ACI 318 Concrete Footing Design Calculator
ACI 318 concrete footing design calculator for square or rectangular footings. Compute required plan size, service and factored bearing pressures, two-way (punching) and one-way shear checks, minimum thickness, and reinforcement per ACI 318-19.
Footing inputs
Enter loads, soil bearing, and geometry to validate punching/one-way shear and flexure.
Design summary
Note: φv = 0.75 (shear), φb = 0.9 (flexure), z ≈ 0.9d.
How to Use This Calculator
Enter your service and factored loads, soil bearing, geometry, and material strengths. Units convert automatically at the top. Click Calculate to refresh the results panel.
- Service axial load with allowable bearing defines the required plan area: Areq = Pservice / qa.
- Factored axial load Pu is used to check punching, one-way shear, and flexural steel per ACI 318-19.
- Column dimensions, cover, and bar size determine the shear perimeter and effective depth (d).
- Use auto thickness for preliminary sizing; switch to manual when you need to test a specific depth.
Methodology
The calculator follows ACI Committee 318-19. Service sizing uses allowable soil pressure, while shear and moment checks rely on factored loads and strength reduction factors. The tool maintains unit consistency as you toggle between US and SI.
Operiamo con dati di ACI 318-19 e traduciamo ogni formula in modo coerente. Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti dalla fonte ufficiale.
Glossary of variables
- Pservice: Service axial load at footing level (kips or kN).
- Pu: Factored axial load per governing load combination.
- qa: Allowable soil bearing pressure (net).
- B, L: Footing plan dimensions along x and y.
- c1, c2: Column side dimensions along B and L.
- h: Overall footing thickness.
- d: Effective depth to tension steel centroid.
- f′c: Concrete compressive strength.
- fy: Steel yield strength.
- bo: Critical punching perimeter at d/2 from the column face.
- Vu: Factored shear demand (punching or one-way).
- φVc: Design shear capacity after φ reduction.
- As: Required steel area per foot/meter.
Step-by-step example
Given: US units, square footing; Pservice = 200 kips, qa = 4 ksf, Pu = 300 kips; column 24 × 24 in; f′c = 4000 psi, fy = 60 ksi; cover = 3 in; bars #5.
- Area: Areq = 200 / 4 = 50.0 sf. Square size B = L = √50 ≈ 7.07 ft ≈ 85 in.
- Factored pressure: qu = 300 / 50.17 ≈ 5.98 ksf (using 85 in sides → 50.17 sf).
- Punching: assume h ≈ 18 in → d ≈ 14.69 in. bo ≈ 154.8 in. A0 ≈ 10.4 sf. Vu ≈ 237.8 kips, φVc ≈ 431.8 kips > Vu.
- One-way shear (x direction): ax ≈ 30.5 in. Vux ≈ 55.8 kips, φVc ≈ 118.5 kips > Vux.
- Flexure (per ft strip): Mu ≈ 19.3 kip-ft, z ≈ 0.9d, As ≈ 0.32 in²/ft. Use As,min ≈ 0.39 in²/ft → spacing ≈ 9.5 in (round to practical spacing).
Result: 85 in × 85 in footing, h ≈ 18 in, #5 @ 9.5 in each way (adjust spacing per detailing).
FAQ
Which edition of ACI 318 is referenced?
ACI 318-19 is used for shear and flexure provisions; verify with your jurisdiction's adopted code.
What's the difference between service and factored loads?
Service loads size area with allowable soil bearing. Factored Pu (e.g., 1.2D+1.6L) drives shear and flexural checks.
Does the calculator include footing self-weight?
Plan sizing assumes net allowable bearing. If qa is gross, include self-weight in the service load before entering values.
How is the punching perimeter computed?
bo = 2(c1 + c2) + 4d, where d is the effective depth.
Are the reinforcement values exact?
Flexural steel is based on z ≈ 0.9d and per-foot strip design—appropriate for preliminary design but refine for final detailing.
Can I work in metric units?
Yes. Toggle to SI to switch to kN, kPa, and mm. Inputs convert automatically.
Is this a substitute for an engineer?
No. Use it for preliminary checks; final design must be sealed by a licensed structural engineer.