Concrete anchor design
A cast-in anchor rarely fails in the steel — it pulls out a cone of concrete. This checks steel tension, concrete breakout, pullout, steel shear and pryout, and names which one governs, with every coefficient exposed.
| Limit state | Nominal (kN) | φ | Design (kN) | Utilisation |
|---|---|---|---|---|
| Steel tension | 125.6 | 0.75 | 94.2 | 21% |
| Concrete breakout, tension ◂ | 72.0 | 0.70 | 50.4 | 40% |
| Pullout | 96.0 | 0.70 | 67.2 | 30% |
| Steel shear | 75.4 | 0.65 | 49.0 | 20% |
| Pryout | 144.0 | 0.70 | 100.8 | 10% |
Nb = 10·λa·√f'c·hef^1.5, ANc/ANco = 1.00, ψed = 1.00.
φNsa = 0.75 · Ase · futa · The anchor itself.
Ncb = (ANc/ANco)·ψed·Nb, Nb = 10·λa·√f'c·hef^1.5 · kc = 10 in SI for cast-in anchors.
Np = 8 · Abrg · f'c · Headed anchor bearing on the concrete.
A clear path from inputs to results.
A cast-in anchor rarely fails in the steel. It pulls out a cone of concrete whose size scales with the embedment to the power of 1.5, which is why depth buys far more capacity than diameter. This checks the five limit states that decide it.
How to use
- Enter the anchor diameter, embedment and edge distance.
- Set the concrete strength and the anchor steel strength.
- Read the governing state — breakout usually wins at shallow embedment.
The formula
Reference: ACI 318-19 §17.6.2 (breakout), §17.6.3 (pullout), §17.7 (shear), φ = 0.70.
Worked example
M16 at 120 mm embedment in C30
- Ase 157 mm², futa 800 MPa, edge distance 180 mm.
- 1
Steel φNsa = 0.75 × 157 × 800 = 94.2 kN. Nb = 10 × √30 × 120^1.5 = 72.0 kN; with the full cone ANc/ANco = 1 and ψed = 1, φNcb = 50.4 kN. Pullout φNp = 0.70 × 8 × 400 × 30 = 67.2 kN.
Concrete breakout governs at 50.4 kN.
Interpreting the result
Breakout is a concrete failure, not a steel one: doubling the embedment multiplies the capacity by 2.8, while doubling the diameter only doubles the steel strength.
Assumptions & limitations
- Cast-in anchors with kc = 10 (SI).
- Tension and shear checked separately; interaction is noted, not computed.
- Shear breakout and side-face blowout are not included.
- No supplementary reinforcement is counted.
Frequently asked questions
Why does embedment matter more than diameter?
Breakout capacity scales with hef^1.5 because the cone surface area grows with the square of the depth.
What is pryout?
A shear load levering a shallow anchor out of the concrete; kcp doubles once hef reaches 65 mm.
When does pullout govern?
For a headed anchor with a small head bearing area relative to the embedment.
Can I use this for post-installed anchors?
Only with the manufacturer’s qualification data; the kc and pullout values differ for adhesive and mechanical anchors.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.