In California, what is the maximum allowable story drift for a building assigned to Seismic Design Category D when subjected to design earthquake forces?
Correct Answer
C) 0.020hsx
CBC Section 1617.3 and ASCE 7 Table 12.12-1 specify that buildings in Seismic Design Category D have a maximum allowable story drift of 0.020hsx, where hsx is the story height. This stricter drift limitation for higher seismic design categories helps ensure structural integrity during seismic events common in California.
Why This Is the Correct Answer
CBC Section 1617.3 and ASCE 7 Table 12.12-1 set the maximum allowable story drift at 0.020hsx for structures in Seismic Design Category D, where hsx is the height of the story being evaluated. This 2% drift limit prevents excessive deformation that could damage non-structural components, cause pounding between buildings, and compromise life safety during major seismic events.
Why the Other Options Are Wrong
Option A: 0.025hsx
0.025hsx is the drift limit for Seismic Design Categories A, B, and C (lower seismic risk), not SDC D. Using this less restrictive value in SDC D would be unconservative and violate the code requirement for higher-risk seismic zones.
Option B: 0.030hsx
0.030hsx is more lenient than what's permitted in SDC D. No standard building occupancy in SDC D is allowed a 3% drift ratio under ASCE 7 — this value does not correspond to any standard SDC D limit.
Option D: 0.015hsx
0.015hsx is more restrictive than required for SDC D standard occupancies. This stricter limit may apply to certain critical facilities or specific structural systems, but it is not the standard limit for SDC D buildings.
Memory Technique
Think '2% for D' — SDC D allows a maximum story drift of 0.020hsx (2%). The letter D is associated with '2' — 2nd-to-last letter tier (D, E), and 2% is the limit. Higher danger = lower drift allowed.
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