UP Mackinaw River Bridge (Green Valley)

View historic articles discussing the construction of this line (digitalized by Internet Archive)

Located along Illinois Route 29 north of Green Valley, this quadrangular lattice through truss bridge carries the former Chicago & North Western Railway over the Mackinaw River. Originally constructed in 1912, the bridge first consisted of a truss span, set onto timber pile piers and approached by timber pile trestle spans on both ends. Timber substructures and approaches were often used during the initial construction of this line, as they allowed the approach embankments to properly settle without damaging steel spans or concrete substructures. In 1921, new concrete piers were constructed, and a 36-foot deck plate girder span added to the north side of the truss span. The most recent upgrade came in approximately 1975, when the 35-foot deck plate girder span was removed, and a new 50-foot deck plate girder span installed. In addition, the north timber approach was completely rebuilt with steel stringer spans set onto new concrete substructures, giving the bridge its current configuration.

Currently, the bridge consists of a 132-foot riveted quadrangular lattice through truss span, set onto concrete piers. The north approach of the bridge consists of a 50-foot deck plate girder span, followed by a 45-foot and six 40-foot steel stringer spans, set onto concrete substructures. This deck plate girder span was reused from an unknown location, and appears to have originally been fabricated in approximately 1900. The south approach of the bridge consists of a lengthy timber pile trestle approach. King Bridge Company fabricated the truss span, while an unknown contractor fabricated the deck girder and the steel stringer spans. Unknown contractors completed the concrete substructures of the bridge. The truss span uses a standard C&NW design, which uses a heavily constructed floor, a combination of built-up and solid members and an A-frame portal bracing. The deck plate girder span consists of three girder lines, and was likely rebuilt upon reuse at this location. The girders have a number of empty holes, and appear lighter than typical spans. Typical of late 20th Century bridges, the steel stringer spans use bolted connections. Railroads often reused steel spans where feasible, as it provided a cost effective way to repair and replace bridges. Often, when a deck girder span was reused, it would be strengthened by adding additional girder lines.

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. Overall, the bridge appears to be in fair to poor condition, with significant section loss noted throughout the truss span. The author has ranked this bridge as being moderately significant, due to the truss design.