Risk%20Safety%20Liability2010C0

# O factors are determined in such a way that the

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o Factors are determined in such a way that the “probability of failure” of each limit state (failure mode) is maintained at some uniform value.

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AASHTO LRFD Bridge Design Example: Design for Vessel Collision o Bridges in navigable waterways shall be designed for vessel impact, considering... o waterway geometry, o size, type, loading condition, and frequency of vessels using the waterway o water depth, o vessel speed and direction, and o structural response of the bridge to collision .
Bridge Design for Vessel Collision (cont’d) o Bridges are classified as “regular” or “critical”. o “Critical” bridges shall continue to function after more severe collisions than the collision limiting “regular” bridges

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Bridge Design for Vessel Collision (cont’d) o An analysis of the annual frequency of collapse is performed for each pier or span component exposed to collision. o The design vessel is selected using a probability-based analysis procedure in which the predicted annual frequency of bridge collapse (AF) is compared to an acceptance criterion.
Bridge Design for Vessel Collision (cont’d) o The Annual Frequency of collapse (AF) is computed by… where... N=annual number of vessels, by type, size... PA=probability of vessel aberrancy PG=geometric probability of a collision by an aberrant vessel PC=probability of bridge collapse due to collision A F N P A P G P C =

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Bridge Design for Vessel Collision (cont’d) o The Annual Frequency of collapse (AF) is limited to a specified acceptable risk... A F N P A P G P C A F = < < - - 1 0 1 0 3 4 f o r " r e g u l a r " b r i d g e s A F f o r " c r i t i c a l " b r i d g e s
Acceptable Risk... o What is an acceptable risk ? Want no risk? Stop living!

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Some Acceptable Risks... o Note that the average American could, if he/she chose, reduce his/her annual risk of death by 173 × 10 -6 by avoiding travel in automobiles or on highways. Since the average American chooses to accept this risk (because of the advantages of automobile transportation), the risk of death associated with automobile travel could be considered an “acceptable risk”, that is one assumed by a reasonable person. o Similarly, the 0.8 × 10 -6 annual risk of death in commercial aviation is accepted by most persons.

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Cost-benefit Risk Assessment Example o The government is proposing legislation to limit formaldehyde emissions to 3 ppm. Industry estimates that to install and operate the necessary scrubbers will cost \$300 million annually. Toxicologists estimate that this new standard will save 30 lives annually. Using cost/ benefit analysis, should the new standard be implemented? o Cost = \$300 million/yr o Benefit = (30 lives/yr)(\$ ??? / life) o What is the dollar value of human life?
What is the value of human life ? o Some measures of the “value” placed on human life o purchasing decisions involving safety (safest car often more expensive) o future earnings of the individual o extra pay offered for risky jobs (e.g. house painter vs. smokestack painter)

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Problems with using studies of purchasing decisions to determine the value of life...
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• Spring '08
• RUSSELL
• Informed consent, Limit state design

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