Crane Steel Structure Fatigue Assessment per ISO 24035:2020
ISO 24035:2020, "Cranes — Steel Structures — Fatigue Assessment Methods," is the dedicated standard for fatigue life assessment of crane steel structures. It specifies a stress-life-based fatigue assessment procedure, covering S-N curve selection, stress cycle counting, fatigue damage accumulation calculations, and a safe-life evaluation process.
Fatigue Assessment Procedure: Step-by-Step
The fatigue assessment procedure defined in ISO 24035:2020 comprises the following steps: load spectrum collection (acquiring load-time history data and load cycle statistics over the crane's lifecycle), structural stress analysis (performing finite element analysis or strain gauge measurements at the assessment locations to obtain stress spectra under various operating conditions), S-N curve selection (choosing the appropriate S-N curve based on steel type, joint detail, and stress ratio), fatigue damage accumulation calculation (using the Palmgren-Miner Linear Cumulative Damage Rule to compute total damage D = Σnᵢ/Nᵢ), and safe-life evaluation (assessing fatigue life based on cumulative damage D, where D ≤ 1.0 indicates structural safety).
Selection of assessment locations — the fatigue assessment must cover all stress concentration zones in the steel structure (weld seams at joints, cross-section transitions, openings, weld toes, etc.) as well as areas subjected to significant alternating stresses (main girder mid-span, end carriage connections, outrigger roots, etc.). At least 3 to 5 fatigue-sensitive locations should be evaluated for each crane. The assessment results are used to establish periodic inspection schedules and fatigue life management strategies for the crane.
Selecting the Right S-N Curves
The standard classifies S-N curves into several FAT grades based on steel structure joint details: parent material (no weld) uses the FAT160 curve (160 MPa @ 2×10⁶ cycles); butt welds ground flush use the FAT125 curve; butt welds in as-welded condition use the FAT112 curve; fillet welds and T-joints use the FAT100 curve; cruciform joints and stiffener plate welds use the FAT80 curve. The fatigue limit corresponds to 2×10⁶ stress cycles (in steel structures, 5×10⁶ cycles is commonly taken as the reference number for the fatigue limit).
S-N curve slope — the relationship between stress range Δσ and fatigue life N is represented as a straight line with slope m on a log-log plot, where m is typically 3 (as-welded) or 5 (parent material and ground welds). The S-N curve for high-stress regions exhibits a fatigue limit beyond 2×10⁶ cycles (below this stress level, fatigue failure no longer occurs). Under variable-amplitude loading, the equivalent stress range Δσeq = (Σnᵢ × Δσᵢ³ / N_total)^(1/3) should be used to calculate the equivalent damage stress.
| connection details | FAT class | mvalue | Fatiguelimit | typical location |
|---|---|---|---|---|
| base material | 160 | 5 | 73MPa | main girder web plate |
| butt joint(ground flush) | 125 | 3 | 57MPa | Flange platebutt joint |
| butt joint(as-welded) | 112 | 3 | 51MPa | Web platebutt joint |
| Fillet weld | 100 | 3 | 45MPa | Stiffener plateconnection |
| cruciform joint | 80 | 3 | 36MPa | main girder and end carriageconnection |
| plus Stiffener plate | 80 | 3 | 36MPa | Stiffener / Stiffening Ribend |
Stress Cycle Counting and Damage Accumulation
Establishing the stress spectrum begins with cycle counting of the load-time history using the Rainflow Counting method, which extracts the stress range and mean stress for each individual stress cycle. Rainflow counting is widely recognized as the standard method for fatigue cycle counting. The counted results are then used to construct a stress range histogram (Δσi-ni distribution). Fatigue life is verified using the Palmgren-Miner Linear Cumulative Damage Rule (D = Σ n_i/N_i ≤ 1.0 for Design Life Verification), where ni is the measured number of cycles at the i-th stress range level and Ni is the allowable number of cycles for that stress range, obtained from the S-N curve.
The structure is considered safe when D < 1.0, while D = 1.0 indicates that the fatigue design life has been fully consumed. Fatigue safety factors are applied as follows: when calculated load spectra are used instead of measured load spectra, a safety factor of ≥1.25 is required. For welds that cannot be visually inspected (e.g., welds inside closed sections), the safety factor is increased to ≥1.5. For cranes intended for personnel lifting, the safety factor is set at ≥2.0.
Assessment Report and Life Management
A comprehensive fatigue assessment report should include the following: the standards used for the assessment, technical parameters and service history of the crane under evaluation, the load spectrum (including its source and statistical methodology), stress analysis results (from finite element model analysis or strain gauge measurements), the S-N curve selection rationale and FAT class, damage accumulation calculations, remaining safe life conclusions, and maintenance recommendations with key inspection points and inspection intervals. Kelude Heavy Industry offers fatigue assessment services for crane steel structures.
| Safety factor | Coefficientvalue | applicable conditions |
|---|---|---|
| conventional | 1.25 | Design loadspectrum |
| not visually inspectable | 1.5 | enclosed Weld Seam |
| man-riding | 2.0 | personnel Lifting and transport |
| elevated temperature/Corrosion | 2.0 | special environment |
FAQ
Q: What does the fatigue assessment procedure per ISO 24035:2020 involve?
A: The procedure covers load spectrum collection, structural stress analysis, S-N curve selection, damage accumulation calculation (Miner's rule), and safe-life assessment. The evaluation should cover at least 3–5 fatigue-sensitive locations, including weld concentration zones and stress concentration areas. A damage value of D ≤ 1.0 is considered safe.
Q: How do I select the appropriate FAT class from the S-N curve?
A: FAT class values are as follows: base material FAT160, butt weld ground flush FAT125, as-welded FAT112, fillet weld FAT100, and cruciform joint with stiffener plate FAT80. The slope m is taken as 3 for as-welded conditions and 5 for base material. The fatigue limit corresponds to 2×10⁶ cycles.
Q: What method is used for stress cycle counting?
A: The Rainflow Counting method is used, as it is the recommended approach in current fatigue assessment standards. Stress ranges Δσi and mean stresses are extracted to build a stress spectrum histogram. Miner's linear damage rule is then applied: D = ∑(ni/Ni), where D ≤ 1.0 indicates a safe design.
Q: How is the fatigue safety factor determined?
A: A factor of 1.25 is applied to the design load spectrum, 1.5 for weld seams that cannot be visually inspected, and 2.0 for cranes used for personnel lifting. Higher safety factors require a corresponding adjustment in FAT class. Kelude provides fatigue assessment and life management services for steel structures.
