Steel Fatigue Design — AISC 360 Appendix 3 Stress Categories and S-N Curves

Fatigue is the progressive fracture of steel under repeated cyclic loading at stresses well below the static yield or ultimate strength. AISC 360-22 Appendix 3 provides fatigue design provisions based on stress categories (A through E'), each with a characteristic S-N curve relating stress range to the number of cycles to failure. Fatigue design is required when members or connections experience more than 20,000 cycles of significant live load stress (AISC Appendix 3, Section 3.1).

When fatigue design is required

Fatigue must be checked for structures subjected to repeated loading:

• Crane runway girders -- crane passage cycles (50,000-2,000,000 over structure life)
• Bridge girders -- truck loading cycles
• Sign and signal structures -- wind-induced vibration
• Machinery supports -- vibrating equipment (compressors, fans, presses)
• Conveyor supports -- repeated material loading/unloading
• Parking structures -- typically exempt (< 20,000 significant cycles)

Most building structures are exempt from fatigue because the live load cycle count is too low. AISC Appendix 3 Section 3.1 explicitly exempts structures with fewer than 20,000 cycles of application of live load.

Fatigue stress range

The fatigue check uses the unfactored live load stress range, not the factored design load:

f_SR = f_max - f_min    [stress range per cycle]

Where f_max and f_min are the maximum and minimum stresses at the detail location due to the unfactored live load. If the stress reverses sign (tension to compression), the full algebraic range applies. Residual stresses from welding keep the effective stress in tension even when the applied stress is compressive, so the full range is always used.

Important: Dead load stresses do not contribute to the stress range (they are constant and do not cycle). Only the fluctuating portion of the load produces fatigue damage.

Stress categories (AISC 360 Appendix 3, Table A-3.1)

AISC defines eight stress categories based on the type and location of the structural detail:

S-N curve equation

FSR = (Cf / N)^(1/3) >= FTH

Where FSR = allowable fatigue stress range (ksi), Cf = fatigue constant for the category, N = number of cycles, FTH = fatigue threshold (constant amplitude fatigue limit). If f_SR <= FTH, the detail has infinite fatigue life regardless of the number of cycles.

Worked example -- crane runway girder

Given: W24x76 crane runway girder, top flange fillet welded to rail clip (Category C detail). Crane capacity produces a live load stress range of f_SR = 8.0 ksi at the weld toe. Expected crane passages: 500,000 over 30-year life.

Check: FSR = (44 x 10^8 / 500,000)^(1/3) = (8800)^(1/3) = 20.6 ksi. FTH = 10.0 ksi.

Allowable stress range = min(20.6, unlimited) = 20.6 ksi (above FTH, so threshold does not govern).

Result: f_SR = 8.0 ksi < 20.6 ksi -- PASS. The detail has adequate fatigue life.

What if N = 2,000,000? FSR = (8800000/2000000)^(1/3) = (4.4)^(1/3) * 10 = wait, let me recalculate: FSR = (44*10^8/2*10^6)^(1/3) = (2200)^(1/3) = 13.0 ksi. Still > 8.0 ksi. PASS.

What if the detail were Category E (short attachment)? FTH = 4.5 ksi. FSR = (1110^8/500000)^(1/3) = (2200)^(1/3) = 13.0 ksi. Still passes. But at N = 2,000,000: FSR = (1110^8/2*10^6)^(1/3) = (550)^(1/3) = 8.19 ksi. Still > 8.0, but barely. At N = 2,200,000 it would fail.

Detail category selection guide

Selecting the correct detail category is the most critical step in fatigue design. Common situations:

Category A (best): Plain rolled base metal with no attachments, connections, or holes. This is the unmodified base metal condition -- rarely applicable in practice because all connections introduce a detail.

Category B (common): Base metal at standard bolt holes; base metal at the toe of stiffener-to-web fillet welds that are ground smooth; CJP groove welds ground flush with the base metal surface.

Category C (typical for welded attachments): Base metal at the toe of transverse fillet welds (stiffener-to-flange, bearing stiffener-to-web). This is the most common fatigue-critical detail in crane girders and bridge girders.

Category E/E' (worst): Base metal at the ends of cover plates or long welded attachments. The stress flow around the attachment end creates a severe stress concentration. Avoid these details in fatigue-critical structures.

Multi-code comparison

EN 1993-1-9 (Eurocode 3 Fatigue): Uses a similar S-N approach with detail categories expressed as the reference fatigue strength at 2 million cycles (Delta_sigma_C). Categories range from 160 MPa (base metal) to 36 MPa (worst details). The Eurocode uses partial safety factors gamma_Mf = 1.0-1.35 depending on consequences and inspectability.

AS 4100-2020 Section 11: Uses fatigue detail categories (f_rn) from Table 11.5.1, with S-N curves similar to AISC. The approach is compatible with the Eurocode framework. Fatigue limit state uses a load factor of 1.0 on the fatigue load.

AWS D1.1 Section 2.20: Provides fatigue provisions for welded structures that are largely harmonized with AISC Appendix 3. AWS adds specific requirements for weld quality inspection and acceptance criteria for fatigue-loaded welds.

Practical tip: improving fatigue performance

The most effective way to improve fatigue life is to improve the detail category, not to increase the member size:

1. Grind weld toes to smooth the stress concentration. Can improve the detail by 1-2 categories.
2. Use bolted connections instead of welded connections in fatigue-critical locations. Bolt holes are Category B vs. Category C-E for welds.
3. Avoid welded attachments on tension flanges. Move attachments to the neutral axis or compression zone where fatigue is less critical.
4. Use CJP groove welds ground flush instead of fillet welds where full-strength connections are needed. Ground CJP welds can achieve Category B.
5. Avoid cover plates. They create Category E' details at the termination point. Use a heavier rolled section instead.

Fatigue categories A through E' -- detailed descriptions

Each fatigue category corresponds to a specific structural detail type with a characteristic stress concentration. Selecting the correct category is the single most important step in fatigue design because the allowable stress range varies by a factor of 6 from Category A to Category E'.

Category A -- Base metal with rolled or cleaned surfaces

The best fatigue performance. Applies to base metal with no welds, attachments, holes, or stress risers. Includes:

• Plain rolled shapes and plates with surfaces in the as-rolled condition
• Members with flame-cut edges that have been ground smooth to SSPC-SP 5 (white metal blast)
• Base metal in members without attachments, away from connections

Category A rarely governs design because most structural members have connections or attachments that create a lower category. The fatigue constant Cf = 250 x 10^8 and threshold FTH = 24.0 ksi give the highest allowable stress range of any category.

Category B -- Bolted connections and ground welds

Applies to base metal at the net section through bolt holes and at the toes of longitudinal fillet welds that are ground smooth:

• Base metal at bolt holes (net section), standard holes
• Base metal at the toe of transverse stiffener-to-web fillet welds (where the stiffener is not welded to the flange)
• CJP groove welds ground flush with base metal surface, inspected by RT or UT
• Base metal at the toe of longitudinal web-to-flange fillet welds in built-up sections

Cf = 120 x 10^8, FTH = 16.0 ksi. This is a common category for bolted connections and well-prepared welded connections.

Category B' -- Longitudinal weld terminations

Applies to base metal at the termination of longitudinal welds where the weld does not continue around the member:

• Base metal at the ends of longitudinal fillet welds
• Gusset plate terminations where the weld ends create a stress concentration
• Longitudinal web-to-flange weld terminations in plate girders

Cf = 61 x 10^8, FTH = 12.0 ksi. The stress concentration at the weld termination point reduces the fatigue life compared to Category B. Providing runout tabs or extending the weld to a less-stressed region can mitigate this detail.

Category C -- Transverse fillet welds and attachments

One of the most common fatigue-critical details in steel structures:

• Base metal at the toe of transverse fillet welds (stiffener welds to flanges, bearing stiffener welds)
• Base metal at the toe of groove welds in transverse joints (without grinding)
• Base metal at the toe of fillet welds attaching gusset plates, connection plates, or other attachments perpendicular to the stress direction

Cf = 44 x 10^8, FTH = 10.0 ksi. This category is frequently encountered in crane girders (stiffener welds), bridge girders (cross-frame connection plates), and industrial structures.

Category C' -- Longitudinal fillet weld shear

Applies to shear stress on the throat of longitudinal fillet welds:

• Shear stress in longitudinal fillet welds (the throat of the weld, not the base metal)
• Plug and slot welds in shear

Cf = 44 x 10^8, FTH = 12.0 ksi. Note that this category uses shear stress on the weld throat, while all other categories use normal stress (tension or compression) in the base metal.

Category D -- Groove welds to transverse members

Applies to base metal at CJP or partial-joint-penetration (PJP) groove welds connecting members at right angles:

• Base metal at groove welds connecting transverse members (e.g., beam-to-column flange CJP welds)
• Base metal at groove welds in T-connections and cross-connections
• Members with groove-welded attachments perpendicular to the primary stress direction, where the attachment thickness exceeds 2 inches

Cf = 22 x 10^8, FTH = 7.0 ksi. The stress concentration at the intersection of the primary member and the transverse attachment creates a moderate fatigue detail.

Category E -- Short welded attachments

Applies to base metal at the ends of short fillet-welded attachments (length a <= 2 inches in the stress direction):

• Base metal at the termination of fillet welds on gusset plates, connection angles, or other attachments with length 0.5 to 2 inches
• Base metal at ends of cover plates (cover plate width less than flange width)
• Base metal at intermittent fillet weld terminations

Cf = 11 x 10^8, FTH = 4.5 ksi. The stress flow must divert around the attachment end, creating a severe stress concentration. Short attachments are particularly damaging because the stress gradient is concentrated over a short length.

Category E' -- Long welded attachments and cover plates

The worst fatigue performance. Applies to base metal at the ends of long welded attachments:

• Base metal at the termination of fillet welds on attachments longer than 4 inches in the stress direction (a > 4 in.)
• Base metal at ends of cover plates wider than the flange
• Base metal at the ends of longitudinal attachments on girder flanges

Cf = 3.9 x 10^8, FTH = 2.6 ksi. The combination of a long attachment (which creates a large zone of stress concentration) and a weld termination creates the most severe fatigue detail in the AISC system. At 2 million cycles, the allowable stress range is only 3.9 ksi for Category E', compared to 24.0 ksi for Category A.

Threshold stress ranges and cycle count table

The table below shows the allowable fatigue stress range FSR at common cycle counts for each category. If the actual stress range is below FTH, the detail has infinite life regardless of cycle count.

How to read this table: For a Category C detail with 500,000 expected cycles, the allowable stress range is 12.1 ksi. If the actual stress range at the detail is 10 ksi, the design passes (10 < 12.1). If the stress range is 14 ksi, the design fails (14 > 12.1) and the detail must be improved or the member size increased.

When fatigue design is required -- decision framework

AISC Appendix 3 Section 3.1 requires fatigue design when ANY of the following conditions are met:

1. The number of full-range live load stress cycles exceeds 20,000 over the design life of the structure
2. The structure supports cranes of any capacity with more than 5,000 lifting cycles
3. The structure supports vibrating machinery that produces stress ranges exceeding FTH
4. The member is subject to wind-induced vibration in flexible structures (signs, towers, stacks)

Conversely, fatigue design is NOT required for:

• Typical building floor and roof framing (< 20,000 cycles over 50-year life)
• Parking structures (traffic loading produces many cycles but stress ranges are usually below FTH)
• Members in compression-only zones (compression does not propagate fatigue cracks; however, residual welding stresses may keep the effective stress in tension)

Fatigue design decision flowchart:

  1. Is the member/connection subject to cyclic live load?
     NO --> Fatigue design not required
     YES --> Continue

  2. Are there more than 20,000 full-range stress cycles expected?
     NO --> Fatigue design not required (AISC A3.1 exemption)
     YES --> Continue

  3. Is the stress range at the detail below FTH for its category?
     YES --> Infinite fatigue life; no further check needed
     NO --> Calculate FSR at the expected number of cycles N
            Check: f_SR <= FSR

Practical examples of fatigue-prone details

Crane runway girder -- stiffener weld toes (Category C)

The most common fatigue problem in industrial buildings. Each crane passage applies a stress cycle to the top flange. For a 25-ton overhead crane making 50 passes per day over 30 years: N = 50 _ 250 work days _ 30 years = 375,000 cycles. The stress range at the stiffener weld toe (Category C) must satisfy FSR = (44 x 10^8 / 375,000)^(1/3) = 13.5 ksi.

Bridge girder -- cross-frame connection plate weld (Category C)

Cross-frame connection plates are fillet welded to the girder web. Under truck loading, each truck passage produces a stress cycle. For a highway bridge with 5,000 trucks per day over 75 years: N = 5,000 _ 365 _ 75 = 137 million cycles. At this cycle count, FSR for Category C drops to (44 x 10^8 / 137,000,000)^(1/3) = 3.2 ksi, which is below FTH = 10 ksi... wait -- actually if N is very large, FSR approaches (Cf/N)^(1/3) which can be below FTH. In practice, variable-amplitude fatigue (different truck weights) is considered per AISC Appendix 3 Section 4, using the rainflow counting method and Miner's rule.

Conveyor support -- welded bracket (Category E)

A conveyor support bracket is fillet welded to a beam flange with a 3-inch attachment length. Conveyor belt tension cycles produce stress at the bracket weld end. For 200 cycles per hour, 8,000 hours per year, 25-year life: N = 200 _ 8,000 _ 25 = 40,000,000 cycles. FSR for Category E = (11 x 10^8 / 40,000,000)^(1/3) = 2.8 ksi. Even modest stress ranges will exceed this, requiring either bolted brackets (Category B) or redesigned connection details.

Sign structure -- anchor rod wind vibration (Category E')

Wind-induced vibration in cantilevered sign and signal structures produces millions of low-amplitude stress cycles at the base plate connection. The anchor rods welded to the base plate create Category E' details. Even though individual stress ranges are small (2-5 ksi), the extremely high cycle count (millions) can produce fatigue failure. Several states have issued advisories requiring fatigue-resistant details for sign structures.

How to improve fatigue resistance

Fatigue resistance can be improved at three levels: reducing the stress range, improving the detail category, or reducing the number of cycles. The most effective strategies target the detail category.

Detail improvement strategies

Stress range reduction strategies

• Increase member size to reduce the nominal stress range at the detail. This is the least efficient approach because doubling the member weight reduces the stress range by only 50%, while a detail category improvement (e.g., C to B) can double the allowable stress range at no weight penalty.
• Redistribute load through parallel load paths to reduce the stress range on any single member.
• Add intermediate supports to reduce span and therefore the moment range.

Cycle count reduction strategies

• Isolate vibrating equipment with spring isolators or neoprene pads to prevent stress transmission into the structural framing.
• Reduce crane travel frequency through operational changes or multiple cranes for lower individual cycle counts.
• Add dampers to wind-sensitive structures to reduce vibration amplitude and effective cycle count.

Common mistakes

1. Using factored loads for fatigue. Fatigue checks use unfactored live load stress ranges. Using LRFD factored loads is overly conservative and may lead to unnecessary member upsizing.
2. Ignoring the stress range threshold (FTH). If the stress range is below FTH for the detail category, the member has infinite fatigue life -- no further check needed regardless of cycle count.
3. Selecting the wrong detail category. The category depends on the specific detail at the location of maximum stress range, not on the member type. A W-shape with a welded stiffener is Category C at the stiffener toe, not Category A.
4. Not checking all details on the member. A crane girder may have Category B at bolt holes, Category C at stiffener toes, and Category E at rail clip weld ends. Each detail must be checked separately.
5. Assuming buildings are exempt without checking cycle count. Structures supporting vibrating equipment, cranes, or repetitive loading may exceed the 20,000-cycle threshold even in buildings.

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Related references

• Weld Joint Types
• Weld Electrode Reference
• Residual Stress
• Stress-Strain Relationship
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against AISC 360-22 Appendix 3 and the governing project specification. The site operator disclaims liability for any loss arising from the use of this information.