Bridges have been essential to America's growth, and countless types were devised to carry highways, railroads, and even canals. Location, materials, cost, traffic, and the ingenuity and creativity of bridge engineers all have influenced the evolution of American bridge technology.
The Colonies and the Early Republic
Large-span bridge building in North America began with the Charles River Bridge at Cambridge, Massachusetts, in 1662. Its pile and beam construction was not unlike that used for centuries in Europe. The design placed heavy timber beams across piles that were hand driven into the riverbed. Side members were then tied together by cross beams, and wood decking was attached to stringers running parallel to the sides. Spans built this way were limited by the length of available timber and the depth of the water.
A more versatile bridge form, the truss, first came into use in the United States during the late eighteenth century. Trusses composed of a series of triangles were assembled from short lengths of timber that, depending upon their location, resisted the forces of either compression or tension. Because of the way in which it was assembled, the truss bridge could be lengthened to span distances far greater than the simple pile and beam bridge. Structural integrity in these bridges came from a balance of the opposing forces inherent in their construction. Because these spans were not self-supporting during construction, however, they were built on false work or framing that was later removed. As with all bridges regardless of type, they had to be designed to support their own weight or dead load, as well as the moving weight or live load that passed over them.
During the first two decades of the nineteenth century, Theodore Burr of Connecticut was one of the best-known American bridge builders. His 1817 patent for a combination arch and truss design was widely used in covered bridges. He had erected some forty-five highway spans in New York, New Jersey, and Pennsylvania by the time of his death in 1822. The wooden walls and roofs that were typical of covered bridges were necessary to protect the truss's countless wooden joints from the ravages of the weather.
By far the most lasting bridges built in the eighteenth and early nineteenth centuries were those made of masonry, but masonry construction was expensive and a shortage of qualified masons in the early Republic limited the number that were constructed. Those bridges were most often built in the form of an arch and assembly proceeded on timber falsework, for these bridges were self-supporting only after the last stone was put in place. When America's first railroads began laying out their routes in the 1820s and 1830s, their bridges were apt to be of stone, since once erected such bridges required little attention and were highly durable. As the pace of railroad expansion quickened toward the mid-nineteenth century, however, the need for bridges burgeoned and permanency was abandoned for expediency. The prevailing attitude was that quickly assembled and even temporary timber bridges and trestles could be replaced at some later date with more lasting structures once a rail line was producing revenue. The railroads' need for quickly constructed spans spurred the development of the truss bridge, constructed primarily of wood, throughout the first half of the nineteenth century.
The Expanding Nation
Individual types of truss bridges can be identified by the way their members were assembled. A large number of designs were patented during the nineteenth century. Some trusses were of no practical value, while others were over-engineered or too expensive to build. Those popular during the nineteenth century did not necessarily find similar acceptance in the twentieth century. In time, the broad range of trusses was gradually reduced to a few basic types that proved to be the strongest and most economical to build. By the early twentieth century, the Pratt and Warren were the most commonly used trusses and into the 1920s the truss was the most common bridge type in America.
As the railroads used ever heavier and faster rolling stock, it was necessary to replace wooden bridges with heavier and sturdier construction. Beginning in the 1840s, cast and wrought iron were being substituted for wood members in some bridges and during the 1850s railroads began turning to bridges made entirely of iron. During the 1870s, steel production increased greatly and the price fell to levels that made it reasonable for use in bridges. By 1930, the expansion of American railroads was over and their influence on the structural development of bridges was at an end.
Two significant advances in bridge technology took place in the late 1860s and early 1870s with the construction of the long-span metal arch bridge across the Mississippi River at St. Louis. First, not only was this bridge the first major spanning of North America's largest river, but engineer James B. Eads specified the use of steel in the bridge's arch members. The three tubular arches rested on masonry piers built on wooden caissons sunk in the riverbed. Second, this was the initial use in the United States of the technique in which excavation work inside a caisson took place in an atmosphere of compressed air. Prior to that workers had labored under water or in areas where water was diverted in some way. Air pressure within the caisson equaled the force exerted by the river water outside and the shell did not flood as excavation work inside progressed down toward bedrock. This technology was crucial in the successful execution of all subsequent subaqueous foundation work.
Limitations imposed by location have forced bridge builders to be innovative. It would be impractical, if not impossible, to erect a bridge across wide, deep ravines if the bridge required the support of extensive false work during construction. As a result, a method evolved that avoided the use of staging. In October 1876, engineer Charles Shaler Smith embarked on the construction of the first modern cantilever railroad span to bridge the 1,200-foot-wide and 275-foot-deep valley of the Kentucky River. Smith refined a bridge-building technique little used outside of ancient China. Cantilever construction employed counter balancing forces so that completed segments supported ongoing work as it progressed inward toward the span's midpoint.
Although the suspension bridge was not new in 1842, its future form was forecast when Charles Ellet's Philadelphia wire suspension bridge was opened to highway traffic that year. Suspension bridges in which the roadway or deck was suspended from heavy wrought iron chains had been built for years. For the first time in a major American span, the deck was suspended from relatively lightweight wire cables. Ellet used a European cable-making technique in which cables were composed of a number of small-diameter parallel wires. The shape of each cable was maintained and its interior protected when the bundle's exterior was wrapped with additional wire. The scale of the bridges that followed increased tremendously, but the basic technology for cable making remained the same.
Civil engineer John A. Roebling was the preeminent suspension bridge designer of the nineteenth century. His career began with a suspension canal aqueduct at Pittsburgh in the 1840s, and each of his following projects reflected a growing skill and daring. His combined railroad and highway-carrying suspension bridge across the Niagara River gorge was completed in 1855. In it a suspended double-deck wooden truss carried the two roadbeds. Although other types of bridges would be built to carry highway and urban rail systems, this was the lone example of a suspension bridge constructed to carry both. The overall design and appearance of his Ohio River suspension bridge at Cincinnati in 1867 foretold of his plans for New York City's even larger and monumental Brooklyn Bridge of 1883. The extensive use of steel throughout the bridge, and especially for its cables, was a watershed in bridge technology.
The Early Twentieth Century
A number of large suspension bridges were built during the first half of the twentieth century. They were ideal for spanning the broad waterways that seemed to stand in the way of the growth of modern America. Neither their construction nor their final form posed an impediment to the nation's busy waterways. During the first four decades of the century, New York City was the focus of much of that construction. The city's boroughs were joined by the Williamsburg (1903), Manhattan (1909), Triborough (1936), and Bronx-Whitestone (1939) suspension bridges. However, none compared in size to the magnificent George Washington Bridge, completed in 1931. It was the first bridge linking Manhattan and New Jersey, and represented a remarkable leap forward in scale. Its 3,500-foot-longsuspended span was double the length of the next largest. The bridge's four massive suspension cables passed over towers soaring more than 600 feet high and its roadway was suspended 250 feet above the Hudson River. It was never truly completed, as the masonry facing called for on each of its steel towers was omitted because of the Great Depression.
The federal government responded to the depression by funding many massive public works projects. Partially as a form of unemployment relief, San Francisco undertook the construction of two great suspension bridges. They spanned greater distances than any previously built bridges. Strong Pacific Ocean currents and the depth of the water in San Francisco Bay made the construction of both the San Francisco–Oakland Bridge (1936) and the Golden Gate Bridge (1937) particularly challenging, and no part of the project required more technical expertise than building the subaqueous tower piers.
Triumphs in bridge building have been tempered by failures and perhaps no span has received more notoriety because of its collapse than did the Tacoma Narrows Suspension Bridge across Puget Sound in Washington State. Beginning in the late 1930s, its construction progressed uneventfully until the bridge was completed in July 1940. The valley in which the bridge was built was subject to strong winds and gusts that set the bridge in motion even while it was under construction. These wind-induced undulations increased in frequency as the bridge neared completion. So noticeable were the span's movements that they earned the bridge the sobriquet Galloping Gertie. Several months after its opening, the bridge was subjected to a period of intense high winds, during which it literally tore itself apart. The rising, falling, and twisting of the deck was so violent that it broke loose from its suspender cables and crashed into the sound. It was later determined that the failure resulted from the bridge being too flexible. The narrow deck and the shallow profile of the steel girders supporting the deck provided little resistance to aerodynamic action. The bridge's collapse prompted a reevaluation of suspension bridge design and resulted in a move away from flexible designs toward much stiffer and wind-resistant construction. A redesigned span across Puget Sound was completed in 1950.
Of the few suspension spans built in the United States after the middle of the twentieth century, one of the more remarkable was the Verrazano-Narrows Bridge, which opened in November 1964. The bridge, situated across the entrance to New York Harbor, connected Staten Island to Brooklyn. It was designed by engineer Othmar H. Ammann, who during his career designed a number of New York City's bridges, including the George Washington Bridge. While no new techniques were introduced in its construction, the bridge is remembered for it huge overall dimensions and the unprecedented size of its individual parts as well as the speed—five years—with which it was erected.
The Late Twentieth Century
Although the first cable stay bridges appeared in seventeenth-century Europe, this type of bridge emerged in a rationalized form only during the 1950s. Their con-figuration may vary in appearance and in the complexity of the tower or towers as well as in the symmetry and placement of the cables. The most recognizable spans are characterized by a single tower or mast and multiple diagonal cables that, if arranged in a single vertical plane, pass over the tower and are affixed at opposite points along the center line of the deck. Decks can be assembled in cantilever fashion from sections of pre-cast, pre-stressed concrete. They offer many of the advantages of suspension bridges, yet require neither the lengthy and costly process of cable spinning nor large cable anchorages. Their overall load-bearing capacity is less than the more complex suspension bridge. One of the most notable American examples is the Sunshine Skyway Bridge completed across Tampa Bay, Florida, in 1987.
With the expansion of American railroads nearing its end, highways became a major factor in bridge design and construction during the 1920s. As a result, the majority of spans constructed during the remainder of the twentieth century were relatively light, reinforced, and prestressed concrete highway bridges. Reinforced concrete bridge construction, in which steel imbedded in the concrete controls the forces of tension, was introduced in the United States in the late nineteenth century. Eventually, a variety of reinforcing systems were patented. The interstate highway system's rapid growth during the 1950s and 1960s fostered the widespread use of pre-stressed concrete beam bridges. Beams fabricated in this way were strengthened by built-in compressive forces. These bridges became the most common type of span in late-twentieth-century America.
Bibliography
Condit, Carl W. American Building Art: The Nineteenth Century. New York: Oxford University Press, 1960.
———. American Building Art: The Twentieth Century. New York: Oxford University Press, 1961.
Jackson, Donald C. Great American Bridges and Dams. Washington, D.C.: Preservation Press, 1988.
McCullough, David. The Great Bridge. New York: Simon and Schuster, 1972.
Petroski, Henry. Engineers of Dreams: Great Bridge Builders and the Spanning of America. New York: Knopf, 1995.
Scott, Quinta, and Howard S. Miller. The Eads Bridge. Columbia: University of Missouri Press, 1979.
Van der Zee, John. The Gate: The True Story of the Design and Construction of the Golden Gate Bridge. New York: Simon and Schuster, 1986.
Wittfoht, Hans. Building Bridges: History, Technology, Construction. Dusseldorf, Germany: Beton-Verlag, 1984.
—William E. Worthington Jr.
.jpg)
