particularly those from the British Isles, knew stone to be an effective material for building bridges, but such structures were costly and time-consuming to build. There were too few skilled stone masons and builders to cross more than a few of the smaller streams, and crossing major rivers was out of the question. While true that stone bridges predominated in the British Isles, Britain had mostly small streams and modest rivers to cross.
More abundant and more easily worked was wood from America’s vast forests, which were filled with old growth trees of every kind. Perhaps a few of the German speakers had seen wooden bridges in central Europe, particularly in what are now Germany, Austria, and Switzerland, but if any were experienced bridge builders, we do not know their names. Thus, the settled immigrants in the British colonies, soon to become the United States of America, had to use their own ingenuity to solve the most pressing problem that blocked transportation: learning how to build bridges.
Among them were skilled wood joiners with experience building mills, barns, and houses. Sawmills were already well established. Builders understood how to create strong frameworks of beams and how to span distances, especially for roofs. Small foundries could make nails, bolts, and straps of iron. Stone masons could build abutments and piers for bridges using patterns similar to those in houses and mills. However, bridge builders found that wooden beams alone are neither long enough nor strong enough to span more than a limited distance, requiring in most cases a series of stone piers, and the techniques for building such structures within the moving waters of a mighty river were as yet unknown.
How much American bridge builders knew about wooden bridge building in central Europe remains uncertain, but the solutions that developed in the young United States suggest Yankee ingenuity rather than a continuing tradition. If there was an American timber framing tradition, it relied on fundamental principles rather than wholly learned bridge patterns. The two principles that form the basis of virtually all bridge solutions that emerged in America are, one, the rigidity of the triangle, and two, the strength of the arch. Beyond these, there was only trial and error, experience, and common sense since scientific analysis of stress in bridge design was unknown until the middle of the nineteenth century. In fact, Civil Engineering as a named field offering training in bridge design and construction developed long after that.
An understanding of what came to be the “American covered bridge” requires, first, an understanding of the basic solution to the stream-crossing challenge—the bridge truss. Builders realized that by joining timbers into patterns consisting of little more than triangles or by creating continuous arches they could build structures rigid enough and strong enough to span great distances, eliminating the need for extra stone piers, which were obstructions to the flow of the rivers and to navigation. Although some small iron fasteners and straps were available, most of the joinery required only wooden dowels, called trunnels (“treenails”).
American craftsmen used wood arranged into rigid patterns to span rivers. Ashulot, New Hampshire’s Upper Village Bridge, built in 1864, uses Ithiel Town’s patent design from 1820 that consists of planks held together with wooden dowels, called trunnels (from “treenails”). (A. Chester Ong, 2010)
A pair of extra long trunnels pierce both pairs of lower chords and the lattice planks of Georgia’s Cromer’s Mill Bridge built in 1906 by James M. “Pink” Hunt. (A. Chester Ong, 2011)
The heavy cross beams supporting the deck of this bridge in Washington County, Pennsylvania, are hung below the lower chord from an iron eye-bar above to a nut fastener below. (A. Chester Ong, 2011)
One of many of New Hampshire’s iconic bridges, the Haverhill-Bath Bridge crosses the Ammonoosuc River in Woodsville where water cascades over both rocks and an old industrial dam. Built in 1829 by the town supervisors for about $2,400, the two-span 256-foot-long Town lattice structure carried NH 135 until 1999. An earlier restoration added massive arches and a pedestrian walkway on the upstream side. (A. Chester Ong, 2010)
The centerpiece of Sullivan County, Pennsylvania’s tiny community of Forksville (pop. 145) is its bridge over Loyalsock Creek built in 1850 by Sadler Rodgers, then only 18 years old, using the Burr truss. (A. Chester Ong, 2012)
Builders came to find endlessly creative ways of combining timbers into individually named trusses based on the triangle principle, whether combined with an arch or not. Moreover, being entrepreneurs they sought to protect as well as capitalize on their 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
