Ballast or open frames

[bshire284]

Hey folks. Long time no post. I'm modeling in the late 60's and earlier era and I need some bridges for a 220 ft. double track bridge span that typically starts (approach) at the exit of a tunnel. At the approach, I would like to use two 80' open deck bridges then onto some Warren trusses and exit onto the deck plate girders.. I'm undecided on building the OD 80'ers as a double track or 2 singles and thought I would get some opinions on this. Has anyone else used the 80' or shorter open decks as singles (open framework) on a double line or do they typically build them as doubles with ballast? This is my first attempt at using bridges on the layout so hope this makes sense and any help on this one is welcome and appreciated. I scratchbuilt some open decks and plate girders I will be using.

 

[Selector]

If the approach, through route, and exit are along a curve, a double-wide girder or truss might be used, depending on the degree of curvature, or they'd simply use a deck girder or a deck truss and probably take their pick of ballast or ties.

There really is no rationale for twin bridges unless you think one could fail and you'd like another option right beside it. Generally, if there's twin tracks, you would either gantlet the bridge passage or make the bridge a double-wide. Depends on budget. Two bridges are generally going to be somewhat more costly than a single that accommodates the requirement. A double requires double the cribbing and forms for abutments since you have to make four abutments instead of one larger one on each end of the crossing. A single requires fewer vertical and diagonal braces, whereas each of two bridges requires those members, plus you're doubling the cords over the top.

Even today, though, there are a great many bridges that are not ballasted. I can show you photos taken in the past 11 years of two different CN bridges in the Thompson River Valley in southern British Columbia as examples.

This first image is at Lytton, a town completely destroyed by a wild fire this past July. Nothing much more than a few chimneys and puffs of smoke when they went back to look. The bridge didn't do well.

This bridge is about 90 km east along the river valley, just a few minutes east of Ashcroft.

 

[PowrCab]

I'd only like to add that, whatever you do, if it's plausible, looks right, that's all that matters..Unless you are depicting real scenes like so many clubs do via copying photos of them, it's your RR..Do what you think it needs for ANY place along the line and don't worry about being judged..
You can change it later, too, if it doesn't seem to work...What you have there look fine !...Just put 'em in and fuggeddaboudit !!

 

[cv_acr]

There really is no rationale for twin bridges unless you think one could fail and you'd like another option right beside it. Generally, if there's twin tracks, you would either gantlet the bridge passage or make the bridge a double-wide. Depends on budget. Two bridges are generally going to be somewhat more costly than a single that accommodates the requirement. A double requires double the cribbing and forms for abutments since you have to make four abutments instead of one larger one on each end of the crossing. A single requires fewer vertical and diagonal braces, whereas each of two bridges requires those members, plus you're doubling the cords over the top.

Gonna kind of disagree here.

If you have double track and you need a double track bridge, well, you need a double track bridge. You can single-track the bridge to reduce engineering and cost, but then you have a bottleneck in otherwise double track territory (and this does happen realistically but when the bottlneck becomes too much of an operational issue, $$ will be spent to upgrade or "twin" the bridge).

You'll pretty much need the same number of girders per span either way, and the same support structure (both girder spans can be supported on the same piers - you definitely DON'T need to make "four abutments instead of one larger one at each end"). A "single" double track [under track] deck-girder span will still have four girders to support the tracks, and will basically indeed just be two side-by-side bridges. The Ties have to be on something and can't just hang in mid-air over light diagonal bracing between the outside girders. Each track needs to be evenly supported. A through-girder bridge can reduce the number of main support girders by sharing the central one. For example: https://railfan.ca/cgi/viewer.cgi?2003-07-25.3944.Guelph.jpg

 

[Selector]

The matter depends on the engineering, and the engineering reacts to the requirement. Two bridges will always be more costly than one, no matter of what type, and that is because, as you show, while some common elements can be shared, some cannot. If you twin a truss, for example, you must make two complete trusses, with all their chords, stringers, braces, and tensioners, and shoes. If they're the same height and width, because of the twin gauge loading, you'll have to make two complete sets of everything (except possibly sharing one abutment), making the enterprise that much more costly.

 

[bshire284]

Thanks for the help on this fellas. I'm going to use the open decks and add a plate and ballast them. I forgot to mention this bridge is on a slight radius. Since the track comes out of the tunnel portal at an angle, this puts the bridge ends and support shoes at an angle also. How would you go about setting these on a pier? With this style of open framework, would they make the pier wide enough to support the offset of the four support shoes of both bridges or would they shorten the panels to align the shoes?

 

[Selector]

You would either lengthen one of the girders or stringers, or you'd engineer and craft the abutment so that it can safely bear the bridge shoes if the two girders must be the same length. This would mean an angled surface presented to the gap or chasm.

 

[bshire284]

Does anyone know if the Micro Eng. tall steel viaduct kits allow for or explain building them for a double track bridge? Thanks again!!

 

[cv_acr]

Does anyone know if the Micro Eng. tall steel viaduct kits allow for or explain building them for a double track bridge? Thanks again!!

The trestle towers in that kit are designed for a single track bridge. It would be a very extensive rework (basically scratchbuilding with some potential use of kit components for the box girder bracing).

I heavily kitbashed these short towers from one of these kits (unpainted here, so you can see the white parts which were made from raw styrene strip and sheet, and all the grey kit parts were modified to make a much shorter tower.

I don't know that I've ever seen a prototype example of a double track version of this type of steel trestle viaduct to use as an engineering example.

The trestle bents "might" look something like this if such a thing existed:

(Normal single track trestle bent example at left. "Possible" bent design for double track bridge at right.)

 

[cv_acr]

Or maybe, probably something like this, with heavy steel box girder support members, and a heavy steel support girder beam across the top of the tower to support the actual bridge girders:

You'd probably want to use beefier components than the ME viaduct kit though for the tower structure.

 

[bshire284]

Great, thanks. Found a pic where someone used what looks like ME viaducts for double track. Looks like it works out well for each track to have their individual viaduct supports.

 

[cv_acr]

Yep that works great. You can see that if the bridge was much taller, though, they'd have to either be spaced further apart (at the top) for the towers to have room (at the bottom), or combine them into a single support structure.