This steel bridge is one of the most unusual bridges ever to be encountered. It is an arch bridge that does not have a traditional arch shape. Rather than having a uniformly curved arch rib, or even an elliptically curved arch rib, the arch rib is not curved and is instead polygonal, with long straight sections that change angles at connection points. However, the most unusual detail is at the center two panels of the structure. At these panels, the top chord of the superstructure bends down sharply. At the same panels, the arch rib bends up. As a result, at these panels, the angle of the top chord lines up with the arch rib. The top chord and the arch rib meet together at the central crown hinge pin of the arch. The crown hinge pin is about six feet below the center of the horizontal portion of the top chord. Because of this bizarre detail, the bridge has the appearance of being pinched in the center, and has the appearance of being two halves. To a casual viewer, it might appear like a truss that is cantilevered out to the center. However, unlike a cantilevered truss, each half of this arch bridge requires the other half in order to stand. Despite the unusual shape that the arch rib follows, the bridge does function as a three hinged arch bridge. Further evidence of the arch function of the bridge is found at the abutments. The arch ribs terminate at hinges and skewbacks, rather than resting flat on a bearing as a truss bridge would do.
The aforementioned unusual detail at the center of this bridge was decided upon after consideration of other designs. The unusual design reduced the materials required for the bridge, while also avoiding unwanted stresses that were present in other designs.
In general, this unusual bridge appears to be a little known secret. It does appear that an arch bridge was chosen for this location because of the topography. The river here cuts a fairly steep and rocky gorge (on that makes photography of the bridge difficult for those without a boat). This topography lends itself well to the function of a steel deck arch bridge. The Historic American Engineering Record reported that the bridge is probably the second bridge at this location, since the Chicago, Milwaukee, and St. Paul Railroad reached Iron Mountain in 1886. A period article about the bridge stated that the previous bridge was a Pratt deck truss with a 225 foot span. A historical photo of the arch bridge under construction appears to show the previous bridge in the background as a double intersection Pratt (Whipple) deck truss. The arch bridge was built in 1902. It includes a series of approach spans. Each end has two 75 foot deck plate girder approach spans. The main span was listed in the Historic American Engineering Record as 180 feet (54.9 meters) long, and in period article about the bridge as 207 feet (63.1 meters). The bridge width center-to-center of the arches is 22 feet (6.7 meters). The abutments for the bridge are concrete. The approach spans also have steel bents for support, which in turn rest on concrete foundations.
A 1910 Handbook of Cost Data for Contractors and Engineers provided the following information:
A single track, three-hinged steel arch bridge was finished in 1903 across the Menominee River, Michigan, for the Chicago, Milwaukee & St. Paul Ry., replacing a steel bridge built 17 years previously, which had grown too light for the traffic. The bridge is 355 ft. long, consisting of a three-hinged arch of 207 ft. span and four plate girder approach spans of 39% ft. each. The trusses are 22 ft. c. to c. The arch has a rise of 52 ft. The bridge is designed according to Cooper's specifications for a live load of two consolidation Class E-50 locomotives and 7,000 lbs. per lin. ft. behind the engines. The weight of steel in the arch span is 480,000 lbs., and in the approach spans it is 150,000 lbs.
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