Fixed and hinged at one end, the medieval drawbridge functioned as a protective system to block out unwanted visitors from entering a town or castle entrance. It also served to permit access across a castle's surrounding mote or ditch, capable of being removed in the case of an attack. Typically constructed out of heavy timber, the medieval drawbridge exploited a system of rope pulleys, later replaced by chains, to raise and lower its unfixed end. An example of a medieval drawbridge is the one at the Fort of Ponta da Bandeira in Lagos, Portugal.
The design of the bascule bridge accommodates both the passage of water and ground traffic. While allowing for the passage of ground traffic, the bascule bridge lies flush with the road, creating an uninterrupted path for cars and trucks. Similar to the medieval drawbridge, the bascule bridge has both a free mobile end and a hinged fixed end. The bridge's mobile end or ends, as in the case of a double-leafed bascule bridge, swing upward and allow the passage of water traffic. Due to the bridge's limited energy requirements for operation, the bascule bridge is the most commonly used adaptation of the drawbridge. An example of a bascule drawbridge is Burnside Bridge in Portland, Oregon.
Also referred to as the "vertical lift bridge," the lift bridge operates by raising and lowering a mobile span of bridge parallel to the road, raising to allow for water traffic and lowering to allow for road traffic. Because the design only requires the bridge's counterweights to equal the weight of the mobile deck, the bridge can be constructed out of heavier material, thereby allowing for safe passage of rail transportation. The main disadvantage of this bridge is the height restriction on how high up the mobile deck can be suspended, making the lift bridge less suitable for the passage of large ships. An example of a lift bridge is Bridgewater Bridge in Australia.
Operationally, the submersible bridge has strong similarities to the lift bridge. However, instead of raising the deck above the roadway, the submersible bridge lowers its deck into the water to approximately 30 feet below water. Ideal for sailing vessels, the submersible bridge has no height restriction on the boats passing through. However, the 30-foot submersion of the deck limits the passage of vessels with hulls that extend far below the water line. An example of a submersible bridge is Corinth Canal Submersible Bridge.
The swing bridge's main structure is supported at its center by a vertical support extending into the water. The mobile deck then swings 90 degrees and stops in a position perpendicular to the roadway to allow for water traffic. Requiring no counterweights, the swing bridge is lighter than other bridges and can be constructed from less material. However, the center supporting pivot does reduce the span of passable waterway. An example of a swing bridge the Belize City Swing Bridge.
