Henry Petroski. American Scientist. Volume 86, Issue 6. Nov/Dec 1998.
An era of great bridge building in America ended with the 1930s, when the George Washington, Golden Gate, San Francisco-Oakland Bay and other structurally significant but lesser known bridges were completed. The first two had record-setting main suspension spans of 3,500 and 4,200 feet, respectively, whereas the third, at 75 million Depression dollars, was the most expensive bridge project to date. The decade also saw the completion in 1931 of the Bayonne Bridge, at 1,652 feet then the longest arch bridge in the world, bettering, but only by 2 feet, the more well-known Sydney Harbour Bridge, completed the following year. The Bayonne, which connects New York’s Staten Island with New Jersey, also won the American Institute for Steel Construction’s award for most beautiful bridge, beating out the George Washington in the 1931 competition. (The Bayonne held the record for the world’s largest steel-arch bridge until 1977, when the New River Gorge Bridge near Fayetteville, West Virginia, was completed with a span of 1,700 feet.) The 1930s also marked the completion of some of the most distinctive Oregon Coast bridges (see Engineering May-June 1996) and many other familiar landmarks in the American highway infrastructure.
The era ended in 1940, not coincidentally, with the collapse of the Tacoma Narrows Bridge, whose 2,800-foot main span made it the third largest suspension bridge in the world. Other factors that contributed to the hiatus in bridge building that would last for a decade and a half were the Second World War and the rationing of crucial materials that accompanied it, as well as the fact that many of the most critically needed fixed links for carrying the country’s growing automobile traffic were then in place. Although a bridge crossing between the lower and upper peninsulas of Michigan would have alleviated long lines of cars waiting for ferries, the traffic there comprised mainly hunters and vacationers, and was thus seasonal and not so pressing. Another obvious location for a bridge was at the entrance to New York Harbor, between Brooklyn and Staten Island, but the latter borough of New York City was not very highly populated at the time, and so that bridge also could wait.
The Mackinac Straits Bridge eventually was built, opening in 1957, with special design attention paid to how the deck would behave in the wind. Unlike the Tacoma Narrows Bridge, and the George Washington and Golden Gate bridges whose state of the art it followed, the Mackinac used a very deep deck truss and a gridded roadway portion to allow wind to pass through the bridge rather than wreak havoc on it. The Verrazano-Narrows Bridge across New York Harbor was completed in 1964, with its lower deck in place from the beginning- not because traffic demanded it but as a conservative design measure to stiffen the world-record 4,260-foot central span. Even without the completion of the Verrazano-Narrows, America had all the top-ranking suspension bridges in the world, but it would not retain that distinction for long.
The End of the American Era
When the 4,626-foot Humber Bridge was completed in England in 1981, America lost its claim to having the world’s greatest suspension bridge, at least as measured by length of main span, and it is not likely to recover the title in the foreseeable future. The country’s great rivers, straits and harbors have by and large all been bridged, and, although arguments could be made for additional spans across, say, the Hudson River or San Francisco Bay, the best locations technically have already been developed. Thus new spans would face great design challenges, which often translate into prohibitively expensive solutions. Furthermore, the changed regulatory climate might not allow new spans to be built at all. Conditions may change in the future, but the present climate suggests that it is likely to be a distant future.
The future has already arrived elsewhere, however, and the year 1998 has seen the opening of what are now the two largest suspension bridges in the world. In June, Denmark officially opened its Store Balt Bridge, which crosses the 4.2-milewide Great Belt sound between the islands of Fyn and Sjolland, where Copenhagen is located. Since Fyn was already connected across the Lille Balt (Little Belt) to Jutland, Copenhagen is finally linked to the mainland. It will also be connected to Sweden in a few years, when the Oresund Crossing-a 10-mile-long fixed link consisting of a tunnel, an artificial island and bridge sectionsis completed. Part of the Store Balt Bridge is, with a 5,328-foot-long main span, the record-holding suspension bridge in Europe. But it is not the longest in the world, because the bridge holding that distinction was opened in Japan only months earlier. The Store Belt’s 8,838-foot total suspended span between anchorages would also have made it the longest in the world by that measure, a distinction held for four decades by the Mackinac Straits Bridge, but this too was not to be.
Akashi-Kaikyo Bridge, which opened in April near Kobe, is likely to be the longest suspension bridge for some time. The span across the Akashi Strait is only one part of the massive HonshuShikoku Bridge Project, which links the islands of Honshu, on which Kobe is located, and Shikoku by way of the small island of Awaji. The towers of the bridge were completed when the 1995 earthquake hit, and it left them a couple of feet farther apart but otherwise undamaged. Since the roadway had not yet been hung from the cables, its components were redesigned to fit properly into the main span, which is 6,529 feet between towers. The 12,828-foot length of the total suspended span means that drivers travel almost 2- 1/2 miles between the 350,000-ton anchorages. The bridge has been designed to withstand an earthquake of magnitude 8.5, and its survival of the 7.5 Kobe earthquake, albeit while still under construction, provides some confirmation of the design’s integrity but also raises serious questions about the wisdom of building colossal structures in earthquake zones.
Future Fixed Links
Such experience may have severe implications for the design of a fixed link in another seismic zone-across the Messina Strait between the Italian peninsula and Sicily, a project that was discussed at least as early as 1870, when a rail tunnel was proposed. An international competition was held in 1969 to encourage conceptual designs, and more than 150 entries were submitted, about 90 percent of them from Italian engineers and firms. Among the schemes to cross the strait between the mythological Scylla and Charybdis, those taken most seriously were tunnels and suspension bridges. Tunnels, however, are costly and take a long time to construct, so the bridge proposals continue to attract the most attention. One design calls for a bridge with a 9,500-foot main span, necessitated by the tower foundations sitting on the shores. The Messina Strait Bridge design would also be three times as long as the longest previous suspension bridge designed to carry railroad traffic, the Tagus River Bridge in Lisbon, Portugal, showing further the uncharted waters into which such daring projects enter.
The deck design for the potential record-setting span is of the wing-like box-girder type favored by the British. This is not surprising since among the structural consultants to the Italian state-owned concessionaire was William Brown, who for many years was with the firm of Freeman Fox and Partners, which developed the girder design for its Severn and Humber Bridges in England and copied it on such other projects as the Bosporus Bridge in Istanbul. (The AkashiKaikyo Bridge, on the other hand, adopted the American deck-stiffening structure characterized by such familiar bridges as the Golden Gate and the Verrazano-Narrows.)
With structures as far beyond experience as the proposed Messina Strait design, unusual features sometimes have to be adopted. For example, with the large torsional deflections possible in such a long bridge span, railroad tracks must be located along the centerline. The outside vehicle roadways are designed so that traffic approaching the bridge in the continental European mode of driving on the right will be repositioned to proceed across the bridge on the left side, in the British way The rerouting was also proposed so that vehicles would move in the same direction as adjacent rail traffic so as to minimize wind effects from passing trains. Vehicles would, of course, have to be rerouted again after they leave the bridge. Although such an arrangement is clearly unorthodox, whether this or any bridge across the Messina Strait will be built soon will depend, not surprisingly, on the political and economic climate in Italy when the question next surfaces. One for the Rock
A much more ambitious proposal is for a fixed link across the Strait of Gibraltar, an intercontinental link that is certain to be even more politically complex to bring to fruition than was the Channel Tunnel. However, the economic and social implications of uniting Africa and Europe across the entrance to the Mediterranean led the United Nations, through UNESCO, to initiate in the late 1970s studies for a Strait of Gibraltar crossing. An interim report prepared under the auspices of the separate economic commissions for Africa and Europe, with the cooperation of the governments of Morocco and Spain, was transmitted to the U.N.’s Economic and Social Council in the mid-1980s, and it summarized the findings of consultants to the study and the status of ongoing studies. International congresses were held to discuss the various aspects of such an unprecedented project, and discussion continues.
As with large engineering projects generally, there are endless possibilities for the location and design of a link to cross the Strait of Gibraltar, but experience and engineering judgment have narrowed the field to a few realistic choices to be studied in some detail and compared. In the case of a Gibraltar crossing, this task fell to the lead consultant, Freeman Fox and Partners, who identified three technically and geographically sensible locations, ranging from a 9-mile route between Spain and Morocco, passing over water as deep as 3,000 feet, to a 27-mile route over the more manageable water depths of 1,150 feet. In the final analysis, both of these routes were rejected in favor of one that covers a distance of 19 miles but at depths not exceeding 1,150 feet. This still presents enormous challenges well beyond firm engineering precedent for either a tunnel, floating bridge or fixed-support bridge-or a combination of these structures. Since it is recognized that technological advancement in the state of the art of construction proceeds even as studies are carried out, it has been recommended that the final choice of structure not be made until the decision to go ahead with the project is at hand.
Since engineering, like science, progresses through peer review, the report of Freeman Fox was reviewed by an equally prestigious consultant, the Bechtel Corporation, which in turn engaged the world-class bridge-designing firm of T. Y. Lin International to study the feasibility of constructing a bridge or bridges across the strait. Because of the extreme depths of the water and the high volume of shipping at the location, a clear objective was to have as few piers as practicable, which obviously meant having the longest possible bridge spans. This led Lin to propose a bridge across the 9-mile route that would have only three piers, but in water as deep as 1,500 feet, and have spans as large as 16,000 feet. Although well beyond the length of the main span of the Akashi-Kaikyo Bridge, the generally agreed-on feasibility of an almost 10,000-foot suspended span across the Messina Strait supported the hybrid design put forth by Lin.
The design combines features of suspension and cantilever bridges, with the suspended span being about 10,000 feet long and the cantilever arms reaching out about 3,300 feet each. The design is reminiscent of the proposal for an ungainly cantilever-suspension bridge (see Engineering, March-April 1991) that nonetheless enabled Joseph Strauss to have formed a Golden Gate Bridge and Highway District, a crucial step in getting that bridge built. The functional design evolved into what many consider still to be the most beautiful bridge in the world. Lin also proposed as an alternative design for Gibraltar a combination suspension and cable-stayed bridge but recognized, as Strauss eventually did, that “the aesthetics of this system can stand some improvement.” The fundamental thing in the conceptual design stage is to establish feasibility within, or even reasonably and defensibly beyond, the state of the art of bridge building.
As an alternative to a bridge, a tunnel beneath the Strait of Gibraltar was proposed as early as 1869, the year the Suez Canal was completed. Large projects have always emboldened engineers to propose still larger projects, and in recent years the completion of the Channel Tunnel has renewed interest in a tunnel scheme between Spain and Morocco. Exploratory borings have been made in Morocco, to depths of the order of 1,000 feet. The clay found is common to the two coastlines, but what lies in a tunnel’s path under the strait cannot be known with absolute certainty until the actual tunnel is bored. Likewise, even though the technology of offshore oil platforms may be adopted by bridge builders to set piers in unprecedented depths of water, what surprises might be in store at those depths will remain unknown until construction actually begins. Such uncertainties, not to mention the multi-billiondollar cost estimates, are likely to escalate as the realities of a unique construction project reveal themselves, as they did with the Channel Tunnel, keeping a fixed crossing of the Strait of Gibraltar on the drawing board.
Closing the Cold War Gap
T.Y. Lin may have made proposals for a bridge across the warm waters of the Mediterranean as a subconsultant, but his involvement with a projected bridge across the frigid, ice-laden waters of the Bering Strait has been as a leader. The idea for a fixed link between Alaska and Siberia arose when the Alaskan territory was about to be acquired by the U.S. Because the Western Union Telegraph Company did not believe a transatlantic cable was practical, it dispatched survey and construction crews to find an overland route for a 5,000-mile telegraph line between the U.S. and Siberia, which would have required the construction of a bridge over the Bering Strait. The plan was set aside when, in 1866, Cyrus Field did demonstrate the feasibility of a transatlantic cable, but the ascendancy of the railroad also provided a continuing impetus for pursuing the bridge idea.
A “cosmopolitan railway,” which had the object of “compacting and fusing together all the world’s continents,” was the subject of an 1890 book by William Gilpin, the first territorial governor of Colorado. That same year, on the occasion of the opening of the Forth Bridge in Scotland, a souvenir program showed a locomotive labeled “Progress” pulling a passenger coach whose route was identified as “Aberdeen to New York, via Tay Bridge, Forth Bridge, Channel Tunnel, and Alaska.” A young Joseph Strauss, long before he would become the mastermind and chief engineer of the Golden Gate Bridge, proposed an international railroad bridge across the Bering Strait in his 1892 University of Cincinnati graduation thesis.
The 1887 decision to extend the Russian railway system into Siberia and the further step, in 1891, to begin construction of a Trans-Siberian Railway (finally completed a century later) encouraged the idea of a bridge. On the other side of the strait, the discovery of gold in Alaska made a fixed crossing even more desirable, as did the continuing development of the railroads. Yet 50 years later, the route that is believed once to have held a land bridge by which people, plants and animals migrated to the North American continent still remained water locked. Enthusiasm for the idea was reawakened during the Second World War, when the U.S. and the Soviet Union were allies, but the Cold War that subsequently developed cooled people to the prospect of a link across the international date line and through the Iron Curtain.
It was in 1958, the year after Sputnik accelerated competition between the superpowers, that bridging the Bering Strait again came to the fore. Lin, in conjunction with Washington Senator Warren Magnuson, who was chair of the Senate Committee on Commerce, suggested a bridge across the Bering Strait “to foster commerce and understanding between the United States and the Soviet Union.” (Shortly thereafter, Russian engineers suggested a dam be built across the strait to regulate the flow of water between the Arctic and the Pacific oceans and thus control weather, but there was disagreement as to which way the water should be pumped.) A decade later, Lin organized Intercontinental Peace Bridge, a charitable corporation created “to initiate and perpetuate efforts toward the eventual building of this hemispheric link.” As just about every other major bridge proposal has had its tunnel counterpart, and vice versa, so the Bering Strait Bridge effort has the Bering Strait Tunnel & Railroad Group, headed by the engineer George Koumal, who in the late 1980s “decided to pursue the concept with a vengeance.”
Although the successful completion of the Channel Tunnel renewed interest in massive projects, the cost overruns and tenuous financial condition of the Chunnel have made it more difficult for tunnel and bridge schemes alike to raise the capital, even if they did have full political authorization to go ahead with detailed design and construction. This is not to say promotion does not proceed. Yet in order to promote a bridge, say, one does have to have more than a fuzzy idea-one has to have workable conceptual designs, at least. In the case of Lin’s Peace Bridge, the relatively shallow depth of the water-180 feet maximum over the 50-mile route-places the project well within design and construction experience. Precedent, or at least arguable models, exist in the 25-mile long bridge across Lake Ponchartrain near New Orleans, and the more recently opened 7-mile-long Confederation Bridge across the ice-forming Northumberland Strait (see Engineering, January-February 1997), but special attention has to be paid to ocean-going shipping, significant ice forces and protection of bridge traffic from the elements of wind, snow, ice and cold. Lin’s design, for example, has the railroad trains running inside a huge box girder, reminiscent of the scheme used in the Britannia Tubular Bridge (see Engineering, May-June 1992) thus shielding them from the elements.
Whether some of these dream bridges will be realized within the foreseeable future depends at least as much on political and economic conditions as on technical details. In the meantime, bridge watchers will have to content themselves with more down-to-earth projects, such as the replacement of the East Bay crossing of the San Francisco-Oakland Bay Bridge. This is the portion of the bridge that suffered damage in the 1989 earthquake. Subsequent reassessment of the bridge led to a realization that retrofitting the structure to make it earthquake tolerant not only would cost on the order of a billion dollars but also would make the bridge even less aesthetically pleasing, especially compared to its near neighbor across the Golden Gate. When it was established that a cornpletely new structure could be had for about the same amount of money, that became the way the Bay Area wished to go. Among the proposals was one employing “an array of horizontally tensione cables to support a lightweight road deck” and thought to be capable of spanning in excess of 2 miles. However, the panel of engineers and architects who reviewed the more than 20 design options in the end chose a single-towered suspension bridge, a design proposed for other locations in the mid-19th century, suggesting that even the technical community is not always fully sure of how far or how fast it can or wants to go.