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What is a Viaduct?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Last Modified Date: 25 November 2016
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A viaduct is a bridge constructed from a series of spans and designed to go across dry land, rather than a body of water. Viaducts can be used in a wide variety of ways, and numerous examples of this type of bridge design can be found around the world, executed in a variety of materials. Viaducts have been constructed since ancient times to carry foot and cart traffic, and today they can be used for cars, pedestrians, cyclists, trains, and trucks.

One common site for viaducts is in valleys. Building a viaduct allows a road or railway to quickly span a valley or chasm, rather than forcing engineers to lay out a path for the roadbed to reach the valley floor, and then establish a route out of the valley. While building a viaduct can be initially very expensive, it is generally cheaper and safer than keeping the road on the flat ground.

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A viaduct may also be used in an area where several different streams of traffic are overlaid, allowing traffic to pass on and under the viaduct so that there is less interruption of traffic. Viaducts are common features in regions with lots of freeways, accommodating multiple streams of traffic without forcing traffic to stop. Viaduct construction is also especially common in areas where trains and cars need to travel in roughly the same places. Trains can be run under a viaduct, while cars may drive on it, or vice versa. In either case, car traffic does not have to stop for passing trains, and the risk of train/car collisions is greatly decreased.

In the classic viaduct design, engineers start by constructing a series of towers, and then connecting the towers with arches and installing a roadbed. In the case of a viaduct which is designed so that traffic can flow underneath, the arches must be built especially high to ensure that passing traffic has enough clearance. Viaducts over low valleys and in other situations where traffic underneath is not expected may have much lower arches, since clearance is not an issue.

Many historic viaducts are made from stone and brick, but metal and concrete can be used as well. In communities with older viaducts, preservation organizations sometimes work to keep the viaduct in good working order, as stone and brick viaducts are considered aesthetically valuable. It is also possible to find examples of abandoned viaducts which were left behind when roadways moved or became impassable, and these sites sometimes become popular with walkers.

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B707
Post 6

I'm kind of curious about the viaducts that go across valleys. I've probably been on one, but just don't recall.

I can see that it would make a lot of sense to construct these. I suppose the early roads had narrow switchback roads going down into the valley and up the other side. But with a viaduct, you have a straightaway road, where traffic can move fast, no stopping for trains or other cars at crosssroads.

The big problems are the expense and the time it takes to plan and build these and, of course, political issues.

sweetPeas
Post 5

In my city there is an "ancient" viaduct that desperately needs to be replaced. It's something like an early freeway. This viaduct was built in the 1940s. It was built to route traffic around the city, which was built on the waterfront. There are two levels - one for each direction.

It has served the city well and survived a couple of large earthquakes.

But a decision has to be made soon about what to put in its place.

JaneAir
Post 4

@ceilingcat - Good point. Viaducts are great on highways also. There is one area where I live that would probably really adversely affect traffic if a viaduct wasn't there. For once, it seems like whoever designed that highway really planned ahead!

However, I happen to hate bridges. Even though they don't go over water, to me, viaducts feel just like bridges. So while the one near me helps traffic it certainly doesn't help my stress level when I'm driving to work in the morning.

ceilingcat
Post 3

I think viaducts sound like a really good idea when cars and trains are trying to share the same space. I've been in a few situations where it looked like a car was going to get hit by a train because of reckless driving.

Also, there is the question of traffic. There is one area where I live that is always backed up because cars have to stop to let trains go by. A viaduct would definitely solve this problem with the added bonus of being much safer!

kylee07drg
Post 2

A really narrow county road in my hometown has a railroad viaduct. It crosses over the road, and the metal girders are really rusty.

I remember being terrified of riding under it when a train was crossing over. The bridge is obviously very old, and it just doesn’t seem like it could support the weight of something as massive as a train.

The railroad is no longer in use, and the tracks have been removed. However, the viaduct was left behind, because the former railroad is now a walking trail. I feel much better going under it knowing that the only weight overhead is that of a person or two.

wavy58
Post 1

The highest viaduct I have ever crossed is on the Natchez Trace Parkway in Tennessee. It crosses over a large valley and State Route 96.

Because of the slope of the valley near one end of the viaduct, the second arch is asymmetrical. This bridge does not have columns supporting the deck. Instead, the bridge’s weight is focused at the arch’s crown.

If you are driving under the viaduct, it looks very streamlined. I have never seen another bridge made this way. I learned from a park ranger on the Natchez Trace that this viaduct is the first arch bridge in the U.S. to be segmentally constructed from concrete.

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