Rebar Anchorage Length Calculator
Basic and seismic anchorage length from bar grade, diameter and concrete class.
Inputs
Result
Formula
| Basic anchorage | la = α × (fy ÷ ft) × d |
| Shape factor α | 0.16 plain bars, 0.14 deformed bars |
| Corrections | ζa = 1.10 (d>25) × 1.25 (epoxy coated) |
| Seismic | laE = ζaE × la (1.15 for class I-II, 1.05 for III-IV) |
| Minimum | la not less than 200 mm |
HRB400, 20 mm, C30, non-seismic: fy = 360 and ft = 1.43 give la = 0.14 × 251.7 × 20 ≈ 705 mm, about 35 bar diameters.
Basic anchorage length la for HRB400 tension bars (mm, non-seismic)
| Bar | C25 | C30 | C35 | C40 |
|---|---|---|---|---|
| HRB400 Φ12 | 480 | 425 | 390 | 355 |
| HRB400 Φ14 | 560 | 495 | 450 | 415 |
| HRB400 Φ16 | 635 | 565 | 515 | 475 |
| HRB400 Φ18 | 715 | 635 | 580 | 535 |
| HRB400 Φ20 | 795 | 705 | 645 | 590 |
| HRB400 Φ22 | 875 | 780 | 710 | 650 |
| HRB400 Φ25 | 995 | 885 | 805 | 740 |
FAQ
How does anchorage length differ from lap length?
Anchorage length is the minimum embedment for one bar to develop its force through bond with the concrete. Lap length transfers force between two bars; each carries only part of it and a joint factor of 1.2-1.6 applies, so a lap is always longer than the anchorage.
Why the 1.1 factor above 25 mm?
Large bars have a lower bond-area-to-section ratio and are harder to encase properly during placing, so bond is relatively poorer. The code compensates with 1.1. Congested heavy bars also tend to leave voids underneath, so vibration and aggregate size matter as much as extra length.
Why longer anchorage in seismic design?
Under earthquake action, bars in joints may yield and even overstrength, exceeding design forces, while reversed cyclic loading degrades bond. Seismic detailing multiplies anchorage by 1.15 for class I-II and 1.05 for III-IV, with confining hoops. Frame joints are where this fails most often.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.