Concrete Bearing Strength (ACI 318-19 22.8)
The concrete-on-concrete bearing check that forgets the confinement bonus: Bn is not just 0.85 f'c x A1.
Example
You enter
- Loaded area A1 (in²) 144
- Supporting area A2 (in², >= A1) 1296
- Concrete strength f'c (psi) 4000
- Factored load Pu (kip, 0 = capacity only) 500
You get
- Confinement factor sqrt(A2/A1), capped at 2.0 2.000 (at the 2.0 cap)
- Nominal bearing strength Bn 979,200 lb (979.2 kip)
- Design bearing strength phiBn (phi = 0.65) 636.5 kip
- Demand / capacity 0.79 (OK)
Checked against published worked examples from: American Concrete Institute.
Details, formula, and sources
ScopeWhen the supporting surface is wider than the loaded patch, the surrounding concrete confines it and the strength is multiplied by sqrt(A2/A1), capped at 2.0. A 12x12 in column (144 in^2) on a 36x36 in footing earns the full 2x -> phiBn = 636.5 kip (phi = 0.65); a pad flush to its support edge (A2 = A1) earns none -> 318.2 kip, exactly half. It returns the capped factor, nominal and design strengths, and the demand-capacity ratio. A design aid, not the engineer of record's stamped design.
Formula or tablesqrt_ratio = min(sqrt(A2 / A1), 2.0); Bn = 0.85 f'c A1 sqrt_ratio; phi Bn = 0.65 Bn; DCR = Pu / phi Bn.
Source and editionThe ACI 318-19 22.8.3 bearing strength of concrete, with the 22.8.3.2 confinement multiplier sqrt(A2/A1) capped at 2.0 and phi = 0.65 for bearing (21.2), by name.
Free accessACI 318 is readable free through the ACI online reading room at concrete.org; the 22.8 bearing provisions are in the published code.
What governsEstimate. AHJ and licensed professional govern.
API field namesloaded_area_in2, support_area_in2, fc_psi, factored_load_kip, sqrt_ratio, bn_lb, phibn_kip, dcr
- Nominal strength Bn = 0.85 f'c on the loaded area A1ACI 318-19 22.8.3.2
- Confinement bonus multiply by sqrt(A2/A1) when the support is wider on all sides, capped at 2.0ACI 318-19 22.8.3.2
- Strength reduction phi = 0.65 for bearingACI 318-19 21.2