Anchorage Length Modification Calculator
lab times correction factors, applying the no-combination rule for 0.8 and 0.7.
Inputs
Result
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 |
lab = 800 mm, Φ20 plain, undisturbed, cover 5d: multipliers are 1.0 and the cover factor 0.70 gives la = 560 mm, i.e. 28d. If it were Φ28 epoxy coated, 1.10×1.25 = 1.375 and la = 770 mm.
FAQ
Why can 0.8 and 0.7 not combine with the other factors?
The mechanisms differ and combining them would over-credit the bar. Thick cover improves confinement and anchorage, while coating and large diameter weaken bond itself. Allowing 1.25×0.7 = 0.875 would assume the thick cover offsets the coating, which it does not - the coating also degrades the confinement benefit. Hence they act alone.
Why do bars over 25 mm need longer anchorage?
Their relative bonded area falls as diameter grows, so the same embedment provides less grip; their greater stiffness also causes more compatibility stress and splitting. Hence the 1.10 factor. Large bars often use mechanical anchorage (bearing plates, couplers) instead, which is cheaper than extending the embedment.
What counts as 'easily disturbed'?
Slipform and climbing formwork, bars moved while surrounding concrete is still plastic, pile cages and underwater placements - cases where the initial bond is repeatedly broken and needs more embedment to compensate. Ordinary beam, slab and column bars do not qualify. This is a design decision, not a site call.
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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.