Which factor most contributes to the collapse of lightweight steel trusses during a fire?

Which factor most contributes to the collapse of lightweight steel trusses during a fire?

• A. The amount of heat generated by the fire
• B. Water weight
• C. Wind loading
• D. Structural steel cross-section

Answer: (A)

Heat input determines how hot the steel gets, and elevated temperature drastically weakens steel. Lightweight trusses use thin members that absorb heat quickly, so their temperature rises rapidly during a fire. As steel heats, its strength and stiffness fall, so it can’t carry the same loads as before. With less capacity, the truss members deform more, connections loosen, and a progressive failure follows, leading to collapse. The other factors—water weight, wind loading, or the cross-sectional size of the steel—don’t drive the failure as directly; their effects are either secondary or change as a result of the steel’s weakened state. The key idea is that the amount of heat generated by the fire governs how much the steel’s strength is reduced, which in turn drives collapse.

Heat input determines how hot the steel gets, and elevated temperature drastically weakens steel. Lightweight trusses use thin members that absorb heat quickly, so their temperature rises rapidly during a fire. As steel heats, its strength and stiffness fall, so it can’t carry the same loads as before. With less capacity, the truss members deform more, connections loosen, and a progressive failure follows, leading to collapse. The other factors—water weight, wind loading, or the cross-sectional size of the steel—don’t drive the failure as directly; their effects are either secondary or change as a result of the steel’s weakened state. The key idea is that the amount of heat generated by the fire governs how much the steel’s strength is reduced, which in turn drives collapse.