UK Bolt Grades -- BS EN ISO 898-1 Classes for Structural Steelwork
The selection of bolt grade directly determines the strength, ductility, and cost of every bolted connection in UK structural steelwork. EN 1993-1-8 references BS EN ISO 898-1 for the mechanical properties of carbon steel bolts, and the UK National Annex confirms the design provisions for all permitted bolt grades. This reference covers the complete specification of Classes 4.6, 8.8, and 10.9 as used in UK construction, including the material designation system, chemical composition requirements, mechanical properties, product marking, and guidance for grade selection in typical UK building and bridge applications.
The ISO 898-1 Grade Designation System
The bolt grade number encodes the minimum ultimate tensile strength and the yield ratio. The first number, multiplied by 100, gives the minimum ultimate tensile strength fub in MPa. The second number, divided by 10, gives the yield ratio (fyb/fub). Thus:
- Class 4.6: fub = 400 MPa, fyb = 400 x 0.6 = 240 MPa
- Class 8.8: fub = 800 MPa, fyb = 800 x 0.8 = 640 MPa
- Class 10.9: fub = 1000 MPa, fyb = 1000 x 0.9 = 900 MPa
Intermediate classes -- 5.6, 5.8, 6.8 -- exist in BS EN ISO 898-1 but are rarely specified for structural steel connections in the UK. Class 4.8 is sometimes used for light secondary member connections.
The yield ratio is an indirect measure of ductility. A higher ratio (e.g., 0.9 for Class 10.9) indicates less post-yield strain capacity, which is why alpha_v reduces for high-strength bolts when designing for shear with threads in the plane. The UK NA adopts these distinctions without modification.
Chemical Composition per BS EN ISO 898-1
The bolt grade dictates the permissible ranges for carbon, manganese, phosphorus, sulphur, and alloying elements. For structural bolting, the key concern is the carbon equivalent value (CEV), which governs weldability and hardenability.
Class 8.8 Chemistry (typical for UK structural bolts)
| Element | Range (%) | Purpose |
|---|---|---|
| C | 0.25 - 0.55 | Strength through carbon content and heat treatment |
| Mn | 0.60 - 1.00 | Deoxidiser, strength, and hardenability |
| P | max 0.025 | Controlled to prevent temper embrittlement |
| S | max 0.025 | Controlled to prevent hot shortness |
| Si | 0.15 - 0.35 | Deoxidiser |
| Cr | -- | May be present as residual |
Class 10.9 bolts typically contain higher carbon and may include intentional additions of chromium and molybdenum to achieve the 1000 MPa strength requirement while maintaining adequate ductility. The CEV for Class 10.9 bolts is higher, which has implications for weldability if bolts are inadvertently welded (which should never occur in properly detailed UK connections).
Mechanical Properties
The mechanical property requirements for structural bolts extend beyond UTS and yield strength to include elongation after fracture, reduction of area, and impact toughness where specified for low-temperature applications.
| Property | Class 4.6 | Class 8.8 | Class 10.9 |
|---|---|---|---|
| Minimum tensile strength fub (MPa) | 400 | 800 | 1000 |
| Minimum 0.2% proof stress fyb (MPa) | 240 | 640 | 900 |
| Minimum elongation A5 (%) | 22 | 12 | 9 |
| Minimum reduction of area Z (%) | -- | 52 | 48 |
| Hardness (HV, min-max) | 120-220 | 250-320 | 290-360 |
| Minimum Charpy energy at -20 C (J) | -- | 27 | 27 |
The elongation requirement decreases with increasing strength: Class 4.6 offers substantial ductility (22% elongation), typical of mild steel, while Class 10.9 is relatively brittle (9% elongation). This is why the EN 1993-1-8 alpha_v factor reduces for higher-strength classes -- there is less plastic redistribution capacity to justify the full 0.6 factor.
For UK bridge structures and exposed connections where brittle fracture risk is elevated, supplementary Charpy V-notch testing at the minimum service temperature is mandatory per BS EN 1993-1-10. Class 8.8 bolts with Charpy verification at -20 C (27 J minimum) are the standard specification for UK highway bridges.
Marking and Identification
BS EN ISO 898-1 requires that each bolt be indelibly marked with the manufacturer's identification symbol and the property class. For structural bolts (diameter 5 mm and above), the marking is on the top of the head:
- Class 8.8: Marked "8.8" on the head
- Class 10.9: Marked "10.9" on the head
- Class 4.6: May or may not be marked, though good practice dictates marking
Nuts are marked correspondingly: Class 8 for use with Class 8.8 bolts, Class 10 for use with Class 10.9 bolts. Washers for preloaded assemblies are marked with the manufacturer's identification.
In UK practice, the BCSA National Structural Steelwork Specification requires that all structural bolts, nuts, and washers be delivered with a test certificate to BS EN 10204 Type 3.1, providing traceability from the mill certificate through to the delivered product.
Grade Selection for UK Applications
The selection of bolt grade is project-specific but follows well-established UK conventions:
Class 8.8 is the default for all building frame connections in the United Kingdom. It provides the optimum balance of strength, ductility, cost, and availability. Over 90% of UK steelwork connections use M20 Class 8.8 bolts in either non-preloaded (shear-bearing, Categories A and C) or preloaded (slip-resistant, Categories D and E) configurations.
Class 10.9 is specified for heavily loaded connections where the bolt count would otherwise be impractical. Common applications include bridge bearing connections, crane girder splices, wind turbine tower flange connections, and heavily loaded moment-resisting end plates in multi-storey frames. The cost premium over Class 8.8 is approximately 40-60%.
Class 4.6 is specified for lightly loaded secondary connections, purlin cleats, and service brackets. Its low cost and high ductility make it appropriate where strength requirements are modest but robustness against over-tightening is beneficial.
For preloaded connections in UK practice, Class 8.8 is the standard. Class 10.9 preloaded bolts are specified only where the slip resistance requirement cannot be met with Class 8.8, because the higher preload of Class 10.9 requires proportionally higher tightening forces, which increases the risk of bolt fracture during installation.
Worked Example -- Grade Selection for a Moment Connection
Consider a portal frame knee connection transferring MEd = 350 kN.m. The connection uses an extended end plate with bolts in the tension zone. The designer must select between Class 8.8 and Class 10.9.
Option 1 -- Class 8.8 M24 bolts: Tension capacity per bolt: Ft,Rd = 0.9 x 800 x 353 / 1.25 = 203.3 kN. Lever arm of tension bolt group: h = 600 mm (centre of top three rows to compression flange). Resistance per bolt row: assuming linear distribution and two bolts per row. Moment capacity: approximately 4 bolts x 203.3 kN x 0.60 m = 487.9 kN.m > 350 kN.m. OK. Bolt count: 8 M24 in the tension zone.
Option 2 -- Class 10.9 M20 bolts: Tension capacity per bolt: Ft,Rd = 0.9 x 1000 x 245 / 1.25 = 176.4 kN. Moment capacity: 8 bolts x 176.4 kN x 0.55 m = 776.2 kN.m. OK. Bolt count: Same (8 bolts) but M20 is smaller and lighter.
The decision between these options is typically driven by cost, availability, and contractor preference. Class 8.8 M20 bolts are the most common and therefore cheapest per bolt. Class 10.9 bolts cost more per bolt but may reduce the total number of bolts required, potentially reducing end plate size and simplifying erection. For most UK building frames, the Class 8.8 option with M20 or M24 bolts represents the most economical solution.
UK National Annex Provisions
The UK NA to BS EN 1993-1-8 references BS EN ISO 898-1 for bolt material properties and adopts all recommended values without modification. The key UK NA confirmations are:
gamma_M2 = 1.25 applies to all bolt strength classes for shear and tension checks.
The alpha_v values (0.6 for 8.8, 0.5 for 10.9 with threads in shear plane) are adopted as recommended.
For bolts in tension, the UK NA notes that the reduced k2 factor of 0.63 applies when the bolt is loaded through the head (prying action) rather than through the shank (direct tension). This is automatically accounted for in the T-stub method of Clause 6.2.4.
The UK NA confirms that bolt assemblies must comply with BS EN 14399 (for preloaded assemblies) or BS EN 15048 (for non-preloaded assemblies), which impose additional requirements beyond BS EN ISO 898-1 for the complete bolt-nut-washer assembly.
Interaction with CE Marking and UKCA Marking
Since the UK's departure from the European Union, structural bolts placed on the UK market must carry UKCA marking under the Construction Products Regulation as retained and amended in UK law. The transition period for CE-to-UKCA conversion has been extended, and at the time of writing, CE-marked products continue to be accepted in parallel with UKCA-marked products for most applications in Great Britain (England, Scotland, Wales). Northern Ireland operates under the Northern Ireland Protocol and may require both marks. Specifiers should verify the current acceptance status with the project's building control body.
Design Resources
- UK Steel Grades Reference -- EN 10025-2 grade selection for UK projects
- UK Steel Mechanical Properties -- fy, fu, and elongation tables
- UK Bolt Capacity Tables -- Class 8.8 and 10.9 bolt resistance
- UK Bolt Pretension Guide -- Preloaded bolt design
- UK Connection Design Guide -- EN 1993-1-8 bolted and welded joints
- All UK Steel Design References -- complete library
Frequently Asked Questions
What bolt grade is standard for UK building connections?
Class 8.8 is the standard bolt grade for UK structural steel building connections. It provides the optimum combination of strength (fub = 800 MPa), ductility (12% elongation), cost, and widespread availability. M20 Class 8.8 is the default bolt for beam-to-column connections, bracing cleats, and splice connections in UK multi-storey frames. Class 10.9 is reserved for heavily loaded connections where space constraints or high demands require higher per-bolt capacity.
How does the UK NA modify bolt grade selection for structural steelwork?
The UK NA to BS EN 1993-1-8 does not modify the permitted bolt grades -- Classes 4.6, 5.6, 8.8, and 10.9 are all permitted as in the Eurocode recommended text. The UK NA confirms gamma_M2 = 1.25 for all grades and the alpha_v factors (0.6 for 8.8, 0.5 for 10.9). Supplementary guidance in the UK NA references BS EN 14399 and BS EN 15048 for assembly-level requirements, which impose stricter dimensional and material certification requirements than BS EN ISO 898-1 alone.
What is the meaning of the numbers in a bolt grade designation?
The first number, multiplied by 100 MPa, is the minimum ultimate tensile strength fub. The second number, divided by 10, is the yield ratio fyb/fub. For Class 8.8: fub = 8 x 100 = 800 MPa, fyb/fub = 8/10 = 0.8, therefore fyb = 800 x 0.8 = 640 MPa. For Class 10.9: fub = 1000 MPa, fyb = 900 MPa. For Class 4.6: fub = 400 MPa, fyb = 240 MPa. The grade designation is per BS EN ISO 898-1:2013.
Are stainless steel bolts covered by the same grade system?
No. Stainless steel bolts for structural applications are specified under a different standard -- BS EN ISO 3506. The grades are designated by the steel type (A2, A4, etc.) followed by the property class (50, 70, 80). For example, A4-80 indicates austenitic stainless steel (A4 = grade 1.4401 or 1.4404, molybdenum-bearing for enhanced corrosion resistance) with UTS = 800 MPa. EN 1993-1-4 covers the design of stainless steel connections and references BS EN ISO 3506. The gamma factors differ from carbon steel.
Educational reference only. All design values are per BS EN 1993-1-8:2005 + UK National Annex and BS EN ISO 898-1:2013. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY -- NOT FOR CONSTRUCTION without independent professional verification.