A contractor is installing a concrete foundation on volcanic ash soil with 15% moisture content. The soil bearing capacity is 2,000 psf. For a total building load of 180,000 lbs, what minimum footing area is required?
Correct Answer
A) 90 square feet
180,000 lbs ÷ 2,000 psf = 90 square feet minimum footing area required.
Why This Is the Correct Answer
The formula is: Required Area = Total Load ÷ Soil Bearing Capacity. Substituting: 180,000 lbs ÷ 2,000 psf = 90 square feet. The volcanic ash soil detail and moisture content are contextual — the bearing capacity given (2,000 psf) already accounts for soil conditions.
Why the Other Options Are Wrong
Option B: 120 square feet
120 sq ft would correspond to a bearing capacity of 1,500 psf (180,000 ÷ 1,500). This is the wrong denominator — the problem states 2,000 psf.
Option C: 108 square feet
108 sq ft does not result from any straightforward application of the given values. It may stem from misapplying a safety factor or rounding incorrectly.
Option D: 72 square feet
72 sq ft would require a bearing capacity of 2,500 psf (180,000 ÷ 2,500), which is higher than stated. Choosing this answer means ignoring the given bearing capacity.
Memory Technique
Remember the formula as 'LOAD over LAND': Required area = Load ÷ Land (bearing capacity). Keep it simple: 180,000 ÷ 2,000 = 90.
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