EN 1993-1-8 Bolt Pretension — Slip-Resistant Connections Categories B and C

Complete reference for preloaded bolts in European steel connections per EN 1993-1-8 Clause 3.6.1 and 3.9. Design preload force Fp,Cd tables for Class 8.8 and 10.9 bolts M12 through M36, slip resistance Fs,Rd per Category B (SLS) and Category C (ULS), ks surface preparation factors, and EN 14399 torque method requirements.

Also includes practical tightening torque values for installation, slip factor mu for surface classes A through D, and the relationship between preload force and applied torque.

Quick access: EN 1993 Bolt Capacity → | EN 1993 Bolt Grades → | EN 1993 Connection Design → | Bolt Torque Calculator →

Why Preloaded Bolts?

Preloaded bolts (Categories B and C per EN 1993-1-8 Table 3.1) apply a clamping force that compresses connected plies together, developing friction resistance that prevents slip. This is required when:

• Slip at service load would be unacceptable (aesthetic, alignment, fatigue)
• The connection is subject to load reversal or impact
• The joint must behave as rigid (slip-critical at ULS)
• Prying forces in tension connections must be controlled
• Fatigue-sensitive locations

Design Preload Force — Fp,Cd (Clause 3.6.1)

EN 1993-1-8 Clause 3.6.1 defines the design preload force:

[ F*{p,Cd} = \frac{0.7 \times f*{ub} \times As}{\gamma{M7}} ]

Where 0.7 accounts for relaxation and scatter in preload application, fub is the bolt ultimate tensile strength (800 MPa for 8.8, 1000 MPa for 10.9), As is the tensile stress area, and (\gamma_{M7} = 1.10) is the partial factor for preloading.

Preload Force — Class 8.8 (fub = 800 MPa)

Preload Force — Class 10.9 (fub = 1000 MPa)

Class 10.9 provides 25% higher preload than Class 8.8, directly translating to 25% higher slip resistance.

Tightening Torque Values

The relationship between tightening torque T and preload force Fp is:

[ T = k \times d \times F_p ]

Where k is the nut factor (typically 0.15-0.25 for lubricated bolts) and d is the nominal bolt diameter.

Recommended tightening torques for standard lubricated bolts (k = 0.20):

Note: These torques are for installation with calibrated torque wrenches. The actual torque-preload relationship depends on lubrication, thread condition, and washer type. Always verify with the bolt manufacturer's data. The k-factor (nut factor) should be determined by torque-tension testing per EN 1090-2 for critical applications.

Slip Resistance — Category C (ULS)

[ F*{s,Rd} = \frac{k_s \times n \times \mu \times F*{p,Cd}}{\gamma_{M3}} ]

Slip Factor by Surface Preparation

Shot-blasted surfaces with light rusting (Class A, (\mu = 0.50)) are the optimal faying surface preparation for slip-critical connections and should be specified whenever possible. Painted surfaces reduce slip resistance substantially.

Slip Resistance Tables — Category C ((\gamma_{M3} = 1.25))

Single shear (n = 1), normal clearance holes (ks = 1.0), Class A surface ((\mu = 0.50)):

Double shear (n = 2): double all values above.

For Class B surface ((\mu = 0.40)): multiply by 0.80. For Class C ((\mu = 0.30)): multiply by 0.60.

EN 14399 Assembly Requirements

Preloaded bolt assemblies must comply with EN 14399 as matched systems:

1. Matched assemblies: Nuts and washers from the same manufacturer — mixing voids certification.
2. k-class: K1 or K2 per EN 14399-2 (defines the torque-preload scatter).
3. Lubrication: The assembly must be lubricated per manufacturer specification.
4. Tightening methods:
   • Torque method (HR system, EN 14399-3): Calibrated torque wrench
   • Combined method (HV system, EN 14399-4): Torque + part-turn
   • Direct tension indicators (DTI, EN 14399-9): Load-indicating washers
   • Tension control (TC) bolts: Spline shear-off at calibrated torque
5. Inspection: 5% random sample per EN 1090-2 must achieve the calibrated criterion.

Worked Example — Slip-Critical Bracing Connection

Problem: Design a slip-critical (Category C) connection for a bracing member with design force NEd = 600 kN. Two connected plates in double shear with shot-blasted surfaces (Class A, (\mu = 0.50)). Normal clearance holes. Use M20 Class 8.8 bolts.

Step 1 — Slip resistance per bolt: (F_{s,Rd} = 1.0 \times 2 \times 0.50 \times 124.7 / 1.25 = 99.8) kN per bolt (double shear, n = 2)

Step 2 — Number of bolts required: (n = 600 / 99.8 = 6.01 \rightarrow) use 6 bolts (2 rows of 3)

Step 3 — Bearing check (Category A at ULS, also required for Category C): (F*{b,Rd}) per EN 1993-1-8 Table 3.4. For S355 plate, M20 bolt, 10 mm plate, e1 = 40 mm, e2 = 30 mm: (\alpha_b = \min(40/(3 \times 22), 800/510, 1.0) = \min(0.606, 1.57, 1.0) = 0.606) (k_1 = \min(2.8 \times 30/22 - 1.7, 2.5) = \min(2.12, 2.5) = 2.12) (F*{b,Rd} = 2.12 \times 0.606 \times 510 \times 20 \times 10 / 1.25 = 104.8) kN (> 99.8) kN — bearing OK.

Step 4 — Torque specification: M20 Class 8.8: T = 0.20 (\times) 20 (\times) 137,200 / 1,000 = 549 Nm. Specify 550 Nm installation torque with calibrated wrench, lubricated condition. Verify with DTI washers on 5% of bolts per EN 1090-2.

Design Resources

• EN 1993 Bolt Grades
• EN 1993 Bolt Capacity
• EN 1993 Bolt Hole Sizes
• EN 1993 Connection Design
• EN 1993 Weld Sizes
• All European References

Frequently Asked Questions

What is the design preload force Fp,Cd for an M20 Class 8.8 bolt? Fp,Cd = 0.7 (\times) 800 (\times) 245 / 1.10 = 124.7 kN. The partial factor (\gamma_{M7} = 1.10) accounts for relaxation and scatter. The nominal preload target at installation is 137.2 kN (0.7 (\times) 800 (\times) 245). The difference between nominal and design represents the safety margin for long-term relaxation.

What is the difference between Category B and Category C slip-resistant connections? Category B checks slip at SLS only ((\gamma*{M3,ser} = 1.10)); at ULS the connection reverts to bearing checks. Category C checks slip at ULS ((\gamma*{M3} = 1.25)) and is the most demanding category. Category C is required for bracing connections with load reversal, impact loads, and seismic frames. Category B is adequate for moment connections where slip at SLS is the primary concern.

How does surface preparation affect slip factor mu? The slip factor ranges from 0.20 (as-rolled mill scale, Class D) to 0.50 (shot-blasted, Class A). Shot-blasting provides 2.5 times the slip resistance of untreated surfaces. Painting reduces (\mu) to 0.30-0.40. For critical Category C connections, specify Class A (shot-blasted, no faying surface paint). Always verify by testing per EN 1090-2 Annex G.

What tightening torque is required for M24 Class 10.9 preloaded bolts? For M24 Class 10.9 with k = 0.20: T = 0.20 (\times) 24 (\times) 247,100 / 1,000 = 1,186 Nm. In practice, specify 1,200 Nm. The k-factor should be confirmed by torque-tension testing. For the combined method (HV system, EN 14399-4), the torque is applied to a snug-tight condition followed by a specified part-turn (typically 180-240 degrees).

Can preloaded bolts be used in tension connections (Category E)? Yes. Category E (EN 1993-1-8 Table 3.1) covers preloaded bolts in tension connections. The preload controls prying action and prevents separation of the connected plies under service loads. For tension connections, the preload force does not directly contribute to the tension resistance (Ft,Rd) — the bolt is checked for tension per Clause 6.2.4 (T-stub model). However, the preload ensures the connection remains tight under fluctuating loads. Class 8.8 is standard; Class 10.9 provides higher Ft,Rd and better prying control but requires hydrogen embrittlement precautions.

Reference only. Verify all values against the current edition of EN 1993-1-8:2005 and EN 1090-2. Preloaded bolt assemblies must comply with EN 14399. Educational reference only.