Negative Eigenvalue

[omalvarez]

Hi dear all, I hope someone can help me, I am running an eigen analysis (bridge structure) but i get a negative eigen value (the first one) I do not know why (I am running an analysis with elastic elements and I am ussing rigid diaphragms each span) I have built the model repeatedly, but I get the same value, the other values ​​seem reasonable. Can anyone help me?

[mhscott]

Do you get negative eigenvalues when you do not have the rigid diaphragm? If so, check your boundary conditions and element connectivity. If not, try using Penalty or Transformation constraint handler. Lagrange can lead to negative eigenvalues.

[omalvarez]

Thanks professor Scott, I have used Transformation constraint handler, and got the negative eigenvelue, then I used Penalty constraint handler and still got the negative eigen value and large periods. I am going to try without rigid diaphragm. Thanks a lot.

[fmk]

does a static or transient analysis run?

[omalvarez]

Yes, static analysis runs, but I keep getting negative eigenvalues (now there are three negatives eigenvalues, first three, this analysis I did with rigid diaphragm)

[mhscott]

Do you get negative eigenvalues when there is no rigid diaphragm?

[omalvarez]

Thanks professor fmk and professor Scott, I finally ran my eigenvalues analysis successfully.I am using Navigator and I changed eigSolver type from "fullGenLapack" to "genBandArpack" and got reasonable results for the eigenvalues (not negative and not large periods [close to 1]). I really appreciate your help. Than you very much.