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America's Covered Bridges. Ronald G. Knapp
Читать онлайн.Название America's Covered Bridges
Год выпуска 0
isbn 9781462914203
Автор произведения Ronald G. Knapp
Жанр Книги о Путешествиях
Издательство Ingram
The old Miller Ferry crossed Alabama’s Tallapoosa River before a covered bridge was built. Its peaceful appearance belies an exceptionally violent battle fought here on March 26, 1814 when troops under General Andrew Jackson fought the Red Stick tribe of the Creek in a war for control of much of the South. Jackson’s troops, aided by a rival tribe, killed nearly 800 of the 1,000 Red Stick warriors, a tragic event memorialized in a national park. (Horseshoe Bend National Military Park)
Even as late as 1797, famed American portrait painter Charles Willson Peale warned: “Easy and safe passages over the waters of the United States are much wanted—even our post roads are deficient; often the affrighted traveller stops, and surveys the turbulent torrent that hides an unknown bottom, he hesitates—doubts whether to risk a passage or not; at last, by delay grown impatient, he with fear and trembling cautiously moves forward and perhaps arrives in safety on the opposite bank; but alas! Too frequently the rash, or fool-hardy driver, is carried down the stream, and all is lost!” (1797: iii).
Fording was only possible when water levels were low enough and the stream bed solid enough to support hooves or wagon wheels. This could change suddenly after a hard rain, making travel extremely unpredictable.
Since deeper and wider rivers, especially the broad estuaries of the great rivers approaching the sea, could never be forded, toll ferries appeared when individual proprietors found it possible for them to be economically viable. When the water level was too high, too low, too swift, or the river was frozen, ferry services usually ceased operation, leading citizens to wish for a better solution—a bridge.
Peale also commented on this matter: “Legislatures, and you men of influence in the counties of each State! Turn your attention to this important object—shorten the distance to market for the sale of the product of your lands. I offer you a cheap and easy mode of building Bridges, the principles of which are so simple, and the mechanism so easy, that any ingenious man may execute them” (p. iii).
Even then, of course, humankind already had long and extensive experience building bridges. European immigrants to America knew bridges from home and had doubtless crossed many in their lifetime. But standing there on the wooded shore of one of America’s wild and hitherto unbridged rivers, they had to determine for themselves how to get people, animals, and freight across safely. Some of the pioneers knew stone masonry and wood joinery from the “old country,” principally parts of the British Isles or one of the German-speaking areas of central Europe. Few, however, had experience building bridges.
Stone arch bridges typified the English solution to crossing rivers, but they were expensive and slow to build. Seen here is one half of a two-span bridge over the River Tees leading to the twelfth-century Barnard Castle in England’s County Durham. (Terry E. Miller, 1987)
Looking about, they had only two materials at hand: stone and timber. Europeans, particularly those from the British Isles, knew stone to be an effective material for building bridges, but such structures were costly and time-consuming to build. There were too few skilled stone masons and builders to cross more than a few of the smaller streams, and crossing major rivers was out of the question. While true that stone bridges predominated in the British Isles, Britain had mostly small streams and modest rivers to cross.
More abundant and more easily worked was wood from America’s vast forests, which were filled with old growth trees of every kind. Perhaps a few of the German speakers had seen wooden bridges in central Europe, particularly in what are now Germany, Austria, and Switzerland, but if any were experienced bridge builders, we do not know their names. Thus, the settled immigrants in the British colonies, soon to become the United States of America, had to use their own ingenuity to solve the most pressing problem that blocked transportation: learning how to build bridges.
Among them were skilled wood joiners with experience building mills, barns, and houses. Sawmills were already well established. Builders understood how to create strong frameworks of beams and how to span distances, especially for roofs. Small foundries could make nails, bolts, and straps of iron. Stone masons could build abutments and piers for bridges using patterns similar to those in houses and mills. However, bridge builders found that wooden beams alone are neither long enough nor strong enough to span more than a limited distance, requiring in most cases a series of stone piers, and the techniques for building such structures within the moving waters of a mighty river were as yet unknown.
How much American bridge builders knew about wooden bridge building in central Europe remains uncertain, but the solutions that developed in the young United States suggest Yankee ingenuity rather than a continuing tradition. If there was an American timber framing tradition, it relied on fundamental principles rather than wholly learned bridge patterns. The two principles that form the basis of virtually all bridge solutions that emerged in America are, one, the rigidity of the triangle, and two, the strength of the arch. Beyond these, there was only trial and error, experience, and common sense since scientific analysis of stress in bridge design was unknown until the middle of the nineteenth century. In fact, Civil Engineering as a named field offering training in bridge design and construction developed long after that.
An understanding of what came to be the “American covered bridge” requires, first, an understanding of the basic solution to the stream-crossing challenge—the bridge truss. Builders realized that by joining timbers into patterns consisting of little more than triangles or by creating continuous arches they could build structures rigid enough and strong enough to span great distances, eliminating the need for extra stone piers, which were obstructions to the flow of the rivers and to navigation. Although some small iron fasteners and straps were available, most of the joinery required only wooden dowels, called trunnels (“treenails”).
American craftsmen used wood arranged into rigid patterns to span rivers. Ashulot, New Hampshire’s Upper Village Bridge, built in 1864, uses Ithiel Town’s patent design from 1820 that consists of planks held together with wooden dowels, called trunnels (from “treenails”). (A. Chester Ong, 2010)
A pair of extra long trunnels pierce both pairs of lower chords and the lattice planks of Georgia’s Cromer’s Mill Bridge built in 1906 by James M. “Pink” Hunt. (A. Chester Ong, 2011)
The heavy cross beams supporting the deck of this bridge in Washington County, Pennsylvania, are hung below the lower chord from an iron eye-bar above to a nut fastener below. (A. Chester Ong, 2011)
One of many of New Hampshire’s iconic bridges, the Haverhill-Bath Bridge crosses the Ammonoosuc River in Woodsville where water cascades over both rocks and an old industrial dam. Built in 1829 by the town supervisors for about $2,400, the two-span 256-foot-long Town lattice structure carried NH 135 until 1999. An earlier restoration added massive arches and a pedestrian walkway on the upstream side. (A. Chester Ong, 2010)
The centerpiece of Sullivan County, Pennsylvania’s tiny community of Forksville (pop. 145) is its bridge over Loyalsock Creek built in 1850 by Sadler Rodgers, then only 18 years old, using the Burr truss. (A. Chester Ong, 2012)
Builders came to find endlessly creative ways of combining timbers into individually named trusses based on the triangle principle, whether combined with an arch or