Fillet Welds and Lap Joints

A second example of “nothin’ welds” occurs when fillet welds are put on the edges of lap joints where the member with the vertical edge of the fillet weld is relatively thin, typically less than 3/8 in (10 mm).

While this is less of an issue with the commonly used semi-automatic welding processes of today, welders using SMAW electrodes (with their inherently broader arc) can inadvertently melt away the top edge of the member. This creates an illusion of a full-sized fillet weld equivalent to the thickness of the top plate. In reality, and as illustrated in Figure 2, the resulting weld throat may be much smaller than the designer intended.

The AWS D1.1 Structural Welding Code addresses this by calling for maximum fillet weld sizes on the edges of lapped members as stated in paragraph 2.4.5: “The maximum fillet weld size detailed along edges of material shall be the following: (1) the thickness of the base metal, for metal less than 1/4 in (6.4 mm) thick; (2) 1/16 in (1.6 mm) less than the thickness of base metal, for metal 1/4 in (6.4 mm) or more in thickness…” This is not applied to the thinner members because, from a practical point of view, these welds normally achieve the full throat thickness. The most straight-forward way to avoid the creation of this “nothin’ weld” is to leave the 1/16 in (1.5 mm) unwelded portion above the upper weld toe. Additionally, welders should be taught of the implications of this practice and be discouraged from melting the top edge.

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