EN 1993-1-8 Weld Strength — Eurocode 3 Fillet & Butt Welds

EN 1993-1-8 — Design of joints

Page last updated 18 October 2026

EN 1993-1-8 is Part 1-8 of Eurocode 3 — the European standard for the design of steel structures — and is the governing document for the design of structural joints, including fillet welds, butt welds, bolted connections and moment-resisting frames. Its weld design method defines how to resolve the stress components on the throat section of a fillet or penetration weld (normal stress σ⊥, shear stress τ⊥ and in-plane shear τ∥), combine them via a von Mises equivalent, and check the result against the directional resistance that depends on the parent-metal ultimate strength fu, the weld correlation factor βw and the partial factor γMw. EN 1993-1-8 is mandatory across the European Union and widely adopted in the Middle East, Asia and any project that references Eurocode.

MechanixCalc runs a simplified version of the EN 1993-1-8 directional method (§4.5.3.2) for fillet and butt welds, covering direct shear, bending in the joined plate's plane, torsional (eccentric) loading and combined load cases. A full-penetration butt weld is additionally capped at the parent metal's yield strength, since such a weld cannot be stronger than the section it joins; a partial-penetration butt weld is checked by the fillet method on its nominal depth of penetration. The engine runs server-side so the governing method never ships to the browser. Results include the equivalent throat stress, the utilisation ratio, the safety factor and — on the same page — a fatigue endurance check to EN 1993-1-9 and a heat-input / t8/5 cooling-time calculation to EN 1011-1/-2 Annex D. It does not design bolted connections, base plates or beam-to-column joints. Weld checks also include a conservative linear shear screen, the long-joint reduction β_Lw, and a flag for fillet welds shorter than max(30 mm, 6·a) (§4.5.1), and the partial factor can be raised from 1.25 to 1.50. A branded, clause-cited PDF report is generated for design files and reviewer submissions.

Calculators that implement EN 1993-1-8

Video walkthrough (3:36)

Read the transcript

Here's a bracket held by two fillet welds, each 120 mm long. At 50 kN the tool says it passes, with a safety factor of 5.52.

Put 200 kN on just one of those welds, and it fails.

This is the MechanixCalc weld calculator. It checks fillet and butt welds to EN 1993-1-8, and their fatigue life to EN 1993-1-9. Let's go through it, step by step.

A fillet weld carries its load through the throat: the narrowest section, from the root to the face.

There the stress has three parts: a normal stress, a shear across the weld, and a shear along it. Together they make one equivalent stress, checked against the weld's strength.

Step one is the weld: a fillet with an 8 mm leg. The specified throat is 5 mm, and the tool uses the smaller of that and 0.7 times the leg.

Two weld lines, 60 mm apart, each 120 mm long, on a 10 mm plate.

Step two is the material: S355 steel, with the standard partial factor of 1.25. That sets the weld's strength at 398.2 MPa.

Step three is the load: a direct shear of 50 kN, carried along the welds.

Step four shows the checks. The throat area is 1200 mm², and the equivalent stress is 72.2 MPa.

Against the 398.2 MPa strength that's a safety factor of 5.52, and the verdict is pass.

The tool also checks the 3 mm minimum throat and the minimum weld length.

Now raise the load to 200 kN.

The equivalent stress climbs to 288.7 MPa. That's still below the strength, but the safety factor is 1.38, under the 1.5 minimum, so the verdict is review.

Now take away one of the two weld lines.

The throat area halves to 600 mm², the stress doubles to 577.4 MPa, and the weld fails.

The fix is a bigger weld. Back to two lines, with a 10 mm leg and a 7 mm throat.

And longer runs, 160 mm each.

At 200 kN the safety factor is now 2.58, and the strength check passes.

But this load repeats. Step five is fatigue: detail category 80, a stress range of 80 MPa, for a million cycles.

The weld lasts 812,884 cycles. The damage is 1.23, above 1, so the joint fails in fatigue.

The strength check still passes, but the verdict takes the worse of the two, and it fails.

Lower the stress range to 60 MPa: the life rises to 1.93 million cycles, the damage drops to 0.52, and the verdict turns green.

What the tool does not check is listed too: a moment about the weld's own axis, bolts, the plates beside the weld and lamellar tearing. So silence never reads as a pass.

Every input, every formula and every check goes into a PDF report. A report your reviewer can follow.

That's a weld, checked. Try it on your own design, on MechanixCalc.com.

What EN 1993-1-8 covers

Fillet weld and butt weld resistance — directional method (§4.5.3.2): stress components σ⊥, τ⊥, τ∥ on the effective throat and two-condition capacity check
Correlation factors βw for S235–S460 structural steel grades (Table 4.1) and the partial factor (γMw = 1.25 standard in the tool, with an optional 1.50)
Combined loading cases: direct shear/axial, bending in the joined plate's plane, torsion from eccentric load, and full F+M+T combinations
Weld group properties — single weld line and two parallel lines (throat area Aw, section modulus Ww, polar moment Jw)
NOT implemented in MechanixCalc: bolted connections and base plates (clauses 3 and 6) — joint classification (rigid/semi-rigid/pinned), bolt-group design and connection detailing rules
NOT implemented in MechanixCalc: moment-resisting beam-to-column joints — component method (§6.2) for rotational stiffness and moment resistance

Parts of the standard

EN 1993-1-8 §3Connections made with bolts, rivets or pins
EN 1993-1-8 §4Welded connections — throat resistance and weld group design
EN 1993-1-8 §5Analysis, classification and modelling of joints
EN 1993-1-8 §6Structural joints connecting H and I sections (component method)
EN 1993-1-8 §7Hollow section joints

Governing formulas

Von Mises equivalent stress on weld throat (§4.5.3.2, condition 1)

σ_eq = √(σ_⊥² + 3·(τ_⊥² + τ_∥²)) ≤ fu / (βw · γ_Mw)

where σ_⊥ = normal stress perpendicular to the throat plane (MPa); τ_⊥ = shear stress perpendicular to the weld axis on the throat (MPa); τ_∥ = shear stress parallel to the weld axis on the throat (MPa); fu = nominal ultimate tensile strength of the weaker part joined (MPa); βw = weld correlation factor from Table 4.1 (0.80 for S235, 0.85 for S275, 0.90 for S355, 1.00 for S420/S460); γ_Mw = partial factor for weld resistance (1.25 for standard EN 1993 joints)

Second condition — normal stress limit (§4.5.3.2, condition 2)

σ_⊥ ≤ 0.9 · fu / γ_M2

where σ_⊥ = normal stress perpendicular to the throat (MPa); fu = ultimate tensile strength of the parent metal (MPa); γ_M2 = 1.25 (partial factor for resistance of cross-sections in tension to fracture)

Directional shear resistance per unit throat area

f_vw = fu / (√3 · βw · γ_Mw)

where f_vw = design shear resistance of the weld per unit throat area (MPa); fu = ultimate tensile strength of the weaker connected part (MPa); √3 appears from the von Mises yield criterion in pure shear; βw and γ_Mw as above

Frequently asked questions

What is EN 1993-1-8 used for?

EN 1993-1-8 is the Eurocode 3 standard for the design of structural steel joints. Its weld design clauses (§4) define how to check fillet and butt welds against the applied stress resultants using the directional method — resolving the throat stress into σ⊥, τ⊥ and τ∥, forming a von Mises equivalent and comparing it to the resistance set by the parent-metal strength fu, the correlation factor βw and the partial factor γMw. It is also the reference for bolted connections (§3) and moment-resisting beam-to-column joints via the component method (§6).

What is the EN 1993-1-8 directional method?

The directional method (§4.5.3.2) resolves the force resultant on each unit length of weld throat into three stress components: σ⊥ (normal stress perpendicular to the throat), τ⊥ (shear perpendicular to the weld axis) and τ∥ (shear parallel to the weld axis). These are combined as σ_eq = √(σ⊥² + 3·(τ⊥² + τ∥²)) and checked against fu/(βw·γMw). A second independent condition checks σ⊥ against 0.9·fu/γM2 — and, for a full-penetration butt weld, against the parent metal's yield strength as well. The utilisation is the maximum of the conditions; the weld meets the code when utilisation ≤ 1.0, and MechanixCalc additionally flags anything below a safety factor of 1.5 for review.

How does the correlation factor βw work in EN 1993-1-8?

The correlation factor βw (Table 4.1) reduces the weld resistance to account for the fact that the weld metal and heat-affected zone are never as strong as the nominal parent plate. βw = 0.80 for S235, 0.85 for S275, 0.90 for S355 and 1.00 for S420 and S460. A higher-strength steel has a higher βw, reflecting the narrowing margin between the parent-plate fu and the achievable weld strength. The factor always appears in the denominator, so a higher βw gives a lower (more conservative) weld resistance relative to fu.

Does MechanixCalc implement the EN 1993-1-8 weld method?

It implements a simplified §4.5.3.2 directional method — the two-condition capacity check, all four load cases (direct shear, bending, torsion and combined F+M+T), and weld group properties for single and twin parallel weld lines — plus a parent-metal cap for full-penetration butt welds, a conservative linear shear screen, the long-joint factor β_Lw and a short-weld flag (§4.5.1). The second condition uses the tool's selected partial factor (1.25, or 1.50 if chosen). It does NOT implement the whole of EN 1993-1-8: the standard also covers bolted connections (§3), hollow-section joints (§7) and the component method for moment-resisting beam-to-column joints (§6), none of which this calculator performs. The governing clause, which of the checks actually governs, βw, γM2 and all intermediate stresses are shown in the generated PDF report. Fatigue endurance (EN 1993-1-9) and heat input / t8/5 cooling time (EN 1011-1/-2) are included in the same session.

Is the EN 1993-1-8 weld calculator free?

You can use it during a free 30-minute preview with no sign-up required, and a free 14-day account trial unlocks every calculator with no credit card. The branded PDF engineering report citing EN 1993-1-8 clause references and saved calculations are included in the free 14-day trial and in every paid plan.

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