Under CBC provisions, what is the primary difference between 'ordinary' and 'special' moment frames in seismic design?
Correct Answer
C) Detailing and ductility requirements
Special moment frames require enhanced detailing for ductility, including special reinforcement patterns, closer stirrup spacing, and specific connection details to ensure ductile behavior during earthquakes.
Why This Is the Correct Answer
Detailing and ductility requirements are the primary difference. Special moment frames (SMF) require enhanced reinforcement detailing — tighter stirrup/hoop spacing, specific connection geometry, and confinement reinforcement — to ensure ductile behavior and energy dissipation during seismic events. Ordinary moment frames (OMF) use standard detailing with lower ductility capacity.
Why the Other Options Are Wrong
Option A: Foundation depth requirements
Foundation depth requirements are not the distinguishing factor between ordinary and special moment frames. Both frame types can sit on similar foundations; the distinction is in the frame members themselves.
Option B: Fire resistance ratings
Fire resistance ratings are governed by occupancy and construction type, not by the seismic moment frame classification. Special frames have no different fire rating than ordinary frames.
Option D: Material strength requirements
Material strength requirements (e.g., concrete compressive strength, steel yield strength) may be similar between ordinary and special frames. The key distinction is not material strength but rather how the material is detailed and confined to achieve ductility.
Memory Technique
Special = Special Detailing. The word 'special' in structural engineering always means extra care in detailing for seismic ductility. Think: 'Special forces get special training' — SMFs get special detailing to survive earthquakes.
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