UK Bolt Grades — BS EN 15048 & BS EN 14399 Guide
Structural bolts in UK are classified by material grade. This reference covers bolt property classes, minimum tensile and yield strengths, and the applicable clauses of BS EN 15048 and BS EN 14399.
Bolt grade selection affects connection capacity, ductility requirements, and installation method. Higher-grade bolts reduce bolt count but may require thicker plates to develop bearing capacity.
Code Reference: BS EN 15048 and BS EN 14399
BS EN 15048 and BS EN 14399 specify bolt material properties and strength grades.
Bolt Property Classes — Mechanical Properties
| Property Class | Material | Min Tensile fub (MPa) | Min Yield fyb (MPa) | Elongation (%) | Core Hardness (HRC) |
|---|---|---|---|---|---|
| 4.6 | Low carbon steel | 400 | 240 | 22 | — |
| 5.6 | Low carbon steel | 500 | 300 | 20 | — |
| 6.8 | Medium carbon steel | 600 | 480 | 16 | — |
| 8.8 | Quenched & tempered | 800 | 640 | 12 | 22-32 |
| 10.9 | Quenched & tempered | 1000 | 900 | 9 | 32-39 |
Naming convention: The number before the decimal × 100 = ultimate tensile strength (MPa). The number after the decimal × the first number × 10 = yield strength (MPa). Example: 8.8 → fub = 8 × 100 = 800 MPa, fyb = 8 × 8 × 10 = 640 MPa.
UK Standard Bolt Assemblies
| Standard | Assembly Type | Grades | Application | Washers |
|---|---|---|---|---|
| BS EN 15048-1 | Non-preloaded | 8.8 | Category A (bearing) connections | 1 flat washer per bolt |
| BS EN 14399-3 | HR — hexagon head | 8.8, 10.9 | Preloaded (slip-critical) | 1 flat + 1 taper washer |
| BS EN 14399-4 | HV — large head | 10.9 | Preloaded with larger bearing face | 2 flat washers |
| BS EN 14399-5 | HRC — tension indicator | 8.8, 10.9 | Direct tension indication (DTI) | 2 flat washers |
| BS EN 14399-6 | Countersunk head | 8.8, 10.9 | Flush surface required | 1 flat washer |
Nut Property Classes (BS EN 15048)
Nuts must be compatible with the bolt grade:
| Bolt Grade | Nut Grade | Nut Proof Load (MPa) | Nut Marking |
|---|---|---|---|
| 8.8 | 8 | 800 | 8 |
| 10.9 | 10 | 1000 | 10 |
Washer Hardness Requirements
| Bolt Grade | Washer Hardness | Standard |
|---|---|---|
| 8.8 | 200-320 HV | BS EN 14399-5, BS EN 15048-2 |
| 10.9 | 300-400 HV | BS EN 14399-5 |
Marking Requirements
All structural bolts must be marked per BS EN 15048 or BS EN 14399:
Grade 8.8 marking: "8.8" on bolt head + manufacturer's identifier + "S" for structural grade Grade 10.9 marking: "10.9" on bolt head + manufacturer's identifier + "S" for structural grade Nut marking: Property class number on one face + manufacturer's identifier
Selection Guide for UK Practice
| Connection Type | Bolt Grade | Standard | Typical Application |
|---|---|---|---|
| Beam-to-column (simple) | 8.8 | BS EN 15048 | Flexible end plate, fin plate |
| Beam-to-column (moment) | 8.8 or 10.9 | BS EN 14399 | Full-depth end plate |
| Column splice | 8.8 | BS EN 15048 | Cover plate splice |
| Bracing connection | 8.8 | BS EN 15048 | Gusset plate to column |
| Base plate anchors | 8.8 | BS EN 15048 | Column base plates |
| Bridge connections | 10.9 | BS EN 14399 | Slip-critical joints |
Dimensional Standards for UK Bolts
| Bolt Size | Thread Pitch (mm) | Across Flats (mm) | Across Points (mm) | Height (mm) |
|---|---|---|---|---|
| M12 | 1.75 | 19 | 21.7 | 8 |
| M16 | 2.0 | 24 | 27.7 | 10 |
| M20 | 2.5 | 30 | 34.6 | 13 |
| M24 | 3.0 | 36 | 41.6 | 15 |
| M30 | 3.5 | 46 | 53.2 | 19 |
| M36 | 4.0 | 55 | 63.5 | 23 |
Worked Example — Grade Selection for a UK Office Building
Given:
- Beam-to-column connections for a 6-storey office
- Typical end plate connection: 6 × M20 bolts per connection
- Design shear per bolt: 55 kN
Option 1 — Grade 8.8 (BS EN 15048): Fv,Rd = 0.6 × 800 × 245 / 1.25 × 10⁻³ = 94.1 kN per bolt > 55 kN — Satisfactory UT = 55/94.1 = 0.58
Option 2 — Grade 10.9 (BS EN 14399): Fv,Rd = 0.5 × 1000 × 245 / 1.25 × 10⁻³ = 98.0 kN per bolt > 55 kN — Satisfactory UT = 55/98.0 = 0.56
Verdict: Grade 8.8 uses a simpler supply chain (non-preloaded, BS EN 15048) with adequate capacity. Grade 10.9 offers minimal advantage for shear capacity but may be needed if the connection is also subject to high tension or requires slip resistance.
Design Resources
- UK Bolt Capacity — Capacity tables for Grade 8.8/10.9
- UK Bolt Hole Sizes — Hole dimensions per EN 1993-1-8
- UK Bolt Pretension — Preloaded bolt installation
- UK Connection Design — Full connection guidance
- UK Steel Properties — Plate material properties
- All UK References
Frequently Asked Questions
What bolt standards apply in UK steelwork?
UK uses BS EN 15048 for non-preloaded structural bolts (Grade 8.8) and BS EN 14399 for preloaded assemblies (HR/HRC/HV grades). Both are harmonized European standards. BS EN 15048 bolts are marked with "8.8" and "S" for structural use. BS EN 14399 bolts have additional requirements for thread length, washer face diameter, and surface finish to ensure proper preload development.
What is the difference between HR and HV bolt assemblies?
HR (BS EN 14399-3) = hexagon head bolts with nut and washer. HV (BS EN 14399-4) = hexagon head bolts for preloading with larger washer face diameter. HR is more common for UK building work. HV bolts have a larger head bearing area to reduce embedment under high preload forces. HR bolts are available in both 8.8 and 10.9 grades, while HV bolts are typically Grade 10.9 only.
Can Grade 10.9 bolts replace Grade 8.8 in UK connections?
Not without checking the connection design. Grade 10.9 bolts have higher strength (fub = 1000 MPa vs 800 MPa) but are more susceptible to hydrogen embrittlement, especially in galvanized or corrosive environments. EN 1993-1-8 Clause 3.4 gives αv = 0.5 for Grade 10.9 (vs 0.6 for Grade 8.8). For tension: k2 = 0.9 applies to both. Grade 10.9 also requires hardened washers (300-400 HV vs 200-320 HV for Grade 8.8) and more careful installation.
What bolt grades are used for anchor bolts in UK foundations?
Column holding-down anchor bolts in UK practice are typically Grade 8.8 threaded bar (BS EN 15048) in lengths of 300-600 mm for pinned bases, or Grade 10.9 for moment-resisting bases. Anchor bolt design follows EN 1993-1-8 Clause 6.2.6 with γM2 = 1.25. The bond to concrete (pull-out resistance) is governed by EN 1992-1-1, not the bolt grade. For base plates subject to uplift, the anchor bolt tension capacity directly affects the holding-down arrangement.
How are bolts certified for UK construction projects?
All structural bolts must have CE marking (or UKCA marking post-Brexit) under the Construction Products Regulation. Batch testing certificates to BS EN 15048 or BS EN 14399 are required for each delivery. Testing includes: tensile strength (full-size product test), wedge tensile test (bolts), proof load test (nuts), hardness test, and dimensional checks. Project specification typically requires execution class EXC2 or EXC3 per BS EN 1090-2, which mandates specific certification and traceability requirements.
Reference only. Verify all values against the current edition of BS EN 15048 and BS EN 14399. This information does not constitute professional engineering advice.