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      Lancaster County in Pennsylvania is home to both a large Amish population and numerous covered bridges. Erb’s Bridge over Hammer Creek, built in 1887, is typical, having a Burr truss and stout stone parapets securing the approach. (A. Chester Ong, 2011)

      The Diffusion of Covered Bridges Throughout North America

      By the 1820s, knowledge of American timber bridge truss technology was not just spreading to a rapidly growing corps of young builders but over an increasingly wide geographic area. Palmer and Burr had evolved designs based on the fundamental principles of triangle and arch framed in wood, but Wernwag experimented both with design and materials, increasingly using elaborate iron elements such as spacers that separated pieces of his arches to prevent dry rot. Timber truss bridges thus evolved rapidly in the hands of the first generation of builders from 1792 to the 1820s. The idea of covering them, though discussed as early as 1769, became a documented reality only in 1805 with Palmer’s celebrated Permanent Bridge. After that, “covered timber truss bridges” became the norm. The earliest bridges had been built where the population was concentrated, from Massachusetts south to Maryland and New Jersey and in eastern New York and Pennsylvania. Movement to the west and much of the south was obstructed by the Appalachian Mountain range. In mountainous or hilly regions, towns were founded along rivers because these provided transportation, power, and water supplies, though sometimes also ice jams and floods. The same held true as explorers and pioneer families explored the territories to the west and south, establishing towns in areas formerly occupied by the native peoples who were mostly forced off their lands, either into reservations or migration to the barren lands farther west.

      Westward expansion naturally pushed past Pittsburgh into Ohio, then Indiana and Kentucky, and on to Indiana, Illinois, and Missouri. Towards the south, people migrated into the Appalachians, including what became West Virginia, western Virginia, the Carolinas, and then Georgia, Alabama, and Mississippi. As they settled in these newly populated areas, they encountered the same problems earlier generations had struggled with in New England and the Middle Atlantic states: vast roadless forests, unbridged rivers, and mountain terrain. As was true earlier in the east, there were no “bridge engineers” per se among them, that science not yet having developed in the young United States, but many were experienced in building homes, mills, and churches, and they too—though perhaps having learned something from the examples set by Palmer, Burr, and Wernwag—set about teaching themselves to build bridges. The covered bridge became the norm, though generally on a more modest scale.

      Throughout the rest of the nineteenth and well into the twentieth century, timber truss covered bridges proliferated throughout the eastern half and to a lesser degree in the western half of the contiguous forty-eight states. Covered bridges were also constructed in Hawaii in the late nineteenth century and in Alaska during the first decades of the twentieth. Over time, they became merely “normal,” as newsworthy as concrete and steel highway deck bridges are today. Until the advent of iron bridge technology, especially after the Civil War, there were few alternatives: open wooden bridges, pony truss bridges, stone arch bridges, and simple stone culverts.

      Adjacent to Greisemer’s Mill in Berks County, Pennsylvania, the covered bridge built in 1868 over Manatawny Creek is 140 feet long. (A. Chester Ong, 2010)

      By 1902, Pittsburgh had at least a dozen bridges over its two rivers, only one being covered. When the covered Union Bridge was built in 1875, Pittsburgh already had its first iron suspension bridge, which was built in 1859 by John Roebling, the German-born engineer who later built the Brooklyn Bridge. The covered bridge, too close to the water, was a nuisance well before it was damaged in the flood of 1907 and replaced. (Library of Congress)

      Spanning the Allegheny River at Pittsburgh’s “Point,” where it joined the Monongahela River to form the Ohio River, the Union Bridge was the first at this location, having been built in 1875 to carry pedestrians, carriages, and the trolley. (Thomas and Katherine Detre Library and Archives, Senator John Heinz History Center)

      Pittsburgh’s Union Bridge consisted of five spans supported by especially powerful Howe trusses with triple rods reinforced with massive laminated arches. Such a structure was needed to support such a wide bridge with long spans and heavy traffic. (Carnegie Museum of Art, Pittsburgh; Gift of the Carnegie Library of Pittsburgh)

      The first, and perhaps the only covered bridge over the Ohio River, connected Wheeling’s island over the west channel to Bridgeport, Ohio, on the National Road. Designed by Lewis Wernwag, the bridge, consisting of three double-lane spans using doubled Burr trusses, was constructed from 1833 to 1836 (some say 1837) by Noah Zane. It was not replaced until 1893. (NSPCB Archives, R. S. Allen Collection)

      We can never know just how many covered bridges were built in the United States (and later in Canada as well), but two individuals, Bill Caswell and Trish Kane, are seeking to do just that. Still a “work in progress,” their website, www.lostbridges.org, seeks to list every covered bridge that exists now or existed in the past in the United States and Canada. As of April, 2012, they had listed 12,895, but certainly this inventory is incomplete. Some states, such as Ohio, have been thoroughly researched while others have not, which likely skews

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