AS 4100 Fatigue Design — Clause 11, Stress Categories & Crane Runway Example

Complete AS 4100:2020 Clause 11 fatigue design walkthrough: S-N curve methodology, detail categories for welded connections, thickness and life correction factors, Miner's cumulative damage rule for variable-amplitude loading, and a worked example for a 20-tonne overhead crane runway girder.

The Steel Calculator WASM engine performs fatigue checks per AS 4100 Clause 11, AISC 360 Appendix 3, EN 1993-1-9, and CSA S16 Clause 26. Fatigue assessment is mandatory for crane runway girders, bridges, offshore structures, and any structural element subject to more than 20,000 load cycles during its design life.

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a Chartered Professional Engineer (CPEng) or RPEQ before use in any project.

Design Problem Definition

A 20-tonne overhead travelling crane in a steel fabrication workshop is supported on a simply supported runway girder spanning 8.0 m between building columns. The crane operates at service class A5 (medium-duty, 500,000 to 1,000,000 full-load-equivalent cycles per AS 1418.1). The runway girder is a 610UB125 Grade 300PLUS with a 25 mm thick continuously welded cap channel (250PFC) on the top flange to form the crane rail seat.

Design Data:

• Girder span: L = 8.0 m, simply supported
• Section: 610UB125 Grade 300PLUS
• Cap channel: 250PFC Grade 300PLUS, continuously fillet-welded to the UB top flange
• Crane capacity: 20 tonnes (SWL), plus 5 tonnes crane bridge self-weight
• Crane wheel loads: P_max = 185 kN (factored), P_min = 42 kN (light hook)
• Wheel spacing: 4.2 m centre-to-centre
• Design life: 2,000,000 cycles (heavy-duty, 50-year workshop operation)
• Service class: A5 per AS 1418.1 (medium-duty)

Critical Fatigue Details to Check:

1. Top flange-to-web fillet weld (Category 80)
2. Transverse stiffener-to-top-flange weld at the load application point (Category 71)
3. Cap channel-to-top-flange continuous fillet weld (Category 80)
4. Web stiffener-to-web weld at the bearing stiffener (Category 71)

AS 4100 Fatigue Design Methodology (Clause 11)

S-N Curve Fundamentals

AS 4100 Clause 11.6 adopts the S-N curve approach: the number of cycles to failure N is related to the applied stress range Delta-sigma by:

N x (Delta-sigma)^beta = C (constant for each detail category)

where beta = 3 for welded details (normal stress range). The detail category number (e.g. 160, 125, 90, 71, 56, 36) is the stress range in MPa that the detail can sustain for 2 x 10^6 cycles at a constant amplitude.

The fatigue strength F_sc for a given number of cycles N* is:

F_sc = k_L x k_s x f_3c

where:

• f_3c = detail category stress range at 2 x 10^6 cycles
• k_L = (N_sc / N*)^(1/beta) = (2 x 10^6 / N*)^(1/3) — the life correction factor
• k_s = (25 / t)^(0.25) for t > 25 mm — the thickness correction factor (t in mm)

For N* <= 5 x 10^6 cycles (the constant-amplitude fatigue limit, CAFL):

F_sc = f_3c x (2 x 10^6 / N*)^(1/3)

For N* > 5 x 10^6, the cut-off limit applies: F_sc = f_cutoff = 0.5 x f_3c (variable-amplitude) or f_cutoff = 0.67 x f_3c (constant-amplitude). At N* = 10^8 cycles (the constant-amplitude endurance limit), all details are assumed to have infinite life if the stress range is below f_cutoff.

Step 1 — Select Detail Categories (AS 4100 Table 11.6.1(1))

For the 610UB125 crane runway girder with a continuously welded cap channel, the critical details are:

Step 2 — Determine Maximum Stress Range

Maximum Moment Range

The crane wheel loads produce maximum bending when the wheels are positioned such that the resultant of the wheel pair is equidistant about the mid-span. For two wheels at 4.2 m spacing on an 8.0 m span:

R_A = (185 x (8.0 - 1.9) + 185 x (8.0 - 6.1)) / 8.0 = (185 x 6.1 + 185 x 1.9) / 8.0 = (1,128.5 + 351.5) / 8.0 = 185.0 kN

Maximum moment under the left wheel (at x = 1.9 m from left support):

M*_max_wheels = 185.0 x 1.9 = 351.5 kN.m

Minimum moment (light hook + bridge self-weight only): assuming the light hook produces P_min = 42 kN and the empty bridge adds a distributed 15 kN over the span as a moving load:

M*_min = P_min x L / 4 + (w_self x L^2 / 8)

P_min = 42 kN, w_self = 610UB125 self-weight 1.23 kN/m + rail 0.60 kN/m = 1.83 kN/m

M*_min_self = 1.83 x 8.0^2 / 8 = 14.6 kN.m M*_min_wheels = 42 x 8.0 / 4 = 84.0 kN.m (two wheels approximated as a single load at mid-span)

Total M*_min = 14.6 + 84.0 = 98.6 kN.m

Moment range: Delta_M = 351.5 - 98.6 = 252.9 kN.m

Stress Range at Critical Details

For the 610UB125 section (Z_x_bottom = 3,220 x 10^3 mm^3 at the tension flange, which governs for fatigue):

Delta_sigma_max = Delta_M / Z_x = 252.9 x 10^6 / (3,220 x 10^3) = 78.5 MPa

This stress range applies at the bottom flange (maximum tension). The top flange stress range is similar in magnitude for a symmetric section but is slightly lower due to the cap channel adding area — approximately 72 MPa at the top flange-to-web junction.

Step 3 — Fatigue Life Calculation for Each Detail

For each detail, check whether the applied stress range Delta_sigma exceeds the detail's fatigue strength for the design life of N* = 2,000,000 cycles.

Detail: Top Flange-to-Web Fillet Weld (Category 80)

f_3c = 80 MPa at N_sc = 2 x 10^6 cycles

Life correction factor: k_L = (N_sc / N*)^(1/3) = (2 x 10^6 / 2 x 10^6)^(1/3) = 1.0

Thickness correction: The flange thickness t_f = 17.3 mm < 25 mm, so k_s = 1.0.

F_sc = 1.0 x 1.0 x 80 = 80 MPa

Delta_sigma = 72 MPa (at the top flange-to-web weld)

Fatigue utilisation: Delta_sigma / F_sc = 72 / 80 = 0.900 — the detail is adequate for 2 x 10^6 cycles with a 10% reserve.

Detail: Transverse Stiffener-to-Flange Weld (Category 71)

f_3c = 71 MPa

F_sc = 1.0 x 1.0 x 71 = 71 MPa

Delta_sigma = 72 MPa (same as the top flange stress — the transverse stiffener experiences the same flange stress range)

Fatigue utilisation: 72 / 71 = 1.014 — the detail marginally fails at 2 x 10^6 cycles.

This means the transverse stiffener weld detail (Category 71) is the controlling fatigue detail for this runway girder. The options are:

1. Reduce the stress range — increase the section size to lower Delta_sigma
2. Improve the detail category — use a full-penetration butt weld ground flush to upgrade to Category 90
3. Accept a lower fatigue life — recalculate the fatigue life at the current stress range

Step 4 — Fatigue Life Recalculation for the Critical Detail (Category 71)

For the Category 71 detail, determine how many cycles it can sustain at Delta_sigma = 72 MPa:

N = N_sc x (f_3c / Delta_sigma)^3 = 2 x 10^6 x (71 / 72)^3 = 2 x 10^6 x (0.9861)^3 = 2 x 10^6 x 0.959 = 1.918 x 10^6 cycles

The detail can sustain approximately 1.92 million cycles at the applied stress range. The design life of 2 million cycles is 4.3% above the predicted fatigue life. This margin is insufficient for a safety-critical industrial application — the detail must be upgraded.

Mitigation: Upgrade Transverse Stiffener Weld to Category 90

By grinding the transverse stiffener-to-flange fillet weld to a smooth transition (1:3 slope) and performing magnetic particle inspection (MPI), the detail category upgrades to Category 90 per AS 4100 Table 11.6.1(1) — treated as a transverse butt weld ground flush.

Category 90: f_3c = 90 MPa

N = 2 x 10^6 x (90 / 72)^3 = 2 x 10^6 x (1.25)^3 = 2 x 10^6 x 1.953 = 3.91 x 10^6 cycles

Fatigue utilisation at 2 x 10^6: 72 / 90 = 0.800 — the detail is now adequate with a 20% reserve.

Step 5 — Variable-Amplitude Fatigue (Miner's Rule, Clause 11.8.2)

The crane operates at multiple load levels, not just the maximum wheel load. A realistic load spectrum for a Class A5 crane (medium-duty fabrication workshop) per AS 1418.1:

The cumulative damage per Miner's rule (Clause 11.8.2):

D = sum(n_i / N_i) for all stress range levels

For Category 90 (upgraded stiffener weld):

Total cumulative damage: D = 0.0373 + 0.0394 + 0.0280 + 0.0061 = 0.111

D = 0.111 << 1.0 — the detail has ample fatigue capacity for the Class A5 load spectrum over 50 years. The critical full-load cycles (5% of the total) contribute the most damage despite being the smallest fraction, because the S-N curve is cubic (damage scales with stress cubed).

Step 6 — Bearing Stiffener Fatigue Check (Category 71)

The bearing stiffeners at the supports experience a stress range from the support reaction range:

Delta_R = R_max - R_min = 185.0 - 52.7 = 132.3 kN (reaction range from the moving crane)

The bearing stiffener is subject to axial stress from the reaction. The stiffener area:

A_st = 2 x 150 x 12 = 3,600 mm^2 (two 150x12 stiffener plates)

Delta_sigma_stiffener = Delta_R / A_st = 132.3 x 10^3 / 3,600 = 36.8 MPa

For Category 71 (bearing stiffener web weld):

F_sc = 71 MPa (at 2 x 10^6)

N = 2 x 10^6 x (71 / 36.8)^3 = 2 x 10^6 x (1.929)^3 = 2 x 10^6 x 7.18 = 14.4 x 10^6 cycles

With 2.92 million total cycles over 50 years, the bearing stiffener has infinite fatigue life — utilisation = 2.92 / 14.4 = 0.203.

Step 7 — Thickness Correction Factor for Thick Flanges

If the crane runway girder used a heavier section with flange thickness t_f > 25 mm, the thickness correction factor k_s would apply:

k_s = (25 / t_f)^(0.25)

For a 610UB125 with t_f = 17.3 mm < 25 mm, no thickness reduction applies. For a crane runway using a 800WB146 with t_f = 32 mm, k_s = 0.955 would reduce the Category 80 fatigue strength to 76.4 MPa.

AS 4100 vs AISC 360 vs EN 1993 Fatigue Design Comparison

Frequently Asked Questions

How does AS 4100 Clause 11 handle fatigue design for steel structures?

AS 4100 Clause 11 follows the S-N curve approach based on detail categories. Fatigue design checks that the stress range f*_sr at a welded or bolted detail does not exceed the fatigue strength F_sc for the specified number of loading cycles. The relationship is N x (Delta-sigma)^3 = constant for each detail category, with a cut-off limit at 5 x 10^6 cycles. The fatigue strength is the product of the detail category, a thickness correction factor k_t = (25/t)^0.25 for t > 25 mm, and a life correction factor k_L = (N_sc / N*)^(1/beta) where beta = 3 for most details. If the calculated fatigue life is less than the design life, variable-amplitude loading with Miner's cumulative damage rule must be used.

What are the most common AS 4100 fatigue detail categories for welded connections?

AS 4100 Table 11.6.1(1) lists the stress categories for welded details. Category 160 applies to base metal away from welds and continuously welded longitudinal butt welds with full NDT. Category 125 covers transverse butt welds ground flush and longitudinal fillet welds away from ends. Category 90 applies to transverse fillet welds (attachment welds) and transverse stiffener-to-flange welds. Category 71 covers intermittent fillet welds and cope holes. Category 56 applies to gusset plate welds and attachment plates longer than 100 mm in the direction of stress. Category 36 covers welded shear studs on beams in bending. For a crane runway girder, the top flange-to-web fillet weld (Category 80-90 depending on NDT) and the stiffener-to-top-flange weld (Category 71) are the most critical details.

How does the Miner cumulative damage rule work in AS 4100 fatigue assessment?

AS 4100 Clause 11.8.2 applies Miner's linear cumulative damage rule for variable-amplitude fatigue loading: D = sum(n_i / N_i) where n_i is the number of cycles at stress range Delta-sigma_i and N_i is the endurance at that stress range. Failure is predicted when D >= 1.0. For a crane runway girder, the loading is broken into stress-range bins: 50% of cycles at 25% of max range, 30% at 50% range, 15% at 75% range, and 5% at full range. Each bin contributes damage n_i / N_i, and the total damage fraction must be less than 0.50 for a crane with a finite design life (the factor of 2 of safety accounts for uncertainty in both load history and fatigue detail classification).

What fatigue design life is specified by AS 4100 for industrial crane runway girders?

AS 4100 Clause 11.4 specifies fatigue design lives based on the structure classification. For crane runway girders in industrial buildings (Class 2 structure per AS 4100 Table 11.4.1), the recommended design life is 500,000 cycles for light-duty cranes (under 10t capacity, under 20 lifts per day) and 2,000,000 cycles for heavy-duty cranes (20-50t, over 50 lifts per day). AS 1418.1 (Cranes, Hoists and Winches) classifies cranes into service classes A1-A8 based on load spectrum and total working cycles, and AS 4100 Annex I provides the mapping between crane classification and stress categories. A 20t overhead crane in a fabrication workshop corresponds to AS 1418 Class A5, which corresponds to 500,000 to 1,000,000 full-load equivalent cycles.

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Related Pages

• AS 4100 Beam Design — 310UB40.4 Worked Example — Flexure, shear, LTB
• AS 4100 Column Design — 200UC46 Worked Example — Axial and buckling
• AS 4100 Bolt Design Guide — Clause 9.3 — Bolted connections
• AS 4100 Fillet Weld Design — Clause 9.7 — Weld capacity SP/GP method
• AS 4100 Load Combinations — ULS/SLS/STB combinations
• Australia AS 4100 Steel Design Guide — Complete standards reference
• Crane Runway Girder Design — Multi-Code — AISC, EN 1993, AS 4100
• Fillet Weld Sizes — International Reference — Multi-code comparison

This page is for educational reference. All resistance formulae are per AS 4100:2020. Fatigue design data such as load histories, crane classification, and stress range estimates must be obtained from the equipment manufacturer, AS 1418.1 crane duty classification, and on-site operational records. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent review by a registered structural engineer (CPEng/RPEQ).