Movable Bridges

Moveable bridge is a bridge that can change position (and even shape in some cases) to allow for passage of boats below. This type of bridge has a lower cost of building because it has no high piers and long approaches but its use stops the road traffic when the bridge is open for river traffic.

The oldest know movable bridge was built in the 2nd millennium BC in the ancient Egypt. History also knows for one early movable bridge built in Chaldea in the Middle East in 6th century BC. Since then they were almost forgotten until Middle Ages when they again appeared in Europe. Leonardo da Vinci designed and built designed and built bascule bridges in 15th century. He also made designs and built models of swing and a retractable bridges. Industrial revolution allowed for new technologies like mass-produced steel and powerful machines and it is no surprise that new types of modern movable bridges appeared in 19th century. They are built even today but many movable bridges that are still in use in United States are built in early 20th century. In time, some of them are repaired with lighter materials and their gears are replaced with hydraulics. 

Types of Movable Bridges

 

Various types of movable bridge are available, but three of them are significantly desirable and practical are discussed in the following sections and number of special types of movable bridge will be discussed as well:

• Bascule bridge
• Swing bridge
• Vertical lifting bridge
• Special types of movable bridge

Bascule Bridge

Bascule bridge, which is also called as drawbridge, is fixed and supported on an axis which is perpendicular to the bridge longitudinal centerline axis.

The horizontal line on which the bridge is pivoted is commonly located at the center of gravity of the bridge to create a balance between the weight of the bridge on either side of the horizontal pivotal axis.

It should be known that the weight balance is not accurate and slight inaccuracies are provided based on the future utilization of the bridge, for example, if the tendency is to open the waterway, then the weight is distributed to help opening the bridge and this weight distribution is termed as counterweight heavy, but if the trend is to employ the weight for closing the bridge, then it is called heavy span.

There are two major types of bascule bridges including single leaf and double leaf bascule bridge. In addition to tripe and quadrable types which are occasionally constructed. The term leaf is used for the part of the bridge that moves and opens the waterway consequently.