EN 1993-1-8 Bolt Grades — Classes 4.6, 5.6, 8.8, 10.9 Structural Bolts
Complete reference for bolt grades used in European structural steel design per EN 1993-1-8 (Eurocode 3: Design of Joints). Including mechanical properties (fub, fyb) per EN 1993-1-8 Table 3.1, tensile stress areas, bolt head markings per EN 14399 and ISO 898, and practical grade selection for Category A, B, and C bolted connections.
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EN 1993-1-8 Bolt Grade System
EN 1993-1-8 Clause 3.1.1 permits structural bolts in material grades conforming to ISO 898-1 (steel bolts). The European grade designation follows the ISO system:
X.Y:
- X = fub / 100 (first digit x 100 = minimum tensile strength in MPa)
- Y = fyb / fub x 10 (second digit indicates the yield-to-tensile ratio)
For example, 8.8 means: fub,min = 8 x 100 = 800 MPa, and fyb,min = 8 x 8 x 10 = 640 MPa (yield ratio = 0.8).
Class 4.6
Low-strength carbon steel bolts. Class 4.6 bolts are used for secondary connections, non-structural applications, and temporary works. They have a yield-to-tensile ratio of 0.60.
- Minimum tensile strength (fub): 400 MPa
- Minimum yield strength (fyb): 240 MPa
- Elongation after fracture: 22% min
- Use: Non-structural connections, handrail brackets, purlin attachments, temporary erection bolts
- EN 1993-1-8 status: Permitted in Category A (bearing type) connections per Table 3.1
Class 5.6
Medium-strength carbon steel bolts. Class 5.6 is permitted under EN 1993-1-8 but is uncommon in structural steelwork; Class 8.8 is the de facto minimum for structural connections.
- Minimum tensile strength (fub): 500 MPa
- Minimum yield strength (fyb): 300 MPa
- Elongation after fracture: 20% min
- Use: Lightly loaded structural connections, purlin splices, secondary bracing
- EN 1993-1-8 status: Permitted in Category A bearing connections
Class 8.8
The standard structural bolt grade for EN 1993-1-8 design. Class 8.8 bolts are medium-carbon alloy steel, quenched and tempered. They account for the vast majority of bolted connections in European building structures.
- Minimum tensile strength (fub): 800 MPa
- Minimum yield strength (fyb): 640 MPa
- Core hardness: 22-34 HRC (ISO 898-1)
- Elongation after fracture: 12% min
- Use: All standard structural connections — beam-to-column, column splices, bracing, end plates, fin plates
- EN 1993-1-8 status: Permitted in all categories (A, B, C) and can be used as preloaded bolts per EN 14399-3
Class 10.9
High-strength quenched and tempered alloy steel bolts. Class 10.9 provides approximately 25% higher tensile strength than Class 8.8 and is used where connection geometry is constrained.
- Minimum tensile strength (fub): 1000 MPa
- Minimum yield strength (fyb): 900 MPa
- Core hardness: 32-39 HRC (ISO 898-1)
- Elongation after fracture: 9% min
- Use: Heavy moment connections, highly stressed tension connections, connections with limited bolt space, slip-resistant Category C connections
- EN 1993-1-8 status: Permitted in all categories (A, B, C) and commonly used as preloaded bolts per EN 14399-3
Complete Bolt Grade Properties Table — EN 1993-1-8
| Property | Class 4.6 | Class 5.6 | Class 8.8 | Class 10.9 |
|---|---|---|---|---|
| fub (MPa) | 400 | 500 | 800 | 1000 |
| fyb (MPa) | 240 | 300 | 640 | 900 |
| fyb/fub ratio | 0.60 | 0.60 | 0.80 | 0.90 |
| Core hardness (HRC) | — | — | 22-34 | 32-39 |
| Elongation A5 (%) | 22 | 20 | 12 | 9 |
| alpha_v (shear coeff.) | 0.60 | 0.60 | 0.60 | 0.50 |
| k2 (tension coeff.) | 0.63 | 0.63 | 0.90 | 0.90 |
| gamma_M2 (partial factor) | 1.25 | 1.25 | 1.25 | 1.25 |
Note the reduction in alpha_v for Class 10.9 bolts: EN 1993-1-8 uses alpha_v = 0.60 for Classes 4.6, 5.6, and 8.8, but reduces to alpha_v = 0.50 for Class 10.9. This reduction acknowledges the lower ductility of Class 10.9 bolts; the high fub provides a net benefit despite the lower coefficient.
Metric Bolt Dimensions — M12 to M36 (ISO Metric Coarse Thread)
EN 1993-1-8 references ISO 898-1 for bolt properties and ISO 4016/4017 for dimensions. The tensile stress area (As) is used for tension resistance calculations per Clause 3.6.1.
| Bolt Size | Thread Pitch (mm) | Body Diameter d (mm) | Tensile Stress Area As (mm2) | Shear Area (body, mm2) |
|---|---|---|---|---|
| M12 | 1.75 | 12.0 | 84.3 | 113 |
| M16 | 2.0 | 16.0 | 157 | 201 |
| M20 | 2.5 | 20.0 | 245 | 314 |
| M22 | 2.5 | 22.0 | 303 | 380 |
| M24 | 3.0 | 24.0 | 353 | 452 |
| M27 | 3.0 | 27.0 | 459 | 573 |
| M30 | 3.5 | 30.0 | 561 | 707 |
| M36 | 4.0 | 36.0 | 817 | 1018 |
The tensile stress area As = (pi / 4) x ((d2 + d3) / 2)^2, where d2 is the basic pitch diameter and d3 is the minor diameter, per ISO 898-1. For shear calculations with the shear plane through the unthreaded portion, the body cross-sectional area A = pi x d^2 / 4 is used.
Bolt Markings — Grade Identification per ISO 898-1 and EN 14399
European structural bolts are identified by head markings for traceability:
| Grade | Bolt Head Marking | Nut Marking | Identification System |
|---|---|---|---|
| 4.6 | 4.6 or manufacturer's mark | 4 or 5 | ISO 898-1 |
| 5.6 | 5.6 or manufacturer's mark | 5 or 6 | ISO 898-1 |
| 8.8 | 8.8 | 8 | ISO 898-1 / EN 14399-3 |
| 10.9 | 10.9 | 10 | ISO 898-1 / EN 14399-3 |
For preloaded assemblies to EN 14399, bolts are marked with the grade and the system designation:
- HR = Hexagon bolt and nut system, preloaded by torque method
- HV = Hexagon fitted bolt and nut system, preloaded by combined method
- HRC = Hexagon bolt and nut system with calibrated wrench
Additionally, the manufacturer's identification mark (e.g., "UB", "N", "PG") is stamped on the bolt head. For tension control (TC) bolts, the grade is marked on the spline end.
Preloaded Bolt Assemblies — EN 14399 System
EN 1993-1-8 distinguishes between ordinary (snug-tight) bolts and preloaded bolts. Preloaded bolts (Classes 8.8 and 10.9) must comply with EN 14399 Parts 1-10, which specifies the bolt assembly, nut, and washer as a matched system.
| EN 14399 System | Designation | Preloading Method | Grade Coverage |
|---|---|---|---|
| HR | Hexagon bolt, nut, washer assembly | Torque method | 8.8, 10.9 |
| HV | Hexagon fitted bolt assembly | Combined method | 8.8, 10.9 |
| HRC | Calibrated wrench assembly | Torque + angle control | 8.8, 10.9 |
| TC | Tension control bolt (twist-off) | Spline shear-off | 8.8, 10.9 |
The design preload force Fp,Cd per bolt is:
Fp,Cd = 0.7 x fub x As / gamma_M7
Where gamma_M7 = 1.10 (EN 1993-1-8 Clause 3.6.1). For an M20 Class 8.8 bolt: Fp,Cd = 0.7 x 800 x 245 / 1.10 = 124.7 kN (unfactored design preload).
Bolt Grade Selection Guide — European Practice
| Application | Recommended Grade | EN 1993-1-8 Category | Preloaded? |
|---|---|---|---|
| Simple beam shear connections (fin plate, web cleat) | 8.8 | A (bearing) | No |
| End plate moment connections | 8.8 or 10.9 | A or B/C | Depends |
| Column splices (multi-storey) | 8.8 | A or B | Optional |
| Bracing connections | 8.8 | A | No (usually) |
| Slip-resistant shear connections (SLS) | 8.8 or 10.9 | B | Yes |
| Slip-resistant shear connections (ULS) | 8.8 or 10.9 | C | Yes |
| Tension connections (hangers, tension rods) | 8.8 or 10.9 | D or E | Yes |
| Fatigue-loaded connections | 8.8 or 10.9 | B or C | Yes |
| Temporary works / erection | 4.6 | A | No |
In practice, Class 8.8 is the default structural bolt grade across Europe. Class 10.9 is specified only where higher capacity per bolt is required (constrained geometry, heavy loads) or when slip resistance at ULS (Category C) demands the higher preload force that 10.9 provides.
Frequently Asked Questions
What is the difference between Class 8.8 and Class 10.9 bolts in EN 1993-1-8? Class 10.9 bolts have approximately 25% higher tensile strength than Class 8.8 (fub = 1000 MPa vs 800 MPa). This provides higher shear and tension resistance for the same bolt diameter. However, EN 1993-1-8 uses alpha_v = 0.50 for 10.9 vs alpha_v = 0.60 for 8.8, so the net shear resistance gain is approximately 4% rather than 25%. Class 10.9 achieves its greater benefit in tension (Ft,Rd = k2 x fub x As / gamma_M2) where the full 25% strength increase is realised. Class 10.9 bolts are also more expensive and have lower elongation at fracture (9% vs 12%) — they are less forgiving of detailing errors.
Why does EN 1993-1-8 use alpha_v = 0.50 for Class 10.9 bolts instead of 0.60? EN 1993-1-8 Table 3.4 specifies alpha_v = 0.60 for Classes 4.6, 5.6, and 8.8, but reduces to 0.50 for Class 10.9. This reduction accounts for the lower ductility of Class 10.9 bolts (minimum elongation 9% vs 12% for 8.8). In a bolted shear connection, bolt ductility is essential for load redistribution among bolts in a group. The more brittle 10.9 material is penalised with a lower alpha_v coefficient to reflect this reduced redistribution capacity. Despite the lower coefficient, the 25% higher fub of 10.9 still provides a modest net gain.
What are the EN 14399 bolt assembly standards for preloaded bolts? EN 14399 is a ten-part standard specifying preloaded bolt assemblies. The key parts are: EN 14399-1 (general requirements), EN 14399-2 (suitability for preloading), EN 14399-3 (HR system — hexagon bolts), EN 14399-4 (HV system — hexagon fitted bolts), EN 14399-8 (HV system for 10.9), EN 14399-10 (HRC system — calibrated wrench). Preloaded bolt assemblies must use matched nuts and washers from the same manufacturer; mixing components from different systems or manufacturers is not permitted under EN 1993-1-8.
What is the design preload force Fp,Cd for an M24 Class 8.8 bolt? Fp,Cd = 0.7 x fub x As / gamma_M7, where gamma_M7 = 1.10 per EN 1993-1-8. For M24 Class 8.8: fub = 800 MPa, As = 353 mm2. Fp,Cd = 0.7 x 800 x 353 / 1.10 = 179.6 kN. This compares to approximately 137 kN for an M20 and 254 kN for an M30. The achieved preload in practice depends on the tightening method (torque, combined method, or TC spline shear-off) and the k-class (k-factor) of the bolt assembly.
Can I use Class 5.6 bolts in structural connections per EN 1993-1-8? Class 5.6 bolts are permitted under EN 1993-1-8 Table 3.1 for Category A (bearing type) shear connections. However, they are rarely used in practice because the cost saving over Class 8.8 is negligible compared to the significant strength penalty (fub 500 MPa vs 800 MPa — 37.5% lower). For a given connection, more Class 5.6 bolts would be needed, or the plate dimensions would have to increase, which typically costs more than using standard Class 8.8 bolts. Class 5.6 is primarily used for secondary members and non-structural applications.
Related Pages
- EN 1993-1-8 Connection Design Guide — Bolt categories, welds, joint classification
- EN 1993-1-8 Bolt Capacity Tables — Fv,Rd and Ft,Rd values M12-M36
- EN 1993-1-8 Bolt Hole Sizes — Clearance, oversized, slotted holes
- EN 1993-1-8 Bolt Pretension Reference — Slip-resistant connections Cat B/C
- EN 1993-1-1 Beam Design Guide — Section classification, Mc,Rd, LTB
- European Steel Properties — EN 10025 S235-S460 Fy and Fu
- Bolted Connections Calculator
- Bolt Torque Calculator
- Beam Capacity Calculator
- Column Capacity Calculator
Educational reference only. All bolt grade values and formulas are per EN 1993-1-8:2005 (Eurocode 3: Design of Joints). Verify all values against the current edition of the Eurocode and the applicable National Annex for your project jurisdiction. Bolt assemblies must comply with EN 14399 for preloaded applications and EN 15048 for non-preloaded structural bolting. Results are PRELIMINARY — NOT FOR CONSTRUCTION. All designs must be independently verified by a licensed Professional Engineer or Chartered Structural Engineer.