Effective Mass

Bi.EMS

Hi all,
I am learning about Modal Mode set up in Optistruct. When learning about this type of problem, I know the concept of "effective mass". So how does "effective mass" affect the model's motion in Optistruct and how from "effective mass" to choose the right number of NDs in EIGRL.
Thank you.

Johan Dahlberg_20306

Hi Jim
You can use this card to get out the effective mass to the .out file
control card PARAM,EFFMAS,YES.

and for the out file above you can see that X-trans 1.00 indicating a rigid boy motion in X
For mode #2 most of the mass is moving in Z-trans, 64.17%

Number of modes that you should extract depends on the frequency of your load, typically 1.5 time the highest frequencey in your load (at least 1,5 times).

For free-free motion you will have 6 rigid body modes, so normally I uese 20 modes for such cases.

We have really nice eBooks on the community describing dynamic analysis 
eBooks - FREE eBook: Learn Dynamic Analysis with Altair OptiStruct

Hope this helps
Kind Regards
Johan

Bi.EMS

Johan Dahlberg_20306 said:

Hi Jim
You can use this card to get out the effective mass to the .out file
control card PARAM,EFFMAS,YES.

and for the out file above you can see that X-trans 1.00 indicating a rigid boy motion in X
For mode #2 most of the mass is moving in Z-trans, 64.17%

Number of modes that you should extract depends on the frequency of your load, typically 1.5 time the highest frequencey in your load (at least 1,5 times).

For free-free motion you will have 6 rigid body modes, so normally I uese 20 modes for such cases.

We have really nice eBooks on the community describing dynamic analysis 
eBooks - FREE eBook: Learn Dynamic Analysis with Altair OptiStruct

Hope this helps
Kind Regards
Johan

Thank you very much