Multiple RBE2 on Single Dependent Nodes Set

Experimenter
Experimenter
In many circumstances, it can be necessary to apply multiple loads through multiple rigid elements (say RBE2) where only independent nodes will be different (i.e. dependent nodes are same). For example, because the point of application is not the same for each load but they still end up being transfered to the same nodes in the model.
 
Attempts to do that does not work for Nx Nastran which will throw error message 5289. It is possible to enable PARAM,AUTOMPC to reverse dependent/independent nodes definition. Nx Nastran will then solve. This is very well documented online and in the user manuals. But:
 
Is there any drawback in doing that in a static linear analysis?
Is this going to change in any way the behavior of the RBE 2 elements involved?
 
Thanks in advance!
5 REPLIES 5

Re: Multiple RBE2 on Single Dependent Nodes Set

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

Dear Grothen,

The use of PARAM,AUTOMPC is very dangerous, I suggest to solve the potential conflict of "double-dependency" at user level and not let the problem in hands of the nastran solver, my suggestion is "never use PARAM,AUTOMPC,YES", the result will change?, of course!!.

If you need to define different RBE2 elemenets sharing the same dependent nodes but different location of  the independent nodes, simple define groups with different configurations and activate the portion of the model you require for solving, and done!.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Multiple RBE2 on Single Dependent Nodes Set

Experimenter
Experimenter

Blas, 

 

Thanks for your reply.


Let's talk of simultaneous loads only in order to narrow the range of solutions.


If I understand correctly, you suggest creating an analysis and, in the Bulk Data Options, under "Portion of Model to Write", select a group which contains only 1 Rigid element (hence 1 load).

Repeat that step with different groups corresponding to the other rigid elements. Repeat as many times as I have rigid elements sharing same dependent nodes.

Then combine results with the Data Surface Editor.

Correct?

 

That's a solution and it will allow me to check if there is a difference in results when compared to using “PARAM,AUTOMPC,YES”.

 

However, do we understand the effect that swapping dependent/independent nodes can have on the linear static solver?

 

Thanks again,

Re: Multiple RBE2 on Single Dependent Nodes Set

Siemens Phenom Siemens Phenom
Siemens Phenom

I will leave the dependent/independent question to the NASTRAN experts.  With regards to Blas' suggestion, he is suggesting multiple analysis sets, and in each one a reference to a group that contains your full model, and the one active rigid element, excluding the quasi-duplicates.  You would create as many analysis sets as rigid/load configurations you have.  Once you have the first group defined, you can copy it and flip in the next RBE.  You will also need a load set for each configuration, and connect those into each analysis set when their node is included.

 

Mark.

Re: Multiple RBE2 on Single Dependent Nodes Set

Siemens Phenom Siemens Phenom
Siemens Phenom

I will respectfully disagree with Blas. In most cases, and the exceptions would be very unique, you should be fine with using AUTOMPC. The issue with independent/dependent dof's is strictly a book keeping issue in Nastran. Using this parameter will allow Nastran to rearrange the MPC equation that gets written internally, but the equation does not change, only the order of the terms in the equation. Take a look at documentation for MPC equation. Notice the first term of the input is identified as dependent, then followed by all of the other terms and coefficients. You can have the same equation, but use a different term for the first one, it is exactly the same relationship, therefor the model behaves the same, but a different node/dof is dependent.

Also, if you look at the RBE3 documentation, you will see that Nastran provides a "manual method" to rearrange this equation using the optional  "UM" input. Unfortunately, there is no "manual option" for changing an RBE2, you must use AUTOMPC or change your model.

 

All that being said, using RBE2 to apply loads might not such a good choice since the equations that written internally result in no relative motion between the nodes that you connect. It is a true rigid connection and can result in a much stiffer model at those locations.

Traditionally, for the use case of spreading loads to nodes, the RBE3 is a better choice since it does not add any stiffness to the model. The equations that are written internally are very different for the RBE3.

 

Regards,

 

Joe

 

 

Re: Multiple RBE2 on Single Dependent Nodes Set

Solution Partner Phenom Solution Partner Phenom
Solution Partner Phenom

My best Kudos for Mark & Joe, thanks!.

Regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/