Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Flexible and rigid response in modal transient analysis

Flexible and rigid response in modal transient analysis

Flexible and rigid response in modal transient analysis

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.


Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Flexible and rigid response in modal transient analysis

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close