Retractable bridge

Retractable bridge

Vindbron (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 or retractile 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.

Some retractable bridges are also floating bridges, such as the Hood Canal Bridge. Two other floating bridges in Washington state used to have retractable spans but were replaced with new floating bridges without retractable spans: the Evergreen Point Floating Bridge (replaced in 2016) and Lacey V. Murrow Memorial Bridge (replaced after it accidentally sank in 1990). This design is popular in Guyana, which has the Demerara Harbour Bridge and the Berbice Bridge.