I’ve found something worrying in the mean rotation MPC which affects rotational DOF constraints and moments on beam elements as well as general use of meanrot in *MPC. It seems to be more about the underlying design of the mean rotation MPC rather than a bug, so I hope @dhondt can chime in or anyone else can help me understand it better.
It appears that this MPC constrains displacements in the plane normal to the rotation axis, ignoring the coordinate parallel to the axis. That’s OK when the nodes all lie in that plane, but when they’re at an angle to that plane it seems to have a side-effect on rotations about other axes.
For example, here’s a beam element oriented at 45 degrees to the z axis with (correctly working) boundary conditions at the top to make it rotate about a y-parallel axis. Without any constraints at the bottom end, it rotates as expected. Adding a z rotation constraint to the bottom node should have no effect because it’s not rotating about z anyway. However, that increases the displacement by about 10% and creates a bending moment in the beam. I think the reason for this is that on the expanded solid element, the two nodes at the +z extreme are at a greater radius from the y rotation axis so they have a greater x displacement than the other two expanded nodes. Those different x displacements look like a rotation about z so the mean rotation MPC moves the 4 nodes around to cancel that out.
*NODE
1,0,1,-1
2,0,0,0
3,0,1,-1.1
*ELEMENT,TYPE=B31,ELSET=1
1,1,2
2,1,3
*MATERIAL,NAME=Material
*ELASTIC,TYPE=ISOTROPIC
200000000000,0
*BEAM SECTION,ELSET=1,MATERIAL=Material,SECTION=RECT
0.1,0.1
1,0,0
*BOUNDARY
1,1,3,0
2,6,,0
3,2,,0
3,6,,0
*STEP
*STATIC
*BOUNDARY
3,1,,0.001
*NODE FILE
U
*END STEP
I put this as an issue on Github as while ago Wrong displacement for beam with a single rotational DOF constraint. · Issue #69 · Dhondtguido/CalculiX · GitHub .