IAIS South Skunk River Bridge

Located between Newton and Colfax near the former station of Metz, this through truss bridge carries a former Chicago, Rock Island & Pacific Railroad (Rock Island) mainline over the South Skunk River. The first bridge at this location was likely a timber truss bridge, constructed when the line was first built. In 1888, the bridge was replaced with a 155-foot quadrangular lattice through truss span, set onto stone abutments. Starting in the late 1930s, a series of improvements were made to the bridge, which completely reconstructed and lengthened the structure. Between late 1937 and early 1938, the original abutments were reconstructed with concrete piers, and new concrete approach piers were constructed. Starting in October 1938 and continuing through December 1939, the original truss span was replaced by a heavier secondhand truss span and three steel stringer spans were constructed on either end. It is unclear if the original truss span was reused at another location. The most recent alteration to the bridge came in 1945, when the east end was further lengthened by installing two additional steel stringer spans on a new concrete pier and concrete abutment. Currently, the bridge consists of a 155-foot, 6-panel, riveted Warren through truss span, approached by five 28-foot steel stringer spans on the east end and three 28-foot steel stringer spans on the west end. The main piers (piers #5 and #6), east approach pier (pier #1) and both abutments consist of a concrete design, while the remaining piers are constructed with concrete piles. The truss span was originally fabricated in approximately 1905 for use at an unknown location. During the late 1930s and early 1940s, a number of similar spans were reused throughout the Rock Island system, although it is unclear where they were originally constructed. American Bridge Company fabricated all of the superstructure, while much of the substructure was constructed by Gould Construction Company. An unknown contractor constructed the east abutment and east pier (pier #1).

The truss span follows a standard Rock Island design that was used in the early 20th Century, with heavy members, riveted connections and a traditionally composed floor. Both the top chord and endposts consist of heavily constructed members, built of two U-shaped channels connected by solid plates on the top/exterior and V-lacing on the bottom/interior. The bottom chord is constructed using two U-shaped channels, connected by widely set V-lacing on both sides. The vertical members are also constructed of solid beams, while the diagonal members are composed of built-up beams which use V-lacing on both sides. The floor system is composed of heavy plate girder floorbeams placed at the panel points and two heavy plate girder stringers. The portal bracing uses a standard Rock Island design, consisting of a lattice design with four "X"s. At the corners, a triangular heel bracing with a rounded edge connects the bottom of the portal bracing to the endposts. The sway bracing uses a similar design to the portal bracing, with 4 "X"s and plates at the connections. The upper lateral bracing is composed of light V-laced beams, while the lower lateral bracing is constructed of L-shaped bars. All of the approach spans consist of six shallow beams, spaced equally and covered by a concrete ballast deck. The main piers use a standard rectangular shape with an angled upstream nose, and it is unclear if portions of the original abutments were reused in these piers. The east pier (pier #1) uses a standard rounded rectangular shape, and the east abutment uses a standard U-shaped design. The remaining approach piers consist of three octagonal concrete piles with a concrete cap. The west abutment also uses a standard design for the era, with short sloped backwalls.

Warren trusses became popular in the early 20th Century as an alternative to pin-connected Pratt truss spans. Throughout the late 19th Century, the Rock Island primarily preferred using lattice trusses for through truss spans. However, once heavier riveted Warren trusses were introduced around 1900, the Rock Island quickly switched to using this design as the preferred truss bridge configuration. Dozens of similar spans were constructed by the Rock Island between 1900 and 1920. Riveted Warren trusses were economical and simple, while also being far stronger than pin-connected Pratt spans constructed in the 19th Century. Railroads often reused steel and iron spans, as it provided a cost effective way to rebuild bridges without requiring a large amount of new material. The Rock Island preferred reusing spans, as the railroad was often in financial distress. Since the initial construction, the bridge has seen only minor alterations, including the encasement of the abutments in a concrete facing. Today, the bridge continues to carry railroad traffic. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. This bridge is likely to be replaced in the coming years, as Iowa Interstate Railroad is seeking to replace all truss bridges on their mainline. The author has ranked this bridge as being moderately significant, due to the truss design.