Dear all,
Let me briefly go through the problem I am facing.
Currently, I have data ( of ground acceleration) obtained from the "seismic accelerograph instrument system" which was placed at the basement of the building and the plot is shown below. According to the plot, it is showing a random waveform up to a certain time and it starts decaying (damping occurs). However, it again gets another waveform (sinusoidal, as shown in the figure) after 300 sec. It looks very unusual to me. I suspect the sinusoidal part to be a building response. But I couldn't decide whether my assumption is valid or not.
So, my questions are:
Thank you so much.
Hello Ashish Sapkota ,
If i understand well, this a signal obtained with an accelerometer in situ. I think it is, as you mentioned, response of the building. Maybe you can try to know in a simple way the resonance frequencies of the building and compare them with the values obtained with the signal you have.
It would be helpful to look at an FFT plot (response vs frequency). The building response would show as resonant natural frequencies (peaks at very specific and possibly multiple specific frequecies). These natural frequency responses would also be present during the random frequency time period.
I think you are getting the result of a soil structure interaction (the "sinusoidal" waveform you get after each random excitation) but I seriously doubt that 5 minutes later (300 seconds) there is any relation at all between them.
What is the source of your random vibrations? Wind? Surface traffic? Underground traffic? Is there any reason why they should come with a 5 minutes interval?
Have you integrated (numerically) the acceleration history twice to observe the displacement history? This can be very beneficial in determining what is actually going on. This may require baseline adjustment and filtering to eliminate drift typical in unfiltered acceleration records. And observe the velocity (integrate once) to see if there exists a pulse-type character which could produce the nearly harmonic response at the end.
Paulo Candeias : Dear sir, the source of random vibration is the earthquake itself. The data is from the Luzon Earthquake (April 22, 2019) which was measured by the sensors placed at the bottom of the structure.
"Is there any reason why they should come with a 5 minutes interval? "
- I am also not clear on how to interpret it and searching for possible reasons. And, yes, soil-structure interaction could be one.
Timothy Huff : Thank you for your insight, sir. I haven't integrated the acceleration history twice. Maybe I can try this technique and understand in a much better way.
There is free software which will do this, as well as adjustment and filtering, for you.
http://sem.inha.ac.kr/prism/
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