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Author Topic: Straight Wall over Slant Wall Advantages/ Disadvantages  (Read 1241 times)

roadman65

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Straight Wall over Slant Wall Advantages/ Disadvantages
« on: October 11, 2022, 07:49:11 AM »

https://goo.gl/maps/Z2BCD89cvWeUN1WA9
Here is the latest PA Route 61 overpass across both I-78 and US 22 near Hamburg, Pennsylvania.

Here is the former courtesy AA Roads.
https://www.aaroads.com/guides/i-078-east-pa/

Notice the older uses the slant walls, but the replacement structure uses a straight wall while removing the bridge piers also previously present there.

What is the advantages to a straight support wall to the slant walls?
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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #1 on: October 11, 2022, 09:50:46 AM »

https://goo.gl/maps/Z2BCD89cvWeUN1WA9
Here is the latest PA Route 61 overpass across both I-78 and US 22 near Hamburg, Pennsylvania.

Here is the former courtesy AA Roads.
https://www.aaroads.com/guides/i-078-east-pa/

Notice the older uses the slant walls, but the replacement structure uses a straight wall while removing the bridge piers also previously present there.

What is the advantages to a straight support wall to the slant walls?

Just link to an older GSV...  https://goo.gl/maps/eCBVpC8aguL1L7uu6   :pan:
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Dirt Roads

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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #2 on: October 11, 2022, 01:15:55 PM »

What is the advantages to a straight support wall to the slant walls?

It is the result of a "game" that bridge designers play when comparing right-of-way width, travel lane width and median location against the various lengths of standardized stringers (bridge beams).  In general, a short slopebank is more stable and less expensive that a retained fill (ergo, retaining wall with fill on the other side).  But it doesn't always fit within the right-of-way.
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Tom958

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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #3 on: October 12, 2022, 06:25:10 PM »

What is the advantages to a straight support wall to the slant walls?

It is the result of a "game" that bridge designers play when comparing right-of-way width, travel lane width and median location against the various lengths of standardized stringers (bridge beams).  In general, a short slopebank is more stable and less expensive that a retained fill (ergo, retaining wall with fill on the other side).  But it doesn't always fit within the right-of-way.

The "slant wall" isn't a wall at all. It's called slope paving. It's there to prevent erosion and simplify maintenance, not to provide any structural support--- the abutment is most likely supported by piles driven into the ground. As this example near Nashville shows, it can be replaced with riprap (pan left to see it) or omitted if the ground is rocky.

The real question is why they went with end bents and end spans on the older bridge and closed abutments on the new. And why they didn't go for open abutments and no end spans, like this. That's where the tradeoffs that Dirt Roads was explaining come into play. I'll add that technology and prevailing construction practices are a factor, too. The invention of precast concrete MSE walls made retaining walls a lot more economical, for one thing.
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Dirt Roads

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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #4 on: October 13, 2022, 06:17:07 PM »

What is the advantages to a straight support wall to the slant walls?

It is the result of a "game" that bridge designers play when comparing right-of-way width, travel lane width and median location against the various lengths of standardized stringers (bridge beams).  In general, a short slopebank is more stable and less expensive that a retained fill (ergo, retaining wall with fill on the other side).  But it doesn't always fit within the right-of-way.

The "slant wall" isn't a wall at all. It's called slope paving. It's there to prevent erosion and simplify maintenance, not to provide any structural support--- the abutment is most likely supported by piles driven into the ground.

There is indeed a structural function to the slopebank beneath abutments, but you are correct that this has very little to do with the abutment structure.  In many cases, the approach to the overpass is constructed on fill.  There are two means to hold the fill in place: (1) a retaining wall (the OP says "straight wall"); and (2) a slopebank (the OP says "slant wall" and Tom958 says slant paving, which is more related to the use of rip rap to hold the fill in place). 

As this example near Nashville shows, it can be replaced with riprap (pan left to see it) or omitted if the ground is rocky.

Actually, the only way to omit the slopebank or retaining wall atop of solid rock is to construct the abutment further back away from the rockwall face.  If you look carefully, the new abutment on the right side is located way back away from the rockwall face (whereas the old abutment was probably located up closer to the edge).  Unless it is sheer granite, the rockface is typically unable to support the forces of a bridge abutment (and even if it is, I doubt that a bridge engineer would ever depend on accuracy of the geology data).

Anyhow, this brings me to a nuanced point about how the slopebank or retaining wall actually "assists" the abutments.  Due to the normal motion of traffic on the overpass, there is a bunch of lateral forces imposed on both the piers/columns and the abutments.  There is also a natural frequency that those structures need to be stiffened against (somewhere around 3 to 10 Hz, if I recall correctly).  The lateral width of those structures need to be designed to withstand all of that.  Unless, of course, an abutment is surrounded by fill that is retained or held by a slopebank.  I might be wrong here, but it is my understanding that in such cases, only the abutment itself needs to withstand the lateral forces and be stiffened against the natural frequency (and not the substructure, which is indeed usually driven or drilled piles).
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kphoger

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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #5 on: October 13, 2022, 07:01:49 PM »

If you look carefully, the new abutment on the right side is located way back away from the rockwall face (whereas the old abutment was probably located up closer to the edge).

Here's a decent view:
https://goo.gl/maps/i3DJY35P9rHSnCqK6
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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #6 on: October 14, 2022, 06:59:24 PM »

If you look carefully, the new abutment on the right side is located way back away from the rockwall face (whereas the old abutment was probably located up closer to the edge).

Here's a decent view:
https://goo.gl/maps/i3DJY35P9rHSnCqK6

That abutment still has piles beneath it, but they are probably drilled into the rock.
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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #7 on: October 18, 2022, 03:37:43 PM »

Notice the older uses the slant walls, but the replacement structure uses a straight wall while removing the bridge piers also previously present there.

I'll leave it to the better qualified people to discuss the walls themselves... but I will note that most replacement and new bridges al over PA are done removing the outside piers adjacent to right shoulders.

As a different spot example, when they replaced the PA-982 overpass at the US-30/PA-982 cloverleaf outside of Latrobe, the old one had outer piers and no median pier, and the new one has no outside piers and a median pier.

New:  https://goo.gl/maps/jWsRWAZ85zPrYUdy7
Old:   https://goo.gl/maps/dZPWX8Wrsnt7wKEY6.


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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #8 on: October 18, 2022, 04:36:27 PM »

I can give you one big disadvantage: bridges with straight walls are magnets for vandalism.

https://goo.gl/maps/Xd5gA6krJba8g9hN7
https://goo.gl/maps/dtPvQAdz74Q2bNsW6
https://goo.gl/maps/tRf1q3n33QKJL88y7
https://goo.gl/maps/Qs6o2A4aTm19mdQ5A

There's a bonus sign goof on the last one if you can spot it.
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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #9 on: October 18, 2022, 04:39:14 PM »

I can give you one big disadvantage: bridges with straight walls are magnets for vandalism.
But they take away a place for homeless people to hunker down.
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kphoger

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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #10 on: October 18, 2022, 04:41:25 PM »

I can give you one big disadvantage: bridges with straight walls are magnets for vandalism.

I see plenty of graffiti on slanted ones too.
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roadman65

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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #11 on: October 18, 2022, 04:58:33 PM »

I can give you one big disadvantage: bridges with straight walls are magnets for vandalism.
But they take away a place for homeless people to hunker down.

Orlando did that by installing facades over the side piers to make the overpass look like itís been built with a straight wall on the. SR 408 bridges over local streets.

https://goo.gl/maps/fjie9SpUBWwsHBv46

Behind the walls are piers and a slant wall. You would never know it if you didnít see the bridge before the walls installed.

https://goo.gl/maps/XBvWkX9wwFXZStoB6
Edit, found an old GSV showing conversion.
« Last Edit: October 18, 2022, 05:02:10 PM by roadman65 »
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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #12 on: October 22, 2022, 06:45:46 PM »

I can give you one big disadvantage: bridges with straight walls are magnets for vandalism.

https://goo.gl/maps/Xd5gA6krJba8g9hN7
https://goo.gl/maps/dtPvQAdz74Q2bNsW6
https://goo.gl/maps/tRf1q3n33QKJL88y7
https://goo.gl/maps/Qs6o2A4aTm19mdQ5A

Yeah, I'm going to say there's a few other elements at play here that contribute to the graffiti.
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csw

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Re: Straight Wall over Slant Wall Advantages/ Disadvantages
« Reply #13 on: October 22, 2022, 08:24:46 PM »

"Straight wall" = MSE (mechanically-stabilized earth) wall. In the simplest terms, dirt is cheaper than concrete. So it's cheaper to construct an MSE wall for an abutment than a concrete pier and additional bridge span.
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