I read journals and book but still I am confused about the ground motion selection procedure and non-linear time history analysis
1. Why do we have to match the ground motion to the response spectra? And if it is necessary to match the ground motion with the response spectra, is it compulsory to match the frequency content?
2. Do we need to match the ground motion with the response spectra in every case?
3. Can't we do the analysis with direct raw ground motion?
4. And my last question is if I perform incremental dynamic analysis on different pga level like 0.1 pga , 0.12pga, 0.14pga, 0.16pga So we have to take current Pga from matched time history or take from raw ground motion. I am using Scale factor formula for time history is =(required pga / current pga )g
Experts please pay attention to my question, your answer will be very helpful for me .
Thank you
You may misunderstand and mix up between two terms. The ground motion history is recorded in the green-field condition and the response spectrum is the output of the structural system/element, 1D model used often. It's hard to further discuss this topic without seeing your referenced documents.
Let me add some fundamentals first. The critical parameters of a ground motion that is of interest to a structural/ earthquake engineer are the amplitude, frequency content and duration of the motion. From a seismologist's purview, the characteristics of ground motion expected at a site are controlled by a host of factors such as the fault type , epi-central distance, focal depth , the magnitude of earthquake , very specifically the local geology and soil deposit characteristics. This mean that the ground motions selected for time history analysis should have characteristics similar to the one expected at site.
Now coming to matching of ground motions with response spectrum . To answer why it is required, it is important we go back to its definition. The response spectra is an envelope of peak spectral (structural) accelerations when a single degree freedom system (SDOF) with different time periods is excited by a ground motion. This mean that shape of response spectra is strongly governed by the above cited important characteristics of ground motion (amplitude, duration & frequency content). That is by using spectrum compatible ground motions we respect the kind of ground motion parameters expected at a site in case of an earth-quake event. The spectrum compatible ground motions shall be used in every case where the engineer is interested in the structural analysis, design and performance based design.
Though there are thousands of real recorded ground motions available in databases such as PEER, COSMOS etc, it is still a big challenge to select ground motions that could be a typical of the site. These databases allow the user to select the motions based on a ny=umber of factors like magnitude , fault type , peak ground acceleration , duration etc.
To help practicing engineers, international codes such as ASCE 7, EC8 provide guidance regarding the minimum number of motions to be considered for spectrum matching and the criteria required for marching. Unfortunately IS 1893(2016) do not provide any such criteria for the selection of spectrum compatible ground motions. Any analysis done with a raw ground motion is of no use to an engineer.
As there is nothing special about Incremental Dynamic Analysis(IDA), we shall use only spectrum compatible records which are to be scaled up gradually to enable the structure to pass through elastic, elasto-plastic and plastic zones.
I would also request you to go through some international journal papers in this regard (eg. Julian Bommer etc).
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