Static condensation and RBE3

[adamas]

I would like to use static condensation (Guyan reduction) to calculate the normal modes of a wing box structure reduced to a set of grids along the main axis. These grids are connected to the structure through RBE3 (master grid) but doing like this NASTRAN gives a fatal error. Do you know if there's a way to use the indipendent grids of RBE3 elements in the A-SET?

Thanks

 

[BlasMolero]

Dear Adamas,
Post here the error exactly (copy & paste the error from the F06 file). The most common user error in RBE3 element specification results from placing 4, 5, or 6 in the Ci (independent DOF) field in addition to the translation components. The rotations of the dependent point are fully defined by the translational motion of the independent points. The ability to input 4, 5, or 6 in the Ci field is only for special applications, such as when all of the connected points are colinear.

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/

 

[adamas]

Hello,

the error form the f06 is this one (where GRID 1 is the indipendent grid of the RBE3):

*** USER FATAL MESSAGE 2101 (GP4)
GRID POINT 1 COMPONENT 1 ILLEGALLY DEFINED IN SETS UM UBF
*** USER FATAL MESSAGE 2101 (GP4)
GRID POINT 1 COMPONENT 2 ILLEGALLY DEFINED IN SETS UM UBF
*** USER FATAL MESSAGE 2101 (GP4)
GRID POINT 1 COMPONENT 3 ILLEGALLY DEFINED IN SETS UM UBF
*** USER FATAL MESSAGE 2101 (GP4)
GRID POINT 1 COMPONENT 4 ILLEGALLY DEFINED IN SETS UM UBF
*** USER FATAL MESSAGE 2101 (GP4)
GRID POINT 1 COMPONENT 5 ILLEGALLY DEFINED IN SETS UM UBF
*** USER FATAL MESSAGE 2101 (GP4)
GRID POINT 1 COMPONENT 6 ILLEGALLY DEFINED IN SETS UM UBF

The dependent grids have only translational DOFs activated.

 

[BlasMolero]

Dear Adamas,
You have prescribed a constrain in "dependent" grid point#1 of RBE3 element, and this is a violation not permited with NX NASTRAN: "dependant" nodes should be free of any constraint.

User information:
The above grid point and component are defined in each of the above
dependent subsets. A point may belong to a maximum of one dependent
subset. This error occurs when a DOF is defined as belonging to
two mutually exclusive sets.
A common example of this occurs when a DOF is defined as dependenton
an MPC (M-set) as well as being constrained (s-set) on an SPC entry.
The message for this states that the component is illegally defined in
the um (user-defined m-set) and us (user-defined s-set) sets. These two
sets are mutually exclusive because all MPC equations are processed
before the SPCs are applied (exception is SOL 24) and the m-set DOFs
are removed from the matrix. When the program attempts pply the SPC,
the DOF is no longer available, and the FATAL message is issued. The
normal correction for this is to modify the MPC so that the DOF in
question is independent (N-set). Then there is no conflict.

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/

 

[adamas]

So there isn't any way to overcome this error and use RBE3 indipendent grids in the A-SET for static condensation?

 

[BlasMolero]

Dear Adamas,

Using Static Condensation in NX NASTRAN
--------------------------------------------------------------------------------
In NX Nastran, the ASETi and OMITi bulk data entries define the degrees-of-freedom in the analysis and omitted sets, respectively. You can use either the ASET/ASET1 or the OMIT/OMIT1 entries to specify only the a-set (with ASET/ASET1 entries) or o-set (with OMIT/OMIT1 entries) degrees of freedom. The software automatically places the remaining f-set DOFs in the complementary set. However, if you specify both the a-set and o-set DOFs, then the unspecified remaining f-set DOFs are automatically placed in the o-set.
Then please note: If you specify the same DOF on both the a-set and o-set, the job fails with "User Fatal Message (UFM) 2101".

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/

 

[nlgyro]

you can try using param,automset,yes in your bulk.. Any kind of double-dependencies at the m-set level is automatically rectified by the solver.