UP Rock River Bridge (Janesville)

Known as the Monterey Bridge, this large deck truss bridge carries a former Chicago & North Western Railway line over the Rock River on the south side of Janesville. The history of the predecessor structures at this location is unclear. The first bridge at this location likely consisted of a wooden deck truss bridge, constructed when the line was first built. In 1873, the bridge was replaced by a an iron Post deck truss bridge, set onto stone substructures. This bridge was strengthened in 1886 by adding an additional truss line of a different pattern between the two Post truss lines. By the late 1890s, the bridge has become too light for traffic and would be reconstructed. In 1896, the Post truss bridge was replaced by four double intersection Warren deck truss spans, reusing the original abutments and apparently set onto new stone piers. In addition, a short 18-foot deck plate girder span was installed on the south end to allow for reuse of the original south abutment. Further alterations to the bridge came in approximately 1901, when the 18-foot deck plate girder span was replaced by a longer span, and the bridge extended with spans of an unknown design to span the new Janesville & Southeastern Railway line and adjacent bridge. In 1907 and 1908, the bridge would again be altered, and a second track added to the west side of the structure. As part of the work, a new through plate girder span was constructed over the railroad on the south end, although it is unclear if the south abutment and south pier (pier #1) were constructed as part of the work.

Currently, the bridge consists of a 61-foot skewed through plate girder span, an 87-foot deck plate girder span, two 142-foot, 8-panel, riveted double intersection Warren deck truss spans, a 130-foot, 7-panel span of the same design, a 105-foot, 6-panel span of the same design and a 50-foot deck plate girder span carrying the west track adjacent to the original north abutment. With the exception of pier #6, the entire bridge is set onto stone substructures, and pier #6 is constructed of concrete. The south abutment, piers and north abutment of the west track appear to be constructed using stone quarried at Duck Creek, Wisconsin; while the north abutment of the east track is constructed using a yellow limestone quarried at an unknown location. The south shore pier, pier #2 is braced to the original south abutment with concrete beams. The original south abutment is unused, and span #2 of the eastern track is located over the structure. Originally, the four deck truss spans were known as Wisconsin Division Bridge #138, while span #2 was known as Bridge #137 1/2 and the southernmost span (span #1) was known as bridge #137 1/4. Prior to 1920, the three separate structures began being considered as a single structure, known as Bridge #138. Lassig Bridge & Iron Works fabricated the east track deck truss spans, while the King Bridge Company fabricated span #2 for the east track. American Bridge Company fabricated all east track spans, as well as the southernmost span (span #1). The south abutment, south pier (pier #1), east track piers and east track north abutment were constructed by unknown contractors, while the west track piers, north pier (pier #6) and west track north abutment were constructed by the Bates & Rogers Construction Company.

Due to the different construction dates, a number of unique features are present on this structure. Both tracks use heavily constructed double intersection Warren deck trusses, which extensively use built-up members and heavily constructed floors. The top chord of the west track spans consist of heavily built-up beams, connected by a solid plate on top and tight X-lacing on the bottom. The bottom chord of the spans are composed of parallel channels, connected by thin plates. The diagonal members are constructed of a combination of rolled members, thin L-shaped bars and V-laced built-up beams. Two different types of V-laced members are used, with tightly V-laced beams forming the inner compression members and a more broadly laced beam forming the tension members. Vertical members connecting the middle of the truss to the top chord are present mid-panel, and are composed of V-laced beams. Floorbeams for the spans are located on top of the top chord, and are composed of I-beams. Four stringers are present on each span, which are arranged into two sets of two and also composed of I-beams. The upper lateral bracing is composed of steel bars, while the lower lateral bracing is composed of V-laced transverse members at the panel points and steel bar diagonals beneath the bottom chord. Additional transverse steel bars connect the two truss lines at the mid-panel points. The interior bracing is composed of two steel channels, which form an "X" shape. The west track deck truss spans use a similar design, although the top chord is constructed with V-laced beams and the bottom chord is constructed with steel channels connected by a tight V-lacing. Diagonal members use a similar pattern, although the use of rolled beams is more extensive in these spans. The diagonal members in the west track span are generally heavier, and are constructed of thicker steel and larger channels. Similar to the east track spans, tightly V-laced beams form a vertical member connecting the center of the truss to the top chord. The floor system uses a similar construction to the east track spans, as do the upper and lower lateral bracing.

The southernmost span (span #1) uses a heavily constructed through plate girder span, consisting of three heavy plate girders, square girder ends and a traditionally compsoed floor. This span is heavily skewed to accommodate the unusual geometry of the railroad crossing underneath. The floor is constructed of floorbeams placed throughout the span, and four stringers arranged into two sets of two. The floorbeams are composed of plate girders, while the stringers are constructed of shallow I-beams. The two deck plate girder spans used for span #2 are similar, consisting of traditionally composed spans. These spans use two heavily constructed girders, connected by traditional bracing. The northernmost span (span #7) for the west track uses a similar design. These spans are supported by steel bents abutting the truss spans. These bents are constructed of a combination of plate girders, rolled beams and built-up V-laced beams. The four main piers (piers #2 through #5) use a typical design, and are constructed using a diamond shape with angled noses on the upstream side. The southernmost pier (pier #1) uses a unique rectangular design, with three taller sections to support the transition between the through plate girder and deck plate girder span. The south abutment uses a standard design, with stepped wing walls extending perpendicular to the structure. The north abutment under the east track and old south abutment both use a lengthy parapet design, which was commonly used in the 19th Century to help stabilize tall fills while minimizing stone required. Timber stringers were likely used to support the tracks on these abutments. Pier #6 and the north abutment of the west track both use standard rectangular concrete designs.

While lattice truss spans were relatively uncommon through the United States, the C&NW used the design extensively. Other railroads in the Midwest also occasionally used the design, although not as frequently. Engineers for the C&NW favored the design, due to its resilience and greater strength without sacrificing economy. Metal lattice truss designs were initially developed for railroad use in the 1870s, often using deep spans with numerous intersections. Through the mid-1880s, short to medium length spans were simplified into double intersection spans, as seen on this bridge. Later additions included vertical members at key points to increase loading capabilities and the improvement of the floor systems. Most lattice spans through the 1880s and 1890s were constructed by the Lassig Bridge & Iron Works, which produced a vast majority of iron and steel bridges for the C&NW during this era. The C&NW continued to use lattice truss designs nearly exclusively into the 1920s, and was among the only railroads in the United States still using the design into the 20th Century. Since the 1907-08 reconstruction, the bridge has seen few alterations. Today, only the west track remains in use, while the ties and rails have been removed from the east track. Overall, the bridge appears to be in fair condition, with several areas of severe section loss and corrosion to various steel plates of the superstructure. The author has ranked this bridge as being regionally significant, due to the truss design and unusual configuration.