Concrete · Special elements
Development & lap length calculator
Development length of deformed bars in tension and tension lap length.
Standard: AS 3600-2018 Cl 13.1.2, 13.2.2
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 |
|---|---|---|
| Bar diameter | 20 | mm |
| fsy | 500 | MPa |
| f'c | 32 | MPa |
| Cover to bar | 40 | mm |
| Side cover | 40 | mm |
| Clear spacing between bars | 100 | mm |
| Bar position | Other | |
| Lap factor k7 | 1.25 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| k1 | 1.30 | |
| k2 = (132 − db)/100 | 1.12 | |
| cd | 40 | mm |
| k3 | 0.850 | |
| Lsy.tb (calculated) | 872 | mm |
| Minimum 0.058 fsy k1 db | 754 | mm |
| Development length | 872 | mm |
| Tension lap | 1,090 | mm |
Method, formulas and assumptions
- Lsy.tb = 0.5 k1 k3 fsy db / (k2 √f'c) ≥ 0.058 fsy k1 db (f'c ≤ 65 MPa)
- k3 = 1 − 0.15(cd − db)/db within 0.7…1.0; cd = min(a/2, c1, c)
- Lap = k7 Lsy.t ≥ 29 k1 db (k7 = 1.25 typical; 1.0 where As ≥ 2As,req and ≤ 50% lapped)
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.