Today marks 95 years of service for Chicago’s Michigan Avenue Bridge (officially named the DuSable Bridge in 2010). Arguably the most recognizable bridge in Chicago, this downtown landmark carries Michigan Avenue across the main stem of the Chicago River.
Opened on May 15, 1920, the Michigan Avenue Bridge is the second of four double-deck trunnion bascule bridges built in the Chicago Loop (note that all of the Loop’s 18 bridges are steel!). It was designed to carry automotive traffic on both decks – six lanes of boulevard traffic on the upper deck and four lanes of commercial traffic on the lower deck. What appears to be one large bridge is essentially two smaller bascule bridges tied together to operate as one, but they can also be operated independently. The steel was fabricated by the American Bridge Company (an AISC Member/Advanced Certified Steel Erector), which has been in business since 1900.
A trunnion bascule is a balanced horizontal seesaw with unequal arms – the longer arm is the bridge leaf over the river, and the shorter arm supports the much larger counter weight on the shore. The leaf weight of the Michigan Avenue Bridge is approximately 4,100 tons, while the counter weight is around 12,000 tons. Because it is balanced, only a small amount of external energy is required to raise and lower the bridge. Two 108-horse-power electric motors and a simple gearing system generate the power necessary to raise each half of the bridge in about one minute.
While the nature of Chicago River traffic at the bridge has changed dramatically over the last 95 years, the bridge is still in operation and lifted about 50 times each year for flotillas of migrating sailboats. You can watch the gates come down, see lights flash, hear the bells, watch the gigantic leaves move skyward and wave to the masted vessels as they sail by; a reflection of Chicago's maritime past.
If you’re planning to spend Memorial Day weekend in the Windy City, Chicago Loop Bridges is sponsoring a free admission day at the McCormick Bridgehouse & Chicago River Museum on Sunday, May 24 from 10 a.m. to 5 p.m. to recognize the three Chicago Loop bridges celebrating milestone anniversaries in 2015, including the Michigan Avenue Bridge. The museum will feature a special display about the interwoven history of these fascinating movable bridges and the Chicago River, and free souvenir magnets will be offered to the first 300 visitors. Jim Phillips of Chicago Loop Bridges (who provided this commentary on the Michigan Avenue Bridge) will also be available to answer questions.
The museum is located at 376 N. Michigan Ave in the SW bridgehouse of the Michigan Avenue/DuSable Bridge. Beginning at river level and spiraling five stories up, the museum provides a one-of-a-kind opportunity to explore the inner workings of the bridge. For more information about the event, contact Jim Phillips at 312.540.0696 or admin@chicagoloopbridges.com.
Michigan Avenue Bridge rising: Photo courtesy of Chicago Loop Bridges