A swing bridge is a type of movable bridge, which is swung open by pivoting on a horizontal plane to allow tall boats and vessels 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 navigation channel. When the bridge swings open, it moves horizontally and opens two separate channels on either side of the central pivot. While closed, a swing bridge allows motor traffic to pass across a body of water. When a boat needs to pass, motors located within the bridge rotate the swing span 90 degrees horizontally to the open position.
Swing bridges have existed for at least 200 years, and the earliest examples were constructed of wood. One of the most well-known swing bridges was the Pennsylvania Main Line Canal Bridge, which was built in the early 19th century. However, these early bridges have been largely replaced, due to their insufficient width for two-lane motor traffic and their limited weight capacity. Most of the old swing bridges still being used today are railroad 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 the bobtail swing. The center bearing variety is supported by a circular disc attached to the bottom of the structure, which allows the bridge to spin like a top while being supported at a single, central point. The rim bearing swing bridge sits on top of a large piece of steelwork, consisting of a set of rollers which surround the central pivot, and a rim girder, which allows the bridge to roll in a circle on a track while pivoting into the open position. The bobtail swing is an asymmetrical bridge, typically made of wood, which is operated by rope from the shore, and is rarely used in modern times.
Swing bridges are used around the world in areas where automobile traffic and water vessels are likely to meet, and are often preferred in these situations over other designs. Unlike other types of movable bridges, the swing bridge does not lift or move into the air while opening. This results in less wind load on the structure, and less structural stress. A lack of counterweights also reduces the overall weight of the bridge, and the time and materials needed for construction are significantly less than other movable bridges.
Although it is a good solution for many waterways, the swing bridge does have disadvantages. It has more moving parts than other moving bridges, and relies heavily upon motors and mechanical parts to move the swing span. If the bridge does not receive regular maintenance, or any of its parts malfunction, serious safety hazards may result. However, if well-maintained, a swing bridge will function safely for many years without incident.