UP Main Street Bridge (Rockford)

View an article describing some early C&NW solid floor designs

Located in Rockford, this unique bridge carries the former Chicago & North Western Railway over Illinois Route 2 (Main Street). The first bridge at this location was likely a timber pile trestle, constructed in the mid-19th Century. In 1876, the bridge was lengthened, and a new iron bridge and stone abutments constructed. This bridge likely utilized either an iron through truss or pony truss design. The bridge was again reconstructed in 1893, in conjunction with a new passenger station immediately east of the bridge. At this time, two additional tracks were added to the structure, giving the bridge its current configuration. The bridge consists of a 73-foot span, set onto stone abutments. The center track utilizes a heavy riveted, 7-panel double intersection Warren pony truss span, while the outer tracks are set between a truss line and a through girder. The floor of the bridge is constructed of steel troughs, which were fabricated by the Pencoyd Iron Works. American Bridge Works fabricated the superstructure, and it is unknown if the substructures were constructed by contract or by railroad forces. Stone for the abutments appears to have been quarried at Duck Creek, Wisconsin.

During the late 19th Century, the C&NW was an innovator of solid floor bridge spans. Solid floors were preferred by some railroads, as they helped to protect superstructure members. The floor of this bridge consists of heavy trough sections and inverted box beams. It was reported that this was the first solid floor bridge designed and constructed by the C&NW. These early solid floor spans helped shape early designs of bridges used for grade separations throughout the C&NW system. The unusual combination of trusses and girders may have been required to allow for sidewalks on the structure. This feature gives the bridge an unusual loading, where most of the load from the three tracks is concentrated in the two center truss lines. These truss lines use a standard C&NW design, although they are considerably heavier than other examples constructed. The critical defect of this type of floor was the pooling of water, which could accelerate deterioration of the steel substructure. Despite this defect, the bridge appears to be in fair to good condition, with no significant section loss noted to the steel components. The author has ranked this bridge as being regionally significant, due to the unusual and innovative design.