|Retractable bridges, such as this one located in Demerara Harbour, are popular in Guyana.|
Bridges are a common feature of the built environment and one of the key elements of civil engineering. The basic principles of bridge design are dependent on the load-bearing structure. These are generally beam, arch or suspension structures.
Some bridges are also defined by other characteristics, for example a movable bridge is defined by its functionality. These bridges are designed in a way that allows them to accommodate different situations and different types of traffic.
 Retractable bridges
The Summer Street Bridge in Boston, Massachusetts, is a retractile bridge that was built in 1899. The bridge was the site of a major traffic accident in 1916. It is still standing over the Reserved Channel, but the retractile function was decommissioned in 1970.
Some retractable bridges in the Netherlands are floating bridges known vlotbrugs. These floating bridges are positioned across canals, and when necessary, the road is retracted into structures that have been built into the canal banks.
An alternative example of a floating retractable bridge is the Hood Canal pontoon bridge in Puget Sound, Washington. When the bridge needs to be opened for waterway traffic, a section of the floating roadway is hydraulically raised up so the retracting portion can slide underneath it.
This copper-plate engraving from ‘Le diverse et artificiose machine’ (The various and ingenious machines) illustrates the concept proposed by Agostino Ramelli (1531-c 1600).
One early example was created in 1588 by Agostino Ramelli. The Italian engineer gained favour with King Henry III of France after providing a strategic method of military engineering that resulted in the breach of enemy defences. In his book of engineering designs, Ramelli proposed a retractable bridge for crossing a moat.
 Noteworthy retractable bridges
Built in 1889, the Carroll Street Bridge in Brooklyn, New York is one of the oldest retractable bridges in the United States. It is one of New York City’s last bridges built with wooden planks to allow automobiles to cross (although there are strict height and weight restrictions). It was designated as a New York landmark in 1987.
The Littlehampton ferry footbridge was built in the 1980s. Known by locals as The Red Bridge, this retractable pedestrian bridge over the River Arun in Sussex is made from concrete, brick and steel. Powered by electricity, the structure retracts to accommodate ships that require access to adjacent industrial areas.
The Littlehampton retractable bridge underwent a refurbishment in April 2021. The project included repainting the bridge - in its trademark red colour - with a product designed to reduce maintenance costs.
The Red Bridge in Littlehampton, Sussex.
The Bridge of Scottish Invention is a retractable footbridge situated over the River Irvine. It was built to provide access to The Big Idea science centre in North Ayrshire, Scotland. The museum closed in 2003 after just three years in operation. However, the bridge still retracts to allow tall ships to pass through.
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