Along the Chicago River, the Calumet River, and the Sanitary and Ship Canal. There are many examples of the so called bascule bridge. The word bascule is derived from the French name for a balance scale.
These bridges are a type of draw bridge with a counterweight that balances a span throughout its upper course. There are multiple types but they all share common features. They have either one or two spans or leafs. They have counter weights. On the single leaf bascule these are above the bridge. On the double leaf bascule bridges the counter weights are located beneath the bridge on the bank of the river.
There are three types:
The rarest in the Rall type. It combines rolling lift with longitudinal motion on trunnions during opening. It was developed and patented by Theodore Rall in 1901. There are no examples in the Chicago area.
Rolling lift trunnion. It raises the span by rolling on a track resembling a rocking chair. Scherzer rolling lift was a refinement developed and patented by William Scherzer in 1893. The eight track bascule bridge on the Sanitary Ship Canal uses this type on some of its spans.
Another type of bascule bridge is a fixed-trunnion. It rotates around a large axle that raises the span. This is also known as the Chicago type. This was developed by the Chicago Bridge Department. The trunnions of these bridges are on the river bank. These bridges uses two leaves to cross the river. These bridge also use counterweights.
Another rare type bridge is a Page bascule bridge. There were only 4 of these type bridges. There is one in the Chicago area. It is a single leaf Warren truss. It was designed by William Hughes. The gearing and motors are supported on longitudingal girders. The counter weights are cast iron blocks attaches to steel plates on the approach structures. This bridge design was patented by Page and Schable.