Engineers of Dreams: Great Bridge Builders and the Spanning of America
Page 47
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 scal
e, 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 ca
ntilever-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.