Observed initial 6 rigid body modes in modal analysis, when super element model is assembled with different structure

Balaji_21730

Observed initial 6 rigid body modes in modal analysis, when super element model is assembled with different structure.

1. Received super element FE model and modal results from external source (Nastran).
2. The model is fixed in all 6DOF at required locations.
3. Modal analysis performed using optistruct and natural frequencies are requested.
4. The modal frequencies of super element model are cross checked with Nastran results, the frequencies are matching.
5. The super element model assembled with structure and modal run submitted.
6. Observed initial 6 modes as rigid body modes and structure modes starting from 7th mode.

Please clarify on this, why rigid body modes are observed, the model connections, materials and properties are thoroughly checked.

Xiaolei

Hi Balaji,

I believe you have fixed the model as you described in the 1st picture. 

Can you please try opening h3d results of residual run(the run with Base Superelement) using HyperView to check the animation? I doubt if there is any flying elements ex. a floating shell element or bar element. 

 

If you still can't find the cause, please share the model then Altair team can have a look at it? In that case, you can send this information to hwsupport@altair.com to create a ticket. 

 

Thanks. 

Xiaolei