Which ACI equation does this use?

It uses the concrete breakout strength equation from ACI 318-19 Chapter 17 (formerly Appendix D): Nb = kc * lambda * sqrt(f'c) * hef^1.5 for cast-in anchors. The single-anchor projected area ANc is taken as the full 9*hef^2 cone with an edge-distance reduction factor when applicable.

kc is the breakout coefficient: 24 for cast-in anchors and 17 for post-installed anchors when using US customary units (pounds and inches). The tool selects the right value based on your anchor type choice.

Does this cover steel and pullout failure modes?

No. This tool isolates the concrete breakout mode, which often governs for shallow embedments near edges. A full design must also check steel tensile strength, pullout, side-face blowout, and any combined shear interaction before accepting a capacity.

What edge distance triggers a reduction?

ACI sets the critical edge distance for breakout at 1.5*hef. When your edge distance is less than that, the projected breakout area is clipped, so the tool reduces the strength proportionally. At or beyond 1.5*hef there is no edge reduction.

Is this a substitute for an engineer?

No. It is an estimating aid for one failure mode. Anchorage to concrete is life-safety work and must be checked and stamped by a licensed engineer against the full ACI 318 provisions and the manufacturer's evaluation report.

Concrete Anchor Pullout Strength Calculator

Anchor bolts in concrete can fail by snapping the steel, pulling out, or by prying out a cone of concrete called a breakout. For shallow anchors close to a slab edge, the concrete breakout mode usually governs. This calculator estimates that breakout capacity using the ACI 318-19 Chapter 17 method.

How it works

The basic single-anchor breakout strength in tension is:

Nb  = kc * lambda * sqrt(f'c) * hef^1.5
ANc = 9 * hef^2                 (full cone, no edge effects)
psi_ed = 0.7 + 0.3 * (ca1 / (1.5*hef))   when ca1 < 1.5*hef, else 1.0
Ncb = (ANc_actual / ANc) * psi_ed * Nb
phi*Ncb  with phi = 0.65 (Condition B, no supplementary reinforcement)

Here kc is 24 for cast-in and 17 for post-installed anchors (US units), lambda is 1.0 for normal-weight concrete, f'c is in psi, and hef and the edge distance ca1 are in inches. The edge factor psi_ed and the clipped projected area both penalise anchors set near a free edge.

Example and notes

A 4 in cast-in anchor in 4000 psi concrete, far from any edge, gives Nb = 24 * 1 * sqrt(4000) * 4^1.5 ≈ 12,140 lb, and phi*Ncb ≈ 7,890 lb. Move that same anchor to 3 in from an edge and both the projected area and the edge factor drop the capacity sharply. Always confirm the controlling mode is breakout and not steel or pullout, and verify with the anchor manufacturer’s evaluation report for post-installed products.