Which time range is associated with catastrophic failure in conventional and legacy construction?

• A. 5-7 minutes
• B. 15-20 minutes
• C. 25-30 minutes
• D. 30-35 minutes

Answer: (B)

Understanding how a fire affects load paths in older, conventional buildings helps explain why a 15–20 minute window is the typical moment for catastrophic failure in this type of construction. In conventional and legacy structures, there isn’t the amount of fireproofing or advanced fire-resistant detailing found in newer systems. As a fire progresses, heat penetrates quickly into steel members and wooden elements, softening connections and weakening the ability of beams, columns, and floors to carry loads. By roughly the 15–20 minute mark, critical members have lost enough strength and stiffness that the primary load paths can no longer be maintained, leading to a sudden, progressive collapse. Early in the fire, you might see localized damage or exposure effects, and fires that burn longer can be mitigated or slowed in modern, protected systems, but for unprotected or minimally protected conventional construction, catastrophic failure is most closely associated with that 15–20 minute range.