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Design foundation footings with precision. Calculate required footing area, dimensions, concrete volume, and rebar requirements based on structural loads and soil bearing capacity.
Typical range: 1500-4000 psf
Typical: 2.5-3.0
Where SBC = Soil Bearing Capacity, SF = Safety Factor
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Soil bearing capacity varies significantly by soil type. Always have soil tested by a geotechnical engineer.
| Soil Type | Bearing Capacity (psf) | Notes |
|---|---|---|
| Clay (hard, dry) | 3000-4000 | Good bearing capacity, compressible |
| Clay (stiff) | 1500-3000 | Adequate for most structures |
| Clay (soft) | 500-1000 | Poor bearing, specialized design needed |
| Silt | 1200-2000 | Medium bearing capacity |
| Sand (dense) | 2000-4000 | Good bearing, minimal settlement |
| Sand (medium) | 1200-2000 | Acceptable for most structures |
| Sand (loose) | 500-1200 | Requires deeper footings |
| Gravel/Rock | 4000+ | Excellent bearing capacity |
Isolated square pads under columns or posts. Most economical for point loads.
Pads with different length and width. Used when loads are non-symmetric.
Long strip footings supporting walls or lines of posts. Most common foundation type.
Important: All designs must comply with local building codes and IBC standards. Soil testing and structural engineer review are required for commercial or critical applications.
Soil bearing capacity must be determined by a geotechnical engineer through soil testing. The engineer performs boreholes, lab analysis, and provides bearing capacity recommendations specific to your site. Never assume bearing capacity without professional testing.
The frost line is the maximum depth soil freezes in winter. Footings must extend below this depth to prevent frost heave (upward movement from freezing soil). Frost depths range from 6 inches in warm climates to 60+ inches in northern regions. Check local codes for your area.
Residential wall footings are typically 16-24 inches wide and 8-12 inches deep. Deck post footings are often 16×16 inches to 24×24 inches depending on load. These are general guidelines; specific sizes depend on load, soil capacity, and local codes.
Most building codes require rebar in footings. Continuous wall footings typically use two horizontal #4 bars running the length. Pier footings use #4 bars in a grid pattern (typically 12" on center). The exact pattern depends on footing size and local codes.
Working Stress Design (WSD) uses an allowable bearing capacity reduced by a safety factor. Ultimate/Limit State Design uses higher loads with reduced material capacity. Most modern codes use a combination approach. This calculator uses working stress design methodology.
Settlement occurs when footing loads compress soil. Differential settlement (uneven settlement) causes structural damage. Proper footing design minimizes settlement through adequate bearing capacity and depth. Soft soils may require deep footings, piles, or special reinforcement.
For residential construction on good soil, prescriptive code tables may be acceptable. For any other situation—commercial buildings, poor soil, heavy loads, or non-standard conditions—a structural engineer must design footings. Many jurisdictions require engineer certification regardless.
Sloped ground requires stepped footings or adjusted designs. Footings on slopes may need larger bearing areas, deeper embedment, or special anchoring. A structural engineer must design footings for sloped sites to ensure stability and prevent sliding.
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