Retractable bridge

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Retractable bridge
Ultunabron in Uppsala (retracted)
Ancestor:	Plate girder bridge
Related:	Lift bridge, submersible bridge, folding bridge
Descendant:	None
Carries:	Automobile, pedestrians
Span range:	Short
Material:	Steel
Movable:	Yes
Design effort:	Medium
Falsework required:	No

A retractable bridge is a type of movable bridge in which the deck can be rolled or slid backwards to open a gap for crossing traffic, usually a ship on a waterway. This type is sometimes referred to as a thrust bridge.

Retractable bridges date back to medieval times. Although never a very popular design, they were used in the mid-19th century for narrow crossings where maximum horizontal clearance was required. Today the Carroll Street Bridge in Brooklyn and the Borden Avenue Bridge in Queens are the only remaining examples of this design in New York City.

A recent example can be found at Queen Alexandra Dock in Cardiff, Wales, where the bridge is jacked upwards before being rolled on wheels. Helix Bridge [1] at Paddington Basin, London is a more unusual example of the type, consisting of a glass shell supported in a helical steel frame, which rotates as it retracts.

Many retractable bridges are also floating bridges, such as the Hood Canal Bridge, where a retractable span can be withdrawn between two lines of pontoons in the shape of a "U". A similar arrangement exists on the Evergreen Point Floating Bridge and Lacey V. Murrow Memorial Bridge.

Historical examples of designs for retractable bridges include those by Leonardo da Vinci [2], and Agostino Ramelli. [3]

[edit] See also

• Movable bridge for a list of other movable bridge types
• Guthrie rolling bridge