DAIR Big Sioux River Bridge (Oak Ridge)

Located between Fairview and Hudson, this pony truss bridge carries a former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) line over the Big Sioux River at Oak Ridge. 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 1914, the bridge was reconstructed with three secondhand through plate girder spans, set onto timber pile piers. This bridge consisted of a 65-foot type "C4" through plate girder span, originally fabricated for Bridge #Z-1284 at Dedham, Iowa in approximately 1900 and two 40-foot type "C3" through plate girder spans, originally fabricated for Bridge #Z-644 at Elwood, Iowa in 1900. In 1932, the bridge would be reconstructed with a longer main span and new approach span to prevent damage from floods and ice jams. It is unknown if the previous through girder spans were reused elsewhere after replacement. The most recent alterations to the bridge came in approximately 2010, when the north approach bents were replaced with steel piles. Currently, the bridge consists of a 123-foot, 6-panel, pin-connected Pratt pony truss span, approached by a 50-foot deck plate girder span on the south end. The north end is approached by 43 14-foot steel stringer spans, while the south end is approached by three 14-foot timber pile trestle spans. The north pier and north approach substructures consist of steel piles, while the south piers and south approach substructures consist of timber piles.

The 123-foot truss span was originally fabricated in 1900 by the Lassig Bridge & Iron Works for use at Bridge #Z-1534 near Neola, Iowa. This span uses a standard design, with a traditionally composed floor, heavy members and large pin connections. Typical of spans of this design, the endposts and top chord consist of built-up beams, which use a plate on the top side and X-lacing on the back side. The bottom chord is composed of built-up beams with X-lacing on both sides. The vertical members of the truss are composed of solid beams, which are connected to the floorbeams by large triangular plates. The diagonal members consist of eyebars, and the counters are composed of square rods. The floor is composed of two plate girder stringers and plate girder floorbeams. The deck girder span was originally fabricated in 1904 by the Wisconsin Bridge & Iron Company for use at Bridge #I-794 across the Crooked River at Elmira, Missouri. This span follows a standard design, with two modest sized plate girders and an open deck. The steel stringer spans use four beams, arranged into two sets of two.

This style of truss span was used extensively by the Milwaukee Road. First developed in the mid-1890s, the design provided a strong and cost effective span for medium length spans. A number of different span lengths were developed using this design, although the spans largely used very similar design features. Typical of Milwaukee Road bridges, a number of different contractors built these spans, depending on which firm offered the best price. Dozens of examples of similar spans were constructed throughout the Midwest, and many continue to survive today. Railroads often reused steel and iron spans, as it was a cost effective way to rebuild bridges without requiring large amounts of new material. Often, when a span was no longer sufficient for mainline use, it could be reused on a branch line. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked this bridge as being moderately significant, due to the truss design.