What Rhymes With ‘Fast Mule Onion?’ Of Course, It’s ‘Bascule Trunnion’

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Every day, hundreds of worker bees and tourists scurry over the Michigan Avenue Bridge. But how much do you know about the bridge?

Those intrepid souls who ventured into the McCormick Bridgehouse during the Chicago Architecture Foundation’s 2015 Open House Chicago learned quite a bit about the bridge and the gears and widgets that control it.

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Ten cool insights about the bridge include:

1. The Michigan Avenue bridge is one of 37 movable bridges in the City of Chicago.

2. There are seven Chicago movable bridges (including the Michigan Avenue Bridge) that are manned 24 hours a day.

3. When the bridge was built electricity was expensive so the motor was designed to use very little juice.

4. Because it has a counterweight, the motor is small, about the size of the engine of a 1950 Volkswagen Beetle. Without a counterweight, the bridge would need the equivalent of an aircraft carrier motor to open it.

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5. The bridge is a double-leaf, double-deck bascule trunnion. Bascule is the French word for “seesaw” and that’s exactly how the bridge works.

6. Balance is the key to the bridge’s design, and the Michigan Avenue Bridge is in almost perfect balance. That means a very small motor can lift it.

7. The total weight of the bridge is 4,100 tons, and its counterweight is 12,000 tons. The bridge balance is incredibly precise—a small change due to ice, debris or even a new coat of paint will screw it up.

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8. Chicago bridges are reddish in color, a mixture of red lead paint which prevents rusting and a black powder.

9. It takes anywhere from eight to ten minutes to open the bridge.

10. Bridges in the city open nearly 100 times a year. Calumet River bridges open far more often—up to 6,000 times each year—because of the many freighters that ply the waters.

Author: Bill Motchan

Bill Motchan is a writer and photographer, and a former resident of the West Loop. He can be reached at bill@ChicagoArchitecture.org.

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