Sauk City Railroad Bridge (Main Channel)

Once located in Sauk City, this unique pony truss and through girder bridge carried the former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) Prairie du Sac Branch over the main channel of the Wisconsin River. Initially, this bridge was part of a much longer structure, crossing the main channel, an island and the east channel. The first bridge at this location likely consisted of a wooden through truss swing bridge, constructed when the line was first constructed. In 1893, the bridge would be renewed with a new wooden bridge. This bridge consisted of 95 spans of timber pile trestle approach, three 103-foot wooden Howe through truss spans over the east channel, 40 spans of timber pile trestle, a 103-foot wooden Howe through truss span, a 191-foot wooden Howe through truss span with curved top chords and a 114-foot wooden Howe through truss span. The entire bridge was set onto timber substructures, and the swing span was necessary as this portion of the Wisconsin River served as part of the Fox-Wisconsin waterway. Timber truss bridges were often used for branch lines, as the material was economical. By the early 20th Century, timber truss spans fell out of favor for any use, and a new steel and concrete bridge would be constructed in 1909-1910. At this time, the center trestle was filled, and the bridge split into separate structures.

Because this bridge was located on a branch line, it was decided to reuse secondhand spans that were originally constructed for mainline use at other locations. Originally, the replacement bridge consisted of a large Whipple through truss main span, approached by two through plate girder spans on the east end and a through plate girder swing span on the west end. The Whipple truss span was originally fabricated in 1881 by the Edge Moor Bridge Works for a bridge across the Mississippi River at Sabula, Iowa. This span consisted of a 217-foot, 12-panel, pin-connected Whipple through truss span, constructed with light members and a bi-level lattice portal. This span was one of a number of spans reused from the Sabula Bridge, with two other spans known to have been reused at Marinette, Wisconsin. In the early 1940s, the Badger Army Ammunition Plant began construction north of Prairie du Sac, and the bridge required upgrades to allow for heavier trains. In 1943, the main through truss span was replaced by two secondhand pony truss spans, giving the bridge its final configuration.

From 1943 to 2002, the bridge consisted of from east to west, two 55-foot type "B" through plate girder spans, a 97-foot 6-inch, 6-panel, pin-connected Pratt pony truss span, a 112-foot 10-inch, 6-panel, pin-connected Pratt pony truss span and a 123-foot counterbalanced through plate girder swing span. With the exception of the center pier, all substructures were constructed with concrete founded onto timber piles. A timber pier was used between the two pony truss spans to minimize expenses when replacing the original center span. The unique mix of spans present on this bridge was due to the spans available, as well as the different dates the bridge was erected. Additional repairs were made in 1946, when the swing pier (pier #5) was reinforced with new concrete at the base. Portions of the east abutment also appear to have been jacketed with concrete in the 20th Century. Typical of Milwaukee Road bridges, the concrete substructures were constructed by railroad company laborers.

The swing span was originally fabricated in 1892 by the Lassig Bridge & Iron Works for use at Bridge #D-562, which crossed a canal connecting the Fox River and Wisconsin River east of Portage, Wisconsin. This span consisted of a 74-foot arm and a 49-foot counterweight arm. The 74-foot arm used a tapered design, while the counterweight arm was mainly square. The counterweight for the span was located underneath the counterweight arm, and consisted of scrap iron rails encased in concrete. The swing span was reportedly operated by use of a pulley system, in which a rope anchored on the west bank turned a round spool underneath the span, turning the span. This span also used a rim-bearing design, with wheels directly affixed to the span which turned on a metal track located on top of the round swing pier. The lift mechanisms at the end used a wedge system, which appears to have been operated manually. These consisted of a bell crank which lifted the span from the bearing blocks. The girders were heavily constructed, and used an unusual design similar to the east approach girders. Floorbeams were spaced throughout the girders, and the ties sat directly onto plates riveted the length of the span. This unusual feature gave the span an appearance similar to a deck plate girder. Counterbalanced or "bobtail" swing spans were used extensively by the Milwaukee Road, as the design allowed spans to be constructed while being mindful of nearby obstructions and minimizing excessively long spans for small canals. These spans required precise engineering, as the counterweight had to make up for the difference between the short (counterweight) arm and the main arm. While the Milwaukee Road built more than a dozen counterbalanced swing spans of varying designs, very few spans were constructed by other railroads in the United States. The swing span stopped operating after the construction of the Prairie du Sac Dam, and it appears that some of the machinery was removed from the span in the early 20th Century.

The 112-foot 10-inch span consisted of a standard 6-panel, pin-connected Pratt pony truss span, set onto a concrete west pier and timber center pier. The truss used a standard Milwaukee Road design, with built-up members and heavy pinned connections. The endposts and top chord of the span consisted of built-up beams with heavy X-lacing. The bottom chord used eyebars, with the bottom chord diagonal at the outer panels. The upper angle connections used large plates to help support the pinned connections. The diagonal members used steel bars, with turnbuckles on some members to adjust the tension. The floor used a typical design, with two stringers and floorbeams placed at the panel points. The 97-foot 6-inch span was similar, although the trusses were not as deep. In addition, the bottom chord was horizontal throughout the span and used a built-up beam with heavy X-lacing instead of eyebars. Both spans were fabricated by the Lassig Bridge & Iron Works in approximately 1899 for use at unknown locations. This design was a standard for the Milwaukee Road, and this design of pony truss was constructed in a number of different lengths. The 112-foot 10-inch and 97-foot 6-inch spans appear to have been unique sizes.

It is possible that the 112-foot 10-inch span was originally part of bridge #H-204 across the Wapsipinicon Oxford Junction, Iowa. This bridge was abandoned and removed in approximately 1940, and is believed to have been reused elsewhere. This bridge used two 112-foot 10-inch spans and one 113-foot 3-inch span. It is believed that two of the spans were reused at Bridge #Y-38 near Adel, Iowa in 1944. One possible location the 97-foot 6-inch span was reused from is Bridge #G-1904 over the New Wood River in rural Lincoln County, Wisconsin. This span was originally fabricated in 1900 as part of a three span bridge at Bridge #Z-708 across the Wapisipinicon River at Olin, Iowa; before being replaced by a double track bridge and reused at Bridges #G-368, G-1900 and G-1904 1914. The bridges at G-1900 and G-1904 were removed in the early 1940s. The span at G-1900 was reused at Bridge #M-538 in Menomonie, Wisconsin. It seems likely that the span from G-1904 also was reused. Both these spans used unique lengths, which the two bridges proposed match and the timelines match. Research into the history of these spans is ongoing.

The two 55-foot through plate girder spans were originally fabricated in 1897 by the Wisconsin Bridge & Iron Company for use at Bridge #Z-1586 across Mosquito Creek at Chautauqua, Iowa. When that bridge was replaced with a new bridge in 1909, the two spans were reused at this location. These two spans used a typical design for Milwaukee Road type "B" girders, with shallow girders, an unusual floor system and an open deck. Typical of spans of this design, the floor was placed approximately halfway up the girders. The floor was composed of plate girder floorbeams, placed throughout the span. Unique to this design, no stringers were used to carry the ties, and instead the ties are set directly onto plates riveted to the girders. The eastern of these two spans used a tapered end and a square end, while the western span used both squared ends. It is unknown why the bridge was originally designed in this fashion, as this style bridge typically had tapered ends at the end of the structure.

The bridge remained in regular use until 2002, when the pier between the western truss span and the swing span began to sink. As a result, the western truss span was demolished with explosives in April 2022. The remaining spans remained intact until 2017, when the pier between the eastern truss span and east approach became undermined. The east truss span, through girder spans and associated substructures were removed in early 2018. After years of planning, a project was approved by Sauk and Dane Counties to construct a new trail bridge at this location. Work on the bridge began in early 2025, which included the removal of the swing span. It is anticipated that the new bridge will open in 2027. Overall, the bridge appeared to be in exceptionally poor condition at the time of removal, particularly in regards to the substructures. Boring information on the blueprints for the bridge indicate that a thick layer of quicksand was present underneath the streambed. It is possible that this contributed to the eventual scour of the two piers that failed. Despite the poor condition of the substructures, the superstructure was in fair condition, with no significant deterioration noted. The author has ranked this bridge as being highly significant, due to the unique history and mix of spans present. In addition, the swing span was among the earliest counterbalanced swing spans constructed by the Milwaukee Road, and represented a major innovation in movable bridge design.