Frequency analysis of space frame with distributed mass

Dear all,

I want to do a frequency analysis of a 3d frame which has three beam elements (2 columns and one beam). There are no lumped masses in the model. I want to assign mass to all three elements equal to m per unit length of the members.

When I assigned nonzero density in the beam section definition (equal to m/A, where A is the cross section area),
When I changed from beam elements into frame elements
When I changed from beam of linear geometric order to beam of quadratic geometric order (standard element libraries)
When I tried to assign *Nonstructural mass (via the Special->Inertia option in Interaction module)
the result was that the eigenfrequencies extracted by abaqus were not correct (they were not consistent with the results of a frequency analysis with consistent mass matrix formulation and classic euler-bernoulli beam stiffness matrix formulation)

Another idea is to investigate whether the mass matrices involved in the frequency analysis for the various elements are assembled using the lumped mass formulation or the consistent mass formulation.
Could anyone make clear about the mass formulations of the various beam elements?
What about the coupled degrees of freedom?
What about the rotary inertias as entries of the mass matrix?

It is mentioned also that when I analysed exactly the same frame with point masses assigned directly to its top nodes the eigenvalues extracted were identical to the results of the frequency analysis with consistent mass matrix formulation and classic euler-bernoulli beam stiffness matrix formulation.

Any other ideas/comments about what can be going wrong?

I can send the input file if somebody wants so.

Thanks to everyone in advance for your help

Best regards,

George Papazafeiropoulos
_____________________________________________
First Lieutenant, Infrastructure Engineer, Hellenic Air Force
Civil Engineer, M.Sc., Ph.D. candidate, NTUA

Mailto:gpapazafeiropoulos@yahoo.gr
Website:http://users.ntua.gr/gpapazaf/
Linkedin:http://goo.gl/MtB3Y
Xing:http://goo.gl/gbvFB
Facebook:http://goo.gl/NJD0z