CN Boyer River Bridge #5

Located alongside US Route 30 and the former Chicago & North Western Railway bridge southwest of Denison, this through truss and through plate girder bridge is the fifth bridge carrying a former Illinois Central Railroad mainline over the Boyer River. When the railroad originally was constructed, the first bridge at this location consisted of a 91-foot and two 45-foot through plate girder spans, set onto concrete substructures. The Boyer River had long experienced severe floods, and by the early 20th Century it was decided to channelize the river. In the late 1940s, the river was channelized, requiring the original bridge to be rebuilt. In 1948, the bridge was rebuilt by removing the 91-foot girder span, installing a secondhand through truss span, shifting the 45-foot girder spans and constructing new concrete piers and timber approaches. It is believed that the truss span was reused from Bridge #WA500.3, which crossed Honey Creek near Ascot, Iowa; approximately 50 miles west of here. That bridge used a truss span of the same length, which was replaced in 1947. It is also unclear if the 91-foot span was reused at another location. Currently, the bridge consists of a 127-foot, 5-panel, pin-connected Pratt through truss span, approached by a 45-foot through plate girder span on either side. The south end of the bridge is approached by two timber pile trestle spans, while the north end is approached by one span. The bridge is set onto concrete piers and timber pile bents. American Bridge Company fabricated the steel spans, while it is unclear if the concrete substructures were constructed by railroad company forces or an unknown contractor. It is believed that the timber components were constructed by railroad company forces.

The truss span follows a typical design for the era, with built-up members, a standard floor system and a double intersection lattice portal. Both the top chord and endpost use a built-up design, consisting of two channels joined by a solid plate on top and X-lacing on the bottom. The bottom chord is constructed out of a similar style beam, which uses V-lacing on both sides. The vertical members of the truss use V-laced beams, while the diagonal members use an eyebar design. The floor system uses plate girder stringers above the bottom chord, as well as two plate girder stringers. The portal bracing consists of an IC-standard design, with L-shaped bars formed into a double intersection lattice design with four bays. The sway bracing consists of tightly X-laced beams. Both the upper and lower lateral bracing consist of L-shaped bars formed into an X shape. The two approach spans use a standard design, with two girders, square girder ends and a standard floor system. The piers use a standard IC design for the era, consisting of two concrete columns, connected by an arched concrete beam at the top. It is believed this style of pier was used to reduce material and improve channel flow.

The Pratt truss pattern was arguably the most popular railroad truss bridge design during the second half of the 19th Century. This design provided a durable and strong structure, while keeping the design simple and economical. Through plate girder spans were popular for railroad use, as they were durable and easy to construct. American Bridge Works had the contract for all bridges along this line that were constructed between 1899 and 1900. This firm fabricated many bridges for the IC during this era for all portions of the system. Railroads often reused steel and iron spans, as it provided a cost effective way to upgrade bridges without requiring large amounts of new material. During the late 1940s and early 1950s, the IC rebuilt many bridges along this line, shuffling many of the spans between locations. Since the 1948 reconstruction, the bridge has seen no significant alterations, and remains in regular use. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked the bridge as being moderately significant, due to the truss design.