How to define an elastic boundary condition? Rotational in my case 1

[alegio20]

Dear all,
I have a quite simple problem but I cannot figure out how to solve it.
I have a cantilever hollow square section beam made with S4 shell elements. At one end, I need to create a rotational spring for simulating a non-perfect encastre; I have created a RF and a coupling costraint to simulate a punctual boundary condition and to simplify the problem. A very low-quality skecth of what I am trying to simulate is attached to the post.
I have already try to use the Sping element -> "connect spring to ground" -> DOF 4 together with a "perfect" boundary condition on displacements but it does not work.
Any ideas are appreciated.

Thanks.

 

[FEA way]

What do you mean when you say that it doesn't work ? Does the analysis fail or is there no impact on the results ?

 

[alegio20]

The analysis fail. It seems that the rotational degree of freedom was not involved. To test that I have try to modify it to 4, 5 or 6, but the analysis aborted anyways. Moreover, changing the load from a concentrated force to a prescribed displacement makes the beam rotate rigidly.. So it is like the the rotational degree of freedom was not involved.
The .inp file is attached.

 

[FEA way]

The force is too large and geometrically nonlinear analysis doesn't converge. If you reduce the force and keep changing the stiffness of the spring, you will see the differences in results.

 

[alegio20]

Thank you! Actually, I was getting the spring stiffness wrong by a couple of order of magnitude!!