How can I include mass elements in only one load case?
Is there a point mass I can apply that doesn't apply to all load cases? Or should I just apply a force to simulate such masses?
"Apply force to simulate a mass" is a dangerous procedure not recommended at all: the resultant FE model is not valid at all to run a normal modes analysis or any dynamic response analysis, so I suggest to avoid it, the mass should be captured using either a concentrated mass CONM2 element to define a concentrated mass at a grid point about its center of gravity, or use FEMAP command Connect > NonStructural Mass Region. This command creates individual REGIONS representing non-structural mass regions, which are used to represent the mass of “non-structural components” which must be considered for a model (i.e., paint, coatings, wiring, etc.). NonStructural Mass Regions may be used in conjunction with or as an alternative to specifying non-structural mass via Properties in FEMAP.
Depending on definition, NonStructural Mass Regions may create NSM1, NSML1, and/or NSMADD entries in Nastran input files. This capability is available for all Nastran Solution Sequences supported by FEMAP.
An interesting feature of NonStructural Mass Regions is that the mass regions can be enabled or disabled at any moment you like (RMB on the model info tree), then you can generate the NX Nastran input deck and perform any analysis considering the mass effect or not.
The CONM2 element is part of the FE model, is not a load, please note the mass GRAVITY effect in a LINEAR STATIC Analysis is activated only if you issue command MODEL > LOAD > BODY. In summary, you can "play" with all of the above features to get what you need, OK?.
Thank you Blas.
But, these options still do not allow me to define mass which is only present in a single load case. Is that not possible in NX Nastran?
I agree generally with the comment about representing load as mass. However I still need a way to include mass in a single load case (it happens to be a lifting load case). I will probably just use force if there is no single-loadcase mass option available.
The CONM2 concentrated mass element will act only in the LOAD CASES where a BODY LOAD is defined, then the effect is the same: if not body load defined, then no gravity effects.
Please note the reason to define a CONM2 concentrated mass element is because the position of the CoG of the mass element is very important in a normal modes/eigenvalue analysis, not only the mass value is important, also the position of its center of gravity is critical.
Hello mr. Blas,
I have a similar problem: a mass that can move on a structure, so it can have several positions.
I must consider the self weight of the structure so the BODY LOAD is always ON. But I'd like to use the same model for more than one study, that is for more than one position of the mass. Is it possible? or the mass element, as part of the model and not of the load set, cannot be changed from ona analysis to another?
Thanks in advance
Simply edit the node of the mass element and reposition when you need. Then run the analysis. Move the node of the mass element to another position and re-run the analysis, and so on .. you have different solutions.
Nastran does not support changing the mass properties between subcases.
You really have 2 options. As Blas pointed out, if nonstructural mass regions work for you, then you can create multiple regions in the modfem , one for each mass distribution, then enable/diasable the regions as desired before each run.
If you need to use concentrated masses, then you will need to update the CONM2 and its location for each run.
You can create a group that excludes the mass elements and send that to the NASTRAN solver:
It's under "Bulk Data Options":
Just be sure your group is "complete". I.e. it must include all nodes, properties, materials that define the elements.
I manage this by creating a new group using elements, excluding those I don't want and then use the tree and right-click -> add related entities:
You can simulate a "moving" mass by using multiple groups too. All mass positions are contained in the complete model, but create mulitple groups; one group per mass position. It'll be tedious if you have more than a couple of positions, but it'll allow you to use the same model and same load sets without creating mulitple model files
There is another option if you have a bunch of cases without the mass element and only one with it and you don't want to edit your other analysis sets to now exclude this mass element.
Create a new model file with the mass element, run that one case and then import the results.
As long as the element and node numbering stays the same FEMAP will map the results.