Wrong displacement for beam with a single rotational DOF constraint.

[victorkemp]

These two beams which are fixed at one end and have a single Z rotation constraint at the other should have the same displacement at the point where the load is applied but they don't. The one with an extra beam attached seems to be correct (zero mean rotation angle of its nodes about Z and displacement agrees with another solver). It looks like the rotational DOF constraint is not being applied correctly unless another beam shares the node.

I'm wondering if this is because the extra beam being attached might cause a knot to be created and knots on beams seem to generally work better for constraints and loads than non-knots.

*NODE
1,0,0,0
2,1,0,0
3,1,1,-1
4,0,1,1
5,0,1,-1
*ELEMENT,TYPE=B31,ELSET=EALL
1,5,1
2,1,4
3,3,2
*MATERIAL,NAME=Material
*ELASTIC,TYPE=ISOTROPIC
200000000000,0
*BEAM SECTION,ELSET=EALL,MATERIAL=Material,SECTION=RECT
0.1,0.1
1,0,0
*BOUNDARY
1,6,,0
2,6,,0
3,1,6,0
5,1,6,0
*STEP
*STATIC
*CLOAD
1,1,10000
2,1,10000
*NODE FILE
U
*END STEP