The Covered Bridge
While framed timber bridges first developed in Europe at least as far back as Roman times, American bridge builders quickly set the standard for timber-bridge construction, producing a succession of innovative designs that made possible the construction of sturdy and cost-effective bridges capable of spanning long distances and safely carrying heavy, moving loads.
At the same time, these builders recognized the value of covering a timber bridge’s framework to ensure the structure’s longevity. By the end of the first quarter of the nineteenth century, the covered bridge was a ubiquitous part of the American landscape. At the height of covered bridge building, around 1870, there were well over 10,000 covered bridges in the United States.
The historic era of covered bridge building — the period when timber bridges were built because wood was abundant and cheap, and when practical knowledge of timber-framing techniques was passed down through successive generations of builders — began in the early 1800s and ended in the 1950s in the United States. The dates defining this era vary from region to region, but the practice of building covered bridges lasted about a century in most parts of the country.
By the 1950s, the economic competitiveness of timber bridges had disappeared, and the historic era of covered bridge building ended. While some covered bridges were built after this time—and they are still occasionally built today in a few scattered parts of the country—these revival-era covered bridges differ from their predecessors in that they are not part of the continuous building tradition that characterized the historic era of covered bridge construction, and the impetus behind them is nostalgia, rather than economy.
Origins of the Covered Bridge
Timber bridges have been built in forested regions of the world for centuries. Wood is an excellent material for building; it is strong, relatively lightweight, and can be worked with common hand tools. Since most species of wood suitable for structural applications deteriorate rapidly when exposed to the weather, early bridge builders quickly learned the value of covering wood bridges with roofs and siding to protect the underlying framework. A wood bridge left uncovered may last fifteen years, but when properly covered and cared for, it can last indefinitely. A few covered bridges in Europe have survived for well over four centuries. Schlossbrücke (1514) in Zwingen, Switzerland, and Spreuerbrücke (1568) in Luzern, Switzerland, are two fine examples.
Carpenters in the forested regions of Europe began to refine the technology of framed timber bridges during the eighteenth century. Swiss brothers Johannes and Hans Ulrich Grubenmann built some noteworthy spans during that time, including the Schaffhausen and Wettingen bridges in Switzerland. European architects, scholars, and aristocrats studied these bridges, and descriptions and illustrations of the spans appeared in contemporary travel publications and technical literature.
Although covered bridges had been build for centuries, American engineers appear to have developed them independently to make the wooden spans more durable.
The Schaffhausen Bridge (1758) across the Rhine River was a two-span bridge of about 360 feet whose length was achieved by an intricate strut-braced beam design. The Wettingen Bridge (1766) across the Limmat River was a 200-foot single-span structure and is believed to have been the first use of a true arch in a timber bridge. While the builders relied on empirical methods and their bridges were heavily built and complex structures, the Grubenmann brothers did boldly demonstrate the potential for long-span wood bridges, earning them international renown and lucrative contracts. Napoleon’s forces destroyed the Schaffhausen and Wettingen bridges in 1799, but several other Grubenmann covered bridges still survive in Switzerland, including Hundwilertobel (1778) at Hundwil and Kubelbrücke (1780) at Kubel.
American builders were undoubtedly aware of — and perhaps inspired by — European covered bridges, but there is scant evidence to document a direct transfer of knowledge from Europe to North America; rather, it is likely that Americans independently developed the practice of covering wood bridges as the need for durable spans arose. Shortly after construction of long-span timber bridges began, the practice of covering them was rapidly adopted here.
Early American Bridges
Bridges were rare in colonial America. Logs, stone slabs, and, occasionally, stone arches spanned small streams, but larger waterways had to be crossed by ford or ferry. Overland travel was hazardous, slow, and uncertain, with delays and accidents common.
A few ambitious river crossings were constructed as timber pile-and-beam structures, but, with few exceptions, long-span bridges were not built in the United States until after the Revolutionary War, when the growing volume of transportation and improving economic conditions justified the expenditure of material and labor. This environment, coupled with a spirit of ingenuity and access to an abundant and cheap supply of timber, spurred the development of timber-bridge design in the United States.
Col. Enoch Hale took a bold first step in advancing American timber-bridge building in 1785 when he erected the nation’s first long-span framed timber bridge across the Connecticut River between Walpole, New Hampshire, and Bellows Falls, Vermont. This bridge was part of the overland route from Boston to Montreal and was heralded as a great achievement in its day. Hale’s braced beam bridge (as distinct from a truss bridge) was supported on rubble stone abutments and a timber pier rising from a small island in the middle of the river.
The Massachusetts Spy stated, “Col. Enoch Hale hath erected a Bridge across Connecticut River, on the Great-Falls, at his own expense. This bridge is thought to exceed any ever built in America, in strength, elegance and publick utility.” Although Colonel Hale’s bridge successfully carried traffic for a decade, its design was uniquely suited to its site and could not be easily replicated elsewhere. Within just a few years, more sophisticated techniques would be used to span America’s waterways.
Creating structures of lengths much greater than a single log or beam was one of the initial challenges facing timber-bridge builders and required construction of a frame structure known as a truss. The truss, which utilizes the stable geometry of linked triangles to carry a load over a void, has been used for centuries for centering masonry arches and for constructing roofs. Introduced into European bridge building in the Middle Ages, the truss is the most efficient way to build long spans using wood. Italian architect Andrea Palladio popularized the concept of truss bridges in his influential treatise The Four Books of Architecture, which was first published in 1570.
Born in Rowley, Massachusetts, in 1751, housewright Timothy Palmer is credited with making a quantum leap forward in bridge building by introducing long-span truss bridges to North America. As a young man, he apprenticed with architect Moody Spofford, best known for his New England churches. In 1792, Palmer took up bridge building, erecting America’s first long-span truss bridge across the Merrimack River at Newburyport, Massachusetts. Containing over 6,000 tons of timber, the 1,030-foot-long structure had pile-and-beam approaches, a draw span over the main channel, and two trussed arch spans, the larger of which measured 160 feet in length.
One contemporary writer described it as follows: “The two large arches (one of which is superior to any thing of the kind on the continent) . . . appear to unite elegance, strength and firmness beyond the most sanguine expectation.” Palmer’s trusses closely resemble one of the plans published in Andrea Palladio’s book, which was available in the United States at the time, revealing, perhaps, the inspiration for the design. Palmer patented his truss design in 1797 and was much in demand as a bridge builder, erecting major timber spans across the Merrimack, Kennebec, Connecticut, Piscataqua, Schuylkill, Potomac, and Delaware rivers.
Proliferation of Covered Bridges
Internal improvements were one of the first priorities of the young nation, with transportation networks desperately needed to improve communication, expand commerce, and unite the country. The Louisiana Purchase of 1803 doubled the land area of the United States, and, over the next half-century, American settlers headed west in increasing numbers. Timber bridges were an ideal solution to some of the many transportation hurdles facing settlers. Constructing a timber bridge only required readily available materials and common hand tools, and skilled carpenters could erect the superstructure of an average-sized bridge within a few weeks. Consequently, hundreds of covered bridges were constructed, first in major urban centers and then in increasingly rural areas as people moved westward.
By 1810, adding roofs and siding to timber bridges was common practice in the United States, and there were covered bridges at most major crossings in southern and western New England, southeastern New York, eastern Pennsylvania, and New Jersey. From this core area, covered bridges spread to the northeast, south, and west at a rapid pace. By 1820, covered bridge building had reached northern New England, Maryland, southwestern Pennsylvania, and southeastern Ohio; by 1830, western Ohio and the Carolinas; by 1840, southern Indiana and the deep South; and, by 1850, there were covered bridges in most regions with European settlements in the United States and Canada.
The number of covered bridges continued to increase until about 1870, by which time there were well over ten thousand of them in the United States. Covered bridges would eventually be found in forty-one states. The few areas where covered bridges were not found include the heart of the Rocky Mountain region, and the northern and southern plains. The reasons for this vary from region to region but include the absence of major river crossings and readily available timber, topography more suited to other types of bridges, late-period settlement, and low population density.
Covered bridges helped achieve the safe, efficient, and economical overland transportation that was a key aspect of the nation’s growth and economic development. They were adapted to the needs of many types of transportation corridors, including turnpikes, canals, and railroads. The rapid growth of the railroads in the mid-1800s placed new demands on bridges. In particular, the increasing weight of locomotives and rolling stock and the need for rigidity encouraged technical advancements in the design of timber-truss bridges.
Developments in Timber-Truss Design
The social and economic climate of the nineteenth century favored the flowering of timber-bridge building in America. The demand for bridges was great, wood was plentiful, and ambition was high. As a result, American builders produced, in rapid succession, a series of remarkable advancements in the design and construction of timber bridges. Between 1790 and 1840, timber bridge forms progressed from rudimentary pile-and-beam spans to scientifically designed, long-span trusses capable of carrying railroad loadings.
These advancements addressed the overall challenge confronting bridge builders: to create economical and efficient structures that could span long distances, that were easy to erect and maintain, and that were strong enough to carry heavy, moving loads. Over time, this led to increased standardization in bridge design.
The earliest significant covered bridges depended on arch construction, as builders sought to capitalize on the inherent strength of that structural form. Within a decade, however, truss construction came to dominate the field of American bridge building. Since the truss is the most efficient way to build long spans of wood, the majority of surviving covered bridges in the United States (more than 98 percent) are truss bridges.
America’s early bridge builders relied on simple truss designs that had been used for roof framing since at least the Middle Ages, specifically the kingpost, queenpost, and multiple-kingpost trusses, which were sufficient for modest spans. These truss types continued to be used into the twentieth century. Approximately 24 percent of the extant covered bridges in the United States use one of these simple truss types.
Later, these simple truss types became the basis for more sophisticated timber bridge designs, like the Burr, Town, and Howe trusses, that could span even greater distances. Dozens of patents were granted for timber bridges before 1850, but only a small percentage of these designs were ever successfully built. Fewer still gained widespread acceptance. More than 60 percent of the surviving covered bridges in the United States use trusses developed by a small group of American bridge builders in the first half of the nineteenth century.
A major figure in the history of covered bridge building is Theodore Burr, credited with the invention of the Burr-arch truss and with building a number of significant covered bridges throughout his twenty-year career. Born in Torrington, Connecticut, in 1771, Burr learned construction at an early age from his father, who was a miller and millwright. In 1800, he built his first bridge, a simple timber-stringer span, across the Chenango River on the Catskill Turnpike at Oxford, New York. He subsequently experimented with a wide variety of timber arch designs for bridges that spanned the Hudson, Mohawk, Delaware, and Susquehanna rivers.
In 1806 and again in 1817, Burr received patents for the bridge design that bears his name. Burr’s masterpiece was the short-lived McCall’s Ferry Bridge (1815), which, with a clear span of 360 feet, 4 inches, was the longest timber arch span erected during the historic era of covered bridge building. In 1822, Burr died under mysterious circumstances while supervising construction of a bridge at Middletown, Pennsylvania. The Union Bridge (1804), which spanned the Hudson River at Waterford, New York, was the last survivor of the bridges Burr designed; it was destroyed by fire in 1909.
Burr’s patented arch-truss configuration was not entirely new, as a similar plan had been published in Columbian Magazine in 1787, yet he was apparently the first bridge builder to make practical use of this design. The Burr-arch truss was an innovative design in which a separate segmental arch was superimposed on a multiple-kingpost truss. Its structural action was such that the arch bolstered the truss, while at the same time being stabilized by it, resulting in a complex interaction.
A major advantage of this design, and a contributing factor to its popularity, was that it allowed for a level deck (in contrast to the arched decks of earlier spans built by Timothy Palmer and Lewis Wernwag), an important feature for multiple-span bridges, and, later, for railroad bridges. The Burr-arch truss was the first patented bridge truss to gain widespread acceptance among bridge builders, although Burr reportedly collected few royalties from it. It was also one of the most popular timber-truss types of the nineteenth century and beyond, as it was still used in some areas until about 1920.
Of the thousands of Burr-arch truss covered bridges that were built during the historic era of covered bridge building, about 185 examples survive in the United States, with some of the finest examples located in Pennsylvania and Indiana.
The Architecture of Covered Bridge
The earliest covered bridges were magnificent structures, both in terms of engineering and architecture. Financed with private capital, the early turnpike bridges were all custom-built structures, wide enough for two lanes of travel, with highly ornamented exteriors. The Schuylkill Permanent Bridge was probably one of the most extravagant, but other grand covered bridges once existed at major river crossings along the East Coast.
By 1820, covered bridges were much more common and less ornate. Covered bridges built by towns and counties were often quite plain in appearance. Architectural variations occurred, often on a regional level, reflecting the use of local materials and building traditions. For example, covered bridges in southeastern Pennsylvania often had long stone-masonry approaches and stepped gables, while many of the covered bridges in Madison County, Iowa, featured flat roofs and arched portals.
Covered bridges erected by the Kennedy family of Rush County, Indiana, bore ornamental scrollwork and cornice brackets, and those built during the 1930s in Blount County, Alabama, were covered with corrugated metal roofing and siding. In Oregon, where covered bridges were built according to state-issued plans into the 1950s, each county developed its own distinctive bridge-housing style.
Because the housing was considered expendable and was expected to be periodically replaced, the majority of covered bridges were left unpainted until the mid-twentieth century. Those that were painted might be red or white, but other colors were also used. By the 1950s, Americans decided, for reasons yet unknown, that covered bridges should be red, and within two decades, approximately one-third of the covered bridges in the United States were painted that color.
The term “covered bridge red” even slipped into modern advertising to market such items as paint, yarn, and wine. A popular Christmas card image of the Bedell Bridge, which spanned the Connecticut River at Newbury, New Hampshire, from 1866 to 1979, was even tinted red, although the bridge itself had never been painted.
In addition to regional architectural styles, the height of the trusses and location of the bridge deck could also dictate the bridge housing. Pony-truss (or low-truss) bridges, which were often used for shorter spans, required no overhead bracing, so each of the trusses might be individually housed (these are referred to as “boxed” trusses).
Sometimes, through-truss (or high-truss) bridges were also “boxed,” or housed, in a similar manner, by enclosing the trusses without adding a roof. Deck-truss bridges, in which the deck is carried on the upper chords, were typically used in locations with steeply sloping terrain, where placing the trusses below the travelled way allowed for shorter and less expensive substructure components. Timber deck-truss bridges often had a roof and siding located below the deck.
Covering a wood bridge is the most reliable way to protect it from decay, but in some instances builders did away with the housing in favor of treating the timbers with chemical preservatives or protecting the upper chords with sheet metal. Non-housed timber-truss bridges share a common history with covered bridges, but very few have been preserved.
Wood versus Iron
Throughout the nineteenth century, interest grew in a new structural material: iron. The tensile strength of iron, coupled with its resistance to fire, rot, and insects, made it an appealing material, particularly on railroads where rigid bridges that were easy to erect and maintain and capable of carrying increasingly heavy loads were desired.
Between 1836 and 1839, Capt. Richard Delafield of the U.S. Army Corps of Engineers designed and oversaw construction of America’s first iron bridge across Dunlap’s Creek on the National Road at Brownsville, Pennsylvania. That innovative structure was an 80-foot, cast-iron arch consisting of five tubular arch rings supporting the roadway. Other engineers tinkered with ideas for iron-truss bridges in the 1840s, but enthusiasm for the use of this new material in structures was tempered by periodic structural failures, which led to lingering suspicion of iron in the public mind. Some designers merely substituted iron parts directly for wood, but ignoring the differences in the structural properties of the two materials figured prominently in the 1876 collapse of an iron bridge in Ashtabula, Ohio, that killed eighty-three people.
Due to these periodic failures, the adoption of iron occurred relatively slowly. As is often the case when a new building material is introduced, there were successes and failures—radical advancements and technological dead ends. As engineers worked to better understand the behavior of iron structures, bridge builders continued to alternate between wood and iron for much of the nineteenth century.
For all of its benefits, iron also had its drawbacks: it was expensive, difficult to manufacture, susceptible to corrosion, and it often failed without warning. Ultimately, the advantages that iron construction afforded, combined with the disadvantages of timber construction (susceptibility to fire and rot and the need for periodic maintenance), led many engineers and public officials to view timber bridges as obsolete.
In the post–Civil War era, covered wood bridges were still economical, except for very long spans, and there were still opportunities for innovative timber-bridge builders to remain competitive, especially in regions where timber was readily available. The surest route to reducing the cost of any construction is to minimize the quantity of materials that goes into it, and many timber bridges were overbuilt for the loads they carried.
By employing the methods of mathematical stress analysis described by engineer Squire Whipple in his 1847 Essay on Bridge Building, builders could proportion structural members in the most efficient manner possible, and, in doing so, cut costs. From the 1860s onward, several inventors addressed this challenge and used the advances in civil engineering to build timber bridges less expensively so they could compete with iron-bridge manufacturers.
Robert W. Smith, born in Tippecanoe City, Ohio, in 1833, was the inventor of the highly successful Smith truss. Smith was educated at home until he was 15 and only attended public school for six weeks to study geometry. He learned carpentry from his father and older brother, who were barn builders. Early in his career, Smith ran a woodworking shop and lumberyard but eventually turned his attention to bridges.
In 1867, Smith patented a bridge truss that, for a short time, allowed wood bridges to successfully compete with iron ones. By 1869, Smith had established the Smith Bridge Company factory in Toledo, Ohio. Bridges were prefabricated to order, shipped to their sites, and erected under the supervision of company agents. The company built hundreds of covered bridges during the 1870s and successfully made the transition to the manufacture of iron bridges. In 1890, the Smith Bridge Company ceased operations, and the plant was sold to the Toledo Bridge Company. Of the hundreds of Smith-truss covered bridges built during the historic era of covered bridge building, twenty-three examples survive in the United States, primarily in Ohio and Indiana.
The Smith truss featured parallel chords connected by a series of intersecting inclined posts and braces and was notable for being both economical and strong. Smith subsequently made several minor modifications to his design, and he received a second patent in 1869, but all Smith trusses followed the same general layout, with one, two, or three web planes, depending upon the length of the span.
Other inventors followed Smith’s lead, and at least two received patents that were geometrically similar to the Smith truss. In 1870, Isaac H. Wheeler of Sciotoville, Ohio, patented a modified Smith truss with offset compression members and an intermediate chord along the midline of the truss. Two years later, Reuben L. Partridge of Marysville, Ohio, patented another modified Smith truss with metal bearing shoes that were intended to reduce the amount of material required for the chords. Neither the Wheeler nor the Partridge trusses ever entered the mainstream of covered bridge building, but they illustrate a mid-nineteenth-century flowering of innovation in American timber-bridge design.
Decline of Covered Bridge Building
By the 1880s, bridge builders had learned how to best utilize the strengths of iron, and the material came to dominate bridge construction. Iron and steel bridges began rapidly replacing timber bridges across the country in the last two decades of the nineteenth century. Many towns and counties debated the costs of construction and maintenance of different types of bridges and were influenced by what neighboring towns and counties were building, which were often the prefabricated metal bridges widely marketed by bridge manufacturing firms. Railroads enabled manufacturers to ship prefabricated metal bridges to distant markets and thereby compete with local builders for contracts. The cost-effectiveness of iron, and later steel, was largely responsible for the decline of timber-bridge building in the early twentieth century.
In addition, the last decades of the nineteenth century saw the rise of the Good Roads Movement, as farmers and bicyclists and, later, increasing numbers of automobile users began to agitate for paved roads and modern bridges. Most engineers viewed wood as a material outmoded for modern purposes.
Lacking incentives to maintain aging timber spans, state highway departments across the country began bypassing or demolishing covered bridges and replacing them with steel and concrete structures that were wider, more open, and better suited for heavy, motorized vehicles traveling at increasingly faster speeds. Thus, during the first decades of the twentieth century, most Americans considered covered bridges neither particularly useful nor worthy of devotion; instead, they often viewed them as archaic, unsightly, and dangerous.
Birth of a Cultural Icon
The automobile brought physical changes to the landscape and was also the means by which growing numbers of individuals experienced their rapidly changing world. In the face of the technological and industrial advances of the early twentieth century, covered bridges within pastoral landscapes generated powerful feelings of nostalgia, and public interest in these picturesque landscape features began to grow.
In the 1930s and 1940s, covered bridges reemerged in the public imagination, becoming the subject of folklore and legend. They also regularly appeared in American popular culture as nostalgic, romantic, or mysterious elements. Many examples can be found in mid- twentieth-century literature, cartoons, music, radio, film, and television. Advertisers, hoping to associate their products with positive images of an idealized America, used covered bridges to market everything from insurance and thermal underwear to cigarettes and beer. A number of well-known companies, including Coca-Cola, Ford, McDonald’s, and Sears, used covered bridges in mid-twentieth-century advertising campaigns.
Beginning in the 1950s, people built small-scale covered bridges for their backyards and businesses, sometimes even demanding that “replicas” of old covered bridges be built on public roads. Curiously, while the housing of covered bridges was traditionally a practical consideration to extend the life of a timber bridge, it often became the primary focus of folk-art bridges and replicas. In many instances, the covering was merely a shed built over a concrete slab or steel-stringer bridge, creating the illusion of a covered bridge.
By the 1960s and 1970s, the iconic status of the covered bridge in the American collective memory resulted in communities across the country recognizing the historical importance of covered bridges and beginning to take steps to preserve them for future generations. In the last few decades, many covered bridges have become cherished local landmarks and symbols of community pride. Although covered bridges continue to be lost to floods, fires, neglect, and vandalism, far fewer are destroyed in the name of progress than previously, and many have been the objects of intensive preservation efforts.
Of the more than ten-thousand covered bridges that once existed throughout the country, nearly seven- hundred covered bridges built during the historic era of covered bridge construction still survive in twenty-nine states. Many of those are still in use on public roads, reaffirming that these venerable structures are more than just quaint relics of a bygone era; they are an integral part of our political, social, cultural, and engineering heritage that merits preservation.
