Metal truss bridges first appeared in the United States in 1840, and are still being used today. The metal truss bridge was the most common bridge type built between the 1870s and 1925. In 1999, when this project began, there were thirty metal truss bridges, dating from 1887 to 1942, remaining in Bradford County, Pennsylvania.  Now, twenty-four of those metal truss bridges remain.

Beginning in the mid-nineteenth century most of the bridges in Bradford County were constructed from wood (Watson 200:1). Natural causes, such as fire, flooding, and ice, damaged many of the wooden bridges. Bradford County Commissioners naturally turned to metal truss bridges to replace damaged wooden bridges. Most of the county’s metal truss bridges were built in the twentieth century, and half of those were erected in the century’s first decade.

Metal Truss Bridges

The truss form dates to ancient times. In America, wood truss bridges date to at least the early nineteenth century. Wood was readily available, and a more trusted material for bridge construction. A transition to metal trusses began in 1840, when William Howe patented a composite truss with wooden and wrought iron members. All-metal truss designs appeared within just a few years, but they were not widely used until after the Civil War, when they were embraced by the rapidly-expanding railroad industry. Iron had become more available, and the safety standards for building metal truss bridges had improved. Truss bridge technology transferred to the nation’s road and highway system in earnest in the 1870s (Comp and Jackson 1977:1-3).

Until the 1920s, metal truss bridges faced little competition from other bridge types. The metal trusses were stronger and more rigid than wooden bridges, and they were fire resistant. Due to competition among dozens of bridge firms then in operation and standardization of bridge designs, metal truss bridges were relatively inexpensive and easy to construct. Steel beam technology and reinforced concrete technology had not yet fully developed, so that metal truss bridges were the best way to span great distances.

A metal truss bridge uses many small pieces, or members, to make a beam long and strong enough to span distances. The individual components are connected in a series of triangles. Each truss member is either in compression or tension, and connections at the panel points can be either pinned or riveted.

Diagram taken from David Weitzman's book "Windmills, Bridges, and Old Machines: Discovering Our Industrial Past"

Most metal truss bridges constructed prior to 1900 used pinned connections. In a pin-connected bridge, a cylindrical metal bar joins the truss members together. This connection method required holes in the ends of each member that were then aligned together, so that the pin could be driven through all the holes to form a structural connection. Pinned connections were popular because they allowed the rapid erection of the trusses and made it easier to analyze the stresses in the truss members. However, they were susceptible to loosening, especially under the shaking caused by fast-moving vehicular loads (Jackson 1988:24)

Fueled by improvements in pneumatic field riveting equipment, builders of metal truss bridges made the transition from pinned connections to riveted connections around 1890 (Simmons 1997).

           Riveted connections also benefited from stronger steel, developed in the closing decade of the nineteenth century.  Where pin connections permitted stress points to move, the new stronger, riveted steel could accept the stress without fracture.  

            Concrete arches and beams would eventually supplant metal truss bridges as common waterway crossings.  By the turn of the twentieth century, concrete had been experimentally used in several waterway crossings.  By the advent of World War II, very few metal truss bridges were built; most were concrete.  Concrete had the advantage of being more weather-resistant.  In addition, most concrete bridges were deck structures, that is, the deck, or roadway, lay on top of the arch or beams.  Vehicle crashes into trusses could cause the bridge to fail.  Deck structures largely eliminate such a problem.  

            In the latter half of the twentieth century, very few truss bridges would be built.  Because the existing truss bridges were often built for traffic either before or in the early stages of fossil-fuel-powered vehicles, many have become to narrow or unable to carry modern loads.  In addition, their many exposed metal parts result in corrosion problems, further weakening them.  As a result, metal truss bridges, once ubiquitous, are now becoming a rarity on the landscape.

 
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Note: This site was produced as mitigation for the replacement of the historically significant Wyalusing Creek Bridge in Stevens Township , Bradford County .  Mitigation is required as a result of Section 106 of the National Historic Preservation Act of 1966, as amended.