Residential Building Inspector B1Footings and FoundationsHard
A geotechnical report for a residential construction site indicates that the soil has a presumptive load-bearing value of 1,500 psf (72 kPa). The proposed continuous footing will support a wall with a design load of 1,000 pounds per linear foot (14.6 kN/m). What is the minimum required width of this footing, according to the 2021 IRC?
- A12 inches (305 mm)
- B10 inches (254 mm)
- C6 inches (152 mm)
- D8 inches (203 mm)
Show answer & explanationAnswer & explanation
Correct answer: D. 8 inches (203 mm)
To calculate the minimum footing width, divide the design load per linear foot by the presumptive load-bearing value. 1,000 psf / 1,500 psf = 0.6667 feet. Convert to inches: 0.6667 feet * 12 inches/foot = 8 inches. The IRC requires the footing width to be at least the wall thickness, but the calculation governs here.
Why the other options are wrong
- A. This width is significantly larger than the minimum calculated requirement.
- B. While acceptable, this width is greater than the calculated minimum required.
- C. This width is insufficient; it would result in a bearing pressure exceeding the soil's capacity.
Footing Width Calculation
Determining the minimum width of a footing to safely distribute the structural load onto the soil without exceeding the soil's allowable bearing capacity.
- Width = (Wall Load per Linear Foot) / (Soil Bearing Capacity).
- IRC R403.1.3 specifies presumptive load-bearing values.
- Footing must also meet minimum thickness and projection requirements.
Memory trick: Load on the soil, width must align; bearing capacity, a critical sign.