AWS D1.1 - Table 2.3 Fillet Welds?

[mojo41]

I don't understand the following requirement under fillet welds in AWS D1.1 Table 2.3 (Allowable Stresses). See image.

"Except that the base metal net section shear area stress shall not exceed 0.40 x yield strength of the base metal."

Does this mean I need to only calculate the direct shear stress on the base metal section and compare to 0.4 x yield strength? (This doesn't make sense to me, but I can't prove it doesn't say that)

Or, does it mean I should calculate the maximum shear stress (i.e. Mohrs circle) from all of the stress components and compare that to 0.4 x yield strength? (This seems conservative and correct, but I think like a mechanical engineer and not a civil engineer).

Or, does it mean something completely different because welds are treated uniquely, and thought of only in terms of shear stress?

Thanks

 

[JAE]

Read the footnote d below the table as this explains it a bit.

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[mojo41]

Comment "d" is just as vague as the table. I originally read "d" and it offers no insight as to the type of stress to evaluate.

"d" The limitation on stress in the base metal to 0.40 × yield point of base metal does not apply to stress on the diagrammatic weld leg; however, a check shall be made to assure that the strength of the connection is not limited by the thickness of the base metal on the net area around the connection, particularly in the case of a pair of fillet welds on opposite sides of a plate element.

 

[gtaw]

The limitations prevent the newbie from selecting a filler metal that overmatches the base metal in an attempt to reduce the size of the fillet welds. In short, it doesn't "buy" the engineer anything if he selects a 90 ksi filler metal to join base metal that only has a yield strength of 50 ksi.

Best regards - Al