UK Bolt Pretension — BS EN 1090-2 & EN 1993-1-8

Preloaded (slip-resistant) bolted connections require specific minimum bolt tension to develop friction resistance between connected plies. BS EN 1090-2:2018 Clause 8 and EN 1993-1-8 specifies installation requirements.

Installation methods include torque control, combined method (torque + turn), and HRC direct tension indicators per BS EN 1090-2.

Code Reference: BS EN 1090-2:2018 Clause 8 and EN 1993-1-8

BS EN 1090-2:2018 Clause 8 specifies minimum pretension force Fp,C = 0.7 fub As. Verification of pretension is required for slip-critical connections.

Minimum Pretension Forces

Fp,C = 0.7 × fub × As. For Grade 8.8: fub = 800 MPa. For Grade 10.9: fub = 1000 MPa.

Slip Resistance per EN 1993-1-8 Clause 3.6

Category B (SLS slip resistance): [ F*{s,Rd,ser} = \frac{k_s n \mu}{\gamma*{M3,ser}} F_{p,C} ]

Category C (ULS slip resistance): [ F*{s,Rd} = \frac{k_s n \mu}{\gamma*{M3}} F_{p,C} ]

Where:

• ( k_s ) = hole size factor (1.0 for standard holes, 0.85 for oversized, 0.6 for long slotted)
• ( n ) = number of friction interfaces
• ( \mu ) = slip factor (0.5 for blast-cleaned steel, 0.4 for hot-dip galvanized, 0.3 for zinc spray)
• γM3 = 1.25 (UK NA), γM3,ser = 1.10 (UK NA)

Slip Factors for UK Practice

Installation Methods (BS EN 1090-2:2018 Clause 8)

Torque Control Method:

• Calibrated torque wrench set to produce Fp,C
• Calibration required for each bolt batch (5 samples minimum)
• Nut factor k varies with lubrication condition
• Torque: ( T = k \times d \times F_{p,C} )
• Typical nut factor k = 0.15-0.25

Combined Method (Torque + Turn):

• Apply 75% of installation torque
• Then turn nut by specified angle (typically ½ turn for standard grip length)
• Less sensitive to friction variations
• Preferred method for UK bridge and heavy civil work

HRC Direct Tension Indicator (DTI) Method:

• Special washer with protrusions that flatten at target tension
• Visual gap check with feeler gauge (0.1 mm or 0.4 mm depending on type)
• Applied to the nut face (under the nut)
• No torque calibration needed — tension verified directly

Pretension Method Comparison

Slip Resistance Examples — M20 Grade 8.8, Standard Hole

Single interface (n = 1), blast-cleaned (μ = 0.5):

Category B: Fs,Rd,ser = 1.0 × 1 × 0.5 / 1.10 × 137.2 = 62.4 kN per bolt Category C: Fs,Rd = 1.0 × 1 × 0.5 / 1.25 × 137.2 = 54.9 kN per bolt

Double interface (n = 2), blast-cleaned (μ = 0.5):

Category C: Fs,Rd = 1.0 × 2 × 0.5 / 1.25 × 137.2 = 109.8 kN per bolt (2 friction planes)

Worked Example — Slip-Critical Splice Connection

Given:

• Column splice with 8 × M20 Grade 8.8 preloaded bolts
• Design shear: VEd = 400 kN (ULS)
• Blast-cleaned surface preparation (μ = 0.5)
• Standard round holes (ks = 1.0)
• Double interface (splice plates both sides: n = 2)

Step 1 — Minimum pretension: Fp,C = 0.7 × 800 × 245 × 10⁻³ = 137.2 kN per bolt

Step 2 — Slip resistance at ULS (Category C): Fs,Rd per bolt = 1.0 × 2 × 0.5 / 1.25 × 137.2 = 109.8 kN Total slip resistance: 8 × 109.8 = 878 kN > 400 kN — Satisfactory

Step 3 — Bearing check (EN 1993-1-8 Clause 3.6): Even when slip is checked, bearing must also be verified at ULS: Fb,Rd = (k1 × αb × fu × d × t) / γM2 Assuming 12 mm splice plates in S355: Fb,Rd per bolt ≈ 155 kN (typical) 8 × 155 = 1240 kN > 400 kN — Satisfactory

Design Resources

• UK Bolt Capacity — Bolt shear and tension
• UK Bolt Grades — Property classes
• UK Bolt Hole Sizes — Hole dimensions
• UK Connection Design — Connection types
• UK Steel Properties — Plate material data
• All UK References

Frequently Asked Questions

What pretension methods are accepted in UK practice?

BS EN 1090-2 Clause 8 accepts three pretension methods: torque control, combined (torque + turn), and HRC (direct tension indicator). Torque method is most common for site work due to simplicity and lower cost. The combined method is preferred for critical connections (bridge joints, heavy civil) where more reliable tension control is needed. HRC/DTI washers are specified when direct verification of tension is required without relying on torque measurements.

What is the minimum pretension force for preloaded bolts?

Minimum pretension force Fp,C = 0.7 fub As per EN 1993-1-8 Clause 3.6. For M20 Grade 8.8: Fp,C = 0.7 × 800 × 245 × 10⁻³ = 137.2 kN ≈ 137 kN. This is approximately 70% of the bolt's ultimate tensile capacity, ensuring the bolt remains in the elastic range while providing adequate clamp force for slip resistance. The pretension is applied during installation and is not an external applied load — it is internal to the bolt.

When should slip-critical connections be specified in UK steelwork?

Slip-critical connections are required when: (a) the connection is subject to load reversal or vibration that could loosen bolts, (b) the connection must not slip under service loads to maintain alignment (e.g., bridge joints with fatigue-sensitive details), (c) connections using oversized or slotted holes where bearing alone is insufficient, (d) connections where hole elongation under service loads is unacceptable, and (e) connections requiring full tension (preload) for stiffness, such as moment-resisting splices.

What is the difference between Category B and Category C slip resistance?

Category B checks slip at serviceability limit state (SLS) using γM3,ser = 1.10, while Category C checks slip at ultimate limit state (ULS) using γM3 = 1.25. For Category B, service-level shear forces must not cause slip, but at ULS, the connection can bear (similar to a bearing-type connection). For Category C, slip must not occur even at ULS — the connection remains non-slip under ultimate loads. Category C is more onerous and is typically used for bridge connections.

How is pretension verified on site?

Pretension verification per BS EN 1090-2 Clause 8.5: for the torque method, at least 10% of bolts per connection are checked with a calibrated torque wrench (nut rotated in tightening direction to confirm torque threshold). For the combined method, nut rotation is inspected. For HRC/DTI, a feeler gauge checks the gap under the DTI washer. If failures exceed 5%, checks are extended to 100% of bolts in that batch. Records of pretension verification must be included in the project quality documentation per execution class EXC2 or EXC3.

Reference only. Verify all values against the current edition of BS EN 1090-2:2018 Clause 8 and EN 1993-1-8. This information does not constitute professional engineering advice.