A lift bridge is a bridge which is designed to allow passage of boats beneath the bridge by lifting part of the bridge's span above the grade of the rest of the bridge. Lift bridges are simply one version of the moving bridge, but they have a number of advantages which make them popular in some regions of the world. Various examples of lift bridges can be found all over the world, providing passage for cars, people, boats, and various craft of all shapes and sizes.
Movable bridges are installed when engineers want to build a bridge low, to save costs, but also allow for the passage of traffic on the waterway beneath the bridge. Low bridges are much less expensive to build, require less engineering, and are faster to build. These are all appealing advantages to bridge builders which make low bridges very common choices when a waterway is being bridged, but when a bridge is low, traffic may not be able to pass underneath it, necessitating a solution which allows part of the bridge to move to allow traffic to pass.
With a lift bridge, a section of the span is designed to rise vertically. The lift bridge includes a hoist which moves the section of the span, usually with the use of counterweights which drop, pulling the span up, with the span being held in place until it is needed again, when the weight of the span is allowed to pull it back into place to complete the bridge. Lift bridges commonly have a pair of towers which houses the mechanism to lift the bridge and lock the span in place.
Like other moving bridges, a lift bridge uses a system of traffic lights to control passage across the bridge, and to alert people when the span is going to be moved. Access to the span is restricted when it is about to be lifted, ensuring that people are not driving or standing on the span, which could be dangerous while it is in motion. The lift bridge will be temporarily closed to traffic while the segment is lifted, and then reopened once the span is back in place.
Some lift bridges use a system of stairs and walkways to allow pedestrians to walk across the bridge while the span is up. This system can also be used to isolate pedestrians from traffic while the bridge is in use, by providing them with a safe area to walk.
