A residential foundation wall in Michigan will support a wood-framed structure. The wall is 8 feet high with 5 feet of unbalanced backfill. What is the minimum wall thickness required for poured concrete construction?
Correct Answer
C) 8 inches
Michigan Building Code requires minimum 8-inch thickness for concrete foundation walls up to 8 feet high with up to 5 feet of unbalanced fill. Greater heights or fill depths require thicker walls.
Why This Is the Correct Answer
Michigan Building Code (based on IRC Table R404.1.1) requires a minimum 8-inch thickness for poured concrete foundation walls when the wall height does not exceed 8 feet and unbalanced backfill does not exceed 5 feet. The 8-inch wall is the code-prescribed minimum for this common residential condition.
Why the Other Options Are Wrong
Option A: 6 inches
A 6-inch poured concrete wall is permitted only for very limited conditions — typically walls no taller than 4 feet with minimal backfill. The 8-foot wall height with 5 feet of fill in this question exceeds the threshold where a 6-inch wall is acceptable.
Option B: 12 inches
12 inches is more than what code requires for this scenario. It would be needed for taller walls (over 8 feet) or greater unbalanced fill depths, or for masonry construction in some configurations. Choosing 12 inches wastes material and cost.
Option D: 10 inches
10 inches is between the code minimum of 8 inches and the over-specified 12 inches. It is not a standard IRC/Michigan code table value for this scenario and does not correspond to a code-required thickness for the given wall height and backfill depth.
Memory Technique
Associate 8 ft wall + 5 ft fill = 8-inch wall. The wall thickness (8 inches) mirrors the wall height (8 feet) in this scenario. When height increases beyond 8 feet or fill exceeds 7 feet, thickness requirements jump.
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What is the maximum allowable span for a 2x8 floor joist (Grade 2 Douglas Fir) at 16" O.C. with 40 psf live load and 10 psf dead load according to Michigan Building Code span tables?
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