How Does a Swing Bridge Work?

Classes of Swing Bridges

• Swing bridges can be classified based on their supporting structures, according to William Henry Warren in the book "Engineering Construction." These classifications fall into four main groups: rim-bearing turntables, center-bearing turntables, hydraulic presses, and floating swing bridges. The floating types of bridges are partially supported by a floating structure, which relieves the pressure on the pivot structure. The rim-bearing and center-bearing turntables use rollers or bearings to allow the bridge to turn.

Power

• Early swing bridges used manual power to open and close the bridge. Today, the only swing bridge still in operation that uses manual power is located in Belize City, Belize. Historically, most swing bridges were designed to move using hydraulic or steam power, although some eventually used electricity to power their rotation. Warren notes that the type of power used to operate the bridge has little impact on the overall design of the bridge.

Pivot Point Location

• The pivot point for a swing bridge may be placed at any point along the length of the swinging span. For many swing bridges, the pivot point is placed on a pier at the center of the span. This allows the bridge to spin without requiring a counterweight to balance the moving arms. When the pivot point is placed nearer to one end of the bridge than the other, the shorter arm must be weighted to balance the longer arm.

The Ends of the Bridge

• Unlike other types of movable bridges, swing bridges aren̵7;t lifted above their resting horizontal plane in order to allow ships to pass. The ends of the bridge, therefore, must be freed from their resting position in order for the bridge to move freely. This can be accomplished using several different mechanical methods. Jack and shoe arrangements, retractable rollers, or wedges are used to support the ends of a swing bridge, and to support the ends and keep them properly aligned when traffic passes over the bridge.