What is the average time to collapse of lightweight wood trusses when exposed to fire?

What is the average time to collapse of lightweight wood trusses when exposed to fire?

• A. 3-5 minutes
• B. 5-7 minutes
• C. 7-9 minutes
• D. 10-12 minutes

Answer: (B)

Lightweight wood trusses fail quickly in a fire because their slender members and connector plates heat up fast, losing strength and stiffness as the wood chars and the metal connections weaken. The load path that keeps the roof supported shortens rapidly, so the structure collapses after a short period of flame exposure. Across typical conditions, the most common average collapse time is about five to seven minutes. Times shorter than this can occur under harsher loading or less protection, while longer times usually reflect protective measures or lighter loads, but the mid-range five-to-seven minutes best captures the typical scenario.

Lightweight wood trusses fail quickly in a fire because their slender members and connector plates heat up fast, losing strength and stiffness as the wood chars and the metal connections weaken. The load path that keeps the roof supported shortens rapidly, so the structure collapses after a short period of flame exposure. Across typical conditions, the most common average collapse time is about five to seven minutes. Times shorter than this can occur under harsher loading or less protection, while longer times usually reflect protective measures or lighter loads, but the mid-range five-to-seven minutes best captures the typical scenario.