How long will metal trusses withstand direct flame impingement before failure?

How long will metal trusses withstand direct flame impingement before failure?

• A. 5 minutes
• B. 10 minutes
• C. 15 minutes
• D. 20 minutes

Answer: (B)

When metal trusses are exposed to direct flame, their temperature rises rapidly and their strength and stiffness drop as heat increases. Unprotected steel loses yield strength and eventually can’t carry the loads above it, so failure can occur once heating reaches a point where the remaining capacity is exceeded. Under typical direct flame impingement, this loss of strength and subsequent collapse tendency is seen around ten minutes, making ten minutes the best-known estimate for time to failure in this scenario. Five minutes would usually be too short for many fully involved-fire conditions, while fifteen or twenty minutes would imply better insulation, lighter loading, or less aggressive heat exposure than what direct flame impingement represents.

When metal trusses are exposed to direct flame, their temperature rises rapidly and their strength and stiffness drop as heat increases. Unprotected steel loses yield strength and eventually can’t carry the loads above it, so failure can occur once heating reaches a point where the remaining capacity is exceeded. Under typical direct flame impingement, this loss of strength and subsequent collapse tendency is seen around ten minutes, making ten minutes the best-known estimate for time to failure in this scenario.

Five minutes would usually be too short for many fully involved-fire conditions, while fifteen or twenty minutes would imply better insulation, lighter loading, or less aggressive heat exposure than what direct flame impingement represents.