America's Covered Bridges. Ronald G. Knapp
George Washington at Yorktown on October 19, 1781, Philadelphia continued to play a critical role as the seat of new national government, and from 1790 to 1800 it served as the temporary capital of the young United States while Washington, DC, was being built. In spite of a deadly outbreak of yellow fever, the city prospered, but its growth and communication with all points east were stymied by the Schuylkill River and the limitations of the ferry service first established in 1723. A movement to build a bridge on High Street, later called Market Street, began in 1750, but for numerous reasons—financial, technological, and political—only succeeded fifty-five years later when Palmer’s Permanent Bridge opened on January 1, 1805.
During this period, the best architectural and engineering minds of the city proposed numerous solutions. The first plan, submitted in 1767, was for a wooden bridge in a single arch some 400 feet in length and 47 feet above the water, but that proved unrealistic.
Two years later, on January 31, 1769, a Philadelphia architect named Robert Smith proposed building at least one wooden arched span, “. . . well covered to secure it from the Weather” (Griggs, 2009: 507). Though his proposal was ignored by the city assembly, it was the first known mention of covering a bridge in the colonies and foreshadowed Palmer’s Permanent Bridge. Another pioneering proposal appeared in 1774 when Thomas Gilpin of Philadelphia proposed building a suspension bridge with chains 400 feet long over the main channel approached by 300-foot-long abutments from each shore. Politician Thomas Paine, a man of many interests—both common and necessary in that day—proposed in 1786 both wooden and iron bridges, even building a 13-foot-long model of the iron bridge which he offered to Benjamin Franklin, who displayed it in his garden. The following year, Paine took that model to France, hoping for approval from the most highly respected bridge engineers in the world. Paine’s knowledge of iron bridges is all the more amazing considering that iron bridge technology had only just been introduced in England, first in a couple of obscure minor bridges and then prominently in Abraham Darby III’s amazing cast-iron bridge built over the River Severn in Coalbrookdale, England, between 1779 and 1781, only five years earlier. Because this was only possible due to the availability of iron from Darby’s nearby foundry, Paine’s idea for anything comparable in the United States was not yet feasible. Indeed, the city did not seriously consider Paine’s iron span because of concerns about its cost, the difficulty of procuring materials, and whether the abutments could withstand the weight.
In 1787, a plan with a drawing published in The Columbian Magazine (discussed earlier in this chapter) for a four-span wooden covered bridge with arches and a truss was likely a reworking of Smith’s 1767 plan. Who offered this proposal is unclear since Smith had died in 1777. Finally, two visionaries offered unrealistic plans for gigantic single spans, Frenchman Godofres Du Jareau in 1796 for a 300–400 foot wooden arch, and Charles Willson Peale’s own single arch, also discussed earlier. It is fair to say that rationality prevailed in most of these cases. Smith’s proposal for a wooden arched truss bridge, however, was the most realistic, but the main drawback was the impossibility of building the necessary piers in such deep water.
The term “permanent bridge,” in reference to the bridge Palmer eventually built on Market Street in Philadelphia, strikes some as odd, since clearly no bridge is ever permanent. The term derived from the desire to build a long-lasting solution to crossing the Schuylkill in contrast to earlier temporary solutions, and proposals, such as a military pontoon bridge, a floating log bridge, or a low stone bridge only passable in low water. Because this river, like most in the eastern United States, was prone to flooding and massive ice jams that swept all before them, even some of the more practical solutions that involved approaches restricting the river’s flow could not be considered. A complete history of the project appeared in 1806 that permits us not just to know the facts of Palmer’s bridge but, through the inclusion of numerous letters and other documents to hear the arguments for and against various aspects, especially the matter of covering the bridge. Originally a report written in 1806 titled “A Statistical Account of the Schuylkill Permanent Bridge, Communicated to the Philadelphia Society of Agriculture, 1806,” it was reprinted in 1815 in the Memoirs of the Philadelphia Society for Promoting Agriculture, Vol. 1.
Though the solution was not yet clear, Judge Richard Peters convinced the state legislature and governor to pass an “act for incorporating a Company for erecting a Permanent Bridge over the river Schuylkill, at or near the City of Philadelphia” on March 16, 1798, with Peters elected as President (Peters, 1815: 19–21). The Company offered 15,000 shares at $10 each for a total capitalization of $150,000, plus another 7,500 shares for unforeseen problems. Although still holding many unsold shares, the directors began searching for an affordable plan. Those proposed included one by William Weston, a British stone mason and engineer, who offered to build a series of stone arches. Another by Benjamin H. Latrobe, also of England, proposed a brickwork arch bridge. Still without a plan, the company hired a contractor to begin building the eastern abutment on October 18, 1800. Thanks to Weston’s advanced knowledge of hydraulics and coffer dams, the company decided to proceed with a pier in the shallower eastern channel, still hoping they might be able to bridge the western portion of the river (and the deeper channel) with a single span. Although challenging, the workers completed it during the fall of 1801. Judge Peters later wrote: “We know that no iron superstructure of such a span had been erected. We sent for Mr. Timothy Palmer, of Newbury Port, a celebrated practical wooden bridge architect. He viewed our site, and gave us an excellent plan of a wooden super-structure. But he pointedly reprobated the idea of even a wooden arch extending farther than between the position of our intended piers, to wit, 187 ft. He had at the Piscataway bridge, erected an arch of 244 feet; but he repeatedly declared, that wherever might be suggested by theorists, he would not advise, nor would he ever again attempt extending an arch, even to our distance, where such a heavy transportation was consistently proceeding” (1815: 71). There being no way to span the channel from the eastern pier to the western shore without an intervening pier, Weston and his masons proceeded to construct one in the deepest water—around 40 feet—beginning (oddly) on Christmas Day, 1802 and finishing it during the spring of 1803. Peters later wrote of this problem: “I have never in the course of my experience, or reading, heard of a pier founded in such a depth of water, on irregular rock, affording little or no support to the piles” (1815: 44).
Even though the piers were in place, Palmer, assisted by Mr Carr and other experienced workmen, only began building the superstructure in 1804, and although they expected the project to proceed quickly, in fact it took nearly a year to complete. But Palmer’s bridge, now benefitting from more than ten years of experience/ experiments in New England, was to be his magnum opus, a judgment accepted by observers at the time as well as the many travelers who later passed through the bridge and wrote glowing accounts of its great beauty.
The “Statistical Account” of 1806 summarizes the bridge’s dimensions: total bridge length was 550 feet, consisting of three spans, the outer two of which had clear spans of 150 feet each and the middle span an amazing 194 feet 10 inches. Adding the abutments and wing walls, the bridge covered 1,300 feet. The width was 42 feet, with an interior clearance of 13 feet for each of two lanes plus footways 5 feet wide on each side separated from the carriageway by “turned posts and chains.” The three trusses (two outer, one separating the roadways) each consisted of double arches rising from the masonry walls to the lower chords beneath the roadway plus slightly curved lower and upper chords rising 8 feet in a continuous arc (and avoiding the rise and fall of each span encountered in Palmer’s earlier bridges). The arches of the outer spans rose 10 feet while that of the middle span rose 12. The kingposts were not only radial but extended below the lower chords to the arches, as was also true of the braces. Each of the shorter spans had eight panels while the center span had ten. While the report says the carriage-way was 31 feet above the water, we are uncertain whether that was at the base or apex of the arches. In all, the bridge required 22,000 perches of stone—each equivalent to 24.75 cubic feet—and 1,500,000 board feet of timber. Its total cost came to nearly $300,000, a huge sum in those days.
Although the bridge opened on January 1, 1805, it remained unfinished in some ways and also uncovered. Judge Peters, though President, had been forced to accept the wishes of the Board in leaving the structure exposed to the weather. Some believed that covering it would cause water to be retained and prohibit drying, though Peters and Palmer believed that sealing the timbers