How to use the anchor concrete breakout calculator
Concrete breakout strength in tension follows ACI 318-19 Chapter 17. The basic single-anchor strength is Nb = kc·λa·√f'c·hef^1.5 in pounds with psi and inches, where kc is 24 for cast-in anchors and 17 for post-installed anchors unless a qualification test in the product evaluation report gives a higher value. Embedment enters to the 1.5 power, so going deeper buys more capacity than going to a larger diameter.
Three modifiers turn Nb into Ncb. The projected area ratio ANc/ANco compares the actual failure area with the full 9hef² square, clipping it wherever an edge is closer than 1.5hef. The edge factor ψed,N drops to 0.7 + 0.3·ca,min/(1.5hef) once the nearest edge is inside 1.5hef. The cracking factor ψc,N is 1.25 for uncracked concrete and 1.0 where cracking is expected at service load. For deep cast-in anchors with hef between 11 in and 25 in, ACI 318 also permits the alternative Nb = 16·λa·√f'c·hef^(5/3), which gives a lower value.
The Ncb shown is a nominal strength. Design strength requires a strength reduction factor: roughly 0.70 to 0.75 for cast-in anchors depending on whether supplementary reinforcement is present, and a value set by the anchor category in the ESR for post-installed anchors, so check the evaluation report and your load condition. Anchor groups, combined tension and shear, and anchor reinforcement all need separate treatment, and safety-critical baseplates should be confirmed by a licensed structural engineer.
Frequently Asked Questions
No. ACI 318 Chapter 17 requires steel strength, pullout, side-face blowout, and concrete breakout in tension, plus a separate set of shear limit states and a tension-shear interaction check. This tool covers concrete breakout only, so design to the smallest governing value.
The breakout cone spreads at roughly 35 degrees and projects a square area 3hef on a side. When an edge is closer than 1.5hef that area is clipped, and the ψed factor of 0.7 + 0.3·ca,min/(1.5hef) reduces the strength further.