truss structure normal modes: low frequencies?

[MBD26]

I have a 3D solid model of a truss, and I'm using Nastran to determine the normal mode frequencies. Nastran is giving me results for modes 1-10. The first 6 modes are effectively 0 Hz, but my understanding is that those correspond to rigid body translations and rotations and that 0 is the expected result for a 3D model. Modes 7-10 all fall between 1-2Hz. I'm not sure what to make of this, as I was expecting much higher numbers. Is this result typical for a the normal modes of a complex structure like a truss? Is this telling me that I don't need to be concerned about, say, a 100Hz disturbance exciting the fundamental modes of the structure?

Thanks

 

[GregLocock]

I wonder what the effect of relaistic boundary conditions would be on that model, unless you are designing a space station?

Cheers

Greg Locock


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[rob768]

Apart from conneting it to the world, as suggested by GregLocock, are you sure you got your units correct? Yuo shouldn't see zero modes, and some apart fromt he geometrie, i expect some higher frequencies.

 

[MBD26]

Greg is actually pretty close with the space station comment... I'm modeling a UAV in flight, so it's an unconstrained model. I've since learned that, for an unconstrained model, it makes sense that the first 6 modes would be 0Hz rigid body modes. I'm not familiar with the nuances of modal analysis for contrained vs. unconstrained models. If the structure is unconstrained, as for a small rigid airframe like this, is dynamic response generally less of a concern? How are unconstrained structures typically modeled for modal analysis?

Thanks

 

[inline6]

unconstrained models and usually left unconstrained.

what are the low freq modeshapes like (i.e. i mean the non rigid ones)?

 

[jhardy1]

Student post?
(It's a bit disconcerting if there is an aerospace company somewhere, where the dynamic analysts are unfamiliar with the concept of unconstrained modes!)

http://julianh72.blogspot.com