I have a panel modeled with shell FEM elements. I want simply supported ends so I applied 3 translational constraints to a small rectangular surface at both ends. How would I go about constraining the rotational DOF. The problem I´m experiencing is that I tried to constrain the rotational constraints on the edges such that I didn´t over constrain my model and make it a clamped clamped constraint, but it doesn´t seem to matter what I do. It´s always brick walling both sides. This makes no sense to me. I even tried splitting the surface and applying all rotational contraints on a small surface near 1 edge, but that still resulted in clamped clamped. Makes no sense to me.

I´m doing a buckling analysis. I want 0% fixity at the two ends. The other two sides are free edges.

Please be gentle with me, it´s my first post here. Yes, I have searched my an answer prior but there doesn´t seem to be much info on simply supported panel buckling analysis.

I have attached some documentation. to show my work and how I´ve tried to constrain my part.

If the panel is bolted at both ends and completely unsupported through the middle I would change the way I would fix it. I try to get as close to "real Life" as I possibly can. If I had a plate supported by angles fixed in location like in the shot below I would use split lines on the plate at the edges of the angle leg. Depending on what you are evaluating, you can treat the angle as immovable and fix the split faces leaving the angles out of the model. I would then select the inside of the bolt holes and fix them as well. Or if the angles are welded securely but you want to account for the flex in the angle you can fix the outside leg of the angle and constrain the bolt holes to the holes in the angle using the bolted feature. It depends on what you are trying to evaluate.

Using your simplified 2D sketch and the way you ended the plate at the bolt holes I would select the edges where the bolts are and constrain translation in all 3 directions from both sides but that would allow rotation unless you want to again consider the edge of what ever the plate is bolted to and use split lines again. There will be differences in the results for all of these evaluations. Your job is to determine how the part will react in real life and what constraints can be simplified and what needs to be considered. Experience is the best guide here.

Does this help?