Which statement correctly describes the relationship of the collapse zone distance to building height for URM walls?

Which statement correctly describes the relationship of the collapse zone distance to building height for URM walls?

• A. It equals two times the height
• B. It equals four times the height
• C. It equals three times the height
• D. It is independent of height

Answer: (C)

The main idea here is that for unreinforced masonry (URM) walls, the area that must be kept clear in the event of a collapse grows directly with the wall height. URM is brittle and can fail suddenly, sending debris outward and downward as the wall and any parapets or upper portions give way. Because heavier debris can travel farther and the build-up of momentum during collapse increases with height, the standard guidance sets the collapse zone at three times the wall height. This keeps rescuers and bystanders outside the range where falling bricks and rubble could reach them. Two times the height would not provide enough clearance for the debris field typical of URM failure, while four times would be more conservative than necessary for most URM scenarios. Saying the zone is independent of height ignores the reality that taller walls generate larger collapse zones, so the three-times-height rule captures that proportional relationship.

The main idea here is that for unreinforced masonry (URM) walls, the area that must be kept clear in the event of a collapse grows directly with the wall height. URM is brittle and can fail suddenly, sending debris outward and downward as the wall and any parapets or upper portions give way. Because heavier debris can travel farther and the build-up of momentum during collapse increases with height, the standard guidance sets the collapse zone at three times the wall height. This keeps rescuers and bystanders outside the range where falling bricks and rubble could reach them.

Two times the height would not provide enough clearance for the debris field typical of URM failure, while four times would be more conservative than necessary for most URM scenarios. Saying the zone is independent of height ignores the reality that taller walls generate larger collapse zones, so the three-times-height rule captures that proportional relationship.