I am trying to do a static condensation (EXTSEOUT) in MSC NASTRAN for a simple box structure as shown in the attached “RBE3_dependent_dof_conflict_with_ASET.docx”. I have applied a multi point constraint using RBE3 to the outer four nodes (independent nodes) to the inner node (dependent node). This will assign the RBE3 dependent dof (i.e 123456) to the M-Set. I need the model to be reduced with respect to the RBE3 dependent nodes(i.e ASET will contain both RBE3 dependent nodes). When I run the model in MSC NASTRAN there is a conflict between the ASET containing the RBE3 dependent node. According to my understanding, MSC NASTRAN would not accept the same node to be in ASET and MSET simultaneously. Due to this, I am unable to reduce the box structure using MSC NASTRAN.
I have seen in MSC NASTRAN superelements user guide(Ref https://simcompanion.mscsoftware.com/infocenter/index?page=content&id=DOC10184&actp=RSS ) that this conflict can be resolve using the UM option in RBE3. But I could not understand how to define the UM node and dof to be reassigned. Can anyone help me with the correct usage of UM option with respect to my model attached (Bar_box_static_condensation.bdf attached).
Any help will be appreciated.
Thank You,
Paul Thomas
Hi Thomas,
I am working on this as well. Just curious to know if you have got some solution by now. I will update you if i find a breakthrough.
Regards,
Prasad
Hi Paul,
If you are still looking out for a solution, I have one. I use this work around all the time and it works.
1) Instead of UM option you can create a co-incident grid at your dependent grid of RBE3.
2) Next, create a stiff spring (celas element) connecting the new grid and your RBE3 dependent grid in all DOF's. I usually use stiffness of 1e9 lb/in, you can use the same in your equivalent units.
3) You will have to now use this co-incident grid for ASET condensation.
That is all. Hope this helps others too.
Amique.
Hello Amique and Prasad,
I did the same thing. Instead of creating the CELAS in all 6 directions, create an infinite stiffness CBUSH (k=1e9 units). This will be the simplest way to deal with this problem. Thank you Amique for your answer. Prasad you can do the same thing as well.
paul, Like this additional celas may add the stiffness to the component, better way is to use PARAM AUTOMSET YES option instead of UM option manually for RBE3. This also will work nicely instead of adding additional celas or CBUSH.
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