In the following example, I have two separate shell meshes, green and beige. Both meshes consist of S8 shell elements. The node-set 1 is used for a fixed boundary condition. The node-set 2 is used to apply a concentrated load of 1 in the negative z-direction (a force of 1 is working on each node in the set). Node-sets 3 and 4 are used for a symmetry boundary condition where the displacement in the x-direction and the rotations about the y and z-axis are equal to 0. The node-set 4 is a copy of the node-set 3 without two middle nodes of the middle finite elements of the green mesh.

I tried to run the simulation without the symmetry boundary condition (result a), with the symmetry boundary condition applied to the node-set 3 (result b) and with the symmetry boundary condition applied to the node-set 4 (result c).

It is obvious that the simple symmetry boundary condition applied to all the side nodes (node-set 3) does not work for the green mesh. The results a) and c) are similar but not exactly the same. I thought that the nodes where the finite elements are connected are causing problems but it turns out that the middle nodes of the middle finite elements are the ones preventing the deformation of the model. Very strange.

Is there any explanation for this? Is there any way to better understand how shell finite elements work in CalculiX in order to not waste time testing out each possible combination of keywords? Or are there some bugs in the code?

The model using node-set 3 can bee found at: https://www.dropbox.com/s/ehm155tv2o4ajrz/Shell_2.inp?dl=0