EAS 140 Engineering Solutions

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EAS 140Engineering Solutions

• Lecture 27
• How Engineers Learn from Failure

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Announcements A, B

• Mon. 12/3 Lecture - Lessons from Failure
• Wed. 12/5 Lecture - Course Review
• Thurs. 12/6 In-lab quiz for Thursday labs
• Mon. 12/10 Final Examination
• Will make sample questions available after
  Thanksgiving
• Will emphasize material from lecture and case
  studies no computing from lab

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Announcements C

• Tues. 12/4 In-Class Ethics Case Study
• Thurs. 12/6 Lecture - Lessons from Failure and
  Course Review
• In-lab quiz for Thursday labs
• Mon. 12/10 Final Examination
• Will make sample questions available after
  Thanksgiving
• Will emphasize material from lecture and case
  studies no computing from lab

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Announcements - D

• Wed. 12/5 Lecture - Lessons from Failure
• Thurs. 12/6 In-lab quiz for Thursday labs
• Fri. 12/7 Course Review
• Mon. 12/10 Final Examination
• Will make sample questions available after
  Thanksgiving
• Will emphasize material from lecture and case
  studies no computing from lab

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Learning from Failure - Overview

• Engineers design systems so that systems will
  fail
• Remember too expensive to design a building so
  it will never fall down
• But Sometimes systems fail before we intend for
  them to fail
• Although sometimes tragic, engineers must learn
  from such failures

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Why Do Engineered Systems Fail?

• Many Reasons for Failure ideas from class?
• Engineers Error
• Kansas City Hyatt Regency
• Mars Climate Orbiter (see text)
• Warning Signs Ignored
• Space shuttle Challenger
• evidence of a leaking o-ring at low temperature
  ignored
• Cycle of Overdesigning and Underdesigning
• Will discuss in more detail

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Overdesign/Underdesign Cycle

• Because consequences of failure are high,
  engineering is a conservative discipline
• Most first attempts are overdesigned
• Example In structural engineering, this means
  that the capacity is much, much greater than the
  load
• Capacity is removed in subsequent systems why?
• save money, looks better
• Eventually, system is unstable and failure occurs
• Cycle restarts with overdesigned systems

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Example Suspension Bridges

• What is a suspension bridge?
• Roadway is suspended on cables
• Cables pass over high towers
• Towers are anchored in the bedrock
• Example Golden Gate Bridge

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Example Suspension Bridges

• Failure Modes
• Suspension bridges are susceptible to failure
  from wind loading
• Can oscillate in the wind
• Solution
• Key design factor is the ratio of the depth of
  the girder to the length of the span between the
  towers

span length
girder depth
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Example Tacoma Narrows Bridge

• Statistics
• Completed 1940
• Span length about 2800 ft
• Third longest suspension bridge in the world when
  built
• Built over a very windy narrows in Puget Sound
• Design
• To make the bridge appear slender, used a very
  small girder depth-to-span length ratio
• Ratio was about one-half of the ratio for the
  Golden Gate Bridge

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Example Tacoma Narrows Bridge

• Result
• Called Galloping Gertie
• Four months after construction

Click me
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Example Tacoma Narrows Bridge

• Aftermath
• When reconstructed, greatly increased the depth
  of the girders
• Note how deep the girders are

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Overdesign/Underdesign Cycle - Summary

• Overdesign at first
• Underdesign later to save money or for aesthetics
• System becomes unstable and failure occurs
• Cycle restarts with overdesigned systems