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 -- when 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. The tile 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 and source
sqrt_ratio = min(sqrt(A2 / A1), 2.0); Bn = 0.85 f'c A1 sqrt_ratio; phi Bn = 0.65 Bn; DCR = Pu / phi Bn.
The 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.
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