FEATURE image: Looking east, a portion of the pony truss bascule Chicago Avenue Bridge in May 2016 before its demolition with its partially obscured bridge house. Author’s photograph.
Built in 1914 by Ketler-Elliot Erection Company of Chicago, the Chicago Avenue Bridge was one of the oldest pony truss bascule bridges in Chicago. Connecting River North and River West, the steel bridge was, after 104 years, demolished in 2018 and replaced, in 2019, by a temporary bridge. A new, permanent immovable concrete bridge is expected to open over the Chicago River in this location in 2021.
A pony truss bridge is a steel truss bridge that allows traffic over and through the truss, but with no cross brace across the top connecting its two sides. The truss bridge assembly of the Chicago Avenue Bridge was made of riveted steel beams—a witness to the early 20th century industrial manufacturing might of Chicago. In addition to being “Hog Butcher For The World, Tool Maker, Stacker of Wheat, Player with Railroads and the Nation’s Freight Handler” as Carl Sandberg wrote in his 1914 poem, “Chicago,” published in the then-new (1912) Poetry magazine the same year the Chicago Avenue Bridge was built, Chicago was also at that time a world leader in steel production and bridge design.
The basic design of any bascule bridge is similar to a medieval castle drawbridge—a leaf or span that rises and descends so to permit traffic upon it—and, in the case of the Chicago Avenue Bridge, traffic also below it on the navigable—and today mainly recreational—Chicago River.
There are more than 50 movable bridges in Chicago. Single-leaf (truss) bascule bridges were constructed where the river was not very wide and often used for train traffic (Chicago is the railroad capital of the U.S.) where a single bridge deck goes up and down between abutments.
The more common double-leaf (truss) bascule bridge, which included the Chicago Avenue Bridge, consists of two leaves or spans which meet in the middle over the river. Counterweights on each side of the bridge beneath it in a river pit (or pits) balances, stabilizes and fortifies the vertical movement of the bridge deck. If the bridge deck is one leaf, the “Chicago Style” bridge rises in a piece vertically to one side of the river; if two leaves, each rise to their side of the river and descend to close again by meeting in the middle of the bridge deck.
Bascule bridges are the most commonly found moveable bridges in the world because they operate quickly and efficiently. The Chicago Avenue Bridge was operated from a companion pitched-roof bridge house with rounded corners and rows of windows clad in decorative (today oxidized green) copper. The bridge house portion of the structure was not demolished in 2018.
There are numerous variations and designs of the bascule bridge which in Chicago includes the trunnion (“pivot point”) bascule (“seesaw’) bridge. The nation’s first such bridge started operation in Chicago in 1902 over the north branch of the Chicago River at Cortland Street which can still be seen in operation today. The bridge design became known as the “Chicago Style” as its leaf or leaves, suspended on axles (trunnions) with massive concrete counterweights located below the bridge in the riverbank pit, opens and lifts a single or dual bridge deck to clear the river for traffic without blocking the waterway with a central pier.
Chicago’s bascule bridges—and the Chicago Avenue Bridge was one of them—were designed to its specific location. Each was designed to take on heavy loads and the attendant vibration which also included the ice and snow pack of Chicago’s winters. The design and construction into bedrock took into account wind resistance, whether the bridge leaves were open or closed, and to wind speeds of 100 miles per hour in any and all directions.
By 1920, improvements in bascule bridge design allowed for the construction of a double deck trunnion bascule bridge where car, truck and foot traffic could be carried simultaneously on its upper and lower decks. The first such double deck trunnion bascule bridge in Chicago was near the site of the old Fort Dearborn on the Chicago River at Michigan Avenue—today’s busy Michigan Avenue Bridge. In October 2010, the bridge was renamed DuSable Bridge in honor of Haitian-born Jean Baptiste Point du Sable (c.1750-1818), Chicago’s first permanent resident who established a trading settlement nearby.
Solzman, David M., The Chicago River: An Illustrated History and Guide to the River and Its Waterways, Wild Onion Books, Chicago, 1998.
“Carter H Harrison Mayor marker – Chicago Avenue Bridge” by swanksalot is licensed under CC BY-NC-SA 2.0
Author’s Photograph. Taken May 6, 2016. All rights reserved.
“File:Architecture Tour 25 Chicago Avenue Bridge (185544768).jpg” by discosour is licensed under CC BY-SA 2.0
“Chicago – Chicago Avenue bridge, north branch of Chicago river” by ukdamian is licensed under CC BY-NC 2.0