For steel trusses, __________ degrees F is the critical temperature

• A. 800
• B. 900
• C. 1,000
• D. 1,100

Answer: (C)

Steel loses strength and stiffness as it heats, and engineers use a practical benchmark called the critical temperature to gauge when a member can no longer safely carry loads in a fire. For typical structural carbon steel used in trusses, that point is about 1000 °F (roughly 538 °C). Reaching this temperature means the yield strength and overall stiffness have declined enough that the member’s load-carrying capacity is severely reduced, increasing the risk of excessive deflection or collapse unless protective measures are in place. Lower temperatures like 800 °F or 900 °F are below this threshold, where capacity remains relatively high, while a temperature around 1100 °F indicates even more severe degradation beyond the critical point. So, 1000 °F is the commonly accepted critical temperature for steel trusses.