By 1900 Chicago had 1,698,575 people. Almost everybody lived and worked along the Chicago River. Getting across was a big problem. There weren’t enough bridges, and those bridges that existed were more suited for a small town than the country’s second largest city. Most of them were swing bridges.
Swing bridges balanced on a large pier in the center of the river. The center piers obstructed the increasing number of large ships that docked downtown, or at the many industrial sites scattered along the North and South Branches of the river. The newly opened Sanitary and Ship Canal added even more ships to the river. Swing bridges opened with a slow horizontal swing, and were easily damaged.
A better bridge was needed. Chicago engineers modified the bascule (French for seesaw) bridge to come up with the perfect design. The trunnion bascule design, also known as the Chicago Bridge, is popular in river cities throughout the world.
Trunnion bascule bridges have two leaves that swing vertically on large pivots. The large counter weight is hidden in the bank of the river under the bridge. This type of bridge is quick opening and does not obstruct the river with a center pier.
The first trunnion bascule bridge, the Clybourn Place (Cortland Street) bridge, opened in 1902. Over the next 30 years most Chicago bridges were replaced to the relief of both road users and ship captains.
For more information see the trunnion bascule bridge booklist.