Fillet Weld Size for a Load per Unit Length

This is the same relation as the throat-stress page with the length divided out of both sides, and that small algebraic move is why weld design is actually done in this form.

Once the load is expressed PER UNIT LENGTH of weld, the length disappears from the sizing calculation entirely. That is why weld capacity tables are published as force per millimetre or per inch of weld per unit of leg size, and why a fabricator can read a required leg straight off a chart. It also makes the linearity obvious: a 6 mm fillet carries exactly twice what a 3 mm one does on the same electrode, because the throat is proportional to the leg.

Getting the load per unit length right is the part that takes thought, and it is where the real errors live. A weld carrying pure direct shear is straightforward — the total load over the total effective length. A WELD GROUP carrying a moment is not. The load on each element of the group grows with its distance from the group's centroid, exactly as a bolt group does, so the governing point is the far corner and not the average. Size the weld for that corner. Where a group carries both a direct load and a torsional one, combine the two components as VECTORS at the critical point; adding their magnitudes is wrong, sometimes badly so, because the two are rarely aligned.

"Effective" length is a term of art. Codes discount the ends of a weld, because the stress does not develop instantly at a start or a stop and craters at the ends are a common defect. Welds returned around a corner — end returns — are credited differently again, and an intermittent weld is measured in its actual deposited segments rather than over the run they are spread across. When a code and a calculation disagree about a weld's length, the code is describing what the weld can be relied on to do rather than what geometry says it is.

Two habits worth carrying out of this page. First, size for the corner and not for the average; the difference on an eccentrically loaded bracket is easily a factor of two. Second, when a weld comes out too small to be practical — below the code minimum for the plate thickness — do not simply write the minimum and move on without checking the base metal, because a minimum-size weld on heavy plate means the plate, not the weld, is deciding what this joint can carry.