Eurocode 3 (EN 1993-1-1)

The EC3 engine implements EN 1993-1-1:2005/A1:2014 - Design of Steel Structures, Part 1-1: General Rules and Rules for Buildings. It supports the default Recommended Values and the UK National Annex for partial factor overrides.

Open Eurocode 3 Calculator

Standard Reference

Supported Section Types

• I-Shapes (UKB, UKC, IPE, HEB, HEA - rolled and welded including plate girders)
• Channels (PFC/UPN/UAP where available from the section library or imported section data)
• Tees (cut from UKB/UKC parents - beam tees and column tees)
• Rectangular Hollow Sections (SHS and RHS - hot-finished and cold-formed)
• Circular Hollow Sections (CHS)
• Angles (equal and unequal leg)

Checks Performed

The design-check table shows the specific row and clause used for each member result. The documentation keeps the same clause groups at a summary level:

• Section classification (5.5 / Table 5.2): I/H, channel, and RHS/SHS classes are evaluated from signed N, My, and Mz at every real force station. Internal-part Class 3 limits use psi from gross elastic plate-edge stresses; force-envelope maxima are not combined into a synthetic stress state.
• Class 4 effective sections (EN 1993-1-5): I/H, RHS/SHS, and supported channel component checks use pure-compression Aeff or pure-bending Weff. If the member is Class 4 and any real station has concurrent axial-flexural or biaxial demand, load-state-consistent effective properties are unavailable: 6.2.9 and 6.3.3 interaction rows are INCOMPLETE and suppressed while component rows remain. Tee/angle compression uses Aeff, but Class 4 tee/angle bending is INCOMPLETE; gross elastic section moduli are not substituted. CHS Class 4 shell resistance is OUT_OF_SCOPE.
• Section resistance (6.2.3-6.2.10): Tension, compression, bending, shear, shear-bending, and N+M section checks use the reported clause row.
• Member buckling (6.3.1 / 6.3.1.4): Flexural and supported torsional or flexural-torsional compression buckling checks are reported separately.
• LTB (6.3.2.1(3) / 6.3.2.2 / 6.3.2.3): LTB resistance uses the general or modified I/H reduction method according to the section family.
• Combined stability and torsion (6.3.3 / 6.2.6(4) / 6.2.7): Annex B stability interaction and closed-hollow-section torsion rows are reported where applicable.

Calculator Inputs

The standalone EC3 calculator accepts the following inputs. All values are in metric units.

Fabrication Type

Select the fabrication method. This affects buckling curves, section classification limits, and the applicable shear-area path under 6.2.6:

• Rolled - standard hot-rolled sections (IPE, HEB, HEA, UKB, UKC)
• Welded / Built-up - fabricated plate girders and welded sections
• Cold Formed - cold-formed hollow sections

Section Geometry

Select a shape group, then enter dimensions or pick from the built-in section library (European/UK shapes).

Material Properties

Partial Safety Factors (γ)

These can be adjusted to match national annex requirements:

Member Lengths & Bracing

A Continuously Restrained checkbox sets χ_LT = 1.0. L_b, C₁, M_cr, k_c,LT, and load height are then inactive and hidden, but Clause 6.3.3 interaction and the independent Clause 6.3.1.4 torsional length still apply.

Load height is available for I-shapes, channels, and tees. For a tee with top-flange loading and active LTB, supply M_cr; otherwise the unsupported LTB capacity and its dependent 6.3.3 interaction fail closed. Continuous restraint makes that M_cr requirement inactive.

For EC3 cantilever LTB cases, verify C₁, L_b/KLT, and load-height assumptions from a suitable NCCI/SCI reference or elastic buckling calculation. The engine does not automatically infer fixed-free cantilever boundary conditions from the model.

Design Actions

Partial Safety Factors

Partial factors can be adjusted in the calculator via the advanced settings panel.

Limitations & Notes

• Station-based Class 3/4 classification: For I/H, channel, and RHS/SHS members, signed concurrent N, M_y, and M_z are evaluated at every real force station. Internal-part Class 3 limits use ψ from gross elastic plate-edge stresses; separate force-envelope maxima are not combined into a synthetic classification state.
• I/H, channel, and RHS/SHS Class 4 combined actions: Pure-compression A_eff and pure-bending W_eff remain available for independent component checks. If the member is Class 4 and a real station has concurrent axial-flexural or biaxial demand, a load-state-consistent effective section is unavailable: the engine publishes INCOMPLETE and suppresses 6.2.9 and 6.3.3 rather than substituting gross or unrelated pure-action properties.
• Custom-section axis convention: The section-library convention uses Izz as the strong-axis inertia and Iyy as the weak-axis inertia. Raw section-library Szz/Syy fields are plastic section modulus values; the loader may expose those plastic values internally as Pzz/Pyy before EC3 reports Wpl,y/Wpl,z. Displayed/report Wel,y/Wel,z elastic values are derived separately. EC3 results are labelled in Eurocode notation, so the library strong axis is reported as y-y and the library weak axis as z-z. Custom or imported sections with these axes swapped can produce incorrect bending, buckling, and LTB axis assignments.
• Net section: Tension rupture under 6.2.3 uses entered A_net when supplied; blank A_net defaults to A_g. Bolt-hole and staggered-path geometry are not inferred.
• Torsional-flexural buckling (compression): Clause 6.3.1.4 torsional or flexural-torsional buckling is checked for supported open-section families. Angles are excluded because the doubly-asymmetric formulation is outside this engine scope.
• Tee/angle Class 4 bending: Axial compression may use the EN 1993-1-5 effective area. Effective bending properties are not implemented, so any major-axis, minor-axis, biaxial, or axial-flexural bending demand returns INCOMPLETE. Numeric bending resistance, LTB, and dependent interaction rows are not published, and gross elastic section moduli are not substituted.
• CHS Class 4: Not supported. EN 1993-1-6 shell buckling is required for CHS with D/t exceeding Class 3 limits; the member path returns OUT_OF_SCOPE and does not publish gross-property capacities.
• Tee stem classification: Tee classification follows the active stress state and Table 5.2 Sheet 2 limits for the reported station.
• C_m factors: Moment-gradient factors for 6.3.3 are computed from the moment diagram where available and can be overridden in the standalone calculator.
• Tee shear: Tee shear follows the relevant EC3 6.2.6 path for rolled or welded tees, with shear routed to the active stem or flange direction based on member orientation.
• M_cr methodology: Uses a supplied M_cr when present; otherwise uses the project analytical LTB method. Some benchmark discrepancies can arise where a reference uses a numerical eigenvalue model.
• Cantilever LTB: C₁ is auto-computed from the moment diagram or can be manually overridden. Fixed-free/cantilever LTB boundary conditions are not auto-detected, so confirm the C₁ and effective length assumptions for EC3 cantilevers separately.
• Local transverse forces and reactions: The EC3 member engine does not perform EN 1993-1-5 local transverse force, patch loading, web bearing, web buckling, end-post, or transverse-stiffener checks, and it does not derive these checks from FEA reactions, point loads, or connection forces. Check support bearing and concentrated load introduction regions separately from the member-strength result.
• National Annex: Default values follow the EN 1993-1-1 Recommended Values used by most European countries. The UK National Annex (γ_M2 = 1.10, η = 1.0) is auto-applied when a UK section is selected from the library. Other national annexes can be matched by adjusting γ_M and η in the advanced settings.
• Torsion (6.2.6(4)): Not an input in the standalone calculator. In the FEA tool, closed hollow sections (RHS/SHS/CHS) use a torsional shear stress check when a torsional section modulus is supplied or derivable. Warping torsion for open sections (I-beams, channels, angles, tees) is not performed.
• Serviceability and special design states: Structural analysis results may report displacements separately, but the EC3 design engine does not perform serviceability limit comparisons, vibration checks, fatigue, fire, corrosion/durability, connection detailing, or execution checks unless handled by a separate workflow.

Verification

The engine is benchmarked against 126 independent test cases from SCI P364, the Designers' Guide to EN 1993-1-1, EUR 22898 EN, and Worked Examples in Eurocode 3 & 4, with an average difference of 0.43%. The higher average compared to other engines reflects differences in M_cr computation methodology (analytical vs. numerical), which are documented in individual benchmark notes.