BRIDGES

Bridge building as a human activity predates recorded history, and bridges are among the earliest structures described in the historical record. In the fifth century b.c.e. Herodotus reports on a bridge over the Euphrates River made of timber resting on a stone foundation. Roman stone bridges at Segovia (Spain) and N?ˆmes (France) are still standing 2,000 years after their construction. In the Middle Ages, bridge building became the province of specialist monastic orders. Medieval bridges were conceived as places to live, not just as a means of passage from one side of a river to another. London Bridge in 1594 supported 100 houses and shops.

Bridge Engineering

In the nineteenth century, bridge building became a scientific discipline, after a backlash brought about by notorious disasters in which bridges failed to endure mathematically predictable loads. A fascinating 1887 monograph by George L. Vose (1831–1910) reflects the period in which bridge building crystallized into a scientific and mathematical discipline. Vose complained that any charlatan could proclaim himself a bridge builder and find customers, while ignoring the mathematics that made the calculation of safety margins simple. "There is at present in this country absolutely no law, no control, no inspection, which can prevent the building and the use of unsafe bridges" (p. 12). He pointed out that the science of bridge loads was well understood: A dense crowd of people creates a load of up to 140 pounds per square foot, while soldiers walking in step double the strain; snow and ice can create a load of 10 to 20 pounds per square foot, while heavily loaded freight trains can create a strain of 7,000 pounds per square foot.

Vose was a pioneering proponent of safety margins. He argued that bridges should be designed to carry a load four to six times greater than the actual loads they are likely to carry under any foreseeable circumstances. Many existing bridges did not meet these standards; some, in fact, were capable of carrying only the predictable load. Of these, Vose acerbically noted that such a bridge is warranted "to safely bear the load that will break it down" (p. 55). The country, in his estimation, was full of highway bridges "sold by dishonest builders to ignorant officials" and awaiting only "an extra large crowd of people, [or] a company of soldiers" to collapse (p. 16).

According to the structural engineer David P. Billington, however, a second transformation occurred when bridges (along with tall buildings) became uniquely modern works of art by exploiting the properties of new structural materials such as steel and reinforced concrete. In the period after 1880 engineers began "to explore new forms with these materials," the first maturity of which occurred in the period between the two world wars (1983, p. 7). The bridge designs of the Swiss engineer Robert Maillart (1872–1940) are archetypical achievements of this new era.

In the contemporary world the Clifton (Bristol, 1864), Brooklyn (New York, 1883), Golden Gate (San Francisco, 1937), and Tsing Ma (Hong Kong, 1997) Bridges are indeed considered works of art, objects whose function is intertwined with their beauty. For the engineer Henry Petroski "there is no purer form of engineering than bridge building" (1995, p. 14). Whereas houses and buildings are designed for appearance, and then engineered, the process followed in bridge construction is the opposite. A bridge must be designed to perform its function successfully; its beauty emerges from the engineering.

Ethics and Bridges

The ethical issues pertaining to bridges span a range of questions. Is a particular bridge really needed? What impacts do bridges have on the social and natural environments where they are constructed? What levels of safety are appropriate in bridge design?

NEEDS. Insofar as they are major public works projects, the need for bridges has to be obvious and they often have to pass a hurdle of criticism before being constructed. At the same time bridges are sometimes built so that powerful politicians can create jobs and funnel money to their districts, or reward political contributors. According to environmental groups in Alaska, the proposed Gravina Island Bridge is an example. Designed to be 1.6 kilometers long and 24 meters higher than the Brooklyn Bridge, the $200 million structure would link the depressed town of Ketchikan (and its 7,500 residents) to an island that has fifty residents and an airport with six flights a day in the busy season. The island is already well served by ferry, and the bridge would bisect a channel used by shipping and floatplanes.

SOCIAL AND ENVIRONMENTAL IMPACTS. Most people do not want a bridge in their own backyards, with the concomitant loss of views and increases in local traffic, leading to a decrease in property values. Illustrating the NIMBY (not in my backyard) syndrome, even citizens who will benefit prefer that a bridge be sited in someone else's neighborhood. The site originally studied for the George Washington Bridge in New York City was at West 110th Street in Manhattan. Two powerful local institutions, St. Luke's Hospital and Columbia University, strenuously opposed this location. Columbia's president, Nicholas Murray Butler, said that the proposed site was "little short of vandalism" (Petroski 1995, p. 242). The bridge was eventually built (1927–1931) on unused land much further north at West 179th Street.

Robert Moses (1888–1981), the motivating force behind many of New York's best-known bridges and parks, is famous for his ruthless treatment of opponents and of local communities that stood in the way. His beautiful Verrazano-Narrows Bridge, built 1959–1964 with either end in a highly populated neighborhood, caused the seizing and demolition of 800 buildings in Bay Ridge, Brooklyn, displacing 7,000 people. On the Staten Island side, 400 buildings were taken by eminent domain, displacing 3,500 residents.

Moses's determination, and his willingness to counter his opponents in the same visceral language they used to attack him, is evident in a series of monographs issued at his direction. In 1939, when the New York Herald-Tribune opposed his proposed Brooklyn Battery Bridge, Moses had the Triborough Bridge Authority publish a brochure entitled "Is There Any Reason to Suppose They Are Right Now?" It ridiculed the Herald-Tribune, excerpting two decades of editorials opposing previous Moses park and highway projects. Moses painted the newspaper as the voice of millionaires who did not want their neighborhoods tainted by projects that would benefit the common folk.

Another organization opposing the Brooklyn Battery Bridge was the Regional Plan Association, which argued that it was not a natural site for a bridge and would deface the land- and cityscape. In his counterattack, Moses noted that the association had backed a proposal for the construction of a 200-meter obelisk in the Battery, which Moses claimed would obscure the view much more than his proposed bridge. In the end, however, Moses lost the battle, and a tunnel was built in lieu of the bridge. Tunnels are frequently proposed as alternatives to bridge projects; underground, they have the virtue of not being seen, but tend to be more expensive to build and are of necessity narrower, carrying less traffic and freight.

BRIDGE SAFETY. Bridges collapse for one of two reasons. Either their design and construction fail to meet contemporary industry standards, or those standards are inadequate to ensure safety in the face of unexpected circumstances. An example of negligent construction was West Gate Bridge, in Melbourne, Australia, which fell while being erected on October 15, 1970. Thirty-five workers were killed in the collapse. The bridge was being assembled in sections, which were elevated and then bolted to one another. It was discovered that two adjoining sections were not flush with one another as designed; the difference in "camber" was about 3 inches, while the specifications called for a difference of no more than 1 inch. In order to fix the problem, the builders should have lowered the two pieces to the ground again, but this would have caused a delay and a cost overrun, so instead they decided to fix them in place.

They applied a very primitive solution, one of placing 8-ton concrete blocks on the higher span, to push it back into line with the other one. This then caused the steel plates to buckle out of shape by as much as 15 inches. In an ill-fated and foolhardy attempt to eliminate the buckling, the builders decided to remove the bolts holding the steel plates in place. After the first sixteen bolts had been removed, the plates had slipped so much that the remaining bolts were jammed and could not be unscrewed. The workers then tightened each of these until they broke, removing the pieces. Like a man sawing off a tree limb upon which he is sitting, they continued removing bolts, until the entire structure collapsed, killing many of them. A Royal Commission appointed to investigate the disaster concluded that what had happened was "inexcusable" and that the builder's performance "fell far short of ordinary competence" (Royal Commission 1971, p. 97).

An example of a structure that arguably was designed acceptably by contemporary standards, but that fell anyway, was the Tacoma Narrows Bridge (built 1938–1940), popularly known as "Galloping Gertie" because of the alarming way it flailed around under high winds before eventually tearing apart. While most bridge disasters occur when a load crosses the bridge that exceeds its carrying capacity, the Tacoma Narrows Bridge had more than an adequate margin of safety for any traffic load. What the architect had failed to anticipate was that the long and thin bridge had "aerodynamic qualities somewhat like the wing of an aeroplane" (Rastorfer 2000, p. 33). Buffeted by heavy winds on November 7, 1940, the whole span began to twist. Finally, hours after Gertie began its last gallop, the bridge tore itself apart and fell.

Petroski notes that bridge failures follow an approximately thirty-year cycle. A notorious failure leads to the use of a new model, which at first is designed conservatively, but then extended and overextended, until a new failure results, and then a new model emerges. The "high girder" design led to the collapse of the Tay Bridge (Dundee, Scotland, 1879), which resulted in the new cantilevered design, which was responsible for the double collapse (in 1907 and 1916) of the Quebec Bridge, which brought about the suspension model, of which Galloping Gertie was an example. In this sense bridges may illustrate a general dynamic, one society should always take into consideration when attempting to make informed ethical use of science and technology.

JONATHAN WALLACE

SEE ALSO Dams;Water.

BIBLIOGRAPHY

Billington, David P. (1979). Robert Maillart's Bridges. Princeton, NJ: Princeton University Press. Billington has written two other books exploring the work of this famous bridge engineer.

Billington, David P. (1983). The Tower and the Bridge: The New Art of Structural Engineering. New York: Basic.

Kharbanda, O. P., and Jeffrey K. Pinto. (1996). What Made Gertie Gallop? Lessons from Project Failures. New York: Van Nostrand Reinhold.

Mock, Elizabeth B. 1972 (1949). The Architecture of Bridges. New York: Arno Press, published for the Museum of Modern Art.

Petroski, Henry. (1995). Engineers of Dreams: Great Bridge Builders and the Spanning of America. New York: Knopf.

Rastorfer, Darl. (2000). Six Bridges: The Legacy of Othmar H. Ammann. New Haven, CT: Yale University Press.

Royal Commission (E. H. E. Barber, F. B. Bull, and H. Shirley-Smith). (1971). Report of Royal Commission into the Failure of West Gate Bridge. Melbourne, Australia: C. H. Rixon, Government Printer.

Triborough Bridge Authority. (1939). "Is There Any Reason to Suppose They Are Right Now?" New York: Author.

Vose, George L. (1887). Bridge Disasters in America: The Cause and the Remedy. Boston: Lee and Shepard.