DAIR Big Sioux River Bridge (Hudson)

Located on the north side of Hudson, 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 with timber pile trestle approaches, constructed when the line was first built. In 1913, the center of the bridge was replaced with a pair of secondhand 70-foot through plate girder spans, set onto timber pile piers and retaining a timber pile trestle approach. These spans were originally fabricated in 1890 for use at Bridge #Z-484 across the Plum River at Savanna, Illinois. In 1932, the bridge was reconstructed with a pony truss span, set onto concrete piers and retaining the timber pile trestle approaches. It is unclear if the girder spans were reused elsewhere or were scrapped. The most recent alterations to the bridge came in 2007, 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 138-foot, 8-panel, riveted Warren pony truss span, approached by 25 16-foot steel stringer spans on the north end and five 16-foot steel stringer spans on the south end. The truss span is set onto concrete piers, while the approaches are supported by timber pile bents. When the approaches were replaced, the original timber bent caps were replaced with concrete, retaining the timber piles.

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 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. It is believed that 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. 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.