Limiting simulation volume/area or displacement for mechanical model

Hey,

while doing a buckling study of an optical fiber I tried including a fixed box around the fiber to constrain the movement of the fiber within this box. Sadly this increases the required simulation time a lot so I replaced the box with 'beams' that would effectively constrain the fiber between the beams.
When converging (the model seems quite unstable for certain parameter values) I noticed that, eventhough I specified contact pairs between the fiber and the inner boundaries of the constraining beams, the simulation tends to give me a result where the fiber ignores the beams and buckles in another direction.
It seems the fiber ignored it's contact pairs with the beams and somehow went through them.

So I was wondering if there is a way to limit the displacement/deformation of an object, in a 3D stationary mechanical model.
If this "box-approach" is the only way to do this: is there a way to either reduce the simulation time by telling COMSOL that you're not interested at all in what happens with the objects that form this constraining box, or make sure the contact pairs are respected during a deformation.


thanks

Evert