CPKC Little Sioux River Bridge

Located on the west side of Spencer, this pony truss bridge carries a former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) line over the Little Sioux River. The previous bridge at this location was likely either a timber pile trestle or a wooden truss bridge, which had been periodically renewed since it was first constructed. In the late 1890s and early 1900s, the Milwaukee Road invested significant capital into this line, replacing timber bridges with steel and concrete structures. The present structure was constructed in 1901. Currently, the bridge consists of a 105-foot, 6-panel, pin-connected Pratt pony truss span, approached by two 50-foot type "C4" through plate girder spans on the east end. The entire bridge is set onto concrete substructures, which are founded on timber piles. It is unclear if the truss span and girder spans were fabricated by the same unknown contractor, or if the spans were fabricated by different unknown contractors. The concrete substructures were constructed by labor employed by the Milwaukee Road Bridge & Building Department, as was customary for this railroad.

The truss span was constructed using a standard Milwaukee Road design, although the length appears to be a slight deviation from more common sizes. 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 through girder spans also follow a standard design for "C4" girders, with two heavy stringers and girders with one rounded girder end and one square girder end. "C4" girders differ from "C3" girders in the construction of the floor. "C3" girders use four equally spaced stringers, while "C4" girders use a more traditional floor design. The substructures also follow a standard design, with rectangular piers and sloped wing walls at the abutments.

The Pratt truss design was arguably the most common truss type used for railroad bridges during the second half of the 19th Century. The design provided an economical, simple and durable design. The Milwaukee Road standardized the Pratt design for pony trusses in the mid-1890s, with the standard design largely developed by Alan Riechmann and Onward Bates. The design was refined over a short period, eventually using a heavy built-up bottom chord by the 20th Century. Many examples of this design are still present on former Milwaukee Road lines. By 1905, the Milwaukee Road had largely switched to using riveted Warren pony truss spans. Through plate girder spans were also popular for railroad use, as they were durable and easy to construct. Since the initial construction, the bridge has seen no significant alterations, and remains in use. 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 common design.