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A bascule bridge is a drawbridge with two counterweighted leaves that pivot upward to allow vessel traffic to pass. They originated in medieval Europe and were operated by men using a winch and counterweight. Most bascule bridges are double-leaf and have two main types: single leaf and double leaf. There are numerous variations and designs, but the heel pin bridge is the most popular and widely constructed.
A bascule bridge is a type of drawbridge consisting of two counterweighted leaves or spans that pivot upward on a horizontal axis to allow vessel traffic to pass underneath. Counterweights help balance each side of the bridge as it swings up, which aids in the lifting motion. Drawbridges are the most commonly found drawbridges in the world, most likely because they open quickly and easily once established and operate on very little power.
The bascule bridge originated in medieval Europe to help defend castles and towns. These early drawbridges were operated by men using a winch and counterweight. Part of Old London Bridge was actually a bascule bridge or drawbridge, and was occasionally raised to allow tall-masted vessels to pass. However, it was not until the 19th century that these bridges became popular for aiding the navigation of large vessels. The first large bascule bridge, the Blagoveshchensky Bridge, was built in St. Petersburg in the 19.
There are two main types of movable bridges: single leaf and double leaf. A single leaf bridge is made up of several connected elements and its counterweights are elevated above the bridge. A double-leaf bridge is constructed similarly, but the counterweights are located under the bridge. Most bascule bridges are double-leaf and consist of two leaves or spans, which meet in the middle above the navigation channel. Single leaf bridges consist of a single leaf or span, which connects to the opposite side of the channel.
There are numerous variations and designs of the movable bridge, including heel-pin and movable bridges. The heel pin bridge, also known as a fixed pin, has the counterweight and bridge on separate pins. When opening, the drawbridge swings back and forth on a rail. There are also some bascule bridges that have no counterweight, the most famous being the 16th Street Bascule Bridge in Milwaukee, Wisconsin in the United States. Many other variations have been patented since the 19th century, but the heel pin continues to be the most popular and widely constructed.
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