Anchorage Length Modification Calculator
lab times correction factors, applying the no-combination rule for 0.8 and 0.7.
With the default inputs: Basic anchorage lab 800 · Bar diameter 20 · Diameter over 25 mm Not over 25 mm etc.Anchorage length la 800 mm
Edit the inputs below to recalculate.Inputs
Result
| Diameter factor | 1.00 |
| Coating factor | 1.00 |
| Disturbance factor | 1.00 |
| Combined ζa | 1.000 |
| Cover factor | 1.00 |
| Modified la | 800mm |
| In bar diameters | 40.0d |
Per GB 50010-2010 clause 8.3.2: ribbed bars over 25 mm take 1.10, epoxy coated 1.25 and bars easily disturbed during construction 1.10, which may be multiplied together. Cover of 3d gives 0.80 and 5d gives 0.70, and these two never combine with the others. lab itself must come from the code tables or formula for the concrete grade, steel grade and seismic level - this tool only applies corrections. la must also satisfy the minimum-reinforcement requirement, and seismic design uses laE with an additional factor.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
Formula
| la = ζa × lab | ζa is the correction factor |
| Multipliers | >25 mm 1.10 · epoxy 1.25 · disturbed 1.10 |
| Cover reduction | 0.80 at 3d, 0.70 at 5d, never combined with others |
| Seismic | laE = ζaE × la |
How to work it out
- Start from the basic anchorage length lab, from tables or lab = α·(fy/ft)·d.
- Apply modification factors ζa: 1.10 for bars over 25 mm, 1.25 for epoxy coating, 1.10 where construction disturbs the bar.
- Reduce where cover is generous: 0.80 at 3d cover, 0.70 at 5d.
- Key rule: the reduction factors (0.8, 0.7) do not combine multiplicatively with the others. Use either the reduction or the increase — never multiply 1.25 by 0.7.
- Keep la at or above the tabulated minimum (generally 200 mm); in seismic design laE = ζaE × la.
Where it goes wrong
- Multiplying 0.8 by 1.25 — the product is 1.0, which looks like a wash, but the code forbids combining them; take 1.25 (conservative) or 0.8 (only if the cover is actually achieved). This is the single easiest mistake here.
- Applying 1.10 at exactly 25 mm — the rule is 'greater than 25 mm', so Φ25 does not qualify; boundary cases are commonly misread.
- Treating epoxy-coated bar as ordinary — the coating cuts bond substantially and 1.25 is mandatory; omitting it is a real safety issue.
Material and cost
Every 10% of anchorage length adds roughly 2-4% to steel tonnage, since anchorage lengths carry no load but cannot be avoided. Epoxy-coated bar costs 15-25% more than plain and needs coating repair wherever damaged.
On site
- 'Disturbed during construction' means slipforming or climbing formwork, not ordinary formwork — do not apply it casually.
- The cover reduction only applies if that cover is actually achieved; 20 mm built against a 25 mm drawing cannot be reduced.
- Anchorage is measured from the critical section, not the member end, and that starting point is often mistaken.
Basis
- GB 50010-2010 Code for design of concrete structures, clause 8.3.2 (anchorage modification factors)
- 16G101-1 Standard drawings for flat-slab detailing, pages 57-58
- GB 50666-2011 Code for construction of concrete structures, Section 5.4
FAQ
Why can 0.8 and 0.7 not combine with the other factors?
Why do bars over 25 mm need longer anchorage?
What counts as 'easily disturbed'?
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Anchorage Length Modification Calculator. 2026-10-25. https://jianzhujisuan.com/en/rebar-anchor-modify