DAIR Big Sioux River Bridge (Elk Point)

Located between Elk Point and Akron, this through truss bridge carries a former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) line over the Big Sioux River. The previous bridge at this location was likely a wooden truss bridge, approached by timber pile trestle spans. In 1908, it was decided to rebuild the bridge with a secondhand steel through truss span, retaining a timber pile trestle approach on the south end. In 2011, the south approach was replaced with steel stringers, reusing the original timber pile trestle bents. The most recent alteration to the bridge came in 2019, when a precast concrete modular girder span was added to the north end of the bridge after a flooding related washout. Currently, the bridge consists of a 152-foot, 6-panel, pin-connected Pratt through truss span, set onto concrete piers. The north approach consists of two 22-foot precast concrete modular girder spans, set onto steel pile piers. The south approach consists of fifteen 16-foot steel stringer spans, set onto timber pile bents which have been reinforced with concrete caps. As part of the 2019 repairs, the original north abutment was altered to form a pier, and additional steel piles were added behind the abutment. The truss span was 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. One truss span was relocated here, two truss spans were relocated to Bridge #T-252 across the Big Sioux River at North Sioux City, South Dakota and a 77-foot through girder span was relocated to Bridge #G-316 1/2 at Wausau, Wisconsin.

The truss span follows a standard design for the era, with built-up members, a standard floor and decorative portal bracing. The top chord and endposts both consist of built-up beams, with X-lacing on the bottom and solid plates on the top. The bottom chord also consists of a built-up beam with X-lacing on both sides. Vertical members consist of V-laced built-up beams, while the diagonal members consist of rectangular steel eyebars. The floor system consists of two plate girder stringers, connected to plate girder floorbeams. The portal bracing uses a decorative design, with a double intersection lattice beam and rounded solid heel bracings. These heel bracings contain a star shaped cutout. The sway bracing uses a built-up beam with V-lacing at the center and a single X at the ends. They are connected to the vertical members by a diagonal bracket, formed out of an L-shaped plate. The upper lateral bracing is composed of rods, which are pinned to the top chord. The lower lateral bracing is composed of L-shaped bars. The approach steel stringer spans consist of four beams, arranged into two sets of two. The concrete substructures also follow a standard design, with a diamond shaped pier and sloped wing walls at the former abutment.

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. 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 truss design.