IAIS Walnut Creek Bridge (Ashworth Park)

Once located in Ashworth Park in Des Moines, this through truss bridge carried a former joint Chicago, Rock Island & Pacific Railroad (Rock Island) and Minneapolis & St. Louis Railway mainline over Walnut Creek and the Bill Riley Trail near Valley Junction. The first bridge at this location likely consisted of a timber pile trestle bridge, constructed when the line was first built. It is unclear if the original bridge was a double track bridge, or if the Rock Island and Des Moines Valley Railway shared a single track. In 1880, the bridge was reportedly rebuilt with an iron truss span, set onto a stone pier and abutment and approached by timber pile trestle spans on the west end. In 1897, the abutments of the bridge were altered for unknown reasons, and it is unclear if any superstructure alterations occurred at this time. In 1901, the bridge was rebuilt with a new 111-foot, riveted quadrangular lattice through truss span for the north track, and a 44-foot deck plate girder span for both tracks was added to the west end of the bridge. In 1908, the south track truss span was reportedly replaced by a new 113-foot Warren through truss span, set onto the original substructures. The north track was removed sometime between 1990 and 2000, giving the bridge its final configuration. Historically, the Rock Island used the north track, while the M&StL used the south track. It appears that both portions of the bridge were constructed to standard Rock Island designs. The bridge consisted of a 113-foot, 6-panel, riveted Warren through truss span, approached by a 44-foot deck plate girder span on the west end. The bridge was set onto a combination of concrete and encased stone substructures. An unknown contractor fabricated the truss span, and an additional unknown contractor fabricated the girder span. The substructures were also likely constructed by multiple unknown contractors.

The truss span followed 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 consisted 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 was constructed of a solid beam. The vertical members were also constructed of solid beams, while the diagonal members were composed of built-up beams which use V-lacing on both sides. The floor system was composed of heavy plate girder floorbeams placed at the panel points and two heavy plate girder stringers. The portal bracing used 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 connected the bottom of the portal bracing to the endposts. The sway bracing used a similar design to the portal bracing, with 4 "X"s and plates at the connections. The upper lateral bracing was composed of light V-laced beams, while the lower lateral bracing is constructed of L-shaped bars. The approach span consists of a standard design, with two modest sized girders and an open deck. The abutments both use a modified design, with short sloped wing walls and numerous alterations throughout. The pier consists of a standard rectangular structure, which has been reinforced with sheet piling and additional concrete.

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. In June 2017, the truss span was replaced by a modern through plate girder span, while the substructures and west approach were left intact. While the truss was not in critical condition, it is believed that it was replaced to allow for greater vertical and horizontal clearances. At the time of replacement, the bridge had seen only minor alterations, and was in regular use. Overall, the bridge appeared to be in fair condition at the time of replacement, with some deterioration noted to the substructure and superstructure. The author has ranked this bridge as being moderately significant, due to the truss design.