America's Covered Bridges. Ronald G. Knapp

Cooper provided detailed drawings of Burr’s experimental design at Trenton, New Jersey. The gigantic arches supported a combination of vertical iron chains and diagonal wooden beams to secure the deck, with only a partial roof over the arches. (Cooper, 1889; reproduced in Nelson, 1990: 17)

      Originally built uncovered, then covered, the wooden suspension design of the Mohawk bridge required additional piers by 1828. This photo, taken during its dismantling in 1873, allows us to see its deteriorated trusses. (NSPCB Archives, R. S. Allen Collection)

      Far less successful was Burr’s Delaware River bridge between Schenectady and Scotia, New York, built in 1808 in four spans following a failed attempt to cross the river in two spans. (NSPCB Archives, R. S. Allen Collection)

      Burr’s next project, aptly described by Cooper as “a curiosity,” demonstrates both Burr’s originality and his daredevil nature. According to Cooper, Burr’s first attempt to bridge the Mohawk River in 1808 between Schenectady and Scotia, New York, resulted in disaster when one of the two spans, each apparently 450 feet long, either collapsed or was swept away. His second attempt, according to the contract quoted by Cooper, placed the bridge at the same site but with additional piers. A double-lane bridge tightly covered under roof and batten siding at some point, it was an experimental wooden suspension bridge. Says Cooper: “The curved ribs are formed of eight 4 x 11-inch planks spiked or bolted together” (1889: 9). Cooper’s drawing, copied from that attached to the contract seen by him, shows an undulating wooden “chord” rising like an arch above the piers and sweeping to the deck level between, in the manner of a suspension bridge. While chain suspension bridges were already well established, it is doubtful anyone had tried to use laminated wooden “cables” in extreme tension. Additional bracing, both vertical and diagonal (the latter from the piers), could not maintain stability. According to an old photo, engineers had tried to reinforce the bridge with additional bracing (Mohr, 1957b: 5). By 1828, six years after Burr’s death, the spans were sagging dreadfully and had to be shored up with new piers between the original ones. Though it looked like a wet blanket stretched over supports and must have given passengers seasickness as they rose and fell over each span, this bizarre-looking crossing lasted until 1873. Though it served sixty-five years, it was clearly a design failure from the start.

      Perhaps recognizing that the Mohawk River plan had flaws, his next project was a three-span bridge over the Schoharie River at Esperance, first contracted in 1809 but not completed until January 12, 1812. An interior photo of this last Burr bridge to survive (until 1930) shows a double-lane bridge with three trusses. The downstream photo clearly shows that Burr had constructed three spans with gently curved arches below the roadway, a feature standard in Palmer’s bridges. The truss, however, bears no resemblance to Burr’s later design, having as it does double panels that resemble half-sized kingpost trusses. Such trusses were probably intended to strengthen the arches, which likely bore most of the weight.

      Following his work in New York State, Burr was lured to Pennsylvania eventually to build four superlative bridges over the mighty Susquehanna and a fifth over the same river in Maryland. While all but one were conventional covered bridges, using some form of what had come to be known as the “Burr truss,” none was of modest proportions, not surprising considering that the Susquehanna is, as some joke, “a mile wide and an inch deep.” The various bridge companies began applying for state approval in 1807. As each was approved, contracts were drawn up with builder Burr, all of which are discussed in Cummings’ 1956 article, which also includes a photo of a small wooden bridge model probably made by Burr himself and now preserved by the Pennsylvania Historical and Museum Commission. Between late February 1812 and 1815, Burr completed the five bridges in Pennsylvania, and before 1818 the sixth one in Maryland. From north to south, each was remarkable.

      This photo, taken shortly before the Mohawk bridge between Schenectady and Scotia was removed in 1873, shows how badly it sagged in spite of the extra piers, demonstrating that wood does not work well as a suspension member placing extreme tension on a material that works better in compression. (Ronald G. Knapp Collection)

      The Mohawk crossing was a failure in spite of added piers and an immense number of extra braces, giving the trusses the look of some of Europe’s heavily timbered covered bridges. (NSPCB Archives, R. S. Allen Collection)

      Built in 1812, Theodore Burr’s bridge over Schoharie Creek at Esperance, New York, used arches below the deck in the manner of Timothy Palmer. (NSPCB Archives, R. S. Allen Collection)

      This rare photo allows us to see the under structure, with the arches below the deck and a heavy deck system. (NSPCB Archives, R. S. Allen Collection)

      Burr’s bridge over Schoharie Creek at Esperance, New York, with only three spans and a single lane was among his simplest. It lasted until 1930. (NSPCB Archives, R. S. Allen Collection)

      1. Perhaps the most modest of Burr’s bridges, and the least known, is the multi-span bridge over the North Branch of the Susquehanna between Nescopeck Falls and Berwick, southwest of Wilkes-Barre.

      2. The bridge between Northumberland and Sunbury also crossed the North Branch near its confluence with the West Branch and was separated into two portions of three spans each by an island. According to the contract, there were two carriageways, each 11 feet 6 inches wide with a 4-foot-wide footway between. In the contract, the bridge had to be roofed with shingles and sided.

      3. The bridge at Harrisburg, like that at Northumberland, was divided by an island and in total consisted of twelve 210-foot spans in two segments of six each. Begun in 1812 and completed in 1817, the eastern portion survived until 1902 and the western until 1903. It was the western portion, however, which caught everyone’s attention because the roadway gently rose and fell with the gigantic arches that were embedded into the abutments and piers far below the deck. Because of this feature, the bridge was widely known as the Camelback Bridge.

      4. Although Burr is credited with designing the bridge between Columbia and Wrightsville, the contractors were Jonathan Walcott, Henry Slaymaker, and Samuel Slaymaker. It is hard to imagine how much stone and timber went into constructing a bridge consisting of twenty-six spans of 200 feet each, for a total of 5,690 feet, more than a mile in length and the longest covered bridge ever built. At that time, Pennsylvania was still covered with primeval forest and timber was easily and cheaply available. By the end of the century, however, with the expansion of the tanbark industry and the proliferation of tanneries dependent on a steady supply of hemlock bark and fuel, the building of tens of thousands of wooden houses in towns throughout the east, and countless mills and covered bridges built, including several replacements for Burr’s original crossing, Pennsylvania’s once magnificent hills and mountains had become denuded.

      5. If Columbia-Wrightsville was grandiose in proportions, the McCall’s Ferry Bridge, farther south where the river is squeezed through its narrowest point, was stunning for both its span length and the radically innovative manner of its construction. The river, racing between hills that brought about weather more turbulent than normal, had a deep narrow channel along with a shallow area in York County. At low water, that portion was exposed, and building a pier at this point was simple. The remaining distance to the Lancaster County shore, however, was 360 feet, and no one had built a successful span of this magnitude before. Theodore Burr, nonetheless contracted to do just that. But with the river bottom 100 feet below the bridge deck, there was no way to erect scaffolding. Burr decided to build his scaffolding on floats instead, and the arch was built standing