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Converting 5% damped response spectrum to 2% damping

Converting 5% damped response spectrum to 2% damping

Converting 5% damped response spectrum to 2% damping

(OP)
The USGS spectral response graphs are all based off of 5% damping, however IEEE-693 (6.9) and ASCE 113 (5.6.2.1) for electrical substation design both state that a maximum damping value of 2% must be assumed for all structures.

These structures are basically inverted pendulums supporting relatively massive equipment and so 5% damping is very unlikely.

Is there a good way to convert the USGS 5% damped response spectrum into a 2% damped one?

I have found Figure 2-1 of Fema273 which shows how to build a generalized response spectrum.
(http://files.engineering.com/getfile.aspx?folder=8...)

These factors are the same as ASCE 7-10 (and ASCE 7-05) table 18.6-1

But this paper here indicates that there is a very large scatter to the data which produced these damping modification factors:
(http://files.engineering.com/getfile.aspx?folder=2...)

Does anyone have any comments? Why doesn't the USGS tool allow you to specify the damping ratio? It seems like they would have the capability if they already have all of the earthquake data to create 5% spectra.

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