How to get the vertical frequency after dynamic time history?

[honestliang]

Hello,everyone! I can define the vertical mass and stiffness to get the vertical frequency,but during the timehistory analysis, horizontal and vertical mass and stiffness need to be appended at the same time to simulate the real reaction of structure, so the problem is coming, after time history analysis, how to stay the state(structural damage) and get the vertical frequency?
I can not delete the horizontal mass and stiffness and get the vertical frequency because doing that will cause time history analysis results loss, only get the original vertical frequency of structure.
Thanks for your answers!

[fmk]

you are going to have to do an eigen analysis with both horiz and vertical masses, look at the eigenvectors and determine those which correspond to the vertical frequencies you seek. if there is liitle to no interaction between horiz and vertical movemnt of the structure they should be similar to what you got with no horizontal masses so look at eigenvectors around those frequencies obtained with no horiz mass if this is the case.

[norahcackle]

You could try performing a modal analysis on the structure after the time history analysis to capture the updated vertical frequency while keeping the horizontal mass and stiffness intact. This way, you can isolate the vertical frequency without losing the time history results.