Merrimac Railroad Bridge

Authors Note: This page is intended to document the bridge in its 1930-2023 configuration. The bridge was significantly altered in 2023-2025.

Located between Merrimac and Okee, this large deck truss and deck plate girder bridge carries the former Chicago & North Western Railway mainline over Lake Wisconsin, which was created by impounding the Wisconsin River. When the C&NW began constructing this line, quickly reaching the Mississippi River at Winona was a top priority for the railroad. The Wisconsin River at Merrimac (then Merrimack) presented a formidable obstacle for the railroad, as the river was wide and surrounded by rugged terrain. Early bridges along this line were constructed using timber, which provided a cheaper option that could be constructed faster. The first bridge consisted of a 1,900 foot long structure. The bridge consisted of three fixed wooden 150-foot Howe deck truss spans, a 150-foot wooden swing span of the same design and a lengthy wooden trestle on either side. The entire bridge was set onto timber substructures, and cost approximately $65,000 to construct. A drawspan was required at this location, as there was still significant steamboat traffic which navigated the Wisconsin River. In the late 1870s and early 1880s, the C&NW invested significant capital into this line, replacing the original timber bridges with stone and iron structures. Work on a new bridge across the Wisconsin River began in 1878, and continued through 1880. Construction on the new bridge was slow going, with spans replaced one at a time as work progressed through the structure. As part of the work, new stone abutments and piers were constructed, using stone quarried at Ablemans (Rock Springs), Duck Creek, Wisconsin and Winona, Minnesota. Iron spans, fabricated by the Leighton Bridge & Iron Works were installed on the new stone substructures.

The second bridge consisted exclusively of riveted lattice deck trusses with a center lattice through truss. From east to west, the bridge consisted of a 65-foot deck truss span (Span A), a 144-foot deck truss span (B), twelve 49-foot deck truss spans (C-N), two 107-foot deck truss spans (O and P), a 118-foot deck truss span (Q), a 154-foot through truss span (R), a 204-foot deck truss swing span (S) and two 123-foot deck truss spans (T and U). The longer deck truss spans were constructed using a quadruple intersection lattice deck truss design, while the smaller deck trusses were constructed of shallow double intersection Warren deck truss spans. All of the truss spans consisted of solid members and lightly built-up beams. The through truss span was constructed using a quadrangular lattice through truss design, with arched lattice portals that was typical for this era. These truss designs became popular in the late 1870s, and many examples of these spans were constructed by the Leighton Bridge & Iron Works for railroads throughout the United States. In addition to the stone piers, the shallow deck truss spans were set onto iron towers, founded on stone pedestals. The bridge became a signature structure constructed by the Leighton Bridge & Iron Works, and was used in many promotional advertisements. This company also constructed most of the iron bridges for the C&NW between approximately 1877 and 1881, when the company went defunct.

As trains became heavier in the late 19th and early 20th Century, upgrades to this bridge would be required. The first significant upgrade was made in 1895, when spans C through N were replaced by new deck plate girder spans, reusing the original iron towers. These spans were the first to be replaced, as they were the lightest constructed spans on the bridge and were likely overloaded. To facilitate the changing of the spans, falsework was constructed along three spans at a time, and the new spans erected on one side of the existing spans. The old span was then shifted to the falsework on the opposite side, and the new span installed. At the time, this method of changing spans was novel, and allowed for minimal interruption to railroad traffic. This method would be recreated at numerous bridges throughout the United States, and became particularly popular for replacing large structures. Further upgrades to the bridge came in 1903, when the remaining spans were replaced by new steel spans. It is unknown if any of the original spans from this bridge were reused elsewhere, as was common practice with the C&NW. Further alterations were made to the bridge in 1909, when the original iron towers under spans C through N were replaced by new steel towers. After the construction of the Prairie du Sac Dam and the formation of Lake Wisconsin, these towers were largely encased in concrete to prevent deterioration of submerged steel.

Additional alterations were made to the bridge throughout the 20th Century. Pier #17, which originally consisted of a stone pier constructed in 1878, was replaced by a concrete pier in 1923. The most significant alterations to the bridge came in 1930, when several components of the bridge were heavily modified. As part of the modifications, span B was shortened, converted to a cantilevered span and a concrete counterweight installed on the east end. In addition, span O was shifted west, span P was shortened, spans C and N were replaced by new deck girder spans and piers #3, #15 and #16 were replaced by new concrete piers in new locations. Later additions came in 1943, when the bearings for the swing span (span S) were modified, and it is believed that the mechanism for turning the span was removed at this time. In 2018, emergency repairs were made to span B. Further alterations were made to the bridge in 2019, when steel components of nine additional spans were replaced, giving the structure its final configuration before a major overhaul occurred in 2023-2025. The final configuration of the bridge consisted of eight truss spans and thirteen deck plate girder spans, set onto a variety of substructures. Between 2023 and 2025, thirteen bridge spans were replaced, including ten deck plate girder spans (spans A and D through M) and two truss spans (spans Q and R). As part of the work, many of the substructures were heavily modified, including the addition of concrete encasements to several piers. Further alterations include the complete replacement of the timber deck, new rails and new handrails. The components of the bridge prior to the reconstruction are detailed below:

Span A was composed of a 65-foot deck plate girder span, constructed in 1903 by an unknown contractor. The span is traditionally constructed, consisting of two heavy girders and an open deck. No significant alterations were made to the span between its initial construction and replacement in 2023. The replacement span consists of a modern steel deck plate girder span of similar dimensions.

Span B was originally constructed in 1903 by an unknown contractor, and was heavily modified during the 1930 reconstruction. Originally the span was composed of a 144-foot, 7-panel, riveted double intersection Warren deck truss span, constructed at a 20-foot 4.5-inch depth. In 1930, the span was heavily altered by removing the westernmost panel, constructing a new pier to create a cantilevered panel at the west end, and installing a reinforced concrete counterweight over the east end. It is believed that these alterations were required due to an unstable pier #3, which was located on the bank of the overflow channel of the river. The current span is comprised of a 104-foot, 6-panel truss span. It is unknown if the work of altering the superstructure was completed by American Bridge Company or by railroad company forces. The counterweight was constructed by the Jutton-Kelly Company, which also was responsible for reconstructing pier #3. This span uses a similar design to other truss spans on this bridge, with heavy members and a traditionally composed floor. The top and bottom chords are composed of V-laced members, with the bottom chord of the outermost eastern using a V-laced design on all four sides. The floor is constructed using four steel stringer floorbeams, arranged into two sets of two. Flooorbeams are composed of trapezoidal plate girders, with the ends riveted directly to the gusset plates. The diagonal members are composed of a combination of built-up and rolled members, with the built-up members consisting of parallel beams connected by thin plates. The interior bracing also uses a typical design for spans seen on this bridge, with two levels of X-bracing between the diagonal members. Additional work to the structure in 1930 included the addition of a mid-level transverse beam and longitudinal members at the west end of the modified span, which help support span C. The additional longitudinal bracing is constructed of X-laced beams. The concrete counterweight is set onto two transverse beams, which are set onto vertical beams resting on the bottom chord. An additional riveted connection and diagonal brace has been added to the east end to support the counterweight. Recent alterations to the span came in 2018 and 2019, when some diagonal members were repaired or replaced with new material. In addition, it is possible that portions of the floor have been repaired or replaced, including the stringers and possibly outer floorbeams. With the recent repairs, it is expected that the life of this span has been extended by approximately 40 years.

Span C consists of a large 69-foot deck plate girder span, constructed in 1930 to replace a 48-foot deck plate girder span originally constructed in 1895. The span was fabricated by American Bridge Company, and uses a typical design, with two heavy girders and an open deck. The east end of the span is set directly onto the cantilevered end of span B, while the west end is set onto a tower (pier #4). Longitudinal plates are used to connect the span to span B and pier #4. No significant alterations have been made to this span since its initial construction, and the span remains in use today. The previous 48-foot span was reused at Galena Division Bridge #1204 near Dalzell, Illinois; and has since been removed.

Spans D through F consisted of 48-foot deck plate girder spans, while spans G through M consisted of 49-foot deck plate girder spans. Despite the different lengths, all ten spans were constructed in 1895 by the Lassig Bridge & Iron Works, and used the same unusual design. Each span had two modest sized girders spaced far apart and a floor system installed between the girders. The floor system consisted of floorbeams, attached approximately midway up the girders with rivet plates. In addition, a transverse bar was used at the bottoms of the girders beneath the floorbeams, to help stabilize the girders. Each span used two stringers, set nearly directly under the rail. These stringers were comprised of shallow plate girders, which were affixed to the floorbeam by use of a hanger. Additional X-bracing was present on each span, which consisted of bars attached to the rivet plates holding the floorbeams. No significant alterations were made over the life of the spans. All ten spans were replaced by modern steel stringer spans in 2023.

Span N consists of a large 79-foot deck plate girder span, constructed in 1930 to replace a 48-foot deck plate girder span originally constructed in 1895. The span was fabricated by American Bridge Company, and uses a typical design, with two heavy girders and an open deck. The east end of the span is set onto a tower (pier #14), while the west end is set onto a trapezoidal bent constructed of built-up beams. Longitudinal plates are used to connect the span to pier #14. No significant alterations have been made to this span since its initial construction, and the span remains in use today. The previous 48-foot span was reused at Galena Division Bridge #1204 near Dalzell, Illinois; and has since been removed.

Span O was originally constructed in 1903 by an unknown contractor, and was shifted to its present location in 1930. The span is composed of a 107-foot, 7-panel riveted double intersection Warren deck truss span, constructed at a 15-foot 6-inch depth. No alterations were made to the superstructure during the 1930 shift, which placed the span approximately 30 feet further west. This shift was required to accommodate a longer span N and new pier #15. It is believed that the original pier #15 may have been unstable due to its location on the bank of the original river channel. This span uses a similar design to other truss spans on this bridge, with heavy members and a traditionally composed floor. The top and bottom chords are composed of V-laced members, with the bottom chord of the outermost panels using a V-laced design on all four sides. The floor is constructed using four steel stringer floorbeams, arranged into two sets of two. Flooorbeams are composed of trapezoidal plate girders, with the ends riveted directly to the gusset plates. The diagonal members are composed of a combination of built-up and rolled members, with the built-up members consisting of parallel beams connected by thin plates. The interior bracing also uses a typical design for spans seen on this bridge, with two levels of X-bracing between the diagonal members. Recent alterations to the span came in 2018 and 2019, when some diagonal members were repaired or replaced with new material, and the end floorbeams replaced with new steel beams. With the recent repairs, it is expected that the life of this span has been extended by approximately 40 years.

Span P was originally constructed in 1903 by an unknown contractor, and was shortened as part of the 1930 reconstruction. The span was originally comprised of a 107-foot, 7-panel riveted double intersection Warren deck truss span, constructed at a 15-foot 6-inch depth. In 1930, the two eastern panels were removed, giving the span its current 77-foot, 5-panel configuration. This alteration was required to accommodate shifted span O and pier #16 in new location. This span uses a similar design to other truss spans on this bridge, with heavy members and a traditionally composed floor. The top and bottom chords are composed of V-laced members, with the bottom chord of the outermost panels using a V-laced design on all four sides. The floor is constructed using four steel stringer floorbeams, arranged into two sets of two. Flooorbeams are composed of trapezoidal plate girders, with the ends riveted directly to the gusset plates. The diagonal members are composed of a combination of built-up and rolled members, with the built-up members consisting of parallel beams connected by thin plates. The interior bracing also uses a typical design for spans seen on this bridge, with two levels of X-bracing between the diagonal members. Recent alterations to the span came in 2018 and 2019, when some diagonal members were repaired or replaced with new material, and the end floorbeams replaced with new steel beams. With the recent repairs, it is expected that the life of this span has been extended by approximately 40 years.

Span Q was constructed in 1903 by an unknown contractor, and was composed of a 118-foot, 7-panel, riveted double intersection Warren deck truss span, constructed at a 16-foot 6-inch depth. This span used a similar design to other truss spans on this bridge, with heavy members and a traditionally composed floor. The top and bottom chords were composed of V-laced members, with the bottom chord of the outermost panels using a V-laced design on all four sides. The floor was constructed using four steel stringer floorbeams, arranged into two sets of two. Flooorbeams were composed of trapezoidal plate girders, with the ends riveted directly to the gusset plates. The diagonal members were composed of a combination of built-up and rolled members, with the built-up members consisting of parallel beams connected by thin plates. The interior bracing also used a typical design for spans seen on this bridge, with two levels of X-bracing between the diagonal members. No significant alterations were made to this span over its lifetime. Span Q was replaced by a modern deck plate girder span in 2024.

Span R was constructed in 1903 by an unknown contractor, and was composed of a 153-foot, 8-panel, riveted double intersection Warren deck truss span, constructed at a 22-foot depth. A longer span was required at this location, as this span replaced the through truss span. Due to the longer span, a deeper design is used than other spans, which gave the span an unusual appearance and presented a hazard to boaters. This span used a similar design to other truss spans on this bridge, with heavy members and a traditionally composed floor. The top and bottom chords were composed of V-laced members, with the bottom chord of the outermost panels using a V-laced design on all four sides. The floor was constructed using four steel stringer floorbeams, arranged into two sets of two. Flooorbeams were composed of trapezoidal plate girders, with the ends riveted directly to the gusset plates. The diagonal members were composed of a combination of built-up and rolled members, with the built-up members consisting of parallel beams connected by thin plates. The interior bracing also used a typical design for spans seen on this bridge, with two levels of X-bracing between the diagonal members. No significant alterations were made to this span over its lifetime. Span R was replaced by a modern deck plate girder span in 2024.

Span S consists of a rim-bearing swing span, constructed in 1903 by an unknown contractor. The span is composed of a 204-foot, 13-panel riveted double intersection Warren deck truss span, constructed at a 15-foot 6-inch depth. Each arm consists of 6 panels, with the east arm constructed at a length of 104 feet, while the west arm is 100 feet long. The rim bearing design was the most common swing span design, which consisted of a superstructure supported by a metal drum, which rotates along a nest of rollers and is turned by a gear system. The superstructure uses a similar design to the fixed truss spans on this bridge, with heavy members and a traditionally composed floor. The top and bottom chords are composed of V-laced members, and the bottom chord over the swing pier is a much deeper steel beam. Unlike the fixed spans, there are no end panels with V-lacing on all four sides. The floor is constructed using four steel stringer floorbeams, arranged into two sets of two. Flooorbeams are composed of trapezoidal plate girders, with the ends riveted directly to the gusset plates. The diagonal members are composed of a combination of built-up and rolled members, with the built-up members consisting of parallel beams connected by thin plates. The interior bracing also uses a typical design for spans seen on this bridge, with two levels of X-bracing between the diagonal members. The diagonal members at the center of the span use a tightly V-laced design. While the swing span is no longer operational, the original drum, roller nest, roller gears and roller end bearings were still intact until 2024. The shaft, keyhole and turning gear were removed from the span in 1943. Recent alterations to the span came in 2018 and 2019, when some diagonal members were repaired with new steel. The remaining swing machinery was removed from the span as part of the 2024 reconstruction, and replaced with new steel bearing blocks. With the recent repairs, it is expected that the life of this span has been extended by approximately 40 years.

Spans T and U were originally constructed in 1903 by an unknown contractor, and consist of 123-foot, 8-panel, riveted double intersection Warren deck truss spans, constructed at a 15-foot 6-inch depth. These spans use a similar design to other truss spans on this bridge, with heavy members and a traditionally composed floor. The top and bottom chords are composed of V-laced members, with the bottom chord of the outermost panels using a V-laced design on all four sides. The floor is constructed using four steel stringer floorbeams, arranged into two sets of two. Flooorbeams are composed of trapezoidal plate girders, with the ends riveted directly to the gusset plates. The diagonal members are composed of a combination of built-up and rolled members, with the built-up members consisting of parallel beams connected by thin plates. The interior bracing also uses a typical design for spans seen on this bridge, with two levels of X-bracing between the diagonal members. Recent alterations to the span came in 2018 and 2019, when some diagonal members were repaired with new material, and some components of the floor replaced with new material. With the recent repairs, it is expected that the life of this span has been extended by approximately 40 years.

The east abutment (substructure #1) and the west abutment (substructure #23) both consist of standard stone abutments, constructed in 1878-1879. Both appear to be constructed using stone quarried at Ablemans (Rock Springs), Wisconsin; and were constructed by an unknown contractor. The east abutment uses a standard U-shaped design, with no wing walls. The west abutment uses a standard flared design, with wing walls extending diagonally from the track. Recent repairs to the abutments include the replacement of mortar at the joints, and additional concrete repairs to the east abutment.

Pier #17 consists of a standard diamond shaped concrete pier, constructed in 1923 by an unknown contractor to replace the previous stone pier. It is likely that the stone began to fail from being submerged, or the lake bottom was no longer stable. Much of this pier was encased in 2024, although the north face appears to have not been encased. Piers #3, 15 and 16 consist of standard concrete piers, constructed in 1930 by the Jutton-Kelly Company as part of the reconstruction project. All three piers use a standard diamond shape, with large riser blocks on the top. The east abutment uses a standard U-shaped design, with no wing walls. The west abutment uses a standard flared design, with wing walls extending diagonally from the track. Recent repairs to the abutments include the replacement of mortar at the joints, and additional concrete repairs to the east abutment. Pier #3 is located one panel inside of the west end of span B, and serves as the pier for the cantilevered end of the span. The main portion of this pier is submerged and not visible. All three piers were encased in concrete as part of the 2024 repairs.

Piers #4 through #14 consist of steel towers, which are partially encased in concrete. The towers were constructed in 1909 by an unknown contractor to replace the older iron towers, which had initially been reused from the previous bridge. The steel for the towers consists of two trapezoidal bents, arranged into a triangular shape. The members of the bent consist of a combination of rolled beams and built-up beams with heavy V-lacing. The bottoms of the towers were encased with large blocks of concrete in 1915 as part of the construction of Lake Wisconsin. It is unknown what contractor completed the concrete encasements. Since the 1915 encasements, only minor repairs have been made to these piers. The piers were reused for the new steel spans D-M during the 2023 reconstruction, and only concrete caps were added to the towers/piers.

Large bridges like this were often built or modified in stages, as evidenced by this bridge. In particular, this bridge has more partial replacements and modifications than most railroad bridges in the Midwest. In the 21st Century, the Merrimac Bridge has become an important piece of railroad infrastructure in Wisconsin. After the removal of the bridge at Sauk City, this bridge is the only structure crossing the Wisconsin River, and allows a sole railroad line to serve Sauk County. Because the industries served, it was deemed necessary to rehabilitate the bridge to allow 286,000 railcars. Repairs to the bridge were mainly funded by the Wisconsin Department of Transportation, with Wisconsin & Southern Railroad, the Wisconsin River Rail Transit Commission (WRRTC) and Sauk County also contributing to the project. While the most recent project replaced sections of this historic bridge, the repairs will allow the structure to hopefully last at least another 40 years. Prior to the rehabilitation, much of the bridge was in fair to poor condition, with significant damage to the substructures and some deterioration to the superstructure. The author has ranked this bridge as being highly significant, due to the unique mix of spans, designs, ages and features noted on this structure.