One rectangular pad footing measures 24 inches by 30 inches and carries a column load the approved plans give as 7,000 pounds, and the presumptive load-bearing value being used for the site is 1,500 pounds per square foot. Judging the bearing pressure alone, what should the inspector conclude?
- The pad is adequate, because 5 square feet of bearing area holds the pressure to 1,400 pounds per square foot.
- BThe pad is inadequate, because 4 square feet of bearing area produces 1,750 pounds per square foot.
Squaring the short dimension throws away the extra 6 inches of length and understates the bearing area by a full square foot.
- CThe pad is inadequate, because 7,000 pounds spread across the 2-foot width works out to 3,500 pounds per square foot.
Dividing a total load by one width gives a load per lineal foot, not a pressure, so it cannot be compared with the soil value.
- DThe pad is adequate, because a pad at least 24 inches in each direction may be used with any soil listed in the presumptive table.
No minimum pad size in the code substitutes for a bearing check against the soil value in use.
Why A is correct
Area equals 2.0 feet times 2.5 feet, or 5.0 square feet. Pressure equals 7,000 pounds divided by 5.0 square feet, or 1,400 pounds per square foot, which is below the 1,500 presumptive value. The pad satisfies the bearing check and the inspector can release it.
What this question is testing
Whether the candidate computes area from two different dimensions instead of assuming a square, and keeps the distinction between pressure in pounds per square foot and load in pounds per lineal foot.
On the job
Interior pad footings are often poured before anyone knows what column lands on them, and it is common to find the same size pad used everywhere on a plan that has three different column loads. The check takes fifteen seconds with a tape and the load from the plan. Where the pad is short, the fix before the slab and columns go in is simple: pour a larger pad or add one. After the girder is loaded it is a jacking and underpinning job.
Memory technique
Pressure is load over area, and area needs both sides of the rectangle.
Exam tip
Rectangular pads are worth a second look; the exam expects you to multiply, not to square one side.
Where to look it up
IRC Chapter 4, the footing provisions at R403.1 together with the presumptive load-bearing value table in R401.4.