Need a book? Engineering books recommendations...
Return to index: [Subject] [Thread] [Date] [Author]
Re[2]: Probabilistic Definition of Seismic Hazards
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaoc(--nospam--at)seaoc.org
- Subject: Re[2]: Probabilistic Definition of Seismic Hazards
- From: "Haninger, Ed" <ed.haninger(--nospam--at)fluordaniel.com>
- Date: Tue, 31 Dec 1996 08:43:00 -0800
A little bit on exceedance probability verses return period. These two are related but it is not a simple linear equation. The relationship between the two is given by the equation: EP = 1 - e ^(-n/T). (one minus e to the power (-n/T)) where: EP is the exceedance probability e is the constant 2.718... n is the design life T is the return period For example, with a return period (T) of 72 years (a common earthquake) and a design life (n) of 50 years the exceedance probability (EP) is 0.50. In other words, if you have a structure designed for a 72 year event there is a 50% probability that this level will be exceeded in the normal structure design life of 50 years. Another good example is the UBC design earthquake. It has a return period of 475 years which equates to a 10% probability of being exceeded in a 50 year period. Ed Haninger Fluor Daniel ______________________________ Reply Separator _________________________________ Subject: Re: Probabilistic Definition of Seismic Hazards Author: seaoc::(SEAOCAA) at ~FABRIK Date: 12/30/96 8:54 PM ...Please clarify what the difference is between annual frequency of occurrence and annual frequency of exceedance - aren't these different ways of expressing the same thing? I do prefer the method of expressing events in terms of probability of exceedance during a given period, since it gives a better sense of the risk involved, but I am not clear on what the mathematical difference is. If an event has an annual frequency of occurrence of 0.002, then it has a 500 year expected return period. Then in 50 years, doesn't it have a 10% probability of being exceeded (0.002x50 = 0.10)? and statistically, wouldn't it have a 100% probability of occurring in 500 years? The problem with using return periods to describe probabilistic events is that it seems to give a false sense of security. For example, consider design for flotation resistance against a 100-year flood. Such a flood would have a 0.01 annual probability of occurrence. For a 50 year design period, is seems to me it would have a "50% probability of being exceeded" (0.01/year x 50 years). If you are concerned with flotation of a structure, designing for a 100-year flood for a 50-year design life sounds conservative. But having a 50% risk of exceedance during the design life sounds very risky. From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
- Prev by Subject: Re: Probabilistic Definition of Seismic Hazards
- Next by Subject: RE: Probabilistic Seismic Hazards & Public Policy
- Previous by thread: Re: Probabilistic Definition of Seismic Hazards
- Next by thread: Concrete Crack Concerns
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]