UK Bolt Pretension Methods -- EN 1993-1-8 Clause 3.6 & BS EN 1090-2

Achieving the specified design preload in high-strength structural bolts is not simply a matter of tightening until it feels tight. EN 1993-1-8 Clause 3.6 requires that preloaded bolts be tightened in accordance with BS EN 1090-2, which specifies four approved methods. Each method has distinct advantages, limitations, and quality control requirements that affect both the reliability of the installed preload and the cost of site operations. This reference covers the four tightening methods, the k-class torque coefficient system, preload force Fp,C per Clause 3.6.1(2), and preload verification procedures for UK structural steelwork.

Design Preload Force Fp,C per Clause 3.6.1(2)

Before selecting a tightening method, the target preload must be established. Per EN 1993-1-8 Clause 3.6.1(2), the design preload force for slip-resistant connections is:

Fp,C = 0.7 x fub x As

Where:

• fub = bolt ultimate tensile strength (800 MPa for Class 8.8, 1000 MPa for Class 10.9)
• As = tensile stress area of the bolt
• 0.7 = preload-to-ultimate ratio, providing 30% margin against bolt fracture

The verification preload Fp,C* for BS EN 1090-2 compliance testing is taken as 1.0 x Fp,C (no reduction). The UK NA to EN 1993-1-8 does not modify the 0.7 coefficient.

Standard Preload Values for UK Bolts

Note: Fp,C = 0.7 x fub x As / 1000. These are the target preload forces, not the design slip resistance. For slip design, the design preload Fp,Cd = 0.7 x fub x As / gamma_M7, where gamma_M7 = 1.10 per the UK NA.

Tightening Methods per BS EN 1090-2

Method 1 -- Torque Control (k-Class System)

The most widely used method in UK building construction. The bolt is tightened with a calibrated torque wrench to a target torque value derived from pre-installation verification testing (PIVT).

Target torque: T = k x d x Fp,C

Where k is the torque coefficient (nut factor) determined experimentally, d is the bolt nominal diameter, and Fp,C is the target preload force.

k-Classes per BS EN 1090-2 Annex H:

K0 (Premium): This is the most tightly controlled class. Bolt assemblies must be from a single production batch with a certified lubricant system. The coefficient of variation V = standard deviation / mean k is limited to 0.06. K0 is specified for bridge construction, crane runway girders, and connections where the reliability of preload is critical and the cost of PIVT is justified.

K1 (Standard): The default class for UK building construction. The k-mean range is the same as K0 (0.10-0.16) but the allowed variability is larger (V <= 0.10). K1 is suitable for moment-resisting frames, braced frames, and any preloaded connection in multi-storey buildings. PIVT must be conducted for each bolt batch used on site.

K2 (General): Allows a wider k-mean range (0.10-0.23) and higher variability (V <= 0.15). K2 is used for secondary members, purlin connections, and applications where preload is required but the consequence of under-preload is lower. The wider range accommodates bolts from multiple production batches with less stringent lubrication control.

PIVT requirements for torque control:

• Minimum 5 bolt assemblies tested per batch per diameter per k-class
• Each assembly tightened through 3 cycles (seating, then 2 test tightenings)
• k determined from the mean of the second and third cycles
• Calibrated torque wrench verified daily against a torque analyser
• Bolt assemblies on site must be from the same batch as the PIVT sample

Worked example -- M20 K1 torque: For an M20 Class 8.8 bolt with PIVT-determined k = 0.14 (K1 class): T = 0.14 x 20 x 171.5 = 480 N.m (target torque) Site verification: torque analyser must confirm 480 +/- 5% (456-504 N.m)

Method 2 -- Direct Tension Indicator (DTI) Washers

DTI washers (also known as load-indicating washers or HRC washers) have raised protrusions on one face. As the bolt is tightened, the protrusions crush progressively. The gap between the DTI washer and the bolt head (or nut) is measured with a feeler gauge; when the gap reduces to the specified thickness, the required preload has been achieved.

Advantages over torque control:

• Direct measurement of bolt tension, not torque (eliminates friction variability)
• Visual inspection -- the feeler gauge either fits or it does not
• No PIVT required (the DTI washer is the tension calibrator)
• Works with any tightening tool (impact wrench acceptable)

UK application: DTI washers are increasingly specified for UK bridge construction, wind turbine tower connections, and nuclear structures. In building construction, they are used where access prevents reliable torque wrench use (e.g., blind bolt installations) or where the highest preload reliability is required (Category C slip-resistant connections in stability-critical frames).

DTI gap specification example for M20: For an M20 Class 8.8 bolt requiring Fp,C = 171.5 kN: Select DTI washer with indicator protrusions calibrated to 171.5 kN at 0.40 mm feeler gauge gap. At installation: gap <= 0.40 mm in at least 50% of feeler gauge positions indicates preload achieved. If gap > 0.40 mm at any position: continue tightening.

Method 3 -- Turn-of-Nut (Part-Turn from Snug-Tight)

The bolt is first brought to the snug-tight condition (plies in firm contact, achieved with a few impacts of an impact wrench or the full effort of a person using a standard spud wrench). A prescribed additional nut rotation then develops the required preload based on the linear relationship between nut rotation and bolt elongation in the elastic range.

Required rotation from snug-tight per BS EN 1090-2:

UK practice: Turn-of-nut is less common in UK building construction than torque control. It is used for large-diameter bolts (M30 and above) where torque control becomes less reliable due to thread friction variability, and for site conditions where calibrated torque wrenches are impractical (e.g., steel erection at height with limited access). The method requires the erector to mark the nut and bolt tail to verify rotation -- missing marks are a common inspection finding.

Method 4 -- Combined Method

An enhanced version of the torque method where, after achieving the torque-controlled preload, a final part-turn check is applied. If the nut rotates through the prescribed angle at the target torque, the bolt assembly is rejected (the k-factor has shifted). This provides a cross-check that the torque-tension relationship established in the PIVT remains valid for each bolt on site.

Combined method procedure:

1. Tighten to target torque per PIVT (Method 1)
2. Mark nut position relative to bolt tail
3. Apply additional rotation check: if nut rotates more than 10 degrees before torque re-attains the target, reject the assembly
4. If nut rotates less than 5 degrees, the bolt may be under-torqued (re-tighten)

The combined method is the most robust and is specified in UK NA guidance for Category C slip-resistant connections in critical applications.

Preload Verification

On-site verification frequency per BS EN 1090-2:

Torque audit procedure:

1. Apply increasing torque in the tightening direction with a calibrated torque wrench
2. Record the torque at which the nut first moves (breakaway torque)
3. Breakaway torque must be >= 1.0 x target torque
4. If breakaway torque < target torque, the bolt is under-tensioned and must be re-tightened or replaced

Design Resources

• UK Bolt Pretension -- Slip-Resistant Design -- Fp,Cd and Fs,Rd per EN 1993-1-8 Clauses 3.8 and 3.9
• UK Bolt Grades Guide -- Class 8.8 and 10.9 properties
• UK Bolt Capacity Tables -- Fv,Rd and Ft,Rd for M12-M36
• UK Bolt Torque EN 14399 -- HR/HV assembly torque values
• UK Connection Design Guide -- EN 1993-1-8 bolted and welded joints
• All UK Steel Design References -- complete library

Frequently Asked Questions

What k-class should I specify for a standard UK multi-storey building?

K1 is the default specification for UK building construction. It provides the reliability needed for moment-resisting and braced frames without the premium cost of K0. Specify K0 only when the connection is fatigue-critical (crane girders, bridges), subject to load reversal in a stability-critical frame, or when the UK NA specifically requires it. K2 is acceptable for secondary steelwork where the consequence of lower preload reliability is limited.

Do I need PIVT for DTI washer installations?

No. PIVT is not required for DTI washer installations because the DTI washer provides direct tension indication. The calibration of the DTI protrusions is established by the manufacturer through compression testing per BS EN 14399-9. The site verification is the feeler gauge check, which is simpler and more reliable than torque auditing. However, the DTI washers must be from the same production batch and the manufacturer's calibration certificate must be available on site.

How does temperature affect torque-controlled pretension?

Ambient temperature affects the lubricant viscosity and therefore the torque coefficient k. BS EN 1090-2 requires that PIVT be conducted at a temperature representative of site conditions. For UK winter construction (temperatures below 5 degrees C), the PIVT must be conducted at a comparable temperature or a temperature correction factor applied. As a practical measure, UK steelwork contractors often switch to DTI washers for winter erection to eliminate temperature-dependent torque variability.

Can preloaded bolts be re-used in UK steelwork?

No. BS EN 1090-2 prohibits the re-use of preloaded bolt assemblies. Once a bolt has been fully tensioned, the threads have yielded locally, the protective coating on the nut has been disrupted, and the lubricant has been displaced. Re-tensioning would produce an unreliable preload. If a preloaded bolt is found to be under-tensioned during verification, it must be replaced, not re-tightened.

Educational reference only. All values are per BS EN 1993-1-8:2005 + UK National Annex, BS EN 1090-2:2018, and BS EN 14399. Verify all values against current editions of the standards. Preload verification must be carried out by qualified personnel with calibrated equipment. Designs must be independently verified by a Chartered Structural Engineer registered with IStructE or ICE. Results are PRELIMINARY -- NOT FOR CONSTRUCTION without independent professional verification.