Isolated footing design
A footing is sized twice for two different reasons. The plan area comes from service load and allowable soil bearing — a geotechnical limit. The thickness comes from factored load and shear — a structural limit, and it is almost always punching shear that sets it. This iterates the depth until both checks pass, then designs the steel.
| Check | Demand | Capacity |
|---|---|---|
| Service load | 160.0 kip | — |
| Factored load | 216.0 kip | 1.2D + 1.6L |
| Footing self weight | 9.8 kip | 1.08 ft thick |
| Gross bearing | 2.83 ksf | 3.00 ksf allowable |
| Net pressure, qu | 3.60 ksf | factored load over area |
| One-way shear, Vu | 63.9 kip | 88.2 kip |
| Punching shear, Vu | 196.4 kip | 212.5 kip |
| Flexural moment | 136.1 kip·ft | at the column face |
| Steel required | 3.12 in² | 2.18 in² minimum |
| Steel provided | 3.12 in² | #5 @ 10 in |
| Column bearing | 216.0 kip | 1432.1 kip |
Punching shear almost always sets the thickness. It is checked at d/2 from the column face on a perimeter of 4(c + d), and it falls off much faster with thickness than one-way shear does, so the two cross over as the load rises.
A = (P_service + W) / qa · Geotechnical limit.
Vu = qu · B · (B/2 − c/2 − d) · φVc = 0.75 · 2√f’c · b · d.
Vu = Pu − qu(c + d)² · φVc = 0.75 · 4√f’c · bo · d.
Mu = qu · B · ((B − c)/2)² / 2 · Floored by 0.0018 b h.
A clear path from inputs to results.
A footing is sized twice for two different reasons. The plan area comes from service load and allowable soil bearing — a geotechnical limit. The thickness comes from factored load and shear — a structural limit, and it is almost always punching shear that sets it.
How to use
- Enter the column dead and live loads.
- Set the column size and the allowable soil bearing.
- Choose the concrete and steel strengths.
- Read the footing size, thickness, governing shear and steel callout.
The formula
Reference: ACI 318-19 for one-way shear (§22.5), two-way shear (§22.6), flexure and the shrinkage minimum; allowable bearing is a geotechnical input.
Worked example
160 kip on 3 ksf soil
- 100 kip dead and 60 kip live on an 18-inch square column.
- 3 ksf allowable bearing.
- 4,000 psi concrete, Grade 60 steel.
- 1
The service load is 160 kip, so the required area is 176/3 = 58.7 ft² and the footing rounds to 7.75 ft square.
- 2
The factored load is 1.2 × 100 + 1.6 × 60 = 216 kip, giving qu = 3.60 ksf.
- 3
The depth is increased until both shear checks pass; punching governs at about 20 inches.
- 4
Punching demand is Pu less the pressure inside the critical perimeter, against 0.75 × 4√f’c × bo × d.
- 5
Flexural steel at the column face is floored by the 0.0018 shrinkage minimum.
7.75 ft square · about 23 in thick · punching governs.
Interpreting the result
Punching shear almost always sets the thickness, and it falls off much faster with depth than one-way shear does, so the two cross over as the load rises. That is why a heavily loaded footing gets thicker rather than wider once the soil is fixed.
Assumptions & limitations
- A square, concentrically loaded footing on uniform soil.
- Three inches of cover, cast against earth.
- The punching capacity uses the simplified 4√f’c expression.
- Uplift, moment, sliding and frost depth are not modelled.
Frequently asked questions
Why is the footing sized on service load but checked on factored load?
Because bearing is a geotechnical limit expressed at working load, while shear and flexure are structural limits at ultimate. Mixing the two is the most common error.
How thick should the footing be?
Whatever makes both shear checks pass — usually 12 to 24 inches for ordinary loads. There is no point making it thicker than the shear requires.
Why 3 inches of cover?
Because the footing is cast directly against earth, so §20.5.1.3 requires 3 inches rather than the 2 inches used for formed surfaces.
What if the column is rectangular?
The critical perimeter becomes 2(c1 + d) + 2(c2 + d) and the β reduction in §22.6.5.2 starts to matter. This model assumes square.
Do I need top steel?
Not for a concentrically loaded footing on soil; the moment is at the bottom face. Top steel appears when there is uplift or a large moment.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.