Types of Moving Bridges

The first movable bridges in history were defensive drawbridges. They were first conceived in Medieval Europe, around Normandy.

Movable bridges have been a constant in human history. Right from the drawbridges over crocodile-filled moats in medieval castles to the curling 'Rolling Bridge' in London, man has always created inventive ways of simply getting across water.

Movable bridges have many purposes. Medieval drawbridges were primarily defensive structures; they could be raised if an enemy approached, making the castle difficult to breach. Modern movable bridges are, more often than not, built to accommodate both road and boat traffic.

Modern architectural and engineering progress means that movable bridges can also become popular tourist sites. Bridges such as the Tower Bridge become iconic symbols of their location, and help attract tourists.

Drawbridges work on a relatively simple principle. The bridge is hinged at one end, and connected to retractable metal chains at the other, making it possible to 'pull it up'.

The word 'drawbridge' is often colloquially used to describe any type of moving bridge, but historically, drawbridges adhere to the description given above.

Bascule bridges are a subgroup of drawbridges. Bascule bridges can be single- or double-leaf. Single-leaf bascule bridges are raised as a single entity, like a drawbridge. These are also calledrolling bascule bridges. 

Double-leaf bascule bridges, on the other hand, split in the middle, and both halves are raised separately. The London Tower Bridge is a famous example of the latter, while bridges such as the Johnson Street Bridge in Victoria and the Pegasus bridge in Caen are examples of rolling bascule bridges.

The Rolling Bridge in Paddington, London, is an (incorrectly named) example of this type of bridge. When the hydraulic pistons are activated, the eight-sectioned bridge curls into an octagon. As mentioned, it doesn't actually 'roll', but curls together

The span of a vertical lift bridge is raised from both ends, staying parallel to its original position. There are two ways to achieve this.

Most vertical lift bridges incorporate towers at both ends. These towers contain pulleys and counterweights that pull up the respective ends of the bridge. In contrast, table bridges are supported on submersed hydraulic supports that raise the span above its usual level. 

The most important difference between the two is that vertical lift bridges are pulled up, whereas table bridges are lifted or pushed above.

This is one of the simplest type of moving bridges, and consists of a span section that is supported by a movable base. The span is thus, simply retracted to let boats through, and is pushed out again to its original position to resume land travel. The famous William A. Bugge Bridge (Hood Canal Bridge) is a floating thrust bridge.

Submersible bridges are held on supports that can be depressed, so that the bridge goes underwater. Since submersible bridges are by definition very low, high vessels can easily pass over it. However, since there is a limit to how low the bridge can go, ships with a high draft can't pass over it.

Tilt bridges are a wonder of modern architecture. Tilt bridges consist of two arcs grounded at the same points but extending in opposite directions. One of the arcs is horizontal, and carries the road travel. The other arc is connected to the horizontal arc by cables, and extends up into the air, balancing the outward curve of the horizontal arc.

When ships approach, the entire structure is tilted to the side of the vertical arc, so that the horizontal arc is raised above the ground. The two arcs thus, rise at a similar angle, creating a clear waterway for ships.

The former creates two separate waterways in the body of water, whereas the latter allows halves of the bridge to align with the respective shore, allowing for the passage of wider vessels.

Transporter bridges consist of gondolas that carry road traffic across the water body, instead of constructing seamless connections that can then be moved. This is a primitive, but cheap, solution, and is rarely used. Only about 10 bridges of this kind are operational today.

While technically not bridges, pulley-operated ferries can be considered movable bridges, though they are very rare, and hardly ever used in the modern world. For very wide water bodies, movable bridges are not a viable option, since huge portions would have to be moved.

In these situations, various other types of bridges are used. Some of these bridges are architectural marvels in themselves, and, when designed liberally, can be utterly beautiful. These are the types of moving bridges seen in today's architecture.