? I see nothing gained by modelling with CQUADs only to trick them to be CSHEAR by their property and I don't see how to do this. If you want shear only behaviour, use CSHEAR.
The difference between CQUAD and CSHEAR is that CQUAD reacts shear and membrane (and plate bending) loads; whereas a CSHEAR reacts only shear loads. Ie a CSHEAR is a CQUAD with the in-plane loads removed (and only the shear stiffness remaining. Now CSHEARs require 4 axial elements along the perimeter, in order for them to work; so you add the axial stiffness of the panel (usually taken as effective width = 30t) to whatever structure members bound the panel (like stringers).
Modelling this way is very olde school.
So returning to your original question, there is no thickness reason to transition from CSHEAR to CQUAD, it is a modelling approach (to choose to use CSHEAR in the first place). I would recommend using CQUAD everywhere, but this probably is a decision above your paygrade and my opinion is of little importance. I suspect you're working to some modelling guidelines that say "use CSHEAR for webs", and if the guidelines don't limit the thickness (I suspect they don't, otherwise you wouldn't ask the question !) then use CSHEAR for webs !?.
Possibly a logical transition is when the panel is fully effective, W = 2*30t … a 3" wide panel becomes fully effective at 0.05"; here W is the short side of the panel. Transitioning between CSHEAR and CQUAD is tricky in that you have to change the effective area of the perimeter members (dropping the web effective area where you use CQUAD).
another day in paradise, or is paradise one day closer ?
