RI Little Sioux River Bridge (Webb)

Located between Webb and Rossie, this large deck truss bridge carries a former Chicago, Rock Island & Pacific Railroad (Rock Island) secondary line over the Big Sioux River. When the Gowrie & Northwestern Railway constructed a line between Gowrie and Sibley, the largest structure required on the line was a tall bridge over the Little Sioux River. To save on earthwork costs, it was decided to instead construct a large deck truss bridge with timber pile trestle approaches. Work on the bridge began and was completed in 1900. Currently, the bridge consists of a 150-foot, 10-panel, riveted double intersection Warren deck truss span, set onto concrete piers. The east approach consists of 21 timber pile trestle spans, and the west approach consists of 15 timber pile trestle spans. An unknown contractor fabricated the superstructure, while the concrete piers were constructed by labor employed by the Rock Island, which funded the construction of this line.

The truss span uses typical details for a span of this design, including a combination of solid and built-up members, a standard floor and an open deck. The top chord and endposts both consist of built-up beams, which are composed of two channels connected by V-lacing. The bottom chord is also constructed of a built-up beam, which is connected by thin plates throughout the member. The compression members use solid I-beams, while the tension members use L-shaped bars, which are connected by thin plates. The floor system is composed of two stringers, connected to plate girder floorbeams. The ties rest on both the stringers and the top chord. The bottom transverse bracing is composed of V-laced beams, while both the upper and lower lateral bracing is composed of L-shaped bars. The internal lateral bracing is also composed of L-shaped bars. The concrete piers follow a standard design, using large rectangular structures founded on timber piles. The approach trestles also follow a standard design, with the tall bents constructed at three levels tall.

While lattice truss spans were relatively uncommon through the United States, the Rock Island occasionally used the design in the late 19th Century. Other Midwest railroads, such as the Chicago & North Western Railway (C&NW) used the design nearly exclusively from the 1870s into the 1920s. Some railroad engineers favored the design, due to its resilience and greater strength without sacrificing economy. Metal lattice deck truss designs were initially developed for railroad use in the 1870s, often using deep spans with numerous intersections. Through the mid-1880s, short to medium length spans were simplified into double intersection spans. By the 20th Century, the design had been refined further, and was constructed using significantly heavier members. The type of lattice truss seen on this bridge was used for structures constructed between 1900 and 1905, and balanced member size with strength. The Rock Island largely stopped using lattice trusses around the turn of the 20th Century, and instead began using more traditional single intersection designs. Since the initial construction, the bridge has seen no significant alterations. After abandonment, the bridge remained in railroad ownership until 2024, when the property was acquired by a private landowner. Adjacent properties are part of an Iowa DNR Wildlife Management Area, and it is somewhat surprising that the railroad property would not have been acquired by the DNR. Today, the bridge remains abandoned but fully intact. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked this bridge as being regionally significant, due to the size and truss design.