Catastrophic failure typically occurs in conventional and legacy construction in how many minutes?

• A. 15-20 minutes
• B. 5-7 minutes
• C. 20-25 minutes
• D. 25-30 minutes

Answer: (A)

In conventional and legacy construction, catastrophic failure tends to occur about 15 to 20 minutes after a fire starts. The reason is that these buildings often lack modern fireproofing, redundancy, and compartmentation. As the fire grows, unprotected steel members lose strength rapidly and wooden members burn away from the inside out, reducing the structure’s load-carrying capacity. Once heat and damage reach critical members, the structure can no longer sustain the loads, leading to a sudden, progressive collapse within this time window. This isn’t exact for every building, but 15–20 minutes is the typical ballpark used to gauge when interior operations become extremely risky in older construction. Modern Fire safety measures can extend this window, while heavier or differently protected systems may shift it.