Sine wave question

Alexandros · Post by **Alexandros** » Tue Nov 02, 2010 12:20 pm

Dear Silvia or anyone else who can help.

I run example 5 with the Inelastic Fiber RC section under the sine wave excitation (Ex5.Frame2D.analyze.Dynamic.sine.Uniform file). I record the displacement at the free end node (node 41) and plot this displacement with time. My building is vibrating between two limit points until the end of the excitation.
After the end of the excitation I have a peak in my plot indicating that energy is coming in my building. I don't understand why is this happening. I think that after the excitation the building should be in free vibration starting from the point where it was when the excitation stopped. The peak becomes more obvious if you add mass on the joints ( for example take 2*$Qlslab) .
I am really confused. Does it have anything to do with the nonlinearities?

Thanks a lot, looking forward to a reply.

vesna · Post by **vesna** » Wed Nov 03, 2010 10:36 am

I looked at the example (downloaded from OpenSees Wiki) you have problem with and did not encounter any problem. After the sine wave I see the free vibrations of the system. Did you make some changes of the example files? If you did not make any changes I suggest you to download these files again.

Alexandros · Post by **Alexandros** » Wed Nov 03, 2010 11:25 am

Thanks for the fast response.

I downloaded the example again and run it but i still have the same problem.
In my model I only change the amplitude of the sine wave (try it with GMSineAccAmpl = 0.5, it happens with smaller values too, but it is more obvious with larger) and the duration of the sine wave (DurationSine = 20, TmaxAnalysis = 30). If you plot the displacement of any of the nodes 21,31,41 with time you will see what I am talking about.

Thanks a lot again.

vesna · Post by **vesna** » Wed Nov 03, 2010 1:25 pm

What I see after I run Example 5 with your input parameters is slight increase in displacement response after duration of sine wave (after t=20 sec), that is followed by decrease of displacement with time (free vibrations). This small increase in displacement (after t=20 sec) is due to the displacement (u(t=20 sec)) and velocity (v(t=20 sec)) at the end of the sine wave duration. They initiated free vibrations following the duration of the sine wave. The result of the analysis is something I would expect.