Cherry Avenue Railroad Bridge

View the Historic American Engineering Record documentation of this bridge
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Located immediately south of North Avenue, this unique through truss swing bridge carries the former Chicago, Milwaukee & St. Paul Railway across the North Branch Canal, part of the North Branch Chicago River. In the mid-1850s, a large canal was constructed, providing a straighter channel for the Chicago River between North Avenue and Chicago Avenue. The first bridge at this location was a wooden truss bridge, constructed for a predecessor of the Milwaukee Road. This wooden truss utilized a bowstring pony truss design, and was set onto a pivot pier. The line provided a critical link to numerous industries on Goose Island. By the late 1890s, the War Department began to study alternatives to the numerous swing bridges across the Chicago River. Swing spans often presented a hazard to river traffic, as large center piers and low clearances resulted in numerous strikes.

During the early part of the 20th Century, alternative bridge designs were implemented to cross the Chicago River. Bascule and lift type spans provided a larger horizontal and vertical clearance, desirable to the War Department, and railroads who sought to minimize delays from river traffic strikes. While most railroads were transitioning to using patented designs for movable spans in Chicago, the Milwaukee Road retained a large and innovative in-house engineering department, which developed plans for most railroad structures, cars and locomotives through the late 19th Century and early 20th Century. The Bridge and Building Department was overseen by Onward Bates, Superintendent of Bridges and Buildings. Bates oversaw the design and construction of various improvements on the railroad. During his tenure at the Milwaukee Road, the railroad prepared nearly all bridge and building plans in house, with Bates signing the plans. In addition, the railroad took a firm stance on constructing, erecting and maintaining as many bridges as possible, without the use of contractors. After Bates resignation to open the Bates & Rogers Construction Company, C.F. Loweth became the Superintendent. It appears that Onward Bates remained a consulting engineer on bridges he oversaw the design on through 1902.

In 1900, the Milwaukee Road submitted plans to the War Department for a replacement bridge. Because of the experience with designing and maintaining swing bridges, the railroad decided to design and construct a new swing bridge. An initial approved plan showed a smaller bridge, with slightly different structural features. A revised design, which was constructed, was approved in February 1901. Construction began on the bridge later in 1901, and steel was erected in the winter of 1902. Typical of the Milwaukee Road, the substructures of the bridge were constructed by railroad forces. 215 tons of steel was furnished for the superstructure by the Wisconsin Bridge & Iron Company, at a cost of approximately $50,000. The bridge was completed in early 1902.

A counterbalanced design was chosen due to the space constraints and unusual angle of the river underneath. The railroad sought to minimize interfering with adjacent properties. Counterbalanced, or "bobtail" spans, were infrequently used by railroads. These spans provided most of the drawbacks of a symmetrical swing span, with additional weaknesses in the counterbalanced design. Counterbalanced spans required precise calculations, additional costs and often required difficult engineering. This type of swing span utilizes a large counterweight on one arm (the counterweight arm), which offsets the longer length of the main arm of the span. Due to the different lengths, a counterbalanced swing span cannot rotate a full 360 degrees, and instead must come to a stop and rotate the opposite direction during opening and closing operations. While the additional time needed to stop and restart amounts to mere minutes, the additional time was often considered inadequate for railroads. The Milwaukee Road designed and implemented a number of counterbalanced swing spans throughout the system, constructing the most known spans of any railroad in the United States. The counterbalanced design was acceptable at this location, as the line carried exclusively freight, which was not bound by the same tight schedules as passenger trains.

The unique swing bridge at the north end of Cherry Avenue consists of a single track, 230-foot, pin-connected counterbalanced Pratt through truss swing span. The pivot pier of the bridge is located on the north shore of the canal, and all substructures of the bridge were constructed of concrete. The south arm, or river arm of the bridge consists of a 120-foot, 6-panel, pin-connected Pratt through truss. The counterweight arm of the truss consists of a 110-foot truss structure, which holds a large concrete counterweight approximately 20 feet above the ground. The truss span utilizes typical Milwaukee Road design features, including built up members, pinned connections, and lattice portal and sway bracings. Rolled beams, built up members, riveted and pinned connections are all used on the counterweight arm. While constructed for a single track, the bridge is 17 feet wide. The unusual width of the bridge may indicate the bridge was once planned to carry Cherry Avenue. The span swings to the east, which allowed the span to only rotate 75 degrees. This small difference in turning radius allowed for quicker operations and less possibility of strikes from river traffic. In addition to using standard inclined endposts, the south arm of the bridge uses an inclined upper chord, which reduces the amount of material needed to properly cantilever the span when opened. The concrete counterweight of the bridge is set as far back as practical, which reduces the amount of concrete needed in the counterweight.

The Cherry Avenue Bridge varies from traditional swing bridges in the lifting, locking and rotating mechanisms. When operating, the rail lifts at the end of the bridge are first operated. These mechanisms lift the rail, and allow the bridge to swing freely. Next, the locking mechanism is lifted, and the span is rotated slightly, before the operator engages the motor of the rotating mechanism. The operator then stops the rotation by cutting power to the motor or by applying a brake. Many traditional swing spans use wedges driven at the end to lift the span after the locking mechanism was disengaged. The Milwaukee Road designed a different mechanism for this function. At the end of the span, a cam shaped end lift shoe rotates about an axis perpendicular to the railroad, causing the span to raise or lower. In closed position, this shoe acts as a rocker bearing, absorbing small expansions and contractions. A shaft running longitudinally under the bridge carried power from the motor to the end shoes. The pivot mechanism on this bridge utilizes a rim-bearing design, where a drum constructed of plate girders rides on a circular track of wheels, which carries the bridges entire weight. A pin at the center of the track maintains the proper alignment of the bridge. A series of radial plate girders extend from the center of the track to the drum girder. The main disadvantage of this design is the bridge must be constructed nearly flawlessly to avoid shifting.

Since the initial construction, the bridge has seen a number of changes. The swing mechanism of the bridge was removed in the 20th Century, and a portion of the floor near the south end was replaced soon after. Despite the inoperable status, the roller drum, end shoes and other features remain intact. The bridge remained active for light railroad use through the 20th Century. In 2008, the bridge was acquired by the Chicago Department of Transportation. The structure received an extensive rehabilitation in 2009. As part of the rehabilitation, the superstructure of the bridge was painted, the concrete substructures and counterweight were repaired, and a pedestrian deck added to the structure. Additional repairs were made to the superstructure, which utilized steel plates and bolts to repair weak areas of the members. As the bridge remained sparsely used, the deck was constructed with a surface similar to railroad grade crossings, which allows for the use of pedestrians without large gaps around the rails. Overall, the bridge appears to be in good condition, and retains a high level of historic integrity. Trains no longer use the bridge, and the grade crossing at North Avenue was removed in 2015. The author has ranked this bridge as being highly significant, due to the unique design and history of the bridge.