I have this problem, but I am not sure if 3DEC allows to solve it by default.
I would like to know how the modal frequencies change while the model is accumulating damage. Therefore I would consider either two options:
A) performing an eigenvalue analysis, considering tangential or secant stiffness change, after only 2/3 seconds of a seismic input, therefore considering already some damage.
B) evaluating the natural frequencies while running the transient analysis, therefore per each time step.
I guess that the B) option is quite demanding and time consuming. But It would be great to hear your opinion.
Thank you in advance.