Which construction type is especially vulnerable to early collapse 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)

Lightweight or combustible wooden/steel truss construction is especially prone to early collapse in a fire because the members are slender and rely on a few load paths that heat up quickly. When exposed to flames, these trusses lose strength and stiffness fast, and connections can fail, snapping the roof or floor system’s support and causing the structure to give way sooner than heavier, more mass-dominated construction. Fire can also burn through concealed areas between members, hiding the progression of damage until the collapse happens. In comparison, heavy timber has more mass and a protective char layer that helps preserve strength longer, concrete masonry walls resist heat well, and steel frames with brick veneer can maintain integrity with proper fire protection, making them less susceptible to rapid, unexpected collapse.