Triple track curved bridge: N scale

I need to build two bridges on a curve that are triple track. Track is Peco code 55 flex. I plan on just spreading ballast over the plastic base of the spans, no cork roadbed.

One is short, around 6" in length, not a huge problem. The other is 16" in length that will have a offset center support. I plan on using 1/8" black ABS plastic for a base and custom extended length sections of Peco truss girder bridge sides (NB-38) for the upper support. Not any prize winner, it surely won't make the cover of MR.

Attached are photos of the two bridges from the side and aerial shots with the longer span with track in place. The trusses you see have not been lengthened. Nothing is attached or secured, just placed there for a overall idea what it would look like. That curved turnout is a approximate location on the short span. I'm also think since I have ample of vertical clearance of adding I beams or some type of narrow channel below. Also, that white center support will be changed due to lack of side clearance. It was a piece of 3/8" ply scrap left over from the table.

One question; will running traffic over a plastic base as what I plan be noisy w/o any cork roadbed?

Questions? Suggestions?

 

Just a bit more noise. The noise is actually quite prototypical. If you ever notice a real train leaving ballasted track to cross a bridge you will realize the sound difference. On your railroad the noise will be no big deal. I like the triple track. Jim

 

You do have a point there. I am mostly concerned about appearance, especially with the longer span since it is most noticeable. It is indeed needing more then just those upper trusses.

 

Most of your situations look more appropriate for a deck truss rather than a through truss. Given the choice (and if the clearances are there) a railroad will always opt for a deck design - because of the lack of obstructions. That's really true on a curve design. I've never seen a curved through truss bridge, lots of curved deck trusses (series of short spans). I suppose they exist. If the span is reachable with a deck girder that's the first choice, if the span is longer it may be a deck truss. You need 22' top of rail to bottom of bridge for modern clearance, if you've got it, go to a deck design. If you don't, then it's a through design.

 

Randy is correct. Ditch the through truss bridge concept and use deck girder bridge pieces. I just answered the noise question, but this 'bridge' did bother me and here I am back again. Through truss bridges would not have been build as curved bridges. Steal from the smelters could be bent of course, but the strains and weight of locos and rolling stock would soon take its toll on any curved beam. If you just have to use through trusses then they will have to be cut at angles to make the curve. They will look odd at best. Good luck, Jim

 

I fully understand all of that, but what I showed was all I could find without buying whole bridges and cutting them apart which I didn't want to do.
Are there any full side panels available for through truss?

BTW, the actual 'bridge' is not curved, just the track. The sides are straight, with a angle offset from center to match with that offset support.
The shorter bridge I would think should not be a appearance problem.

 

Micro engineering makes 40' and 80' plate girders, 4 to a pack for less than $6. I think this is what was used in the bridge on the Maria's Pass layout shown in the current N Scale Railroading (May-June). If you have this mag you can see how the bridge was made. It's pretty easy and I don't think noise would be a problem.

 

I just did a search and Fifer Hobby has both the 40' and 80' girders. Hope this helps.

 

Having done a fair amount of bridge research for my own layout, I agree with the above. Your six inch bridge is a no-brainer with 80 foot girders. You just need two, one for each side of the bridge.

The longer bridge is more problematic but still calls for girders in a deck bridge configuration. From the photos is looks like you have tight side clearances. Use
one of your trusses to construct a pier. Steel piers are narrower than concrete and using steel is something that realroads would do in areas with tight clearances like yours.

It looks like you have enough vertical clearance to go with an 80 foot and a 120 foot span. For the 120 you need girders with a deeper chord than the 80 footers, probably 50% deeper or more. You could bash together one such girder from 3 80-footers, or make one up with sheet and angle styrene.

Another option would be to move some track a bit and use two 80 and one 40 footer with two piers.

 

I just remembered that I had two single track Atlas high truss bridges I couldn't use, left over from my 1st layout 15+ years ago.
I have already cut them apart, separating the sides from the base and top. They are long enough. All I will need is something for the top cross supports and some support where the base of those trusses meets that plate I have cut for the bottom of the bridge.

 

Well Bruce, as if you didn't get enough opinions (with most of which I concur), here's another one.
My guess is that the railroad would build separate structures rather than a super wide deck in a situation like this, on a curve, especially if the tracks were not all laid at the same time. I would probably go with double track bridges on the two outer tracks and single track bridges on the inner line (siding?) this would require pulling the inner track in a little. I'm thinking the shorter spans should be deck bridges because you have plenty of vertical clearance, but the longer span (~110'?) should have a much deeper deck and could justify through deck or even a through truss, especially on the double track. Or you could just wing it ...
Have fun, Otto

 

OC Engineer JD; Nice, but it would have to be angled on both ends one side narrower than the other and that is well beyond by experience level.

For a plate girder span, elsewhere this was sugested, but it is HO which I would think would be out of proportion;
http://www.valleymodeltrains.com/ca...cts_id=8565&osCsid=ioso4adhn4bllk7b59h8dmjop2

Ok, how about this. From a Atlas single track truss bridge. The pieces are just 'sitting' there, they would have to be cut down in length.
Though, I don't know what to do about the top cross supports.

 

The SP trestle at Redding, CA shows the basic design elements and logic of similar curved spans.

If you can space the support members close enough for a deck girder, and have the clearance, its a deck girder. If the span gets wide enough, it's a deck truss.

http://www.flickr.com/photos/80651083@N00/97380774/

And if you don't have any clearance, and its a long span, it's a through truss:

http://bridgehunter.com/pa/jefferson/bh42303/

I did a design like the Redding bridge on my first N layout, scratchbuilt the entire thing out of Plastruct. I needed a two-track curved bridge, a long one; 13" and 11" radius around about 90-degrees. Through girder over the tracks underneath, deck girder on the rest. It looked a little wierd, but it did the job. Make the deck out of thick styrene and determine your clearance points, glue the track down, glue the girders down, etc. Hey, it's just plastic. Get a Zona saw and some clue and start hacking and fitting. Between Model Engineering Works and some of the plastic truss bridge kits you probably don't need to scratchbuild much except the piers.

The girders I made out of that have re-emerged in several bridges since. That was a lot of girders!

 

Get one of these

http://www.google.com/products/cata...a=X&ei=I9DET6adLoWQ8wT2vMWrCw&ved=0CHkQ8wIwBA

Cut the sides off and make your own floor out of styrene. You can get I beams and make it look realistic or just sheet and get the railroad running. One day you can rip it out and put in a better version when you have time and money.

Ryan

 

I can't for either requirement per the photo.

That was my 2nd idea per the 2nd photos. Neither looks good due to having three tracks. I also can't do a double bridge and a single.

What do you think of using a HO scale girder per someones else's suggestion?

 

One thing we really need here to help.... measure from the lower track (top of rail surface) to the bottom of the track on the upper levels.

Remember that 22' is only just under 1 3/4" and that's for maximum stack train clearance. It sure looks to me like you've got plenty of space for a deck structure, but the ruler will settle that one pretty quickly.

Also measure that bridge with the pier in it as individual span lengths - inches are OK. Your center styrofoam pier can be replaced with a very narrow steel girder pier if necessary. Same thing with additional steel vertical beams as piers, doesn't take much, maybe have to tweak the track over just a hair.

To make sure I understand....there are two bridges here, right? The one with the pier and the one without it?

FWIW I do work for a railroad bridge engineering firm. "We have the technology". If you want this thing to hold up to scrutiny, and even a simulated load test, it can be done...!

 

Clearance is 2 3/8" as it stands if I use the 3/32" black ABS sitting on top of the rigid foamboard as I originally planned and showed in all photos so far.
I now cut an additional piece of 3/16" Plexi clear, since I thought it would be a better base for support reasons. If I use this 3/16", it will drop the clearance accordingly since I will have to lower it slightly due to the thickness difference of the two pieces.

Yes, there are two bridges. The 1st two photos are the shorter span, the others are this (not to scale);

 

OK, so the first logic check is to figure out if it is even possible to use a deck girder with a 120' clear span underneath.... that's right on the upper limits of steel bridge design for a girder, rather than a truss. Fairly rare.

Surprise, here's a great example. And a 120' span, I used an average of 9" on your sketch as a test. (9/12)*160=120'
http://www.flickr.com/photos/canfielddave/978077418/

Notice how deep it is. There's a direct relationship between span and depth on a plate girder.

So.... you need 22' of clearance; 22/160 = .1375*12 = 1.65"; figure 1 5/8" You have 2 3/8 clearance, so you have .725 clear, that is 116" or a 9'6" girder (....wow - .7125 inches, about 23/32) . OK.... Remember that the track support crossmembers are hidden inside, you only see the outside, so close enough.

In this situation (multiple tracks) you'd simply have additional girders in there (underneath) where you couldn't seem anyway, to compensate for width and load and they would all be latticed together. That's the beauty of using girders under a curved structure, you just add more girders. And its nice in a model because you only have to put under there what you can see.

But I still think this is within design limits and clearances for a deck girder.
http://www.trainweb.org/s-trains/davis/girder2.gif

Remember that the top edge of the girders is - or can be - right under the ties. And you don't really have to think about scale, just size, you need something around 9" long by 23/32 high. It will be harder to design the piers and the abutments than the bridge.

If the tracks were wide enough apart for clearances, you could even consider a through girder. Multiple-track, curved through girder bridges do exist - there's one six miles away from me here - 2 track, 3-span, with a center girder. I will tell you though that the railroad absolutely hates it, it's been a clearance nightmare since the 1920's. Whacking that center girder and the clearance problems are why railroads would do almost anything but that if they could figure another way. And I'll be interested in seeing if anybody can produce a picture of a truss bridge with a curve in the middle of the span - even a short one. I don't think I've ever seen one in the field or can recall one