Risk, Safety and Liability

1. Risk, Safety and Liability PHIL/ENGR 482 Ethics in Engineering: Chapter 7

2. An engineering responsibility • Codes of ethics require the engineer to prevent exposure of the public to unacceptable risks.

3. NSPE Code • “hold paramount the safety, health and welfare of the public” design to “accepted engineering standards” • not “complete, sign, or seal plans and/or specifications that are not of a design safe to the public health and welfare in conformity with accepted engineering standards” • in “circumstances where the safety, health, property or welfare of the public are endangered” engineers must “notify their employer or client and such other authority as may be appropriate”

4. Understanding and managing risks • What is risk? • How do we design to reduce risks? • What are acceptable risks?

5. Risk • One definition of “risk” is: • “Exposure to the chance of injury or loss; a hazardous or dangerous chance” * • This definition involves both • the probability of an event occurring • the consequences of the event * Webster’s Dictionary

6. An engineering definition of risk • Example: When playing Russian Roulette, a single bullet is loaded into a six-chambered revolver. The chamber is spun and the trigger is pulled. There is a random 1/6th chance that the revolver will fire. Risk = (1/6) x (Death)

7. Risk is inherent in engineering • Engineering involves innovation which necessarily has risk. • Examples: • Tacoma Narrows Bridge--collapse • Three Mile Island Power Plant--radiation release • Concorde airliner--crash

8. Probabilities in Independent Events • Example: A coin is flipped three times. What is the probability heads will show three times in a row? • Use an “event tree” Heads Heads 0.5 Heads 0.5 Tails 0.5 Trials Probability = 0.5 x 0.5 x 0.5 = 0.53=0.125 0.5 Tails 0.5 0.5 Tails 0.5

9. Risk Assessment Example • A nuclear reactor will “meltdown” if the control rods fail and the cooling pump fails. What is the probability of this occurring?

10. Risk Assessment

11. Problems with Event-Tree Analysis: • assigned probabilities are sometimes conjectural • cannot anticipate all failure modes: • pipe rupture, • pipe corrosion, • terrorist attack, • human error, • etc.

13. Management of risk in design • Develop inherently low-risk designs • Incorporate redundancy in design • Design with appropriate Safety Factor • Allowable Stress Design (ASD) • Load Factor Design (LFD) • Probabilistic design methods (LRFD)

14. Safety Factors accommodate uncertainties in... • Applied loads • Material strength • Simplified methods of analysis • Construction quality • Maintenance • etc...

15. Safety Factors also... • Reflect different consequences • Safety factors are higher for critical infrastructure (fire stations, hospitals) than for other facilities (warehouses)

16. Safety Factors in design • An engineer working for Otis Elevators determines that a fully loaded passenger elevator will weigh 6450 lb. The elevator is supported by a double-sheaved cable so that the cable tension is 1/4 of the elevator weight. The elevator is expected to experience dynamic load factors of approximately 1.35. The design code requires a safety factor of 7.6. What cable diameter should the engineer specify?

17. Safety Factors in design (cont’d)

18. Breaking strength for 6x19 wire rope...

19. Safety Factors in design (cont’d) • Choosing 1/2-in. cable...

21. Allowable Stress Design example • Julio is designing a portable cylindrical compressed air tank for use by motorists with flat tires, based on the calculated hoop stress...

22. Allowable Stress Design example--(cont’d) • He calculates that the design pressure (125 psi) will cause the allowable stress (20,000 psi) in the 12 in. diameter steel tank if the wall thickness is 0.0375 in. • He then increases the calculated wall thickness by 0.060 in. to allow for corrosion, and chooses the next larger available thickness 0.105 in (12 ga).

23. Allowable Stress Design example--(cont’d) • The Factor of Safety of a new tank against exceeding allowable stress is then… The factor of Safety of a corroded (0.045 in. wall thickness) tank against yielding (36,000 psi) is...

24. Design difficulties... • All failure modes must be anticipated and evaluated... • buckling • fatigue • corrosion • fire • Different failure modes have different risk (uncertainty x consequence), so different FS may be appropriate for each mode

25. Load Factor Design example • A steel link is being designed to carry a 1,000,000 lb load. What is the required cross-sectional area?

26. Load Factor Design example, (cont’d) “Ultimate Strength” “Allowable Stress” Safety factor against ultimate stress: 3.0 load factor (not typical value) 0.233 stress reduction factor (not typical) FS= (3.0)/(0.233) =12.8

27. Probabilistic design philosophyex.--AASHTO Bridge Design Specs. • Load factors and resistance factors are not fixed, but chosen in each design based on the specifics of the instance. • Factors are determined in such a way that the “probability of failure” of each limit state (failure mode) is maintained at some uniform value.

28. Cost-benefit risk assessment example • 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? • Cost = $300 million/yr • Benefit = (30 lives/yr)($?????/ life) • What is the dollar value of human life?

29. What is the value of human life ? • Some methods to place a value on human life • purchasing decisions involving safety (e.g. car purchase) • future earnings • extra pay needed for risky jobs (e.g. house painter vs. smokestackpainter)

30. Problems with using studies of purchasing decisions to determine the value of life... • wealthy people are willing to pay more • people will pay 7 times more to reduce risk of cancer than to reduce risk of death in an automobile • decisions are based on perceptions (values) • women value their lives more than men, i.e., men are more willing to engage in risky behavior • A 1984 study by Shualmit Kahn indicates that people typically valued their lives at $8 million (Note: this figure is higher than is typically used in public policy analysis.)

31. Quantifying Risk: Example • Suppose a toxicologist is asked to determine if Compound X is a carcinogen at a loading of 5 mg/kg of human body weight. To answer this definitively, the ideal experiment would have to be performed as follows: • Have a control group of 100,000 humans consisting of 1000 100-subject groups. They receive a placebo. • Have a test group of 100,000 humans consisting of 1000 100-subject groups. They receive a pill containing the suspected carcinogen.

32. Quantifying Risk (cont’d.) • Wait 20 years. • Plot the data as histograms and analyze. • For ethical reasons,a surrogate (e.g. rat) would be used instead of humans

33. More realistic test data might be... • Because of cost constraints, a much smaller group of rats would be tested. • Suppose the test results are as follows: • Control: Number of rats = 100 Number of cancer deaths = 12 • Test: Number of rats = 100 Number of cancer deaths = 14 • Question: How would the toxicologist know if Compound X causes cancer in rats?

34. One solution... • If we assume the binomial result (cancer--yes/no) is a random normal variable, we can estimate the mean (1=np=12) and the standard deviation (1=(npq)1/2=3.24) of the control group. • Likewise we can assume the exposed group is normal with a mean (2=14) and a standard deviation (2=(npq)1/2=3.46). • Then the standard deviation associated with the difference of the two means is

35. One solution... • Applying the t-test, with a 95% confidence level, we can say that the increase in cancers is not random. • Specifically, we can say with a confidence of 95% that the exposure will cause an increase in cancer rates of 1.8% to 2.1% (about 20,000 new cases for each million persons exposed).

36. Implications... • These are the possible outcomes to the question: “Does Compound X cause cancer in rats?”

37. Public Policy Expert’s Approach to Risk • First priority is to protect the public. • Consequences: • False Positive A chemical is banned as being carcinogenic, when in reality it is not. The producer loses potential profits from the sale of this chemical. • False Negative A dangerous chemical is approved as safe and sold to the general public. The death rate from cancer increases.

38. Public policy expert approach (cont’d). • The public policy expert is not interested in “scientific truth” per se. He prefers to err on the “safe side”, i.e., protect the public from false negatives. • She presumes that the chemical is dangerous, and places the burden on the manufacturer to prove the chemical is safe. • In a democracy, the government policy makers respond to the public’s wishes. As a result, we tend to allocate differing amounts of money to save lives by different measures...

39. Allocation of Money

40. Layman’s approach to risk • Respect for Persons Approach • Key Issues: • is the risk distributed equitably? • are those assuming the risk compensated? • is the risk voluntary? • does the person assuming the risk understand it? • does the person assuming the risk have control?

41. Layman’s approach to risk... • Laymen often overestimate low probability risks • Willing to accept higher voluntary risks than involuntary risks (by factor of 103) • Laymen don’t compare a risk to already accepted risks • Laymen overestimate risks of human origin compared to risks of natural origin • Laymen’s approach more closely follows Respect-for- Persons approach than the Utilitarian approaches used by many experts

42. An Acceptable Risk is one that is... • freely assumed with informed consent • equitably distributed • properly compensated

43. Informed Consent • RP says we should treat people as “moral agents” (autonomous, self-governing individuals)…thus we should seek “informed consent” before assigning risk • Criteria for informed consent • consent must not be coerced* • person must be accurately informed* • person must be competent* to assess information *there are possible conceptual and applications issues to be resolved

44. Problems with informed consent • difficulty getting informed consent • consent must be obtained before the risk is assumed • consent requires negotiation • holdouts or unreasonable preferences • parties must be well informed and reasonable • people are often hysterical regarding dramatic or catastrophic risk • people underestimate the consequences of risks that have never happened before

45. When it isn’t possible to get informed consent... • Only expose people to risks they would consent to, if they were informed of all known risks.

46. Or, ... • As an alternative to gaining consent from everyone affected by the risk, the group leaders can decide to accept the risk for the group.

47. Problems with Informed Consent (cont’d.) • Some people may give informed consent to things that are not in their interests, because of... • misunderstanding information • immaturity • irrationality • Such consent isn’t autonomous.

48. Problems with Informed Consent (cont’d.) • If consent is not autonomous, then you should find a way to make consent autonomous.

49. Risk concepts--Example • The electric power company proposes to build a nuclear power plant near your neighborhood. Given the newly deregulated electricity market, the power probably will be sold out of state because prices are higher there.

50. Risk concepts--Example (cont’d.) • Is the risk voluntary? • Does the person taking the risk understand it? • Does the person taking the risk have control? • Is the risk distributed equitably? • Do those taking the risk get the rewards?