3D Rigid bar element

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Hello,

does someone knows how to create a rigid bar element in CCX? I need to set a point mass and connect it to the main FE structure via rigid element(s). The idea is to capture the inertia effects of a small body (point mass) in a modal analysis of a big structure. The added mass has no effects in terms of stiffness. I placed a point mass in a node at the center of gravity of the small body, but I dont know how to connect it to the main structure.

If I remember correctly, in NASTRAN the RBE3 would be a good solution. I am not sure if I can replicate it in CCX. I thought of using translation equations. One complication I thought is that we expect torsion of the main structure, so the translation of one node of the main structure wont replicate the rotation of the small rigid body.

I also thought I could set 3 beams with zero mass and almost infinite stiffness in an triangular arrangement.

Greetings,
Carlo

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You could try with *RIGID BODY card.

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hi,

as i know RBE3 in Nastran is different compared to *Rigid Body keywords in CalculiX or Abaqus, it has an option to avoid over-constrained condition (e.g stay plane as is as undeformed, poison effect, warping in steel column modeling as solid with concentrated force/moment).

you can use *Coupling keywords to modeling similar conditions and set DOF’s you may consider to constrained/activated. previously i was took some comparison for simple case (links). non-english writen so language translation is required, but did many pictures from screenshot can speak itself?

thank you,

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Just adding to what xyont said, *COUPLING with *DISTRIBUTING is like RBE3. I have used it with a point mass before. But it’s more finnicky than *RIGID BODY so avoid doing anything fancy. It can fail or produce wrong results if you have very large rotations (>~10 degrees), try to couple rotational DOFs, or use *TRANSFORM on it sometimes even though it’s OK other times.

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hi vicmw,

thanks for add some hint. i did not look into specific details, only want to says similarity between Nastran and Calculix in RBE3.

is failure problem comes from *COUPLING with *DISTRIBUTING keywords only? how about DCOUP3D element. any simple examples are worth of me to learn.

thank you,

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I made a mistake, sorry. You can’t use *DISTRIBUTING for this because it neglects the offset of the reference node from the mesh it’s connected to. Also, I’m having trouble getting it to work with *FREQUENCY.

@carlomontec, I think you have to use either:

  • *RIGID BODY which rigidly connects all its nodes.
  • *COUPLING with *KINEMATIC which behaves like *RIGID BODY.
  • Write constraint equations yourself to couple all the DOFs. Yuck.
  • Beam elements with high stiffness.
  • *DISTRIBUTING COUPLING maybe. See below.

@xyont, I don’t have much experience with DCOUP3D and *DISTRIBUTING COUPLING except that it seems to be more low-level than *COUPLING and you have to specify the weights yourself which would be tricky on an unstructured mesh. Hopefully someone else can add more.

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Thank you all for your valuable contributions.