Australian Fatigue Design — AS 4100 Clause 11 Guide
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Reference for fatigue design of steel structures per AS 4100:2020 Clause 11. Covers detail categories, S-N curves, stress range calculation, Palmgren-Miner cumulative damage rule, and fracture-critical applications.
Detail Categories — AS 4100 Table 11.5.4
| Category | Description | Constant Amplitude Limit (MPa) |
|---|---|---|
| 10 | As-welded rolled section, butt weld trimmed flush | 52 |
| 20 | Butt weld in plate (full pen, ground) | 40 |
| 30 | Butt weld in plate (as-welded) | 33 |
| 36 | Longitudinal fillet weld end | 27 |
| 45 | Transverse fillet weld (load-carrying) | 22 |
| 50 | Cruciform joint, full-pen weld | 18 |
| 56 | Cruciform joint, partial pen weld | 15 |
| 63 | Attached plate by fillet welds | 12 |
| 71 | Welded cover plate end | 10 |
| 80 | Welded shear stud | 7 |
Higher category number = better fatigue resistance (lower stress concentration).
Fatigue Strength — S-N Curves
Constant amplitude fatigue limit (CAFL): Δσc at 2×10⁶ cycles
Fatigue life: N = (Δσc / Δσ)³ × 2×10⁶ (for Δσ > CAFL)
Infinite life: No fatigue check required if Δσ ≤ CAFL
| Category | Δσc at 2×10⁶ | ΔσCAFL (5×10⁶) | Δσ (1×10⁷) |
|---|---|---|---|
| 10 | 125 MPa | 52 MPa | 40 MPa |
| 20 | 92 MPa | 40 MPa | 30 MPa |
| 36 | 71 MPa | 27 MPa | 20 MPa |
| 50 | 56 MPa | 18 MPa | 14 MPa |
Cumulative Damage — Palmgren-Miner Rule
D = Σ(ni / Nfi) ≤ 1.0
Where:
- ni = number of cycles at stress range Δσi
- Nfi = number of cycles to failure at Δσi from S-N curve
Example: Two stress ranges: Δσ1=50 MPa (2,000 cycles, Nf1 for cat 36), Δσ2=30 MPa (10,000 cycles)
- D = 2000/Nf1 + 10000/Nf2
- If D < 1.0 → OK for design life
Worked Example
Problem: Crane runway girder with Category 36 detail. Stress range Δσ=60 MPa, 500 cycles/day, 250 days/year. Design life 30 years.
Solution:
- Total cycles: N = 500 × 250 × 30 = 3.75×10⁶ cycles
- For Cat 36: Δσc=71 MPa at 2×10⁶
- Fatigue life: Nf = (71/60)³ × 2×10⁶ = 1.65 × 2×10⁶ = 3.30×10⁶ cycles
- Cumulative damage: D = 3.75/3.30 = 1.14 > 1.0 FAIL
- Solution: Use Category 20 detail (better detail). Nf = (92/60)³ × 2×10⁶ = 7.2×10⁶ cycles → D = 3.75/7.2 = 0.52 < 1.0 OK
Design Resources
- [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
FAQ
When is fatigue design required per AS 4100? Clause 11.1 requires fatigue design when the number of stress cycles exceeds 20,000 and the stress range is significant. Crane girders and bridges are typical.
What detail category is best for fatigue? Category 10 (trimmed flush butt weld) has the highest fatigue strength. Category 80-90 (attached elements) have the lowest. Specify detailed design to minimize stress concentrations.
How is cumulative damage assessed? Using Palmgren-Miner rule: D = Σ(ni/Nfi) ≤ 1.0. Each stress range level contributes proportionally to the total damage.
Educational Use Only — This reference is for educational and preliminary design purposes only. All structural designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) in accordance with AS 4100:2020 and all applicable Australian Standards. Results are not for construction.
Design Applications
Common Design Scenarios
This reference covers structural design scenarios commonly encountered in Australian 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 Grade 300 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.