Engineers of Dreams: Great Bridge Builders and the Spanning of America

by Henry Petroski

  Hundreds of letters poured in with more suggestions. A man from New Jersey proposed “Verrazano Bridge,” after the Florentine navigator who was claimed by some to have been the first European to cross New York Bay, in 1524, but who was “considered an uncertain quantity by many historians.” “Columbus Bridge” and “Hendrik Hudson Bridge” had solid support, but the “volume of letters from school children and women’s clubs” kept the hope for honoring George Washington alive. In the end, the letters showed “a complete lack of unanimity of public opinion,” and the Port Authority decided on “George Washington Bridge,” only dropping the explicit mention that it was a memorial. Pleas for other names continued to be advanced in letters to editors, but to no avail. Though newspaper and magazine editorials expressed resignation to the official naming of the bridge, “what the millions who use it in the years to come will choose to call it is another matter.” Engineering News-Record believed the bridge would “get its workaday name from the people, while its christen [sic] name is likely to remain unknown outside parish records and mortgage papers.” How wrong these prognosticators were. The name “George Washington Bridge”—sometimes shortened in speech to “the George Washington,” “the GWB,” or, within the Port Authority offices, simply “the George”—is used to this day by New Yorkers, New Jerseyites, and travelers to and from all parts of New England.

  The Washington Bridge in 1889, when recently completed and known as the Harlem River Bridge (photo credit 5.12)

  At the dedication, New York Governor Franklin D. Roosevelt and New Jersey Governor Morgan F. Larson formally opened the bridge with a ribbon-cutting ceremony at the center of its span, where grandstands had been erected for five thousand guests. It was a day to celebrate and show unity. Roosevelt and ex-Governor Al Smith arrived together, and they received loud applause. “But probably the greatest and most spontaneous greeting was that accorded to” Ammann and Lindenthal, “who came to the grandstand in the same automobile.” Unlike Lindenthal, who was described as “designer of the great Hell Gate Bridge and dean of American bridge builders,” Ammann needed no identification in the newspaper story covering the ceremonies. His eclipse of his mentor was complete; Lindenthal was not even listed among the consulting engineers, although in the final report on the project Ammann would identify Lindenthal specifically as one who had “rendered special advice on design questions.” Regardless of whether Ammann was setting the record straight or fulfilling a sentimental obligation to an aging colleague, the balance of influence and power had definitely shifted. Future bridge design in America would follow not Lindenthal’s but Ammann’s lead in pursuing longer, lighter, and more flexible bridges that were enormous but not Brobdingnagianly grotesque.

  Among the many ceremonial touches that day was one that both recalled the past and foreshadowed the future:

  Defying the age-old rule that marching troops break step when crossing a bridge, columns of soldiers, sailors, marines and Coast Guard came swinging down the roadway from the Manhattan plaza. Those in the center of the bridge felt the gigantic span vibrate as if shaken by earth tremors, but the crowds were only amused at the strange sensation.

  It had been possible to build the George Washington Bridge at a cost that made it politically acceptable because Ammann had used all the technical ingenuity at his command to reduce the dead weight of the structure; but this also made the bridge perceptibly flexible. He discussed the development of the suspension bridge in his 1923 article in Engineering News-Record, in which he described the evolution of the stiffening truss, the dominant feature of the roadway profile of such recent suspended spans as Howard Baird’s Bear Mountain Bridge and Modjeski’s Delaware River Bridge, each of which briefly held the record for the longest suspended span in the world. In his review, Ammann explained how, a century earlier, Telford had designed his Menai Strait Suspension Bridge with a flatter curve to its chains, which in turn required a flexible deck to accommodate the deflections of the chain due to traffic loads and temperature changes. The deck of the Menai was blown down in the wind in the nineteenth century, but in his characteristically selective and slightly skewed view of bridge-building history, Ammann did not mention this extreme effect of a too-flexible deck. Instead, he described how the Niagara Gorge Suspension Bridge could carry railroad trains because its engineer, whom Ammann misidentified as Washington Roebling, successfully incorporated a stiffening truss. Still, whatever the lacunae in his historical view, Ammann understood fully the importance of having a bridge deck stiff enough to withstand traffic loads vertically and wind loads horizontally, and what was true for spans of moderate length he saw could be extended even further for those of unprecedented length. The stiffness of a bridge deck and cables complicated the calculation of the manner in which the load was distributed among the various components of the structure, however, and the less precisely an engineer knew this distribution, the more conservative he had to be in his design. Conservatism in bridge design translates into using more steel to compensate for uncertainties, which in turn translates into a heavier bridge that costs more to build.

  The required strength of a bridge deck depends not only upon the weight of the deck itself but also upon the weight of the traffic it has to bear. The requirements for Lindenthal’s Hudson River proposals were all dominated by the extreme weight of heavy freight railroad trains, which was ultimately reflected in his prohibitive cost estimates. In the case of the 179th Street bridge, however, Ammann had begun from the premise that it would not carry any heavy railroad trains. As originally planned, the upper deck would be for cars and trucks only, and the lower deck would carry at most only light commuter rail traffic. When the project evolved into two stages, with the first stage having only motor vehicles carried on a single deck, Ammann had been able to make some drastic reductions in the weight of the deck itself, by reasoning about the nature of the traffic on the completed bridge.

  Short-span bridges were typically designed by assuming that they might at some time be loaded with bumper-to-bumper truck traffic, which determined their strength requirements. As longer and longer spans came to be designed, however, it became clear that a solid line of trucks on a bridge deck was a very unlikely occurrence indeed; designing for that condition led to a very expensive structure that might have to support such a load very infrequently, if ever, and then for only a very short time. Since all structures were designed with some additional reserve strength anyway, in order to provide a prudent margin of safety, all bridges were in a sense overdesigned for the vast majority of the uses to which they were put. In his 1916 treatise on bridge engineering, Waddell gave currency to an idea that Modjeski had employed in the Manhattan Bridge—namely, that for the purposes of designing the bridge the unlikely extreme traffic condition be reduced by one-half. By the mid-1920s, such a practice had become somewhat standard in designing long-span bridges, such as the one then under construction over the Delaware River.

  In working out plans for his own bridge in such a design climate, Ammann further reasoned that the mix of automobile and truck traffic at the northerly-Manhattan location would be such that he could make additional reductions in the assumed maximum load. One such reduction was possible because the bridge would have eight lanes, and it was unlikely that all lanes simultaneously would be equally loaded with truck traffic. In the final analysis, Ammann used only one-sixth of the maximum conceivable traffic load as a design load. Though this so-called live load may seem to lead to a drastic reduction in strength, the effect is greatly lessened by the fact that the dead weight of the bridge itself would dominate the total design load. In an insightful analysis, with Jameson Doig, of “Ammann’s first bridge,” David Billington estimates that, had the engineer used the standard reduction of one-half for the traffic load, more than $7 million worth of additional steel would have had to be added to the $23 million that did go into the bridge. Furthermore, according to Ammann, since “every dollar spent for steel in the flooring and stiffening trusses in a span of
this length requires at least an equivalent expenditure for materials in the cables, towers, and anchorages to carry the floor steel, the total saving by the adoption of the flexible trusses is estimated to be almost $10,000,000.” This 25 percent savings of the entire cost of the bridge may have made the difference in the economic attractiveness of Ammann’s proposal to Silzer.

  Ammann’s calculations were supported by the “deflection theory” that Moisseiff had applied to the design of the Manhattan and Delaware River bridges. A consequence of that theory was that as the dead weight of a span increased, the stiffness of the deck could decrease. Since the weight obviously increased with length, longer bridges could be more flexible. This counterintuitive result was due in part to the necessarily heavier cables or eyebars, which alone would provide considerable resistance to deflection, whether caused by traffic or by wind. Whereas in many other suspension bridges, Ammann pointed out, “the stiffening system is heavier than the cables or chains themselves,” in his bridge the weight of the stiffening system would amount to “only about one-eighth of that of the chains.”

  When the structure was completed in 1931, the slenderness of the deck of the George Washington Bridge was one of its most striking features, but some critics found fault with other aspects of its design. The American Institute of Steel Construction’s award for that year’s “Most Beautiful Bridge,” for example, went not to Ammann’s 179th Street suspension bridge but to his Bayonne arch. According to an architect on the jury, “The setting of the Washington Bridge with the lofty Palisades on one side and the low Manhattan shore on the other did not call for symmetrical treatment and might be filled better by an asymmetrical structure.” But engineers by and large embraced fully Ammann’s Hudson River achievement, especially as embodied in the structural aesthetic of its most slender deck.

  At a special dinner meeting of the American Institute of Consulting Engineers held less than two weeks after the opening of the George Washington Bridge, the structure was described as a “great monument to Mr. Ammann,” and to him had fallen the title “Pontifex Maximus” of New York. Amid the mutual admiration of engineers, the flexibility of light bridge decks came in for some light talk. After Ammann spoke about the long history of bridging the Hudson—and had his recollection of the date of the first tunnels corrected from the audience by their engineer, J. Vipond Davies himself—Ammann discussed the question of stiffening suspension bridges. Among his remarks was a reflection on the evolution of his ideas:

  In my preliminary studies for this bridge, it took me a long time to wean away from the rigid system and to find out what would be the required rigidity, but I finally came to the conclusion that if the bridge were built for highway traffic alone the weight would be sufficient to provide ample rigidity without any stiffening whatsoever; in other words, it was feasible to go back to the early English type of suspension bridge, that is, to a practically unstiffened floor suspended vertically from the cables.

  What neither Ammann nor anyone else at the time seemed to recall about the “early English type of suspension bridge” was that many of those light decks were destroyed in the wind. Rather, the mood at the dinner was gleeful, the participants reveling in stories of people who feared the light bridges. After Ammann spoke, it was the turn of guest Colonel E. Vivien Gabriel, of the British Royal Engineers, “a man of vast and wide experience in India.” Among his stories of designing and constructing suspension bridges in that country, he related how he sometimes “drove a flock of sheep or cattle over the bridges before we tried the cars, in order to convince” the local engineers. After listening to Gabriel’s “delightful stories,” Ammann ended the evening with some of his own “about the early bridges built by American engineers.” Presumably recorded verbatim, Ammann’s closing remarks suggest that he had less a sense of lightness in story than in bridges:

  There was one of these flexible suspension bridges in New England, and an engineer who visited that place stood in the middle of the bridge while a team of oxen went over, and he relates that he became perfectly seasick.

  In another instance, an American engineer had built a suspension bridge of one of those thin wires or ropes that the Colonel mentioned, and the Indians who lived in the vicinity were very doubtful about its strength. They were used to the substantial cantilever bridges built of solid wood, and they refused to go over it. Finally they held a council, and decided that before they would go over they would send their squaws over, and if the bridge stood up under them, they would go over, too.

  These stories, told in a time before speeches were vetted for political correctness, must have gotten a laugh, not for their execution, but for their allusion to the George Washington Bridge, whose flexibility was of no concern to these worldly and sophisticated men of scientific engineering.

  Of the forty-odd engineers present at the meeting, only Ammann himself would go on to build more daring bridges than the George Washington, but the success of that structure would also embolden engineers who were not in attendance. These included Leon Moisseiff and Joseph Strauss, who were engaged at the time in the design and construction of a bridge that would have a central span of forty-two hundred feet, a full 20 percent longer than the George Washington.

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  Talk of bridging the dramatic strait between San Francisco Bay and the Pacific Ocean known as the Golden Gate had begun in the nineteenth century, but the social and technical conditions of the early twentieth were needed to advance the project beyond talk. Bay Area movers and shakers were becoming increasingly annoyed by the ferry service between Marin County and San Francisco, and among them was the San Francisco Call Bulletin’s James Wilkins, whose engineering degree and daily ferry rides seem to have destined him to launch, in 1916, a new and relentless editorial campaign for a bridge. Another staunch proponent of a bridge was San Francisco’s city engineer, Michael O’Shaughnessy, who had been hired a few years after the 1906 earthquake to rebuild an infrastructure that was still in shambles. To secure a long-term water supply for the city, O’Shaughnessy wanted to build—almost 250 miles away, in the Sierra Nevada Mountains—a dam across the Hetch Hetchy Valley, which was regarded as a natural setting equal in beauty to nearby Yosemite Valley. Opposition to the project by the environmentalist John Muir and the Sierra Club took the case all the way to the U.S. Supreme Court. When the dam was christened, in 1923, it was named after engineer Michael O’Shaughnessy, in recognition of his dedication to the scheme. Completing the supply line to the city took another decade. Half a century later, environmentalists were trying to have the dam torn down and the Hetch Hetchy Valley restored to a natural state, but San Francisco’s residents have become as dependent upon the water it impounds as they have upon the Golden Gate Bridge.

  Newspaper people and city engineers, no matter how determined, cannot merely will great water supplies or bridges into existence. To bridge the Golden Gate, a proposal that was sound both technically and fiscally was needed, and such a scheme would have to come from an engineer with the background, vision, and time to conceive and execute the first rough calculations on a dream. In the course of dealing with all kinds of infrastructure problems for the city, ranging from tunneling under hills to seeing that all was in order for the 1915 Panama-Pacific International Exposition, O’Shaughnessy encountered many an engineer with whom to talk of things that might be. Among those with whom he raised the question of a bridge across the Golden Gate was Joseph Strauss, one of whose patented bascule bridges with a massive concrete counterweight was the first such one in San Francisco. In addition to being responsible for a nondescript if not downright ugly Fourth Street Bridge, Strauss designed the amusement ride known as the Aeroscope for the 1915 World’s Fair. The ride carried 118 fairgoers about two hundred feet in the air in the equivalent of a modest two-story house mounted on the end of a steel truss that was effectively a revolving bascule bridge. The view provided of the fair and the surrounding area must have been spectacular, for, as the ride progressed in a helical pat
h over the fairgrounds, passengers could see “Alcatraz and the Angel Islands in the bay, and the Golden Gate and Pacific Ocean beyond.” Engineer Strauss no doubt looked out at that view and dreamed.

  Fourth Street Bridge constructed in San Francisco in 1916 by the Strauss Bascule Bridge Company (photo credit 5.13)

  Joseph Baermann Strauss was born in Cincinnati in 1870, the son of a noted portrait artist, Raphael Strauss. It was just three years since traffic had begun to move between that city and Covington, Kentucky, across the Ohio River, on the new suspension bridge designed and built by John Roebling. Though Joseph Strauss may not have grown up literally in its shadow the way engineer David Steinman would in the shadow of the Brooklyn Bridge, the dominant role such a bridge plays in the life of a city does not escape young men who want to realize certain kinds of great dreams and be remembered for them. As an engineering student at the University of Cincinnati, Strauss was well aware that his five-foot frame would not allow him to compete on the football field, and he was recalled as having become determined then “to build the biggest thing of its kind that a man could build.” Ambition was apparent in his becoming class president and class poet by the time he finished school in 1892; in his graduation thesis, he proposed the construction of an international railroad bridge across the Bering Strait. Raphael Strauss gave the new graduate “$100 and told him to go out into the world and make it on his own.”

  Armed with this modest stake and his college degree, young Strauss moved to Trenton, the New Jersey town where the John A. Roebling’s Sons Company had been founded in 1849 to manufacture wire rope, and where the Roebling family had become prominent. Strauss took a job as a draftsman with the New Jersey Steel and Iron Company, a bridge-building firm that dated from 1866 and was owned by the philanthropist Peter Cooper’s New York-based Cooper, Hewitt & Company. After two years in Trenton, Strauss accepted an opportunity to return to the University of Cincinnati to teach for a year in the Department of Engineering, at the end of which time he married his college sweetheart, May Van. They moved to Chicago, where Strauss took a position as detailer with the Lassig Bridge and Iron Works. After gaining further experience as inspector, estimator, and designer with the Lassig firm, Strauss joined the Chicago Sanitary District, and advanced from designer to squad boss. In 1899, he became principal assistant engineer in charge in the office of Ralph Modjeski, which had designed many of the lift- and drawbridges over the Chicago River.