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- From: "Ahmed Nisar (415)896-5858" <SFOAN(--nospam--at)AM.DAMES.COM>
- Date: Tue, 31 Dec 1996 08:54:54 -0700 (PDT)
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Probabilistic methods used to specify ground motion express annual probability of exceeding a certain ground motion level. i.e. a peak ground acceleration or spectral acceleration of say 0.5g means that there is a 0.002 annual probability that this value may be exceeded. This annual probability can be expressed in terms of exceedence probabilities for a given life using the following relationship. This relationship assumes a poisson probability distribution. p = 1 - e^(-LT) where: p = probability of exceedence L = annual probability of exceedence T = time period Therefore for a 0.002 annual probability of exceedence, p for a 50 year period is 0.095 or 9.5%. For p = 0.1 or 10% and T=50 years, L = 0.00210721 or an equivalent average return period of 474.6 commonly rounded off to 475 years. In some codes like the Standard Building Code (primarily used in the southeast) a 10% rpobability of exceedence is referred to as a 90% probability of nonexceedence in 50 year. hope this helps ahmed nisar dames & moore san francisco
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