Chicago’s moveable bridges are a unique part of the city’s character, made necessary by the city’s many crossings of a busy river with low banks in the business district. In the 19th century, the Chicago River downtown was the city’s main harbor, always clogged with Great Lakes vessels during the navigation season. Most commercial vessels now dock in the Lake Calumet area, and the city schedules twice-weekly bridge openings for pleasure boats, to avoid interrupting downtown traffic.

Most of the city’s road bridges are trunnion bascule bridges of a type developed specially for Chicago’s conditions. The moveable leaf is balanced by a hidden counterweight so that it can be quickly lifted by an electric motor of approximately 100 hp. The leaf rotates on an axle or trunnion, which keeps the weight of the bridge on a single spot that can easily be supported by foundations in Chicago’s swampy soil.

Many of the railroad bridges use a similar design, but appearance is less important and the counterweight is often above the railroad tracks rather than hidden beneath it.

A few bridges are of the vertical lift type, in which the entire center section spanning the waterway is lifted by cables running over towers at each end.

A talk on Chicago bridges
was given on May 4th, 2012, at the Chicago Architecture Foundation
download the slide show here
download the podcast here

TO LEARN MORE:
HistoricBridges.org, a website that catalogs notable bridges nationwide
Bridgehunter.com, a similar website
Jim Phillips's website on downtown bridges
The website for the McCormick Tribune Bridgehouse Museum
Learn about a documentary film on Chicago bridges