Steel Fatigue Design — AISC Stress Categories & Cycles
Fatigue failure occurs when steel members are subjected to repeated cyclic loading, even at stresses below the static yield strength. Cracks initiate at stress concentrations (welds, bolt holes, notches) and grow over thousands or millions of cycles until the member fractures. This guide covers the AISC 360-22 fatigue design provisions.
When Fatigue Design Is Required
Fatigue design is required when ALL of these conditions exist:
- The member is subjected to cyclic loading (load varies between maximum and minimum values)
- The number of cycles exceeds N = 20,000 over the design life
- The stress range (Δσ = σmax - σmin) is tensile at some point during the cycle
Common applications requiring fatigue design:
| Application | Typical Cycles | Stress Range Source |
|---|---|---|
| Crane runway beams | 100,000-2,000,000 | Lifting and traveling |
| Bridge members | 2,000,000+ | Traffic loading |
| Vibration-prone members | 10,000,000+ | Mechanical equipment |
| Sign structures | 500,000+ | Wind-induced vibration |
| Stair stringers | 100,000+ | Foot traffic |
| Conveyor supports | 1,000,000+ | Equipment vibration |
| Offshore platforms | 10,000,000+ | Wave loading |
AISC Fatigue Stress Categories
AISC Table A-3.1 defines stress categories based on the type of stress raiser (the "detail").
Key Stress Categories
| Category | Description | Example |
|---|---|---|
| A | Base metal, rolled or cleaned surfaces | Plain plate away from welds |
| B | Base metal at welded connections | Longitudinal welds in web or flange |
| B' | Base metal at gross section of bolted members | Slip-critical bolted connections |
| C | Base metal at transverse weld toes | Transverse groove welds |
| C' | Base metal at transverse groove welds | Full-penetration groove welds with reinforcing |
| D | Base metal at short attachments | Fillet welded attachments ≤ 2 in long |
| E | Base metal at longer attachments | Fillet welded attachments 2-4 in long |
| E' | Base metal at longer attachments | Fillet welded attachments > 4 in long |
| F | Shear on weld metal | Fillet welds in shear |
Categories range from A (best fatigue resistance) to E' (worst fatigue resistance). The more severe the stress concentration, the lower the category.
Design Stress Range
The design stress range must not exceed the threshold stress range:
Δσ ≤ Fsr
where Fsr = threshold stress range from AISC Appendix 3, Table A-3.1.
Threshold Stress Range by Category and Cycles
| Category | N = 20,000-100,000 | N = 100,000-500,000 | N = 500,000-2M | N = 2M-10M | N > 10M (threshold) |
|---|---|---|---|---|---|
| A | 63 ksi | 37 ksi | 24 ksi | 16 ksi | 24 ksi |
| B | 49 ksi | 29 ksi | 18 ksi | 13 ksi | 16 ksi |
| B' | 39 ksi | 23 ksi | 15 ksi | 10 ksi | 12 ksi |
| C | 35 ksi | 21 ksi | 13 ksi | 9 ksi | 10 ksi |
| D | 28 ksi | 16 ksi | 10 ksi | 7 ksi | 7 ksi |
| E | 22 ksi | 13 ksi | 8 ksi | 5 ksi | 4.5 ksi |
| E' | 16 ksi | 9 ksi | 6 ksi | 4 ksi | 2.6 ksi |
Values are approximate. Use AISC Table A-3.1 for exact values.
Constant Amplitude Threshold (CAFL)
Below this threshold, fatigue cracking does not initiate regardless of the number of cycles:
| Category | CAFL (ksi) |
|---|---|
| A | 24.0 |
| B | 16.0 |
| B' | 12.0 |
| C | 10.0 |
| D | 7.0 |
| E | 4.5 |
| E' | 2.6 |
If the actual stress range is below the CAFL for the applicable category, fatigue design is not required.
Design Procedure
- Determine the number of cycles (N) over the design life
- Identify the stress category based on the detail type (AISC Table A-3.1)
- Calculate the stress range Δσ = σmax - σmin (using service loads, NOT factored loads)
- Compare to threshold: If Δσ < CAFL, fatigue does not govern. If Δσ ≥ CAFL, check against allowable stress range for the given N
- Detail improvement: If the check fails, improve the detail (change from fillet weld to groove weld, remove backing bars, grind weld toes)
Important Design Rules
- Use service loads, not factored loads, for fatigue calculations
- Fatigue is a serviceability limit state, not a strength limit state
- The stress range is what matters, not the maximum stress
- Compression-only stress ranges do not cause fatigue (cracks close in compression)
- Weld details govern: Most fatigue failures occur at welds, not in base metal
- Weld toe grinding can improve fatigue life by 1-2 categories
- No fatigue for wind loads on buildings (number of cycles at design wind speed is small)
- Avoid partial penetration welds in fatigue-critical connections
Fatigue Design Checklist
- Cyclic loading identified and cycle count estimated
- Stress range calculated using service loads
- Stress category assigned per AISC Table A-3.1
- Threshold stress range checked for design cycle count
- Critical details identified (welds, attachments, holes)
- Detail improvements considered if check fails
- Specifications note fatigue requirements for fabrication
Frequently Asked Questions
How many cycles before fatigue becomes a concern? Per AISC, fatigue design is required when the number of cycles exceeds 20,000. Below 20,000 cycles, fatigue is not a design consideration.
Do I need to check fatigue for wind loads on buildings? Generally no. Wind loads at the design level occur too infrequently to accumulate significant fatigue cycles. However, wind-induced vibration of flexible members (signs, poles, cables) may require fatigue design.
What is the most fatigue-resistant detail? Category A (base metal with rolled or cleaned surfaces, away from any welds or connections) has the highest fatigue resistance. Any weld or bolt hole reduces the category.
Can I improve fatigue resistance by grinding welds? Yes. Grinding the weld toes smooth (removing the undercut and notch) can improve the fatigue category by 1-2 levels. This is commonly done for crane runway beams and bridge girders.
What is the constant amplitude fatigue threshold? The CAFL is the stress range below which fatigue cracking will not initiate, regardless of the number of cycles. If your actual stress range is below the CAFL, you do not need to perform a detailed fatigue analysis.
Related Pages
- Bolted Connections — Bolt capacity checks
- Welded Connections — Weld capacity checks
- Crane Runway Beam Design — Fatigue-critical beam design
- Steel Stress-Strain Curve — Material behavior
- Connection Types — Connection detail options
Disclaimer
This is a calculation tool, not a substitute for professional engineering certification. All results must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) before use in construction, fabrication, or permit documents. The user is responsible for the accuracy of all inputs and the verification of all outputs.
[object Object]
[object Object]
Frequently Asked Questions
What is the recommended design procedure for this structural element?
The standard design procedure follows: (1) establish design criteria including applicable code, material grade, and loading; (2) determine loads and applicable load combinations; (3) analyze the structure for internal forces; (4) check member strength for all applicable limit states; (5) verify serviceability requirements; and (6) detail connections. Computer analysis is recommended for complex structures, but hand calculations should be used for verification of critical elements.
How do different design codes compare for this calculation?
AISC 360 (US), EN 1993 (Eurocode), AS 4100 (Australia), and CSA S16 (Canada) follow similar limit states design philosophy but differ in specific resistance factors, slenderness limits, and partial safety factors. Generally, EN 1993 uses partial factors on both load and resistance sides (γM0 = 1.0, γM1 = 1.0, γM2 = 1.25), while AISC 360 uses a single resistance factor (φ). Engineers should verify which code is adopted in their jurisdiction.