been substituted at a later date. 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 is 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 over 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 reinforced concrete tailpits that descend below the surface of the river. Each of the two tailpits is supported on nine cylindrical foundation piers. One of these piers was sunk to bedrock, below the river surface, the other 17 piers are sunk to the hardpan, which is 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 . The Michigan Avenue Bridge is made of steel. The bridge can carry about 30,000 people daily. 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 motors open and close each of the bridge leaves. Originally the bridge was staffed 24 hours a day, and opened up to 3,000 times a year to allow ships through, but since the 1970s bridge lifting has been scheduled in the spring and fall, when the bridge is raised