D1.3 thickness of the sheet is regulated as per the PQR essential variables in table 4.2.
Thickness of the supporting structural member is not an essential variable.
In relation to the structural member, D1.1, for example, shows in table 5.7 the minimum allowable fillet weld sizes based on material thickness. Note b however states that the minimum weld size need not exceed the thickness of the thinner part joined.
So, when detailing weld sizes in this case, use a size equal to the sheet thickness and you'll be covered, regardless of supporting structural member thickness
Tim