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 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. 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).
Several examples exist in New York City, (e.g., Carroll Street Bridge (built 1889) in Brooklyn, 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 [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. The Summer Street Bridge over Fort Point Channel in Boston is another variant type. This bridge is oriented northwest-southeast, with the NW-bound lanes of traffic retracting diagonally to the north, and the SE-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Many retractable bridges are also floating bridges, such as the Hood Canal Bridge, where a retractable span can be withdrawn between two lines of pontoons in the shape of a "U".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 [2], and Agostino Ramelli. [3]