Sauk City Railroad Bridge (East Channel)

Located in Sauk City, this through truss and through girder bridge carries the former Chicago, Milwaukee, St. Paul & Pacific Railroad (Milwaukee Road) Prairie du Sac Branch over the east 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 over the island and 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 over the main channel. 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. Railroads often reused steel and iron spans, as it provided a cost effective way to upgrade branch line bridges without requiring large amounts of new material. Currently, the bridge consists of two 147-foot, 6-panel, pin-connected Pratt through truss spans, approached by a 45-foot type "B" through plate girder span on either end. In addition, the east end is approached by several spans of timber pile trestle. The west abutment and piers under the truss spans are constructed of concrete, while the pier between the east approach girder and trestle is constructed of timber piles. The east approach trestle and timber pile pier were last renewed in 1934, and the west pier and west abutment were jacketed with concrete in 1939. Typical of Milwaukee Road bridges, the concrete substructures were constructed by railroad company laborers.

The two 147-foot truss spans were originally fabricated in 1891 for use at Bridge #L-268 across the Mississippi River at Hastings, Minnesota. When the approaches of that bridge were replaced in 1909, the 1890s-era approach spans were freed up for reuse. One source indicates that the Milwaukee Road fabricated these truss spans from raw steel plates themselves, although there is no additional sources confirming or denying this. Some railroads experimented with fabricating their own steel and iron bridge spans in the late 19th Century, as it allowed the company to keep the entire construction process in-house. Blueprints indicate that these spans were designed by the Milwaukee Road Bridge & Building Department under the direction of Onward Bates. The Milwaukee Road was known for completing almost all bridge design in-house, and Onward Bates was a prominent bridge engineer for the Milwaukee Road at this time. By keeping the bridge design in-house, standard spans could be developed, which allowed for easier maintenance, repairs, replacements and reuse.

The two through truss spans use a similar design to other truss bridges designed by the Milwaukee Road in the early to mid-1890s. Both the endposts and the top chord consist of built-up beams, with solid plates on the outside and V-lacing on the interior. The bottom chord is also composed of built-up beams, which use extensive X-lacing on both the top and bottom. This design is somewhat unusual, as truss spans constructed in the early 1890s typically used eyebars for the bottom chord. Vertical members of the truss span are composed of built-up beams, with V-lacing on the inside and outside. The diagonal members are composed of eyebars, formed of metal bars and rods. Built-up crash bars have been included at the end panels, which use a V-laced design. The floor is composed of four stringers, arranged into two sets of two. Floorbeams are placed at the panel points, and consist of plate girders that are set beneath the bottom chord and stringers. Both the lower and upper lateral bracing consist of metal bars, which is connected to the bottom and top chord by plates. The portal bracing is similar to other Milwaukee Road truss bridges from this era, with a lattice beam and curved heel bracing with circular cutouts. The sway bracing is similar, but the lattice beam is shallower and the heel bracing consists of angled metal bars.

The two 45-foot through plate girder spans were originally fabricated in 1892 by an unknown firm for use at Bridge #A-56 across the North Branch Chicago River in Chicago, Illinois. These spans follow a typical design for type "B" girders, with an unusual floor system. 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. Both spans use shallow girders with squared ends. This design was used by the Milwaukee Road in the 1890s. It is believed the main benefit of this design is that it required less material, as it did not contain stringers.

Pratt trusses were the most common design of truss bridge during the second half of the 19th Century. This design combined economy, strength and simplicity. The Pratt truss design was ultimately superseded by riveted Warren truss spans around the turn of the 20th Century. Girder spans were commonly used by railroads, as they were durable and easy to construct. The two girders present on this bridge are unusual in their design, and appear to be excellent examples of unaltered type "B" girder spans. This bridge remained in service until 2002, when the adjacent main channel bridge partially collapsed and was partially removed. After much of the main channel bridge was removed in 2018, Sauk and Dane Counties began working towards reusing this railroad grade as a rail-trail. This bridge was acquired by the two counties, and work on converting the structure to trail use began in 2025. It is believed that the bridge will be opened to the public sometime in 2027. As part of the work, new railings and decking will be added to the bridge, but it is not believed that there will be any alterations that will affect the historic integrity of the structure. 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, age and unknown builders.