Which type of construction is most prone to collapse early in a fire due to lightweight components?

• A. Heavy timber
• B. Concrete masonry
• C. Steel frame with brick veneer
• D. Lightweight or combustible wooden/steel truss construction

Answer: (D)

The main idea here is that lightweight or combustible wooden/steel truss construction loses its load-carrying ability much faster in a fire than heavier, noncombustible systems. These truss systems use slender members that heat up and fail quickly. Steel components expand and soften with heat, and the connections can fail as the metal rises in temperature. Wood members are fuel, so they burn and lose strength rapidly, often with little warning, especially where there are many voids and concealed spaces for the flame to spread. The combination of rapid heating, weakened connections, and burning fuel leads to a sudden loss of stability and a higher likelihood of early collapse.

In contrast, heavy timber builds up a protective char layer that insulates interior members and slows heat penetration, helping the structure retain strength longer. Concrete masonry is noncombustible and resists fire spread, while a steel frame with brick veneer has substantial mass that provides more resistance before failure. But among these, lightweight or combustible wooden/steel truss construction has the least reserve against fire, making it most prone to early collapse.