Canadian Steel Fatigue Design — CSA S16-19 Clause 26 Detail Categories
Complete reference for fatigue design per CSA S16-19 Clause 26 for cyclically loaded steel structures. Covers detail categories from the CISC Handbook, S-N curves, constant amplitude fatigue limit (CAFL), stress range calculation, cumulative damage (Palmgren-Miner), and a worked example for a crane runway beam.
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CSA S16 Fatigue Design Framework
Per CSA S16-19 Clause 26, fatigue design applies when the number of stress cycles exceeds 20,000 over the design life. The S-N approach uses:
Delta_sigma^n × N = C (where n = 3 for slope of S-N curve)
The fatigue life N for a given stress range Delta_sigma is:
N = C / Delta_sigma^3 (for the finite life region)
Detail Categories and Parameters
Per CSA S16 Table 10 (based on CSA S6 and AASHTO):
| Category | Typical Detail | C (×10^12) | CAFL (MPa) | N at CAFL (×10^6) |
|---|---|---|---|---|
| A | Plain rolled base metal, flame-cut edges with ground finish | 8.19 | 165 | 1.82 |
| B | Welded I-sections with continuous longitudinal welds, base metal in welded built-up sections | 3.93 | 110 | 2.95 |
| B' | Full-penetration groove welds with back-up bar removed and ground flush parallel to stress | 2.05 | 83 | 3.59 |
| C | Full-penetration groove welds with back-up bar left in place, transverse stiffener-to-web welds | 1.44 | 69 | 4.39 |
| C' | Base metal at end of longitudinal fillet welds, transverse groove welds (FJP) | 1.44 | 83 | 3.59 |
| D | Transverse groove welds (with backing bar), fillet-welded attachments ≤ 50 mm transverse | 0.72 | 48 | 6.52 |
| E | Base metal at end of longitudinal fillet welds (t > 20 mm), fillet-welded attachments > 50 mm | 0.36 | 31 | 12.1 |
| E' | Base metal at end of longitudinal fillet welds (t > 20 mm, attachments > 100 mm) | 0.18 | 18 | 20.0 |
| F | Fillet welds at weld root failure | 0.17 | 25 | 10.9 |
Fatigue Design Procedure
Step-by-Step
- Identify details: Locate all welded and bolted connection details subject to cyclic stress
- Classify detail category: Per CSA S16 Table 10, determine the appropriate category for each detail
- Calculate stress range: Delta_sigma = sigma_max - sigma_min at each detail
- Determine allowable cycles: N_allowed = C / Delta_sigma^3
- Check fatigue life: N_design (actual cycles) ≤ N_allowed
- Check CAFL: If Delta_sigma ≤ CAFL, infinite life assumed
Stress Range Calculation
For crane runway beams:
Delta_sigma = (M_max - M_min) / S_x
Where M_max = moment from loaded crane, M_min = moment from dead load only (or zero for simply supported).
For welded details on the bottom flange at the beam midspan, the maximum stress range occurs when the crane is directly over the section.
Permissible Stress Ranges
For 2,000,000 Cycles (Frequent Loading)
| Detail Category | Permissible Delta_sigma (MPa) | Application |
|---|---|---|
| A | 165 | Base metal remote from welds |
| B | 110 | Longitudinal welds, beam web to flange |
| C | 69 | Full penetration groove weld at stiffener |
| D | 48 | Transverse groove weld with backing |
| E | 31 | End of longitudinal fillet weld, t ≤ 20 mm |
| E' | 18 | End of longitudinal fillet weld, t > 20 mm |
For 100,000 Cycles (Serviceability Limit)
| Detail Category | Permissible Delta_sigma (MPa) |
|---|---|
| A | 435 |
| B | 340 |
| C | 244 |
| D | 194 |
| E | 154 |
Improvement Factors
Per CSA S16 Clause 26.4, the fatigue resistance can be improved by:
| Improvement Method | Increase in Category | Comments |
|---|---|---|
| Grinding weld toes flush | +1 category | Remove stress raisers |
| TIG dressing | +1-2 categories | Remelt toe region |
| Peening (hammer or needle) | +2-3 categories | Compress surface |
| Shot peening | +50% CAFL | Improves all details |
| Burr grinding transverse attachments | Up to Cat C | For details D through E' |
Worked Example — Crane Runway Beam
Given: W610×125 crane runway beam spanning 9.0 m, simply supported. Bridge crane: 50 kN capacity, 100 kN trolley + bridge self-weight. Estimated 100 lifts/day, 250 days/year, 50-year design life = 1,250,000 cycles.
Step 1 — Determine Critical Details:
- Category B: Beam web-to-flange weld (continuous longitudinal fillet)
- Category C: Crane rail attachment to top flange (transverse stiffener brackets)
- Category E: End of stiffener-to-flange fillet weld at bottom flange
Step 2 — Stress Range (Maximum): Crane wheel loads (assume 2 wheels at 3.0 m spacing). Critical at midspan: M_max_wheel ≈ 375 kN·m (from influence line for maximum moment under one wheel)
Combined moment: M_max = 375 + dead load moment = 375 + 0.5 × 8 × 9.0^2 / 8 / 8 = 375 + 5 = 380 kN·m
S_x (W610×125) = 3,050 × 10^3 mm^3 Delta_sigma = 380 × 10^6 / 3,050,000 = 124.6 MPa (approximately 125 MPa)
Step 3 — Fatigue Check: Category B: CAFL = 110 MPa > 125 MPa? No, 125 > 110, so finite life check: N = C / Delta_sigma^3 = 3.93 × 10^12 / 125^3 = 3.93 × 10^12 / 1.953 × 10^6 = 2.01 × 10^6 cycles N_design = 1,250,000 cycles ≤ 2,010,000 cycles. OK (ratio = 0.62).
Category C: CAFL = 69 MPa. 125 >> 69. Not OK. N = 1.44 × 10^12 / 125^3 = 1.44 × 10^12 / 1.953 × 10^6 = 737,000 cycles N_design = 1,250,000 > 737,000. NOT OK.
Step 4 — Mitigation: For the Category C detail (transverse stiffener bracket):
- Option 1: Redesign bracket to Category B detail (butt weld and grind)
- Option 2: Increase beam to W610×140 (Sx = 3,680 × 10^3 mm^3) Delta_sigma = 380e6 / 3,680,000 = 103.3 MPa Category B: 103.3 < 110 MPa CAFL → infinite life. OK! Category C: N = 1.44e12 / 103.3^3 = 1.44e12 / 1.10e6 = 1.31 million cycles N_design = 1,250,000 ≤ 1,310,000. OK.
Result: W610×125 is adequate for the Category B details but NOT for Category C stiffener details. Increase to W610×140 or redesign stiffener connections to improve detail category.
Fatigue-Resistant Detailing
Design Rules for Fatigue
- Avoid transverse attachments at locations of high tensile stress range
- Provide smooth transitions at changes in cross-section (radius ≥ 50 mm for flange width changes)
- Grind weld toes at critical fatigue locations
- Use full-penetration groove welds with back-gouging for butt splices in tension flanges
- Orient welds parallel to the principal stress direction where possible
- Minimize eccentricity in connections to reduce secondary bending stresses
Fatigue vs Fracture
| Aspect | Fatigue | Fracture |
|---|---|---|
| Mechanism | Crack initiation and propagation under cyclic stress | Rapid crack propagation under tensile stress |
| Governing clause | CSA S16 Clause 26 | CSA S16 Clause 27 |
| Control | Stress range | Fracture toughness (Charpy) |
| Design approach | S-N curves, infinite life | Material selection, temperature zone |
| Critical load | Service load range | Maximum tensile stress |
Frequently Asked Questions
When is fatigue design required per CSA S16? Per CSA S16-19 Clause 26.1, fatigue design is required when the number of stress cycles exceeds 20,000 over the design life of the structure. This applies to: crane runway beams (> 20,000 cycles), railway bridges, highway bridges (per CSA S6), vibrating machinery supports, and wind turbine support structures. For typical building frames, fatigue is not a governing limit state.
What is the constant amplitude fatigue limit (CAFL)? The CAFL is the stress range below which the detail has infinite fatigue life (does not fail in fatigue regardless of the number of cycles). For Category B: CAFL = 110 MPa. For Category C: CAFL = 69 MPa. For Category D: CAFL = 48 MPa. If the calculated stress range is below the CAFL, no further fatigue checking is required — the detail will not develop a fatigue crack.
How do I determine the detail category for a welded connection? Identify the critical location in the connection (where cracking is most likely) and compare with CSA S16 Table 10 (or CSA S6 Table 10). Common categories: (a) Base metal away from welds = Category A, (b) Continuous longitudinal fillet welds = B, (c) Transverse stiffener-to-web weld = C, (d) End of longitudinal fillet weld = E, (e) Transverse groove weld with backing bar = D.
What is the fatigue improvement from grinding weld toes? Grinding weld toes to achieve a smooth concave profile at the weld toe can improve the fatigue category by one level (e.g., from D to C). TIG dressing can improve by 1-2 categories. Peening can improve by 2-3 categories but must be carefully controlled to avoid damaging the base metal. Grinding is the most commonly specified improvement because it is verifiable and does not require specialised equipment.
Related Pages
- CSA S16 Beam Design
- Canadian Steel Charpy Values
- Canadian Steel Properties
- CSA S16 Weld Capacity
- Canadian Weld Inspection
- Beam Capacity Calculator
- All Canadian References
This page is for educational reference. Fatigue design per CSA S16-19 Clause 26. Verify detail category against actual connection details. For bridges, refer to CSA S6 which has more stringent fatigue requirements. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.
Design Resources
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