Steel will support what percent of the designed load and can fail at any time after flashover?

• A. 60%
• B. 40%
• C. 80%
• D. 100%

Answer: (B)

Structural steel loses a lot of its strength when exposed to fire. As heat climbs, the material’s yield strength and stiffness drop dramatically, so the member can carry far less load than it was designed to handle. After flashover, steel is typically able to support only about 40% of its designed load. Because the remaining strength is quite small and heat can cause rapid, uneven expansion, a member can fail suddenly at any time, even if it looks intact. This is why steel-driven collapses in fires can occur with little warning once flashover has happened. The other percentages would imply more remaining capacity than is typically present under fire conditions, which is why 40% is the correct concept to remember.