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Re: Probability..
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- Subject: Re: Probability..
- From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Date: Fri, 3 Jan 1997 10:07:22 +0500
Thanks again to Ahmed Nisar for clarifying terms of probability: >The difference in probability of exceedence and probability of occurrence can >be understood this way.. >Suppose you throw two dice, the probability of getting an exact seven, no more, >no less is the probability of occurrence of seven. the probability of >exceedence will be the probability of getting anything greater than an exact >seven. It occurred to me after asking the question that probability of occurrence refers to the probability of a single event occurring whereas probability of exceedance refers to any event "equal to or exceeding". Thus when ASCE 7 refers to "annual probability" for wind speed, it is referring to probability of exceedance; similarly, for floods we refer to expected return period which relates to exceedance probability. Thus looking back at the original message which prompted my questions, Frank McClure questioned the following statement: "probabilistic methods could help inform the decision, with consideration of the likelihood of a significant seismic event occurring in the time before remedial work, the risk of collapse due to that event relative to the risk of collapse of some other "typical" building during a similar exposure period, etc.". His response was "We start down a very slippery slope if we starting telling our clients or the public that the probability of occurrence can be given in terms of a certain number of years for earthquakes of a certain magnitude or ground motion strength. Earthquake ground motion strength parameters are usually, for building code purposes or earthquake risk assessments, expressed in terms of "MEAN (average) ANNUAL FREQUENCY OF EXCEEDANCE ( not OCCURRENCE). " While I concur that probability of occurrence of a single event is different than probability of exceedance based on the magnitude of an event, I don't see how that prevents us from discussing seismic events based on the probability of a certain magnitude event during a given number of years (if that was the point). Nevertheless, I do think that expressing the probability in terms of the probability of exceedance over a given design life is more meaningful than stating the "mean" return period, as a return period longer than the design life seems to give a false sense of security - e.g., a 100-year flood design for a 50-year design life sounds conservative, but a 40% probability of exceedance in 50-years sounds risky. It would be beneficial if all such events - earthquake, wind, flood, etc - were expressed using consistent terms.
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