Non-prototypical Engineered Systems
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Non-prototypical Engineered Systems

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William Bulleit's fPET-2010 presentation

William Bulleit's fPET-2010 presentation

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Non-prototypical Engineered Systems Non-prototypical Engineered Systems Presentation Transcript

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