Q: My question is about Chicago's drawbridges. How many are there? How many river bridges in total?

We take them for granted. Some of us cross these bridges nearly every day and never stop to think about them. Yet Chicago has more movable bridges than any other city in the world, and the city is recognized as an international innovator in the engineering of bridges.

The Chicago River System -- which includes the North Shore Channel, the Sanitary and Ship Canal and the Calumet-Sag Channel -- is 156 miles long. And, in Cook County alone, there are more than 180 bridges spanning the river. But the history of Chicago's bridges starts with just one.

The first bridge built across the Chicago River was completed in 1832 at Kinzie Street. It was a crude, wooden bridge that only allowed foot-traffic. Two years later the first drawbridge designed to accommodate vehicles was built at Dearborn Street. But the narrow passage created by the drawbridge created major problems for ships navigating the river, so the bridge was torn down in 1839. Then, in the 1840s, the city experienced additional mishaps when several floating bridges were swept away by the Flood of 1849.

In 1856, the first swing bridge was completed at Rush Street. An improvement over the earlier wooden bridges, the steam-powered swing bridge was constructed from both iron and timber. It opened by pivoting 90 degrees on a central pier, thereby swinging out of the way of passing vessels. Until the end of the nineteenth century, the swing bridge became the favored bridge type, but it was not without its problems. Tragedy struck on November 3, 1863 when the Rush Street Bridge collapsed under the weight of a herd of cattle being driven over it. And, because swing bridges turn in the middle of the river, ships again had trouble navigating the narrow passage it created. Collisions were a common occurrence.

As engineers sought a better solution to Chicago's bridge problem, the city entered its experimental phase. A jack-knife bridge that folded back on itself was built in 1891 but was deemed a failure. The first vertical lift bridge, with tall towers at either end controlling counterweights to lift a center span, was completed in 1894. And, in 1895, the Scherzer rolling lift bridge was developed in Chicago, opening at Van Buren Street.

But the bridge type most associated with the city is the trunnion bascule. Trunnion bascule bridges have leaves that rotate on a shaft, or trunnion, located on the shore. A complex system of counterweights, gears and electric motors, operated by a bridge tender, raise the leaves upwards and away from the center of the river. The first trunnion bascule bridge in the United States was completed in 1902 at Cortland Street, and it is still there today. Not only did the design prove effective, but it was copied around the world and became known as the Chicago-type bascule. Most of the bridges you see in Chicago now are of the bascule-type, but examples of the swing bridge, vertical lift and Scherzer lift still exist, though many are now inoperable.

Today Chicago boasts somewhere between 37 and 43 movable bridges. At least 20 of these are in the downtown area. During Chicago's boating season, which runs from April 15 through November 15, over 52,000 boats pass through the Chicago River, and the bridges are opened close to 30,000 times a year.

Very few of the bridges are manned full-time. Instead, bridge tenders with the Chicago Department of Transportation operate the bridges in roving teams. When bridges need to be raised, the teams leapfrog each other, driving trucks or other vehicles, to open the bridges in turn.

The bridges only take about eight minutes to raise and lower. For pedestrians and motorists eager to cross, however, the wait can seem much longer. But the next time you find yourself waiting to cross one of Chicago's bridges, stop a moment to appreciate what beautiful feats of engineering they are.

Bibliography

McClendon, Dennis. "Bridges." The Encyclopedia of Chicago. 2004.

Sinkevitch, Alice, ed. AIA Guide to Chicago. 2nd ed. Orlando, FL: Harcourt, 2004.

Wolfe, Gerard R. Chicago In and Around the Loop. 2nd ed. New York: McGraw-Hill, 2004.