Bollman Suspension Truss Bridge

From www.marylandhistoricaltrust.net:

On an abandoned B & O Railroad spur in the village of Savage, Maryland, is one of America's more significant civil engineering relics. This two-span iron truss bridge is the sole surviving example of a type that played a critical role in railroad development, a story that has characteristically been dominated by the parallel progress of the locomotive. The design, patented in 1852, was undoubtedly inspired by the classical method of strengthening a wood beam by the addition of an iron truss rod below. The Bollman truss was invariably of composite construction. Those members subjected to tensile stresses were of wrought iron, while those in compression were of cheaper cast iron. Each panel point on the lower chord is supported by two diagonal tension members connected to the top of both end posts. In this way each floor beam of the truss is suspended from the cast iron end posts at each abutment. The major structural advantage of the design is that a failure of a diagonal tension member will cause the collapse of only a single floor beam; with most other truss designs, failure of a diagonal will cause the entire span to collapse. Bollman built the first of his trusses for the B & O at Savage in 1850. The structure now at the site, however, was fabricated in 1869 for use on the railroad's main line. The company moved the bridge to Savage in the late 19th century for use on a small spur line serving Savage Mills. It is a two-span bridge with a total length of 160'. Each individual span is 80' long. The bridge was last used in 1966.

The Bollman bridge at Savage, Maryland is the sole surviving Bollman truss in the United States, and possibly in the world. The system of bridge trussing invented by the Baltimore engineer Wendel Bollman (1814-1884) was the first to be used with consistency on an American railroad in which all of the principal structural members were of iron. The direct and intimate relationship of this bridge to two National Historic Landmarks should be noted. The Thomas Viaduct, in Howard and Baltimore Counties, and the Baltimore and Ohio Transportation Museum, in Baltimore City, have been so designated as fitting recognition of the vital role played by the Baltimore and Ohio Railroad in America's internal communication and transportation. The Bollman truss, it can be fairly stated, played as vital a role in the railroad's development as did any other single aspect of its early plant.