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To Engineer is Human by Henry Petroski

Engineer is Human

RATING: 3/10…READ: September 27, 2011

The role of failure in engineering. Reads mostly like an academic textbook with hints of humanity within. Starts strong, but then jumps all over the place content wise. Look elsewhere for a better introductory engineering book.

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It is necessary to understand, at least partly, the nature of engineering design. It is the process of design, in which diverse parts of the “given-world” of the scientist and the “made-world” of the engineer are reformed and assembled into something the likes of which Nature had not dreamed, that divorces engineering from science and marries it to art.

If we could remember those early efforts of ours to raise ourselves up among the towers of legs of our parents and their friends, then we can begin to appreciate the task and the achievements of engineers, whether they be called builders in Babylon or scientists in Los Alamos. For all of their efforts are to one end: to make something stand that has not stood before, to reassemble Nature into something new, and above all to obviate failure in the effort.

In time we walk without thinking and think without falling, but it is not so much that we have learned how to walk as we have learned not to fall.

A bulb that has burned continuously for decades may appear in a book of world records, but to an engineer versed in the phenomenon of fatigue, the performance is not remarkable. Only if the bulb had been turned on and off daily all those years would its endurance be extraordinary, for it is the cyclic and not the continuous heating of the filament that is its undoing.

The exact lifetime of a part, a machine, or a structure, is known only after it has broken.

Structural engineering is the science and art of designing and making, with economy and elegance, buildings, bridges, frame works, and other similar structures so that they can safely resist the forces to which they may be subjected.

A scientific hypothesis is tested by comparing its conclusions with the reality fo the world as it is. Yet, no matter how many examples of agreement one may collect, they do not prove the truth of the hypothesis, for it may be agued that one has not test it in the single case where the theory may fail to agree with reality. Om the other hand, just one instance of disagreement between the hypothesis and reality is sufficient to make the hypothesis incontrovertibly false.

The fundamental feature of all engineering hypotheses is that they sate, implicitly if not explicitly, that a designed structure will not fail if it is used as intended. Engineering failures may then be viewed as disproved hypotheses.

Cantilever beam—this is a beam held only at one end while supporting weight or resisting motion all along its free length. Trees and skyscrapers act as cantilever beams when they resist the pressure of the win tending to bend and topple them.

Galileo came to the correct conclusion that the beam’s strength is proportional to the square of the depth of the beam. This is consistent with our experience that it is much easier to break a piece of lumber by bending it through its small rather than its large dimension, and a one-by-ten does indeed appear to have a hundred times as much resistance to bending through its ten-as across its one-inch depth.

Engineers today, like Galileo three and half centuries ago, are not superhuman. They make mistakes in their assumptions, in their calculations, in their conclusions. That they make mistakes is forgivable; that they catch them is imperative. Thus it is the essence of modern engineering not only to be able to check one’s own work, but also to have one’s work checked and to be able to check the work of others.

What the engineers of the nineteenth century developed and passed down to those of the twentieth was the trial and error of mind over matter. They learned how to calculate to obviate the failure of structural materials, but they did not learn how to calculate to obviate the failure of the mind.

Contrary to their popular characterization as intellectual conservatives, engineers are really among the avant-garde. They are constantly seeking to employ new concepts to reduce the weight and thus the cost of their structures, and they are constantly striving to do more with less so the resulting structure represents and efficient use of materials. The engineer always believes he is trying something without error.

To go through with exotic plans is clearly to be adventuresome, and one can ensure his safe and satisfied return by anticipating all that might go wrong. As the past trips of the astronauts to the moon demonstrated, travel to places without benefit of previous experience need not be domed to failure.

Copying may work for an ordinary highway bridge, but it clearly will not do when the highway is to cross a wider bay or a deeper ravine than ever spanned before. Then there are no examples to copy; there is no proven experience to follow. Thus the history of modern bridges is also the history of the development of a more scientific approach to designing large engineering structures than the pyramids and cathedrals or even the Roman aqueducts.

Dismissing the single structural failure as an anomaly is never a wise course. The failure of any engineering structure is cause for concern, for a single incident can indicate a material flaw of design error that renders myriad apparent structural successes irrelevant.

In engineering, numbers are means, not ends, and it ought rightly to have taken the failure of only a single bridge to bring into question the integrity of every other span.

While some of the details of engineering may be arcane, the principles of design and safety, of risk and benefit, are not, for to build a bridge is no less a human endeavor than to take a trip.

Which innovation leads to a successful design and which to a failure is not completely predictable. Each opportunity to design something new, either bridge or airplane or skyscraper, presents the engineer with choices that may appear countless. The engineer may decide to copy as many seemingly good features as he can from exiting designs that have successfully withstood the forces of man and nature, but he may also decide to improve upon those aspects of prior designs that appear to be wanting. This a bridge that has stood for decades but has developed innocuous cracks in certain spots may serve as the basis for an improved design of a bridge of approximately the same dimensions and traffic requirements. Or an existing design that has suffered no apparent distress after many years of decades of service may lead the engineer to look for ways to make it lighter and thus less expensive to build, for the trouble-free prototype appears to be overdesigned.

The choices of design are ultimately like the choices of life. While the engineer can pursue on paper two or even many different designs that fulfill the requirements of a projected structure, in the last analysis only one design can be chosen to be built, just as finally, only one route can be taken on a single trip from Chicago to New York no matter how many are considered in the planning.

Engineering is like writing a novel—constantly improving, learning from mistakes, and never being satisfied.

The factor of safety is calculated by dividing the load required to cause failure by the maximum load expected to act on a structure. Thus if a rope with a  capacity of 6,000 pounds is used in a hoist to lift no more than 1,000 pounds at a time, then the factor of safety is 6,000 / 1,000 = 6. As used in design of a hoist, a factor of safety of 6 would be determined by experience or judgment.

Although the design engineer does learn from experience, each truly new design necessarily involves an element of uncertainty. The engineer will always know more what not to do than what to do. In this way the designer’s job is one of prescience as much as one of experience.

Engineers increase their ability to predict the behavior of their untried designs by understanding the engineering successes and failures in history. The failures are especially instructive because they give clues to what has and can go wrong with the next design—they provide counterexamples.

A second basic design philosophy to obviate structural failure is called the “safe-life” criterion. Safe-life design, which allows for the inevitability of failure well beyond the service life of the structure, is not so simple to realize.

No matter how well today’s engineer understands the behavior of cracks, he cannot factor in the unknown human element, including that of his own limitations of prescience, into all his calculations. The safe operation of a complex system like w wide-bodied aircraft or a nuclear power plant ultimately depends upon the strength and reliability of all the links, both human and mechanical, in the chain.

Ironically, structural failure and not success improves the safety of later generations of a design. It is difficult to say whether a century-old bridge was overdesigned or how much lighter the frames of forty-year-old buses could have been.

As every cook knows, ingredients vary in quality and exact measure from batch to batch of the same recipe, especially when the recipes calls fro many ingredients whose relatively small amounts come to be taken as more suggestive than precise. And when some stainless steel is made from virgin ore and some from leftover scrap iron, as it often is, the quality of the ingots can be even more uncertain.

The paradox of engineering design is that successful structural concepts devolve into failures, while the colossal failures contribute to the evolution of innovative and inspiring structures. However, when we understand the principal objective of the design process as obviating failure, the paradox is resolved.

For a failed structure provides a counterexample to a hypothesis and dhows us incontrovertibly what cannot be done, while a structure that stands without incident often conceals whatever lessons or caveats it might hold for the next generation of engineers.

Failure analysis is as easy as Monday-morning quarterbacking; design is more akin to coaching. However, the design engineer must do better than any coach, for he is expected to win every game he plays. That is a tough assignment when one mistake can often mean a loss. And when defeat occurs, all one can hope is to analyze the game films and learn from the mistakes so that they are less likely to be repeated the next time out.

Answers are approximations and should only be reported as accurately as the input is known, and, second, magnitudes come from a feel for the problem and do not come automatically from machines or calculating contrivances.

Because structural analysis and detailing programs are complex, the profession as a whole will use programs written by a few. These few ill come from the ranks of the structural “analysts”…and not from the structural “designers.” Generally speaking, their design and construction-site experience and background will tend to be limited. It is difficult to envision a mechanism for ensuring that the products of such a person will display the experience and intuition of a competent designer.

Failure Classifications:

1. Ignorance

–a. Incompetent men in charge of design, construction, or inspection.

–b. Supervision and maintenance by men without necessary intelligence

–c. Assumption of vital responsibility by men without necessary intelligence.

–d. Competition without supervision.

–e. Lack of precedent.

–f. Lack of sufficient preliminary information.

2. Economy

–a. In first cost.

–b. In maintenance.

3. Lapses, or Carelessness

–a. An engineer or architect, otherwise careful and competent, shows negligence in some certain part of the work.

–b. A contractor or superintendent takes a chance, knowing he is taking it.

4. Unusual occurrences—earthquakes, extreme storms, fires, and the like.


Limit States:

++Overload—geophysical, dead, wind, earthquake, etc.; manmade, imposed, etc..

++Understrength—structure, materials instability

++Deterioration—cracking, fatigue, corrosion, erosion, etc.

Random Hazards:

++Fires / Floods / Explosions (accidental, sabotage) / Earthquake, Vehicle Impact

Human-Based Errors:

++Design Error—mistake, misunderstanding of structural behavior

++Construction Error—mistake, bad practice, poor communications

Good judgment is usually the result of experience. And experience is frequently the result of bad judgment. But to learn from the experience of others requires those who have the experience to share the knowledge with those who follow.

It is a great profession. There is the fascination of watching a figment of the imagination emerge through the aid of science to a plan on paper. Then it moves to realization in stone or metal or energy. Then it brings jobs and homes to men. Then it elevates the standards of living and adds to the comforts of lie. That is the engineer’s high privilege. [Herbert Hoover]

Design involves assumptions about the future of the object designed, and the more that future resembles the past the more accurate the assumptions are likely to be. But designed objects themselves change the future into which they will age.