Success and Failure in Engineering

By Petroski, Henry | National Forum, Winter 2001 | Go to article overview
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Success and Failure in Engineering


Petroski, Henry, National Forum


Failure is a central idea in engineering. In fact, one definition of engineering might be that it is the avoidance of failure. When a device, machine, or structure is designed by an engineer, every way in which it might credibly fail must be anticipated to ensure that it is designed to function properly. Thus, in designing a bridge, the engineer is responsible for choosing and specifying the type and size of the piers, beams, and girders so that the bridge does not get undermined by the current in the river that the bridge spans, does not collapse under rush-hour traffic, and does not get blown off its supports. The engineer ensures that these and other failures do not occur by analyzing the design on paper, and the objective of the analysis is to calculate the intensity of forces in the structure and compare them with limiting values that define failure. If the calculated force intensities are sufficiently within the limits of the material to be used, the bridge is assumed to be safe, at least with respect t o the modes of failure considered. (Each separate mode of failure must be identified and checked individually.)

In a suspension bridge, for example, the total force on the main cable depends upon the geometry and material of the bridge and the traffic it must carry. The force that the cable must resist determines how large the cable must be if a certain type of steel wire is used. Because the steel wire, like every engineering material, has a breaking (failure) point, the engineer calculates how far from the breaking point the cable will be when the bridge is in service. If this difference provides the desired factor of safety, the engineer concludes that the bridge will not fail, at least in the mode of the cable breaking, even if the wire installed is somewhat weaker than average and the traffic load is heavier than normal. Other possible ways in which failure may occur must also be considered, of course. These may include such phenomena as corrosion, ship collision, earthquakes, and wind. The collection of such calculations and considerations constitutes a complete analysis of the design.

ENGINEERING HISTORY AND ENGINEERING PRACTICE

The history of engineering, even of ancient engineering as recorded 2,000 years ago by Vitruvius, who wrote in the first century B.C. what is generally considered to be the oldest work on engineering extant, has a relevance to modern engineering because the fundamental characteristics of the central activity of engineering -- design -- are essentially the same now as they were then, have been through the intervening millennia, and will be in the new millennium and beyond. Those characteristics are the origins of design in the creative imagination, in the mind's eye, and the fleshing out of designs with the help of experience and analysis, however crude. Furthermore, the evolution of designs appears to have occurred throughout recorded history in the same way, by incremental corrections in response to real and perceived failures in or inadequacies of the existing technology, the prior art. There also is strong evidence in the historical record that engineers and their antecedents in the crafts and trades have always pushed the envelope until failures have occurred, giving the advance of technology somewhat of an epicyclic character. Thus, according to this view, the fundamental characteristics of the creative human activity we call design are independent of technological advances in analytical tools, materials, and the like.

The way artifacts were designed and developed in ancient times remains a model for how they are designed and evolve today. This is illustrated in a story Vitruvius relates of how the contractors and engineers Chersiphron, Metagenes, and his son Paconius used different methods to move heavy pieces of stone from quarry to building site. The method of Chersiphron -- which was essentially to use column shafts as wheels, into whose ends hollows were cut to receive the pivots by which a pulling frame was attached, as indicated in Figure 1 -- worked fine for the cylindrical shapes that were used for columns, but the method failed to be useful to move the prismatic shapes of stones that were used for architraves.

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