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BS EN 1992-1-1/NA — Rectangular Beam Bending

Tension reinforcement A_s · neutral axis depth x · stress block λx · lever arm z · ductility check x/d

Structure: EN core plus national parameters

Eurocode separates the formulae (identical in every adopting country) from the parameters each country fixes nationally. This platform follows the same split: the UK pack contains no formulae of its own — only the NDP set. Fields marked with an orange bar are NDPs.

α_cc = 0.85 for compression — but tension is a separate matter

The UK NA is reported to adopt α_cc = 0.85 for compression, matching Germany and departing from the EN recommendation of 1.0.
However, the UK is also reported to set a different coefficient for tension (α_ct, commonly quoted as 1.0). The shared engine applies a single α_cc on the compression side, which is what governs the bending calculation offered here. Any check governed by the concrete tensile design strength f_ctd is therefore out of scope for this pack — go back to the official National Annex for those.

⚠ Two notation traps:
1. C30/37 means f_ck = 30 MPa on a cylinder and 37 MPa on a cube. Eurocode designs on the cylinder strength. The Chinese C30 and Russian B25 classes are cube strengths and are not interchangeable.
2. In Eurocode, x is the neutral axis depth and the stress block is λx deep (λ = 0.8). In the Chinese and Russian codes, x already denotes the stress block. Confusing them gives a 20% error.

Input data

Concrete strength class

f_ck cylinder / f_ck,cube cube. Concrete specified to BS 8500 in UK practice.

Reinforcement grade

Width b (mm)

Effective depth d (mm)

Design moment M_Ed (kNm)

From the BS EN 1990/NA combinations

Nationally Determined Parameters

α_cc (compression) UK deviates UK NA to 3.1.6(1)P

Reported UK value 0.85 against the EN recommendation of 1.0. Reduces f_cd by 15%.

γ_c = EN UK NA to 2.4.2.4(1)

γ_s = EN UK NA to 2.4.2.4(1)

x/d ductility limit UK NA to 5.5(4)

⚠ UK NA value not confirmed — EN recommendation used as a placeholder

Results

f_ck / f_ck,cube (MPa)—

f_cd = α_cc·f_ck/γ_c (MPa)—

f_yd = f_yk/γ_s (MPa)—

λ / η §3.1.7(3)—

μ = M/(b·d²·f_cd)—

Neutral axis depth x (mm)—

Stress block λx (mm)—

Lever arm z (mm)—

x/d—

Tension reinforcement A_s (mm²)—

M_Rd (kNm)—

Utilisation M_Ed/M_Rd—

⚠ Not verified: —

CLAUSE REFERENCES

§6.1Equilibrium η·f_cd·b·λx = A_s·f_yd; M_Rd = η·f_cd·b·λx·(d − λx/2)

Eq.(3.15)f_cd = α_cc·f_ck/γ_c

UK NA to 3.1.6(1)Pα_cc = 0.85 for compression (reported UK value)

§3.1.7(3)Rectangular stress block: λ = 0.8, η = 1.0 up to C50/60

Eq.(9.1N)A_s,min = max(0.26·f_ctm/f_yk·b·d ; 0.0013·b·d)

Engine: eurocode.js · Pack: GB-BSEN-UKNA.v1-beta ·

Combinations → NA comparison →

Calculation steps

Two UK regulatory matters this tool does not resolve

1. Post-EU-exit conformity. Product marking has moved from CE towards UKCA with repeatedly extended transition arrangements. The BS EN design standards are unaffected, but product specification and acceptance on site are. Confirm the current position separately.
2. Building Safety Act 2022. The Act created the Building Safety Regulator and a gateway approval regime for higher-risk buildings. For projects in scope, structural design sits inside a stricter duty-holder framework than the Eurocode text alone implies.
Note also that building regulation differs across England, Wales, Scotland and Northern Ireland.

Preliminary calculation tool. Final design, signed responsibility and building control submission require the full official text of BS EN 1992-1-1 and BS EN 1990 with their UK National Annexes, and review by a suitably qualified engineer.