Clive Greenbelt Trail - Walnut Creek Bridge

Located at Campbell Park in Grimes, this through plate girder bridge carries a former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) branch line over Walnut Creek. The first bridge at this location was likely a timber pile trestle, constructed when the line was first built. In the late 1910s, the Milwaukee Road upgraded a couple of bridges along this line, replacing outdated timber structures with secondhand steel and iron structures. In 1917, a secondhand type "B" deck plate girder span was installed, with new timber pile piers and timber pile trestle approaches constructed at either end. In 1929, the main span was reconfigured from a type "B" deck plate girder to a type "C3" through plate girder span to increase clearance underneath and strengthen the span. The most recent alteration to the bridge came in 1937, when the timber pile trestle approaches and timber pile piers were last renewed. Currently, the bridge consists of a 65-foot through plate girder span, approached by six 14-foot timber pile trestle spans on the west end and three 14-foot spans on the east end. The entire bridge is set onto timber pile substructures. Due to the age, it is believed that the girders for the main span are fabricated from wrought iron. Lassig Bridge & Iron Works fabricated the main span, while it is unclear if the 1929 alterations were completed by a contractor or by railroad company forces. It is believed that all timber components were constructed by railroad company forces.

The main span was originally fabricated in 1889 for use as part of a large viaduct at Bridge #Z-1334 in Manning, Iowa. When the present bridge was completed as part of a double tracking and realignment project, the viaduct was disassembled. It is believed this span was one of the two spans which crossed the West Nishnabotna River. It is unclear if other portions of the viaduct were also reused along branch lines. Originally, the girder span was configured as a type "B" deck plate girder. This type of span uses shallow girders with an unusual floor system. The original floor system was composed of plate girder floorbeams, placed halfway up the girders. Unique to the type "B" design, the ties sat directly on flanges riveted to near the top of the girders. When the span was converted to a type "C3" girder, the original floor system was altered by installing stringers, floorbeams and a lateral system at the bottom of the girder. It is unclear if the original floorbeams were reused as part of the new floor, or if the entire floor consisted of new material. Evidence of the original type "B" design is clear throughout the span, with the original flanges still present inside the girders. It also appears that additional strengthening was done to the girders at this time, including the addition of additional angles near the center of the span. In addition, several rivets have been replaced with square bolts, an alteration that is likely nearly 100 years old. Type "C3" spans were a standard Milwaukee Road through plate girder design, which used four lighter stringers instead of the two heavier stringers used by type "C4" through plate girder spans. The timber components of the bridge follow a standard design, with round timber piles, standard caps and standard stringers.

Railroads often reused steel and iron spans, as it provided a cost effective way to rebuild bridges without requiring large amounts of new material. In the early 20th Century, the Milwaukee Road renewed bridges at relatively short intervals, and generally had extra bridge spans on hand. Oftentimes, these "secondhand" spans would be modified prior to reuse. It is somewhat unusual that the main span was modified some 12 years after the original installation here. After the line was abandoned, the bridge was rehabilitated for trail use by installing a concrete deck and railings. Today, it carries the Clive Greenbelt Trail. Overall, the bridge appears to be in fair condition, with minor deterioration noted to the timber components. Unfortunately, the City of Clive is seeking to replace this bridge with a modern pedestrian bridge. This project has been planned since the 2010s, and it is unclear when it may occur. It is hoped that the new pedestrian bridge can incorporate the historic plate girder span or that the plate girder span can be reused at another location. The author has ranked this bridge as being moderately significant, due to the age and design of the structure. This bridge is among the oldest known plate girder spans in Iowa.