A contractor discovers soft clay soil conditions during excavation for a single-family residence. The soil bearing capacity is 1,500 psf. What is the minimum footing width required for a load of 3,000 pounds per linear foot?
Correct Answer
C) 24 inches
Required width = Load ÷ Bearing capacity = 3,000 ÷ 1,500 = 2 feet = 24 inches minimum width needed.
Why This Is the Correct Answer
The footing width formula is: Width = Load per linear foot ÷ Allowable soil bearing capacity. Substituting: 3,000 lbs/ft ÷ 1,500 psf = 2.0 feet = 24 inches. This ensures the footing distributes the structural load over a sufficient soil area without exceeding the soil's bearing capacity.
Why the Other Options Are Wrong
Option A: 30 inches
30 inches (2.5 feet) would require a soil bearing area of 2.5 ft × 1,500 psf = 3,750 lbs/ft capacity, which exceeds the actual load of 3,000 lbs/ft. While it is safe, it is not the minimum required and indicates an incorrect calculation or an added safety margin not asked for.
Option B: 36 inches
36 inches (3.0 feet) similarly exceeds the minimum and would be conservative over-design. A 36-inch footing on 1,500 psf soil could carry 4,500 lbs/ft — 50% more than the actual load. The question asks for the minimum, not a comfortable design width.
Option D: 18 inches
18 inches (1.5 feet) would provide only 1.5 ft × 1,500 psf = 2,250 lbs/ft of bearing capacity, which is insufficient for the 3,000 lbs/ft load. This would cause soil overstress, settlement, and potential structural failure.
Memory Technique
The formula is Load ÷ Capacity = Width. Remember it as 'what you need divided by what the ground gives you equals how wide to spread it.' 3,000 ÷ 1,500 = 2 feet = 24 inches. Always convert to inches for the answer.
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