Root Gap Discrepancy on a Fillet Weld
You are performing visual inspection on a structural steel fabrication. A 6 mm (1/4 in) fillet weld joins a stiffener plate to a web. The WPS specifies a root gap of 0-1.5 mm and a leg size of 6 mm. During inspection you observe that the fit-up gap at one location is approximately 3 mm over a 75 mm length. The welder proceeded without notifying anyone. The weld cap appears convex and the leg on the web side measures about 5 mm while the leg on the stiffener measures about 7 mm. There is visible undercut on the stiffener side of roughly 0.5 mm. The applicable code requires fillet weld leg size not less than specified and limits undercut to 0.8 mm. No dye penetrant has been applied.
1. What visual indicators in this setup suggest the root gap exceeded the WPS limit, and what is your immediate disposition?
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The key indicators are:
- Asymmetric leg sizes (5 mm web vs 7 mm stiffener). When the root gap is excessive, the welder typically has to bridge the gap, depositing more metal on one side and pulling the arc to compensate. The uneven legs are a classic visual signature of a gap that was too wide to close with the specified leg size alone.
- Convex cap with a short leg on one side. A convex profile combined with a leg below the 6 mm minimum means the weld does not meet the specified throat; the extra gap consumed metal that should have gone into leg length.
- Undercut of 0.5 mm on the stiffener side, within the 0.8 mm code limit but consistent with the welder manipulating the electrode to bridge the gap.
Immediate disposition: The weld is nonconforming because the web-side leg (5 mm) is below the specified 6 mm. The root gap of 3 mm exceeds the WPS maximum of 1.5 mm. I would reject the weld, mark it for repair, and require a repair WPS or engineering disposition (e.g., a larger leg size to compensate for the gap). The welder should be counseled on the fit-up notification requirement. I would also verify whether the gap was present at fit-up or opened during welding, since that affects whether this is a fitter or welder issue.
2. The welder argues the 7 mm leg on the stiffener side means the weld is "bigger than required" and should pass. How do you respond using code logic?
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The argument fails because fillet weld acceptance is governed by the minimum leg or throat on each fused face, not by total deposited metal.
- The code requires the leg size not be less than specified (6 mm). The web-side leg is 5 mm - a 1 mm shortfall, which is a rejectable condition regardless of the stiffener-side leg.
- The 7 mm leg on the stiffener side is irrelevant to compliance on the web side; excess metal on one face does not compensate for a deficient leg on the other. The weld's effective throat is controlled by the smaller leg (for a 45° fillet, throat = 0.707 × leg), so a 5 mm leg gives roughly 3.5 mm throat versus the required ~4.2 mm.
- The excessive root gap means the weld was not made to the qualified joint detail. Even if both legs met size, a 3 mm gap outside the WPS range is a procedure nonconformance requiring disposition.
I would document the measurements, reference the specific code clause on minimum fillet size and WPS conformance, and issue a nonconformance report. The repair should restore the minimum leg on the web side, and if the gap cannot be closed, engineering should approve a revised detail.