modal analysis/beam with loads

[spike]

In ANSYS, the only "loads" valid in a modal analysis are displacement constraints. How do you determine the modes of a structure with real loads, such as a beam supporting heavy equipment?
I tried running a static analysis, saved the file, started a new "modal" analysis and requested that Prestress be included, but the results are still for a structure with no loads.

Thanks

 

[Drej]

>How do you determine the modes of a structure with real loads, such as a beam supporting heavy equipment?

By carrying out a free vibration eigenvalue analysis (antype,modal) like everyone else. This is a modal analysis of a system FREE of external loads, and thus shows potentially how the structure would respond (mode shapes - eigenvectors - and corresponding eigenvalues) to a dynamic load input. If you want to see how your system responds dynamically, run a transient dynamic analysis with a dynamic load input, not a free vibration.


>I tried running a static analysis, saved the file, started a new "modal" analysis and requested that Prestress be included, but the results are still for a structure with no loads.

You can include pre-stress if you want, but make sure the pstres,on option is switched for BOTH the static and the modal analyses (not just one).

Cheers,

-- drej --

 

[GregLocock]

and just model the heavy equipment as non structural mass.

Cheers

Greg Locock

 

[DRDearth]

I'm going to visit this site more often.. I've written several artiles regarding Dynamic Reponse problems. One such article is in Machine Design magazine "FEA Upate" column. This article includes a sample problem with detailed hand calcuations and a FEA problem for download. Send me a note and I'll forward you a copy.

David R. Dearth, P.E.
Applied Analysis & Technology
E-mail "AppliedAT@aol.com"