Minimum fillet length

[wadavis]

I'm sizing temporary fillet welds for transport loading and want to ensure that I'm accurately calculating the capacity of these short fillet welds.

The minimum effective length of a fillet weld shall be 38 mm (1-1/2 in) or 4 times the size of the fillet, whichever is larger.
Where the geometry of the joint makes it impossible to deposit the minimum effective length, the effective fillet size shall be 0.25 times its effective length.

The effective length of a fillet weld shall be the overall length of the full-size fillet, including end returns. No reduction in effective length shall be made for either the start or termination of the weld if the weld is full size throughout its length.

AWS D1 has similar wording.

I was concerned that the ends would be weak spots in fillet welds and that short temporary welds would have a reduced capacity but the code does not have any reductions. The code only allows shorter welds if he geometry requires it, and then adds a steep effective length reduction.

As for design; it is clear cut there is a minimum length for fillet welds within the code but I'm here to understand why the code is written this way. Are fillet weld starts and ends robust enough that no capacity reductions are required for 38mm welds? What happens with shorter welds that justifies a 0.25 reduction in capacity that isn't happening with a 38mm length.






Structural, Alberta

 

[racookpe1978]

"Rules of Thumb" are neither "Codes and Standards", nor are they "Design Requirements" ...

For average conditions under average loads and average steel alloys welded by average welders, the average rule-of-thumb is (most of the time) useful in preventing average errors by providing an average-margin-of-safety greater than the average-margin-of-error.

Hence, if an average fillet weld may have an average chance of a starting point flaw, and the average fillet weld may have an average chance of having an average touch-off flaw, then the average thumb rule says that an average fillet weld greater than 1/4 inch fillet height can hold 1000 psf per inch of weld, after subtracting 1/4 inch from each end of the weld. (There are variations in this thumb rule accounting for much thicker fillet heights.)

This possibility of having flaws at the start and stop of an average (untested! and un-inspected/no NDE exam!) fillet weld is the basis for the "shorter effective fillet weld length" of that rule. To avoid the requirement, dye pen exam the temporary welds, or use a sightly thicker/slightly longer weld.