FEATURE image: Portion of the open-grate pony-truss bascule Chicago Avenue Bridge and concrete house, erected in 1914, as it was in its last years of operation. Pointing to increased high traffic volume and load-bearing capacity issues, the 104-year-old bridge was demolished and replaced in 2018. Author’s photograph taken in May 2016.
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. Since this portion of the river is now used mostly for recreational purposes, 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—and a witness to Chicago’s early 20th century industrial manufacturing might.
As well as 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 new (1912) Poetry magazine, 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 rises and descends that permits traffic upon it and— for the Chicago Avenue Bridge— also traffic below it on the Chicago River.
Chicago boasts more than 50 movable bridges. Single-leaf (truss) bascule bridges were constructed where the river was not very wide and a single bridge deck goes up and down between abutments. This was often used for train traffic which was convenient since Chicago was and still remains the railroad capital of the United States.
The more common double-leaf (truss) bascule bridge— including 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 single leaf, the “Chicago Style” bridge rises in a piece vertically to one side of the river. If the bridge deck is 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 foundational portions of the bridge house were constructed of concrete. Upper portions were made of wood with roofing of vitrified tile. So far, the bridge house portion of the structure has not been demolished.
In 1914, the new Chicago Avenue Bridge across a jag of Chicago River abutting the national headquarters of Montgomery Ward whose complex was first built in 1907, made for a sleek, powerful, modern and elegant statement about industry and urban commerce in early 20th century Middle America.
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 included the ice and snow pack of Chicago’s typically harsh 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 DuSable Bridge, formerly Michigan Avenue Bridge which opened in May 1920.
The Chicago Avenue Bridge is also significant as it belonged to a set of Chicago city bridges built between 1910 and 1915 whose designs looked to go beyond their utilitarian function to incorporate artistic and other aesthetic elements. Beginning with the Chicago Avenue Bridge in 1913, the Chicago Planning Commission worked with the city’s Bridge Division at the bascule bridge’s design stage.
Following the demolition of the Chicago Avenue Bridge in 2018, an immoveable temporary steel girder bridge was installed in 2019 over this span of the Chicago River. For the moment, bridge design has returned to utilitarian functionality alone with little or no artistic or aesthetic program which had been one of the progressive features of the Chicago Avenue Bridge over a century ago.
SOURCES:
Solzman, David M., The Chicago River: An Illustrated History and Guide to the River and Its Waterways, Wild Onion Books, Chicago, 1998.
http://chicagoloopbridges.com/Ctype.html
https://chicago.curbed.com/2018/10/29/18038134/construction-chicago-avenue-bridge-traffic
https://www.archpaper.com/2018/06/chicago-offers-historic-chicago-avenue-bridge-free/
https://loc.gov/pictures/item/IL0823/
https://historicbridges.org/bridges/browser/?bridgebrowser=illinois/chicago/
