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.
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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
Design Applications
Common Design Scenarios
This reference covers structural design scenarios commonly encountered in structural steel design practice:
- Strength verification: Check member or connection capacity against factored loads per the applicable design code
- Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
- Code compliance: Ensure design meets all provisions of the governing standard
- Connection detailing: Verify weld sizes, bolt quantities, and edge distances
Related Design Considerations
- System behavior: consider the interaction between members and connections
- Load paths: verify that forces can be transferred through the structure to the foundations
- Constructability: check that the design can be fabricated and erected practically
- Cost optimization: evaluate alternative sections or connection types for economy
Worked Example
Problem: Verify a typical steel member for the following conditions:
Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category
Design Check:
- Determine governing load combination (LRFD or ASD per applicable code)
- Calculate maximum internal forces (moment, shear, axial)
- Compute nominal capacity per code provisions
- Apply resistance/safety factors
- Verify interaction if combined forces exist
Result: Use the results from the Steel Calculator tool to verify design adequacy.
Frequently Asked Questions
What Australian Standard governs structural steel design?
AS 4100-2020 (Steel Structures) is the primary standard for structural steel design in Australia. It covers all aspects of design including member capacity, connections, serviceability, and fire resistance. The standard uses a limit states design philosophy with resistance factors (φ) applied to nominal capacities. Companion standards include AS/NZS 3679.1 for hot-rolled sections, AS/NZS 1554 for welding, and AS/NZS 4600 for cold-formed steel.
What are the common steel grades used in Australian construction?
The most common steel grades for Australian construction are Grade 300 and Grade 350 per AS/NZS 3679.1. Grade 300 (minimum yield 300 MPa for sections > 12 mm thick) is the standard for general structural applications. Grade 350 (minimum yield 340 MPa for sections > 12 mm) is used where higher strength reduces weight. Grade 400 and Grade 450 are available for specialized applications requiring higher strength-to-weight ratios.
How does AS 4100 compare to AISC 360?
Both AS 4100 and AISC 360 use limit states design (LRFD) principles. Key differences include: AS 4100 uses a single "capacity factor" φ approach rather than separate φ for different failure modes; AS 4100 specifies distinct buckling curves for hot-rolled and welded sections; the moment capacity formula in AS 4100 uses αm factor directly rather than Cb; and AS 4100 has more detailed provisions for slender sections and combined actions. Despite philosophical differences, both codes produce similar results for typical members.
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