In California seismic design, what is the maximum allowable story drift for structures with Risk Category II occupancy?
Correct Answer
D) 0.020 times the story height
CBC Section 1617.3 and ASCE 7 Table 12.12-1 specify that Risk Category II structures (standard occupancy buildings) have a maximum allowable story drift of 0.020 times the story height. This limit helps ensure structural integrity and occupant safety during seismic events in California's high seismic zones.
Why This Is the Correct Answer
ASCE 7 Table 12.12-1 (adopted by CBC) sets the maximum allowable story drift for Risk Category II structures at 0.020 times the story height (2.0%). Risk Category II covers standard occupancy buildings (offices, residential, retail) — the most common building type. The 2.0% limit balances structural performance with practical design economy.
Why the Other Options Are Wrong
Option A: 0.015 times the story height
0.015 (1.5%) is the drift limit for Risk Category IV structures (essential facilities like hospitals and emergency operations centers) where stricter performance is mandated. Applying it to Risk Category II is overly conservative.
Option B: 0.030 times the story height
0.030 (3.0%) exceeds the maximum permitted drift for any standard occupancy category. No Risk Category in ASCE 7 Table 12.12-1 permits a 3.0% drift limit for typical structures.
Option C: 0.025 times the story height
0.025 (2.5%) applies to certain structural systems like special moment-resisting frames, not to Risk Category II occupancy in general. Conflating system-based limits with occupancy-based limits is a common error.
Memory Technique
Risk Category II = 2.0% = the 'normal' building gets the '2' value. Risk Category IV (critical facilities) gets the tightest limit (1.5%) — they need to stay open after a quake.
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