In this extract from his new book, American engineering writer Henry Petroski looks at the way in which several high profile disasters have influenced bridge design over the last 150 years.
The history of engineering, as that of civilisation itself, is clearly one of both successes and failures, and paradoxically the failures are really the more useful component of the mix. Although examples of good engineering practice and grand technical successes can certainly serve as paradigms of good judgement and things to emulate, great engineers and great people generally do not become so merely by reading biographies of great men and women. And great new engineering achievements do not come to be so merely by inference from an extrapolation of successful precedents.
Indeed, the history of civil engineering is littered with the wreckage of famous bridges that were designed in a tradition of success: the Dee Bridge in 1847; the Tay Bridge in 1879, the Quebec Bridge in 1907, and the Tacoma Narrows Bridge in 1940. Although each was pushing the limits of the state of the art in terms of length or slenderness – and economy – none of these structures was forging any fundamentally new technology; each seemed to be but an incremental step (albeit, in some cases, a too-large one) in what had already been done more or less successfully.
Pattern of collapse
One very interesting study of the failure of bridges either under construction or recently completed or modified was published some years ago by Paul G Sibly and Alistair C Walker. The study was based on Sibly's doctoral dissertation, in which he found that major bridge disasters that had occurred between the mid-19th and mid-20th centuries had happened with surprising temporal regularity – with significant failures taking place at roughly 30-year intervals.
The pattern established by the Dee, Tay, Quebec, and Tacoma Narrows bridges was continued when in 1970 two steel box-girder spans – one located in Milford Haven, Wales, and one in Melbourne, Australia – collapsed while under construction. (Since Silver Bridge was four decades old when it collapsed in 1967, it does not fall into the same category. Nor may the Minneapolis bridge that collapsed in 2007, though the placement of heavy construction equipment and materials on it may be said to have made it recently modified.)
The striking temporal regularity of the occurrence of major bridge disasters led to the expectation that the pattern would be continued by a failure that would occur around the year 2000.
As the close of the 20th century approached, the most likely bridge type to fulfil the prediction appeared to be the cable-stayed. Throughout the 1990s, cable-stayed bridges had been experiencing ongoing cable-vibration problems and were being retrofitted with vibration-damping devices in a variety of ways. Among the earliest indications of problems with stay cables were observations made in Japan, where the vibrations were aggravated by the presence of rain. Cable casings began to be fitted with raised spiral strakes to break up the rivulets that were blamed for the unwanted motion.
Other examples abound. The record-setting (2,808ft main span) Pont de Normandie, located at the mouth of the Seine, upon completion in the mid-1990s exhibited cable and deck vibrations significant enough to cause it to be retrofitted with orthogonal cable ties and also with shock-absorbing dampers, all of which diminished the appearance of the sleek structure. On a visit to Sydney, Australia, in 1998, I was driven over the new Glebe Island Bridge, whose main span is about 1,140ft long. My host explained to me that it had recently been decided to retrofit it with dampers to check undesirable vibrations. In spite of these behavioural problems, there has been no dramatic collapse of a cable-stayed bridge, though the problems persist and increasingly cable-stayed designs incorporate vibration-ameliorating features. During a 2009 visit to South Korea, I was taken to the recently completed Incheon Bridge, and it too had dampers in place to check cable vibration.
In spite of such examples and more, cable-stayed bridges have continued to be built with increasingly long spans, and cable vibration problems have become commonplace, if not expected. Whether the continued reliance on retrofitting so many of these bridges with damping devices will prevent a colossal failure in the wind remains to be seen. What is incontrovertible is that such measures have reduced dramatically the unwanted vibration problems of the structures, checking their motion to within acceptable limits. However, such retrofitting appears to have developed a sense of confidence within the bridge-building community that if problems arise in a new bridge they can be brought under control. This is very much analogous to the situation that existed with suspension bridges in the late 1930s – until the appearance of the previously unobserved torsional motion of the deck of the Tacoma Narrows Bridge led within hours to its collapse. Although up to the year 2010 there had been no similar catastrophic failure of a cable-stayed structure, a sudden twist in the behavior of one might still be cause for concern.
Pressure on footbridges
Another bridge type was also experiencing a great spurt of creative development during the 1990s, and that is the pedestrian bridge. Footbridges are nothing new, of course, having been perhaps the first bridges, and it is their very pedestrian nature – literally – that seems to have made them appear to be but modest challenges to bridge designers. Indeed, it may be precisely because of their long history and familiarity that their engineering has seldom in recent times been the primary focus of attention. Architecture, aesthetics, and the use of new materials in footbridges have typically been much more discussed than their structural engineering. This all changed with the change of the millennium.
Pedestrian traffic subjects a bridge to quite a different kind of loading than does vehicular traffic or the wind. It would appear to be a lighter and gentler load, but that does not mean it might not be a more difficult load for a particular bridge to bear. Take the day in 1987, when the Golden Gate Bridge was closed to vehicles to celebrate its 50th anniversary, so many people crowded onto the structure that it was subjected to the heaviest load it had experienced in its lifetime. The total weight of the people has since been much discussed, along with the resultant noticeable downward deflection and flattening of the centre of the bridge's main span, but the presence of so many pedestrians also caused the bridge to sway sideways. A similar sideways motion occurred in New Zealand in 1975, when protesters took over the Auckland Harbour Bridge. But such examples of highway bridges swaying underfoot were generally considered anomalies or irrelevant by pedestrian-bridge engineers.
In the years surrounding the change of the millennium, footbridges came to be seen as more than just pedestrian structures. The town of Gateshead sponsored a competition for teams of architects and engineers to design a distinctive footbridge, which resulted in the innovative Gateshead Millennium Bridge, which opens for shipping along the Tyne like an eyelid to the morning light, and closes after a ship has passed in the night. In Japan, the structural engineer Leslie Robertson teamed up with architect IM Pei to produce the dramatic bridge that is part of the ceremonial entrance to the remotely located Miho Museum. In several Western countries, the engineer-architect Santiago Calatrava has designed cable-stayed footbridges dramatically supported by a single mast, such as the span that now carries people from downtown Milwaukee to the new wing of its art museum, also designed by Calatrava. And in London, an engineer, an architect, and a sculptor teamed up to produce the Millennium Bridge linking across the Thames the Tate Modern art gallery and St Paul's Cathedral.
The London Millennium Bridge is, of course, famous for having been shut down just days after its opening in June 2000, when the bridge deck started swaying sideways to an alarming degree. A similar thing had happened the prior year to a footbridge in Paris. The Passerelle Solferino was designed to give pedestrians a serene crossing of the Seine from the Tuileries quay to the Musee d'Orsay. Although tested before opening with 150 people dancing to a beat designed to reveal dynamic susceptibilities, this technologically and aesthetically innovative arch bridge swayed on opening day. The one-inch movement, amplified by French politics, forced the closure of the bridge that same day.
Closing bridges susceptible to swaying is a wise decision, for even just the hint of a bridge's possibly impending fall can trigger panic and tragedy. A classic example occurred on Memorial Day 1883 when, a week after the Brooklyn Bridge opened, a dozen people were trampled to death on its walkway when holiday strollers panicked about the safety of the structure. In 1958, a pedestrian suspension bridge at Kiev in the Ukraine was closed because it swayed a fraction of an inch when crowded on the weekends. More recently, in November 2010, when millions were in Phnom Penh, Cambodia, for an annual water festival, a swaying suspension bridge of modest span was the scene of the death of 350 people who were crushed or suffocated and another 400 who were injured in a stampede that was believed to have been set off by fears that the structure was unstable.
Unfortunately, the bridge was not closed until after the disaster occurred. The architect-engineer of the Passerelle Solferino, Marc Mimram, admitted that designing "a footbridge is more difficult than other bridges, with its conflicting demands of light weight and long span." Still, how could such embarrassing oversights happen, especially at the dawn of the 21st century, when engineers were using powerful computer-based tools to design what their ancestors once sketched out only with sticks in the sand?
As with the other bridge types that Sibly had studied, the design of footbridges had become routine. Unfortunately, the loads considered in their design did not include the sideways forces exerted by people walking, forces that have a frequency of one-half that associated with a person's vertical footfall. Historically, with bulkier bridges, this had not been a problem, but the natural frequency of sideways motion of the slender Passerelle Solferino and London Millennium Bridge was close to the frequency of sideways force that pedestrians exert in the course of moving in their normal gait. Though crowds of people do not generally walk in step, if the bridge beneath them begins to move sideways the people on it tend to fall into step the better to keep their balance. This in turn magnifies the sideways motion of the structure, and a positive feedback loop is developed. The motions can get so violent that public safety concerns dictate that the bridge be closed.
The Passerelle Solferino and London Millennium bridges do not look anything like each other. The former is an arch and the latter a low-profile suspension bridge. Yet although they do not look similar, they shared with other footbridges fundamental design assumptions, which clearly did not take into account the critical sideways loading mode. In this sense the development of footbridges falls into the pattern pointed out by Sibly, namely, that the state of the art, which had developed out of successful experience, was finally being pushed, albeit inadvertently, into realms for which it was inadequate. A previously ignorable characteristic of the design came to dominate structural behaviour. By this criterion, the failure – even though not a dramatic collapse – of these footbridges, in which dynamic phenomena insignificant in bridges of lesser magnitude revealed themselves to be limiting, would appear to have continued the 30-year cycle and fulfilled the expectation of a bridge failure around the year 2000. *
Reprinted by permission of the publisher from 'To Forgive Design' by Henry Petroski, [332-338], Cambridge, Mass.: The Belknap Press of Harvard University Press, Copyright ' 2012 by the President and Fellows of Harvard College