4 3/4 Mile Rd. / Pine River
Location: 4 3/4 Mile Rd. / Pine River
Year Built: 1903
About this Bridge:
Located over the Pine River about a mile east of Gordonville, this medium-span truss crosses the Pine River on the Gordonville Road. This pin-connected Pratt through truss, extending 120 feet, is supported by concrete abutments with angled wingwalls. The web members are built up from rolled steel sections from the Cambria mills of Pennsylvania. They are comprised as follows: upper chord and sloped end posts - two channels with cover and batten plates; lower chord - 2 looped rectangular eyebars; vertical - two channels with lacing (with two looped rectangular eyebars at the hip); and diagonal - two rectangular eyebars. The guardrails are lattice; The 12-foot-wide asphalt-covered concrete deck is supported by I-beam stringers and floor beams, which are field-bolted to the verticals. Other than the relatively recent installation of Armco guardrails and turnbuckled tension rods at the hip verticals, the bridge is unaltered. It is presently closed to vehicular traffic.
From the 1870s through the 1910s, pinned Pratt through trusses were the bridge of choice for medium- and long-span application in Michigan. Patented in 1844 by Thomas and Caleb Pratt, the Pratt design was characterized by upper chords and vertical members acting in compression and lower chords and diagonals that functioned in tension. Its parallel chords and equal panel lengths resulted in standardized sizes for verticals, diagonals and chord members, making fabrication and assembly relatively easy.
"The Pratt truss is the type most commonly used in America for spans under two hundred and fifty feet in length," noted bridge engineer J.A.L. Waddell.
In the manufacturing industry, in which efficiency equated with profit, Pratt trusses received almost universal use. Virtually all of the major regional fabricators manufactured Pratt trusses and marketed them extensively to Michigan's counties and townships in the late 19th and early 20th centuries. As a result, more Pratt trusses were built in the state during the period than all other truss types combined.
Built in 1903 by the Tunnel City Bridge and Iron Works of Port Huron, the Pine River Bridge fits well within the milieu of Michigan bridge construction. This bridge is the only known surviving example of the work of the Tunnel City Bridge and Iron Works of Port Huron, Michigan, one of only a half dozen Michigan-based metal truss bridge companies to operated in the nineteenth and early twentieth centuries.
Moved to private property in the late 1990's.