Thursday, August 21, 2014
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Historical Memory and Engineering Failures

George Santayana, the Spanish-American poet and philosopher, once warned that "Those who do not remember the past are condemned to repeat it." This is especially true in the field of bridge building, where over the past 150 years dramatic failures have occurred at surprisingly regular intervals.

In 1847, the first major structural failure on Britain's expanding railway network occurred at Chester, England. The Dee Bridge, whose cast- and wrought-iron design followed common practice for the period, collapsed under a passing train, killing everyone aboard. Subsequent investigation revealed that the structure, the longest of its kind, simply pushed the limits of railroad-bridge engineering too far.

In 1879, the longest bridge in the world spanned the River Tay at Dundee, Scotland. Composed of many modest spans, the structure involved no radically new design concepts and seemed to be a mere application of proven technology. However, the force of the wind was grossly underestimated and worksmanship was inferior. As a result, the Tay Bridge, vulnerable in a gale, was blown off its supports.

In 1907, the longest span in the world was being constructed over the St. Lawrence River near Quebec, Canada. The bridge was of a relatively new type, known as a cantilever, which had become quite fashionable. Although it was only slightly longer than the highly successful cantilever bridge over the Forth River near Edinburgh, Scotland, the Quebec Bridge was so inadequately designed that it collapsed before it was completed.

In 1940, the third longest suspension bridge in the world was opened in Washington State. The Tacoma Narrows Bridge was designed as state of the art, which included a strong aesthetic preference for slender structures. Within four months of its opening, the bridge was destroyed by winds in a manner totally unanticipated by its engineers.

In 1970, steel box-girder bridges in Milford Haven, Wales, and in Melbourne, Australia, failed spontaneously while under construction. Both were among the longest structures of their kind and were thought to be just natural applications of existing technology.

In 2000, the much-anticipated opening of London's Millennium Bridge over the River Thames was followed only three days later by its closure. The sleek footbridge swayed unexpectedly and excessively under the feet of pedestrians, and it was deemed too dangerous to use. What should have been a mere extension of the millennia-old art of building pedestrian bridges, proved to be a modern engineering embarrassment.

The thirty-year interval between historic bridge failures was first highlighted by the work of Paul Sibly, who wrote a thesis on the subject, and his University of London advisor, A. C. Walker. They noted the cyclical regularity of such occurrences and speculated that it represented a gap in communications between generations of engineers.

Although each of the notable failures involved a different type of bridge, in no case was the structure radically new. Each used technology that engineers had been confidently employing for bridges, and for which the assumed loads and methods of analysis were well established. In every case, engineers believed that they were just building incrementally on successful practice.

In fact, designing in a climate of success can be dangerous for an engineer. Successful experience teaches us only that what has been accomplished in the past has worked. But things that work on a small scale do not necessarily work when slightly larger.

This was known to Vitruvius, who wrote about Greek and Roman engineering more than 2,000 years ago. It was also known to Galileo, who noted that Renaissance engineers who followed successful methods of building ships and moving obelisks were often surprised by the spontaneous failures when tried with larger ships and obelisks.

Failures always reveal weaknesses and provide incontrovertible evidence of our incomplete understanding of how things work. When the failures described above occurred, engineers were sensitized to their own limitations and so approached subsequent designs - no matter of what kind of bridge - with renewed respect for the laws and forces of nature. Unfortunately, human memory fades with time, and new generations of engineers with no vivid experience of past failures can proceed with hubris to design again beyond wise limits.

The history of engineering is no mere adjunct to technical know-how. A historical perspective on bridge building or any other engineering specialty provides a caveat about how our humanity affects our thinking. Building a new bridge following a familiar model can lead to complacency. Building a novel bridge, especially in the wake of a spectacular failure, forces engineers to think from scratch and also to think more deeply and critically. Hence, the paradox that success leads to failure, and failure leads to success.

The cable-stayed bridge is a form that is currently being pushed to limits and beyond those originally imagined to apply to it. Widespread successes with cable-stayed structures have made the type almost commonplace. As such, its development into ever longer spans is following the historic pattern that in the past has led to failures. Whether there will be a major cable-stayed bridge failure soon - or around the year 2030 - will most likely depend not so much on computer analyses as on how well engineers know their history and are determined not to repeat it.

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