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# Grid Point Weight Generator, WTMASS and Super-elements

Good morning

I was wondering whether someone could clarify my understanding of the grid point weight generator.

I have a model comprising of numerous super-elements stored on DMIG cards. The residual structure structure has no mass. I factor all the mass mass matrices with M2GG=1.264*MAAX. The total mass of the structure is 296.36*1.264=374.6kg and this is shown correctly in the GPWG.

When I replace one of the super-elements with the actual sub-model (weighing 4.3467kg), I use a WTMASS factor of 1.2654 to factor the mass of the actual sub-model to reflect the same factoring it would have received as a super-element using the scaling on the M2GG card.

So my structure is now

MAAX : (296.36-4.3467)*1.264=369.1kg

Residual structure 4.3467*WTMASS=4.3467*1.264=5.5kg

Total Mass = 374.6kg

However the GPWG gives me 296.03kg. It appears nastran is dividing by the WTMASS factor once it has assembled the total mass of the stucture (which includes factoring the Residual mass up by WTMASS). Is this correct?

The SPC forces of the model subject to gravity loads are correct, so the mass of the model assembly is correct, however I am confused by the output of the GPWG.

Peter

2 REPLIES

## Re: Grid Point Weight Generator, WTMASS and Super-elements

Peter,

Take a look at the Basic Dynaics Users Guide, Appendix J

It gives all the details on GPWG, but the section below is what you are looking for I believe:

• The scale factor entered with parameter WTMASS is applied to the assembled element mass
before the GPWG. The GPWG module, however, converts mass back to the original input units
that existed prior to the scaling effect of the parameter WTMASS. (Note that the parameter
WTMASS is not applied to M2GG or M2PP input, but the M2GG mass is assembled into the
mass matrix prior to GPWG. Therefore, for GPWG output only, the M2GG mass is scaled by
the same parameter as the element mass. M2GG input may be scaled independently using
the CM2 parameter.)
• The GPWG is performed on the g-size mass matrix, which is the mass matrix prior to the
processing of the rigid elements, MPCs, and SPCs.
• The mass at scalar points and fluid-related masses are not included in the GPWG calculation.
• The GPWG for a superelement does not include the mass from upstream superelements.
Therefore, the GPWG for the residual structure includes only the mass on the residual points.
The center of gravity location is also based on the mass of the current superelement only.
• If a large mass is used for enforced motion, the large mass dominates the structural mass. For
model checkout, it is recommended to remove the large mass and constrain the driving point. A
static analysis is a convenient way to generate a mass matrix and obtain output from the GPWG.
• The output from the GPWG is for information purposes only and is not used in the analysis.

Regards,

Joe

Solution
Solution
Accepted by topic author PeterE
‎03-08-2017 01:54 AM

## Re: Grid Point Weight Generator, WTMASS and Super-elements

Joe

Thanks for the clarification

Peter