Rebar Anchorage Length Calculator
Basic and seismic anchorage length from bar grade, diameter and concrete class.
Quick answer
With the default inputs: Bar grade HRB400 · Bar diameter 20 · Concrete class C30 etc.Anchorage length la 705 mm
Edit the inputs below to recalculate.Inputs
Result
Anchorage length la
705mm
| Design strength fy | 360N/mm² |
| Design tensile ft | 1.43N/mm² |
| fy / ft | 251.7 |
| Shape factor α | 0.14 |
| Correction ζa | 1.00 |
| Calculated value | 705mm |
| Seismic anchorage laE | 705mm |
| In bar diameters | 35.2d |
Uses la = α(fy/ft)d from GB 50010, without the beneficial corrections for cover and confining reinforcement. Structural design must follow the full code and the design documents.
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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 |
How to work it out
- Look up the basic anchorage length lab from bar grade, concrete grade and diameter, per the code table.
- Apply the modification factor: 1.1 for deformed bar over 25 mm, 1.25 for epoxy coating, 1.1 for construction disturbance, with reductions where cover is generous.
- Apply the seismic factor: 1.15 for class 1 and 2, 1.05 for class 3, 1.00 for class 4.
- That gives the seismic anchorage length lae — check it against the drawing.
- Compression anchorage is 0.7 times the tension value, but never less than 200 mm.
Where it goes wrong
- Using la as lae — omitting the seismic factor shortens anchorage by 15% at class 1 and 2, the most common hidden structural defect.
- Using lab for C30 with C25 concrete — one grade down adds 10-15%, and the relationship is not linear.
- Ignoring the diameter factor — above 28 mm it is 1.1, which adds 30-40 mm per anchorage and may simply not fit.
Material and cost
Anchorage length drives steel quantity directly: every 10% added to lae adds roughly 3-5% of end anchorage steel. At about 4000 per tonne, getting one grade wrong on a 30,000 m² project is 80,000-150,000 of material. That is not a saving to be optimised — it is structural safety.
On site
- Check against the standard detail book tables rather than recomputing from the formula.
- In congested beam-column joints the anchorage often will not fit; raise it at detailing stage and agree a bend or headed end.
- Different seismic zones (tower, podium, basement) carry different requirements — do not apply one value to the whole building.
Basis
- GB 50010-2010 (2015 edition) Code for design of concrete structures, Section 8.3 (anchorage)
- 16G101-1 Standard detail drawings for monolithic concrete structures, pages 57-58
- GB 50666-2011 Code for construction of concrete structures, Section 5.3
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Rebar Anchorage Length Calculator. 2026-10-25. https://jianzhujisuan.com/en/rebar-anchorage
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.