Commercial Building Inspector B2Structural SystemsEasy
A building inspector is reviewing the structural plans for a new single-story commercial building with a relatively small footprint. The plans indicate the use of isolated spread footings. According to the 2021 IBC, what is the minimum required depth below grade for the bottom of isolated spread footings, assuming no frost penetration considerations?
- A24 inches (610 mm)
- B12 inches (305 mm)
- C18 inches (457 mm)
- D6 inches (152 mm)
Show answer & explanationAnswer & explanation
Correct answer: B. 12 inches (305 mm)
The 2021 IBC, Section 1809.4, states that the minimum depth of footings below the undisturbed ground surface shall be 12 inches (305 mm). This general minimum depth is required even when frost penetration is not a factor, to ensure adequate bearing capacity, stability, and protection against erosion or shallow soil disturbance.
Why the other options are wrong
- A. 24 inches is often specified for larger structures or deeper frost lines, but exceeds the general minimum.
- C. 18 inches is a common depth, but not the *minimum* required by code in the absence of frost.
- D. 6 inches is generally too shallow and not permitted as a minimum depth for spread footings.
Minimum Footing Depth (IBC)
The minimum depth below grade required for the bottom of isolated spread footings, as specified by the IBC, to ensure stability and bearing capacity.
- Applies even without frost considerations.
- Ensures adequate soil cover and stability.
- 12 inches (305 mm) minimum per IBC 1809.4.
Memory trick: Footings need at least a foot in the ground to stay sound.