Zoo Interchange Railroad Bridge

View an article discussing construction of the Milwaukee, Sparta & North Western Railway (digitalized by Google)

Located near the Zoo Interchange in West Allis, this large through truss carries the former Chicago & North Western Railway mainline over the Hank Aaron State Trail (former Chicago, Milwaukee, St. Paul & Pacific Railroad). In 1910, subsidiary Milwaukee, Sparta & North Western Railway constructed a new belt line around Milwaukee, with the intention of improving operations for the C&NW. As part of the work, several large bridges were required to cross a number of different railroad lines, roadways and waterways. A large through truss bridge would be required to cross the La Crosse Division of the Chicago, Milwaukee & St. Paul Railway (Milwaukee Road). Work on the new double track bridge began in 1910, and was substantially completed by the end of that year. Currently, the bridge consists of a double track 170-foot, riveted quadrangular lattice through truss span, set onto concrete substructures. The truss span is heavily constructed, using large built-up members and a heavily constructed floor system. The endposts are comprised of heavy beams, which use a solid plate on the outside and heavy X-lacing on the inside. The top and bottom chords consist of channels, connected by extensive X-lacing and V-lacing. The diagonal members are constructed of a combination of rolled beams, solid bars and built-up beams comprised of heavily V-laced members. Typical for spans of this design, the portal bracing uses an M-frame design, comprised of built-up beams with heavy V-lacing. The top lateral bracing consist of V-laced plates, and there is no bottom lateral bracing present. The floor uses a ballast trough deck design, with heavily constructed ballast channels formed from steel sheets. In addition, large plate girders inside of the bottom chord contain the ballast. This style of floor became popular in the early 20th Century, as it required less maintenance than typical open deck designs. Drainage holes have been installed throughout the ballast channels, and the ballast channels are filled with a combination of asphalt cement and ballast. Typical of bridges along this line, the abutments are constructed of an elongated U-shaped design. Pennsylvania Steel Company fabricated the superstructure, while the Cleary-White Construction Company constructed the substructure. All substructures are founded on timber piles, typical of construction along this line.

This type of truss design is relatively uncommon throughout the United States. However, a few railroads preferred the design, such as the Chicago, Rock Island & Pacific Railway (Rock Island), the Chicago & North Western Railway (C&NW) and the Omaha Road. Spans constructed in the late 1870s and early 1880s for the C&NW featured an arched and pedimented lattice portal bracing, light sway bracing and laced members. The second generation was nearly exclusively constructed by Lassig Bridge & Iron Works between 1884 and 1900, and featured heavier members and a pedimented portal bracing. While the design fell out of favor for the Rock Island and Omaha Road around the turn of the 20th Century, the design remained popular with the C&NW into the 1920s. 19th Century versions of this design were primarily constructed out of iron, while 20th Century versions of this design used much heavier members and were constructed of steel. The C&NW preferred this design, as it was both strong and demonstrated great resilience in case of a derailment. A handful of spans using this design were constructed along this route, primarily to cross large rivers and other railroads. Since the initial construction, the bridge has seen few alterations. As part of the reconstruction of the Zoo Interchange, a new bridge was constructed immediately west of this structure in 2014. Instead of demolishing the historic structure, the bridge has been left standing and the railroad bypasses the bridge. Currently, the structure does not serve any traffic, and is closed to use. Because the bridge does not serve a function, its future is uncertain, despite the historic nature of the bridge. Ideally, the bridge could be reused as a rest area/overlook for the Hank Aaron State Trail. To reuse the bridge in this manner, minor repairs would be required to the bridge, railings would need to be added and a ramp would need to be constructed to the top of the bridge. In addition, fencing and other security measures would be required to separate pedestrians from the adjacent active railroad. Overall, the bridge appears to be in fair to good condition, with some minor deterioration noted. The author has ranked this bridge as being moderately significant, due to the truss design.