When should I check the "AUTOMPC" box? 2

[legersalazar]

Hello Forum,

I've been using Femap and Patran on and off for years and my memory gets rusty at times.

Could someone refresh my memory on why is it that sometimes we have to check the "AUTOMPC" box in the NASTRAN Bulk Data Options?

Is this everytime we're using MPC elements? What does it do exactly?

Thank you,

Whyowhy

 

[BlasMolero]

Hello!,

A degree-of-freedom can’t be designated as a member of the "m-set" more than once unless PARAM,AUTOMPC,YES is defined. Including PARAM,AUTOMPC,YES in the NX Nastran Bulk Data specifies that the software automatically selects the m-set dofs rather than use the m-set values as specified on MPC or RIGID element cards (RBE2, RBE3, etc.).

This option relieves the need to carefully define rigid elements (or MPCs) so as to not have a conflict of the m-sets between elements. In addition, any redundant constraints will be eliminated.
A fatal error results if the same degree-of-freedom is designated as dependent by two rigid elements or if the first-named degree-of-freedom on any MPC entry is also a designated member of um on a rigid element entry.

But as always is better you take control of the situation and do things right, not relieves "by default" in AUTOMPC.

Best regards,
Blas.



~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/

 

[legersalazar]

So if I understand you correctly, it's better to leave "AUTOMPC" unchecked, get a fatal error message and fix my DOFs, than to let the software do it, that way I know exactly what's going on?

 

[BlasMolero]

Dear Whyowhy,
In fact, when a fatal error exist related with rigid elements in general is because a double dependencies trouble exist, or a constraint is prescribed in any dependent nodes of either RBE2/RBE3 elements. The error occur when two elements share a dependent node. If your model contains double-dependencies, the NX NASTRAN solver may not be able to correctly resolve the degrees of freedom in your model, then instead to use AUTOMPC is better to understand the problem and fix it by hand, meshing correctly with rigid elements.
Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/