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Probabilistic Definition of Seismic Hazards
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: tnhanc(--nospam--at)alaska.net
- Subject: Probabilistic Definition of Seismic Hazards
- From: FEMCCLURE(--nospam--at)aol.com
- Date: Sun, 29 Dec 1996 02:23:05 -0500
- Cc: seaoc(--nospam--at)seaoc.org
Dave Evans, December 28, 1996 In your recent posting to SEAOC List Server, concerning "Seismic Evaluation of Buildings--unsafe for occupancy", you mentioned "probabilistic methods could help inform the decision, with consideration of the likelihood of a significant seismic event occuring 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." Seismic hazards are usually defined by ground motion strength parameters (e.g. PGA or SA) at a specified mean annual frequency of EXCEEDANCE, (not mean annual probability of OCCURENCE) (e. g. 0.002 per year, (1/500) ,or its effective equivalent (e.g. annual probability of exceedance, mean return period, probability of exceedance in n years, e.g., 10% in 50 years), taken and abridged from ATC 35-2, (1995) "National Earthquake Ground Motion Mapping Workshop" page 6-1, "An Advocation: Map Probabilistically Derived Quantities" by C. Allin Cornell. He goes on to state: "I have intentionally emphasized "mean (annual) frequency of exceedance" rather than "probability" in the definitions above, first, because I believe today that this is proper index to use in specfying structural safety norms and more importantly here, because it makes it very clear that Probabilistic Seismic Hazard Analysis (PSHA) results carry with them (no matter how presented) the units of time." In other words, seismic hazards are usually defined using ground motion strength parameters (e.g. Peak Ground Acceleration or SA, such as the 1994 UBC or NEHRP Provisions Response Spectra values) at a specified MEAN ANNUAL FREQUENCY OF EXCEEDANCE, which is the reciprocal of the MEAN RETURN PERIOD for a criterion earthquake with a specific probability of exceedance in n years. The definition of seismic hazards are not usually defined in terms, such as the "likelihood of occurence" in given number of years or the "recurrence interval" (like the return of Haley's Comet) of a criterion earthquake of a certain magnitude or ground motion strength. It is important to remember that we are discussing MEAN RETURN PERIOD ( the average return period) not the RETURN PERIOD. We start down a very slippery slope if we starting telling our clients or the public that the probability of occurence 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 OCCURENCE). Frank E. McClure FEMCCLURE(--nospam--at)aol.com P.S. You did not answer my earlier question to you: "Is it good PUBLIC POLICY for the requirements of FEMA 273 for the seismic evaluation and guidelines for the rehabilitation - retrofit- of existing buildings to be more restrictive than the 1994 NEHRP Provisions for the seismic design of new buildings." I would appreciate your or anyone elses' views on this matter.
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