UK Fatigue Design — EN 1993-1-9 Detail Categories and S-N Curves
Fatigue design of steel structures per BS EN 1993-1-9:2005 with UK National Annex. Covers detail categories for welded and mechanically fastened joints, S-N curves, the Palmgren-Miner damage accumulation rule, equivalent constant amplitude stress range, and a worked example for a UK crane girder in S355 steel.
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Fatigue Design per EN 1993-1-9
EN 1993-1-9 provides the fatigue design framework for steel structures subject to repeated loading. The fatigue strength is based on detail categories (ΔσC) that classify the fatigue resistance of structural details.
S-N Curves and Detail Categories
The detail category ΔσC is the stress range (in MPa) corresponding to 2 million cycles at 97.7 % survival probability:
| Detail Category | Fatigue Strength at 2 × 10⁶ cycles | UK Application |
|---|---|---|
| 160 | 160 MPa | Parent metal, rolled clean surfaces |
| 125 | 125 MPa | Butt weld ground flush, full NDT |
| 112 | 112 MPa | Butt weld as-welded, good quality |
| 100 | 100 MPa | Butt weld on backing bar |
| 90 | 90 MPa | Cruciform joint, K-butt weld |
| 80 | 80 MPa | Fillet welded attachments (load-carrying) |
| 71 | 71 MPa | Longitudinal fillet weld ends |
| 63 | 63 MPa | Transverse fillet weld attachments |
| 50 | 50 MPa | Cover plates, longitudinal attachments (L > 80 mm) |
| 45 | 45 MPa | Slotted holes, shear connectors |
| 40 | 40 MPa | Transverse butt weld in thin plate |
Detail Categories for Common UK Connections
| Connection Type | Detail | ΔσC (MPa) | FAT Class |
|---|---|---|---|
| Plain steel | Rolled surface, as-delivered | 160 | FAT160 |
| Butt weld | Full-penetration, ground flush, UT tested | 125 | FAT125 |
| Butt weld | Full-penetration, as-welded, no NDT | 90 | FAT90 |
| Butt weld | On permanent backing bar | 80 | FAT80 |
| CJP T-joint | Full-penetration, ground toe | 90 | FAT90 |
| Fillet weld (end) | Load-carrying fillet, 50 mm < L < 100 mm | 71 | FAT71 |
| Fillet weld (end) | Non-load-carrying fillet, L < 50 mm | 80 | FAT80 |
| Fillet weld parallel | Longitudinal fillet | 80 | FAT80 |
| Bolted connection | Net section, preloaded, Category D/E | 112 | FAT112 |
| Bolted connection | Net section, non-preloaded | 90 | FAT90 |
| Stud shear connector | Welded stud in shear | 90 | FAT90 |
| Cover plate end | End of cover plate, t > 12 mm | 50 | FAT50 |
Fatigue Assessment Methods
| Method | Description | Application |
|---|---|---|
| Safe Life | Damage tolerance assumed — component replaced at design life | Standard UK approach for crane girders, bridges |
| Damage Tolerant | Periodic inspection detects cracks before failure | Bridges with defined inspection intervals |
| Palmgren-Miner | Cumulative damage: D = Σ(ni/Ni) ≤ 1.0 | Variable amplitude loading |
Constant Amplitude Fatigue Limit (CAFL)
| Detail Category | ΔσD at 5 × 10⁶ (MPa) | ΔσL at 1 × 10⁸ (MPa) |
|---|---|---|
| 160 | 117 | 64 |
| 125 | 91 | 50 |
| 100 | 73 | 40 |
| 90 | 66 | 36 |
| 80 | 59 | 32 |
| 71 | 52 | 29 |
| 63 | 46 | 25 |
| 50 | 37 | 20 |
Stress ranges below ΔσL are assumed to cause no fatigue damage.
Worked Example — UK Crane Girder
Girder: 533UB, S355, supporting an overhead crane Detail: Butt weld in flange (as-welded, NDT verified), ΔσC = 90 MPa Loading: 500,000 cycles per year, design life 25 years Stress range: Δσ = 65 MPa (from crane load analysis)
Step 1 — Fatigue Resistance
For ΔσC = 90 MPa, the fatigue strength at N cycles:
log(N) = log(2 × 10⁶) − 3 × log(Δσ / 90) for Δσ ≥ ΔσD (CAFL)
ΔσD for Category 90: 66 MPa (at 5 × 10⁶ cycles)
Since Δσ = 65 MPa < ΔσD = 66 MPa, the stress range is below the CAFL. The slope changes to m = 5 below the CAFL:
N = 5 × 10⁶ × (ΔσD / Δσ)^5 = 5 × 10⁶ × (66/65)^5 = 5 × 10⁶ × 1.078 = 5.39 × 10⁶ cycles
Step 2 — Damage Check
Total cycles in design life: 500,000 × 25 = 12.5 × 10⁶ cycles
Palmgren-Miner damage: D = 12.5 × 10⁶ / 5.39 × 10⁶ = 2.32 > 1.0 → FAIL
Step 3 — Mitigation
Options:
- Increase detail category — grind the butt weld flush (ΔσC = 125), giving N ≈ 5 × 10⁶ × (91/65)^5 = 5 × 10⁶ × 5.50 = 27.5 × 10⁶ > 12.5 × 10⁶ — OK
- Reduce stress range — deepen the section (610UB) to reduce Δσ
- Increase inspection interval — damage-tolerant design with regular NDT
- Relocate weld — move the butt weld away from the high-stress region
Recommendation: Grind the butt weld flush and perform UT verification (achieving FAT125). This provides adequate fatigue life with a factor of 2.2 on cycles.
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 a detail category (ΔσC) in EN 1993-1-9?
The detail category ΔσC is the fatigue strength of a structural detail at 2 million cycles, expressed in MPa, at 97.7 % probability of survival (mean minus 2 standard deviations). A higher ΔσC means better fatigue resistance. For example, ΔσC = 90 means the detail can withstand 90 MPa stress range for 2 million cycles. The detail category depends on the joint type, welding quality, and inspection level. FAT90 is typical for as-welded butt joints; FAT125 for ground-flush butt joints with full NDT.
How does the UK NA modify EN 1993-1-9 fatigue design?
The UK NA to BS EN 1993-1-9 adopts the S-N curves and detail categories without modification. The UK NA confirms the partial factors: γMf = 1.00 for high consequence of failure (safe life), γMf = 1.15 for low consequence (damage tolerant). For UK highway bridges to BS EN 1993-2, the UK NA specifies more onerous requirements including stricter detail categories for certain details and mandatory NDT for Category ≥ 100 details.
What fatigue detail category applies to an as-welded butt joint?
An as-welded full-penetration butt joint with the weld cap not ground flush and NDT-verified quality has ΔσC = 90 (FAT90). If the weld is ground flush and UT-verified, the category increases to ΔσC = 125 (FAT125). If the butt weld is on a permanent backing bar (typical for beam flange splices), the category reduces to ΔσC = 80 (FAT80). The constant amplitude fatigue limit ΔσD for FAT90 is 66 MPa.
What is the Palmgren-Miner rule for variable amplitude fatigue?
The Palmgren-Miner rule sums the fatigue damage from each stress cycle: D = Σ(ni/Ni) ≤ 1.0, where ni is the number of cycles at stress range Δσi and Ni is the number of cycles to failure at Δσi from the S-N curve. For variable amplitude crane girder loading, the stress range histogram is divided into bins, and the cumulative damage is calculated. If D exceeds 1.0, the detail has insufficient fatigue life. The UK NA requires γMf = 1.00 for the safe life method.
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.