UP Armitage Avenue Bridge (West)

This bridge carries the Union Pacific Northwest Line over Armitage Avenue west of Ashland Avenue in the Logan Square neighborhood of Chicago. During the late 19th Century, railroad traffic within the City of Chicago posed a serious hazard to the general population. Starting in the mid-1890s, the City of Chicago passed a number of ordinances mandating the elevation of tracks within the city. An ordinance was passed on December 27th, 1897 requiring the elevation of the Wisconsin Division of the C&NW between Sangamon Avenue and Wood Street, and the Milwaukee Division of the C&NW between the Wisconsin Division and Diversey Parkway. Construction began soon after, and was largely completed in 1899. This section required extensive retaining walls, which were constructed with stone.

Built in 1899, the bridge consists of a 62-foot and two 20-foot through plate girder spans, set onto steel bents and stone abutments. The structure consists of two parallel bridges, which share abutments but use separate piers. In addition, the bridge is set at a heavy skew. The girder spans are typical for early track elevation projects, consisting of a traditional floor constructed of stringers and floorbeams, covered by a sheet metal deck. Grooves under the rails act as both a ballast channel and stringers, an unusual variation of the sheet metal deck design. In addition, the ends of the girders are a rounded taper, typical of C&NW bridges. The bridge was initially constructed for three tracks, although the westernmost track was removed in the 1980s. Lassig Bridge & Iron Works fabricated the girders for this track elevation project. This company fabricated numerous spans for the C&NW, and had its shop along this line at Wrightwood Avenue. However, the industrial lead into the Lassig shops was too sharp to allow the spans for this project to be shipped whole, and the pieces were instead assembled at the division shops before being installed. Stone for the abutments was quarried at a railroad owned quarry near Duck Creek, Wisconsin, and the abutments and retaining walls were constructed by contract.

This type of structure was initially popular for grade separations in Chicago, but fell out of favor as trough floor spans and concrete spans became more economical. The metal floor of this design of bridge was susceptible to pooling water, which would cause corrosion to the steel. In addition, the through girder design limited horizontal clearances on the bridge. While other bridges along this line have been replaced, it is not known when this bridge will be replaced. Overall, the bridge is in poor condition, with significant section loss and deterioration seen throughout the superstructure. The author has ranked this bridge as being locally significant, due to the common design.