Bridges

Bridges

Sources

Bottlenecks. In the late summer of 1787 George Washington made the mistake of trying to cross the stream at Elkton, Maryland, on what he described as “an old rotten, and long disused” bridge. One of his carriage horses fell through, and only the prompt assistance of some bystanders saved the first citizen of the republic from plummeting into the water. Experiences such as Washington’s were not uncommon in the early national era. “Roads” were often no more than mud tracks, and few streams could boast permanent all-weather bridge crossings. Most travelers relied on ferries and fords, both subject to the vagaries of the weather, even to cross major rivers. The lack of reliable bridges and roads represented more than just an occasional danger or inconvenience to travelers. During the War of 1812 the nation’s armies found the unreliable road-and-bridge system a serious impediment to the timely movement of troops and supplies, especially in the West. In an expanding postwar economy, transportation bottlenecks at river crossings obstructed the free flow of trade goods between the countryside and the ever-growing port and manufacturing cities.

Burr. In the decades after the War of 1812 the threat to national defense and economic growth posed by bad roads and bridges prompted both private investors and the state and federal governments to expend vast sums on infrastructure, including turnpikes, canals, railroads, and the bridges that carried all of these arteries across the nation’s waterways. America could claim few trained civil engineers qualified to design these bridges, but a generation of bridge-building craftsmen arose to meet the unique design and construction challenges presented by America’s great rivers. Theodore Burr was one who learned his bridge-building skills as a carpenter rather than in the engineering classroom. Burr used his innovative wooden arch-truss designs to bridge the Hudson, the Delaware, and the Mohawk, all broad rivers with heavy currents and teeming with boat traffic, before moving on to his masterpiece, the McCall’s Ferry Bridge over the Susquehanna. When completed in 1815 the McCall’s Ferry Bridge was the longest wooden-arch span in the country.

The Truss Design. Burr’s arch-truss bridges impressed the public and professionals alike, but the wooden truss bridges of Ithiel Town were easier to construct, and the cantilevered arch-truss bridges of Louis Wernwag were more successful in the long run. Wernwag finished the “Colossus” of Philadelphia in 1812, a 340-foot clear span cantilevered wooden bridge that became the admiration of artists and engineers from the United States and overseas. The Colossus was the first modern cantilever, and Wernwag built some thirty such bridges over the next three decades throughout the Northeast and Midwest. Town’s bridges, on the other hand, did away with the arch entirely, relying instead on a bridge framework of wooden planks crossed in a diamond pattern and secured with wooden pins (much like a garden lattice) resting on stone piers. Simple in design and made with easily obtained materials, Town’s truss bridge became popular for the highways and earliest railroads of the nineteenth century.

Railroad Bridges. All-wood spans were more than adequate for most of the traffic in prerailroad America. But with the growth of extensive rail networks in the 1840s and 1850s engineers needed bridges that could span wide river valleys, withstand the weight and vibration of massive trains, and still avoid the expense and stream obstruction of multiple-pier wooden spans. These requirements were especially important in bridging rivers such as the Mississippi or the Ohio, where heavy barge and steamboat traffic necessitated wide channels and tall vertical clearances (the smokestacks on some steamboats were over 75 feet tall). A new generation of engineers, including William Howe, Caleb and Thomas Pratt, and Squire Whipple, managed to solve these problems by fine-tuning Town’s basic truss design and by utilizing more cast and wrought iron in the planning and construction of railroad bridges. As Pittsburgh foundries increased the volume and quality of their iron (and eventually steel) production, all-metal Pratt, Howe, and Whipple truss bridges became the standard railroad spans nationwide.

Suspension Bridges. In the winter of 1816 the first wire suspension bridge in America, a passenger toll path only 2 feet wide and 408 feet long spanning the Schuylkill River at Philadelphia, collapsed under snow and ice. Almost two decades earlier James Finley, a western Pennsylvania judge, had come up with the idea of using iron chains stretched over stone piers to hold up a level floor over a span of water. While Finley’s first suspension bridges never exceeded 70 feet, he claimed that they would someday safely cross open spaces and waterways 1, 000 feet wide. By the last half of the nineteenth century bridge designers in the United States and Europe were proving Finley’s predictions practicable. Suspension bridges offered strength and safety (when braced for wind) over long stretches of water, with the added advantage of fewer piers and thus less obstruction to navigation. The latter benefit became especially important when bridging active shipping channels at busy ports such as New York.

THE MCCALL’S FERRY BRIDGE

Theodore Burr bragged that the wooden arch-truss bridge over the Susquehanna River that he designed in 1815 contained “the greatest [arch] in the world … at three hundred and sixty feet four inches” It was certainly one of the most difficult bridges in America to build. At the spot Burr chose, the Susquehanna was swift, almost one hundred feet deep, and subject to ice floes during the spring thaw. Instead of working in the dangerous main channel Burr’s laborers constructed the bridge’s central arch in upright sections at a spot a quarter-mile downstream, then used the frozen river of January 1815 to swing it upstream and raise it into place. Unfortunately, the river was not entirely frozen, and blocks of ice mixed with slush had stacked up at the narrows to a depth of sixty to eighty feet where the bridge piers stood ready for the deck. Somehow his workmen manhandled the huge arch into place with only one accident: a worker who plunged fifty-four feet to the river yet survived. Bonfires and ample amounts of liquor heralded the completion of the span, but the bridge fell victim within two years to another ice jam and was never replaced.

Sources: Llewellyn N. Edwards, A Record of History and Evolution of Earty American Bridges (Orono: Maine University Press, 1959);

Richard Shelton Kirby and others, Engineering in History (New York: McGraw Hill, 1956).

Sources

Llewellyn N. Edwards, A Record of History and Evolution of Early American Bridges (Orono: Maine University Press, 1959);

Donald C. Jackson, Great American Bridges and Dams (Washington, D.C.: Preservation Press, 1988);

Lee H. Nelson, The Colossus of 1812: An American Engineering Superlative (New York: American Society of Civil Engineers, 1990);

Ted Ruddock, Arch Bridges and Their Builders, 1735–1835 (Cambridge, U.K.: Cambridge University Press, 1979).