Swing bridges rotate horizontally to allow tall boats to pass and have been around for 200 years. There are variations, including center bearing, rim bearing, and bobtail swing. They are preferred over other designs due to less wind loading and structural stress, but require regular maintenance.
A swing bridge is a type of movable bridge, which is opened by rotating in a horizontal plane to allow tall boats and ships to pass. The movable part of the bridge is known as the swing span, which has two arms extending from the pivot to each side of the shipping channel. As the bridge opens, it moves horizontally and opens two separate channels either side of the center pin. When closed, a swing bridge allows automobile traffic to pass across a body of water. When a boat needs to pass, motors located inside the deck rotate the swing arc 90 degrees horizontally to the open position.
Swing bridges have been around for at least 200 years and the earliest examples were built of wood. One of the most famous swing bridges was the Pennsylvania Main Line Canal Bridge, built in the early 19th century. However, these early bridges have largely been replaced, due to their insufficient width for two-lane automobile traffic and their limited weight capacity. Most of the old swing bridges still in use today are railway bridges, built in the late 19th and early 20th centuries.
There are numerous variations of the typical swing bridge, including center bearing, rim bearing, and bobtail swing. The central bearing variety is supported by a circular disc attached to the underside of the structure, which allows the bridge to rotate like a rope while being supported at a single central point. The rim bearing swing bridge sits atop a large piece of steel, consisting of a series of rollers surrounding the center pivot and a rim beam, which allows the bridge to roll in a circle on a track while rotating in the open position. The bobtail swing is an asymmetrical bridge, typically made of wood, which is operated by a rope from the shore, and is rarely used in modern times.
Swing bridges are used worldwide in areas where automobile traffic and nautical vessels are likely to be encountered and are often preferred in these situations over other designs. Unlike other types of swing bridges, the swing bridge does not lift or move in the air when opening. This results in less wind loading on the structure and less structural stress. The lack of counterweights also reduces the overall weight of the bridge, and the time and materials required to construct it are significantly less than with other movable bridges.
While a good solution for many waterways, the swing bridge does have drawbacks. It has more moving parts than other movable bridges and relies heavily on motors and mechanical parts to move the swing arc. If the lift does not receive regular maintenance or if one of its parts malfunctions, serious safety risks can occur. However, if well maintained, a swing bridge will operate safely for many years without incident.
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