Truss bridges have a long pedigree. The first ones were made of wood and erected in various European countries at least as early as the 16th century. Beginning in the late 1700s, many of them were built throughout the United States. “Combination trusses,” which included both wood and metal bridge components, began to appear in the 1840s. Beginning in the late 1860s, all-metal truss bridges became widespread, particularly within the railroad industry.
Trusses are assemblies of beams or other supports, typically arranged in combinations of triangles. Truss bridges take many forms. One early truss-bridge design, the bowstring-arch truss, was patented in 1841 by Squire Whipple, a Massachusetts engineer who is considered the father of iron bridge building in the U.S. He also invented the Whipple Truss, a design employed on the Cairo Rail Bridge, which was completed in 1889 and spanned the Ohio River, near Cairo, Ill. Its two longest spans each measured 158 meters. At 3,220 m overall, it was, at the time, the longest metallic structure in the world. Truss bridges were a popular choice of bridge designers in the U.S. from the 1870s through the 1930s.
One variety of truss bridge played an important role in World War II. A civil engineer working in the British War Office, Donald Bailey, designed a portable, prefabricated truss bridge in 1941. Its modular components were 10-ft x 12-ft rectangular units that, once assembled, formed bridges that were strong enough to support 35-ton Sherman tanks. They did not require special tools or equipment to assemble. It was first used by British forces in North Africa in 1942. Known as the Bailey Bridge, it was widely used by British, American and Canadian forces on the move, often in situations in which retreating German and Italian armies had destroyed bridges.
Over 3,000 Bailey Bridges were employed by the advancing Allied armies in Sicily and Italy alone, and many others enabled Allied forces to advance on battlefronts in Asia against Japanese forces. Gen. Eisenhower, commander of Allied forces in Europe, gave this tribute: “The three major inventions of the war were radar, the heavy bomber, and the Bailey Bridge.” Bailey Bridges continue to be used widely, both by military units and civilian contractors.
“Truss bridges have long been important to the rail industry—six Ohio River crossings and one Mississippi River crossing are all functioning trusses that are over 100 years old,” says James N. Carter Jr., chief engineer of bridges and structures at Norfolk Southern Corp. “After 100 years, we see that the main trusses are in pretty good condition, but we have problems with the brace members and the floor systems. [With proper rebuilding] I believe they’ll be there 100 years from now. The last major river crossing we redid was a bridge across the Mississippi River at Hannibal, Mo., with six truss spans." The spans ranged in length from 180 ft to 409 ft, and the project was completed in 1993.
“The essential value of a truss is that it’s a very efficient use of a material: You use the distance between the material to give you strength. [The distance] makes it more stable, more rigid," he adds. "Also, trusses lend themselves to repair and replacement of individual members. Members are riveted together and replaced with bolts, and it can be done fairly easily. In the rail industry, we don’t have the luxury of putting up a detour sign. When we have to do a repair, we’re looking for a way to get in and out quickly to reduce interruption to our network."
"We’re to the point in the highway industry where we’re pushing steel girders out to 150 ft. In the rail industry, over 150 ft, we’re probably going to choose a truss. Below that, girders are much more economical,” says Carter.
Truss bridges continue to play a significant role in highway construction. “There’s still a place for truss bridges for highways, but less so than in the past," says Jamey Barbas, a long-span bridge expert. “Truss bridges are very stiff bridges. There’s a lot of steel in them and, therefore, inherently, a lot of maintenance. [When] comparing your options, cable-stayed or suspension bridges are usually more economical choices.”
“Usually, truss bridges are older and, oftentimes, conservatively designed with extra capacity, allowing for the flexibility of adding to them. Also, trusses may be favored for double-decker or combined highway-rail bridges,” Barbas adds.
Barbas has worked in the New York City area on several major bridge rehabilitation projects involving bridges with truss components. “On the Williamsburg Bridge [a suspension bridge with truss components], we had to reinforce the truss chords with reinforcing plates to give it extra strength to deal with increased truck weight and comply with modern codes. The Williamsburg Bridge is a utilitarian workhorse. It was able to be retrofitted 100 years later for an additional 100 years of service life."
“The Bronx-Whitestone Bridge was a suspension bridge that utilized a stiffening truss," Barbas also notes. "When they wanted to reduce the load, they removed the truss and added wind-faring elements for aerodynamic stability.”