Ponakin Road Bridge, Spanning Nashua River on Ponakin Road, Lancaster, Worcester County, MA
Significance: The single span Ponakin Road Bridge is 100 feet long and 20 feet wide, with vertical end posts. The bridge has diagonal compression members and diagonal eye-bar tension members that extend two panels, with secondary tension rods extending over one panel. The upper chord lateral bracing is riveted and additional bracing is provided by diagonal cross bars. This truss was developed by Simeon S. Post in about 1865, with a configuration characterized by compression members inclining towards the center of the bridge. The post truss was an important bridge form in the second half of the 19th century, and widely used for transcontinental railway construction. The Ponakin Road Bridge is the only all metal Post truss bridge surviving in the U.S. This type of bridge is commonly referred to as the Post Patent Truss, but research in the Patent Office records failed to uncover any patents taken out by S.S. Post that describe a truss with inclined compression members. Nevertheless, this type of truss was widely known as the Post truss and it was built in great profusion throughout the East and Midwest between 1865 and 1880. After this time its popularity waned as the standardized Pratt truss began being built in great numbers. The Ponakin Road Bridge is listed on the National Register of Historic Places. The Ponakin Bridge is the only known surviving iron bridge to incorporate all of the design features of Simeon S. Post's patent for an "improved iron truss bridge." Post trusses enjoyed a brief period of popularity in the late 1860s and early 1870s. Railroads often chose Post's bridge for long-span river crossings. The Ponakin Bridge is an unusual example of a Post truss used for a relatively short-span highway bridge. The Ponakin Bridge served a small cotton manufacturing village on the west bank of the North Nashua River. The bridge has sustained some structural damage but has not been significantly altered.
Unprocessed Field note material exists for this structure: FN-12
Survey number: HAER MA-13
Building/structure dates: 1871 Initial Construction
Building/structure dates: 1973 Subsequent Work
Building/structure dates: 1978 Subsequent Work
National Register of Historic Places NRIS Number: 79000378