America's Covered Bridges. Ronald G. Knapp

earlier, Richard Sanders Allen had featured the bridge in his Covered Bridges of the Middle Atlantic States (1959) and provided a color reproduction of the painting by Thomas Birch as the frontispiece.

      Even though Palmer’s Permanent Bridge crossed the river nearby, a group of businessmen proposed a second crossing on January 30, 1811 and called for proposals. At first, only two proposals came in, an implausible one by the visionary Thomas Pope, and two designs—now lost—by architect Robert Mills, one uncovered, one covered. Only later did Wernwag step forward with a design for a triple-arched structure reinforced with a truss with an unprecedented clear span of 340 feet with two lanes. On December 5, 1811, the company contracted with Wernwag based on the believability of his presentation and the use of a well-constructed scale model to build the bridge. Wernwag demonstrated great wisdom in anticipating a number of problems and designing solutions, especially the problem of rot developing between members: “This Bridge has a superiority of any other, having near 100 feet span [sic], more than any in Europe or America. The dry rot is entirely prevented by the timber being sawed through the heart, for the discovery of any defect & kept apart by iron links & screw bolts, without mortice or tennon, except the king posts & truss ties. No part of the timber comes in contact with each other, & can be screwed tight at any time when the timber shrinks. Any piece of the timber can be taken out & replaced if required without injury to the Superstructure” (quoted in Nelson, 1990: 21).

      Wernwag’s design is known to such an extent that it could be reproduced today. The main structural element was a series of massive wooden arches, each composed of six members joined in pairs to create three arches. Because of his concern for dry rot, Wernwag designed an ingenious iron bracket, two per panel, that separated the members and yet preserved their working in union. The arches together acted as the lower chord. Above it were panels having double 6 x 12 inch kingposts and each made rigid with braces and counterbraces, apparently of equal size. There were systems of iron bars, rods, and clamps, including ones that anchored the entire bridge diagonally back into the abutment walls, throughout the structure. Lateral bracing was anchored to what he called iron “boxes” placed on the tops of the kingposts. The upper chord did not run parallel to the arches, having a flatter curve, giving the bridge a flared shape, with the middle narrower than the ends. Overall, the design was as sophisticated as any ever proposed in the United States or Europe, and the ease with which Wernwag completed the construction was also amazing.

      Lewis Wernwag’s Colossus, opened on January 7, 1813, was 340 feet long in a single span, the longest such bridge at that time. While it could have served many more years, arsonists destroyed the structure in just twenty minutes in 1843. (Yale University Art Gallery)

      “A View of Fairmount and the Water-Works, c. 1837” (watercolor, pencil and gouache) was painted by John Rubens Smith from the perspective of a hotel verandah looking towards Wernwag’s monumental Colossus. It clearly shows the bridge within the context of human life at the time in a city that was still quite compact. (Bridgeman Art Library)

      Wernwag designed a bridge primarily supported by three compound arches that doubled as lower chords with an upper chord that, while curved, was non-parallel. These were made rigid with posts and braces. (US Patent Office)

      William Strickland’s painting, “A View of a Bridge over the Schuylkill River” (oil on canvas), actually shows America’s two most important bridges of the time, Palmer’s Permanent Bridge in the foreground and Wernwag’s Colossus upriver in the distance. Although part of Philadelphia’s city streets, the surroundings indicate how much open countryside remained. (Bridgeman Art Library)

      The bridge, which was not covered at first, was officially opened on January 7, 1813. As related years later by John Wernwag, Lewis Wernwag’s son, there was great apprehension that when the scaffolding was removed, the bridge would fall. Wernwag famously brought the nervous company managers to the bridge before the ceremony, who asked, “Well, Lewis, do you think our bridge will stand the test today?” Wernwag answered, “Yes, gentlemen, it will” (quoted in Nelson, 1990: 24–5). He then showed them that he had removed the blocks between the bridge and scaffolding the day before, and the bridge was already supporting itself. It was not until March 13 that the company agreed to “roof and finish” the bridge with a shingled roof and ten windows with shutters at a cost of about $90,000.

      Although Wernwag’s record for building the longest single-span wooden bridge fell to Burr’s short-lived McCall’s Ferry Bridge in 1814, nature was kinder to the “Colossus of Fairmount” than to the McCall’s Ferry structure, destroyed by flood waters only three years later. A hurricane that struck Philadelphia on September 3, 1821 tore off the roof and siding of the Colossus. But just four years before Wernwag’s passing, in 1843, arsonists accomplished what nature had failed to do: by setting fire to the bridge and causing it to fall into the river within only twenty minutes. The crossing remained without a bridge for four years, until Charles Ellett Jr built an innovative suspension bridge using woven wire rope.

      Having begun his bridge building career with his capstone achievement, Wernwag could devote himself to less ambitious projects. As related by Richard Sanders Allen: “From then on, Wernwag practically had to fight off agents from bridge companies. Usually they would offer a big block of stock in their infant organizations, but sometimes there was cash on the barrelhead. The now famous inventor chose only the best offers, and the next few years found him building bridges across the Delaware at New Hope, over the Schuylkill at Reading, and spanning the Susquehanna at Wilkes-Barre. Out in Pittsburgh in 1816, he erected that growing city’s first bridges across the Allegheny and the Monongahela” (1959: 16).

      Of these, one in particular stands out, the bridge over the Delaware River at New Hope, Pennsylvania, which was built between 1813 and 1814. The six spans of 175 feet each, for a total of 1,050 feet, were constructed using an innovative truss designed by Wernwag and apparently not used again. As was typical in that day, the bridge was 32 feet wide, with two carriageways and flanking pedestrian walkways. According to an engraving that showed this design, along with that of the Colossus and a swing bridge, the structure consisted of a laminated or multi-sectioned arch resting on the lower chords at each end, and a panel truss of wooden kingposts and iron rods apparently in tension, because they are placed diagonally from the center towards the ends—the reverse of the wooden braces found in more usual trusses. There appear to be numerous iron separators and clamps similar to those he proposed for and used on the Colossus.

      After this, Wernwag became involved in a number of industrial projects, but the business practices of the day made such investing risky, and Wernwag’s involvement in what came to be the Phoenix Works, a large mill making iron implements, came to naught, when, as a result of the war of 1812, the firm went bankrupt. He also invested in a lumber mill at Conowingo, Maryland, which manufactured precut timbers for trestle bridges in Tidewater Virginia, and there he built a ten-span covered bridge over the Susquehanna to facilitate customers crossing to his mill. When Burr’s Port Deposit Bridge was heavily damaged by fire in 1823 and 1828, Wernwag was called in each time to rebuild the lost spans. Finally, in 1824, he moved to Harpers Ferry, Virginia (now West Virginia), where he purchased an island in the Shenandoah near its junction with the Potomac. Emory Kemp sums up his career: “In 1824 he purchased the Isle of Virginius at Harpers Ferry, Virginia . . . and there established a manufacturing center. This move to Virginia brought him into contact with the Baltimore and Ohio Railroad (or B&O), for which he built bridges, the most notable being a Y-shaped bridge over the Potomac at Harpers Ferry [1842]. Ironically, one of Wernwag’s smallest structures was to become famous as a result of John Brown’s raid in 1859—the engine house at Harpers Ferry that sheltered Brown and his men during the abortive raid” (2005: 11).

      As with Burr and Palmer, none of Wernwag’s bridges have survived, though the engine house remains as part of the Harpers Ferry National