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4 3/4 Mile Road Bridge

   

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Bridge Documented: 2006

Primary Photographer(s): Nathan Holth

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Key Facts

	Bridge Name	Facility Carried / Feature Intersected	Location	Structure Type	Construction Date and Builder/Engineer
	4 3/4 Mile Road Bridge	4 3/4 Mile Road Over Pine River	Rural: Midland County, Michigan	Metal 8 Panel Pin-Connected Pratt Through Truss, Fixed	1903 By Builder/Contractor: Tunnel City Bridge and Iron Works of Port Huron, MI

Technical Facts

Main Span Length	Structure Length	Roadway Width	Main Spans	NBI Number
116 Feet (35.4 Meters)	120 Feet (36.6 Meters)	15 Feet (4.6 Meters)	1	56314H00009B010

View Archived National Bridge Inventory Report - Has Additional Details and Evaluation

This bridge is the only known bridge in existence built by the Tunnel City Bridge and Iron Works of Port Huron, Michigan. It is thus extremely important as what may be the last physical remnant of a little-known Michigan bridge company. In case you are curious why the company named themselves "Tunnel City" consider Port Huron's history during the period the bridge company was in operation. In 1891s, the first full-size sub-aqueous tunnel in North America was completed, running from Port Huron, MI and Sarnia, ON to serve trains. In 1903 when the 4 3/4 Mile Road Bridge was built, this key international tunnel would have been a major source of fame for Port Huron. Engineers would have possibly nicknamed Port Huron "Tunnel City" back then. Click here to visit HAER's page for this historic tunnel.

This bridge has been moved for a number of years, as MDOT's website mentions, and it took some searching to find who moved the bridge and where it moved. It turns out that a nearby resident, Jim Hyatt, moved the bridge to a nearby private drive and restored it. HistoricBridges.org is appreciative that this beautiful piece of Port Huron heritage was saved from the dumpster, which is where the Midland County Road Commission was going to put it.

Constructed in 1903, this bridge is a classic example of a pin connected Pratt through truss. The bridge is composed of eight panels yielding a 120 foot span. V-lacing is present on the vertical members, and lattice is present on the portal bracing. The original railings on the bridge are also lattice.

The bridge has been altered, most notably with extensive plate steel welded to bottom chord elements. In addition, the flooring system was also replaced, but with a wooden deck. Although MDOT's website mentions that the deck of the bridge was originally concrete, wood was a common surface for pin connected through truss bridges.

The unusual, simple plaque on the bridge which is composed of a thin plate with punched holes spelling out the bridge builder and date might at first glance appear to be not original. However there are a couple other bridge companies who created plaques of this type. After careful consideration, HistoricBridges.org believes the plaque to be original.

HistoricBridges.org thanks Jim Hyatt for allowing a visit and and photo-documentation of this important bridge, and also for choosing to save this historic bridge from certain doom!

Information and Findings From Michigan Historic Bridge Inventory Narrative Description 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 cloes 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.

Photos and Videos: 4 3/4 Mile Road Bridge

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Bridge Photo-Documentation

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