Sunday, July 27, 2008

Colossus Bridge, Schuylkill River

The Upper Ferry bridge built at Fairmount near Philadelphia in 1812 and tragically destroyed by fire in 1838 was the longest single-trussed wooden arch in the United States, spanning over 340 feet (102 meters). It caused a sensation in its day and was inevitably labeled a new “wonder of the world,” “the Colossus at Philadelphia,” and “the Colossus at Fairmount.” This covered bridge, responding to new constraints, took timber engineering to its limits.

At the beginning of the nineteenth century, driven by the need for agricultural growth, the population of the narrow coastal plain of the northeastern United States was spreading beyond the “tidewater” region. Before then, many short streams and estuaries had adequately met communication needs, but the inland farmers demanded roads, fords, and bridges. Water mills, increasing in number as farming increased, were of necessity sited where rivers could not be forded, and they also needed transportation routes. There were good supplies of building lumber in the region and the harsh climate was better suited to wooden construction than to masonry. The earliest bridges were merely logs carried on timber stringers; their spans were limited to the available lengths. As bridge technology developed, longer spans were achieved by joining stringers and employing trusses and arches. Climate was an important factor and the covered bridge soon became not only popular but also necessary. The roof protected the structural timber from alternate wetting and drying, discouraging rot and extending the life of the bridge. There is a story, perhaps apocryphal, of a Virginia builder who observed that bridges were covered “for the same reason that our belles [wear] hoop skirts and crinolines: to protect the structural beauty that is seldom seen, but nevertheless appreciated”—a delightful analogy.

The first covered bridge in the United States replaced a pontoon across the Schuylkill River in Philadelphia and was therefore optimistically called the Permanent Bridge. A stone bridge was originally intended, but when the abutments and piers were completed in 1804, the decision was made to span the river with timber. The New England bridge architect Thomas Palmer designed a structure braced with three arches and multiple king posts, and it was constructed by Owen Biddle, a Philadelphia architect and builder. When it was opened to traffic in 1805 it had no cover, but on Palmer's advice and the prompting of Permanent Bridge Company shareholders, a roof and clapboard siding were soon added. Palmer believed the covering would extend the life of the structure from twelve years to perhaps forty; it was still sound when replaced forty-five years later.

Within five years there was a demand for another bridge across the Schuylkill, to be built at Upper Ferry and connecting the area then known as Fairmont with the western bank. The design was put in the hands of Lewis Wernwag, an immigrant carpenter from W├╝rttemburg, Germany, who had already built bridges over Neshaminy and Frankford Creeks.
Wernwag’s new bridge, built in 1812, was an elegant single-trussed arch spanning over 340 feet (102 meters)—certainly the longest of its kind in the United States and (according to some sources) the second-longest single-span bridge in the world at the time. The totally enclosed, elegant, low-arch bridge terminated in classical loggias at each end. Ten rectangular windows on each side provided light and ventilation for travelers. Graceful as it was, its achievement does not lie in its appearance but in the genius of its timber engineering. The wooden road deck was supported on five laminated arch beams that rose a little over 3 feet (1.07 meters) at midspan. On each side of the deck the river was spanned by a bow lattice beam, shallower at midpoint than at the ends and stiffened along its length with twenty-eight sets of double diagonal bracing. Iron tension ties anchored the beams to the ground at the masonry abutments, and others complemented the bracing along their entire length.

Wernwag’s reputation was established as a builder of long-span wooden truss bridges, and he built several more, including the Hickman Covered Bridge (1838) in central Kentucky. Also known locally as the Wernwag Bridge, it was the longest cantilever wooden bridge in the country. The practice of building wooden covered bridges spread quickly throughout the United States, and literally thousands were built during the nineteenth century. The Covered Bridge Society of America identifies over 1,500 extant covered bridges throughout the world. Over two-thirds of them are in North America. Pennsylvania has 219, over half of which are still in use on public roads.

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