ACI 318-19

ACI 318-19 Chapter 13 | Shallow Foundation Design

Spread footings transfer structural column loads into supporting soil strata. Design requires satisfying two independent criteria: geotechnical soil bearing capacity without excessive settlement, and structural reinforced concrete strength against punching and flexure.

Code Equation

Governing Formula

q_{\text{ max }} = \frac{P}{B \cdot L} \left( 1 \pm \frac{6e}{L} \right) \le q_{\text{ allow }}, \quad V_u \le \phi V_c

Standard: ACI 318-19Dual US Customary & SI Metric

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Notation & Variables

q_{\text{ max }}

Maximum factored soil bearing contact pressure

ksf or kPa

P

Total axial column load applied to footing

kips or kN

B

Footing width perpendicular to eccentric axis

ft or m

L

Footing length along eccentric axis

ft or m

e

Load eccentricity (e = M / P)

in or mm

q_{\text{ allow }}

Allowable soil bearing capacity from geotechnical report

ksf or kPa

Worked Example

Determine required footing area

For an axial service load of P = 200 kips and allowable soil bearing capacity qallow = 4.0 ksf: Areq = 200 / 4.0 = 50 sq ft.

Select plan dimensions

Select an 8.0 ft x 8.0 ft square footing (Area = 64 sq ft). Service bearing pressure q = 200 / 64 = 3.125 ksf < 4.0 ksf (PASS).

Check punching shear at d/2

Verify that factored column load minus upward soil pressure within the critical perimeter b0 does not exceed phi * Vc.

Governing Criteria

When does this govern?

Always governs base column reactions and determines the required plan dimensions of all building foundation layouts.

Common Pitfall

Common Mistake

Using factored (ultimate) loads to size footing plan dimensions instead of unfactored service loads, leading to over-designed footing footprints.

Related Resources

Notation & Variables

q_{\text{ max }}

Maximum factored soil bearing contact pressure

ksf or kPa

P

Total axial column load applied to footing

kips or kN

B

Footing width perpendicular to eccentric axis

ft or m

L

Footing length along eccentric axis

ft or m

e

Load eccentricity (e = M / P)

in or mm

q_{\text{ allow }}

Allowable soil bearing capacity from geotechnical report

ksf or kPa

Worked Example

Determine required footing area

For an axial service load of P = 200 kips and allowable soil bearing capacity qallow = 4.0 ksf: Areq = 200 / 4.0 = 50 sq ft.

Select plan dimensions

Select an 8.0 ft x 8.0 ft square footing (Area = 64 sq ft). Service bearing pressure q = 200 / 64 = 3.125 ksf < 4.0 ksf (PASS).

Check punching shear at d/2

Verify that factored column load minus upward soil pressure within the critical perimeter b0 does not exceed phi * Vc.