Modal analysis of rods with nonlinear material

[adampar]

Hello i am a new member at this forum and i would like to get some help regarding a question i have. I have a truss structure with cables which i model as rod elements with nonlinear material behavior (high tension stiffness and very low compression stiffness). I would like to have a boundary free condition modal analysis of this structure. Now, i have tried what Blas Molero suggested in another thread Link but i seem to have the same natural frequencies as i get from a SOL103 analysis (normal modes analysis). Normally i should expect that some eigenfrequencies would be lower due to the fact that the cables can't resist to compression, but this is not the case here. What should i do to get correct results?

Thanks in advance,

Vasilis