America's Covered Bridges - Ronald G. Knapp

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began. (A. Chester Ong, 2012)

      Ultimately, it is up to the editors of publications listing covered bridges to set standards for inclusion. The World Guide is more problematic in that changes in policy could wreak havoc on each edition if new editors make major changes such as including or excluding all those “fake” bridges we love to hate. Designating a bridge with an official number (state-county-bridge) lends it legitimacy. Once a number has been assigned to a bridge, you cannot eliminate that listing or replace it without creating a certain amount of chaos.

      We favor distinguishing “classic” historical bridges from those which are less so. But we also recognize that a full discussion of the covered bridge in America (including Canada) cannot take place without the inclusion of bridges that have been modified or even reconstructed. In many cases, the World Guide adds #2 to the bridge’s number. Indeed, let that be the thread that unifies the narrative: our changing perspectives on the covered bridge.

      Eberly’s Mill Bridge, built in 1846 of Burr truss construction, is typical of Lancaster County, Pennsylvania. Though retaining a high density of Amish who continue using their horse-drawn buggies, as seen here, the county’s spectacular growth has altered its countryside from rural to suburban, forcing the Amish to live within urban development and congestion. (A. Chester Ong, 2011)

      This three-span Howe truss deck railroad bridge crossed the Allegheny River at Foxburg on the western edge of Clarion County, which is north of Pittsburgh. Approached on both banks by long, curving trestles, the upper deck carried trains and the inside deck appears to have carried wagons, while a pedestrian walkway runs outside the trusses. Such structures came about in response to America’s growing industrialization. (Smithsonian Institution)

      The last remaining covered canal aqueduct in the United States, Metamora, Indiana’s Duck Creek Aqueduct, was built in 1847 to replace an 1842 predecessor destroyed in a flood. Sixty feet long and of Burr truss construction, it carried the Whitewater Canal over Duck Creek for only a brief period. The 76-mile-long canal, built between 1836 and 1847, was so heavily damaged again by flooding in 1847 that little of it remained open. After falling into disrepair, the state restored the bridge in 1946, and today it carries tourists seeing the village of Metamora from a newly built canal boat. (A. Chester Ong, 2011)

      The Juniata Division of the Pennsylvania Canal—actually a system—was opened in 1832. Starting from Duncan’s Island on the Susquehanna River, the canal crossed the Juniata River through a 600-foot multi-span Burr truss-covered aqueduct with interior towpaths. This canal branch, with 86 locks and 25 aqueducts, never made a profit, and the right of way was eventually sold to the Pennsylvania Railroad. (Pennsylvania Canal Society)

      Built in 1838 by John Hough to cross the Scioto River, the Circleville Aqueduct was the longest covered aqueduct on the Ohio and Erie Canal. After the canal ceased operation in 1878, the bridge served as an ice skating rink in the winter and as a dance hall at other times until it burned under mysterious circumstances in 1915. (Miriam Wood Collection)

      Bridge Basics

      Covered bridges are essentially wooden (or mostly wooden) trusses that carry a roadway over a body of water. While most such bridges today carry (or carried in the past) vehicular traffic, many similar bridges also carried railroads. Less common were aqueducts designed to carry a canal and canal boats.

      When the roadway passes between trusses whose base is level with the roadway, engineers call this a “through” bridge. When the roadway passes above the trusses, engineers call this a “deck” bridge. Because covered bridges by definition have a roof, all covered bridges are “through” bridges, but a great many wooden through truss bridges, especially those built for railroads, were left uncovered. Deck bridges could be covered in that the sides were protected with siding, but just as many were left open. Some deck bridges, sided or not, carried vehicular traffic, but the majority carried railroads.

      Through bridges have the advantage of being higher over the water, and deck bridges, because the trusses are below grade, tend to be closer to the water and therefore more vulnerable to floods and ice jams. Aqueducts could be either through or deck, covered or uncovered. Some through aqueducts were fully covered. Through aqueducts, however, were necessarily quite wide to allow for towpaths on each side of the trough. Not surprisingly, the majority of aqueducts were deck trusses with the trough running between the upper portions of the trusses.

      In addition to full-sized bridges and aqueducts, there are also “pony” truss bridges. Some are simply low trusses in an otherwise normal covered bridge, while others are boxed and lack a roof.

      Bridge trusses had to be placed on some kind of foundation. Where stone was plentiful, these foundations consisted of large rectangular blocks laid without mortar. Where stone was difficult to obtain, builders often used heavy wooden posts, but foundations of wood naturally deteriorated quickly. The foundations on each bank of the river or stream are called “abutments,” while supports built between them in the river are called “piers.”

      In most cases, the builders preferred to build the roadway right to the bridge entrance, sometimes held in place by stone parapets or retaining walls. In other cases, builders constructed free-standing abutments at the water’s edge and away from the higher river bank, requiring open wooden approaches.

      Sometimes these approaches were supported by simple wooden trusses, either open or boxed in. While this solution provided the river a wider space during flooding, it made entering the bridge more dangerous for vehicles, and open wooden approaches naturally rotted quickly too. Open approaches are common in the American South and Midwest, especially Illinois and Iowa.

      How the Book is Organized

      The era of the covered bridge in North America spans slightly over two centuries. Chapter One views them both historically and as common sense, logical solutions to the problem of getting people, animals, and goods across rivers. They were simply bridges—utilitarian, functional, commonplace—although sometimes later recognized as engineering marvels or seen as aesthetically pleasing masterpieces.

      During the first half century or so, roughly to 1850, bridge builders worked from their experience in constructing other kinds of wooden framed structures, especially houses, barns, churches, and mills. Although bridge building became a mathematically informed science during the second half of the century, with the appearance of several “treatises” and textbooks on bridge building, many local builders continued to work within local traditions based on an experientially learned body of knowledge passed down from elder to younger. Indeed, some local builders were illiterate, yet could construct wooden bridges capable of carrying heavy traffic for many years.

      The covered canal aqueduct at Taylorsville, Ohio, after its partial collapse following the 1913 flood. Its Burr truss design is clearly visible. (NSPCB, R. S. Allen Collection)

      Built in 1880 and restored in 1998, the Colvin (or Calvin) Bridge spans Shawnee Creek in Bedford County, Pennsylvania, with a half-size (or “pony”) truss. The upper chord is only halfway up, with the flared kingposts above it merely to support the roof. (A. Chester Ong, 2011)

      Tohickon Creek is spanned by a 179-foot-long two-span boxed pony Howe truss in Bucks County, Pennsylvania’s Ralph Stover State Park.

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