EN 1993-1-8 Steel Connection Design — Bolt & Weld Checks
EN 1993-1-8 (Eurocode 3, Part 1-8) is the European standard governing the design of steel connections. It covers bolted connections, welded connections, and their combinations with a rigorous partial safety factor framework. This guide includes worked examples for both bolt shear and fillet weld checks with real numbers, gamma factor tables, and comparisons to AISC 360 and AS 4100. Verify every calculation with our free connection calculator.
What you will learn
- The Eurocode partial safety factor system (gamma_M0, gamma_M1, gamma_M2)
- How to check bolt shear, bearing, and tension per EN 1993-1-8 Table 3.4
- A complete bolt shear worked example with M20 Class 8.8 bolts
- Fillet weld design using both the simplified and directional methods
- A fillet weld capacity worked example with S355 steel
- How EN 1993-1-8 compares to AISC 360 and AS 4100
Copyright and standards notice
This site does not reproduce copyrighted code clauses or proprietary tables verbatim. Discussion of EN 1993-1-8 here is high-level and intended to help you understand verification workflows. Always consult the official published standard and applicable National Annex for authoritative requirements.
The Eurocode safety factor system
Eurocode uses partial safety factors (gamma) applied to resistance rather than the phi factors used in North American and Australian codes. The key factors for connections are:
| Factor | Recommended Value | Application |
|---|---|---|
| gamma_M0 | 1.00 | Resistance of cross-sections (yield) |
| gamma_M1 | 1.00 | Resistance of members to instability |
| gamma_M2 | 1.25 | Resistance of cross-sections in tension (rupture), bolts, welds, pins |
Important: National Annexes can modify these values. For example, the UK National Annex uses gamma_M2 = 1.25 (same as recommended), but some countries use different values. Always check your applicable National Annex.
The gamma_M2 = 1.25 factor applies to all connection checks — bolts, welds, net section rupture. This is equivalent to a phi factor of 1/1.25 = 0.80.
Step 1 — Bolt shear resistance per EN 1993-1-8
Limit states for bolted connections
EN 1993-1-8 Table 3.4 defines the following resistance formulas:
Bolt shear resistance (Cl. 3.6.1): F_v,Rd = alpha_v x f_ub x A / gamma_M2
Where alpha_v = 0.6 for Class 8.8 (shear plane through threaded portion) and alpha_v = 0.5 for Class 10.9.
Bearing resistance (Cl. 3.6.1): F_b,Rd = k1 x alpha_b x f_u x d x t / gamma_M2
Where alpha_b is the smallest of: e1/(3d0), p1/(3d0) - 1/4, fub/fu, or 1.0.
Bolt tension resistance (Cl. 3.6.1): F_t,Rd = k2 x f_ub x As / gamma_M2
Where k2 = 0.9 for standard bolts.
Combined shear and tension (Cl. 3.6.1): F_v,Ed / F_v,Rd + F_t,Ed / (1.4 x F_t,Rd) ≤ 1.0
Bolt shear worked example — M20 Class 8.8
| Parameter | Value |
|---|---|
| Design shear action (V_Ed) | 200 kN |
| Bolt class | 8.8 (f_ub = 800 MPa) |
| Bolt diameter | M20 (d = 20 mm, d0 = 22 mm) |
| Tensile stress area | As = 245 mm² |
| Number of bolts | 4 (2 rows x 2 columns) |
| Shear planes | 1 (single shear, threads in shear plane) |
| Plate grade | S275 (f_u = 430 MPa) |
| Plate thickness | 10 mm |
| End distance (e1) | 40 mm |
| Edge distance (e2) | 35 mm |
| Pitch (p1) | 60 mm |
Shear resistance per bolt: F_v,Rd = alpha_v x f_ub x As / gamma_M2 = 0.6 x 800 x 245 / 1.25 = 94.1 kN per bolt
Group shear resistance (4 bolts): 4 x 94.1 = 376.3 kN
Utilization (shear): 200 / 376.3 = 0.53 (OK)
Bearing resistance per bolt: alpha_b = min(e1/(3 x d0), p1/(3 x d0) - 1/4, f_ub/f_u, 1.0) = min(40/(3 x 22), 60/(3 x 22) - 0.25, 800/430, 1.0) = min(0.606, 0.659, 1.860, 1.0) = 0.606 (end distance governs)
k1 = min(2.8 x e2/d0 - 1.7, 2.5) = min(2.8 x 35/22 - 1.7, 2.5) = min(2.755, 2.5) = 2.5
F_b,Rd = k1 x alpha_b x f_u x d x t / gamma_M2 = 2.5 x 0.606 x 430 x 20 x 10 / (1.25 x 1000) = 104.2 kN per bolt
For end bolts: 104.2 kN. For interior bolts, alpha_b = 0.659, so F_b,Rd = 113.3 kN.
Group bearing: 2 x 104.2 + 2 x 113.3 = 434.9 kN
Utilization (bearing): 200 / 434.9 = 0.46 (OK)
Results summary — bolted connection
| Check | Capacity (kN) | Utilization | Status |
|---|---|---|---|
| Bolt shear (group) | 376.3 | 0.53 | OK |
| Bearing (group) | 434.9 | 0.46 | OK |
| Controlling | 376.3 | 0.53 | OK |
Bolt shear governs. For this geometry, increasing end distance from 40 mm to 50 mm would increase the end bolt bearing capacity by 25%.
Verify this example: Bolted Connection Calculator -- select EN 1993-1-8 code and enter these values.
Step 2 — Fillet weld design per EN 1993-1-8
The two methods
EN 1993-1-8 Cl. 4.5.3 provides two methods for fillet weld design:
Simplified method (Cl. 4.5.3.3): The resultant stress on the weld throat must satisfy: F_w,Ed ≤ F_w,Rd = f_vw,d x a x L_eff
Where f_vw,d = f_u / (sqrt(3) x beta_w x gamma_M2)
Directional method (Cl. 4.5.3.2): Resolves forces into components on the throat plane: sqrt(sigma_perp² + 3 x (tau_perp² + tau_parallel²)) ≤ f_u / (beta_w x gamma_M2) AND sigma_perp ≤ 0.9 x f_u / gamma_M2
The directional method is more complex but can give higher capacity for transversely loaded welds.
Correlation factor beta_w
The correlation factor depends on the steel grade:
| Steel Grade | f_u (MPa) | beta_w |
|---|---|---|
| S235 | 360 | 0.80 |
| S275 | 430 | 0.85 |
| S355 | 510 | 0.90 |
| S420 | 520 | 1.00 |
| S460 | 540 | 1.00 |
Higher-strength steels have higher beta_w values, which reduces the weld design resistance. This is because the weld metal strength does not increase proportionally with the base metal strength.
Fillet weld worked example — 6 mm fillet, S355 steel
| Parameter | Value |
|---|---|
| Design shear action (V_Ed) | 150 kN |
| Weld type | Double fillet weld (both sides) |
| Leg size (a_leg) | 6 mm |
| Throat thickness (a) | 6 / sqrt(2) = 4.24 mm |
| Weld length | 200 mm each side |
| Steel grade | S355 (f_u = 510 MPa) |
| Load direction | Longitudinal (parallel to weld axis) |
Simplified method capacity per mm: f_vw,d = f_u / (sqrt(3) x beta_w x gamma_M2) = 510 / (1.732 x 0.90 x 1.25) = 510 / 1.949 = 261.7 MPa
Design resistance per mm run: F_w,Rd = f_vw,d x a = 261.7 x 4.24 = 1109 N/mm = 1.109 kN/mm
Total weld capacity (two 200 mm runs): 2 x 200 x 1.109 = 443.8 kN
Utilization: 150 / 443.8 = 0.34 (OK)
Capacity per mm — quick reference (simplified method)
| Leg Size (mm) | Throat (mm) | S275 (kN/mm) | S355 (kN/mm) |
|---|---|---|---|
| 4 | 2.83 | 0.812 | 0.741 |
| 5 | 3.54 | 1.015 | 0.926 |
| 6 | 4.24 | 1.218 | 1.109 |
| 8 | 5.66 | 1.624 | 1.479 |
| 10 | 7.07 | 2.029 | 1.849 |
| 12 | 8.49 | 2.435 | 2.219 |
Note: S355 welds have lower capacity per mm than S275 welds for the same weld size because beta_w is higher (0.90 vs 0.85). This is counterintuitive but correct — the higher base metal strength requires a higher correlation factor.
Verify this: Welded Connection Calculator -- select EN 1993-1-8 and enter these values.
EN 1993-1-8 vs AISC 360 vs AS 4100 — connection design comparison
| Feature | EN 1993-1-8 | AISC 360 | AS 4100 |
|---|---|---|---|
| Safety format | Partial factors (gamma_M) | Resistance factors (phi) | Capacity reduction (phi) |
| Bolt shear factor | alpha_v = 0.6 (8.8) | Fnv from Table J3.2 | 0.62 x fuf |
| Bolt safety factor | gamma_M2 = 1.25 | phi = 0.75 | phi = 0.80 |
| Bearing formula | k1 x alpha_b x fu x d x t / gamma_M2 | φ × 2.4 × Fu × d × t (LRFD, φ = 0.75) | 3.2 x df x tp x fu |
| Weld throat | a = a_leg / sqrt(2) | te = a_leg x 0.707 | tt = tw x 0.707 |
| Weld safety factor | beta_w x gamma_M2 | phi = 0.75 | phi = 0.80 SP / 0.60 GP |
| Weld categories | Execution class (EXC1-4) | Single level | SP / GP |
| National Annex | Required for each country | N/A | N/A |
The fundamental engineering is the same across all three codes — the differences are in safety factor placement, terminology, and the National Annex system that allows countries to customize Eurocode parameters.
Eurocode bolt classes compared
| Class | f_yb (MPa) | f_ub (MPa) | Common Use |
|---|---|---|---|
| 4.6 | 240 | 400 | Non-structural, secondary connections |
| 5.6 | 300 | 500 | General structural connections |
| 8.8 | 640 | 800 | Primary structural connections (most common) |
| 10.9 | 900 | 1000 | High-strength connections, pre-loaded bolts |
Key difference from AISC: Eurocode uses the naming convention "X.Y" where the first number x 100 = f_ub, and first x second x 10 = f_yb. So Class 8.8 means f_ub = 800 MPa, f_yb = 0.8 x 800 = 640 MPa.
Step 3 — Slip-resistant connections (Category B and C)
EN 1993-1-8 Cl. 3.9 defines three connection categories:
| Category | Shear Check At | Bearing Check At |
|---|---|---|
| A (bearing) | ULS | ULS |
| B (slip-resistant at SLS) | SLS (slip) + ULS (bearing) | ULS |
| C (slip-resistant at ULS) | ULS (slip) + ULS (bearing) | ULS |
Slip resistance per bolt (Cl. 3.9.1): F_s,Rd = k_s x n x mu / gamma_M3 x F_p,C
Where:
- k_s = 1.0 for standard holes
- n = number of friction surfaces
- mu = slip factor (0.20 to 0.50 depending on surface preparation)
- gamma_M3 = 1.25 (SLS) or gamma_M3 = 1.10 (ULS)
- F_p,C = 0.7 x f_ub x As (preload force)
For an M20 Class 8.8 bolt with mu = 0.40 (blast-cleaned surface): F_p,C = 0.7 x 800 x 245 = 137.2 kN F_s,Rd (SLS) = 1.0 x 1 x 0.40 / 1.25 x 137.2 = 43.9 kN per bolt per friction surface
This is significantly lower than the bearing shear resistance (94.1 kN), which is why slip-resistant connections require more bolts for the same load.
Common mistakes in Eurocode connection design
Forgetting the National Annex. Gamma factors, bolt class restrictions, and execution class requirements can all vary by country. Using recommended values when your country has different NA parameters is a code compliance error.
Using alpha_v = 0.6 for Class 10.9 bolts. Class 10.9 bolts use alpha_v = 0.5 (when the shear plane passes through the threaded portion), not 0.6. This 17% difference matters.
Ignoring the beta_w correlation factor for welds. Omitting beta_w from the weld resistance formula unconservatively overestimates weld capacity. The factor ranges from 0.80 (S235) to 1.00 (S460).
Mixing bearing-type and slip-resistant checks. Category A connections are checked for bearing at ULS only. Category B and C connections must also satisfy slip resistance. Using Category A resistance for a slip-critical joint is unconservative.
Misapplying the bearing formula. The alpha_b factor depends on end distance, pitch, and the ratio f_ub/f_u. End distance often governs — always compute all four terms and take the minimum.
Using gross area for net section rupture. EN 1993-1-1 Cl. 6.2.3 requires net area through bolt holes for tension resistance at ultimate. The 0.9 factor on net area (N_u,Rd = 0.9 x A_net x f_u / gamma_M2) is frequently forgotten.
Frequently Asked Questions
What bolt class should I use for standard structural connections? Class 8.8 is the most common choice for primary structural connections in Eurocode practice. Class 10.9 is used when higher bolt capacity is needed or when pre-loaded (slip-resistant) connections are required.
Do I need to check both SLS and ULS for Category B connections? Yes. Category B requires a slip check at SLS and a bearing/shear check at ULS. Both must be satisfied.
Can I use AISC bolt capacity tables with Eurocode? No. The safety factor systems are different. AISC uses phi factors on the resistance side; Eurocode uses gamma factors that are mathematically reciprocal but applied at different points in the calculation chain.
How do I determine the execution class? EN 1090-2 defines execution classes EXC1 through EXC4 based on consequence class, service category, and production category. The execution class affects inspection requirements, fabrication quality, and weld acceptance criteria.
What is the minimum bolt end distance in Eurocode? EN 1993-1-8 Table 3.3 requires e1 ≥ 1.2 x d0 for the end distance (direction of load transfer). For M20 bolts in 22 mm holes, the minimum is 1.2 x 22 = 26.4 mm.
Key Takeaways
- gamma_M2 = 1.25 governs all connection checks (bolts, welds, net section rupture). Always check the National Annex for your jurisdiction.
- alpha_v differs by bolt class: 0.6 for Class 8.8, 0.5 for Class 10.9. Getting this wrong is a 17% error.
- Bearing resistance depends on end distance. Compute alpha_b from all four conditions and take the minimum.
- Weld capacity in Eurocode depends on steel grade through beta_w. Higher-strength steel can mean lower weld capacity per mm.
- Category A/B/C determines the design model, not just the bolt installation method.
- The simplified weld method is conservative. The directional method in Cl. 4.5.3.2 can give 10-15% higher capacity for transverse welds.
Run This Calculation
Bolted Connection Calculator — bolt shear, bearing, tension, and block shear checks per EN 1993-1-8, AISC 360, AS 4100, and CSA S16.
Welded Connection Calculator — fillet and groove weld capacity per EN 1993-1-8 with directional method and throat area calculations.
Further Reading
- Bolt hole sizes reference — standard, oversize, and slotted
- Bolt capacity table — shear and tension for common bolt grades
- Bolt spacing and edge distance requirements
- Minimum weld size reference
- Fillet weld size chart — capacity per unit length
- Steel Fy and Fu reference — yield and tensile strength by grade
- EN 1993 code notes
- AS 4100 bolt group design — worked example
- AS 4100 fillet weld design — SP/GP worked example
- How to verify calculator results
- Steel grades reference
- Disclaimer (educational use only)
Disclaimer (educational use only)
This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.
All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.
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