---- | ----------- | -------------------------------------------------- | | 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.

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 BS 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 (diametre 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:

1. gamma_M2 = 1.25 applies to all bolt strength classes for shear and tension checks.

2. The alpha_v values (0.6 for 8.8, 0.5 for 10.9 with threads in shear plane) are adopted as recommended.

3. 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.

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.

BS EN 14399 Assembly Types for Preloaded Bolts

BS EN 14399 is the harmonised European standard for high-strength structural bolting assemblies suitable for preloading. It specifies the complete bolt-nut-washer assembly as a system, recognising that preloaded bolt performance depends on the interaction of all three components. BS EN 14399 is referenced by BS EN 1993-1-8 Clause 3.1.2 for preloaded bolted connections (Categories B, C, and E) and the UK NA confirms its applicability without modification.

BS EN 14399 is published in ten parts, with Parts 1 to 6 being the most relevant for UK structural steelwork:

System HR vs System HV

Two distinct assembly systems are defined, and they must not be mixed on the same bolt:

System HR (BS EN 14399-3): The standard UK system. Uses hexagon head bolts and nuts to BS EN ISO 4014/4017/4032 dimensions with a wider bearing face. Nut height is 0.8d. Both Class 8.8 and Class 10.9 bolts are permitted. Preload is typically achieved by torque tightening to a calibrated value. The HR system is the default preloaded bolt system in UK building construction and is available from all major UK fastener suppliers.

System HV (BS EN 14399-4): The Continental European system, also used in UK bridgework. Uses hexagon head bolts and nuts to DIN 6914/6915 dimensions with a narrower bearing face and taller nut (1.0d). Only Class 10.9 bolts are permitted. Preload is typically achieved by the combined method (torque plus part-turn). HV bolts are identifiable by the manufacturer's mark and property class stamped on the head, plus a distinctive washer profile.

Key Distinctions

BS EN 15048 for Non-Preloaded Assemblies

Non-preloaded bolting assemblies (for shear-bearing connections, BS EN 1993-1-8 Category A) are specified under BS EN 15048. This standard is less demanding than BS EN 14399 because preload is not relied upon for design resistance. BS EN 15048 covers Class 4.6, 8.8, and 10.9 bolts with standard nuts and washers, and is the appropriate specification for the majority of UK building frame connections where bolts are designed in shear and bearing only.

Comparison with ISO 898-1 Mechanical Property Grades

BS EN ISO 898-1 defines the material property classes for carbon steel bolts, but it covers only the bolt itself -- not the nut, not the washer, and not the assembly behaviour. The table below compares the BS EN ISO 898-1 bolt property classes with the complete assembly standards used in UK structural steelwork:

The critical point for UK specifiers: specifying "Class 8.8 bolts to BS EN ISO 898-1" is insufficient for a preloaded connection. The correct specification is "Class 8.8 preloaded bolting assemblies to BS EN 14399-3 (System HR)". This single line on the drawing ensures the contractor supplies assemblies that have been tested and verified as a system, not individual components that happen to meet the bolt material standard.

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 relevant designated standard for UKCA marking of structural bolting assemblies is BS EN 14399 (for preloaded assemblies) or BS EN 15048 (for non-preloaded assemblies), not BS EN ISO 898-1 alone.

The UKCA marking requirements include:

• Manufacturer's Declaration of Performance (DoP): Must be issued for each assembly type, declaring the essential characteristics (yield strength, ultimate tensile strength, elongation, Charpy energy, preload performance where applicable).
• UKCA mark on product or packaging: The bolt head, nut, and packaging must carry the UKCA mark alongside the property class marking. For small components where marking is impractical, the packaging label suffices.
• Approved Body certification: For System 1+ products (preloaded assemblies), an Approved Body must certify the factory production control and perform audit testing. This is the UK equivalent of the EU Notified Body.
• Batch testing: Each production batch must be accompanied by a BS EN 10204 Type 3.1 test certificate from the manufacturer, and for preloaded assemblies, a supplementary test certificate from the Approved Body confirming preload suitability.

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.

For projects where both CE and UKCA marks are acceptable, the specifier should confirm that the bolt assemblies have been tested to the same standard under both schemes. The technical requirements of BS EN 14399 are identical whether assessed under the CE or UKCA framework; the difference is purely in the conformity assessment body (EU Notified Body vs UK Approved Body) and the mark applied.

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 -- BS EN 1993-1-8 bolted and welded joints
• All UK Steel Design References -- complete library

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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. BS 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.