What is the maximum spacing allowed for transverse reinforcement (ties) in concrete columns located in seismic design category D per the CBC?
Correct Answer
D) 4 inches
In seismic design category D, the CBC requires transverse reinforcement spacing not to exceed 4 inches in concrete columns to provide adequate confinement and ductility.
Why This Is the Correct Answer
Per CBC (based on ACI 318 Chapter 18 for seismic provisions), concrete columns in Seismic Design Category D require special moment frame detailing. The maximum transverse reinforcement spacing in the confinement zone (typically the end regions of the column) is the least of: 1/4 of the minimum column dimension, 6 times the longitudinal bar diameter, or 6 inches — but in SDC D, the governing value is often 4 inches for the most critical confinement regions. This tight spacing ensures adequate confinement and ductility to resist seismic loads.
Why the Other Options Are Wrong
Option A: 6 inches
6 inches is a common spacing for non-seismic or lower SDC applications, but SDC D requires closer spacing (4 inches) in the critical confinement zones to develop sufficient ductility under cyclic loading.
Option B: 12 inches
12 inches is a standard tie spacing for non-seismic columns (ACI 318 non-seismic provisions allow up to 16db or 48 tie-bar diameters). SDC D seismic requirements are far more stringent and would never allow 12-inch spacing in a confinement zone.
Option C: 8 inches
8 inches is between the non-seismic and seismic requirements. While used in some moderate seismic categories, it does not meet the SDC D requirement of 4 inches maximum in confinement zones.
Memory Technique
SDC D = 4 inches. Think 'D for Demanding — 4 inches.' Higher seismic category = closer ties = more confinement = more ductility. The letter D and the number 4 pair together: D is the 4th letter of the alphabet.
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