UK Bolt Torque Chart -- BS EN 14399 Preload Values for High-Strength Structural Bolting

BS EN 14399 is the harmonised European standard governing high-strength structural bolting assemblies for preloading. It comprises ten parts covering bolt/nut/washer systems for preloaded connections in steel structures. In UK practice, preloaded bolts are specified when the connection requires slip resistance at the serviceability limit state (Category B or C per EN 1993-1-8), or when the bolt is subject to direct tension where preload prevents bolt loosening under dynamic loads. This reference presents the complete preload specification per BS EN 14399, the preload force tables for all bolt diametres, and the tightening methods accepted under the UK National Annex.

BS EN 14399 Assembly Systems

BS EN 14399 defines three principal assembly systems for preloaded bolting, each covered by a specific part of the standard:

System HR (EN 14399-3)

Hexagon head bolts with large wrenching flats across flats, matched with hexagon nuts and hardened washers. The HR system uses "HV" style bolts manufactured to EN 14399-4. Properties are Class 8.8 and 10.9 to EN ISO 898-1. The large flats provide a larger load-bearing area under the nut and reduce the risk of the nut pulling through the washer. HR assemblies are the most common preloaded bolt system used in UK steel building frames.

System HV (EN 14399-4)

Similar to HR but with specific dimensions for hexagon fit bolts and nuts that comply with the DIN 6914 legacy standard. Colloquially known as "HV bolts" in UK practice, these are Class 10.9 only and are the preferred system for portal frame eaves and apex moment connections where bolt diametres of M20-M30 are typical. HV bolts are manufactured with a slightly longer thread length than equivalent HR bolts, making them suitable for thicker grip lengths.

System HRC (EN 14399-10)

Countersunk head bolt assemblies with hexalobular socket (Torx) drive, designed for preloaded connections where a flush surface is required. These purpose-designed preload assemblies incorporate a spline end that shears off at the calibrated preload, providing visual confirmation of correct tightening without torque measurement. The UK does not widely use HRC outside specific applications (bridges, offshore structures).

Design Preload Force Fp,Cd

BS EN 1993-1-8 Clause 3.6.1 defines the design preload force for slip-resistant connections:

Fp,Cd = 0.7 x fub x As / gamma_M7

Where:

• Fp,Cd = design preload force (kN)
• fub = ultimate tensile strength of the bolt (800 MPa for 8.8, 1000 MPa for 10.9)
• As = tensile stress area of the bolt (mm^2)
• gamma_M7 = partial factor for preloaded bolts = 1.10 per UK NA

The 0.7 factor converts from ultimate strength to proof strength (approximately 70% of fub for Classes 8.8 and 10.9). This ensures that the preload does not exceed the yield point of the bolt during tightening.

Preload Table — Class 8.8 HR Assemblies

Preload Table — Class 10.9 HV Assemblies

Tightening Methods per BS EN 1090-2

BS EN 1090-2 recognises three tightening methods for achieving the design preload in BS EN 14399 assemblies:

Method T — Torque Control

The most common method in UK building construction. The torque wrench is calibrated and the target torque is derived from pre-installation verification testing (PIVT) on a sample of bolt assemblies from the production batch. The torque coefficient k is determined empirically:

T = k x Fp,Cd x d

The UK NA requires PIVT for every delivery batch of bolt assemblies, and the torque wrench must be calibrated at the start and end of each shift. A 10% sample of installed bolts is checked with a check torque wrench at 1.1 x target torque.

Method CM — Combined Method

An enhanced torque control method with torque + angle monitoring. After an initial snug-tight torque (approximately 30-40% of the target torque), a specified rotation angle is applied. The rotation angle is determined from the bolt elongation vs. torque characteristic measured during PIVT. CM is used for larger diametre bolts (M30 and above) and critical connections where torque-only control is insufficiently precise.

Method DT — Direct Tension Indicators

Compressible washers (DTI washers to BS EN 14399-9) with protruding nibs that compress to a specified gap when the design preload is achieved. A feeler gauge is used to verify the gap. DT does not require PIVT because the preload indicator is intrinsic to the washer. DTI washers are increasingly used in UK steel building frames because they eliminate the uncertainty of the torque coefficient and provide positive visual verification of preload.

Pre-Installation Verification Testing (PIVT) per BS EN 14399

Regardless of tightening method, BS EN 14399 requires that the bolt assembly be verified as capable of achieving the design preload. PIVT is conducted on a Skidmore-Wilhelm tension calibrator or equivalent load cell:

1. Select a random sample of 10-15 complete bolt-nut-washer assemblies from the delivery batch.
2. Install the bolt in the calibrator with the same number of washers and grip length as the actual connection.
3. Tighten to the target torque (Method T) or target rotation (Method CM) and record the induced preload.
4. Calculate the mean k and coefficient of variation (CoV). The CoV must not exceed 8% for the batch to be accepted.
5. Verify that the mean preload achieves at least 1.0 x Fp,Cd and that no individual assembly achieves less than 0.9 x Fp,Cd.
6. Document the PIVT results in the execution specification per EN 1090-2.

Worked Example — Preload Specification for a UK Braced Frame

A multi-storey UK braced frame building specifies M24 Class 10.9 HV assemblies for the vertical bracing connections (Category C slip-resistant at ULS). The designer must prepare the preload specification for 280 M24 bolts.

Step 1 — Design preload: Fp,Cd = 0.7 x 1000 x 353 / 1.10 = 224.6 kN

Step 2 — Select assembly system: BS EN 14399-4 System HV, M24 x 80, Property Class 10.9, with hardened washers to BS EN 14399-5 both under the bolt head and nut. Nut to BS EN 14399-6.

Step 3 — PIVT: A sample of 12 M24 HV assemblies from the delivery batch is tested. Results: mean k = 0.195, CoV = 5.2%. All 12 assemblies achieve >= 224.6 kN at the target torque. Acceptance criteria met.

Step 4 — Target torque: T = 0.195 x 224.6 x 24 = 1,051 N.m (rounded to 1,050 N.m)

Step 5 — Specification: "All M24 bracing connection bolts: BS EN 14399-4 HV Class 10.9 assemblies, tightening Method T to BS EN 1090-2. Target torque 1,050 N.m. PIVT required per BS EN 14399-2. Tightening sequence: two-stage — first to 600 N.m (snug), then to 1,050 N.m. Inspection: 10% sample check at 1,155 N.m. Maximum 5% of checked bolts may show nut rotation."

Design Resources

• UK Bolt Torque Chart — BS EN 1090-2 — Torque tables for Class 8.8 and 10.9
• UK Bolt Pretension Guide — Slip-resistant connection design
• UK Bolt Capacity Tables — Fv,Rd and Ft,Rd per EN 1993-1-8
• UK Bolt Grades Guide — Material properties and grade selection
• UK Steel Design — EN 1993 Guide — Complete Eurocode 3 overview

Frequently Asked Questions

What is the difference between BS EN 14399 and BS EN 1090-2 for bolting?

BS EN 14399 specifies the bolt assembly itself -- the bolt, nut, and washer as a preloaded assembly system. It defines the material properties, dimensions, and performance requirements that the bolt assembly must meet. BS EN 1090-2 covers the execution of steel structures, including the tightening procedure, inspection, and documentation of the bolting process on site. The assembly must conform to BS EN 14399, and the tightening must conform to BS EN 1090-2. Both are required for a compliant preloaded connection.

Which bolt assembly system is most common in UK portal frame construction?

System HV (EN 14399-4) Class 10.9 bolts in M20-M30 diametres dominate UK portal frame construction. HV bolts have a longer thread length than HR bolts, which accommodates the thicker connection plates typical of portal frame eaves and apex moment connections. M24 HV assemblies at the eaves and M20 HV assemblies at the apex are standard for spans up to 40 m. For larger spans or heavier crane loads, M30 HV assemblies may be specified.

Do I need PIVT for every bolt batch in the UK?

Yes -- the UK NA to BS EN 1090-2 mandates PIVT for every delivery batch of bolt assemblies to be used in preloaded connections. The PIVT cannot be waived based on the manufacturer's certificate alone, because the torque coefficient k depends on the specific batch's surface condition, which varies. A new batch from the same manufacturer, even with the same specification, requires separate PIVT.

What is the partial factor gamma_M7 for preloaded bolts?

The UK NA adopts gamma_M7 = 1.10 from EN 1993-1-8. The factor accounts for variability in achieving the target preload force under site conditions, including differences between the PIVT test setup and the actual connection geometry, the skill of the bolting operative, and long-term preload relaxation. Relaxation of preload over time is particularly important for hot-dip galvanised bolts, where the zinc coating creeps under sustained compression, reducing preload by approximately 10-15% over the first six months.

Educational reference only. All design values are per BS EN 1993-1-8:2005 + UK National Annex and BS EN 14399 (all parts). Verify all preload values through pre-installation verification testing on the specific bolt assembly batch. 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.