Most structural welding happens in statically loaded applications — buildings, platforms, equipment supports where loads are essentially constant and stress reversals are infrequent. But bridges, crane runways, industrial equipment, offshore structures, and certain building connections are subject to cyclic loading: repeated stress cycles that can cause fatigue failure at stress levels well below the static yield point.
AWS D1.1:2025 includes provisions specifically for these structures. CWIs and QC managers working on cyclically loaded jobs need to understand the fatigue provisions because they affect joint selection, WPS requirements, NDE scope, and acceptance criteria — in ways that a standard static-structure WPS does not address.
What is fatigue in structural welds
Fatigue failure is crack initiation and propagation under repeated stress cycling, even when peak stresses stay below yield. Welds are particularly susceptible because:
- Geometric stress concentration: the abrupt transition from weld to base metal at the weld toe creates a local stress concentration factor
- Residual tensile stresses: thermal shrinkage from welding leaves residual tensile stresses at the weld surface, which add to applied tensile stresses and accelerate crack initiation
- Weld discontinuities: porosity, undercut, overlap, and lack of fusion act as crack initiation sites that reduce fatigue life compared to a theoretically perfect weld
A weld that passes all static-structure acceptance criteria — adequate size, full penetration, no rejectable discontinuities per visual or UT criteria — may still have reduced fatigue life compared to a machined connection due to weld toe geometry alone.
AWS D1.1 Chapter 2: structural design provisions
AWS D1.1 Chapter 2 contains the structural design provisions including fatigue requirements. The fatigue provisions apply to structures where the engineer of record (EOR) has determined cyclic loading governs design. The fabricator's role is not to determine whether fatigue applies — that is the EOR's determination, communicated through the design documents — but to execute the WPS selection, joint detail, inspection, and acceptance criteria that fatigue-governed joints require.
Rule library based on AWS D1.1:2025; verify against your governing edition.
Stress categories: Table 2.5
AWS D1.1 Table 2.5 assigns stress categories to specific structural connection details. The categories range from A (best fatigue life) to F and F' (lowest fatigue life):
| Category | Representative detail |
|---|---|
| A | Base metal, plain member remote from welds |
| B | Base metal at butt welds with smooth transitions |
| C | Base metal at transverse stiffener-to-flange welds |
| D | Base metal near fillet or partial-penetration groove welds |
| E | Base metal at weld toes of longitudinal welds or attachments |
| F | Fillet weld shear stress |
Category determines the allowable stress range (Fsr) for a given number of cycles. A connection designed for 2 million cycles at Category C has a significantly different allowable stress range than a Category E detail at the same cycle count.
The implication for WPS selection: the joint geometry that achieves a given category is specified by the engineer. Using a different joint type — even one that is structurally adequate — can drop the connection to a lower category and invalidate the fatigue design. Substituting a fillet weld for a specified CJP butt weld, for example, typically drops the stress category and requires the EOR to re-evaluate the design.
Joint design requirements for cyclically loaded structures
For fatigue-sensitive connections, AWS D1.1 imposes additional requirements beyond the standard static-structure joint design provisions:
Smooth transitions: butt weld reinforcement that is left in place (not ground smooth) reduces the connection to a lower stress category than one with the crown ground flush. Where the design assumes a flush-ground butt weld (Category B), the WPS must specify grinding as a required step, and inspection must verify compliance.
Weld toe geometry: for connections where Category C or better is required, the weld profile at the toe is a critical quality attribute. Weld toes with sharp angles, undercut, or cold lap (overlap) are crack initiation sites. Visual acceptance criteria for cyclically loaded structures should be more stringent than for static connections on these profile attributes.
Transverse welds in tension zones: butt welds transverse to the primary stress direction in tension zones require full CJP welding and specific quality levels. The tension zone weld repair article covers the constraints on repairing discontinuities in these locations.
Avoidance of backing bars in fatigue-critical butt joints: permanent steel backing left in place creates a notch at the root that is a fatigue crack initiation site. For Category B or better fatigue classifications, the design often specifies backing removal and root pass grinding — which must appear explicitly in the WPS. See CJP groove weld backing removal requirements.
WPS implications for cyclically loaded structures
The WPS for a fatigue-governed joint is not fundamentally different from a standard WPS — the same Table 6.6 essential variables apply, the same PQR or prequalified basis applies. But several aspects of the WPS require explicit attention:
CVN supplementary essential variables (Table 6.8): when CVN impact testing is required — as it frequently is for cyclically loaded structures in cold-climate or seismic applications — Table 6.8 adds essential variables to the list. Heat input range, preheat, and interpass temperature become essential (not just nonessential parameters to record) when CVN is in scope. See CVN supplementary essential variables for the full variable list.
Filler metal for CVN applications: when CVN toughness is required, the filler metal must be documented with its CVN classification in the WPS and PQR. Not all filler metals within an F-number group have the same CVN performance. See CVN filler metal selection for demand-critical welds for guidance on selecting and qualifying filler metals for toughness applications.
Ground cap passes: where the design requires a flush-ground weld surface for fatigue category classification, the WPS should specify grinding as a required operation, grinding direction, and final profile requirements. Grinding is not a standard element of most WPS documents — if it is required, it must be stated.
Peening restrictions: peening of cap passes is prohibited under AWS D1.1 regardless of whether the joint is statically or cyclically loaded. On cyclically loaded connections, cap-pass peening is particularly problematic because it can mask the surface discontinuities — undercut, cold lap, porosity — that are most detrimental to fatigue life. The inspector must examine the cap pass before any post-weld surface treatment.
NDE requirements for cyclically loaded structures
The NDE scope for cyclically loaded structures is typically more extensive than for static structures:
UT on CJP butt welds: ultrasonic testing of complete-joint-penetration butt welds in tension zones is standard for cyclically loaded structures. The UT acceptance criteria for structural welds covers the evaluation procedure and rejection criteria.
MT at weld toes: magnetic particle testing (MT) at weld toes on fatigue-critical connections can detect surface and near-surface cracks before they propagate. Some contracts require periodic MT inspection during service on high-cycle connections.
More stringent visual acceptance: AWS D1.1 Table 8.1 (visual inspection criteria for statically loaded structures) and Table 8.2 (cyclically loaded structures) differ. Table 8.2 imposes tighter limits on undercut, surface porosity, and weld profile in the tension zone. CWIs inspecting cyclically loaded work need to apply the correct table — applying static criteria to a cyclic joint is a quality failure even if the weld "looks fine."
Seismic vs. fatigue: overlapping requirements
AWS D1.8 (Structural Welding Code — Seismic Supplement) governs demand-critical welds in seismic moment frames. Seismic loading is a form of cyclic loading with specific amplitude and ductility requirements. AWS D1.8 requirements overlap with but are not identical to the fatigue provisions in D1.1 Chapter 2. For moment frame work in seismic zones, the governing document is typically AWS D1.8; for crane girders and bridge work, the fatigue provisions of D1.1 Chapter 2 (or AASHTO for bridges) govern. AWS D1.8 seismic supplement covers the demand-critical weld requirements.
Practical checklist for fatigue-governed work
Before welding a connection classified as cyclically loaded:
- Confirm the stress category from the design drawings — it determines acceptance criteria and joint geometry requirements
- Verify whether CVN toughness testing is required and, if so, that Table 6.8 essential variables are captured in the WPS
- Review the WPS for backing removal, grinding, and weld toe requirements specific to the fatigue category
- Apply the correct visual acceptance table (Table 8.2 for cyclically loaded structures, not Table 8.1)
- Establish NDE hold points appropriate to the fatigue classification — CJP UT, weld-toe MT where specified
- Confirm that cap-pass inspection is a hold point before any post-weld treatment
Managing WPS documentation across static and cyclic applications — tracking which procedures qualify for which joint categories and CVN requirements — is where a structured welding procedure library pays for itself. A mixed-use shop running both building and bridge work cannot afford to apply the wrong acceptance criteria to the wrong job. See our qualification tracking features for how software can flag the mismatch before the inspector does.