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A retractable bridge is a type of movable bridge in which the deck can be rolled or slid backwards to open a gap while traffic crosses, usually a ship on a waterway. This type is sometimes referred to as a thrust bridge. Retractable bridges date back to medieval times. Due to the large dedicated area required for this type of bridge, this design is not common. A retractable design may be considered when the maximum horizontal clearance is required (for example over a canal). Two remaining examples exist in New York City (the Carroll Street Bridge (built 1889) in Brooklyn and the Borden Avenue Bridge in Queens). 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 at Paddington Basin in 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.〔()〕 The Summer Street Bridge over Fort Point Channel in Boston is another variant type. This bridge is oriented northwest–southeast, with the northwest-bound lanes of traffic retracting diagonally to the north, and the southeast-bound lanes retracting diagonally to the west. Many retractable bridges are also floating bridges, such as the Hood Canal Bridge, where a retractable span can be withdrawn between two lines of pontoons in the shape of a "U".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〔 ()〕 and Agostino Ramelli.〔()〕 == See also == *Movable bridge for a list of other movable bridge types *Guthrie rolling bridge 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Retractable bridge」の詳細全文を読む スポンサード リンク
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