EAS 140 Engineering Solutions Lecture 27 How Engineers

EAS 140 Engineering Solutions Lecture #27 How Engineers Learn from Failure

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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Announcements – A, B

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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Announcements – C

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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Announcements - D

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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Learning from Failure Overview

Many Reasons for Failure – ideas from class? Engineer’s 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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Why Do Engineered Systems Fail?

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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Overdesign/Underdesign Cycle

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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical 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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Example – Suspension Bridges

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 EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Example – Tacoma Narrows Bridge

Result: – Called “Galloping Gertie” Four months after construction: Click me EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Example – Tacoma Narrows Bridge

Aftermath: – When reconstructed, greatly increased the depth of the girders – Note how deep the girders are EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Example – Tacoma Narrows Bridge

Overdesign at first Underdesign later to save money or for aesthetics System becomes unstable and failure occurs Cycle restarts with overdesigned systems EAS 140: Engineering Solutions Chemical Civil Computer Industrial Electrical Mechanical Overdesign/Underdesign Cycle - Summary