According to CBC Chapter 23, what is the maximum spacing for #4 rebar in a concrete retaining wall that is 8 feet high in a seismic zone?
Correct Answer
C) 18 inches on center
CBC limits reinforcement spacing to the lesser of 3 times the wall thickness or 18 inches for retaining walls in seismic zones to ensure adequate reinforcement distribution.
Why This Is the Correct Answer
CBC Chapter 23 limits reinforcement spacing in retaining walls to the lesser of three times the wall thickness or 18 inches on center in seismic zones. The 18-inch maximum governs when it is the more restrictive of the two criteria, ensuring adequate distribution of reinforcement to resist both lateral earth pressure and seismic forces.
Why the Other Options Are Wrong
Option A: 12 inches on center
12 inches on center is more conservative than required. While 12-inch spacing would be structurally adequate and might be specified in high-demand applications, the CBC maximum spacing for this scenario is 18 inches. Using 12 inches exceeds the code requirement but is not what the code mandates as the maximum.
Option B: 24 inches on center
24 inches on center exceeds the 18-inch maximum established by CBC Chapter 23 for retaining walls in seismic zones. Using 24-inch spacing would violate the code requirement and result in insufficient reinforcement to resist seismic and earth pressure loading.
Option D: 16 inches on center
16 inches on center is within the allowable range (less than 18 inches) and would be structurally acceptable, but it is not the maximum spacing. The question asks for the maximum, which is 18 inches — selecting 16 inches shows imprecise knowledge of the code limit.
Memory Technique
Remember '18 inches = 1.5 feet' for seismic retaining walls. Think of a ruler: 18 inches is the span of a standard ruler and a half. That is the maximum rebar spacing CBC allows for seismic zone retaining walls under Chapter 23.
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