America's Covered Bridges. Ronald G. Knapp

designs through the newly developed patent system, which promised an inventor a fee for the use of his design. As was typical of what many identify as the “American spirit,” designs proliferated, some becoming wildly successful, others having little or no application, and some appearing downright whimsical. What came to be called the “American covered bridge” initially was simply a “bridge,” and the story of the American covered bridge is essentially one of innovations in framing trusses.

      The United States conducted its first census in 1790, ascertaining that there were slightly fewer than four million people living in what had recently been thirteen colonies, plus the districts of Kentucky, Maine, Vermont, and Tennessee. All but around 200,000 citizens were classified as “rural,” for America’s cities were then mere towns compared to what they would become. In 1790, New York City had but 33,131 people, Philadelphia only 28,522, and Boston a mere 18,320. Even with immigration, the US population would not exceed 100 million until the 1920 census. What this tells us is that relatively few people had to perform the tasks required to build the new nation, including its bridges.

      Thus, the self-appointed, amateur “civil engineers” who took it upon themselves to solve the river crossing problem were forced to improvise based on common sense and experience. Having no known direct knowledge of Germanic timber bridges, they had at their disposal few documents that might provide clues. One of these was the writings of Italian architect Andrea Palladio (1508–80), the person most responsible for codifying classical architecture and designing numerous significant buildings and churches that now characterize the Italian Renaissance style. His I quattro libri dell’architettura [The Four Books of Architecture] was first published in 1570 and came to be known to the English-speaking world after an illustrated translation was published in London in 1738. In this document, Palladio described both in words and drawings four proposed wooden bridges, only one of which is known to have been built. All were simple trusses based on two of the most fundamental patterns, the kingpost and the queenpost, which are described in the next chapter. None was capable of spanning more than about 50–60 feet, however. While we can document the possible availability of Palladio’s drawings to American builders, there is no way of knowing whether they actually replicated his basic designs into their own bridges.

      Two Swiss bridges and their intricate structures were known to potential engineers in places like Philadelphia and New York because numerous travelers had provided detailed diagrams, some published in the United States. The first was the immense two-span covered bridge at Schaffhausen, Switzerland, built by Hans Ulrich Grubenmann between 1756 and 1758. Originally intended to be a single span of 364 feet, the city elders insisted on two more modest but unequal spans measuring 193 and 171 feet respectively, each supported by an elaborate system of overlapping queenposts plus a system of struts reinforcing the ends. The second bridge was built in the 1760s by members of the Grubenmann family at Wettingen over the Limmat near Baden. Its 200-foot span was supported by a massive seven-layered, iron-banded, laminated arch nearly seven feet thick attached to the frame. Although not typical of American bridges then or later, at least one of eminent American builder Theodore Burr’s massive bridges, the multi-span bridge crossing the Delaware at Trenton, which took three years to build (1804–6), closely resembles the Wettingen bridge. Engravings of both Swiss bridges clearly showed them to have been covered with roofs. From references in several documents, we know that a few Americans knew the European tradition of covering wooden bridges for protection.

      Andrea Palladio included plans for three truss bridges in his 1570 I quattro libri dell’architettura [The Four Books of Architecture], two reproduced here: (left) 1–15, a queenpost design, and (right) 1–16, a multiple kingpost design. (Palladio, English translation, 1738)

      Now long gone, Schaffhausen, Switzerland’s famous bridge over the Rhine, was much noted by travelers and artists, here immortalized in the hand-colored etching by Johann Heinrich Bleuler (1758–1823). The 364-foot two-span bridge was built between 1756 and 1758 by Hans Ulrich Grubenmann, whose complex design involving numerous polygonal arches and various struts attracted the attention of early civil engineers. (© Trustees of the British Museum)

      Although there is the possibility that America’s self-made bridge builders could have known the work of Palladio and the Grubenmann families, there is no known direct evidence to connect them to those specific designs or bridges. What is so remarkable about the nation’s first generation of bridge engineers is that they not only, as it were, had to “reinvent the wheel” but did it with an unparalleled boldness bordering on the brazen. Civil engineering historian Lee H. Nelson’s concise but definitive study, The Colossus of 1812: An American Engineering Superlative (1990), focuses on the work of four exemplary early builders and their bridges: Charles Willson Peale (1741–1827), Timothy Palmer (1751–1821), Theodore Burr (1771–1822), and Lewis Wernwag (1769–1843).

      Among the first problems encountered by early bridge builders was that many of the young American towns needing bridges sat beside broad tidal rivers which often flowed deep. Some were more than a thousand feet wide, others even half a mile, and in one case, the Susquehanna in central Pennsylvania, more than a mile wide at some points. All were major impediments to transportation, trade, and urban expansion. Frances Manwaring Caulkins’ History of Norwich, Connecticut sums it up well: “. . . roads could not have been opened and rendered safe for traveling in any direction without spanning a multitude of small streams with some kind of stone-work, or with timber and plank, and these perhaps the next spring flood would sweep away. Consequently the work of building and repairing bridges was always beginning, ever going on, and never completed” (1874: 343).

      Partial plan of Schaffhausen Bridge showing both trusses and cross sections. (Fletcher and Snow, 1934)

      Until the French developed deep-water caisson technology during the nineteenth century, builders of multiple-span bridges had to contend with the difficult task of placing stone piers mid river, sometimes utilizing simple coffer dams, which were temporary enclosures within a stream. It was not until 1872 that James B. Eads, the builder of the Brooklyn Bridge, took out his path-breaking patent for a watertight caisson structure that greatly facilitated the construction of deep-water bridge piers. Until then, builders were only able to construct piers in relatively shallow rivers and consequently were forced into creating (or at least imagining) grandiose spans for other situations. Some early builders proposed and even built some of the most daring bridges in American history.

      Thomas Pope patented his “Flying Pendent Lever Bridge” in 1807 and published plans in his 1811 A Treatise on Bridge Architecture. There he claimed (imagined) that he could build an 1,800-foot covered arch over the East River in New York City. (American Philosophical Society)

      Charles Willson Peale published his proposal for a 390-foot laminated arch bridge over the Schuylkill at Market Street in Philadelphia in An Essay on Building Wooden Bridges. (Peale, 1797; reproduced in Nelson, 1990: 7)

      To get an idea of their boldness, consider New York “architect and landscape gardener” Thomas Pope, who patented in 1807 his “Flying Pendent Lever Bridge,” a rainbow-like “grand parabolic arc,” which is described in detail in the last 50 pages of his 288-page A Treatise on Bridge Architecture published in New York in 1811. That same year he came to Philadelphia and exhibited a model of half of such a bridge which was, by his account, some 50 feet long and weighed 10 tons (1811: 23).

      During this time, Pope proposed to build several bridges, but only that for the Lancaster-Schuylkill or Upper Ferry, the future location of a bridge by Wernwag, has survived. Presumably using a plan like his “Flying Pendent Lever Bridge,” he proposed a single span of 432 feet, 46 feet above the water, and estimated to cost $50,000 exclusive of the abutments and covering. That the city fathers ignored the proposal