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

Brooklyn

Brooklyn is the most populous of New York City's five boroughs, with nearly 2.6 million residents, and the second-largest in area. Since 1896, Brooklyn has had the same boundaries as Kings County, which is now the most populous county in New York State and the second-most densely populated...

, Borden Avenue Bridge in Queens). A recent example can be found at Queen Alexandra Dock in Cardiff

Cardiff

Cardiff is the capital, largest city and most populous county of Wales and the 10th largest city in the United Kingdom. The city is Wales' chief commercial centre, the base for most national cultural and sporting institutions, the Welsh national media, and the seat of the National Assembly for...

, Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

, where the bridge is jacked upwards before being rolled on wheels. Helix Bridge http://en.structurae.de/structures/data/index.cfm?ID=s0012099 at Paddington Basin

Paddington Basin

Paddington Basin is an area of Paddington, London named after the nearby canal basin.The junction of the Regent's Canal and the Grand Junction Canal is close to this point but the basin itself is the terminus of the Paddington Arm of the Grand Junction Canal. It was opened in 1801...

, London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 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

Fort Point Channel

Fort Point Channel is a maritime channel separating South Boston from downtown Boston, Massachusetts, feeding into Boston Harbor. The south part of it has been gradually filled in for use by the South Bay rail yard and several highways...

 in Boston

Boston

Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...

 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 bridge

Pontoon bridge

A pontoon bridge or floating bridge is a bridge that floats on water and in which barge- or boat-like pontoons support the bridge deck and its dynamic loads. While pontoon bridges are usually temporary structures, some are used for long periods of time...

s, such as the Hood Canal Bridge

Hood Canal Bridge

The Hood Canal Bridge is a floating bridge located in the U.S. state of Washington that carries Washington State Route 104 across Hood Canal and connects the Olympic and Kitsap Peninsulas. At long, The Hood Canal Bridge (officially William A. Bugge Bridge) is a floating bridge located in the U.S....

, 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

Evergreen Point Floating Bridge

The Governor Albert D. Rosellini Bridge—Evergreen Point is the longest floating bridge on Earth at and carries State Route 520 across Lake Washington from Seattle to Medina.The Evergreen Point of the bridge's original name is the westernmost of the three small Eastside...

 and Lacey V. Murrow Memorial Bridge

Lacey V. Murrow Memorial Bridge

The Lacey V. Murrow Memorial Bridge is a floating bridge that carries the eastbound lanes of Interstate 90 across Lake Washington from Seattle to Mercer Island, Washington. It is the second longest floating bridge on Earth at , whereas the longest is the Evergreen Point Floating Bridge just a few...

.

Historical examples of designs for retractable bridges include those by Leonardo da Vinci

Leonardo da Vinci

Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...

 http://brunelleschi.imss.fi.it/genscheda.asp?appl=LIR&xsl=modello&lingua=ENG&chiave=101089, and Agostino Ramelli

Agostino Ramelli

Agostino Ramelli was an engineer who designed the "book wheel" or "reading wheel".During the Siege of La Rochelle , Agostino successfully engineered a mine under a bastion and breached the fortification, making him popular with his commander, Henri d'Anjou, who later became Henri III of France.In...

. http://www.scienceandsociety.co.uk/results.asp?image=10423506

See also

• Movable bridge for a list of other movable bridge types
• Guthrie rolling bridge
  Guthrie rolling bridge
  A Guthrie rolling bridge was a kind of retractable bridge, an 18th century version of the drawbridge. It was commonly installed as the access across the narrow steep sided ditches characteristic of the polygonal forts of this era....