Understanding Foundation Design
Foundation design requires careful consideration of loads and soil mechanics. Here are the critical factors:
- Load Total structural load combines dead load (structure: 40-80 psf/floor), live load (40-100 psf residential, 50-150 psf commercial), and environmental loads (snow: 20-40 psf, wind: varies by region).
- Soil Bearing capacity ranges: Dense sand (6,000-8,000 psf), Stiff clay (4,000-6,000 psf), Soft clay (1,500-3,000 psf). Factor of safety typically 3.0 for residential, 4.0 for commercial.
- Depth Minimum depth determined by frost line + 6". Increase depth for poor soils or high loads. Typical range: 3-6 ft residential, 4-8 ft commercial.
- Area Foundation area = Total Load / Allowable Bearing Pressure. Add 10-15% for eccentricity. Width typically 16-24" for walls, 3-6x column width for isolated footings.
Design Factors
Capacity
Ultimate bearing capacity = cNc + γDNq + 0.5γBNγ. Apply safety factor for allowable capacity. Check both bearing and structural capacity.
Settlement
Maximum allowed: 1" total, 0.75" differential. Calculate immediate settlement using elastic theory, long-term using consolidation theory.
Stability
Check overturning (FSot > 1.5), sliding (FSsl > 1.5), and global stability. Consider lateral earth pressure and surcharge loads.
Safety
Design per ACI 318: φ=0.65 for bearing, φ=0.75 for shear. Include load factors: 1.2D + 1.6L + 0.5(Lr or S or R).