Reply To: Negative frequencies during eigenvalue analysis

[Tonmoy]

Hi Reno!

Thank you for your response. I applied fixed boundary conditions (BCs) at the bottom of the foundation pad, but I didn't apply any BCs for the cylinder as we want to understand the response of the it during earthquake excitation.

The model should clearly exhibit some rigid body modes, and it's expected to find responses such as the cylinder moving left to right or up and down in the x-y plane during the first few modes. However, sometimes I encounter modes that seem unusual or unexpected. For instance, in one case, instead of the expected left-right rigid body mode, the cylinder showed a tilted response (as if the contact surface was holding a corner of the cylinder while allowing the rest of the body to move freely). In another case, the cylinder was moving in an inclined direction in the x-y plane, which is not something we expect in the first couple of modes.

In these situations where the rigid body modes appear to be incorrect or unrealistic, I've noticed that LS-DYNA reports negative eigenvalues. So, I wanted to understand why the negative eigenvalues generally appear and if there are any documents reflecting the theory of computing eigenvalue that ls-dyna employs in this kind of cases.

Thanks a lot again for your time and response.

 

Regards, Tonmoy.