For a built-up roof system in California seismic zone, what is the maximum allowable aggregate surfacing weight per square foot according to CBC requirements?
Correct Answer
D) 400 pounds
California Building Code Section 1511.4.4 limits aggregate surfacing to 400 pounds per square foot in seismic design categories D, E, and F. This limitation prevents excessive roof loads that could compromise structural integrity during seismic events common in California.
Why This Is the Correct Answer
CBC Section 1511.4.4 sets the aggregate surfacing limit at 400 pounds per square foot for seismic design categories D, E, and F. This cap prevents excessive dead load on the roof structure, which in a seismic event gets amplified by ground acceleration and can cause catastrophic structural overload.
Why the Other Options Are Wrong
Option A: 600 pounds
600 pounds per square foot exceeds the CBC limit and would represent a hazardous dead load in a high-seismic zone. Heavier aggregate increases the inertial forces on the structure during an earthquake, raising the risk of roof collapse.
Option B: 300 pounds
300 pounds per square foot is below the actual 400 lb/sq ft limit and is not the threshold specified in CBC Section 1511.4.4. Choosing this would lead a contractor to unnecessarily restrict their design without code basis.
Option C: 500 pounds
500 pounds per square foot is between the limit and a higher unsafe value. It is not a recognized CBC threshold; the actual maximum is 400 lb/sq ft for seismic categories D, E, and F.
Memory Technique
Remember '4 for seismic safety': California has 4 seismic design categories of concern (D, E, F — but just remember the number 4), and the limit is 400 pounds. Four hundred — four — seismic. Alternatively, imagine a '400 lb cap' like a weight limit sign at a seismic fault.
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