Okay, I think I understand the problem, but I'd like to share my thoughts so someone can confirm for me.
This is what I see. The top surface and the bottom rails have grain going in the same direction. That means that the middle of the table should shrink the same amount on the bottom and top:
So even if the shrinkage on the top is pulling the sides of the table in, the same is happening at the bottom, so it shouldn't cause stress. But there would be stress at the joint itself. I think the bottom rail is a more usual joint:
I have it set up here as a wedge mortise and tenon, but I don't know if I actually designed it right so let's pretend it's a through mortise and I glue the tenon in. The glue contact surface looks like this:
In the rail, this 1" will shrink however amount inside the mortise, while the mortise's grain is perpendicular so it won't shrink in the same direction. But from what I understand, a joint like this is fine and can just be glued and there's no problem.
The top, meanwhile, has a contact surface that looks like this:
(Sorry about the crappy view, but I wasn't sure how to visualize it well). The grain of the top is going in the same direction as the 1/2" axis of the tenon, so if 1" is fine from a shrinkage perspective in the bottom rail, I would suspect that 1/2" is fine in the top. But the rail supporting the top has its grain going in the same direction as the 4" axis of the tenon, which is the same grain direction as the tenon. That's great for the tenon and rail, but that means the tenon is shrinking perpendicular to the top, along a 4" contact surface. I can see why 4" might be a lot, but I've seen really large glue-ups of joints this size. For example, you can see in this episode of New Yankee Workshop (https://youtu.be/g2j9Of6Hbrg?t=4m43s
) Norm builds a door using floating tenons roughly this big.
But even if one of those 4" tenons is okay in the top (which I'd still like confirmation of because I'm not sure) there's more to the story, because the upper rail supporting the top connects the 2 tenons:
So while each 4" tenon might be fine (which needs confirmation), there is actually a 13" section undergoing shrinkage, and 13" might be too much? Is that an accurate assessment?
Thank you so much for helping. I know this is a lot of pretty basic stuff I'm asking.