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REFRACTIONS - By Henry Petroski

Engineering Problems

A flawed illustration blurs the Grand Challenges vision.

Photo: HENRY PETROSKI - Engineers and engineering students are less likely to be inspired than confused by depictions of the impossible.Not too long ago I came across an article titled “Engineering Solutions” that appeared in a magazine published by a professional engineering society. The article was about the Grand Challenges for the 21st Century put forth by the National Academy of Engineering, and about how the list had developed into “a rallying cry to launch initiatives uniting academia, industry, government, and entertainment” and “to excite students about engineering.”

What caught my eye first was the graphic image used to illustrate the article — a full-page drawing of an incomplete stone arch, whose keystone is being lowered into place by a handful of well-dressed people who, by implication, are young engineers or engineering students. Any engineer or potential engineer who looks more than cursorily at this image is likely to be more confused than convinced that engineering solutions will work for any problem, let alone the grandest challenges of our day.

Since ancient times, true stone arches have been erected on timber scaffolding, usually called centering or falsework, because the individual wedge-shaped stones, known as voussoirs, cannot support each other as an arch until it is complete. It is the placing of the keystone that marks the completion, at which time the centering is knocked down to leave the arch gracefully spanning otherwise empty space.

In the magazine image, there is no falsework at all, leaving the two sides of the incomplete arch cantilevered out from the piers, with no visible means of support. Although modern reinforced concrete bridges are constructed without falsework by a so-called balanced cantilever method, the method relies upon steel cables that tie concrete segments together to form a pair of monolithic cantilevers that will be united midspan by a keystonelike final segment. The magazine image gives no indication that such a method is being used.

Not only is the incomplete stone arch as impossible structurally as an Escher drawing of stairways going nowhere is architecturally, but also the depiction of the placing of the keystone is not consistent with current reality. The people involved are probably not wearing steel-toed shoes, and they clearly are not wearing hard hats or safety harnesses. Nevertheless, assuming that everything is drawn to scale, the volume of the stone is about 200 cubic feet. Assuming further that it is made of limestone or some similar material, the keystone weighs about 16 tons. The handful of people about to install it must then each be capable of lifting in excess of 5,000 pounds.

In other words, if a bright high school student ponders this image as symbolic of what engineers do, he or she can only conclude that they must perform superhuman feats that defy the laws of physics. While it may be desirable to depict engineers as capable of overcoming the greatest of challenges, it serves no good purpose to suggest that they can ignore the laws of nature and the limitations of humans.

The graphically striking image can certainly be interpreted metaphorically, as the editors must have intended, but engineers and engineering students tend to think literally. They are less likely to be inspired than confused by the depiction of an impossible situation. The purported solution itself becomes the problem.

The drawing of the keystone placement can, in short, raise more questions about how realistic engineers and engineering solutions are. The students that we might wish most to attract to the profession — those who are critical thinkers able to see intuitively a difficulty with a design, whether a technical or artistic one — might very well be turned off by this cartoon of an engineering solution that has no connection to a real problem.


Henry Petroski is the Aleksandar S. Vesic Professor of Civil Engineering and a professor of history at Duke University. His latest books are An Engineer’s Alphabet: Gleanings from the Softer Side of a Profession and To Forgive Design: Understanding Failure.


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