In unreinforced masonry wall buildings, the collapse zone distance is equal to how many times the building height?

• A. Three times the building height
• B. Two times the building height
• C. Four times the building height
• D. Five times the building height

Answer: (A)

The key idea is that unreinforced masonry walls can fail in a dramatic, out‑of‑plane collapse, sending large debris outward from the building. Because this debris can travel a distance well beyond the footprint, the safety guideline uses a collapse zone that is a multiple of the building height. For URM structures, the established distance is three times the building height. So the safe collapse zone is about 3H, where H is the building height. This reflects the tendency of these walls to topple and shed debris outward, rather than just fall straight down.

Two times height would understate how far debris can reach, while four or five times would be more conservative than necessary for typical URM behavior. In practice, recognizing the three‑times rule helps responders stay clear of the area where debris could land during a collapse event.