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Re: Seismic Response Spectrum

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I think you can use probability theory to find the equivalent annual risk for both PGA:
- The annual risk for the first PGa would be: 1-(1-0.1)^(1/10) = 0.0105, which is equivalent to say that a PGA of 0.17g would be exceeded once in  95 years (0.0105 = 1/95).
- Similarly, for the second PGA, I get an annual risk of 0.001053 (=1/950).
Now, if I move to a 50-year window, I get the following for both PGAs:
Prob(PGA>0.17g) in 50 years = 1-(1-1/95)^(50) = 0.41 = 41%.
And, Prob(PGA>0.28g) in 50 years = 1- (1-1/950)^(50) = 0.051 = 5%
That means that you need a larger PGA (more than 0.28g) to meet the 2% probability of exceedance in 50 years that you are looking for.
Arturo Martinez 
----- Original Message -----
From: Rich Lewis
Sent: Tuesday, May 23, 2006 9:50 AM
Subject: Seismic Response Spectrum

I was given seismic design criteria for an overseas project that is not typical of what is used in the IBC code.  The design criteria given to me is:


Peak Horizontal Ground Acceleration:


0.17(g) - 10% over 10 years.

0.28(g) - 10% over 100 years.


I was wondering how this information can be utilized in an equivalent static force analysis.  IBC uses an acceleration response spectrum having a 2 percent probability of exceedance within a 50 year period.  Seems to me I?m mixing apples and oranges with this criterion.  My seismic texts don?t show a method for taking raw data like this and translating it into an equivalent static force system.  Is it possible?






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