Difference between Grid Point Weight Generator and Weight Check? 2

[Francesco Bro]

Hi,

I know it is possible to check the mass properties and CG position of the FEM model by using the Grid Point Weight Generator (PARAM,GRDPNT,0). But it is also available the Weight Check command (WEIGHTCHECK) at the Case Control.

What is the real difference between using one or the other?

When is it convenient to use each one?

Thank you!

 

[dmapguru]

When the model does not contain superelements, the Grid Point Weight Generator output from PARAM,GRDPNT,0 will give you the mass and inertias of the entire model computed from all degrees of freedom (DOF) in the model where mass is defined. A WEIGHTCHECK request, when the G or J SET are in the requested list will report the same thing. Now comes the first thing WEIGHTCHECK can do for you that PARAM,GRDPNT,0 cannot.

If you have some dependent degrees of freedom in your model (e.g. rigid elements or MPC definitions), and/or if the model contains boundary conditions (SPC/SPC1), and/or if you requested static reduction or component mode synthesis (CMS), then if you request SET=ALL, Nastran will compute the mass and inertias of the model after the mass on dependent DOF have been reduced to the independent DOF, after the application of boundary conditions, after static condensation and after CMS. Along with the mass/inertia output, you will also get a table showing the percentage "loss" of mass due to the relevant step. In this way, you have a concise report of how much mass your model just "lost" because you SPC'd some degrees of freedom, for example.

If the model contains superelements, PARAM,GRDPNT,0 will only output the mass/inertia of the current component (including the residual); this output will not include any mass from upstream superelements. WEIGHTCHECK will output the mass/inertia before (J set) and after (G set) upstream superelement mass inclusion.

DG