Engineers of Dreams: Great Bridge Builders and the Spanning of America - Henry Petroski (1996)
The great bridges of the great engineers remain as spectacular today as they were when they were dedicated, but even the greatest bridges may be least appreciated by those who benefit most from them. A search for a more efficient means than ferries to move railroad trains and then motor vehicles impelled the engineers of the century of bridge building that extended from the 1840s to the 1930s to design more and more ambitious spans. However, unless these bridges are approached with a proper perspective, whether by the armchair traveler from behind a book or the actual traveler from behind a steering wheel, their greatness and achievement can hardly be appreciated. The right of way that approaches a massive cantilever head-on affords no view of the bridge to speak of, and the way across can appear from a train window to be little more than a long series of slanted steel obstructions to the view of a majestic river. The whoosh-whoosh-whoosh of relative motion gives no hint of the human dreams and drama that began over a blank and silent drawing board.
The view from the rear seat of an automobile can give us no better a perspective of a highway bridge, especially if it is one on a straight and heavily trafficked road. Who can really appreciate the structural-engineering achievement of a spectacular bridge when traveling in one car among hundreds in one of several lanes and at the same time trying to help the driver pick out the one relevant sign for the next connecting road on the interstate-highway route? Sometimes we can ride over bridges of immense technical achievement without even sensing their magnitude or grandeur or knowing that we are on a bridge at all. This is especially true of arch bridges whose structural muscle lies wholly below the roadway and so is invisible from the road. One such bridge is the Rio Grande High Bridge, which carries U.S. Route 64 over the gorge of the Rio Grande River in northern New Mexico. Traveling west on this route from Taos, one encounters what appears to be a remarkably flat plain extending for miles, essentially unobstructed by vertical vegetation or human artifacts. From the level of the road approaching it, the gorge itself is an invisible cut in the plain, and its great depth can only be appreciated by parking the car and leaning over the parapet on one of the slight outward extensions of the bridge so thoughtfully provided for sightseers, from which the great steel arch below may seem to make the gorge appear even deeper than it is. Such virtually invisible and largely anonymous bridges are numberless on the numbered roads across America, on the winding Pacific Coast road, in the gouged land of the Southwest, in the hilly land of the East, and even in the flat Midwest, where bridge approaches rise like Indian mounds to carry local traffic over the interstate.
The Bixby Creek Bridge, a reinforced concrete arch, on California’s coastal highway (photo credit 7.1)
Ironically, the greatest bridges and the ones that we tend to approach from the most favorable prospects are those in large and crowded cities, where the buildings push the roadways almost into the water, so that they must spiral back toward the bridge as if drawn to its grandeur. In New York City, for example, one has a spectacular view of the George Washington Bridge for what seems like miles when approaching it from the south on the West Side Highway and the Henry Hudson Parkway. In rush hour, the traffic on this road is often stop-and-go, and when it is stopped one can admire Othmar Ammann’s masterpiece and reflect upon how much more convenient than ferries it nonetheless is for crossing the Hudson. At night, looking out the window of a terrace restaurant just east of Columbia University, one can see, over the rooftops of Manhattan to the north, the overwhelming scale of the bridge outlined in lights. Over the rooftops to the east, the lighted outline of the Triborough Bridge looms, and, beyond it, that of the Bronx-Whitestone. As one drives south on the FDR Drive, New York City’s oldest suspended structures—the Williamsburg, Manhattan, and Brooklyn bridges—appear in sequence; driving under their approach spans and beside their towers provides a sense of scale that is missing in a drive over them. In San Francisco, the towers of the Golden Gate and Bay bridges dominate so many views that they have become defining landmarks of the city by the bay.
Although perhaps less appreciated visually, the importance of the Bay Bridge for communication between San Francisco and Oakland was demonstrated when a section of its upper deck fell onto its lower one during the 1989 Loma Prieta Earthquake, closing both roadways for about a month. The Northridge Earthquake, which struck Los Angeles in 1994, demonstrated further the vital link that a major bridge provides in a highway system. With key bridges out, Los Angeles commuters found themselves stuck in day-long traffic jams on detours in the early days following the disaster. Similar frustrations occurred in Connecticut in 1983, when a section of a bridge over the Mianus River fell without warning, leaving a gap in the heavily traveled Interstate 95. Though traffic was rerouted through neighboring towns, drivers were frustrated and residents annoyed until the bridge section was replaced.
Barring accidents, bridges, like health, are most appreciated when they begin to deteriorate and fail. Thus politicians seemed to become interested in bridges when they found that so many of them were structurally deficient. In 1992, for example, this included about one out of every five of the half-million or so bridges in the United States—an improvement over some previous years. Among the most dramatic stories is of a bridge whose condition began to deteriorate almost from its beginning. The Williamsburg Bridge is said to have been “born of a dare” when Leffert Buck was challenged to “build a bridge longer than the Brooklyn Bridge, in half the time and with less money,” and the span appeared to have fulfilled the requirements when it opened in 1903. Buck achieved the time and cost savings in part by coating the wire strands in the bridge’s four cables with graphite and linseed oil rather than galvanizing them with a molten-zinc mixture, as had been done with the Brooklyn. At the time, the procedure was considered radically different, and the wisdom of the decision came under serious question when broken wires and corrosion were discovered in the cables before the bridge was a decade old. Before it was two decades old, the bridge’s cables were wrapped in a galvanized steel cover, but severe rusting continued inside. When the bridge was forty years old, hundreds of gallons of linseed oil were poured onto the cables at the tops of the towers, with the expectation that it would seep down the cables and into their interstices and slow the corrosion. Two decades later, the procedure was repeated, with fish oil and mineral spirits. In the meantime, painting of the steel in the towers and roadway had been neglected, and so they had also developed severe rusting. Other East River bridges—the Brooklyn, Manhattan, and Queensboro—had also been the victims of deferred maintenance, blamed on a period of fiscal crisis that had struck New York, and by the mid-1980s they were undergoing repair and rehabilitation work costing on the order of half a billion dollars.
The serious deterioration of the cables of the Williamsburg Bridge put it in a category of its own, however, and a major decision presented itself: should the bridge be fixed, or should it be torn down and replaced by a new structure? Replacing the cables without closing the bridge was likened to “restringing a pearl necklace while it is around someone’s neck,” but building an entirely new bridge “in this era of environmental impact statements” was thought to invite “legal challenges that could result in substantial delays.” Several proposals that might be described as “radical” were considered. One involved erecting new towers over the old ones and hanging a new deck under the old, which traffic would continue to use during construction. When the new deck was finished, the old one could be closed and demolished, after which the new deck could be moved into position to receive traffic. A second proposal involved first building narrow new bridges on each side of the old, then tearing down the deteriorated bridge and building a third bridge in its place, and finally joining all three to provide a wide new roadway. Still another proposal called for building two larger bridges on either side of the old one, tearing it down, and then moving the two new bridges entire toward each other to be joined together as a unit. While such proposals were being considered, the engineering firm of Steinman, Boynton, Gronquist & Birdsall was monitoring strain gauges that it had installed on the eyebars connecting the cables to the anchorages, in order to have advance warning of any accelerated deterioration of rusted wires. The consulting engineers were effectively using a technique introduced by their progenitor, David Steinman, seventy years earlier, when he wanted to check theory with reality on the Hell Gate Bridge. But just as the Williamsburg situation was described as “a case study of how not to treat a bridge as well as how not to build one,” the Hell Gate itself was at the same time the subject of a different kind of scrutiny.
In the late 1980s, New York Senator Daniel Patrick Moynihan was made aware that the Hell Gate Bridge had not been painted in fifty-odd years, except for the daring work of graffiti artists who had left their marks high atop the stone parapets and steel arches. Since Moynihan had lived for a while as a child in Astoria, known mostly as the site of the Steinway piano factory but also as the eastern approach to the Hell Gate, he took a special interest in the bridge, which he called “a great engineering miracle.” Moynihan, who was chairman of the Subcommittee on Water Resources, Transportation, and Infrastructure of the Committee on Environment and Public Works, was disappointed that no one at either Amtrak or the Department of Transportation appeared to have an interest in the bridge. He was further annoyed when his inquiring letter to the Department of Transportation brought no response, and so he held a special hearing on Capitol Hill to discuss the matter. The nearby Triborough Bridge, he pointed out, was constantly being painted by a crew that did nothing else, and that was the way to take care of a bridge so that it did not rust away. Not all bridges, unfortunately, were under the watchful care of an agency like New York’s Triborough Bridge and Tunnel Authority, which collected sufficient tolls to maintain its works.
Moynihan pointed out that generally we were “disinvesting in the American plant” and that “the national roof is leaking.” The estimate for painting the Hell Gate Bridge was $43 million, however, about a third of which would have to be spent just in removing the accumulated rust and dealing in an environmentally sound way with the lead-based paint that still covered the bridge here and there. Since the heavy steel bridge was deemed structurally sound by W. Graham Claytor, Jr., president and chairman of Amtrak, he had little sympathy for spending so many millions of dollars for “cosmetic purposes.” So reported The New Yorker in a story on the city’s “eighth bridge” in early 1991. Whether or not he needed such a story to push him to persist, before the year was out Senator Moynihan had worked the halls of Congress, and $55 million had been appropriated to repair and repaint the Hell Gate Bridge, “the least famous of the eight across the East River,” but the one dearest to the senator’s heart. Not every bridge has such influential friends.
The appropriation of the money was only the first step in getting the Hell Gate Bridge painted, however. In the spring of the following year, a special train carried Moynihan, Mayor David Dinkins, and other local politicians to the bridge for a ceremonial first brushstroke, as The New Yorker reported. The ceremony had a special twist to it, for the key bridge link in the northeast corridor rail line between Boston and Washington was to be painted “an entirely new color—Hell Gate Red!” The color had been chosen by a “committee of color experts,” which included architects, a representative of the Municipal Art Society, the “minimalist painter” Robert Ryman, and the “color consultants” Taffy Dahl and Donald Kaufman from the firm Donald Kaufman Color. A press kit described the new color as “deep cool red” and placed it in “the family of red colors historically associated with railroads,” so that the Hell Gate would be readily distinguished from the city’s automobile bridges. No mention was made of the distinctive red that had covered the Forth Bridge for a century. The consultants’ color was to “complement the greens and blues of the landscape, adding to the richness of the scene,” which made it sound as if the bridge were in a pristine natural setting rather than the litter-strewn, graffiti-pocked center of New York. Proponents of the color appeared to consider as a plus that it also would “disguise any rust that developed on the bridge”; no one seems to have mentioned that this should not be the point of a paint job. Paint should serve a prophylactic as well as a cosmetic purpose, but, rather than disguise rust, it might be better to highlight any that might begin to develop, so that it could be attended to before it spread too far.
When the train arrived at the bridge, dignitaries and reporters alike noted the considerable number of holes in the walkway of the aging bridge. However, the order of business was not to repair the structure but to apply the ceremonial first strokes of paint. Shortly after the mayor and the senator began wielding their rollers, observers saw that the color was not quite what they had been led to believe was Hell Gate Red. To some it looked like “sort of a vermilion,” to others a pink. The representative of the Municipal Art Society assured Senator Moynihan that after the appropriate number of coats, the paint would dry to the appropriate color, that in fact the ceremonial paint was a benign latex substitute for the real thing, whose fumes, it was feared, would have “knocked out half the Democratic leadership of the city.”
The story of painting the Hell Gate is full of art and artifice, of politics and chicanery. The engineers—Lindenthal and his assistants, Ammann and Steinman—and their engineering of the structure seventy-five years earlier were not a part of the story or the event in the early 1990s. They had by and large been forgotten in the hoopla of color consultants and committees, who will no doubt ignore the bridge for another several decades at least, especially if its rust is disguised by its color. To the engineer, painting a bridge is as necessary as changing the oil in a car; it is neglected at the peril of the machine, which at least one architect, Le Corbusier, understood did not have to have grossly moving parts. Every bridge is a machine of sorts, moving ever so slightly under the action of the traffic, the push of the wind, the heat of the sun, or the growth of the rust that should not be allowed. Arresting rust and other deleterious movements of a bridge are matters for sound engineering, not for sound-byte politics. It has been estimated that as much as 2 percent of new-construction cost should be earmarked each year for maintenance, including painting, for the life of a major bridge structure. Neglect, euphemistically called “deferment,” of maintenance is only postponement of the inevitable, as the cases of the Williamsburg and Hell Gate bridges so forcefully demonstrate.
The Intermodal Surface Transportation Efficiency Act of 1991, behind which Senator Moynihan was the major force, has encouraged aesthetic improvements to infrastructure generally, and these can double as protection against deterioration. In the Baltimore area, for example, Stan Edmister, who calls himself the nation’s first “bridge-maintenance artist,” has used multiple layers of high-gloss paint to provide a protective coating that he claims will last fifteen years, which is about twice the time that conventional bridge paint lasts. In 1992, Edmister began painting bridges on Interstate 83 with “eye-catching, rich colors that break from the light blues and greens traditionally used to make bridges disappear into the landscape.” His idea of painting different structural elements of the spans different colors evokes the practice of Victorian times, when the Crystal Palace, for example, was decorated with such a polychromic scheme, as Alhambra Jones applied his “science of color” to it. London’s light-blue, white, and red Blackfriars Bridge, the blue-and-white suspended approaches of stone-clad Tower Bridge, and the red Forth Bridge near Edinburgh, are fine extant examples of the late-Victorian sense of color. Indeed, from 1890 until recently, the Forth Bridge was conscientiously kept painted “Forth Bridge red,” and the constant job occupied twenty-four painters, who worked steadily in a twelve-year cycle to keep the entire structure covered with five coats of paint. The vastness of the endeavor was so well known in Britain that “painting the Forth Bridge” is still a metaphor there for an endless task.
For some time in America, even before there were modern color consultants and bridge artists, engineers have been receptive to using paint for the decoration as well as for the protection of steel. As early as 1902, Chicago’s Loop elevated structure was painted “a pale buff color” at the request of merchants who wished to brighten the street below. That effect was achieved, “but soot and dirt which collected on the upper portions and mud which spattered the columns soon began to mar the appearance of the structure, so that it was eventually repainted the original dark gray.” In 1920, at the annual meeting of the American Railway Bridge and Building Association, J. R. Shean, of the Pacific Electric Railway, argued that “canary yellow, pearl gray or light olive green” finishing coats on steel bridges would make them “more in harmony with the surroundings.” Though there would be some discoloration from dirt and smoke, he reasoned that the lighter colors would last longer because of their greater “resistance to heat rays.”
David Steinman was a strong advocate of the creative painting of bridges. He introduced the use of color in structures he designed because he had grown tired of seeing bridges painted funereal black and battleship gray and he wanted “to get away from these sad, somber, cold colors and into something warm and bright, to harmonize with and be a part of the landscape.” His Mount Hope Bridge, his first departure from tradition, was painted “a light-greenish tint.” Steinman “progressively became bolder, using verde green, jade green, apple green, foliage green, and forest green” in later bridges. His St. Johns Bridge in Portland, Oregon, for example, whose high roadway provides over two hundred feet of navigable clearance, in 1931 was painted “a pleasing shade of verde green,” to blend in with the trees, rather than the yellow-and-black stripes that had been suggested to warn airplane pilots of its presence. The major suspension span of Steinman’s Thousand Islands International Bridge, dedicated in 1938, had its steelwork painted a “patina green.” His most daring use of color, perhaps, was in the Mackinac Bridge, for which he chose “a two-color combination—foliage green for the spans and cables, and ivory for the towers, to express the difference of function”—namely, tension and compression, something Waddell had suggested in the nineteenth century. The onetime academic Steinman knew he might be “joshed about the ivory towers,” but he felt they were appropriate for the structure. Furthermore, he reflected, “I may be called a dreamer. But the Mackinac Bridge is a wonderful dream come true!”
Among the most distinctively painted of American bridges is, of course, the Golden Gate, and its consulting architect, Irving Morrow, first proposed “using an orange-red color for the towers,” with deeper shades for the suspenders, cables, and approaches. He also thought that the scale of the structure should be “emphasized, rather than played down,” and he thought that a “red, earthy color” would be appropriate for contrast with the alternately foggy-gray and blue skies over the Golden Gate. In the end, a single color was chosen, to tie the bridge into the red-orange rock of the Marin hills. The color finally settled upon has been variously described as “red lead” and “iron-oxide red,” but it is officially known as “International Orange,” and it is in fact strikingly similar to that of Forth Bridge red. Morrow, who was also responsible for the sculptural detailing of the towers, helped make the bridge what has been called “the world’s largest Art Deco sculpture.” However, the Golden Gate Bridge remains a healthy work of structural art largely because it has been properly cared for, being painted continuously since its completion in 1937. The job of covering the ten million square feet of steel surfaces takes a full-time force of painters forty-eight months to complete, after which they, like their onetime counterparts on the Forth Bridge, begin all over again.
The continued collection of tolls on a bridge like the Golden Gate assures its continued maintenance. In his final report, Chief Engineer Strauss countered the call for “free bridges” with the observation that, like free lunches, “there are no free bridges” and “all bridges must be paid for in taxes of some sort.” He called tolls a “users’ tax” and described them as “the only method by which the age-old isolation of San Francisco could be ended.” However, Strauss also noted that the “modest and fair” toll rate of fifty cents set by the Bridge District in 1937 was less than half what he had based his financial calculations on, thus jeopardizing its future. Fortunately, his estimates of use were overly conservative. The first year, the Golden Gate was crossed by about nine thousand vehicles a day; a half-century later, the bridge was being crossed by more than six times that amount, reaching an aggregate of over one billion cars. In 1968, the Golden Gate became the first bridge to institute one-way tolls, thus relieving congestion for about half the traffic. Toll revenue now exceeds $50 million annually, which is enough for “maintenance, repairs, modernization, equipment, supplies and salaries” to operate the bridge, plus some left over to subsidize public transportation. This is a far cry from New York’s toll-free East River bridges, whose engineers have had to fight for money to paint and repair an infrastructure neglected and long forgotten by the vast majority of politicians.
Rust and corrosion may attack a bridge slowly over many years, but an earthquake can do its harm in a few seconds. Besides the damage it did to the San Francisco-Oakland Bay Bridge, the 1989 Loma Prieta Earthquake caused a mile or so of the theretofore undistinguished, if not downright ugly, Nimitz Freeway in Oakland, also known as the Cypress Structure, to collapse on scores of cars and trucks, crushing, trapping, and killing forty-two people. The Golden Gate Bridge survived that quake unscathed, but it has subsequently been ordered to undergo retrofitting to prepare it to withstand the “big one” that continues to threaten California. Earthquakes were not unknown to San Francisco when the Golden Gate was being planned and designed, of course, and there was considerable controversy between geologists and engineers about the nature of the foundation on which the bridge would rest. What most complicates any design that must take earthquakes into account is that there is no single earthquake to design against. As the 1994 quake in Los Angeles and the 1995 one in Kobe, Japan, demonstrated, each time the earth shakes, it may move in a different direction, with a different amplitude, and with a different frequency. The wind is almost more predictable.
In order to design bridges against earthquakes, engineers must make judgments as to the range of directions, amplitudes, and frequencies that are most likely to occur in the vicinity of the structure. Designing against an earthquake that might measure upward of an 8.5 on the Richter scale, for example, would result in a very, very conservative design structurally, the building of which would require an enormous financial investment. In fact, engineers today face almost the same degree of uncertainty and ignorance in designing against earthquakes as mid-nineteenth-century engineers did in designing against the wind. Only the experience gained on real bridges in real circumstances can confirm or refute the soundness of the judgments that have been made. Many of the San Francisco—area bridges and viaducts built from the 1930s through the 1950s were created in a design climate that accounted for earthquake forces in a particular way. For example, Charles H. Purcell, chief engineer of the San Francisco-Oakland Bay Bridge, described in 1934 how earthquakes would be taken into account in its design:
Since the structure is in a region which has suffered, and may again suffer, more or less violent earth shocks, unusual precautions have been taken to safeguard against the hazard. All elements of the bridge are designed for an acceleration of the supporting material [the earth] of 10 per cent that of gravity. It was readily recognized that the usual criteria for earthquake design would not be satisfactory in dealing with this [extraordinary] structure. In view of this, an exhaustive study was made and design methods were evolved which took into consideration the various peculiarities of the problem.
In the case of the channel piers, the horizontal force created by the acceleration of the mass will be augmented by forces due to the movement of the pier through the water and the soft mud immediately below. In fact it is conceivable that the soft mud may have an acceleration of its own in a direction opposite to that of the pier. These forces were incorporated in the analysis. In dealing with the superstructure, and particularly the suspension spans, the elastic and mechanical flexibility of the elements were fully considered. For earthquake design a 40 per cent increase in basic unit stress was permitted.
Though Purcell’s “unusual precautions” to accommodate a ground acceleration of 10 percent that of gravity may have seemed conservative in the 1930s, they proved to be less so as earthquake experience accumulated. The 6.6-magnitude Northridge Earthquake that struck near Los Angeles in early 1994 had both horizontal and vertical ground accelerations far in excess of 10 percent. Even before that earthquake, however, experience subsequent to the construction of the Bay Bridge revealed gaps in contemporary understanding and the resulting weaknesses in designs, and corrective measures were prescribed for it and other older bridges. Unfortunately, identifying engineering needs and raising funds to implement them are not necessarily commensurate. For example, had engineers argued in 1988 for spending tens of millions of dollars to encase the steel piers of the East Bay Crossing in concrete to stiffen them against the sideward swaying of an earthquake, further studies might have been called for to establish why a bridge that had appeared perfectly sound for half a century suddenly needed to have thickened the slender legs that had contributed to the structure’s grace. However, after the 1989 earthquake revealed that the flexibility of the Oakland span caused enough swaying to dislodge one of its deck sections, the money was available fast enough so the work was completed in early 1992.
Over a billion dollars was earmarked for the strengthening of highway structures throughout the state in the wake of the Loma Prieta Earthquake, but that work was incomplete when the Northridge Earthquake struck. The California Department of Transportation, known as Caltrans, had necessarily to engage in a form of triage for bridges, and when the Los Angeles area shook in January 1994, at least one of the major highway spans scheduled for strengthening the next month collapsed. For some time afterward, even geologists were a bit frustrated that they “had not yet pinpointed the source of the powerful earthquake that appears to have emanated from a previously unmapped thrust fault.” There were no great fissures opened up by this quake described as “the costliest natural disaster in U.S. history,” with associated costs estimated as high as $30 billion. Among the highway bridges that collapsed and caused the greatest inconvenience for commuters were necessarily ones that involved the longer spans required at the junctions of major routes.
Some of the bridge failures during the Northridge Earthquake were unusual in that bridge decks appeared to have been bounced vertically as well as slid horizontally. Whereas the San Francisco earthquake of 1989 was characterized by a slow horizontal shaking that shifted the East Bay span off its supports and caused the elevated freeway to collapse like a house of cards on a shaky table, the Los Angeles earthquake involved large vertical motions that dominated the structural response in some locations. All but two of the bridge structures that collapsed in the 1994 quake were built before the 1971 San Fernando earthquake, and Caltrans came in for some criticism that it had given strengthening these low priority. The two newer structures that collapsed were said to have been negligently designed by highway engineers, but a Caltrans spokesman pointed out, “You can’t design a bridge to resist every possible earthquake—and this one came from an unknown fault.” Such charges and defenses will no doubt continue to be made as long as bridges are built—and collapse. But if no bridges ever collapsed, engineers would then come in for criticism because they were designing structures to resist incredibly large earthquakes, storms, and even terrorist attacks that might never happen. And if engineers were demanding and getting, perhaps with the aid of stories and pictures of fallen bridges, all the funds they needed to design against everything, what other needs of society would be neglected? Priorities for health and safety are never easily determined, whether they relate to bridges or to the people who use them.
Must we thus expect, if not allow, a bridge failure to occur now and then? The history and promise of bridges suggest that we must, for reasons that have to do with neglect of the past and its relevance for the future. Neglect of the past is often embodied in a short-term historical memory, thinking, with hubris, that one’s own generation’s engineering science and technology have progressed so far beyond what they were a generation or two earlier that the bridges of one’s professional progenitors, and even one’s mentors, make pretty pictures but not examples or models for modern engineering. A historical perspective on bridges and their engineers reveals not only that such shortsightedness is nothing new, but also that it has led to disaster time and again.
A close reading of the history of major bridge failures is contained in a remarkable piece of scholarship by Paul Sibly and his adviser, then at University College London, Alastair C. Walker. Among the conclusions of their work, published in 1977, was the strong temporal pattern that bridge failures had followed from the middle of the nineteenth century. What Sibly and Walker noted was that the collapses of the Tay, Quebec, and Tacoma Narrows bridges, which occurred in 1879, 1907, and 1940, respectively, were very nearly thirty years apart. A less commonly remembered incident, but one that was equally dramatic and in its own time the subject of investigation by a royal commission, was the collapse of Robert Stephenson’s Dee Bridge in 1847—further reinforcing the observation that a thirty-year cycle was associated with bridge failures. To test their hypothesis, which pointed to a major bridge failure about the year 1970, Sibly and Walker looked at incidents around that time and found that, indeed, in 1970 there were two significant failures of a new type of steel bridge, known as a box girder, then under construction in Milford Haven, Wales, and in Melbourne, Australia.
The pattern of bridge failures laid out by the historical studies of Sibly and Walker revealed several characteristics besides a thirty-year cycle. For example, each of the bridge types involved was of a different kind (trussed girder, truss, cantilever, suspension, and box girder), and each had been evolving within a design climate of confidence and daring when the accident occurred. The stories of the Quebec cantilever and the Tacoma Narrows suspension bridge epitomize this aspect of the thesis. Indeed, the different types of bridges that failed had often been introduced or developed with renewed vigor two or three decades earlier in response to a failure that drove a different kind of bridge out of favor. Thus the cantilever-bridge type, made most famous by the spans over the Firth of Forth, was introduced in the wake of the disastrous fall of the high girders of the Tay Bridge. When the Quebec Bridge collapsed, so did the reputation of the cantilever as a prime competitor of the suspension bridge for long spans.
Among the speculations Sibly and Walker offered to explain the thirty-year cycle was the nature of engineering practice, in which there developed “a communication gap between one generation of engineer and the next.” This can certainly be true when aging engineers like Theodore Cooper remain aloof from their projects, as he did in the case of the Quebec Bridge, and when less experienced engineers associated with a project defer to the presumed infallible experience and judgment of a more eminent engineer, as happened with the Tacoma Narrows Bridge. In the case of Gustav Lindenthal and the succeeding generation embodied in his assistants Ammann and Steinman, the inflexibility of the mentor in failing to modify the plans for his dream bridge, to accommodate the changing nature of transportation across the Hudson River in the twentieth century, not only cast his judgment in doubt but also opened up rifts that prevented substantial communication between professional generations.
But perhaps the most significant factor that widens simultaneously the communication and generation gaps between engineers of different times is the ever-present development of engineering science and the tools of analysis. No twentieth-century suspension-bridge engineer seems ever to have lost a reverence for the works of John Roebling, as epitomized in his Brooklyn Bridge. However, with the growing development of analytical tools, as embodied in the deflection theory that Leon Moisseiff seemed so effectively to have developed and applied, the methods of Roebling, which relied upon physical more than mathematical argument, appeared to have been superseded. Unfortunately, with the relegation to dusty archives of Roebling’s verbal reasoning about his concerns over stiffness and wind, the natural forces and response of bridges to them that so concerned him ceased to be a primary concern to more mathematically minded engineers, who remembered the old master’s bridges primarily as aesthetic models. The limitations of this shortsighted view of engineering history became immediately apparent in the wake of the collapse of the Tacoma Narrows Bridge, and the subsequent revitalization of the suspension-bridge form took place only in light of newly embraced aerodynamic theories and wind-tunnel testing. This new perspective led to such innovations as the winglike decks and inclined suspender cables of the Severn and Humber spans in England, the latter the longest in the world until the completion of a bridge in Denmark and the Akashi-Kaikyo Bridge across Japan’s Akashi Straits. The Severn span, however, has not been without its own problems, and it has had to be “strengthened” to carry the heavier lorries that have been allowed to use Britain’s motorways since the bridge’s original design and construction. No matter how strong the bridges are, users of the United Kingdom’s greatest spans across its wide estuaries have been buffeted by winds so forceful at times that the largest lorries have been instructed to cross in pairs, thus reducing somewhat their chances of being blown over.
The pattern of bridge development established by Sibly and Walker suggests that in the late twentieth century there should be not only another radically new bridge type evolving toward more and more daring lengths and slenderness but also that a major failure can be expected in that type sometime around the turn of the millennium. The genre that seems so eerily poised to continue the thirty-year cycle of major bridge failures has developed from an old type that was rediscovered in Europe in response to the exigencies of rebuilding an infrastructure destroyed by the war. Though the superstructure of many bridges in Germany had been damaged, their foundations and piers were often reusable. The challenge to engineers was to design for these prewar foundations lighter bridge decks that could then carry the heavier postwar traffic. Cable-stayed bridges had been conceived centuries earlier, but they had never before been built on the scale or in the numbers that they began to be in Germany in the 1950s. For some time after that period, such bridges were thought to be the most economical and suitable choice for spans no longer than about twelve hundred feet, or somewhat shorter than the main span of the Brooklyn Bridge. By the 1980s, however, cable-stayed designs were being proposed with span lengths that had previously been thought to be in the exclusive realm of the now more conventional suspension bridge.
The Sunshine Skyway Bridge across Tampa Bay, among the longest and most photographed of the cable-stayed bridges in the United States, has a main span of twelve hundred feet. Completed in 1987, the Florida crossing remained among the top dozen or so longest cable-stayed bridges in the world into the early 1990s, when the genre really took off to new lengths. Many long spans were completed in Japan and Canada in the late 1980s, and in 1991 the longest cable-stayed bridge finished in Europe was the Queen Elizabeth II Bridge across the Thames at Dartford, with a main span of almost fifteen hundred feet. The French, however, had already at that time under construction over the mouth of the Seine a cable-stayed bridge with a main span in excess of twenty-eight hundred feet—the magnificent Pont de Normandie—and Danish engineers let it be known that they were considering a cable-stayed span approaching four thousand feet in length to complete what is known as the Great Belt link between Denmark’s two largest islands. Though this design was eventually rejected in favor of the suspension bridge, itself remarkable with its main span of more than a mile in length, the daringness of the Danish cable-stayed proposal became the topic of some discussion among engineers.
Cable-stayed bridge proposed over the Mississippi at Cape Girardeau, Missouri (photo credit 7.2)
The Sunshine Skyway Bridge across Tampa Bay, shown on the cover of a brochure (photo credit 7.3)
British engineers questioned whether it was wise for the French even to attempt a cable-stayed bridge with a main span almost twice the existing record. Questions of how the incomplete structure at the mouth of the Seine would behave in the wind were central to such endeavors, and there were warnings that scaling up in so large a leap from existing bridges was a prescription for disaster. Engineers proposing the doubling or even tripling of existing spans were confident, however, claiming that the larger bridges were “perfectly” possible because of modern computer modeling and construction techniques. Special devices were fitted to the incomplete spans to stabilize them in the wind during construction, and when the deck was finally completed, in the summer of 1994, many an engineer breathed a sigh of relief. After six years of design and construction, the span was opened to traffic early in 1995.
Regardless of how sophisticated the computer models or construction techniques, whether the cable-stayed Pont de Normandie, with its record main span, would successfully cross the mouth of the Seine depended at least in part on luck. Excessively high or unusual winds that were not factored into the computer model could hold as much of a surprise for the engineer of the Pont de Normandie as the excessive weight of steel that was inadvertently omitted from the calculations for the Quebec Bridge held for its engineer. Any model, whether a simple equation on the back of an envelope or an elaborate numerical one in the gigantic memory of a supercomputer, is only as good as its fundamental assumptions. The Tacoma Narrows Bridge fell because the most sophisticated deflection theory used to design it did not take into account the dynamic effects of the wind. In sum, the undoing of a project will derive not so much from its size or scale as such, as from an imperfect understanding. What is an insignificant detail in a cable-stayed bridge of relatively modest size can grow to surprising importance as the size of a bridge grows. The sage advice to increase the size of bridges slowly reflects an awareness of this scale effect among more experienced engineers, but younger engineers, full of confidence in their computers, often think such caution to be a mark of excessive conservatism.
Making great leaps in size does not, of course, doom a bridge to failure, and daring young engineers can use the historical examples of the Forth and the George Washington bridges to defend their ambitious designs. The cable-stayed bridge that might fulfill the inexorable prophecy of Sibly and Walker’s pattern of failures will not necessarily be the longest of the genre. The Tacoma Narrows Bridge had, after all, only the third longest main suspended span in 1940. But though mere size may not bring bridges down, it may often be the focus of attention with regard to warnings about their behavior. Being only the third-greatest span, with a modest traffic load and in a relatively remote location, the Tacoma Narrows was not a structure that called attention to itself—even though it was the most slender of bridges—until it began to oscillate in the wind and collapsed. It may be similar with cable-stayed bridges. The Pont de Normandie and others in the vanguard of design will be more carefully planned and watched than those that will be almost but not quite as large—those that will be, ironically, of little more than local significance or remarkableness during their design and construction. It may be from the great but not necessarily the greatest that we can expect the most unpleasant surprises.
If a major collapse of a cable-stayed bridge is to be prevented, there must be as much attention paid to the maintenance of the engineering-design infrastructure as to that of the physical infrastructure. This means that engineers should be as sensitive to the historical cycles of success and failure that have plagued the design enterprise as they are to the cycles of freezing and thawing that can plague their physical roadways and bridges. Care of the design infrastructure requires the maintenance of lines of communication between engineering generations, so that new tools and models are not used in ignorance of past experience. The surest way to break the vicious cycle of bridge failures identified by Sibly and Walker must certainly begin with the recognition that neglected patterns from the past become unconscious patterns for the future. Only by bringing these patterns into sharp focus and by seeing the modern engineer as reinventing, albeit with faster and more powerful tools, the bridges of the past and of different cultures, can we hope to realize dreams that do not spiral into nightmares. The bridge-and-structural engineer Henry Tyrrell articulated this almost a century ago, when he wrote the opening lines of the preface to his 1911 history of bridge engineering:
Proficiency in any art or science is not attained until its history is known. Many a student and a designer finds, after weary hours of thought, that the problems over which he studied were considered and mastered by others, years or centuries before, perhaps with better results than his own.
The earlier years of the cable-stayed bridge genre brought to the fore such individual engineering personalities as the German Fritz Leonhardt, who practiced in Stuttgart; the greatest spans today are being designed by firms that carry the names, but not necessarily the personalities, of the older generation. As the presence of Ammann and Steinman continues to be felt through the firms of Ammann & Whitney and of Steinman, Boynton, Gronquist & Birdsall, commonly referred to simply as Steinman, so does that of Leonhardt in the firm of Leonhardt, Andrä und Partner, to which in the early 1990s the patriarch still contributed his philosophy, if not his daily presence. However, not all of the most recent record spans of cable-stayed structures have been designed by firms that tie themselves in name to great engineers of the past. The Danish firm with the trendy name CowiConsult, “one of the world’s leading bridge designers,” was the one designing, in its bridge department in Copenhagen, the world’s longest cable-stayed span. Other great bridge-designing firms, such as Britain’s Acer-Freeman-Fox, and the American Sverdrup Corporation, still tie themselves explicitly to the names of their entrepreneurial ancestors, but the increasingly used anonymous collective designations of “partners” and “corporation” indicate the trend away from the small partnership team or the dominant personality of an individual consulting engineer.
Whether their designs are attributed to forceful individuals or to anonymous corporations, cable-stayed bridges are not the only uniquely twentieth-century bridge type, nor is steel the only twentieth-century bridge material. Among other notable categories that have been very successful, not only as great structures but also as works of art, are the concrete bridges of the Swiss engineers Robert Maillart, in the first half of the century, and Christian Menn, after midcentury. Both have concealed steel, as reinforcement and as cables, in their bridges, which are primarily concrete structures. David Billington has described both Maillart and Menn as structural artists whose works are monumental pieces of sculpture as well as utilitarian works, and he has written in illuminating detail especially about Maillart’s great concrete bridges.
As concrete has challenged steel, so have new materials challenged them both. Advanced composite materials made of glass, carbon, and polymer fibers, originally developed for the aerospace industry, are being introduced into experimental bridges, enabling them to weigh as little as one-tenth as much as conventional steel or concrete designs. One such bridge, a 450-foot-long cable-stayed road bridge over Interstate 5 in San Diego, has been aided in the materials-testing-and-design phase by a grant from the Federal Highway Administration. Because the cost of the new stuff is as much as twenty times that of conventional materials, such bridges can generally be expected to need this kind of financial support, and to remain in the experimental category until the materials become economically competitive. That will not prevent engineers from dreaming of using the newer materials to span such continuing challenges as the Strait of Messina and the Strait of Gibraltar.
In the meantime, others continue to work with conventional materials but in unconventional forms. Among the most talked-about individual bridge designers of the late twentieth century, in any material or form, is Santiago Calatrava, whose training as both an engineer and an architect has given him and his work a special cachet. Calatrava was born in the second half of the twentieth century, in 1951, in Valencia, Spain; he studied architecture there before going to the Swiss Federal Institute of Technology in Zurich, where he became also a civil engineer. Since opening a practice in Zurich in 1981, he has been responsible for structures of dramatic space and volume, mostly connected with transportation, throughout Europe. His best-known structures tend to be his bridges, however, including the canted-towered, cable-stayed Alamillo Bridge, built for the 1992 Expo in Seville, and the Bach de Roda Bridge in Barcelona, whose inclined arches and suspender cables enclose pedestrian paths that broaden midspan to plazas to create a secure yet open space that is both protective and inviting. The engineer-architect Calatrava has been accused of being more the latter than the former, however, for he has said that he wants “to win back engineering objects like bridges for architecture.” Never mind that such talk can reopen old wounds and spark debate between the professions; in the final analysis, Calatrava’s work will be judged by the standards of both, and there are indications that he forgets some of the fundamental principles of structural-engineering art in his pursuit of appearances.
Santiago Calatrava’s Alamillo Bridge in Seville, Spain (photo credit 7.4)
In his Alamillo Bridge, for example, Calatrava employed a massive counterweight under the roadway to add enough tension to the longest cable so that it would be taut and not sag under its own significant weight. This added considerably to the cost of the bridge, and at the same time sacrificed structural honesty. Calatrava may see such compromises as necessary to win back bridges for architecture, yet the great engineers have never felt they were deliberately wresting bridges from architects. Even if modest spans like the ones Calatrava has designed may fairly be viewed as pieces of sculpture as well as utilitarian crossings by those who commission them, it is not necessarily obvious that bridges of record span, which will always remain first and foremost engineering problems requiring engineering solutions, should be saddled with significant extra weight, be it physical or metaphorical, for the sake of appearance alone.
The city of St. Paul, Minnesota, recently commissioned not an engineer but a sculptor, James Carpenter, “to conceive the form for a bridge” across the Mississippi River. Among the forms developed by the New York artist, who worked in conjunction with a German engineer, was a skewed-decked, cable-stayed bridge supported by a V-shaped pylon located on an island six hundred feet from either shore. Though a computer-generated image of the bridge inserted electronically into photos of the site showed the structure to be a striking design, with elements clearly intended to make the bridge distinctive, the cost would have been more than twice that of a conventional span. It fell upon the City Council to decide whether to spend an extra $15 million dollars, even though the bulk of it might have to be requested from federal bridge-replacement funds, for a structure whose principal characteristic might be said to be difference for the sake of being different. The city’s Public Works Department was instructed to work out estimates for a cable-stayed bridge with its two spans in alignment, but the cost for such a structure was still considered prohibitive, because it was learned that “federal funds would not be available for anything beyond the least expensive design.”
Computer-generated image of Calatrava’s unrealized East London crossing of the River Thames (photo credit 7.5)
Though the mayor clearly preferred the artist’s signature design, political and economic realities led him to lean toward a less expensive double-arch option, also produced by the artist, which was not unlike an unrealized Calatrava proposal for a single-arch crossing of the Thames in East London. In fact, this medium-priced design was the public’s choice, according to opinion surveys, but in the end the mayor recommended a third type of bridge to cross the river at Wabasha Street. This was the least dramatic, least distinctive, and least expensive of the original options—a box-girder bridge that could be built for $20 million and could include “pedestrian amenities such as ornamental lighting, windshields, pedestrian outlooks, and stair and elevator towers” to the island in the river. As in so many other cases regarding bridges and their appearance, in the final analysis the politicians and citizens of St. Paul had to settle for what they could afford. The dream of an artist, neither more nor less than that of an engineer, was alone insufficient to dictate reality.
A public tension between means and wants often only highlights a more constant tension between function and form. Though it ebbs and flows as surely as do the waters over which many a monumental bridge is built, the ongoing push and pull between designer and financier, between engineer and architect, between engineering and art, come to our attention mainly when a question of design bobs to the surface. Disagreements over form will no doubt remain as long as there are engineers and artists who see their objectives as different. Bridge design is among the most visible and vulnerable arenas in which such competition has taken and will continue to take place, and it is wise to recall that the greatest bridge engineers have always taken an equal interest in the structural and artistic values of their designs, the mediators more often than not being safety and economics. Not that engineers are more willing than architects to sacrifice beauty for brawn, or looks for lucre; the greatest bridges that engineers have built are clearly the ones that unite and achieve both structural and aesthetic goals, and often with striking strength and economy in their context. Above all, however, engineers know that, first and foremost, their bridges must stand into the future against weight and wind and want. The most beautiful bridge, when negelected in structural design and maintenance, can become, fallen, the most ugly pile of concrete and steel. That is not bridge building.
As new materials, computational techniques, and generations of engineers come to dominate the world of bridge building, as they will especially the projects involving the greatest technical challenges, there will necessarily come to the fore competitions and disagreements among designs and their designers. This is to be expected in any creative endeavor; we should not be surprised that it is heightened in bridge building, which is among the most visible, symbolic, and evocative of all interactions between engineer and engineer, and between engineers and society. Artists and architects may challenge the engineer, but ultimately only the engineer will be able to cantilever out over technically uncharted waters to build bridges greater than any before. Though knowledge of structural principles is of course essential in such an endeavor, a sense of history provides the judgment for engineers to dream effectively beyond the present. Even if dreams come easily to dreamers, bringing a dream to reality takes a view that is firmly founded on experience of what can and cannot be done technically, plus a confidence in what is humanly and economically possible at a given time. Both of these qualities may be necessary to achieve greatness in bridge building, yet they alone seem not to be sufficient to bring a particular dream to reality. There appears to be, as Ammann saw it, a certain element of luck involved in the enterprise.
Whether designed by engineer or architect, artist or Boy Scout, every bridge is a legacy to its environs and to its users. The environment itself, especially when it is cruel by geography or polluted by society, cannot be expected to be any more respectful of a bridge than it is of an automobile or an endangered species. The society of users, who are in fact willy-nilly the stewards of the world’s bridges and of the greater infrastructure, must recognize that every artifact that has been or ever will be created, whether in now traditional steel and concrete or in the composites of the future, must be maintained as well as used. By understanding this and the origins of our bridges and other artifacts of civilization, and the humanness of those who once dreamed of what we now so often take for granted, we not only engage ourselves in the technosocial endeavor that involves engineers at its core, but we also understand how their human natures and their dreams affect the way we experience our cities and towns, our borders, and our open spaces.