Grounded CBUSH Element (Nastran) for Model Boundary Conditions

[enghelp2806]

Hi, I'm trying to understand the use of CBUSH elements with PBUSH properties for grounded constraints on a model. Normally I'd use displacement constraints in the 6 DOFs to constrain my model, but I recently came across "grounded bush" elements. In Hypermesh, these elements are classified as 1D CBUSHs applied at a single node and assigned the PBUSH property card. By single node, I mean the element itself is created using a single node, not two coincident nodes.

How do these "grounded" elements constrain a model? I understand that the PBUSH property card allows for the application of various stiffnesses in the 6 DOFs, but I don't see how a model can be free of rigid body motion without hard displacement constraints?

Thanks!

 

[NRP99]

Request to post Nastran related questions in- forum825. This is general FEA forum for general queries on FEA.

Search entire eng-tips.com/Nastran forum with "CBUSH", may be the question is already posted.

 

[FEA way]

According to Nastran’s documentation, CBUSH element is generalized spring and damper. When you leave the definition of the second connection point blank, a grounded spring and damper are used. That’s how it works internally. PBUSH is just a generalized spring and damper property - it’s used to specify the spring stiffness and damping coefficient.

 

[Ng2020]

Yes it's just a convenient shortcut. Functionally identical to an spc with 6dof constraint on one of the cbush nodes.

 

[rb1957]

why would you not constrain the node in question ?

Or is the CBUSH creating a finite stiffness constraint ?

another day in paradise, or is paradise one day closer ?