Fatigue Design in Steel Structures: Cyclic Loading and Detail Categories

A steel member sized for static strength can still crack in service if it sees enough load cycles, even when every individual cycle stays well under the yield stress. Fatigue is a separate failure mode from the static strength checks that dominate most building design, driven not by peak stress but by stress range, the difference between maximum and minimum stress in a repeated cycle, and by the number of times that cycle repeats over the structure's life. Crane runway girders, highway bridges, and machine support platforms are the structures where fatigue routinely governs; typical office and residential buildings rarely see enough load reversals for fatigue to control member sizing.

Stress Range Governs, Not Peak Stress

A member subjected to a single large overload but almost no repeated cycling is a static strength problem. A member subjected to a modest but frequently repeated stress range, such as a crane girder loaded and unloaded dozens of times a day for thirty years, is a fatigue problem, and the two are checked with entirely different criteria. Fatigue design curves plot allowable stress range against the number of cycles to failure (an S-N curve), and because steel fatigue curves are roughly linear on a log-log plot, a doubling of stress range can reduce fatigue life by a factor of eight, since life scales with stress range raised roughly to the third power for most welded details.

This cubic relationship is the reason fatigue-sensitive members are detailed conservatively even when static capacity has generous margin: a stress range that looks trivial next to yield strength can still exhaust fatigue life within the structure's service life if the cycle count is high enough. AASHTO and AISC both define an infinite-life threshold, a stress range below which a detail can theoretically sustain unlimited cycles without cracking; keeping stress range under that threshold is the simplest way to remove fatigue from the governing checks entirely.

Detail Categories: A to E'

AASHTO's LRFD Bridge Design Specifications and AISC 360's fatigue provisions both classify welded and bolted details into categories, commonly labeled A through E', ranked from best to worst fatigue performance. Category A covers plain rolled or extruded base metal with no attachments, the smoothest stress path and the highest allowable stress range. Category E' sits at the opposite end, covering details like a weld with a groove reinforcement left in place at a re-entrant corner, or a cover plate end weld on a beam flange without proper transition detailing, geometries known to concentrate stress severely and crack at comparatively low stress ranges after relatively few cycles.

Transverse stiffener welds, cover plate terminations, and welded attachments to flanges are common category D and E details in practice, which is precisely why plate girder design and crane girder design pay close attention to stiffener weld termination geometry, discussed further in the context of web panel behavior in plate girder design. Full-penetration groove welds ground flush and inspected to remove flaws can achieve category B, while the same joint left with the weld reinforcement in place, unground, drops to category C or worse purely because of the geometric stress riser the leftover weld bead creates.

Fatigue-Sensitive Structure Types

Crane runway systems accumulate an unusually high number of full load cycles over their service life and are almost always fatigue-governed rather than strength-governed at the girder-to-bracket connections and at flange-to-web welds under the rail. Highway bridge girders see traffic-induced cycling that, while individually low in stress range compared to a bridge's static capacity, accumulates into millions of cycles over a design life measured in decades, making weld detail selection at flange cover plates and cross-frame connections one of the most consequential decisions a bridge designer makes.

Wind-sensitive structures, including tall building outrigger connections and long-span roof members subject to repeated wind buffeting, and support structures for reciprocating or rotating machinery, round out the categories where a fatigue check is standard practice rather than an edge case. In ordinary building structures without cranes, heavy vibrating equipment, or unusual repetitive loading, the number of significant stress cycles over the building's life is typically low enough that static strength and serviceability checks govern member sizing without a separate fatigue analysis being required.

The detail category tables and stress range limits referenced above are published in the AASHTO LRFD Bridge Design Specifications, and the equivalent building-structure provisions are set out in Appendix 3 of the AISC Specification for Structural Steel Buildings. Selecting a base steel grade with adequate toughness for the expected service temperature is a related but separate consideration covered in structural steel grades.