You are building a foundation wall that is 8 feet high and will retain 6 feet of backfill. The wall thickness is 8 inches. Calculate the minimum horizontal reinforcement required per the Michigan Building Code for this concrete masonry unit (CMU) foundation wall.
Correct Answer
A) #4 rebar at 24 inches on center
For CMU foundation walls retaining more than 4 feet of backfill, Michigan Building Code requires #4 rebar horizontally at 24" o.c. maximum.
Why This Is the Correct Answer
#4 rebar at 24 inches on center is the correct minimum for an 8-inch CMU foundation wall retaining more than 4 feet of backfill per the Michigan Building Code (which adopts IRC structural provisions). The 6-foot backfill depth triggers the more stringent spacing requirement, and #4 bar at 24" o.c. satisfies the code's horizontal reinforcement table.
Why the Other Options Are Wrong
Option B: #5 rebar at 24 inches on center
#5 rebar at 24 inches on center uses a larger bar than required. While #5 would provide more strength, the code minimum requires only #4. Specifying oversized rebar unnecessarily increases material cost without code benefit.
Option C: #4 rebar at 32 inches on center
#4 rebar at 32 inches on center uses the correct bar size but insufficient spacing. With 6 feet of backfill (exceeding 4 feet), the code tightens the spacing to 24" o.c. maximum. 32" o.c. spacing is too wide for this backfill depth.
Option D: #5 rebar at 48 inches on center
#5 rebar at 48 inches on center is incorrect on both counts — the bar is oversized and the spacing is far too wide. 48" o.c. horizontal spacing would be inadequate for any significant backfill retention.
Memory Technique
Remember '4 feet of fill = 24 inches apart.' When backfill exceeds 4 feet, space #4 bars at 24" max. The matching number — 4 feet, #4 bar, 24" — makes a pattern: 4-4-24.
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