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truss structure normal modes: low frequencies?

truss structure normal modes: low frequencies?

truss structure normal modes: low frequencies?

(OP)
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

RE: truss structure normal modes: low frequencies?

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.

RE: truss structure normal modes: low frequencies?

(OP)
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

RE: truss structure normal modes: low frequencies?

unconstrained models and usually left unconstrained.

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

RE: truss structure normal modes: low frequencies?

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

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