To be somewhat prototypical, does a Truss or Plate girder bridge have to rest on the abutment shelf or can it be on the top of the abutment? Thank you in advance.
Prototypically, such bridges rest on a shelf, and are supported by what are known as "bridge shoes". The shoes are hinged pedestals with rounded 'feet' that allow the bridge to shift with temperature swings as the bridge deck lengthens and shrinks. The deck, itself, would be level with the rest of the approaches so that the rails maintained the grades at each end.
selector Prototypically, such bridges rest on a shelf, and are supported by what are known as "bridge shoes". The shoes are hinged pedestals with rounded 'feet' that allow the bridge to shift with temperature swings as the bridge deck lengthens and shrinks.
Prototypically, such bridges rest on a shelf, and are supported by what are known as "bridge shoes". The shoes are hinged pedestals with rounded 'feet' that allow the bridge to shift with temperature swings as the bridge deck lengthens and shrinks.
Well, hinges are good for vertical changes but metal plates are fine for horizontal expansions/contractions. You'll frequently see a combination of shoes and plates, or only metal plates where the span isn't great. This is demonstrated below. The first are of a multi-span girder bridge crossing a valley (Alhambra Valley, Martinez, CA) while the last is a single span crossing a two-lane road (Morello Avenue, Martinez) on the BNSF mainline.
Shoes at end of bridge:
Mid-span, no shoes:
Mid-span, combination:
Short girder, just plates at both ends, no shoes (only one end pictured):
Mark
Mark, Nice pics for use of weathering the bridge and abutment.
Modeling B&O- Chessie Bob K. www.ssmrc.org
bogp40 Mark, Nice pics for use of weathering the bridge and abutment.
Thanks. The diffused lighting conditions (in this case caused by nearby grass fires) during the taking of most of these pictures helped. .... Yeah, BNSF places little emphasis on surface maintenance of its bridges with the result of significant corrosion. Makes we wonder if the corrosion has gone far enough to have structural effects.
Some bridges have actual rollers on tbe opposite end from the fixed shoes, versus the sliding plates on these (excellently clear) photos. The Paul Mallory book on bridges, Bridge & Trestle handbook, from Carstens is still after all these years one of the best sources of good info and drawings on this topic.
Dave Nelson
willy6To be somewhat prototypical, does a Truss or Plate girder bridge have to rest on the abutment shelf or can it be on the top of the abutment?
I believe that the end spans of a bridge structure rest on the abutment shelf because the remaining vertical part of the abutment acts to hold back the earth, stone, rock, or whatever terrain that leads up to the bridge.
To look at this question a different way, typically there's a retaining wall at the top of an abutment to allow the subgrade and roadbed to be built right up to its back side, with the track running across the top of this wall and onto the bridge. What appears to be a shelf or step is actually the top of the main structure of the abutment constructed to support the bridge. The height of the retaining wall is equivalent to the depth of the bridge's deck structure plus the height of whatever shoes or pedestals the bridge rests on, so the top of the bridge deck matches the top of the retaining wall, at least approximately. Often the track will be on some sort of fill approaching the bridge, so the abutment will have wing retaining walls extending to either side to support the end of the fill. So long,
Andy
Andy Sperandeo MODEL RAILROADER Magazine
And with longer bridges the rails have to have their chances for expansion too! The bridge rails can move to the side and the land rails are fastened. It look similar to the points of a turnout.
Wolfgang
Pueblo & Salt Lake RR
Come to us http://www.westportterminal.de my videos my blog
Pardon the large file...
Here's an abutment on my N scale layout. You can see how I modeled the retaining wall structure to support the roadbed, with the abutment structure below. It was constructed with Evergreen sheet and strip styrene.
The original WM span over the highway in Ohiopyle was the first part of the Connellsville sub to be removed when the line was taken out of service. Actually it was before it was taken out of service... A farewell excursion had to be diverted to the B&O for most of the trip because the span was taken out. It's a good study of concrete leaching, anyway. In the picture you can clearly see the original abutment, and the newer concrete work that holds up the more recent pedestrian bridge.
At Ilchester Md. here's a detail of the pier between a plate girder through span and a through truss. Note the concrete piers that are built up to support the new track alignment (originally a double track line, now a single track centered on the bridge structure) Interesting how the bridge feet sit at different levels.
On this highway overpass, the bridge sits into a pocket, formed by the art deco pilasters on either side of the bridge. The feet appear to sit in recesses below the actual abutment deck.
Here's a detail of the bridge before I added the trees around it.
Hope this is helpful...
Lee
Route of the Alpha Jets www.wmrywesternlines.net
This short bridge on CN's Grimsby Sub illustrates Andy's answer:
The span is only long enough to accommodate a single lane of street traffic, plus a narrow sidewalk:
As you can see, at some point in the past, the railroad has raised their track by raising the original stone abutments (ca 1850s) with a poured concrete addition:
This photo shows the original stone retaining wall that sat atop the stone abutment (behind the newer concrete addition - note that its height is the same as that of the girder.) The new retaining wall is simply a couple of large timbers, held in place, in part, by the girders themselves:
Because of its short span, there appear to be no bridge shoes or rollers, although I'd guess that the girders would be sitting on a steel plate atop the concrete.
Wayne
On my layout I have the bridge sitting on the abutment. Why? Because the little 'feet' things that came with it made it to high for the track. May not be prototypical but who the heck is going to notice except a bridge engineer. Do whatever works best for you.
'there's something happening here, what it is ain't exactly clear' Modeling the Hard Knox Valley Railroad in HO scale http://photos.hardknoxvalley.com/
Can't you trim 1/8" off the bottom of the abutment so you can include the right detail? It's not brain science, you know...
Nope...too stupid.
mainetrainsOn my layout I have the bridge sitting on the abutment. Why? Because the little 'feet' things that came with it made it to high for the track. May not be prototypical but who the heck is going to notice except a bridge engineer.
Well, lots of model railroaders, who study such things, like engineering, will notice the missing bridge shoes.
I'm with the other poster--why couldn't you shorten the abutment by cutting it to fit?
You know, you guys are absolutely correct, I should have figured out a way to cut off some of the abutment. What would you suggest for cutting rock?!!!!!!!! Maybe a little stick of dynamite. Didn't realize I was building my layout to please the rivet counters. Wait a minute. I'll stick some trees there and then you won't be able to see my abject failure as a modeler. Maybe I should get out of the hobby altogether since it probably gives you some sleepless nights thinking a slipshod modeler as myself is contaiminating your hobby.
mainetrainsYou know, you guys are absolutely correct, I should have figured out a way to cut off some of the abutment. What would you suggest for cutting rock?!!!!!!!! Maybe a little stick of dynamite. Didn't realize I was building my layout to please the rivet counters. Wait a minute. I'll stick some trees there and then you won't be able to see my abject failure as a modeler. Maybe I should get out of the hobby altogether since it probably gives you some sleepless nights thinking a slipshod modeler as myself is contaiminating your hobby.
It's your point to live with your mistake.
My first trestle was without Nut-Bolt-Washers. I've added them later when I learned about them. It was a tricky job, gluing with CA those tiny NBWs flush. I was not able to drill all those holes.
With my newer bridges I add these parts at once.
And I've change the design from my latest trestle, because I made a mistake.
I thought the purpose of the thread was to show how bridge abutments work and offer suggestions as to how to model them accurately... You presented us with the situation on your layout, and I offered a suggestion as to how you could make it better. If you can't cut the abutments, you could trim out the wood underneath it to make room for the appropriate hardware. (Don't raise the bridge, lower the river!)
If you're content with what you've got, more power to you. I like your suggestion of adding some trees to camouflage this detail. I've used that trick myself plenty of times.
In fact, the bridge I showed doesn't have proper feet at all. I just built little stand-offs using bits of styrene to give the appearance of bridge feet.
Anyway, sorry if I lit your fuse... Tell me, how did you make the abutments? They look pretty good!
I would like to say I made them little rock by little rock, but I bought them from Walther's - part number 933-1041.
mainetrainsWait a minute. I'll stick some trees there and then you won't be able to see my abject failure as a modeler. Maybe I should get out of the hobby altogether since it probably gives you some sleepless nights thinking a slipshod modeler as myself is contaiminating your hobby.
The trees idea is a good one.
But looking back over the other posts I don't see anyplace where anyone said anything about abject failure, slipshod work, or hobby contamination. The question was asked how come you didn't just shorten the castings. (Okay, maybe we could find fault with the brain science part.) Part of the hobby is trying to learn how to improve things. The fact that the abutments are castings does make correcting a problem difficult, but possibly the individual who pointed out the lack of the feet could give a suggestion as to how to deal with the casting.
And so far as the feet go, the fact that they're missing is not as big an omission as including them and putting them in an incorrect location. I've seen some models where the feet are placed in a location where the bridge would fail.
A resin casting can be lopped of easily with a razor saw, a dremel tool, or if you have access to one, a band saw. that's how I shortened these:
You just have to take care to keep the bottom level.
wm3798 A resin casting can be lopped of easily with a razor saw, a dremel tool, or if you have access to one, a band saw.
Well, yes and no. You show a bridge pier. You can cut some off the bottom of one of these without affecting the top structure of the casting. With the abutment piece, cutting some off the bottom will correct the position of the shelf. However, the vertical wall behind the shelf will also be shortened and will not be able to (prototypically) hold back the approaching terrain. It would be necessary to build the wall up the same amount that was removed from the bottom. Don't ask me how I found this out.
So, being as it's a Walthers casting and a Walthers bridge, what we have here is a badly designed product!
wm3798what we have here is a badly designed product
No, not exactly. I went back and looked at the model picture. Even though I think that the original question asked if the bridge should sit on the top or on a shelf, I believe that the OP actually used bridge piers instead of abutments. If this is actually the case he could go ahead and trim off the pier bottom, but he'd still be missing that vertical wall that holds back the imagined earth.
Anyway, as I said above, better not to have the feet than to have them in the wrong place.