Understanding Footings
Footings are critical structural elements that transfer building loads to the soil. Design must comply with ACI 318 requirements:
- Types Isolated footings (columns): typically square, 2-4x column width. Strip footings (walls): width = load/(bearing capacity), minimum 12" wider than wall.
- Dimensions Minimum thickness: greater of 6" or L/12 for plain concrete, L/10 for reinforced. L = footing projection beyond column/wall face.
- Soil Pressure Design for uniform pressure distribution. Maximum pressure ≤ allowable bearing capacity. Include 15% additional area for moment transfer.
- Reinforcement Main bars: As = Mu/(φfy×0.9d). Minimum steel ratio: 0.0018 for Grade 60, 0.0020 for Grade 40. Development length ≥ 12".
Design Considerations
Depth
Minimum: Local frost depth + 6". Typical range: 24-48". Check two-way shear at d/2 from column face (punching).
Drainage
Install perimeter drain 6" below footing. Use clean gravel (¾") backfill. Slope grade 1:20 min away from foundation.
Cover
Minimum concrete cover: 3" for soil contact, 2" for exposed surfaces. Use 4000 psi min concrete for freeze-thaw exposure.
Soil
Compact subgrade to 95% Standard Proctor. Place on undisturbed soil or engineered fill. Verify bearing capacity with plate load test.