Footing bearing pressure check per ACI 318-19

Footing proportioning (sizing) is a service-level check: the footing must be large enough that the soil pressure under service loads does not exceed the net allowable bearing capacity. ACI 318-19 Sections 13.2.1 and 13.2.2 require that footings be proportioned for unfactored loads and net allowable soil pressure. This page covers the service-level proportioning checks: bearing, kern, overturning, and sliding. Strength design of the footing section (shear, flexure) uses factored loads and is covered on the hub page and other spokes.

Service loads vs factored loads

Footing design is a two-stage process. First, the footing is sized for service-level soil bearing: the base area is chosen so that the soil pressure under unfactored loads stays within the net allowable bearing capacity. Second, the footing section is checked for factored-load strength (shear, flexure, punching). ACI 318-19 §13.2.1 states: "Base area of footing shall be determined from unfactored forces and moments." The soil does not know about load factors; it responds to the actual (service) loads.

Applicable clauses

Net allowable bearing pressure

The net allowable pressure deducts the weight of the soil overburden and the footing self-weight from the gross allowable:

• q_a = gross allowable bearing capacity from the geotechnical report (ksf)
• γ_soil = soil unit weight in kcf (typically 0.120 kcf)
• D_f = depth of footing below grade (ft)
• γ_c = concrete unit weight = 0.150 kcf
• h = footing thickness (in); divided by 12 to convert to ft

The soil already supports its own weight and the footing weight before the column load arrives. Only the remaining capacity is available for the column load. ACI 318 does not specify q_a; it comes from the geotechnical investigation (R13.2.2) or IBC Table 1806.2 for presumptive values.

Bearing pressure under service loads

For concentric loading (no moment):

For eccentric or biaxial loading (resultant within kern):

• S_L = L · B² / 6 (section modulus about L-axis)
• S_B = B · L² / 6 (section modulus about B-axis)
• All pressures in ksf; moments in kip-ft (converted from kip-in by dividing by 12)

Check: q_max ≤ q_net. When the maximum bearing pressure exceeds the net allowable, the footing is too small and must be resized.

Kern check (middle third)

The kern is the middle third of the footing base. For full-contact bearing (no uplift), the eccentricity of the resultant must stay within:

where e_L and e_B are eccentricities in inches. When the resultant falls outside the kern, part of the base lifts off and the effective bearing area shrinks. calcnote Phase 1 requires full-contact bearing; partial-contact analysis is not yet implemented.

Overturning stability

The footing must resist overturning about any edge:

• M_resist = (P_service + W_footing + W_soil) · dim_ft / 2
• M_overturn = M_service / 12 + V · (h / 12)
• dim_ft = footing dimension perpendicular to the overturning axis (ft)

Checked in each direction whenever the overturning moment in that direction is positive. A non-zero lateral shear V alone can trigger the check (V contributes to M_overturn through its lever arm h/12 even when M_service = 0). The governing (lowest) factor of safety controls.

Sliding stability

The footing must resist horizontal sliding from lateral loads:

where μ is the soil-concrete friction coefficient (typically 0.3 to 0.5). Passive pressure on the footing sides is conservatively neglected.

Wall footings

Wall footings are analyzed per 1-ft strip: P is in kip/ft, M in kip-in/ft. The same bearing, kern, overturning, and sliding checks apply, with L = 1 ft implicitly.

Worked example: concentric isolated footing

Step 1: Net allowable pressure

• γ_soil · D_f = 0.120 × 2.0 = 0.240 ksf
• γ_c · h/12 = 0.150 × 18/12 = 0.225 ksf
• q_net = 4.0 − 0.240 − 0.225 = 3.535 ksf

Step 2: Bearing pressure

• q = 100 / (6 × 6) = 2.778 ksf

Step 3: Check

• q = 2.778 ksf ≤ q_net = 3.535 ksf → OK
• No eccentricity: kern check passes trivially
• No lateral load: overturning and sliding checks not applicable

In practice, the engineer sizes the footing so that q ≤ q_net. A 6×6 footing here uses 79% of the net capacity. If moments or lateral loads are present, eccentricity increases q_max and the kern, overturning, and sliding checks become active.

Frequently asked questions

Sources

• ACI 318-19: §13.2.1 (unfactored forces), §13.2.2 (net allowable from geotech), R13.2.2 (q_a from geotechnical investigation or IBC), §13.3.1.2 (minimum depth).