La Crescent Swing Bridge

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Located at La Crescent, this large swing bridge carries the former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) mainline across the western (main) channel of the Mississippi River. At this location, the Black River flows into the Mississippi River, and the two rivers are separated by a series of swamps, islands and back channels. A bridge was first authorized at this location in 1868, but was not immediately built. The act was renewed in 1872, and allowed for the Milwaukee & St. Paul Railway to construct a bridge at any point between La Crosse and Houston County, Minnesota. In 1875, the location was finalized, and construction on a crossing began. The first bridge was completed in December 1876, and consisted of one long timber pile structure with sections of iron spans over the Black River, east channel and main channel. The original structure across the west channel of the Mississippi River consisted of two 164-foot, 10-panel, pin-connected Pratt through trusses, a 250-foot, 17-panel, pin-connected Whipple through truss span and a 360-foot, 20-panel, pin-connected Whipple through truss swing span with a curved top chord. The west approach consisted of approximately 5,000 feet of timber pile trestle, while the east approach was connected to the east channel bridge by 650 feet of timber pile trestle. The entire bridge was constructed of wrought and cast iron, and the metal spans were set onto stone piers. American Bridge Company (of Chicago) constructed the original superstructure, while an unknown contractor constructed the stone pier.

After the original construction, the only significant alteration to the bridge was made in 1890, when one of the 164-foot spans was replaced by a new span after a derailment destroyed the original span. By the turn of the 20th Century, the original structure had become too light for traffic, and work to replace the bridge began in 1901. As part of the work, the lengthy trestle approaches were filled with earth. Work to replace the superstructure began with rebuilding the stone piers, which involved cutting the stonework down and installing new concrete copings on the piers. The new bridge consisted of new steel truss spans of the same lengths to replace the iron spans, and deck girder spans on either end. Construction was completed in the spring of 1902 under the direction of C.F. Loweth, Engineer of Bridges and Buildings for the railroad. Two of the old spans are known to have been reused at have been reused at Eau Claire, Wisconsin, and it is believed that the original swing span was scrapped. The only significant alteration included the replacement of the original west approach with a larger deck plate girder span in approximately 1930, giving the structure its present configuration.

Currently, the bridge consists of a 40-foot deck plate girder, followed by a pair of 164-foot, 6-panel Pratt through truss spans and a 250-foot, 9-panel, pin-connected Parker through truss span. The swing span consists of a pair of 7-panel, pin-connected modified Pratt through truss leaves, joined over a center pier by a tall tower. Originally, the west approach consisted of a short 26-foot deck plate girder span, which has been replaced with a 75-foot span. Stone is used for most of the piers, while the west pier and abutments are constructed of concrete. It is believed that all substructures are founded on timber piles, sunk deep into the river bed. Phoenix Bridge Company fabricated the truss spans, while an unknown contractor fabricated the girder spans. It is unknown which contractor constructed the stone piers, or where the stone was quarried from. Alterations to the piers were completed by Milwaukee Road company forces, typical for this railroad. The west pier and west abutment also were likely constructed by railroad company forces.

The swing span is constructed using a rim bearing design, where the superstructure is set onto a steel drum, which rotates over a roller nest. This span employs a special type of rail lift designed by the Milwaukee Road, which uses a cam to lift the ends of the bridge to turn. The truss spans are heavily constructed, using a combination of built-up members and solid bars, with a traditionally composed floor consisting of floorbeams and girders for stringers. The top and bottom chords of the truss spans use built-up members, consisting of steel channels connected by heavy X-lacing. All endposts of the spans are laced, with the swing span using a design with X-lacing separated by thin plates, and the fixed spans using a continuous design with heavy X-lacing. The vertical members all consist of built-up beams, composed of tightly X-laced channels. With the exception of the inner diagonals near the center tower, all diagonals are composed of slender steel bars with eyebars at the ends. The inner diagonals of the swing span use a heavily built-up design, and the diagonals at the inner side of each half use a design similar to the endposts. Portal bracing for all spans consists of a lattice design, which uses thin bars riveted to struts. The sway bracing of all spans uses a similar design, which is connected to laced struts. The top strut on the swing span uses a decorative design, with a star shaped cutout in the center. The center tower of the swing span uses a three bay design, with built-up members and solid bars and rods. A wooden machinery house is located on the south side of the center of the swing span. The deck plate girder spans are traditionally composed, with the west approach using heavy girders and the east approach using a shallow design.

Since the replacement of the west approach, the bridge has seen few alterations. Pratt truss spans were arguably the most popular truss design in the 19th Century, as they were durable, strong and economical. The Parker truss design is a variant of the Pratt design, which uses a curved top chord and can be constructed with longer spans. While the French Slough, East Channel and Black River bridges were significantly replaced in the late 1990s and early 2000s, this bridge has been retained with little changes. Due to the amount of barge strikes, it has been proposed to replace the swing span with a new vertical lift span. However, this work is still in the planning phases and will not occur in the near future. Overall, the bridge appears to be in fair to poor condition, with some deterioration noted throughout the steel. The author has ranked this bridge as being highly significant, due to the unique combination of large truss spans.