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

by Henry Petroski

  With regard to the super-structure, I do not pretend to be qualified to analyze and check the design of the long span suspension bridge, but I have studied this design in connection with the designs of other bridges, which have been successfully erected, and are in successful operation. I also have great confidence in the ability and integrity of the Consulting Engineers under whose direction the computations and design drawings for the super-structure have been made. Moreover, these engineers have earned a very enviable reputation as experts in this field, as evidenced by the commendation from other suspension bridge experts which they have received in technical publications.

  I therefore, [sic] feel that with the exception of the unusual narrowness of this bridge with reference to its span length, the super-structure design is technically sound. It is probably technically sound notwithstanding its narrowness, but there are several reasons why it would be of material advantage if the bridge could be widened at a reasonable increase in the cost, and therefore, I recommend that serious consideration be given to the possible increase in the width of this structure, before the contract is let or work begun.

  To Condron, the extreme narrowness of the deck of the Tacoma Narrows design forced him to conclude that “it would be advisable to widen the super-structure to 52 ft.,” which would give the bridge a width-to-span ratio of 1:53.8—still very narrow, but a less radical departure from experience. Had Condron’s recommendation been followed, it is very possible that the Tacoma Narrows Bridge would have been stiffened enough that, even had it exhibited some degree of flexibility in the wind, that might have been within tolerable limits and thus subsequently correctable, as it was to be in other contemporary bridges. Even if the course of suspension-bridge development had gone that way, however, this is not to say that some subsequent slender-bridge design would not have been proposed and approved without the reservations of so conscientious and perceptive an advisory engineer as Theodore Condron.

  Condron could not have made his case more rationally or emphatically, unless perhaps he had appealed to the experience earlier that year of Russell Cone, resident engineer of the Golden Gate Bridge, who had observed not only horizontal but also vertical deflections of that span. According to Cone, during a windstorm on February 9, 1938, “the Bridge was undulating vertically in a wavelike motion of considerable magnitude.” He went back to his office to get his camera and record the motion that “appeared to be a running wave similar to that made by cracking a whip,” but when he returned that motion had stopped, and soon the wind died down. Neither Condron nor the board of consulting engineers, however, seems to have been aware of or excessively concerned about the behavior of the Golden Gate Bridge at the time the Tacoma Narrows was being designed. In any event, Condron’s warning about the width of the Tacoma Narrows Bridge was not heeded, and the report of the consulting engineers prevailed:

  It might seem to those who are not experienced in suspension bridge design that the proposed 2800-foot span with a distance between stiffening trusses [girders] of 39’ and a corresponding width of [sic] span ratio of 72, being without precedent, is somewhat excessive. In our opinion this feature of the design should give no concern.

  The board emphasized its conclusion by asserting that it believed the span could even be “materially increased if it were necessary, keeping the same width without any detrimental effect.” With such an endorsement, the Toll Bridge Authority received a loan for about $3 million and a grant of a like amount from Pierce County. Construction bids were received by October 1938, and the bridge was completed less than two years later.

  The Tacoma Narrows Bridge executing its fatal oscillations in November 1940 (photo credit 5.22)

  Even before the bridge was completed, however, engineers were surprised by its large movements; these were being studied on a model at the University of Washington, by Professor F. B. Farquharson, when a new twist developed in November. Until that time, the bridge deck had moved up and down in waves, and various checking cables and devices had been applied to it, as they had to Ammann’s Bronx-Whitestone and David Steinman’s Deer Isle bridges. However, on November 7, 1940, the clamps holding one of the checking cables at center span slipped, and the bridge began to move in a new way, twisting about its centerline in a wind of about forty miles per hour. The motion became so severe that the bridge was closed to traffic, and Farquharson went to see what was happening.

  Camera equipment from a nearby shop was taken to the bridge, and so its twisting through a total of nearly ninety degrees was caught on the most famous film footage in structural-engineering history. A reporter’s car was the only vehicle on the bridge, abandoned when it could not be controlled, and only Farquharson, the reporter, and his dog felt the full heaving of the steel-and-concrete deck. An attempt to get the dog out of the car was also abandoned in the increasingly violent motion, and the reporter and Farquharson were captured on film crawling, staggering, and climbing back toward the bridge tower and terra firma. Farquharson, apparently knowing more about structural vibrations than the reporter, walked along the centreline of the bridge, which as a nodal line was almost motionless, while the reporter fought along the heaving curbline. Not long after they reached safety, the bridge deck twisted itself apart and fell into the water. The motion had been so violent that the massive steel towers were permanently bent out of shape and had to be dismantled before a replacement bridge could be built—with very deep trusswork providing not a terribly slender profile but a very stiff deck.

  The collapse of the Tacoma Narrows Bridge revealed a classic case of hubris, for the success of bridges like the George Washington and its close antecedents and descendants had given the coterie of major suspension-bridge engineers almost unlimited confidence and license in their designs, even as these were beginning to sway and wave in the wind. Because the new breed of engineers believed they were calculating, with the deflection theory, stresses and strains more accurately than nineteenth-century engineers like Telford and Roebling, their classic works were conveniently taken as aesthetic rather than structural models. The new field of aerodynamics, which was being applied to the development of the airplane in the 1930s, was seen to be largely irrelevant to designing and analyzing generally static structures like bridges.

  There was, however, at least one civil engineer in the mid-1930s who “felt an obligation to make available to the civil engineering profession” the results of tests and theoretical studies being carried out by aeronautical engineers at that time. W. Watters Pagon knew, for example, that the principle of the wind tunnel was valid, because a powered structure flying through the quiescent air is equivalent to wind blowing over a stationary body, and in 1934 and 1935 he had published a series of eight articles on aerodynamics in Engineering News-Record, in which he discussed wind forces and their action on structures. The first article, entitled “What Aerodynamics Can Teach the Civil Engineer,” opened with a recitation of how much was unknown about how structures behaved in the wind, including why a building had twisted in the recent Miami hurricane, but the whole series seems largely to have been ignored by the bridge builders. Only after the collapse of the Tacoma Narrows Bridge were Pagon’s articles described as “must reading.” Such a turnabout was prompted in no small measure by a letter that appeared in Engineering News-Record shortly after the bridge collapse. The letter, from Theodore von Kármán, director of the Daniel Guggenheim Aeronautical Laboratory at the California Institute of Technology, presented a very concise and convincing mathematical analogy between the twisting of an airplane wing and that of a bridge deck in the wind.

  Although half a century later von Kármán would be identified on a commemorative U.S. postage stamp as an aerospace scientist, no doubt in part for his efforts to advance rocketry from “an eccentric study into a reputable discipline,” his training and background were in engineering. Von Kármán was born in Hungary in 1881, and received a mechanical-engineering degree from the Budapest Royal Technical University in 1902. After a year of mi
litary service, he returned to Budapest to teach for a while, but left before long to take a position as a mechanical engineer with a machinery manufacturer. Two years later, he went to Berlin to study mechanics at the University of Göttingen, from which he received his Ph.D. in 1908. He became prominent in Europe in the newly established field of aeronautics, and in the late 1920s divided his time between the University of Aachen, in Germany, and Caltech, in Pasadena. In 1930, he accepted the position of head of the Guggenheim Laboratory and moved permanently to the United States, where he came to lead the country’s first jet-propulsion and rocket-motor program. Von Kármán was in the process of establishing a model supersonic wind tunnel at Caltech when the Tacoma Narrows Bridge collapsed.

  Von Kármán was one of three engineers appointed by the Federal Works Agency to investigate the failure of the Tacoma Narrows Bridge. He was joined by Glenn B. Woodruff, the consulting engineer from San Francisco who had been the engineer of design for the San Francisco-Oakland Bay Bridge, and, not surprisingly, Ammann, who had, of course, dominated suspension-bridge design and had investigated the failure of the Quebec cantilever bridge at the beginning of his career. The committee’s report, issued less than five months after the collapse, concluded that “the Tacoma Narrows Bridge was well designed and built to resist safely all static forces, including wind, usually considered in the design of similar structures.” In other words, the constant sideways push of the wind had been taken into account in the standard way for engineers of the time, according to what is known as the state of the art, and there was no blame to be placed on them. They were simply taken by surprise by the structure’s “excessive oscillations,” made possible by the “extraordinary degree of flexibility.” Ignorance, and not incompetence, was to blame: “It was not realized that the aerodynamic forces which had proven disastrous in the past to much lighter and shorter flexible suspension bridges would affect a structure of such magnitude as the Tacoma Narrows Bridge.”

  That the report read as it did should perhaps not have been surprising, given the composition of the board of engineers, but their relationship and conclusions must have evolved over the months they worked together. Von Kármán was somewhat of a maverick, a confirmed bachelor who seemed as likely to be found posing with a buxom blonde or a world leader as with a wind tunnel, if his bombastic autobiography published a quarter-century later is a fair representation of the man. In the book, written “with” a freelance writer in the manner of a celebrity, von Kármán related how he followed the news reports of the Tacoma Narrows collapse, only to be startled by a news item the following day reporting that the governor of Washington had announced that “the bridge was built correctly and that a new one would be built according to the same basic design.” That evening, the engineer “took home from Cal Tech a small rubber model of the bridge” that one of his mechanics had made for him and demonstrated in his living room with an electric fan and the model an “instability which grew greater when the oscillation coincided with the rhythm of the air movement from the fan.” As he had suspected, “the villain was the Kármán vortices,” or the whirlpools of air, named after the investigator himself, that were shed in the wake behind the moving model and thus buffeted it. Von Kármán wrote to the governor, to Farquharson, and to Engineering News-Record about his discoveries and concerns, an initiative that could not have hindered his being placed on the investigatory board.

  In von Kármán’s recollection of board meetings during the investigation, he mentioned that he was surprised at the “long standing of the prejudices of the bridge engineers,” as embodied in their consideration of static as opposed to dynamic forces and their difficulty in seeing how “a science applied to a small unstable thing like an airplane wing could also be applied to a huge, solid, nonflying structure like a bridge.” This all led to “some definite undercurrents of rivalry”; Ammann was portrayed as especially reluctant to accept such suggestions as wind-tunnel testing of bridge designs.

  In the final analysis, von Kármán may have thought it best to let the bridge engineers worry about bridges, for which they were paid. He admitted that they had won him over on one “difference in thinking” between them and him. Though he was prepared to serve for his standard government consulting fee of fifty dollars a day, the other engineers “bargained for a sizeable percentage of the value of the bridge, which after all was insured for six million dollars.” This indicated to him that aeronautical engineers acted in consulting positions as if they were only “elevated laborers,” and he “learned a good deal in a nonengineering way from this experience” at the bargaining table from the less flamboyant if not outwardly shy bridge engineers.

  Ammann’s thoughts during the investigation could not have been very far from his George Washington and Bronx-Whitestone bridges, to whose design Moisseiff had contributed so much. Woodruff, who had also been associated with Moisseiff in conjunction with the design of the San Francisco-Oakland Bay Bridge, must not have been predisposed to believe that the engineering of the Tacoma Narrows was faulty. In fact, five years earlier, as part of an issue of Civil Engineering focusing on that project, Woodruff had written a short article, “From the Viewpoint of the Bridge Designer,” in which he spelled out the advantages bridge designers then had over their predecessors. However, in spite of the “more complete theory, an immense amount of experimental data, and more reliable materials, as well as the accumulated experience of past years,” he warned that there was the danger that bridge design would come to be considered routine.

  At the same time, Woodruff wrote, as if anticipating von Kármán’s amazement and frustrations at the meetings that would occur in December 1940 in Seattle, bridge building was becoming so highly specialized that there was the “danger of losing contact with the other branches of engineering and with allied sciences.” This all would seem to have predisposed Woodruff to be more of a finger-pointer when it came to blaming engineers, but he did not do so. Rather, he closed his article with a quote from another engineer: “The most perfect system of rules to insure success must be interpreted upon the broad grounds of professional intelligence and common sense.” That these were the words of none other than Theodore Cooper, whose own intelligence and common sense were seriously called into question in the wake of the collapse of his Quebec Bridge in 1907, suggests that Woodruff was not one to hold engineers culpable for not foreseeing problems of an extraordinary kind.

  Though Ammann and Woodruff believed that intelligence and common sense required the designer to “analyze all the assumptions made, estimate the possible errors in them, and also make a careful study of the properties of materials to be employed,” they also appear to have believed that doing this to the best of the designer’s ability satisfied his obligation and freed him of any guilt. Doing all one knew to do was, after all, the best that could be expected of an engineer of bridges or of rockets. It is very likely that, before the final report was drafted by Ammann, the rocket scientist von Kármán came to see this point of view of the bridge engineers.

  If the engineer Moisseiff, along with the profession of engineering, was clearly exonerated by his colleagues, the precise physical causes of the motion of the bridge and the final disaster were left somewhat ambiguous and vague by the failure report. Vertical oscillations of the bridge were “probably induced by the turbulent character of wind action,” but there was “no convincing evidence” that such vertical motions were unstable or even dangerous to the bridge. It was “reasonably certain” that a cable band that had been installed to check some of the deck motion had slipped, and this “probably initiated the torsional oscillations,” which brought the span down. Among the more general conclusions of the report were, not surprisingly, that “further experiments and analytical studies are desirable to investigate the action of aerodynamic forces on suspension bridges.” The report also concluded, however, that, “pending the results of further investigations, there is no doubt that sufficient knowledge and experience exists to permit the sa
fe design of a suspension bridge of any practicable span,” without reference to how wide such a span might be. Such a conclusion might have been subject to ridicule in less turbulent times.

  The onset of World War II would no doubt have interrupted bridge building much the way World War I did even if the Tacoma Narrows collapse had not occurred. In any case, there was less urgency to following up on the report than there might have been. Many unanswered questions about the aerodynamic behavior of bridges remained, however, and it fell largely to Professor Farquharson, in the Structural Research Laboratory of the Department of Civil Engineering at the University of Washington, to continue throughout the 1940s to work on and pull together the results of laboratory and mathematical studies on the stability of suspension bridges in the wind conducted principally at his institution and at Caltech. Farquharson’s work was sponsored mainly by the Washington Toll Bridge Authority, which needed to replace the bridge that had collapsed, in cooperation with the Public Roads Administration of the Federal Works Agency. The Authority’s consulting board included Woodruff and von Kármán, who was identified as “aerodynamicist” and who would on occasion identify himself as “representing the wind.” Ammann, as a consulting engineer to the Port of New York Authority, represented that body on the Advisory Board on the Investigation of Suspension Bridges.

  Whereas Ammann’s selective use of history had been symptomatic of the myopia that characterized suspension-bridge building in the 1930s, Farquharson’s report opened with a broad, inclusive historical survey of the dynamic behavior of suspension bridges. Farquharson began this survey by noting that the collapse of the Tacoma Narrows Bridge “came as such a shock to the engineering profession that it is surprising to most to learn that failure under the action of wind was not without precedent in the history of suspension bridges.” He then proceeded to describe trends and recount the main events of that history, which he summarized in a table that listed “bridges severely damaged or destroyed by wind” between 1818 and 1889, plus the 1940 Tacoma Narrows disaster. After that last collapse, “much old information long forgotten was once again made available to the profession.”