Question of calculating Q for Horiz. Shear

[JAE]

In-office debate about the attached sketch.

The top section is a wide flange with a channel cap on top with welds at the WF flange tips.

The bottom section is a wide flange with the same channel - but the connection is at the WF neutral axis via two figuratively thin
plates and a small gap between WF top flange and channel web. Assume the two connector plates in the second option are very small/thin.

It appears that both would have the same value of Q for determining horizontal shear for the weld attachment calculation.

However it doesn't seem intuitively correct that the welds would carry the same shear as the flexural behavior of the two sections seem very different.



Any thoughts?

 

[KootK]

This thread really got me thinking. I agree that the longitudinal shear demand, as calculated using VQ/I, is independent of the vertical location of the welds. That being said, it seems to me that relocating the welds must have SOME consequence. To try and reason this out, I drew free body diagrams of the connected parts independently, including the shear force imparted to each by the welds (see attached sketch). This led me to the following conclusions:

1) It appears that when two sections are connected in such a fashion they have a tendency to want to either separate or come together vertically. This tendency must be resisted by both the welds and the affected parts of the connected members (thin plates in JAE's example). This is not something that I've considered in the design of welds or bolts for composite members in the past.

2) It seems to me that the magnitude of the force mentioned in point number one IS affected by the vertical location of the connecting welds. This also implies that the portion of the vertical shear carried by each member would also depend on the vertical location of the connecting welds. This last bit certainly causes me grief. It's contrary to the notion that the vertical shear in each member would be static and could be found by integrating VQ/It over each member.

I look forward to hearing others engineers' critiques of these arguments.

KootK