UP Boone River Bridge (Webster City, South)

Located in Briggs Woods Park south of Webster City, this large deck truss bridge carries a former Chicago & North Western Railway line over the Boone River. The first bridge at this location was likely a wooden deck truss, supported on timber pile piers and approached by a lengthy timber pile trestle approach on the north end and a shorter approach on the south end. During the early years of the 20th Century, the C&NW invested significant capital into this line, replacing timber bridges with heavier stone and steel structures. In 1901, the main portion of the bridge was replaced by a steel deck truss span, set onto two new stone piers. In 1909, a second major project was undertaken at this bridge, which replaced the timber pile trestle approaches with new deck girder spans and concrete abutments, filling the remainder of the long north approach. Currently, the bridge consists of a 128-foot, 8-panel, riveted double intersection Warren deck truss span, approached by a 60-foot deck plate girder span on the north end and a 50-foot deck plate girder span on the south end. The two piers are constructed of stone, while the abutments are constructed of concrete. Stone for the piers consists of a yellow limestone, likely quarried at Mankato, Minnesota. An unknown firm fabricated the truss span, while the American Bridge Company fabricated the approach spans. An unknown contractor constructed the piers, while an additional unknown contractor constructed the abutments. It is possible that the piers were constructed by the Widell Company of Mankato, Minnesota; which had large contracts for supplying stone and constructing stone bridges in Iowa for the C&NW in 1901 and 1902.

The truss span follows a standard design for deck trusses constructed for the C&NW during the early 20th Century, with a combination of built-up and solid members, riveted connections and a standard floor system. The top chord consists of X-laced built-up beams, while the bottom chord consists of a similar design which uses V-lacing instead of X-lacing. At the outermost panels, the bottom chord is comprised of a built-up beam which uses V-lacing on all four sides. The diagonal compression members are comprised of solid beams and the tension members are comprised of built-up beams consisting of two bars connected by thin plates. The endposts use a similar design to the diagonal compression members and are composed of solid beams. The floor is constructed of four stringers, which are constructed of steel stringers and arranged into two sets of two. These stringers are connected to plate girder floorbeams, which have a trapezoidal cross section, typical for trusses from this era. The upper lateral bracing, lower lateral bracing and internal bracing are all composed of steel L-shaped. The deck plate girder spans follow a standard design, and are comprised of two heavy plate girders and an open deck. These spans are supported by a heavy bent which is placed on the piers. The piers follow a standard diagonal shape, while the abutments use a standard design with sloped wing walls extending diagonally the bridge.

While lattice truss spans were relatively uncommon through the United States, the C&NW used the design extensively. Other railroads in the Midwest also occasionally used the design, although not as frequently. Engineers for the C&NW 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. By the 20th Century, the design had been refined further, and was constructed using significantly heavier members. The type of lattice truss seen on this bridge was popular for structures constructed between 1900 and 1905, and balanced member size with strength. While most railroads stopped using lattice trusses in the early 20th Century, the C&NW continued to use lattice truss designs nearly exclusively into the 1920s. Since the initial construction, the bridge has seen no significant alterations. Overall, the bridge appears to be in fair to good condition, with no significant deterioration noted. The author has ranked the bridge as being moderately significant, due to the truss design.