In lightweight steel truss construction, the usual cause of collapse during a fire is

• A. The amount of heat generated by the fire in a structure.
• B. Weight of water.
• C. Impact load of firefighters jumping onto roof.
• D. Cooling effects of water.

Answer: (A)

When lightweight steel truss roofs are exposed to fire, heat is the main factor that drives collapse. As temperature rises, steel loses strength and stiffness, and the connections between members can fail. Because these trusses are slender and rely on a small number of fasteners, this weakening progresses quickly, causing sagging and progressive failure of the roof structure. Water applied to the fire can help by cooling the steel, and the added weight of water is not the typical cause of collapse. The idea of firefighters jumping onto the roof or the cooling effects of water are not the primary mechanisms behind failure, whereas the heat-induced loss of strength is.