North Sioux City Railroad Bridge

Located between Sioux City and North Sioux City, this through truss bridge once carried a former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) line over the Big Sioux River. The previous bridge at this location was a wooden truss bridge, approached by timber pile trestle spans. In 1907, it was decided to rebuild the bridge with a pair of secondhand steel through truss span, retaining a timber pile trestle approach on each end. The most recent upgrade came in 1931, when a deck plate girder span was added to each end and new concrete abutments were constructed. The bridge consisted of two 152-foot, 6-panel, pin-connected Pratt through truss spans, approached by a 77-foot deck plate girder span on the south end and a 40-foot deck plate girder span on the north end. The entire bridge was set onto concrete substructures. The truss spans were originally fabricated in 1900 for use at Bridge #Z-854 across the Cedar River near Cedar Rapids, Iowa. When a new bridge was constructed on a new alignment, various spans from the original bridge were reused throughout the Milwaukee Road system. Two truss spans were relocated here, one truss span were relocated to Bridge #T-480 across the Big Sioux River near Elk Point, South Dakota and a 77-foot through girder span was relocated to Bridge #G-316 1/2 at Wausau, Wisconsin.

The truss spans followed a standard design for the era, with built-up members, a standard floor and decorative 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 sides. Vertical members consisted of V-laced built-up beams, while the diagonal members consisted of rectangular steel eyebars. The floor system consisted of two plate girder stringers, connected to plate girder floorbeams. The portal bracing used a decorative design, with a double intersection lattice beam and rounded solid heel bracings. These heel bracings contained a star shaped cutout. The sway bracing used a built-up beam with V-lacing at the center and a single X at the ends. They were connected to the vertical members by a diagonal bracket, formed out of an L-shaped plate. The upper lateral bracing was composed of rods, which are pinned to the top chord. The lower lateral bracing is composed of L-shaped bars. The approach spans followed a standard design, with two heavy plate girders and an open deck. The piers also followed a standard design, using a rectangular shape with an angled upstream nose. The abutments followed a typical 1930s design, with sloped wing walls.

Pratt trusses were arguably the most common railroad truss bridge design during the second half of the 20th Century. This design was simple, economical and durable. By the turn of the 20th Century, railroads began switching to riveted Warren truss spans. The Milwaukee Road continued to use Pratt trusses until approximately 1910, when it followed suit in switching to Warren trusses. Railroads often reused steel and iron spans, as it was a cost effective way to rebuild bridges without requiring large amounts of new material. Often, when a span was no longer sufficient for mainline use, it could be reused on a branch line. When the bridge at Cedar Rapids was replaced, it had only been standing for 7 years, making it a prime candidate for reuse. Additional repairs were made to the bridge in the 20th Century, including installing a sheet pile reinforcement around the center pier. Early on June 24, 2024, the center pier of the bridge became undercut from flooding, causing the two truss spans to collapse. The truss spans were removed later that summer, and a new bridge opened in 2025. The new bridge consists of two modern through plate girder spans, retaining the outer piers and approaches from the previous bridge. Overall, the bridge appeared to be in fair to good condition at the time of documentation, with no significant deterioration noted. The author has ranked this bridge as being regionally significant, due to the truss design.