According to California Building Code requirements for special moment-resisting frames in high seismic areas, what is the maximum story drift limit for structures assigned to Seismic Design Category D?
Correct Answer
C) 2.5% of story height
CBC Section 1620.3.2 limits story drift to 2.5% of story height for special moment-resisting frame systems in Seismic Design Category D. This stringent limit helps prevent structural damage and collapse during major seismic events common in California, ensuring life safety and property protection.
Why This Is the Correct Answer
CBC Section 1620.3.2 (referencing ASCE 7 provisions) sets a 2.5% story drift limit (0.025 × story height) for special moment-resisting frame systems in Seismic Design Category D. This limit is specifically more permissive for SMRFs because these ductile systems are engineered to deform considerably while maintaining load-carrying capacity, absorbing seismic energy through controlled yielding.
Why the Other Options Are Wrong
Option A: 1.5% of story height
1.5% is too restrictive for SMRFs in SDC D. More rigid systems or certain Risk Category IV structures have tighter drift limits, but a 1.5% limit would unnecessarily constrain SMRF performance without safety benefit.
Option B: 2.0% of story height
2.0% (0.020) is the correct limit for standard Risk Category II structures under general SDC D rules, not specifically for special moment-resisting frame systems. Confusing these two limits is a common error.
Option D: 3.0% of story height
3.0% exceeds the code-permitted maximum for SDC D. No standard structural system in SDC D is permitted to drift this much under the CBC/ASCE 7 framework without triggering additional design requirements.
Memory Technique
SMRFs are specially designed to FLEX — they get an extra half percent (2.0% + 0.5% = 2.5%) compared to standard structures because their ductility earns them room to move.
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