UK Bolt Bearing and Tear-Out Resistance — EN 1993-1-8 Clause 3.6 + UK NA
Bearing resistance of bolted connections per EN 1993-1-8 Clause 3.6 with UK National Annex. The k1 and αb bearing coefficients, Fb,Rd calculation for standard UK bolt spacings, edge distance requirements, and a worked example for S275 and S355 plate in UK steelwork connections.
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Bearing Resistance Formula — EN 1993-1-8 Table 3.4
The design bearing resistance of a connected plate per bolt is:
Fb,Rd = k1 × αb × fu × d × t / γM2
Where:
- k1 = factor accounting for edge distance perpendicular to the force direction
- αb = factor accounting for bolt spacing parallel to the force direction and bolt strength
- fu = tensile strength of the connected plate (410 MPa for S275, 470 MPa for S355)
- d = bolt diameter
- t = plate thickness (minimum of connected parts)
- γM2 = 1.25 (UK NA value)
k1 — Edge Distance Factor
| Bolt Position | Formula | Notes |
|---|---|---|
| Edge bolts | k1 = min(2.8 × e2/d0 − 1.7, 2.5) | e2 = edge distance perpendicular to load |
| Inner bolts | k1 = min(1.4 × p2/d0 − 1.7, 2.5) | p2 = gauge spacing perpendicular to load |
αb — Spacing and Strength Factor
| Bolt Position | Formula | Notes |
|---|---|---|
| End bolts | αb = min(e1/(3d0), p1/(3d0) − 0.25, fub/fu, 1.0) | e1 = end distance parallel to load |
| Inner bolts | αb = min(p1/(3d0) − 0.25, fub/fu, 1.0) | p1 = pitch parallel to load |
Standard Values for UK Connections
For typical UK connection detailing (BS EN 1090-2 minimums + standard spacing):
| Parameter | Symbol | Minimum | Standard (UK Practice) |
|---|---|---|---|
| End distance | e1 | 1.2d0 | 40 mm |
| Edge distance | e2 | 1.2d0 | 30 mm |
| Bolt pitch | p1 | 2.2d0 | 70 mm |
| Gauge | p2 | 2.4d0 | 60 mm |
| Hole diameter | d0 | d + 1 mm | d + 2 mm (≤ M24), d + 3 mm (≥ M27) |
αb Values for Standard Spacings — M20 Bolt in S355 Plate
| Position | e1 = 40 mm, p1 = 70 mm, d0 = 22 mm |
|---|---|
| End bolt: αb = min(40/(3×22), 70/(3×22) − 0.25, 800/470, 1.0) | = min(0.606, 0.811, 1.702, 1.0) = 0.606 |
| Inner bolt: αb = min(70/(3×22) − 0.25, 800/470, 1.0) | = min(0.811, 1.702, 1.0) = 0.811 |
k1 Values for Standard Spacings — M20
| Position | e2 = 30 mm, p2 = 60 mm, d0 = 22 mm |
|---|---|
| Edge bolt: k1 = min(2.8×30/22 − 1.7, 2.5) | = min(2.118, 2.5) = 2.118 |
| Inner bolt: k1 = min(1.4×60/22 − 1.7, 2.5) | = min(2.118, 2.5) = 2.118 |
Fb,Rd per mm Plate Thickness — S355 (fu = 470 MPa), Standard Spacings
| Bolt | d0 (mm) | End bolt αb | Inner bolt αb | Fb,Rd end (kN/mm) | Fb,Rd inner (kN/mm) |
|---|---|---|---|---|---|
| M12 | 14 | 0.641 | 0.929 | 28.9 | 41.9 |
| M16 | 18 | 0.606 | 0.815 | 36.5 | 49.1 |
| M20 | 22 | 0.606 | 0.811 | 45.7 | 61.1 |
| M24 | 26 | 0.606 | 0.808 | 54.8 | 73.1 |
| M30 | 33 | 0.594 | 0.788 | 67.1 | 89.1 |
| M36 | 39 | 0.583 | 0.769 | 79.0 | 104.2 |
For a 10 mm thick S355 plate with M20 end bolts: Fb,Rd = 45.7 × 10 = 457 kN. This exceeds the bolt shear resistance (Fv,Rd = 47.0 kN for 8.8 threads in plane), so bolt shear governs — which is the intended design outcome.
Design Resources
- UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
- UK Steel Mechanical Properties — fy, fu, and elongation tables
- UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
- UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
- UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
- UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
- All UK Steel Design References — complete library
Frequently Asked Questions
What is the minimum end distance for bolts in UK steelwork per EN 1993-1-8?
The minimum end distance e1 is 1.2 × d0 (hole diameter) per EN 1993-1-8 Table 3.3. For an M20 bolt with a 22 mm hole: e1,min = 1.2 × 22 = 26.4 mm. UK practice typically uses 40 mm for M20 as the standard end distance. The UK NA does not modify Table 3.3 minimum values.
What is the difference between bearing and tear-out failure?
Bearing failure is plastic deformation of the plate material around the bolt hole (hole elongation). Tear-out failure is shear rupture of the plate from the bolt hole to the plate edge (end tear-out). Both are captured by the same Fb,Rd formula. The αb factor controls whether the failure is limited by end distance (tear-out governed by e1/3d0), bolt pitch (bearing interaction governed by p1/3d0 − 0.25), or bolt strength (fub/fu). For standard UK detailing, e1/3d0 typically governs for end bolts.
What is the maximum k1 value for a bolt bearing check?
The maximum k1 value is 2.5 regardless of the edge distance or gauge spacing. This corresponds to a fully developed bearing stress block. For edge bolts: k1 = 2.8 × e2/d0 − 1.7, capped at 2.5. For e2 = 30 mm and d0 = 22 mm: k1 = 2.118 (< 2.5). To achieve k1 = 2.5 for an M20 bolt (d0 = 22 mm): e2 ≥ (2.5 + 1.7) × 22 / 2.8 = 33 mm.
Does the UK NA modify the bearing resistance formula?
No. The UK NA to EN 1993-1-8 adopts the bearing resistance formula (Fb,Rd) without modification. The UK NA confirms γM2 = 1.25 for bearing. The k1 and αb factors are unchanged from the recommended values. UK practice follows the standard EN 1993-1-8 Clause 3.6 method for bearing and tear-out checks.
Related Pages
- EN 1993 Steel Design Overview
- European Steel Properties
- EN 1993 Beam Design Guide
- EN 1993 Column Buckling
- EN 1990 Load Combinations
- UK Steel Chemical Composition
- UK Steel Charpy Values
Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. 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.