UP E. Fork Des Moines River Bridge (Armstrong)

Located west of Armstrong, this large deck truss bridge carries a former Chicago, Rock Island & Pacific Railroad (Rock Island) line over the East Fork Des Moines River near Iowa Highway 9. When the railroad extended from Armstrong to Estherville, a large bridge was required over the East Fork Des Moines River. Initially, this bridge consisted of a secondhand deck truss span, set onto concrete piers and approached by lengthy timber pile trestle approaches on either end. Throughout the early 20th Century, these trestle approaches were gradually filled and shortened. Significant filling of the approaches was completed in 1907, with required the purchase of adjacent property to create a borrow pit. The most recent alteration to the bridge came in 1992, when the timber pile trestle approaches were replaced by a handful of spare steel stringer spans, set onto new steel pile substructures. Currently, the bridge consists of a 90-foot, 9-panel, riveted double intersection Warren deck truss span, set onto concrete piers. The east approach consists of a 47-foot steel stringer span (span #1) and a 50-foot steel stringer span (span #2). The west approach consists of a 46-foot steel stringer span (span #4) and a 40-foot steel stringer span (span #5). The approach substructures all consist of steel pile bents, with concrete caps.

The deck truss span was originally fabricated in approximately 1885 at an unknown location by an unknown contractor, before being moved here in 1899. The span consists of a light design, with light built-up members, two truss lines, unusual lack of a traditional floor system and an open deck. The top chord and endposts both consist of built-up beams, which are composed of two channels connected by thin plates. The bottom chord is also constructed of a built-up beam, which is constructed of channels connected by a variety of plates and rivets. The compression members consist of both built-up beams and solid beams. The built-up beams consist of two U-shaped channels, connected by a tight V-lacing. The tension members consist of two L-shaped bars per set. Unique to this bridge, the floor system is unusually light, with the ties sitting directly on the top chord. This design is unique, as it does not use typical floorbeams and stringers. The upper transverse bracing and lower transverse bracing both consist of X-laced built-up beams. Both the upper lateral bracing and lower lateral bracing consist of L-shaped bars. The interior lateral bracing also uses L-shaped bars. Some portions of the truss were strengthened with high strength bolts in 1992. The steel stringer spans all use different designs, and consist of a variety of lengths. The eastern steel stringer span (span #1) was fabricated in 1948 by American Bridge Company for use at an unknown location, and consists of six equally spaced shallow steel stringers. The western steel stringer (span #5) was fabricated in approximately 1940 by an unidentified contractor for use at an unknown location, and consists of four equally spaced shallow steel stringers. Spans #2 and #4 appear to have been fabricated in 1992 by an unknown contractor, and consist of bolted steel stringer spans, both of which use four stringers per span. It is possible that span #4 is older and was rebuilt here, but this is unclear. The concrete piers consist of large rectangular structures with an angled upstream nose. It is unclear if the piers were constructed by railroad company forces or by an unknown contractor.

Railroads often reused steel and iron spans, as it provided a cost effective way to construct bridges without requiring large amounts of new materials. While lattice truss spans were relatively uncommon through the United States, the BCR&N occasionally used the design in the 1880s, before switching to more standard designs in the 1890s. Other Midwest railroads, such as the Chicago & North Western Railway (C&NW) used the design nearly exclusively from the 1870s into the 1920s. Some railroad engineers favored the design, due to its resilience and greater strength without sacrificing economy. Metal lattice deck truss designs were initially developed for railroad use in the 1870s, often using deep spans with numerous intersections. Through the mid-1880s, short to medium length spans were simplified into double intersection spans. The type of lattice truss seen on this bridge is an unusual variation of a design used throughout the mid-1880s. The lack of a floor system may have been due to the short size of the span. Since the 1992 reconstruction, the bridge has seen no significant alterations, and is currently out of service. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked this bridge as being regionally significant, due to the unique history and truss design.