The UK National Annex's lower αcc = 0.85 barely moves the needle on an isolated footing's bar count — because the footing's bending is minimum-steel-controlled, not calculated-demand-controlled, and the two shear checks (where αcc = 1.0 in every Annex) are completely unaffected either way.
Every check above runs automatically inside the Structyze Isolated Footing module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.
The Example We'll Use
- Column 400 × 400 mm
- Footing 2.6 × 2.6 m, thickness 450 mm, cover 50 mm
- Axial: permanent 600 kN, variable 300 kN
- Allowable bearing 150 kN/m²
- Concrete C30/37, steel B500; 16 mm bars at 150 mm each way
Working Through It
Service bearing q ≈ 133 kN/m² against 150 kN/m² allowable — the tightest check overall at utilisation 0.89, and entirely unaffected by which National Annex applies, since it's a pure EN 1997 geotechnical check independent of concrete material factors.
One-way shear (utilisation 0.77) and punching shear (utilisation 0.68) both use αcc = 1.0 in every Annex for shear provisions — identical numbers under the UK NA and the CEN base.
Bending, MEd ≈ 113 kN·m/m, gives K ≈ 0.03 — far below K′ under either Annex's ductility limit. Because the section is this lightly stressed in bending, the lower UK NA fcd (17.0 MPa against the CEN base's 20.0 MPa) doesn't change the bar count at all; the footing is comfortably singly reinforced either way.
What Governs, and Why
Bearing is the tightest check at utilisation 0.89 — ahead of one-way shear (0.77), punching (0.68) and bending (0.53). The UK NA's lower αcc truly has no practical effect on this specific footing, precisely because the bending demand sits so far below either Annex's ductility limit that the bar count comes out identical regardless of which one applies.
Where Structyze Takes Over
Every check above runs automatically inside the Structyze Isolated Footing module, free to try online — change any input and every downstream number recalculates instantly, with the governing code clause shown next to each result.
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Quick FAQ
If the UK NA doesn't change this footing's outcome, when would it actually matter?
The UK NA's αcc = 0.85 (versus the CEN base 1.0) only shows up in checks where K is truly close to K′ — a more heavily loaded footing or beam, where the bending demand approaches the ductility limit, would see the lower UK NA capacity potentially force doubly-reinforced design earlier than the CEN base would. For a lightly stressed member like this footing, the difference simply never becomes visible.
Why is bearing unaffected by the National Annex at all?
Soil bearing is fundamentally a geotechnical (EN 1997) serviceability check comparing service-load contact pressure against an allowable soil value — it has nothing to do with concrete material partial factors like αcc, which only enter the structural (EN 1992) strength checks further down the design procedure.
Final Thoughts
This footing is a useful demonstration that a National Annex choice doesn't automatically change every number in a design — whether it matters depends entirely on how close the specific check in question sits to its own governing limit.