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What is a Lift Bridge?

Mary McMahon

Last Modified Date: July 01, 2023

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.

Pittsburgh is known for its many bridges, including several lift bridges.

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.

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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.

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllTheScience researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Mary McMahon

Discussion Comments

anon328651

April 4, 2013

I think it would be fun to ride on a raising lift bridge! Just imagine yourself in a car, and you are driving over a lift bridge, on the section that can be lifted, and and it suddenly starts to rise...

Perdido

February 26, 2012

Has anyone here ever walked across a lift bridge walkway when it was raised? When I first thought about it, I pictured a person slowly being lifted up along with the bridge, but after reading this article, I can see how stairs and a higher walkway would allow passage.

I guess pedestrians must have the option of going up a stairway when the main walkway is closed to traffic. The secondary walkway is probably pretty high up and possibly intimidating. I know I wouldn't want to be walking high over a body of water.

kylee07drg

February 25, 2012

I never really thought about lower bridge building being less expensive. I had always wondered why anyone would build a low bridge that had to be raised for barges to go underneath.

Many cities watch their road and bridge construction budget very closely, so it stands to reason that they would rather construct a low bridge that could be lifted than a high one that would cost much more. I actually prefer driving over lower bridges, so this doesn't bother me one bit. I have a fear of heights, and I don't have to deal with that on lift bridges.

cloudel

February 25, 2012

@Oceana – Lift bridges are intimidating, but I don't think you have anything to fear. Every precaution is taken to make them safe for drivers.

The lift bridges I have seen have crossbars that come down in front of them before they are about to be raised. These are like the ones in front of railroad tracks, so you couldn't drive through them accidentally.

I have never heard of a car falling through a lift bridge. It's a terrible thing to think of, but it's very unlikely to happen.

Oceana

February 24, 2012

Lift bridges are so scary to me! I remember riding in the backseat of my parents' car on the way to the Gulf coast and coming to a stop in front of a drawbridge.

Flashing lights signaled us to stop. We were the first car in line, and we had a good view of everything.

The very asphalt began to rise, and it opened up in the middle. I just had this horror of being right in the center as it lifted and falling into the ocean.

Even after the bridge had lowered back to the horizontal position, I was scared riding over it. For all I knew, it could decide to open as we were crossing, sending us into a watery death trap!