According to CBC, what is the maximum spacing allowed for transverse reinforcement (ties) in concrete columns within potential plastic hinge regions in seismic zones?
Correct Answer
B) 4 inches or 4db
CBC requires closely spaced transverse reinforcement in potential plastic hinge regions, with maximum spacing of 4 inches or 4 times the longitudinal bar diameter (4db), whichever is smaller.
Why This Is the Correct Answer
ACI 318 (adopted by CBC) requires confinement reinforcement in potential plastic hinge zones of concrete columns to be spaced no more than the smallest of: (a) one-quarter of the minimum column dimension, (b) 6 times the longitudinal bar diameter (6db) for some cases, or (c) 4 inches (and no more than 4db) in the most critical seismic detailing. For special moment frames and high-seismic zones, CBC requires the tighter 4-inch or 4db limit. This close spacing provides confinement that prevents core concrete from spalling and enables ductile post-yield behavior during earthquakes.
Why the Other Options Are Wrong
Option A: 6 inches or 6db
6 inches or 6db is a less restrictive spacing requirement applicable to intermediate moment frames or regions outside the plastic hinge zone. Using this spacing in a potential plastic hinge region in SDC D or E would provide insufficient confinement and violate the special moment frame requirements.
Option C: 8 inches or 8db
8 inches or 8db is far too loose for plastic hinge confinement. This spacing is closer to standard gravity column tie spacing and provides negligible confinement for seismic ductility demands. Columns with 8-inch tie spacing in plastic hinge zones are known to fail in compression during major earthquakes.
Option D: 3 inches or 3db
3 inches or 3db is more restrictive than the code minimum and is not a standard codified requirement. While providing more confinement is not harmful structurally, the question asks for the maximum allowed spacing — the code limit is 4 inches/4db, not 3 inches/3db.
Memory Technique
Plastic hinge = '4 and 4.' Four inches maximum, four bar diameters maximum — the tighter of the two controls. Think: hinge zones are the most critical seismic locations, so they get the tightest ties (4), while less critical zones get 6.
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