ENGR 482 Lecture 11
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Risk
All codes protect public from "unacceptable risk"
What is acceptable?
Definiion
Exposure to:
- chance of injury, damage, or loss
- a hazardous or dangerous chance
Involves
- Probability that something will happen
- Consequences of the event
Note: Risk and safety are conceptual inverses
From mathematics:
Example: Bridge Collapse
Bridge fonudation dephts are often governed by the depth of scour, which is related to the size of the fluod, defined in terms of its probability:
- A 100-year flood has a 0.01 chance of accurring in any given year
- A 500 year flood is a flood which has a 0.002 chance of occurring in any given year
Consider bridge designed for a 500-year flood
- If the collapse occurs during rush hour (1/24 probability), 10 lives will likely be lost.
- If the collapse occurse any other time (23/24 probablitiy), suppose only 1 life will likely be lost.
Total risk would be sum of probabilities:
- risk of death during rush hour = (2 × 10−3)(1/24)(10) = 833 × 10−6
- risk of death in any other time = (2 × 10−3)(23/24)(10) = 1917 × 10−6
- total risk = risk of death occurring in rush hour + risk of death in any other time = 2750 × 10−6
Example: Nuclear Meltdown
Control rods and emergency cooling pump prevent reactor meltdown
Event tree:
|-- fails (p1) | |-- fails (p2) | `-- available (1-p2) `-- available (1-p1) |-- fails (p2) `-- available (1-p2)
First level is control rod, second level is cooling pump, and chances of any outcome happening is product of all probabilities involved (e.g. would be the probability that the control rods fail, but the pump works.
In Theer Mile Island:
- feedwater pumps failed
- pressure relief valve opened, but became stuck open
- Signals failed to show valve was stuck.
Notes
- Not all risks can be antiicipated (human error, terrorist attacks)
- All work involves risk
- Innovation in design generally increases risk (Tacoma Narrows bridge)
Exacerbation of Risk
- Tight coupling: if one system fails, other systems will fail quickly if they are tightly coupled.
- Processes or systems interact in unanticipated ways:
- Accelerator sticks on Toyota and crash occurs
- Electric cars present danger due to lack of engine noise
- Columbia foam strike admits hot plasma to wing's inner structure
- Normalizing deviance is an acceptance of a level of risk as normal (allows greater risks to be accepted more readily
Safety: Reducing Risk
- Develop inherently low-risk designs (e.g. using gravity for lowering control rods)
- Incorporate redundancy (multiple sensors in control systems: if one doesn't match, then it's usually a red flag)
- Design failure modes that give warning before catastrophic failure
- Design for appropriate factor of safety (FS)
- Design gradual failure
Factors of Safety
Jet liner risk is mitigated by very close control of
- applied loads
- construction quality
- variations in material properties
- manufacturing variability
- maintenance
Example: Elevator Engineer
Full load weigs 6450 lbs.
Elevator supported by 4 cables so individual tension is 1/4 total load.
Tension (demand) = weight × 1.35 / 4 = 2176 lbs
Use specified factor of safety = 6.5
Choose 1/2-in. cable that has 11.8 ton capacity gives safety factor