DAIR Big Sioux River Bridge (Fairview)

Located in the town of Fairview, this pony truss bridge carries a former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) line over the Big Sioux River. The first bridge at this location was likely a wooden truss bridge, constructed when the line was first built. In 1911, the center of the bridge was replaced with a pair of secondhand through plate girder spans, set onto timber pile piers and retaining a timber pile trestle approach. In 1932, it was desired to reconstruct the bridge to improve flow. As a result, a new pony truss bridge was installed on concrete piers, and the two girder spans were shifted to serve as approach spans. The most recent alterations to the bridge came in 2014, when the original timber pile trestle approaches were replaced by steel stringer spans, reusing portions of the timber bents. Currently, the bridge consists of a 135-foot, 8-panel, riveted Warren pony truss span, approached by a 65-foot type "C4" through plate girder span on either end. The south approach consists of eight 16-foot steel stringer spans, while the north approach consists of thirteen 14-foot steel stringer spans. The truss span is set onto concrete piers, while the remainder of the substructures consist mainly of timber pile piers and bents. Some approach bents have been reconstructed with steel pile structures.

The truss span follows a standard design for the era, with heavy members, a curved upper angle connection and a standard floor. The top chord and endposts both consist of built-up beams, with X-lacing on the bottom and a solid plate on the top. The bottom chord is composed of a built-up beam which uses X-lacing on both sides. Both the vertical and diagonal members use heavy solid beams, and the vertical members are connected to the floorbeam by a thick triangular plate. The floor consists of two plate girder stringers and plate girder floorbeams, typical for spans from this era. At the upper angle connections, a rounded plate connects the top chord to the endposts. The approach spans follow a standard design for "C4" through plate girder spans, with two heavy plate girders, a standard floor and rounded girder ends on one end and square girder ends on the other. C4 girders differ from C3 girders in the design of the floor. While a C3 girder uses four stringers spaced equally under the entire track, a C4 girder utilizes a more traditional floor with two stringers, placed under the rails. The steel stringer spans use four beams, arranged into two sets of two. The concrete piers follow a standard rectangular design, with an angled upstream nose. The original caps of the timber pile trestle bents have been replaced with concrete, and select bents were replaced with steel piles.

The two through girder spans were originally fabricated in 1905 by the Wisconsin Bridge & Iron Company for use at Bridge #B-28 near Elm Grove, Wisconsin. When that bridge was filled in approximately 1910, the spans were released for reuse. Lakeside Bridge Company fabricated the main span following a standard Milwaukee Road design used in the late 1920s and early 1930s. It is unknown if the concrete piers were constructed by a contractor or by railroad company forces. Warren truss spans became popular in the 20th Century, as they were simple, economical and strong. Through plate girder spans were popular for railroad use, as they were durable and easy to construct. Railroads often reused steel spans, as it provided a cost effective way to upgrade bridges without requiring large amounts of new material. 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.