eigen analysis: number of modes

[fran]

Hi,

eigen command does not calculate 3 modes of a 5 DOF frame. There is no problem with:

2 modes of 5 DOF frame
3 modes of 10 DOF frame.

What is the reason of that?

Thanks.

[fran]

I mean with "does not calculate" is that it gives the following error:

BandArpackSolver::Error with _saupd info = -9999
Could not build an Arnoldi factorization.IPARAM(5) returns the size of the current Arnoldi
factorization. The user is advised to check thatenough workspace and array storage has been allocated.
Warning FrequencyAlgo::solveCurrentStep() - the EigenSOE failed in solve().
EigenAnalysis::analyze() - algorithm failed


The eigen solver is after the dead load analysis and before the nonlinear dynamic analysis.

[fmk]

the eigensolver can only determine n-1 eigenvalue, where n is the total number of non-zero eigenvalues available.

it also has difficulty sometimes when user requets a eigenvalues and the a+1'th is the same as the a'th.

so if you expect more eigenvalues try asking for 1 more or 1 less.

[fran]

In order to increase the 5DOF I added 1e-9 mass values in the Y direciton as you proposed in the manual and asked more eigen values. That solved the problem.

Thank you for your response.

[aizen]

Hi

I do not understand well. Why does this type of analysis work in this way?

If I have a problem with 3 DOF ( DOF are all displacements in x direction ) with 3 masses in the x direction I can not ask for 3 eigen values. Do I have to ask only two eigen values?

Do I add 3 masses in the y direction to have 3 eigen values? But in this case the eigen values are 6..If I use 3 y masses very low, I are sure that the new eigen values are the last.

Do I understand well?

???

[fmk]

yes.

[aizen]

Dear fmk

thank you very much.



Aizen