EN 1993 Fatigue Design — Steel Fatigue per Eurocode 3 Part 1-9
Complete guide to fatigue design of steel structures per EN 1993-1-9:2005. Detail categories (36 to 160), S-N curves (log-log slopes m = 3 and m = 5), Palmgren-Miner damage accumulation, partial factors gamma_Ff and gamma_Mf, equivalent constant amplitude stress range. Worked crane runway girder example with Category 71 detail.
Quick access: Welding Procedure | Weld Inspection | Weld Capacity
Detail Categories (EN 1993-1-9 Tables 8.1-8.10)
| Category | Description | Typical Application |
|---|---|---|
| 160 | Base metal, rolled sections away from welds | Plain girder |
| 140 | Base metal with welded attachments, short | Beam with short stiffeners |
| 125 | Full penetration butt weld, ground flush | Welded splices ground smooth |
| 112 | Full penetration butt weld, NDT checked | Welded splices |
| 100 | Fillet welded attachment | Cross-beam connections |
| 90 | Fillet welded cover plate end | Cover plate termination |
| 80 | Longitudinal weld of stiffener | Stiffener-to-web weld |
| 71 | Fillet weld, transverse load, K-joint | Truss K-joints, crane girders |
| 63 | Cruciform joint with partial penetration | Heavy welded sections |
| 56 | Transverse butt weld on backing bar | Site welds |
| 50 | Base metal at weld toe of stiffener | Fillet welded stiffener toe |
| 45 | Base metal at end of welded cover plate | Cover plate end on flange |
| 40 | Base metal at fillet weld on edge plate | Edge beam weld termination |
| 36 | Non-load-carrying longitudinal fillet weld | Longitudinal attachments |
S-N Curves (EN 1993-1-9 Table 8.1)
| Category | Delta_sigma_C (MPa) at 2M cycles | Delta_sigma_D (MPa) at 5M cycles | Delta_sigma_L (MPa) at 100M cycles |
|---|---|---|---|
| 160 | 160 | 118 | 65 |
| 140 | 140 | 103 | 57 |
| 125 | 125 | 92 | 51 |
| 112 | 112 | 83 | 46 |
| 100 | 100 | 74 | 40 |
| 90 | 90 | 66 | 36 |
| 80 | 80 | 59 | 32 |
| 71 | 71 | 52 | 29 |
| 63 | 63 | 46 | 26 |
| 56 | 56 | 41 | 23 |
| 50 | 50 | 37 | 20 |
| 45 | 45 | 33 | 18 |
| 40 | 40 | 29 | 16 |
| 36 | 36 | 26 | 15 |
For constant amplitude stress ranges above Delta_sigma_D, slope m = 3. Below Delta_sigma_D, slope m = 5. Stress ranges below Delta_sigma_L do not contribute to fatigue damage.
Partial Factors (EN 1993-1-9 Table 3.1)
| Assessment Method | gamma_Ff | gamma_Mf |
|---|---|---|
| Damage tolerant (low consequence) | 1.00 | 1.00 |
| Safe life (medium consequence) | 1.00 | 1.15 |
| Fail safe — non-welded details | 1.00 | 1.15 |
| Fail safe — welded details | 1.00 | 1.25 |
Worked Example — Crane Runway Girder
Welded plate girder, 12 m span. Detail category at web-to-flange weld: Category 71. Crane: 100 kN capacity, 5000 cycles/day, 50 year design life (approx 1.25M cycles).
Stress range at weld: Delta_sigma = 90 MPa (tension to near-zero)
| Parameter | Value |
|---|---|
| Category | 71 |
| Delta_sigma_C | 71 MPa |
| Applied Delta_sigma | 90 MPa |
| gamma_Ff | 1.00 |
| gamma_Mf | 1.25 |
Fatigue resistance: Delta_sigma_C / gamma_Mf = 71 / 1.25 = 56.8 MPa Applied: gamma_Ff x Delta_sigma = 1.00 x 90 = 90 MPa > 56.8 MPa
Fatigue life: N_Rd = 2E6 x (71 / 90)^3 = 2E6 x 0.491 = 982,000 cycles Design life: 1.25M cycles > 982k cycles — detail does NOT achieve design life.
Option: Upgrade to Category 100 (ground butt weld): N_Rd = 2E6 x (100 / 90)^3 = 2E6 x 1.372 = 2.74M cycles > 1.25M cycles OK
Frequently Asked Questions
What do the detail categories in EN 1993-1-9 represent?
Detail categories (Delta_sigma_C) represent the fatigue strength at 2 million cycles. Higher numbers (160 for plain rolled sections) indicate better fatigue performance. Lower numbers (36 for longitudinal fillet welds) indicate fatigue-sensitive details. The category depends on geometry, loading direction, weld type, and fabrication quality.
How does the Palmgren-Miner rule apply?
The Palmgren-Miner rule (Clause 8.3) sums fatigue damage: D = sum(n_i / N_Ri) where n_i is cycle count at stress range Delta_sigma_i and N_Ri is the endurance from the S-N curve. Failure when D >= 1.0. For variable amplitude loading, the stress spectrum is binned and damage accumulated. EN 1993-1-9 permits using equivalent constant amplitude stress range for simplified assessment.
Related Pages
Educational reference only. Fatigue design per EN 1993-1-9:2005. Detail categories from Tables 8.1-8.10. Verify fatigue load models with EN 1991 parts. Results are PRELIMINARY - NOT FOR CONSTRUCTION without independent verification.
Design Resources
Reference pages