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[webny99]

How about this one in Harrisburg, PA?

That's definitely one that I'd distinguish as a viaduct rather than an overpass.

I suppose... and I guess the same would apply to State St. just to the north.

[webny99]

How about the new section of PA 576 outside Pittsburgh? And another one here.

[ethanhopkin14]

How about the new section of PA 576 outside Pittsburgh? And another one here.

Interesting you bring up the Pittsburgh area.  I was driving this route last Tuesday and saw this overpass and thought of this thread.

[webny99]

How about the new section of PA 576 outside Pittsburgh? And another one here.

Interesting you bring up the Pittsburgh area.  I was driving this route last Tuesday and saw this overpass and thought of this thread.

Wow, that might be the tallest I've seen in this thread. This view from much further back really shows how tall it is too.

[dlsterner]

Wow, that might be the tallest I've seen in this thread. This view from much further back really shows how tall it is too.

The overpass in the very first post in the thread could be another candidate for the tallest in the whole thread.  It's the I-476 bridge over US 6/11 in Scranton PA.

Post link:  https://www.aaroads.com/forum/index.php?topic=8710.msg202481#msg202481
GSV link from that post:  http://goo.gl/maps/ifFE6

[Mr_Northside]

The overpass in the very first post in the thread could be another candidate for the tallest in the whole thread.  It's the I-476 bridge over US 6/11 in Scranton PA

The I-476 bridge @ Clark's Summit was the tallest on the PA Turnpike owned roads, but the "Joe Montana" bridges on TPK-43 (Mon-Fayette expressway) are now the tallest.
I'm not sure what the tallest in PA would be if you include all the roads/highways/bridges in addition to PTC ones.

That said, I think there was discussion earlier in this thread, about is it still just an "overpass" if the function is more than just a bridge over/under the road itself, where the road is the reason.   Some of those tall bridges would be there whether or not there was a road underneath it or not, just to bridge the valleys.

[SteveG1988]

The overpass in the very first post in the thread could be another candidate for the tallest in the whole thread.  It's the I-476 bridge over US 6/11 in Scranton PA

The I-476 bridge @ Clark's Summit was the tallest on the PA Turnpike owned roads, but the "Joe Montana" bridges on TPK-43 (Mon-Fayette expressway) are now the tallest.
I'm not sure what the tallest in PA would be if you include all the roads/highways/bridges in addition to PTC ones.

That said, I think there was discussion earlier in this thread, about is it still just an "overpass" if the function is more than just a bridge over/under the road itself, where the road is the reason.   Some of those tall bridges would be there whether or not there was a road underneath it or not, just to bridge the valleys.

When does an overpass become a viaduct?

The clarks summit crossing is a viaduct, it's main purpose is to get a road between two hills. The PA43 crossing is the same way, it's main purpose is to allow a road to go between hill tops without having to go down into the valley.

The Arch overpass on I68 and the similar one on I80 in Ohio were built to cross a rock cut afaik. Those might be counted as overpasses since they were built to cross over a road that was placed beneath another road.

[empirestate]

The overpass in the very first post in the thread could be another candidate for the tallest in the whole thread.  It's the I-476 bridge over US 6/11 in Scranton PA

The I-476 bridge @ Clark's Summit was the tallest on the PA Turnpike owned roads, but the "Joe Montana" bridges on TPK-43 (Mon-Fayette expressway) are now the tallest.
I'm not sure what the tallest in PA would be if you include all the roads/highways/bridges in addition to PTC ones.

That said, I think there was discussion earlier in this thread, about is it still just an "overpass" if the function is more than just a bridge over/under the road itself, where the road is the reason.   Some of those tall bridges would be there whether or not there was a road underneath it or not, just to bridge the valleys.

When does an overpass become a viaduct?

The clarks summit crossing is a viaduct, it's main purpose is to get a road between two hills. The PA43 crossing is the same way, it's main purpose is to allow a road to go between hill tops without having to go down into the valley.

The Arch overpass on I68 and the similar one on I80 in Ohio were built to cross a rock cut afaik. Those might be counted as overpasses since they were built to cross over a road that was placed beneath another road.

Yes, basically that's the consensus. As stated before, a viaduct (or in some cases, a long bridge) crosses a road only incidentally. The structure's main purpose is to cross a physical feature such as a waterbody or valley, and were it not for the road, the structure would still exist. Indeed, many viaducts cross multiple roads in one go; another example would be the Castleton-on-Hudson Bridge.

An overpass, on the other hand, is built for the express purpose of crossing a road. Were there no road to be crossed, there would be no structure.

[webny99]

As stated before, a viaduct (or in some cases, a long bridge) crosses a road only incidentally. The structure's main purpose is to cross a physical feature such as a waterbody or valley, and were it not for the road, the structure would still exist. Indeed, many viaducts cross multiple roads in one go; another example would be the Castleton-on-Hudson Bridge.

An overpass, on the other hand, is built for the express purpose of crossing a road. Were there no road to be crossed, there would be no structure.

I don't know, I would say viaducts and overpasses are both just a type of bridge. The PA 43 and I-476 examples in particular are constructed like any other overpass (with beams, not arches), just much taller and longer than usual. I wouldn't have a big issue with calling either one a viaduct, but I still think they should qualify for the thread.

[empirestate]

I don't know, I would say viaducts and overpasses are both just a type of bridge. The PA 43 and I-476 examples in particular are constructed like any other overpass (with beams, not arches), just much taller and longer than usual. I wouldn't have a big issue with calling either one a viaduct, but I still think they should qualify for the thread.

That just depends on what one considers to be interesting about the height. To my mind, a structure built merely to cross another road that happens to be very high is more striking than one that is built to cross a valley, because I'd expect height of the latter but not of the former. This does seem to fit the spirit of the thread: as I read the OP, it talks about highway grade separations that happen to be tall due to surrounding terrain. To me this connotes something distinct from a structure that is tall due to surrounding terrain, and that happens to cross another highway above grade.

But in any event, that's a separate question from what distinguishes viaducts from overpasses and the like. It's true that they're both types of bridge, so there's no controversy between our statements there. But I'm not aware of those terms being defined by any method of construction. A certain method may be typical of one or the other, but the method isn't a defining characteristic. (See also the terminology of rotaries vs. roundabouts and their kin.)

To put that more clearly, you might build something out of beams because it's an overpass. But it wouldn't be an overpass because it's built out of beams.

[webny99]

I guess I'm just not sure there's any examples of an overpass that just randomly happens to be very tall for reasons that have nothing to do with the surrounding terrain.  Now I'm picturing something like the Gateway Arch in road form.. that seems like the only possible way you could get an extra-high, road-only overpass in which terrain is not a factor at all.

[empirestate]

I guess I'm just not sure there's any examples of an overpass that just randomly happens to be very tall for reasons that have nothing to do with the surrounding terrain.

No, I think the heights almost always has to do with terrain. Often, it's because the road on the lower grade passes through a cut (natural or otherwise).

[US 89]

I guess I'm just not sure there's any examples of an overpass that just randomly happens to be very tall for reasons that have nothing to do with the surrounding terrain.  Now I'm picturing something like the Gateway Arch in road form.. that seems like the only possible way you could get an extra-high, road-only overpass in which terrain is not a factor at all.

If the overpass also crosses a navigable river or is right next to a bridge that does, you can get abnormally high bridge heights that way too...

[webny99]

Here's another PA example, not as high as some of the ones already mentioned, but also more like a standard overpass than a viaduct, as some have called other examples: https://goo.gl/maps/4gGWqyuQZsAsZwME8

[Terry Shea]

How about the new section of PA 576 outside Pittsburgh? And another one here.

Interesting you bring up the Pittsburgh area.  I was driving this route last Tuesday and saw this overpass and thought of this thread.

That doesn't look safe at all!

[Rothman]

How about the new section of PA 576 outside Pittsburgh? And another one here.

Interesting you bring up the Pittsburgh area.  I was driving this route last Tuesday and saw this overpass and thought of this thread.

That doesn't look safe at all!

Why not?  It's even relatively new.

[Terry Shea]

How about the new section of PA 576 outside Pittsburgh? And another one here.

Interesting you bring up the Pittsburgh area.  I was driving this route last Tuesday and saw this overpass and thought of this thread.

That doesn't look safe at all!

Why not?  It's even relatively new.

Doesn't appear to be enough support at the top and the bridge sections don't appear to line up properly.

[andrepoiy]

Is this considered tall? Probably not, but I believe it might be one of the taller ones in Ontario.


https://goo.gl/maps/eozEAnQfGKEbdtJF6

[cockroachking]

How about the new section of PA 576 outside Pittsburgh? And another one here.

Interesting you bring up the Pittsburgh area.  I was driving this route last Tuesday and saw this overpass and thought of this thread.

That doesn't look safe at all!

Why not?  It's even relatively new.

Doesn't appear to be enough support at the top and the bridge sections don't appear to line up properly.

The supports seem to have been constructed for 3 lane bridges: decked for 2, but with room for expansion for 3.