Bloomingdale Line Swing Bridge

View the Historic American Engineering Record documentation of this bridge
View an article discussing the design and construction of this bridge
View an article discussing the design and construction of this bridge

Located south of Courtland Street Avenue, this unique deck plate girder swing bridge carries the former Chicago, Milwaukee & St. Paul Railway across the North Branch Chicago River. The first bridge at this location was a wooden truss swing bridge, constructed for a predecessor of the Milwaukee Road. This bridge was reportedly replaced in 1889 by a new 161-foot wooden pony truss swing bridge, with a rounded top chord. The bridge was set onto a stone pivot pier, which sat near the middle of the channel. Due to the location of the bridge, much of the river between the pier and outer bank would become silted in, creating a navigation hazard. 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. At the same time, railroads were opting to upgrade outdated wooden and iron swing bridges.

Throughout much of its existence, the Milwaukee Road retained a large and innovative in-house engineering department, which developed plans for most railroad structures, cars and locomotives. This operation was most prevalent between the early 1880s and the mid 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.

In 1898, 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. A counterbalanced design was chosen due to the space constraints and unusual angle of the river underneath. Counterbalanced, or "bobtail" spans, were infrequently used by railroads. These spans provided most of the drawbacks of a symmetrical swing span, with additional detriments in the counterbalanced design. Counterbalanced spans required precise calculations, additional costs and often required difficult engineering. This type of swing span utilizes a heavy counterweight on one arm (the counterweight arm), which offsets the longer length of the main arm of the span. 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.

Prior to construction beginning on the bridge, the railroad was required to elevate their tracks west of this bridge, in conjunction with the Chicago & North Western Railway project. While the railroad sought to rush construction on the bridge, the specialty spans could not be fabricated in time. As a result, the tracks were raised on the old wooden swing bridge by adding wooden bents to the existing structure. Construction on that project was complete in May 1899. Construction of the concrete substructures for the new bridge was completed during the summer of 1899. On October 6th, 1899; work on erecting the new swing span was commenced, with work being completed on temporary jacks and blocking. The individual sections were delivered to the site, and spliced together on-site. Once fully completed, the old swing span was opened, portions of the truss removed, and the girder installed perpendicular through the old span. The span was ready for placement on October 21st; and a 16-hour change out of bridge spans occurred on October 22nd. The west span was placed a few days later, completing the structure in its current configuration.

Known to the railroad as "Bridge #Z-6", this bridge consists of a 175-foot counterbalanced deck plate girder swing span, approached by a 70-foot deck plate girder span on the west end. The west abutment and west pier are constructed of stone, and were altered and reused from the previous bridge. The east pier and east abutment are constructed of concrete, and were built in 1899. The swing span consists of three spans, carried on an unusual pivot pier and roller nest. The river (west) span consists of a 108-foot deck plate girder span; the center span consists of a 33-foot 6-inch deck plate girder span, which is supported by the roller nest. The counterweight (east) span consists of a 33-foot 8-inch deck plate girder span. The interior of this span utilizes an arrangement of cast iron weights, which counteract the weight of the longer span. The total weight of the counterweight is over 113,000 pounds. The spans were fabricated by the American Bridge Works. The substructures were conducted and the bridge was erected by the railroad company, as was customary for the railroad.

Nearly every swing bridge can be classified as either center bearing, where the weight of the span is concentrated on a column in the center; or rim bearing, where the weight is evenly distributed throughout a drum set onto wheels, which roll on a track. However, this unusual bridge cannot be truly classified as either. Unique to this bridge, the roller nest only forms a 57 degree arc on the pivot pier, and is not directly attached to the superstructure. The center of rotation of the bridge is located at the joint between the middle span and the counterweight span. The east end of the counterweight span is carried by rollers, which are set onto 85-pound rail. In addition, the roller nest only moves at half of the spans rate of rotation, due to it not being fixed to the superstructure. The roller nest is connected to a triangular shaped truss, which lies flat with its apex centered at the pivot. While the turning mechanism on this bridge is unique, this was not the first location that it was used. Onward Bates, who initially developed the design, tried using it at a nearby bridge over the North Branch of the Chicago River near Kinzie Street. The mechanism was unsuccessful, and the span was eventually converted to a traditional style turning mechanism. William H. Finley designed a bridge for the Chicago & North Western Railway near Kinzie Street which used an improved version of the mechanism. The improved version included the use of the flat truss, known as the "spider truss". Despite the competing railroad employers, William Finley and Onward Bates were apparently acquaintances who shared bridge design and experience.

This bridge varies from traditional swing bridges in the lifting, locking and rotating mechanisms at the end of the span. 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.

Since the initial construction, the bridge has seen a few changes. Much of the copper for the bridge operation was stolen in the 2000s, rendering this bridge inoperable . Despite the inoperable status, all of the turning mechanism equipment remains intact. The bridge remained active for light railroad use through the 20th Century. The last train passed over the bridge in approximately 2014, and the bridge has been abandoned since. The bridge has since been acquired by the City of Chicago, and the land around the bridge is planned to become part of the Lincoln Yards project, a mixed use development. The master plans for this development call for the reuse of the bridge as an overlook. The author hopes that the bridge can be restored and preserved into an overlook with proper interpretive signage. Overall, the bridge appears to be in fair condition, and retains a high level of historic integrity. The author has ranked this bridge as being highly significant, due to the unique design and history of the bridge.