Risk%20Safety%20Liability2010C0

O we cant anticipate all failure modes o all pipe

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o We can’t anticipate all failure modes: o All pipe rupture mechanisms o pipe corrosion as a function of use and age o terrorist attack, numerous vulnerabilities o human error (too many options to count!) o Failure modes are not always independent – dependencies are hard to model
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March 1, 2010 News Advisory VERMONT YANKEE SURVEY TO SHOW IF VERMONTERS SUPPORT STATE SENATE’S ACTION ON REACTOR CLOSURE, LIKELY OUTCOME OF TOWN MEETING VOTES TUESDAY Just Completed Survey of 800 State Residents Gauges Support for Reactor, Impact of Tritium Leaks; After State Senate Vote…
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Risk is inherent in engineering o All engineering involves risk. o Innovation in design generally increases risk. More generally, any change (from proven practice) will often increase risk. o Examples: o Tacoma Narrows Bridge--1940 collapse o Three Mile Island Power Plant--1979 radiation release o Concorde airliner--2000 crash in Paris
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Safety: Operation of Engineering Systems to Reduce Risk o Many “engineering” failures involve, at least in part, an operations failure…consider the reactor failure at Three Mile Island: o The main feedwater pumps failed ; a pressure relief valve automatically properly opened, but then stuck open . Signals failed to show that the valve was stuck open. o Because of either administrative or human error , a critical valve in the emergency feedwater system was left closed, delaying the operation of that system for 8 minutes . o Systems are said to be tightly coupled when a failure in one system can adversely and rapidly affect operations in another system. Tightly coupled systems make failures more difficult to predict and control.
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Safety: Design of Engineering Systems to Reduce Risk to Acceptable Levels o Develop inherently low-risk designs o Incorporate redundancy in design o Design for failure modes that give warning before catastrophic failure o Design for appropriate Factor of Safety (FS) o Design for failure modes that are “controlled” and limit public exposure
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Factors of Safety: o To accommodate uncertainties in... o applied loads, o material properties, o manufacturing variability, o construction quality, o Maintenance deficiency o and, to reflect different consequences for different failure modes. FS Failureload Design Load Risk Mitigation
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Factors of Safety in Design o An engineer working for Otis Elevators determines that a fully loaded passenger elevator will weigh 6450 lb. The elevator is supported multiple cables so that individual cable tension is 1/4 of the elevator weight. The elevator is expected to experience dynamic load factors of approximately 1.35. Suppose the design code requires a factor of safety of 6.5. o What cable diameter should the engineer specify?
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Factors of Safety in Design (Cont’d) Computetension T... T = W 1.35 4 = 2176lb Use specified FS = 6.5 Designtension = 2176lb 6.5 = 14,150lb = 7.07ton
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Breaking Strength for 6x19 Wire Rope...
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  • Spring '08
  • RUSSELL
  • Informed consent, Limit state design

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