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In Need of Curved HO Bridge - Ideas?

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In Need of Curved HO Bridge - Ideas?
Posted by kenben on Monday, October 7, 2019 6:17 PM

I have a 22" radi curved track that is elevated at 4" above the track below, which passes underneath it at an angle. The length of the curved bridge Iʻm looking for would need to be about 8" to 10". Ideas please???

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Posted by gmpullman on Monday, October 7, 2019 7:56 PM

You'd probably have to go with a skewed plate girder bridge, half-through type. Since the bridge would be straight and the track curved you would have to widen the distance between the girders to allow for the overhang of cars passing through the girders.

Since the track below passes at an angle you would want to offset (skew) the girders so as not to have too long of a span.

Central Valley makes some bridge girders that may work:

https://www.shop.cvmw.com/72ftPlateGirders-2-1903-1.htm

You could then scratch build a deck either using open beams or cheat a little and use a ballasted deck and simply make the deck out of solid styrene or thin modelers plywood.

Here's a link to a sketch of what I'm thinking of:

https://www.steelconstruction.info/File:R5_Fig8.png

 

Good Luck, Ed

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Posted by selector on Tuesday, October 8, 2019 1:58 AM

You can't curve a bridge deck, nor its supporting structures. You have to build it wide enough so that the ties supporting the rails, and the rails themselves, can curve along the widened deck and still meet the gauge loading.  Or, if you have an intervening supporting pier or pylon, or bent frame, you can run angled stringers that support the ties and rails.  You still curve the tie bed and rails, but the stringers supporting the ties can be lapped and angled between two outer pairs of stringers.  If this sounds like what you'd like to do, I can post a diagram.

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Posted by SPSOT fan on Tuesday, October 8, 2019 7:36 AM

Using a widened deck girder bridge like other have sudgusted seems to be an easy way. However, and  alternative is to use a wood trestle, that you make curves. I’ve seen a number of picture of similar items. Of course this approach requires you to scratchbuild the trestle.

Regards, Isaac

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Posted by Overmod on Tuesday, October 8, 2019 8:06 AM

In practice this is the sort of thing that any savvy railroad would replace with a fill ASAP (with a portal liner for the skew track).  This presupposes enough vertical clearance from the loading-gage limit on the bottom track that you have room for the drainage system under the ballast above the top of the portal liner.  A somewhat extreme example of this is the replacement for the failed Howe truss at Ashtabula over the years -- it may not be pretty, but it is HIGHLY unlikely to fail or even wash out significantly, and 'repair' is simple and straightforward.

If the clearance is restricted, you're looking at one of the bridge designs with relatively thin 'bottom chord' and deck structure.  This is a bit complicated by the combination of curve and skew; the lateral deck members in particular will need to be stronger while kept as 'shallow' as possible, an interesting problem in statics.  Note that this would not preclude you from having a ballasted deck under the track, perhaps built like the proposed ballasted-deck conversion for the Merchants Bridge in St. Louis.

This might be a place for a little 'drama' design-wise: consider adapting bowstring trusses for the vertical members.  It does occur to me that with a little tinkering of abutments (e.g., pockets for the physical hinge supports on a 'long' corner), if you have a span of 8 to 10 inches and not too radical a skew you might be able to build the bridge itself unskewed (there are precedents in actual railroad-bridge engineering for this) between actual hinge supports, with the track skew accommodated entirely under the wider span (with tight clearances on 'contrapositive' abutment corners)

BTW, if you want to see examples of how sharp curves are accommodated on bridge decks, look at some of the aerial photos of the aftermath of the Amtrak 502 Talgo wreck in Washington.  That's almost a textbook example of how you engineer the 'maximum speed' track curve for a given infrastructure situation.

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Posted by BATMAN on Wednesday, October 9, 2019 4:43 PM

I had a curve on a hill so built my own.

 

You just make it wider to accept the curved track.

Brent

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DrW
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Posted by DrW on Wednesday, October 9, 2019 7:18 PM

Curved viaducts are not uncommon in Europe, especially in the Alps. Mostly, they are made from stone and they are truly built in curved form; some have rather dramatic curve radii. If you don't mind the "European look", you can get models from Faller, Kibri, etc. (all sold by US dealers).

 

Image result for curved viaduct faller HO scale

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Posted by jjdamnit on Wednesday, October 9, 2019 7:21 PM

Hello All,

Have you considered a viaduct?

Because these structures are built of stone they can follow a curve. 

If the track below is a single line this would not be difficult to model.

That being said, steel bridges can be curved and even super-elevated.

I can think of several prototypical examples here in Colorado.

The first example of a curved span would be the Horseshoe Curve Bridge on the George Town Loop scenic railroad.

Yes, the structural members are straight but they are arranged to make a curve.

A second, and not so famous curved and super-elevated bridge in Colorado is on the abandoned line between Victor, Colorado and Highway 50; east of Cañon City, on the Phantom Canyon Road.

Just north of the town site of Adelaide, Colorado, there is a railway bridge that is now part of the roadway.

The bridge is a curved structure made from straight members. In addition to being curved it is also super-elevated.

When I was exploring the foundations of this bridge I discovered the bents are in a triangular configuration.

Rather than the bents being parallel to each other and perpendicular to the trackage the bents on the outer radius are on separate footings while the single bent on the inner radius is on a single footing. 

Also the outer bents are taller than the inner bents, thus creating a super-elevated bridge.

This triangular support configuration in steel could be replicated to accommodate a larger radius.

Hope this helps.

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by selector on Wednesday, October 9, 2019 11:57 PM

I said in my earlier post that you can't curve bridges that use stringers and pylons or intermediate piers. This means trusses, girders, and trestles.  Here is an image showing what I mean.  As I said, you CAN curve the rails, but you must widen the deck for clearances.

 

 

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Posted by SeeYou190 on Thursday, October 10, 2019 6:32 AM

On our layouts we do things the prototypes would not for various reasons and considerations.

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Even though curved bridges of some styles do not exist in the real world, reasonable curved bridges can be made for our model railroad layouts, and they might not look the best, but if they make a desired track arrangement work, so be it.

.

If you photograph them from the right angle, they will look OK, and no one needs to be the wiser.

.

-Kevin

.

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by Water Level Route on Thursday, October 10, 2019 8:26 AM

What is your era?  Would a curved trestle be appropriate?

Mike

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Posted by DSchmitt on Thursday, October 10, 2019 4:28 PM

Need to see scale plan and if roadbed/tracks built photos

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Posted by Track fiddler on Thursday, October 10, 2019 5:16 PM

Straight sections with a common angle and support underneath each section create a curved bridge prototypically for all railroad bridges.

Sometimes the width of the bridge has to be a little wider to accommodate that for side to side clearance.

 

 

 TF

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Posted by nwsisu on Friday, October 11, 2019 9:39 AM

If you have access to a 3D printer, you can design your own, or download a model of the internet, and modify it.

Or you can just design the bridge and pay a printing service to print it for you.

I designed and printed my own bridge. Looks ok I guess, but not too prototypical, I might redesign and replace it eventually.

3D-printed HO train bridge

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Posted by SeeYou190 on Friday, October 11, 2019 10:01 AM

nwsisu
Looks ok I guess, but not too prototypical, I might redesign and replace it eventually.

.

I like it. I have not tried to design anything for 3D printing yet, but that was a perfect project for it.

.

-Kevin

.

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by betamax on Saturday, October 26, 2019 7:12 PM

I was at an NMRA division workshop today, and the subject of the group build is bridges.

There was some discussion about various types of bridges, and it was pointed out that there is no such thing as a curved bridge.  A curve is created by using a series of straight segments, which join at a slight angle, and the track is laid as a curve on the bridge deck.  

We are lucky in that one of the members is a civil engineer who knows a lot about bridge constuction. He led a clinic at the last meeting on the issue of bridge constuction, and mentioned the 4Ds: Drainage, drainage, drainage and don't build on organic soil.  The last item you can deal with, the others are out of your control.

One member (also an engineer) is building a curved viaduct using ME viaduct kits, about seven in total. Following the prototype, it is on a curve about 450' long.  So he has to work out exactly how to position and join the various segments to create a deck for the curve.

To make it interesting, you build the bridge upside down, so the deck is true, building on top it that.

 

 

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Posted by doctorwayne on Saturday, October 26, 2019 10:06 PM

betamax

....One member (also an engineer) is building a curved viaduct using ME viaduct kits, about seven in total. Following the prototype, it is on a curve about 450' long.  So he has to work out exactly how to position and join the various segments to create a deck for the curve.

To make it interesting, you build the bridge upside down, so the deck is true, building on top it that.  

In the photo below, the taller bridge is Micro Engineering parts, and it was built upside-down on the cut-out 3/4" plywood roadbed which originally spanned that valley...

While it's only about 350' (HO) in length, using the pre-cut roadbed ensured that the curved bridge would fit properly.

...and likewise for this one, a combination of M.E. towers and deck girders, along   with Atlas deck trusses....

.....and it at least got some scenery around it....

The lower bridge, from the first photo, also used the cut-out roadbed as a pattern, but it was needed only for the curved portion at the high end....

The "concrete" piers supporting the bridge are different heights, due to both the terrain on which they rest and to the fact that like all of the bridges shown, the track is on a grade. 
I made a single multi-piece mould to cast all of the piers in Durabond 90 patching plaster, the mould meant to sit upside down for filling....

The plaster for each pier was poured to a height slightly taller than required for its particular position, then the base of it filed to the proper height.

Some jobs are more easily done working upside down.

Wayne

 

 

 

 

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Posted by betamax on Saturday, October 26, 2019 10:43 PM

Excellent work.  Demonstrates what our group build is trying to accomplish: a hybrid bridge using different components. The goal is to incorporate your structure into a small diorama to be entered at next year's regional convention.

I'm building a Hunterline trestle bridge for this project and not sure exactly how I want to combine it with another bridge. Since my layout is still in planning phase I don't want to make any commitments.

 

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Posted by doctorwayne on Sunday, October 27, 2019 2:10 PM

In response to the OP's original problem, I think that the limiting factor that would preclude a trestle is the lower track being on an angle relative to the curve of the upper track.  In other words, there's room for supports for the needed bridge only at the ends of the span - the lower track is in the way of any intermediate ones.

If kenben were to use the Central Valley girders suggested by Ed, and also skew them, they will provide the required 10" span.  I'd go with a solid deck using .060" sheet styrene, as it will allow a good solid bond with the CV parts using solvent-type cement.

Unless kenben is running high cubes or autoracks, 4" is more than enough clearance for trains on the lower track, so I'd place the girders so than only the top edges with the rounded ends protrude above the styrene deck - this will allow the bridge deck to be narrower than if the majority of the girders' height were above the deck.
Longer cars, with a wider swing of the ends, and overhang in the centre, should be high enough to not hit the girders' tops.

I'd also ballast the deck - makes the construction much simpler, albeit with less detail.

The lowest overhead clearance limit on the main line of my late '30s-era layout is 2.375".  Some equipment exceeds this limit, and is banned from that area - this was a deliberate decision in order to create manageable grades. 
I have other even-lower clearances within some industrial areas, and this was also done deliberately to enhance operations - if Joe Blow's Macaroni Stamping Plant can use only boxcars with an 8'6" inside height, that's what we'll deliver. Smile, Wink & Grin

Wayne

 

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Posted by Eilif on Tuesday, October 29, 2019 3:00 PM

The HO Model Railroading Handbook by Robert Schleicher has a good step-by-step showing how he used strait bridge segments to make a fairly long "curved" bridge in a fairly tight curve.  It's even on the cover.

 The third (last) edition was published in 1998 so some of the info is dated and it's aimed at the beginner/intermediate audience, but it's dirt cheap and I keep finding usefull information in it.

 

Another option.  I'm not sure it's still in production, but it shouldn't be hard to find the Atlas #790 "Flatcar Girder load" .  It was a pack of 4 bridge sides meant as loads on flats, but the four segments may be enough to cover the length you want. 

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