ELS Menominee River Bridge

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Located between Kingsford and rural Florence County, Wisconsin; this unique deck arch bridge carries the former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) mainline over the Menominee River. Due to the unique terrain and the river banks being composed of solid granite, constructing a bridge at this location required a minimal amount of masonry. The first bridge at this location likely consisted of a wooden deck truss span, constructed when the line was first built. In 1886, the original bridge was replaced by a 256-foot, 11-panel, pin-connected Whipple deck truss span, approached by timber pile trestle approaches and set onto chiseled granite piers. This span was fabricated by the Morse Bridge Company of Youngstown, Ohio and used features typical of mid-1880s truss spans, such as light built-up members and a standard floor. By the turn of the 20th Century, this bridge had become too light for traffic and it was desired to replace the bridge with a new steel span. Instead of constructing a new deck truss span, it was decided to replace the bridge with an innovative arch design, which would minimize interference to the existing truss span and reduce the amount of new masonry required. It is believed that work began on the bridge in 1901, and was completed in 1902. Upon completion, the deck truss span was removed, disassembled and stored at an unknown location. In 1906, the span was rebuilt and reused at Dunnville, Wisconsin, where it continues to carry a trail.

Currently, the bridge consists of a single track 207-foot, 9-panel, three-hinged, solid rib deck arch span, approached by two 39-foot deck plate girder spans on either end. The bridge is set onto concrete substructures, and the approaches are supported by steel bents set onto concrete pedestals. While the main span is considered to be an arch, it does not use a traditional arch shape with a uniformly curved arch rib. Instead, the arch rib is polygonal, with long straight segments that change angles at the connection points. The most unusual detail of the arch is the center panel, where the top chord and arch rib bent sharply and cross at a pin connection, known as the crown hinge pin. Due to this unique feature, the bridge has the appearance of consisting of two cantilevered halves. Members of the arch are traditionally composed for the era, consisting of a combination of heavily built-up beams and eyebars. Both the top chord and arch rib consist of built-up beams, consisting of X-lacing on both sides. At the outer panels, the design of the rib changes, and uses a much heavier and tighter X-laced design. The vertical members use a combination of built-up V-laced beams at the outer panels, while the vertical members surrounding the inner panels consist of built-up beams with no lacing. The diagonal members at the outer panels are composed of lighter V-laced members, while the diagonal members near the center of the span consist of eyebars. The truss is heavily braced, with eyebars connecting the two halves of the arch. In addition, the upper lateral bracing uses steel plates, while the lower lateral bracing consists of V-laced beams. Also unique to this bridge, transverse members are present across the outer two panels, and are composed of V-laced beams. The floor system follows a typical design for truss spans from the era, with heavy plate girder stringers and floorbeams. Unique to this bridge, the stringers rest on top of the floorbeams, and the outer floorbeams consist of a lattice girder. The truss span uses a combination of riveted and pinned connections, with the pinned connections located at the center three panels. The approach spans follow a standard shallow design, consisting of two plate girders and an open floor. The bents are composed exclusively of V-laced beams.

While the bridge has an unusual "pinched" shape, the structure functions as a true three hinged arch. At the main piers, the arch ribs terminate at hinges instead of a standard bearing as a truss span would. The center span consists of approximately 240 tons of steel. The unusual pinched design was chosen after consideration of other designs, as this design reduced the total material required and avoided unwanted stresses that had been found in other designs. Typical of Milwaukee Road bridges, the entire bridge was designed in-house by Charles Frederick Loweth, Engineer and Superintendent of Bridges and Buildings. Also typical of Milwaukee Road bridges, the substructures were constructed by labor employed by the Milwaukee Road Bridge & Building Department. Phoenix Bridge Company fabricated the superstructure as part of a large order placed by the Milwaukee Road in 1901. Steel arch bridges are relatively uncommon in the United States, due to the complex designs and amount of materials required. It is even more unusual for a internal engineer to design such an unusual structure, which required precise calculations. Since the initial construction, the bridge has seen no significant alterations, and remaisn in regular use. Overall, the bridge appears to be in good condition, with no significant deterioration noted. Due to the terrain at this location, photographs of this bridge are difficult to come by. The author has ranked this bridge as being nationally significant, due to the extremely unusual design.