Vertical Period Monitoring in 3D Frames

[Ameri]

Hi, I want to monitor the vertical period (translational in "z" direction) of a 3D 12-story structure during dynamic analysis and then plot the vertical period vs. time graph. I know how to write a script so as to record the list of eigenvalues during analysis for each time step. However, the problem is that my desired eigenvalue (the frequency of translational mode in "z" direction indeed) might be in every location in the eigenvalues list! In other words, it might be the 1st component of the eigenvalue list at t=0.06, the 9th component at t=1.08, the 3rd component at t=2.51, ... And I really don't know how to determine this in the script and record the desired eigenvalue during the analysis. Is it possible to do so and record a certain period during dynamic analysis of 3D structures in OpenSees?! I appreciate your consideration in advance.

[fmk]

sounds like at every step you are going to have to have to issue eigen command, record a couple of eigenvalues and eigenvectors and postprocess them later. you could put it in the script, just depends what you want to use for postprocessing tcl, matlab or excel.

[Ameri]

Thanks for your reply Dr McKenna. I know how to issue eigen command at each time step and record the eigenvalues/eigenvectors, but I don't know how to figure out which eigenvalue represents the frequency of the desired mode shape (translational in z direction).Definitely it should be done by the use of eigenvectors, however, it's almost impossible for me to do this manually. I would perform 180 dynamic analyses and each of them contains 2000-2500 steps. By the way, could you tell me please is there any script for establishing and monitoring a certain eigenvalue during dynamic analysis?!

[fmk]

that you will gave to get looking at the eigenvector values .. if vertical mode, the translational values for the vector will be small compared to vertical dof values .. you can put it in the script as you can get the eigenvectors directly from a node.

no script for monitoring an eigenvalue .. people typic get it once, save them and go on. you could place them into a list or add them to end of a file if you want.