IAIS Walnut Creek Bridge (North)

Located west of 73rd Street in Clive, this pony truss 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. It is unclear if this bridge was ever rebuilt with a timber truss span, or if it remained a timber pile trestle into the 20th Century. In 1902, the bridge was rebuilt with a steel pony truss span, set onto new concrete abutments. The bridge remained in this configuration until 1943, when it was decided to reconstruct the substructures to improve the creek channel. As part of the work, the original abutments were demolished, two new piers constructed, the truss span shifted west and new timber pile trestle approaches constructed at either end. Currently, the bridge consists of a standard 104-foot, 6-panel, pin-connected Pratt pony truss span, set onto concrete piers. This span is approached by two 14-foot timber pile trestle spans at either end, which are set onto standard timber pile bents. An unknown contractor fabricated the main span, while the concrete piers were constructed by the Thorgerson & Ericksen Company. It is believed that the timber components were constructed by railroad company forces.

The pony truss span follows a standard Milwaukee Road design that was used in the early 20th Century, with built-up members and a standard floor system. Typical of spans of this design, the endposts and top chord consist of built-up beams, which are composed of two U-shaped channels joined by a solid plate on the top and X-lacing on the bottom. 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 lower lateral bracing is formed by L-shaped bars, and the stringers are braced by additional lateral bracing. The piers consist of an unusual design, with rectangles that are beveled at the noses. The approach spans use a standard design, with timber stringers supported by timber pile bents. Remains of the original east abutment are still intact, while the original west abutment was demolished and the concrete used as rip-rap to stabilize the embankment.

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 Onward Bates and Albert Riechmann. The design was refined over a short period, eventually using a heavy built-up bottom chord by the turn of 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. This span appears to be an example of the second generation of standard pony truss spans, and the design was used at dozens of locations throughout the Milwaukee Road system. Despite appearing similar from afar, this span is significantly different from another pony truss span along the same line. Since the 1943 reconstruction, the bridge has seen no significant alterations, except for the occasional renewal of the timber components. The bridge remained in use until September 2026, when operations over the Grimes Line ceased. As of 2026, communities along the rail line are currently in negotiations to acquire the right-of-way for trail use. It is hoped that this bridge will be reused as part of any potential trail. Overall, the bridge appears to be in fair to good condition, with some minor deterioration noted to the concrete piers and timber approaches. The author has ranked this bridge as being moderately significant, due to the truss design.