A relatively unknown but still active swing bridge crosses the North Branch of the Chicago River at a bend in the waterway roughly one block south of Cortland Street. Built in 1899 by the Chicago, Milwaukee and Saint Paul Railway (commonly referred to as the Milwaukee Road), this bobtail swing bridge is still active. Due to its very low river clearance, it is normally left in the open position so as not to impede barge and river traffic on the North Branch. It is used to carry away train carloads of scrap for recycling from metal dealers on the east side of the river.
World renowned civil engineer Onward Bates—the former Superintendent of Bridges and Buildings for the Chicago, Milwaukee and Saint Paul Railway and past president of the American Society of Engineers—designed this unique steel plate-girder swing bridge. This one-of-a-kind moveable structure replaced a 161-foot wooden swing bridge that rotated upon on a center pier in the middle of the river channel.
Situated at a bend in the river, silt repeatedly built up on the outside of the bend, effectively blocking use of the west draw. So when the bridge was opened, vessel passage to either side of the center pier was effectively limited to passing through the eastern draw. The Army Corps of Engineers, in 1897, characterized this obstruction to navigation on the Chicago River as “one of the worst places on the north branch.” By 1900, the Secretary of War ordered that the center-pier swing bridge be removed.
Most swing bridges rotate at least 90 to 360 degrees to open and close. The Milwaukee Road’s Z-6 bobtail swing bridge has a pedigree that goes back at least four centuries. Leonardo da Vinci’s notebooks log the earliest known recording of a swing bridge design that pivots on one shore to cross a narrow river or canal.
We don’t know whether or not one of these bridges was actually constructed. The advent of steel made this type of bridge a fairly common alternative in the early 1900s. Also, new and larger steel ships, which began to appear a few decades earlier, drove out center pier swing bridges. Because of the enormity and cost, innovating and replacing bridges had a tendency to lag navigational needs by 10 to 20 years or more.
The novel Z-6 bobtail swing has an abbreviated turntable. It’s composed of a cluster of rollers that move in a 28-degree arc; rollers under the bobtail end move 57 degrees to open and close.
The two sets of rollers each rotate at half the speed of the bridge’s movement. Normally open, the bridge rests entirely over land on the east bank of the river. Between the plate girder sides of the bridge, its short tail end holds 56.65 tons of iron counterweights, though it’s not obvious from its profile.
At just over 175 feet long, this may be the longest plate-girder swing bridge ever built since established engineering standards generally limit such spans to 150 feet. However, the unsupported section of the Z-6 Bridge that span the river is only 141½ feet long.
Onward Bates’s Z-6 design was replaced due to frequent operating problems at the Milwaukee Road’s acutely skewed river crossing north of the Chicago, Milwaukee & St. Paul Railroad and Kinzie Street Bridges (above).
In 1898, to the surprise of many of Chicago’s civil engineers, the CM&S built an improved eccentric-pivot bridge at their crossing south of Kinzie Street. C&NW assistant chief engineer William H. Finley, who designed this bridge, also introduced a spider truss that allowed better nesting of the rollers (Bates’s initial design had problems with the rollers falling out of alignment). Bates would later incorporate Finley’s improvements in the Z-6 Bridge.
Like Bates, Finley would go on to serve as president of the American Association of Engineers. Due to a need for improved navigation, Finley would oversee replacement of the eccentric-pivot bridge for the C&NW in 1908 with the now iconic Strauss heel-trunnion bascule bridge.
The Joseph B. Strauss-patented design is recognized for its large suspended concrete counterweights south of Kinzie Street. It still stands today in the open to allow the free passage of watercraft. For more information about this bridge, see the February 4, 2014, post A Chicago Favorite Remains Open For All To See.
As far as we know, the Z-6 Bridge on the North Branch of the Chicago River is the only existing clustered-roller eccentric swing bridge of its kind in the world. It is one of more than a dozen unique moveable bridges strung across Chicago’s waterways. You can find more detail on the design, operation, and development of this bridge in the Historic American Engineering Record No. IL-162.