History of a Truss Bridge
When people reflect on "historic bridges," they most often envision covered wooden truss bridges. With its picturesque design, the wooden truss bridge has a near universal appeal. For many years, travelogues and historians alike have documented them and promoted their preservation, more than any other bridge type.
For centuries, builders used timber as a construction material for trusses, possibly even for truss bridges. However, it was not until 1570 that Andrea Palladio published Four Books on Architecture, the first written documentation concerning wooden truss bridges (Hayden 1976:51). Palladio, the first to promote the use of wooden trusses for bridge design, described several wooden trusses including the basic Kingpost and Queenpost designs. However, builders in Europe erected few wooden truss bridges until the eighteenth century, and then most commonly in heavily wooded countries such as Switzerland.
Beginning in the late 1700s, builders extensively erected wooden truss bridges in the United States, and by the mid-1800s, this country led the world in wooden truss bridge design (Steinman and Watson 1957:114). A combination of factors contributed to the quick rise of the United States in wooden truss design. In the mid-1700s the United States contained a very limited transportation system, and the Revolutionary War virtually destroyed this already inadequate system. By the late 1700s, the recently formed United States needed a much expanded and improved system. Further, while the iron industry did not have widespread influence, timber and men to mill the timber seemed limitless. Wooden truss bridges, which used short timbers built up in sections, seemed an ideal solution.
In the early nineteenth century, a variety of builders devised various bridge designs that they promoted. In a highly competitive and fluid field, every builder tried to devise "the" truss that would be economical, simple to construct, and viable for longer lengths. Out of a large number of builders, Timothy Palmer (1751-1821), Louis Wernwag (1770-1843), and Theodore Burr (1771-1822) led the development of wooden truss bridge construction in the United States (Steinman and Watson 1957:117-120). During this period, builders knew little about the specific mechanics of how truss bridges worked and their exact limitations. Thus, for greater strength and additional length, builders commonly utilized a combination arch and truss design, often called "camelback" or "hump" bridges due to the appearance of an arch (Ortega 1991:2-5). Both Palmer and Wernwag used as their main component an arch supplemented by a truss. In 1806 Burr introduced the first patented bridge system widely used in the United States, a truss supplemented by an arch (DeLony 1994:10). While Burr was the most famous of the three, Palmer also had a lasting and significant impact on wooden truss bridge design.
Contrary to common perceptions, builders did not originally cover wooden truss bridges. Palmer was one of the first builders in the United States to promote covering the wooden truss (the load bearing portion of the bridge) with a barn-like structure. In some cases, the covering provided lateral bracing, making the entire structure more resistant to wind shear. Yet, the covering primarily existed for the truss’s protection from the weatherization process. Noted engineer Henry Tyrrell stated in 1909 that the normal life span of a covered wooden truss bridge was thirty to forty years while an uncovered bridge might last one-third as long (Tyrrell 1911:121). However, chemical preservatives such as creosote applied to the timber members could also provide protection from the weatherization process. By the beginning of the twentieth century, builders increasingly used creosote rather than covering wooden truss bridges.
In 1820 Ithiel Town received a patent for the Town lattice truss, the first true truss that acted independently of any arch action (Hayden 1976:52-54). Due to the truss’s simplicity and ease of construction, many builders chose to erect Town’s lattice truss. Interestingly, it seems that Town more actively pursued selling his truss design than building it. He promoted his truss in a variety of ways, including the publication in 1831 of a pamphlet that his truss could be made from iron, but no builder tried it until 1859 (DeLony 1994:11). Town even employed agents to inspect new bridges and to collect royalties on his design (Allen 1970:4).
In 1840 William Howe patented the Howe truss, another truss that enjoyed widespread popularity. Howe based his design on the limited stress analysis information available at that time, the first designer to do so since previous trusses were unadaptable to analysis (Edwards 1976:156-157). The Howe truss used metal vertical tension rods and timber diagonal compression members. This joint use of metal and wood materials for bridge components, called a "combination truss," was a significant transitional feature in the eventual development of an all-metal truss. The popularity of the Howe truss resulted, in part, from its comparatively simple erection. The Howe truss design eliminated the need for skilled carpenters to notch and peg wooden jointed bridges by using threaded iron rods for verticals and simple junction boxes as connections (Kemp and Anderson 1987:19). As bridge historian Eric DeLony wrote, "The Howe truss may be the closest that wooden-bridge design ever came to perfection. For simplicity of construction, rapidity of erection, and ease of replacing parts, it stands without rival" (DeLony 1994:11).
In 1844 Caleb Pratt, an architect, and his engineer son Thomas designed the Pratt truss, another truss from this period that had widespread significance. While the configuration appears to be the same as a Howe truss, the Pratt truss’s verticals functioned as compression members and diagonals functioned as tension members. The Pratt truss required more iron than a Howe truss, and due to the increased cost and less rigid construction, builders did not extensively use it for wooden trusses. However, as the cost of iron declined, its popularity increased, and it greatly impacted metal truss bridge design. By the early twentieth century the Pratt truss and its derivations had become the most popular metal truss in the United States.
Wooden truss bridges provided a means to span large crossings efficiently. These new bridges not only facilitated transportation but also increased awareness and interest in bridge building. As a result, builders developed a variety of truss types and built numerous wooden truss bridges throughout the nineteenth century, the heyday of wooden truss design. At the same time, the construction of wooden truss bridges heightened awareness of the potential of truss designs and resulted in new variations in iron and later steel bridges. Even though builders erected a small number of wooden truss bridges into the twentieth century, beginning in the mid-nineteenth century, evolving designs in metal eventually eclipsed the use of wooden truss bridges and rendered them virtually obsolete by the end of the nineteenth century.
|Allen, Richard. Covered Bridges of the South. Battleboro, Vermont: Stephen Greene Press, 1970.|
|DeLony, Eric. "The Golden Age of the Iron Bridge." I&T: Invention and Technology, Fall 1994.|
|American Society of Civil Engineers. American Wooden Bridges. New York: ASCE, 1976. (reprint)
Edwards, Llewellyn. A Record of History and Evolution of Early American Bridges. Orono, Maine: University Press, 1959 (original)
|Hayden, Martin. Book of Bridges. New York: Galahad Books, 1976.|
|Kemp, Emory L. and Richard Anderson. "The Reading-Halls Station Bridge." Journal of the Society for Industrial Archeology 13 no. 1 (1987): 17-40.|
|Ortega Consulting. Statewide Covered Bridge Assessment for the Pennsylvania Department of Transportation. Pennsylvania: Ortega Consulting Media, 1991.|
|Steinman, David and Sara Ruth Watson. Bridges and Their Builders. New York: Dover, 1957.|
|Tyrrell, Henry. History of Bridge Engineering. Chicago: Henry Tyrrell, 1911.|