Every structure transfers its weight to the ground through foundations. The footing is the structural element that spreads the concentrated column or wall load over enough soil area to keep the bearing pressure within the soil's allowable capacity. If the bearing pressure exceeds the soil capacity, the footing settles, potentially unevenly, causing structural distress, cracking, and in severe cases, failure.
This guide covers isolated spread footings (the most common type for columns) and continuous strip footings (for walls), with bearing pressure calculations per ACI 318-19 Chapter 13 and allowable soil bearing values from IBC Table 1806.2.
Bearing Pressure: Concentric and Eccentric Loading
For a concentrically loaded footing (load applied at the centroid of the footing), the bearing pressure is uniform across the base:
q = P / A
Where P = total vertical load (including footing self-weight and any overburden) and A = footing area (L × B for rectangular, πD²/4 for circular).
When the load is eccentric (applied off-center, or when a moment is present), the bearing pressure distribution becomes trapezoidal or triangular:
q_max = P/A × (1 + 6e/B) [when e ≤ B/6]\nq_min = P/A × (1 − 6e/B)
Where e = eccentricity = M/P (moment divided by axial load) and B = footing width in the direction of eccentricity.
When eccentricity exceeds B/6 (the "kern" of the footing), the minimum bearing pressure goes negative, meaning tension would be required. Since soil cannot resist tension, part of the footing base loses contact and the pressure distribution becomes triangular with a concentrated peak:
q_max = 2P / [3 × (B/2 − e) × L]
This partial bearing condition is undesirable because the peak pressure can be very high and the footing behavior becomes less predictable. Design should aim to keep eccentricity within the kern (e ≤ B/6).
Footing Bearing Pressure Calculator
Check bearing pressure for spread footings with eccentric loading per ACI 318 and IBC Table 1806.2. One-way shear, two-way punching shear, and flexural reinforcement checks included.
Allowable Soil Bearing Capacity
The allowable bearing capacity comes from one of two sources:
1. Geotechnical investigation (soil report): A geotechnical engineer tests the actual soil at the site and provides an allowable bearing pressure based on soil type, density, moisture content, and depth of footing. This is the most accurate and is required for all significant structures.
2. IBC Table 1806.2 (presumptive values): For smaller structures where a site-specific geotechnical investigation is not required, the IBC provides presumptive load-bearing values:
| Soil Type | Allowable Bearing (psf) |
|---|---|
| Crystalline bedrock | 12,000 |
| Sedimentary rock | 4,000 |
| Sandy gravel / gravel (GW, GP) | 3,000 |
| Sand, silty sand, clayey sand (SW, SP, SM, SC) | 2,000 |
| Clay, silty clay, sandy clay (CL, ML, MH, CH) | 1,500 |
These presumptive values are conservative for well-compacted soils at typical depths but may be unconservative for loose fills, expansive clays, or soils with high water tables. When in doubt, get a soil report.
Footing Bearing Pressure Calculator
Check bearing pressure for spread footings with eccentric loading per ACI 318 and IBC Table 1806.2. One-way shear, two-way punching shear, and flexural reinforcement checks included.
Footing Thickness: One-Way and Two-Way Shear
The footing must be thick enough to resist shear failure. Two shear checks are required:
One-way (beam) shear: Checked at a distance d (effective depth) from the face of the column or wall. The critical section acts like a wide, shallow beam:
Vu ≤ φVc = φ × 2√f'c × b × d
Where b = footing width and d = effective depth (footing thickness minus cover minus half the bar diameter). φ = 0.75 for shear.
Two-way (punching) shear: Checked on a perimeter at d/2 from the face of the column. This is typically the governing check for isolated column footings:
Vu ≤ φVc = φ × 4√f'c × bo × d
Where bo = perimeter of the critical section = 4 × (column width + d) for a square column on a square footing.
If either shear check fails, increase the footing thickness. Adding shear reinforcement to footings is uncommon and impractical, it is almost always cheaper and simpler to pour a thicker footing.
Footing Bearing Pressure Calculator
Check bearing pressure for spread footings with eccentric loading per ACI 318 and IBC Table 1806.2. One-way shear, two-way punching shear, and flexural reinforcement checks included.
Bottom Reinforcement: Flexural Design
The footing acts as a cantilever from the face of the column. The soil bearing pressure pushes upward, and the weight of the footing and soil act downward. The net upward pressure creates a bending moment in the footing overhang that requires bottom reinforcement (tension on the bottom face).
The critical section for flexure is at the face of the column (for concrete columns), at the face of the baseplate (for steel columns), or halfway between the middle and edge of the wall (for masonry walls).
The required reinforcement area is determined by:
Mu = q_net × L_cantilever² / 2\nAs = Mu / (φ × fy × (d − a/2))
Where a = As × fy / (0.85 × f'c × b) is the depth of the equivalent stress block. This is iterative (a depends on As, which depends on a), but converges quickly.
Minimum reinforcement for footings: As_min = 0.0018 × b × h for Grade 60 steel (ACI 318-19 §7.6.1.1 for temperature and shrinkage). This minimum often governs for lightly loaded footings.
Footing Bearing Pressure Calculator
Check bearing pressure for spread footings with eccentric loading per ACI 318 and IBC Table 1806.2. One-way shear, two-way punching shear, and flexural reinforcement checks included.
Footing Bearing Pressure Calculator
Check bearing pressure for spread footings with eccentric loading per ACI 318 and IBC Table 1806.2. One-way shear, two-way punching shear, and flexural reinforcement checks included.