Eigen analysis with -genBandArpack results in error

[Jatkinson]

Hi all,

I have some 3D tall building models, with masses applied to nodes at every floor level. I was having some difficulties with eigen analysis using the -genBandArpack option. Specifically, I was getting an error if I requested more then a few modes:

ArpackSolver::Error with _saupd info = -9999
Could not build an Arnoldi factorization.IPARAM(5) the size of the current Arnoldi factorization: is 15factorization. The user is advised to check thatenough workspace and array storage has been allocated.
WARNING DirectIntegrationAnalysis::eigen() - EigenSOE failed in solve()
Node::getEigenvectors() - eigenvectors have not been set

The -fullGenLapack option would work fine with the same number of modes, but takes significantly longer.

I then added a small mass to all nodes in the modal, and now the -genBandArpack option works fine.

Can someone explain this behavior?