar (web) bent at 90-degree angles and welded to angle irons (chords) at the top and bottom of the bar bends. Classified as type II non-combustible construction, it is used in new commercial buildings. However, it is also used to replace old wooden joists in renovated ordinary-type, brick-and-joist buildings.
Buildings of non-combustible construction have masonry or steel non-bearing enclosure walls and steel column and girder framework, and use lightweight steel open-web bar-truss joists as roof and floor supports. Non-combustible construction using steel in place of wood for interior structural framework is well suited for large-area commercial buildings. Long-span steel truss beams create large open spaces.
The lightweight steel bar joist must be viewed by firefighters as extremely hazardous in roof and floor construction for two reasons: the failure characteristics of unprotected steel and the joist spacing. The Handbook of Fire Protection, published by the National Fire Protection Association (NFPA), states: "When unprotected, the steel bar joist may collapse after five or 10 minutes of fire exposure." Advancing a hoseline inside or operating on a roof of a burning building constructed with steel bar-joist trusses is a high risk. A rapid collapse may occur.
The average response time of an urban fire company is five minutes; a suburban or rural fire company is somewhat longer. If a fire occurring in an unoccupied building with a roof supported by unprotected lightweight steel bar joists or C-beams is so severe that roof venting is required to assist advancement of the first hoseline, then the roof must be considered too unstable to send firefighters there for ventilation and interior attack with a hoseline. Fire chiefs should instead consider horizontal ventilation and defensive firefighting attack after all occupants have been removed.
Some one-story, non-combustible buildings using lightweight steel joists are constructed with large rectangular windows at the upper portion of the masonry or corrugated steel enclosure walls. These windows are effective for smoke venting. Located at the top portion of the walls, where heated smoke would accumulate, these windows have a horizontal length that is greater than the vertical depth of the window area. Ventilating several of these windows can be accomplished faster and more safely than making a four-by-four-foot roof cut. Roof ventilation has been a very effective and relatively safe fire department operation when carried out on a wood-joist roof support system spaced 16 to 24 inches on center.
Roof construction that varies from this standard design is more dangerous. When a different roof construction, such as that using lightweight steel bar joists or C-beams, is introduced into a community in which standard wood construction has predominated, the different collapse characteristics present a new safety hazard to firefighters. A steel bar joist and C-beam will collapse more quickly. An experienced firefighter who has advanced hoselines into burning ordinary-constructed buildings or cut vent openings in a roof of solid-wood joists cannot transfer his or her sense of safe operating time to a building with a lightweight steel roof. Firefighters have developed a sense of how long they can work inside a burning building or on a roof over a fire, based on the type of roof construction used in ordinary construction, not noncombustible construction with steel trusses.