Michigan Avenue Bridge - Design and Operation

Design and Operation

Michigan Avenue Bridge is a double-leaf, double-deck, fixed counterweight, trunnion bascule bridge. It was engineered by the Chicago Department of Public Works, Bureau of Engineering; Edward H. Bennett was the consulting architect and William A. Mulcahy the chief engineer of construction. At the time of construction it was believed to be the first double-deck bridge ever built to have roadway on both levels; faster non-commercial traffic using the upper deck and slower commercial traffic that served the various industries and docks in the vicinity of the river using the lower deck.

Each of the bridge's leaves are divided into two along the axis of the bridge such that it functions as two parallel bridges that can be operated independently of one another; at the time of construction bridges in the Chicago River were frequently struck by vessels, and this duplex arrangement allows for leaves damaged in such a collision to be opened for repair without needing to completely close the bridge to traffic. The counterweights are below the level of the lower deck and when the bridge is opened they swing down into 40-foot (12 m) deep reinforced concrete tailpits that descend 34.5 feet (10.5 m) below the surface of the river. Each of the two tailpits is supported on 9 cylindrical foundation piers. One of these piers was sunk to bedrock, 108 feet (33 m) below the river surface, the other 17 piers are sunk to the hardpan, which is 80 to 90 feet (24 to 27 m) below the water level. On the south side of the river one of the freight tunnels of the Chicago Tunnel Company had to be re-routed to make room for the tailpit. The counterweights are composed partly of concrete and partly of a concrete composite with rivet punchings; each of the four counterweights weighs 1,595 short tons (1,447 t).

The bridge abutments and the facing of the bridge tender houses are made of Bedford stone. There are four bridge tender houses: the northwest and southeast bridgehouses house the controls for operating the bridge; the northeast and southwest bridgehouses are purely decorative. Two 108 horsepower (81 kW) motors open and close each of the 3,750-short-ton (3,400 t) bridge leaves. Originally the bridge was staffed 24 hours a day, and opened up to 3000 times a year to allow ships through, but since the 1970s bridge lifting has been scheduled in the spring and autumn, when the bridge is raised twice weekly to allow sailboats to pass between Lake Michigan and inland boat yards where they are stored for the winter.