UP Rock River Bridge (Nelson)

Located west of Nelson, this large through truss bridge carries the former Chicago & North Western Railway across the Rock River. The first bridge at this location was a wooden truss bridge, constructed when the line was built. This bridge utilized three 120-foot spans with pile trestle approaches. In the spring of 1857, a wreck occurred on the bridge, and the bridge reconstructed with new wooden truss spans. Spans from the old bridge were reused at DeKalb and Cherry Valley. In 1875, the bridge would be reconstructed with a new bridge, consisting of three 120-foot combination through truss spans, approached by timber pile trestle spans. Combination truss spans were commonly used in the 1860s and 1870s, as the design was economical, and the iron components of the truss provided greater strength. In 1889, the bridge would again be reconstructed, using a new double track five span steel through truss bridge. The most recent reconstruction of the bridge came in 1929, when the bridge was strengthened and the original portals reconstructed, giving the structure its current configuration.

Currently, the bridge consists of five 146-foot riveted quadrangular lattice through truss spans, set onto stone substructures. The original portals of the spans consisted of a typical Lassig Bridge & Iron Works design, with a lattice top and decorative heel bracing. The original members of the truss spans consisted of solid and laced members, and the floor was traditionally composed. During the 1929 reconstruction, the original portals were replaced with a steel beam set above the truss lines, to improve the vertical clearance of the structure. In addition, several of the members in the center of the spans were replaced, and the floor system was reconstructed. The original bridge was fabricated by Lassig Bridge & Iron Works, which constructed most steel bridges for the C&NW in the late 1880s and 1890s. The substructures were constructed by an unknown contractor, and appear to be constructed of limestone quarried at Duck Creek, Wisconsin. Wisconsin Bridge & Iron Works was awarded the contract for the strengthening of the truss span.

Although the lattice truss design was popular in Europe, few American railroads utilized the design. While Union Pacific Railroad, the Chicago, Rock Island & Pacific Railway and a handful of eastern railroads utilized the design to various extents, no railroad constructed as many lattice truss bridges as the C&NW. Between 1878 and 1930, through trusses constructed by the C&NW nearly exclusively used this design. Although lattice trusses were not popular with American railroads after the 1880s, the C&NW relied heavily on this design, as it had proven to be resilient towards derailments and damage. The main drawback of the design was the unpredictable nature, as the structure was not statically determinant. The lattice truss design appears to have first been introduced to the C&NW by Leighton Bridge & Iron Works, which constructed some of the first iron lattice trusses for railroad use. Nearly all of the lattice trusses constructed for the C&NW between 1886 and 1900 were fabricated by Lassig Bridge & Iron Works. Relatively few double track examples of this design remain in use today. Overall, the bridge appears to be in fair to good condition, and continues to carry heavy traffic. The author has ranked this bridge as being regionally significant, due to the age and size of the structure.