Retaining & slopes · Anchors, nails & shoring
Ground anchor acceptance test calculator
From the elastic extension between alignment and test load: apparent free length against the acceptance limits, tendon stress at test load and creep over a log cycle of time.
Standard: FHWA-IF-99-015 / BS 8081 style acceptance · apparent free length · creep per log cycle
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Free length | 8 | m |
| Bond length | 6 | m |
| Number of strands | 4 | |
| Area per strand (15.2 mm) | 143 | mm² |
| Tendon modulus Es | 195 | GPa |
| Design (working) load | 600 | kN |
| Test load / design load | 1.33 | |
| Alignment load / design load | 0.1 | |
| Measured elastic extension (alignment → test load) | 56 | mm |
| Creep reading time 1 | 6 | min |
| Creep reading time 2 | 60 | min |
| Movement between the two readings | 0.9 | mm |
| Lower limit factor on free length (FHWA 0.8, BS 8081 0.9) | 0.8 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Test load | 798.0 | kN |
| Alignment load | 60.00 | kN |
| Tendon area | 572.0 | mm² |
| Limit: lower free length | 6.40 | m |
| Limit: upper Lf + 0.5Lb | 11.00 | m |
| Creep per log cycle | 0.900 | mm |
| Tendon stress at test load | 1,395 | MPa |
| Limit: 0.8fpu (fpu 1870 MPa) | 1,496 | MPa |
| Apparent free length A·Es·δe/ΔP | 8.46 | m |
- ✓ Apparent free length ≥ lower limit
- ✓ Apparent free length ≤ Lf + 0.5Lb
- ✓ Creep ≤ 2 mm per log cycle
- ✓ Tendon stress at test load ≤ 0.8fpu
Method, formulas and assumptions
- Test load = 600*1.33 = 798 kN
- Alignment load = 600*0.1 = 60 kN
- Tendon area = 4*143 = 572 mm²
- Limit: lower free length = 0.8*8 = 6.4 m
- Limit: upper Lf + 0.5Lb = 8+0.5*6 = 11 m
- Creep per log cycle = 0.9/LOG10(60/6) = 0.9 mm
- Tendon stress at test load = 798*1000/572 = 1395.1 MPa
- Limit: 0.8fpu (fpu 1870 MPa) = 0.8*1870 = 1496 MPa
- Apparent free length A·Es·δe/ΔP = 572*195*1000*56/((798-60)*1000)/1000 = 8.4637 m
- Too short an apparent free length suggests friction or grout in the free length; too long suggests debonding of the fixed length.
- Use the acceptance criteria in the project specification where they differ.
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.