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# Flexible and rigid response in modal transient analysis

## Flexible and rigid response in modal transient analysis

(OP)
Hi members,
I have an issue in understating how modal transient analysis works free free structures. If I apply a time varying force with frequencies well above zero( say 40hz), and ask Nastran an FE solver to consider only flexible modes in analysis, the result is a mean zero response of accelerations and forces. Does this mean I am losing data from rigid rotations and translations caused by mean force (0hz) force component.

Also, rigid modes do not strain, but in reality any constantly accelerating body deforms means it strains. If I include rigid modes in my solution will I get the strains consequently internal reactions caused by these rigid motions, I find it difficult to comprehend why modes which do not strain will give any reaction force. Can sincerest shed light on this?

### RE: Flexible and rigid response in modal transient analysis

"reality any constantly accelerating body deforms means it strains"

Consider a structure in free fall. No strain. Uniform acceleration. Until you get this right the rest will be inexplicable.

Cheers

Greg Locock

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### RE: Flexible and rigid response in modal transient analysis

(OP)
Greg Locock thanks for reply. A freely falling body is in influence of field of gravity, so every small partical in the body has applied force which is equal to force on every other mass particle, so no particle pulls any other particle. But if situation is like a force at one end of rod with mass "m" and length "l" causing it to accelerate, here portions adjacent to force on rod will pull portions after them causing the rod to elongate. So, if I try to analyze this with modal transient analysis with rigid body modes included, and since force is not varying with time, only rigid modes should participate in response, in this scenario can i expect the solution to have strains which is physically happening or no strains. I may be thinking it wrong, but want to know whether or where, I have gone wrong.

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