The Basic Definitions

 

Bridge tender house is the structure at the entrance to the bridge that houses the control equipment to raise the bridge. There may be as many as four, or only one on a bridge.

 

Rack and pinion is the gearing system that moves the bridge. The pinion is a round gear driven by an electric motor. This gear is meshed in a toothed track (rack) attached to the bridge leaf. As the pinion is rotated by the motor, it moves the rack which in turn raises or lowers the bridge leaf. The rack and pinion can be seen on bridges with through or pony trusses ( N. Clark St., N. La Salle St., N. Wells St., N. Franklin St., W. Lake St., W. Washington Blvd.). Racks are also visible from the lower decks of the N. Lake Shore Dr. and N. Michigan Ave. bridges. To see video of the rack and pinion operating on the W. Lake Street bridge, click here.

 

 

Deck is the horizontal surface that carries the traffic over the river. A double deck bridge carries two levels of traffic over the river.

 

Beams are horizontal structural members that support the bridge decks.

 

A Truss is a type of beam that is made up of triangular elements. Sixteen of the bridges here are supported by trusses. There are four categories employed here.

 

Deck trusses have all supporting members below the roadway (considered the most aesthetic). The bridge at W. Adams is an example of a deck truss. Compare the above deck railings on this bridge with the above deck structure of the other truss types.

 

The W. Adams St. bridge uses deck trusses.

 

 

Through trusses have all supporting members above the roadway (considered the least aesthetic). The bridge at W. Lake is an example of a through truss.

 

The W. Lake St. bridge uses through trusses.

 

Pony and Rail Height trusses have supporting members both above and below the roadway (aesthetic compromise). The difference between these two categories is how far above the deck the top chord of the truss extends. As the name implies, the rail height truss extends above the deck to a height of about 3 feet. As seen at the N. Franklin St. bridge, the pony truss extends much higher above the deck than the rail height truss.

 

 

 

The N. Wabash Ave. bridge (left) uses rail height trusses, while the N. Franklin St. bridges (right) uses pony trusses.

 

 

Box Girders are hollow rectangular beams assembled from four pieces of steel. The bridge at W. Randolph St. uses box girders.

 

Box girders are used on the W. Randolph St. bridge.

 

 

Tail pit is the area below the road surface at the rivers' edge. It accommodates the counterweight and lifting machinery of the bridge.

 

Counterweight is a mass of material that aids in the rotation of the leaf (see concepts).

 

Back to the top of this page