DAIR Big Sioux River Bridge (Canton)

Once located on the south side of Canton, this large truss bridge carried the former Chicago, Milwaukee, St. Paul & Pacific Railroad across the Big Sioux River. The first bridge at this location was likely a timber Howe through truss bridge with timber pile trestle approaches, constructed when the line was first built. In 1911, a 75-foot deck plate girder span, originally fabricated in 1887, would be relocated from Bridge #C-466 at Bangor, Wisconsin. Further upgrades to the bridge were made in 1917, when a new truss span was installed adjacent to the girder. In 1931, the deck girder span would be replaced by a heavier deck plate girder span. It is unknown if the 75-foot span was reused elsewhere or scrapped. The most recent upgrades to the bridge came in 2011, when the original timber pile trestle approaches were replaced by steel stringers, reusing the original bents. The bridge consisted of a 136-foot, 5-panel, riveted Pratt through truss span, approached by a 72-foot deck plate girder span on the north end, seven 14-foot steel stringer spans on the south end and six 14-foot steel stringer spans on the north end. The entire bridge was set onto timber pile substructures. The truss span was originally fabricated in 1909 for use at Bridge #Z-1154 across the Des Moines River at Madrid. A new high bridge was completed in 1914 and the original structure removed soon after. The deck girder span was originally fabricated in 1898 for use across the South Branch Kishwaukee River at Bridge #Z-204 near Genoa, Illinois. An unknown contractor fabricated the truss span, and an additional unknown contractor fabricated the girder span. The timber components of the bridge were constructed by railroad company forces.

The truss span followed a somewhat unusual design for the era, with heavy members, riveted connections and a unique portal bracing. The top chord and endposts both consisted of built-up beams, with X-lacing on the bottom and solid plates on the top. The bottom chord also consisted of a built-up beam, with X-lacing on both the top and bottom. The vertical truss members consisted of a combination of solid beams and built-up beams with V-lacing. The diagonal truss members consisted of a similar design. Similar to other truss bridges from the era, the floor system was composed of two plate girder stringers supported by plate girder floorbeams. Both the upper and lower lateral bracing were composed of L-shaped bars. The portal bracing used a double intersection lattice portal with L-bars forming an angled heel bracing. The sway bracing also used a similar design. The deck plate girder span was simply composed, consisting of two heavy plate girders. Upon relocation here, the girder was converted from a through girder span to a deck girder span by removing the original floor and constructing new internal bracing.

Railroads often reused steel spans, as it provided a cost effective way to upgrade bridges without requiring large amounts of new material. Often, when a span was no longer suitable for one location, it could be installed on a branch line with or without modifications. Pratt trusses were arguably the most common truss design for railroad bridges in the 19th Century. Originally, these spans were constructed with pinned connections. By the turn of the 20th Century, riveted connections had become the standard for railroad truss spans, and the Pratt design was largely superseded by the Warren design. The Milwaukee Road continued to use riveted variations of the Pratt design until approximately 1900, before switching to using the Warren design. The bridge appeared to be in fair condition at the time of replacement, with no critical defects noted. The bridge was destroyed by flooding on March 15th, 2019, and would be replaced by a new steel and concrete bridge. The author has ranked the bridge as being regionally significant, due to the truss design and relocation history.