Michigan Avenue Bridge

by Suzanne Hanney

Michigan Avenue Bridge Michigan Avenue Bridge Opened May 14, 1920, -- 100 years ago -- the Michigan Avenue Bridge is the most celebrated in Chicago, the first double-leaf, doubledeck fixed trunnion bascule bridge ever built, which created the north-south thoroughfare that is now the Magnificent Mile. Since October 2010, the bridge has been named in honor of Jean Baptiste Pointe Du Sable, Chicago’s first permanent settler.

A bascule bridge is essentially a drawbridge, which operates like a seesaw on a pivot point, with its long arm extending over the water, balanced by a counterweight. “Bascule,” the term for counterweight, comes from the French verb baculer, which means to strike on the buttocks, which the lowered counterweight resembles when the bridge is raised, notes Patrick T. McBriarty in “Chicago River Bridges” (University of Illinois Press: 2013). “Trunnion” refers to the pin and supporting bearing that form the hinge that allow the bridge leaves to move up and down. According to the website of the McCormick Bridgehouse and Chicago River Museum on the southwest corner of the bridge, “the balance of the approximately 4,100-ton bridge leaf and 12,000-ton counterweight of the bridge is so precise, it only takes a 108-horsepower motor to open and close each leaf.” The motor powers a series of gears, the last of which is called a pinion gear, [which] grasps grooves in the counterweight, pushing it into a pit and lifting the bridge’s long arm into the air, “an engineering marvel even today.” On May 14, the museum will host local bridge experts in a virtual centennial celebration on its social media (www.bridgehousemuseum.org).

The widest bascule bridge in the world when it was built, the Michigan Avenue Bridge is actually two side-by-side double leaf bridges, so that two northbound lanes, for example, can be used for traffic while the southbound lanes are undergoing maintenance or repair, according to McBriarty. The first bridge designed to carry automobiles on both decks, its two upper decks provide three lanes, each 28 feet wide, and two 14-foot sidewalks. Its lower deck has two lanes in each direction, each 18 feet wide.

Chicago has long hosted the world’s greatest collection of drawbridges, McBriarty notes, because these bridges can accommodate ships while also allowing other forms of transportation, such as horse and buggy, trains or autos (though not at the same time). When it was first built, the Michigan Avenue Bridge would lift more than 3,000 times a year, but today it lifts roughly 40 times a year and visitors can see the bridgeworks in motion at the five-story McCormick Bridgehouse and Chicago River Museum. Located at 99 Chicago Riverwalk, the museum was opened in 2006 by Friends of the Chicago River to show the dynamic relationship between Chicago and its river.

Although narrow and slow-moving, the Chicago River has been an important waterway since Native Americans dominated the area. It is this river that made the city what it is, McBriarty notes. Before there were planes, trains or automobiles, waterways were the only means of travel and transport. In 1673, the explorer Louis Jolliet and the Jesuit priest Father Jacques Marquette learned from Native Americans about a shortcut to the Mississippi River: the Chicago “portage.” When the swampy portage (roughly seven miles of land from what is now roughly 26th Street and Western Avenue to 49th Street and Harlem), was flooded, explorers could float their canoes from the south fork of the Chicago River to the Des Plaines River, which flowed successively into the Illinois River and then the Mississippi. In dry spells, however, the explorers had to drag their canoes over rollers, amid mosquitos and leeches. Jolliet had mused about a canal – a continuous water route -- in place of the portage.

Over 150 years later, in 1822, Congress made the first land appropriation for the Illinois & Michigan Canal, which replaced the portage. Opened in 1848, the I & M Canal connected the Chicago River at Bridgeport with the Illinois River at LaSalle, 96 miles away. There was now a water route from the Atlantic Ocean to the Great Lakes, the Mississippi River and the Gulf of Mexico, with Chicago at its center. In the 1880s, Chicago became the busiest port in the world; its population doubled from 500,000 to one million people by 1890.