Footings and Foundations
Footings and foundations
Footings spread building loads into the soil. Questions usually test whether you understand that size follows load and soil, and that depth follows frost and disturbance-not a single magic number.
Concepts
- Soil bearing capacity: presumptive values may be used for ordinary soils; weaker or fill soils need evaluation.
- Footing width is selected from load and soil capacity; thickness, projection, and reinforcement have minimums.
- Footings bear on undisturbed soil or compacted, approved fill, and extend below the frost line.
- Stepped footings and sloped sites need level bearing and proper step height and length.
- Foundation walls resist soil and surcharge; anchorage connects sill plates and floor framing to the foundation.
- Drainage matters: footing drains, grading away from the wall, dampproofing or waterproofing, and backfill placement.
- Termite and decay protection begins at the foundation.
Traps
- Measuring footing depth from finished grade rather than from undisturbed bearing soil.
- Assuming wider is always acceptable when the real issue is eccentricity, reinforcement, or soil.
- Mixing up dampproofing (resists soil moisture) with waterproofing (resists hydrostatic pressure).
- Forgetting that backfill must not be placed before the foundation inspection.
Worked example A footing trench is dug to 30 inches, but 10 inches of loose fill sits at the bottom, leaving only 20 inches of bearing in undisturbed soil above a 24-inch frost depth. The footing fails: depth is measured to competent bearing, and the loose material must be removed or properly compacted and approved.
Rule of thumb Bear on good soil, below frost, with the footing projection not exceeding its thickness unless designed otherwise.