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.
Illustrative example
A 4-inch cast-in anchor in 4,000 psi concrete, far from any edge:
Nb = 24 × 1 × √4000 × 4^1.5 = 24 × 63.25 × 8 ≈ 12,144 lb- Full cone area:
ANc = 9 × 4² = 144 in² - No edge reduction:
psi_ed = 1.0 Ncb = Nb ≈ 12,144 lb- Design strength:
phi × Ncb = 0.65 × 12,144 ≈ 7,894 lb
Now move that anchor to 3 inches from a free edge (where 1.5 × hef = 6 in):
- Edge factor:
psi_ed = 0.7 + 0.3 × (3/6) = 0.7 + 0.15 = 0.85 - The projected cone is clipped by the edge — the effective area
ANc_actualshrinks below the full 144 in² - The design strength drops noticeably — proximity to an edge meaningfully reduces capacity
Understanding the failure modes
The concrete breakout mode is only one of several ways an anchor can fail. A complete ACI 318 design checks all applicable modes:
| Failure mode | Governing factor | Notes |
|---|---|---|
| Steel tensile rupture | Steel area × Fu | Often governs deep, strong-concrete anchors |
| Pullout | Bond or bearing area | Post-installed anchors need ICC-ES report values |
| Concrete breakout (tension) | Embedment, f’c, edge | This calculator covers this mode only |
| Side-face blowout | Edge distance | Governs for anchors very close to an edge |
| Combined tension + shear | Interaction equation | Required when both forces are present |
For shallow embedments in normal-weight concrete with no edge issues, breakout often controls. For deep anchors in high-strength concrete or anchors installed far from edges, steel rupture may govern instead.
Edge distance and the critical distance
The critical edge distance for concrete breakout is 1.5 × hef. When the anchor is farther from any edge than this, the full 45° breakout cone is unobstructed and the edge reduction disappears. When the anchor is closer, the cone is clipped and both the area term and the psi_ed factor reduce the capacity.
For a 4-inch embedment, the critical edge distance is 1.5 × 4 = 6 inches. Position anchors at least this far from a slab edge or a formed opening whenever the design allows.
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.