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. The short trusses are boxed in from the elements, but there is no necessity for a roof. (A. Chester Ong, 2010)
Rugged cut stone abutments and a pier support Maine’s Porter-Parsonfield Bridge between Oxford and York Counties. This 160-foot Paddleford truss bridge was built in 1876. (Terry E. Miller, 2010)
Like many bridges in the Illinois-Iowa-Kansas area, where streams are muddy and lack defined banks, Sangamon County, Illinois’s Glenarm Bridge sits on four metal cylinders filled with concrete, with short open approaches to the bridge. (Terry E. Miller, 1968)
A buggy emerges from an old and long-forgotten bridge over the Yocona River in Lafayette County, Mississippi, in the early 1900s. The Town lattice design was preferred in most parts of the South. (Lafayette County, Mississippi Genealogy and History Network)
Chapter Two traces the history and development of bridge design. At first using only the materials at hand—mainly timber but small pieces of iron hardware as well—bridge builders developed an increasing array of structural patterns—called “trusses”—which offered greater and greater flexibility, strength, and efficiency. Hundreds of bridge trusses were devised, many of them receiving patents, though practically speaking only about a dozen came into common use.
Chapter Three addresses a question that has long perplexed bridge enthusiasts: exactly how were covered bridges erected? After exploring several theories that have been proposed, we examine a body of evidence comprised of bridge treatises, personal reminiscences, and vintage photographs. We suspect that the answers have been difficult to come by because the methodology of bridge construction was simply taken for granted.
Chapter Four traces the fate of the covered bridge into modern times. As befell the canals and water-powered mills that developed during the same period as the covered bridge, new technology gradually rendered the covered bridge obsolete. As bridge building became an engineering science and foundries produced ever greater amounts of iron in ever larger pieces, wooden bridges gave way to metal bridges. As wood was giving way to metal, bridge building transitioned through a period of “combination” bridges in the 1870s to the 1890s, some being open, some covered. Oddly, though, the building of covered bridges did not end with the development of iron bridges. At the beginning of the twentieth century, when wooden bridges were mostly obsolete in New England, the Middle Atlantic States, and parts of the Midwest, two other areas began building covered bridges in great numbers—the American Northwest, especially Oregon, and the Canadian provinces of Québec and New Brunswick. Both areas were remote from iron foundries and steel mills in regions where wood was more plentiful. The American South, for similar reasons, continued building covered bridges into the 1930s.
In Bartow County, Georgia, travelers crossing the Lowry or Euharlee Creek Bridge first had to cross a wide flood plain on an open approach. While necessary, such structures were subject to rapid deterioration because of exposure to the elements. (A. Chester Ong, 2011)
This chapter also considers the period when covered bridges became, in most areas at least, obsolete, if not nuisances and impediments to progress. If covered bridges were well maintained, they remained capable of carrying most traffic, but increasing vehicle size and heavier loads created problems. Large trucks now carried freight, buses became heavier and larger, and farmers had tractors pulling larger and larger pieces of farm equipment over country roads. Another major change was the increasing use
