Non-prototypical Engineered Systems

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Non-prototypical Engineered Systems

  1. 1. William M. Bulleit<br />Michigan Tech<br />Uncertainty in the Design of Non-prototypical Engineered Systems<br />
  2. 2. Concept<br />Design<br />Prototype – with feedback to design<br />Production<br />QA & Testing<br />(Element 14, Journal 1)<br />Product Development Cycle Electronic Products<br />
  3. 3. Concept<br />Design<br />Construction – feedback to design mostly changes, not necessarily improvements<br />Non-prototypical Systems<br />
  4. 4. Aleatory<br /> Of or related to chance<br />Uncertainty generally not reduced by increased knowledge<br />Flipping a coin - frequentist or subjective<br />Epistemic<br />Of or related to lack of knowledge<br />Uncertainty generally reduced by increased knowledge<br />Flipping a coin - physics<br />Types of Uncertainty<br />
  5. 5. Time – past and future<br />Statistical limits – never enough data<br />Randomness – nothing is one number<br />Human error – screw ups happen<br />Sources of Uncertainty - Basic<br />
  6. 6. Use changes<br />Predict future loads based on past loads<br />Deterioration<br />Increased time causes increased probability of extreme load<br />Time<br />
  7. 7. Only can take so many samples of anything<br />We only have about a 100 years of load data<br />Never sure if the sample represents the population<br />Statistical Limits<br />
  8. 8. Seismic ground motions are random processes<br />Wind pressure is a random process<br />Cross sectional dimensions vary<br />Live load varies spatially<br />Randomness<br />
  9. 9. “To err is human, to anticipate is design.”<br /> Anonymous<br />“Good judgment comes from experience, and experience comes from bad judgment.”<br /> Attributed to Mark Twain<br />Design<br />
  10. 10. Modeling – simplifications or misconceptions<br />Contingency – it does not exist<br />Inconsistent crudeness – one refined, one not…<br />Code complexity – what to choose?<br />Sources of Uncertainty - Design<br />
  11. 11. Occupancy live load is assumed to be uniformly distributed<br />Wind load is assumed to be static<br />Load variability is assumed to be representative of load effect variability<br />Strain distribution assumed to be linear<br />Modeling<br />
  12. 12. “I am persuaded that many more failures of foundations or earth structures occur because a potential problem has been overlooked than because the problem has been recognized but incorrectly or imprecisely solved.”<br /> Ralph B. Peck<br />Human Error/Modeling Error<br />
  13. 13. Tacoma Narrows<br />
  14. 14. Contingent: dependent on something not yet certain.<br />In engineering design contingency refers to the need to visualize a system and perform analysis and design on the envisioned system before it can be built. (Scientists typically analyze existing systems.)<br /> [H. Simon, The Sciences of the Artificial] <br />Contingency increases uncertainty<br />Contingency<br />
  15. 15. Engineers’ designs are not consistently crude.<br />Some portions of a code are well researched and based on engineering science, and some have been in the code for decades (EFW for concrete T-beams).<br />Inconsistent Crudeness<br />
  16. 16. “A heuristic is anything that provides a plausible aid or direction in the solution of a problem but is in the final analysis unjustified, incapable of justification, and potentially fallible.”<br /> B. V. Koen, Discussion of the Method<br />Heuristic<br />
  17. 17. We use them to help solve problems and perform designs that would otherwise be intractable or too expensive to perform.<br />Ex. 1: 0.2% offset method gives the yield stress of the steel.<br />Ex. 2: The dynamics of the wind load can be ignored in the design of buildings.<br />Ex. 3: Occupancy live load is uniformly distributed.<br />Heuristics<br />
  18. 18. Use characteristic values (e.g., 5th percentile)<br />Use design provisions that have stood the test of time, but update as necessary (possibly due to failures)<br />Check designs and inspect construction (Quality control)<br />Make appropriately conservative assumptions in analysis (What is appropriate?)<br />Dealing with Uncertainty<br />
  19. 19. Check complex analyses with simpler methods where possible.<br />Use your own experience.<br />Recognize that heuristics are used in all engineering design and think about their limits <br />Dealing with Uncertainty (Cont.)<br />
  20. 20. “The person who insists on seeing with perfect clearness before deciding, never decides.”<br /> Henri F. Amiel<br />“Choosing not to decide is a decision.”<br /> Anonymous<br />Decisions<br />
  21. 21. Reflection by the engineer on a design may be a way to enhance future similar designs<br />Reflection may also work as a type of feedback (e.g., Citicorp Building, 1978, William Le Messurier)<br />Reflection<br />
  22. 22. Prototypical versus non-prototypical systems are distinguished by the amount and timing of feedback<br />Design of prototypical systems involves relatively rapid feedback during design and more feedback during operation (e.g., automobiles, computers, light bulbs)<br />Non-prototypical systems receive essentially no feedback during design, and only slow feedback during their life (e.g., Tacoma Narrows, Deepwater Horizon)<br />Time and Again<br />
  23. 23. Low probability – high consequence events<br />Black swan events<br />Human/societal limitations<br />Conclusion<br />
  24. 24. Questions?<br />

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