UP East Boyer River Bridge

Located in Denison, this unusual through truss bridge once carried the Chicago & North Western Railway Iowa Division mainline and Wall Lake branch over the East Boyer River. Since the railroad first arrived in the area, the Boyer River had been troublesome, often flooding, washing out the tracks and requiring larger bridges to be constructed. The first bridge at this location likely consisted of a timber truss bridge, constructed when the line was first built. In 1874, the bridge was rebuilt with a 119-foot combination Howe through truss span, set onto timber pile substructures and approached by timber pile trestle spans. In 1883, the bridge was replaced with a single track 175-foot iron through truss span, set onto stone substructures. It is not entirely clear what design this span used, although it is most likely that it was a pin-connected Whipple truss span based on the length. In 1899, a new line was constructed between Denison and Wall Lake, leaving the mainline immediately west of the bridge. Between 1898 and 1902, the C&NW undertook a major reconstruction of this route, double tracking the line between Cedar Rapids, Iowa and Council Bluffs, Iowa. As part of the project, the truss span was replaced by two triple track 60-foot through plate girder spans, set onto stone substructures. The replacement bridge was started in 1901 and completed in 1902, and it is believed that American Bridge Company fabricated the superstructure, while Widell Company constructed the stone substructures.

In mid-1909, a severe flood hit the Denison area, severely damaging roads and bridges. It is unclear what impact the flood had on this bridge, but it is known that the railroad decided to replace an 8-year-old bridge in 1910. During the 1910 construction season, the bridge was completely rebuilt with a larger truss span and an additional girder approach span. In the late 1930s or early 1940s, the river was channelized and levees constructed to protect the town from floods. While the branch to Wall Lake was abandoned in 1942, the northernmost track remained in use until the mid-1960s as a siding. Further alterations were made to the bridge in 1958 and 1973, when the substructures were significantly rebuilt with sheet piling reinforcements,, giving the bridge its final appearance. The bridge consisted of a triple track 120-foot, riveted quadrangular lattice through truss span, approached by a 50-foot through plate girder span on the east end. The entire bridge was set onto concrete substructures, although railroad records indicate portions of the 1902-era stonework may have been reused. American Bridge Company fabricated the truss span at their Lassig Plant in Chicago, while the concrete substructures were constructed by an unknown firm. It is unclear if the 1958 and 1973 alterations were made by railroad company forces or by an unknown contractor.

The truss span used an unusual design for the era, with three truss lines forming a double track and a single track section. In addition, the truss used heavy members, an M-frame/A-frame portal and a trough style floor. Both the top chord and endposts used a built-up design, consisting of two channels joined by X-lacing on the bottom and a solid plate on top. The bottom chord was set above the floor and used a similar design, with V-lacing on both sides. The compression members were generally composed of V-laced beams, while the tension members were generally composed of L-shaped bars. In addition, a V-laced vertical member was present outside of the hip vertical at each end of the truss. The floor system followed a design that was developed in the early 20th Century, which was known as a "trough floor". This type of floor uses numerous transverse hollow beams, which are filled with concrete and hold a ballast deck. Larger transverse plate girder floorbeams were present at the panel points to hold the trusses. Each floorbeam was composed of a double plate girder, which connected to the bottom chord of the truss. The portal bracing consisted of an M-frame design for the double track section and an A-frame design for the single track section. Both were constructed using V-laced beams. In addition, the sway bracing and upper lateral bracing were also constructed using V-laced beams. The through girder span used a design typical for the era, with three heavy plate girders, rounded and tapered girder ends and a trough floor system. The abutments used a standard design, with short sloped wing walls. The pier used a diamond shaped design, which was heavily reinforced with concrete and sheet pile protection at the base.

The lattice through truss design is relatively uncommon throughout the United States. However, a few railroads preferred the design, such as the Chicago, Rock Island & Pacific Railway (Rock Island), the Chicago & North Western Railway (C&NW) and the Chicago, St. Paul, Minneapolis & Omaha Railway (Omaha Road). Spans constructed in the late 1870s and early 1880s for the C&NW featured an arched and pedimented lattice portal bracing, light sway bracing and laced members. The second generation was nearly exclusively constructed by Lassig Bridge & Iron Works between 1884 and 1900, and featured heavier members and a pedimented portal bracing. While the design fell out of favor for the Rock Island and Omaha Road around the turn of the 20th Century, the design remained popular with the C&NW into the 1920s. 20th Century versions were significantly heavier, using solid members, a heavier floor and utilitarian portal design. The C&NW preferred this design, as it was both strong and demonstrated great resilience in case of a derailment. It is believed that this was the only triple-track version of this design constructed for the C&NW. Trough floor spans became popular in the early 20th Century, particularly for use in Chicago track elevation projects. This design minimized maintenance and allowed for a solid deck. In 2021, work on a replacement structure began, which was completed in 2022. While portions of the bridge were offered for reuse, there was no significant interest and the bridge was scrapped. Today, a new modern through girder span is used at this location. Overall, the bridge appeared to be in fair condition at the time of replacement, with some significant deterioration to the steel. The author has ranked this bridge as being regionally significant, due to the unique triple-track design.