FEAP User Forum

FEAP => General questions => Topic started by: M. Kurdi on September 05, 2013, 01:43:29 PM

Title: Natural frequencies of structure
Post by: M. Kurdi on September 05, 2013, 01:43:29 PM

Hello,
I want to compute the natural frequencies of structure from solving eigenvalue problem of the tangent stiffness and mass matrices. I am using following commands to output these matrices and solve the normal mode shapes:

batch
MASS
TANGent
SUBSpace,,3
TANGent,,-1
OUTPUT TANG
OUTPUT MASS
end

When solving the eigenvalue problem using the MASS_001 AND tang_001 matrices the results do not match the ones provided using FEAP analysis. Do I need to use a different command to get these matrices?

Thanks for your help.

Title: Re: Natural frequencies of structure
Post by: FEAP_Admin on September 05, 2013, 02:44:52 PM

This should work just fine.  Perhaps you treated the matricies incorrectly in your external program?
If using MATLAB try something like:

K = full(spconvert(tang_001));
M = full(spconvert(mass_001));
eig(K,M)

You should get an identical match to the EIGENVALUES themselves.  Be careful to not compare circular frequencies
or other quantities that FEAP provides in addition to the eigenvalues themselves.

Title: Re: Natural frequencies of structure
Post by: M. Kurdi on September 05, 2013, 05:34:52 PM

Thank you!