Eigen values- insight required

[ateestructural]

Hi,

I know that one gets complex Eigen values when there is damping in the system. IS it possible that if the system is unstable, one gets complex values?

I have got a structural system wherein I get complex Eigen values. The system is undamped.IT is stable.

I would like to understand the reason for getting complex Eigen values

Sahil

 

[cyt4]

Hi, the most probable answer is that you are asking for eigen values with default settings and the complex eigen values are probably slightly lower than the real solution. Ask for eigen values slightly positive(0.01 or .001) and you wont have to deal with complex numbers. The meaning of complex or negative eigenvalues is that if your load is reversed you would get a solution in the real plane.

 

[namklof]

A "normal modes" analysis neglects damping and produces real eigenvalues and vectors. If damping is included (assuming the damping is less than critical) the eigenvalues and vectors come in complex conjugate pairs. If you look at the last part of chapter 10 in "Funamentals of Structural Dynamics" by Craig, you will find a good description of complex modes analysis.