Movable bridges are used to cross a body of water that is also a thoroughfare for freight and/or passenger boats. While they can be very inconvenient at the time that they open, especially if people are in a hurry, they can also be a beautiful sight. Four types of movable bridges are in operation today.
Bascule bridges are the most popular type of bridge in operation today. Bascule bridges operate by pivoting upon a fixed axis. Bascule bridges are usually split into two parts which move away from each other. These two sides are called leaves. Two types of bascule bridges exist: one is a Rall-type double-leaf bascule bridge. These bridges locate the machinery for lifting the bridge at the very top of the bridge. A famous example of this sort of bridge is the Broadway Bridge in Portland, Oregon. Another major type of bascule bridge is the Strauss-type double-leaf bascule bridge. In this type of bascule, the machinery for opening the bridge is located inside two pier structures on the bridge, where they are better protected from the weather. An example of this type of bridge is the Burnside Bridge, also in Portland, Oregon. In general, bascule bridges are usually counterbalanced by sizable weights, so that very little motor power is necessary to move them.
Retractile bridges are quite different from bascule bridges. They are mounted up on wheels on a track on one side of the body of water. When the bridge needs to move, it is simply moved back along its track until there is sufficient space for the boat to pass through. This type of bridge is no longer widely used, and is considered obsolete. However, an example of a working retractile bridge is found in Brooklyn, New York, where Carroll Street crosses the Gowanus Canal.
Swing Span Bridges
Swing span bridges have an axis at their center, and move by spinning horizontally until the bridge is perpendicular to its previous position, therefore creating two channels through which boats can run. In New York City, the bridge over the Harlem river at 207th Street is considered a classic example of this type of bridge. Swing bridges are also occasionally cantilevered out from one side of the channel when there is not space for the axis to be located right at the bridge's center. Once again, this type of bridge is rarely used today, mostly because of the system's slowness of operation and the enormous amount of space it demands in order to work properly.
Vertical Lift Bridges
Vertical lift bridges are lifted up and down just like an elevator. Bridges like these have a tower on each end of the bridge, each of which encloses an enormous rotating drum. These drums have end cables attached to them that in turn lift the bridge up and down when the drums are turned. The oldest vertical lift bridge in the United States is the Hawthorne Bridge in Portland, Oregon. Other examples include the Roosevelt Island Bridge between Queens, New York, and Roosevelt Island in the East River.
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