Hi Distilled
I believe that we are talking about two different things here.
Firstly the issue that was raised about the weld not "hooking around" to another side on a common plane of contact, is due to the almost certain weld defects and lack of leg length that will be obtained, along with the fact that it makes inspection of the fit-up essentially impossible.
The second issue that I believe you were concerned with all along is whether to allow a weld on the "overlapping" side of your joint. (Without hooking around.) I believe that it will depend on your particular circumstances, and that indeed it is an issue of "force flow" from one member into the other.
If your square tubing had a wall that was thick compared to the flat plate, then there would be little wrong with placing a weld in this position, because the rigidity of the tubing would allow the force to be transferred through all the welds of the assembly. This situation is however unlikely, because the square tubing would typically have a much thinner wall.
Under this situation, (the thinner wall) the force flow would be concentrated through the weld on the "overlapping" side, as a tensile force. This would result in a great stress concentration, and would weaken the design. By placing the welds along the sides and end of the connection, you introduce a much larger area through which the force is transmitted by shear.
Hope this helps.
Regards
Niekie Jooste
Fabristruct Solutions