Why are secondary collapses less likely in light frame buildings that are under three stories tall?

• A. Lighter weight of construction
• B. Heavier materials
• C. More exits
• D. Longer spans

Answer: (A)

Progressive or secondary collapse is driven by the amount of mass and energy the rest of the structure has to absorb after an initial member fails. In light-frame construction, the overall mass is small, and when the building is under three stories tall the amount of weight above any given level is limited. That means if a floor or wall gives way, the redistributed loads and inertia are relatively small, so adjacent members are less likely to be driven past their strength limits. The structure simply doesn’t have enough weight to push the collapse into the remaining framing, making secondary collapses much less likely. Heavier materials or longer spans would add mass and transfer loads more aggressively, increasing the risk, while things like the number of exits don’t directly change how the structure behaves during an initial failure.