The framing plan notes a design load of 60 pounds per square foot carried over a tributary width of 14 feet at this wall, the footing has been formed 16 inches wide, and the soils information for the lot rates the ground at 1,500 pounds per square foot. What bearing pressure does this footing deliver?
- A840 pounds per square foot
This is the line load on the wall in pounds per lineal foot, not the bearing pressure. It is the right intermediate number reported one step too early, and it treats the footing as though it were 12 inches wide.
- 630 pounds per square foot
- C1,260 pounds per square foot
This divides the line load by 8 inches rather than 16, which doubles the answer. It usually comes from using a half width or from mixing up the footing width with the wall thickness.
- D420 pounds per square foot
This divides by 24 inches instead of 16. Using a footing width the plans do not show understates the pressure and can hide a genuinely overloaded footing.
Why B is correct
Sixty pounds per square foot over a 14-foot tributary width gives 840 pounds per lineal foot. Dividing by the 16-inch footing width, which is 1.333 feet, gives 630 pounds per square foot, comfortably within the 1,500-pound presumptive value.
What this question is testing
Whether the candidate can convert an area load and tributary width into a line load, then into a bearing pressure using the correct footing width in consistent units, and compare the result against the allowable value.
On the job
Bearing pressure checks come up whenever a house has a long interior span, a heavy point load, or soil weaker than the plans assumed. The tributary width is the piece most often botched: it is half the span on each side of the wall, not the full span. Inspectors who can run this in the field can decide on the spot whether a soil surprise in the trench matters or not, which saves a trip and a stop-work.
Memory technique
Pounds per foot, then pounds per square foot.
Exam tip
Two steps: psf times tributary width gives plf, then divide by the footing width in feet.
Where to look it up
IRC Chapter 4, foundations, footings and the presumptive load-bearing value table, with loads from the approved plans.