Museum Exhibits - The City of Bridges
The City of Bridges
“Shipping was the main business and the Chicago River was the main street.”
-The Moveable Bridges of Chicago
Today, we take for granted how easy it is to walk or drive across the river. But, in the 1820s, river crossing was no simple task. The only ways across were ferries or floating log bridges.
Residents talked about building a permanent bridge, but objections arose. Merchants on the flourishing south side did not want to lose business to the less popular north side. Shipping interests balked because bridges might interfere with river traffic.
Northsiders solved their situation by privately funding bridges. The introduction of moveable bridges meant that river traffic could thrive. Today, Chicago has more moveable bridges than any other city in North America, and only Amsterdam, the capital of the Netherlands, has more movable bridges in the world.
The Bascule Trunnion Bridge
The Michigan Avenue Bridge is a double-leaf, double-deck bascule trunnion bridge. Bascule means “seesaw” in French, which is how this bridge operates.
The bridge span over the river is balanced by a concrete and steel counterweight. The bridge and counterweight balance on a trunnion—the point on which the bridge pivots.
The bridge is in almost perfect balance with its counterweight, which means that a surprisingly small motor lifts it.
The bridge’s control system—the source from which the bridge receives its signals to open and close—consists of a magnetic primary control and a DC auxiliary control.
The motor requires little electricity because power was very expensive when the bridge was built.
The bridge operates on 108-horsepower 550 V motors, one for each leaf (two per side). Each leaf also features a brake for additional safety.
The motor (the grey box marked “Earle”) rotates a shaft, which turns a series of gears—that increase in size to transfer power easily.
The final gear, the bull gear, has wider teeth that actually rotate the rack, opening and closing the bridge.
Chicago's First Movable Bridge
Chicago’s first movable bridge was a wooden drawbridge, built in 1834 at Dearborn Street. The poorly constructed bridge was often stuck open, stalling traffic for hours or even days. Legend says that once the City decided to tear it down, eager citizens demolished the drawbridge themselves before anyone could change the plan.
In 1928, J.E. Fraser and Henry Hering sculpted the Michigan Avenue Bridge's four bridgehouses’ exterior bas reliefs, which illustrate important moments in Chicago’s history. “The Discoverers” records the arrival of Louis Jolliet and Father Marquette. “The Pioneers” features John Kinzie and other early settlers. “Defense,” which adorns the Bridgehouse Museum, memorializes the Fort Dearborn Massacre. “Regeneration” celebrates the remarkable recovery from the 1871 Chicago Fire.
Gateway to the North
In 1909, architect Daniel Burnham envisioned a monumental gateway between Chicago’s north and south sides: the Michigan Avenue Bridge. Until this time, North Michigan Avenue (then Pine Street) had not experienced the same growth as the southern end, due in part to the inconvenience of crossing the river.
Edward Bennett designed the bridgehouses in the Beaux-Arts style, and construction began in 1917. The bridge opened May 14, 1920, to cannon fire, marching bands, ship horns, and parades-another grand vision realized in the “city of big shoulders.”
Today, Bennett’s bridge is a designated Chicago landmark.
The City of Chicago maintains thirty-seven moveable bridges. Thirty-two are on the Main, North, and South branches of the Chicago River; five are on the Calumet River.
Downtown bridges open approximately 100 times a year from April to November. Calumet River bridges open 5-6,000 times each year for freighter vessels and watercraft.
Bridge openings take eight to ten minutes—from the time the gates lower to the moment traffic is clear to cross. The amount of time depends on how many vessels need to pass.
Seven of Chicago’s moveable bridges are manned 24-hours a day. Of them, five are on the Calumet River because the Calumet harbor continues to be this region’s busiest port.
The red color of Chicago bridges recreates the original hue, a mix of red lead paint (to prevent rusting) and a black powder (to tone down the red).
Bridge Tender, Keeper of the Bridge
“The tramp, the wharf rat, and the river pirate are [the bridge tender’s] neighbors.”
-Chicago Tribune, 1896
Chicago hired its first bridge tenders in the 1840s. The river was busy, and ships could arrive and depart on a moment’s notice. Many tenders lived on the bridges in small houses so they could keep an eye on the river at all times.
The life of a bridge tender was far from dull. Almost daily, they might have to put out a fire, break up fights with tramps, divert suicide attempts, or save the lives of those who fell off bridges.
Although less dramatic, a bridge tender’s job today requires expert knowledge of the bridges. They perform regular maintenance to assure safe, timely openings and closings.
These are the four most popular bridge types in Chicago.
This bridge has a huge weight that counterbalances the bridge leaf at a pivot point (the trunnion). This makes opening and closing the bridge almost effortless.
Center Pier Swing Bridge
The bridge sits on a pier in the middle of the waterway and rotates to allow traffic to pass on each side. This bridge type was not maintained on the Chicago River, which is very narrow. But, a few remain on the Sanitary and Ship Canal.
Scherzer Rolling Lift Bridge
This bridge features a counterweight similar to the bascule trunnion. It rocks back and forth, almost like the legs of a rocking chair.
Vertical Lift Bridge
These bridges use counterweights to lift the bridge span into the air like an elevator.