"Nested" parts and no penetration

[AbeFM]

Sorry if this question is a little vague...

I've got a customer who wants to stiffen a long aluminum rectangular tube with a short length of smaller bit of steel channel centered within it. I thought I'd run a quick sim to just see how it behaves... And I'm having trouble setting up the correct boundary conditions such that the outer tube bends until it contacts the steel center as one would expect. They start off without contact, so I added a "no penetration" condition to them, and they just move as if bonded. I can only assume there's something to do with the "gap" feature, but being that I set that down to a few mils and the parts are separated internally by a hundred times that, I'm at a loss on how to make it behave.

Tweaking a few random options (like global non-penetration) and trying h-adaptive is making it run quite slowly.... Just looking for a quick pointer, thanks!
-Abe.

 

[AbeFM]

So, revisited this when the customer liked the results, but says that I can't constrain the entire section of end. Apparently, the beam is riding against two ~1" pins - meaning it's supported at singularities....

So I'm back to not being able to get a result.

Does someone have a good walk through on how to properly use thin shells? If someone could model a force acting on a square tube, constrained by a pin at each end (just pressing against it) and I could look at the set up, I'd be very very grateful. Somehow I always get errors. Its pretty ridiculous, I've done countless simulations on assemblies on thousands of parts.... but this gives me fits.