IAIS Walnut Creek Bridge (South)

View an article discussing the first generation of Milwaukee Road pony trusses (digitized by Google)

Located near the intersection of Grand Avenue and 63rd Street (Iowa Highway 28) on the west side of Des Moines, 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 In the late 1910s, the Milwaukee Road upgraded a couple of bridges along this line, replacing outdated timber structures with secondhand steel and iron structures.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, the present pony truss span was constructed. Currently, the bridge consists of a 106-foot, 6-panel, pin-connected Pratt pony truss span, approached by a single timber pile trestle span on either end. The entire bridge is set onto timber pile substructures. It is believed that the timber components were last renewed in 1952. The truss span was originally fabricated in 1897 for use at Bridge #R-18; which was the 22nd crossing of Bloody Run near Marquette, Iowa. After a devastating flood along Bloody Run in 1916, the Milwaukee Road shifted bridges around, which released this pony truss for reuse. Wisconsin Bridge & Iron Company fabricated the main span, while the timber components were constructed by railroad company forces. This truss design was originated by Onward Bates; Engineer and Superintendent of Bridges and Buildings for the Milwaukee Road, and the design was refined by Albert Reichmann, Assistant Engineer of Bridges.

The pony truss span uses a an early variation of a standard Milwaukee Road design, and uses eyebars, unique members and an unusual floor system. The top chord and endposts both consist of built-up beams, consisting of two U-shaped channels joined by X-lacing on the bottom and a solid plate on top. The bottom chord is composed exclusively of eyebars, set into two sets of two throughout the structure. Unique to this span, the vertical members use a built-up design, consisting of trapezoidal beams formed from two U-shaped channels covered by a solid plate on either side. The diagonal members consist of a combination of punched eyebars and looped rods. The looped rods consist of a square bar, and are used for the counter members. The floor system is somewhat unusual, consisting of unique plate girder floorbeams and two plate girder stringers. These floorbeams are formed from two relatively light plate girders, which are connected at the stringers and bottom chords. The lower lateral bracing is formed by flat bars, which form an "X" shape at the panels. In addition, the stringers are braced by additional transverse and lateral bars. Both piers are heavily constructed, consisting of three rows of timber piles, capped with standard square lumber. The truss span is set onto rows of heavy timber blocking, located on the caps. The approaches use a standard design, with traditional timber stringers and pile abutments.

In the late 19th Century, spans between 80 feet and 120 feet required either a through truss span or two smaller spans. Large plate girder spans had not yet become popular, due to the amount of material required for a long span. Likewise, through trusses were not preferred, as they were more expensive and came with clearance constraints. This was a significant disadvantage to the Milwaukee Road, which operated in areas that required many medium-length spans. By the mid-1890s, several railroads began working on a solution, commonly called "lattice girders". In reality, these designs used truss technology which had been adapted to reduce the amount of material required and eliminate overhead bracing. One of the designs developed was known as a pony truss. Pony trusses had been used for railroads since the 1870s, but had generally been limited to shorter spans. In 1896, Onward Bates and Albert Reichmann began working on a solution for the Milwaukee Road, which would adapt the Pratt truss pattern for a pony truss design. During the late 1890s, the Milwaukee Road was upgrading dozens of bridges with new metal spans every year. The first generation of spans were fabricated in late 1896 and into 1897, and primarily used in northeast Iowa. The design was refined by late 1897 to eliminate the unusual floorbeams and unique vertical members, and by 1899 the design was refined further to include a built-up bottom chord instead of eyebars. The Milwaukee Road ultimately settled on the 1899 variation as a standard design, and installed dozens of Pratt pony truss spans throughout the system between 1899 and 1905.

Starting in the early 1880s, almost all Milwaukee Road bridges were designed in-house by the Bridge & Building Department. This made the railroad somewhat unique, as many railroads used consulting engineers to design bridges. Onward Bates joined the Milwaukee Road in 1887 after leaving Edge Moor Bridge Works and the Pittsburgh Bridge Company. During his tenure, he innovated several standard bridge designs for the railroad, designed several notable structures and created the letter-number bridge numbering system that the Milwaukee Road used through the remainder of the 20th Century. Bates left the Milwaukee Road in 1901 to start his own contracting firm, the Bates & Rogers Construction Company; which was a notable firm during the 20th Century, surviving into the 1980s. Albert Reichmann joined the Milwaukee Road Bridge & Building Department in late 1894 after previously working for the Lassig Bridge & Iron Works. He remained at the railroad until 1901, when he joined the American Bridge Company as a Division Engineer in charge of all engineering in the western division. Reichmann was appointed Chief Engineer of the American Bridge Company in 1931, and was elected Vice President of the firm in 1935. He served in this role until his retirement in 1938. During his tenure at American Bridge Company, Reichmann was credited with designing some of the most substantial bridges and steel structures constructed by the firm. Wisconsin Bridge & Iron Company was originally founded in 1880, and was reincorporated in 1891. The company was served by the mainline of the Milwaukee Road on the north side of Milwaukee, and provided the Milwaukee Road with a large amount of bridge material between the mid-1890s and the 1930s. The company remained active until the 1970s, when it was sold and disbanded.

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 does not appear that this span was modified in any meaningful way upon relocation. Since the initial construction, 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 unclear what the future holds for this unique truss span. Unfortunately, Walnut Creek has a history of flooding and oftentimes reaches the bottom of this bridge, lodging debris in the floor system. If the line is acquired for trail use, it is possible that the bridge will be removed or replaced with a modern structure. Overall, the bridge appears to be in fair condition, with no critical defects noted. In particular, the truss span appears to be in good condition, with no visible defects. The author has ranked this bridge as being regionally significant, due to the age, design and builder. While several "first generation" pony truss spans were constructed by the Milwaukee Road, it appears that this is the sole remaining example of this innovative design. Despite appearing similar from afar, this span is significantly different from another pony truss span along the same line. Should the bridge be removed, it is hoped that the truss span can be reused in some way, either on a new trail bridge or on a trail elsewhere in the area.