Dynamic simulation (normal modes) of a structure with cables in tension with a different stiffness for tension and compression

Good afternoon,

I am trying to perform the dynamic simulation of a structure which includes cables in tension and I want to determine the eigenfrequencies and eigenmodes.

This is what I did:

• Spring (CELAS - PELAS), it works both for linear static and normal modes, but it sets the same stiffness to tension and compression and this is unreal for me
• Gap (CGAP - PGAP), it works only for non-linear static while it induces a constant stiffness in all the other simulations
• Bush (CBUSH - PBUSH), I tried but it doesn't allow to set two different stiffness in the same direction

I would like to use something like CGAP - PGAP which is also valid in the dinamic (normal mode) simulation.

All the recommendations are welcome! Thank you very much in advance. 

Marco