I have to carry out Linear time history analysis for a 2% damped structure as per Indian code. But Code give spectra for 5% damping only. Code asks to amplify the spectral values by 1.2 for 2% damped spectra. How to consider this in spectral matching. I have tried following cases and all have resulted in different results of matched time history.
Case1. Target spectra defined for 5% damping. 1.2 Scale factor considered in Step3 spectral matching.
Case 2. Target spectral uploaded from input file. Input file contains 5% damped spectral acceleration multiplied with 1.2. While uploading, 2% damping is selected.
Case 3. Target spectral uploaded from input file. Input file contains 5% damped spectral acceleration multiplied with 1.2. While uploading, 5% damping is selected.
In all the three cases the target spectra values are same. But the spectral matching results are different. How?
Damping
Re: Damping
This is not necessarily a surprise. You could get different accelerograms matching the same target spectrum. The key is to examine the accelerogram response spectrum and compare to the target response spectrum in all three cases. If the match between ground motion and target spectra is satisfactory, then you should have three candidate ground motions to consider.
However, the procedure does seem odd. The definition of the code-based spectrum is a 5% damped spectrum. Ground motions matched to that spectrum are viable candidates for use in analysis, provided the baseline adjustment and filtering produces acceptable velocity and displacement histories as well. Those ground motions are design ground motions for the site, and may be used to analyze a structure of any damping - it doesn't have to be the same level of damping as the design response spectrum.
Now, on the other hand, if you were running a response spectrum modal superposition analysis, then yes, I see the need to amplify the design response spectrum. But for response history analysis, I am not sure you need to do that.
So I would double-check the code requirement and make certain you are applying it correctly.
However, the procedure does seem odd. The definition of the code-based spectrum is a 5% damped spectrum. Ground motions matched to that spectrum are viable candidates for use in analysis, provided the baseline adjustment and filtering produces acceptable velocity and displacement histories as well. Those ground motions are design ground motions for the site, and may be used to analyze a structure of any damping - it doesn't have to be the same level of damping as the design response spectrum.
Now, on the other hand, if you were running a response spectrum modal superposition analysis, then yes, I see the need to amplify the design response spectrum. But for response history analysis, I am not sure you need to do that.
So I would double-check the code requirement and make certain you are applying it correctly.
Tim Huff