Small wind / Buyer questions
Structure and civil works
Why can two tower welds with similar static strength have different fatigue resistance?
The short answer
Similar static resistance does not guarantee similar fatigue behavior. Weld geometry, local stress concentration, load direction and the resulting stress range influence crack initiation and growth under repeated loading. A weld that carries one maximum load adequately can still require a separate fatigue assessment for the tower’s expected loading history.
Identify the stress that repeats
Static assessment checks the relevant extreme actions. Fatigue assessment examines cycles of stress at a particular detail. The stress range and number of cycles matter, together with the chosen assessment method and detail classification.
An apparently smooth external weld is not a complete fatigue assessment. Its geometry, internal features and relationship to the loaded plate or shell determine how forces pass through the detail.
Compare details on a consistent basis
Two welds can have equal nominal throat area while different attachment shapes create different local bending or notch effects. A transverse attachment can also interrupt a stress path differently from a longitudinal one.
Ask whether the reported stresses are nominal, structural or local quantities. Applying a fatigue curve intended for one stress definition to another can give misleading results even if the numerical stress range is unchanged.
Resolve changes before fabrication or repair
The tower designer and fabricator should align the assessed detail, welding requirements and inspection scope. Adding a bracket or changing a weld profile can require reassessment even where the added static load is small.
Do not grind, weld over or alter a tower detail from this general explanation. An existing crack requires an authorized condition assessment and repair decision, not an assumption that more weld metal improves fatigue resistance. FHWA steel guidance explains these mechanisms in bridge and support structures; it does not establish a MARSTON tower life, inspection interval or universal weld acceptance limit.
Sources
Examples are illustrative. Confirm product-specific performance, compatibility and supply scope in the applicable documents.