Cliff, great representation of what I tried to put into words. The sketch and model are spot on
Am I understanding correctly? With Parallel tracks, separation should be 7 inches center to center? And if thatās the case any railings should be 3.5 inches off center each side? Would this mean that if I went with a plate girder bridge it would need to be 1 foot 2 inches wide or 36cm wide?
Bill,
That sounds about right. However one thing to note is those calculations are meant for a straight line. If the bridge is built going into a curve or out of a curve you may want to widen it a bit if your on a tighter radii or running longer equipment.
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Sean, hereās what I was thinking of rails, assuming itās hand-spiked onto wood ties. This is showing a 7" CtoC track separation.
The LGB track would replace both the rails and ties of course; but with this dual-track thing, the plastic ties wouldnāt meet at the centerā¦ The gap between them might look odd, and the center joist would probably go awayā¦ Thoughts?
BTW, maybe you could post photos of your jig?
Thanks very much, Bob.
Yes, if 7" is the proper number. I tried to find where I read that, but couldnāt. But hereās an NMRA doc on clearances.
Worst case side clearance (dim A) is on page 6; for āmodern eraā F scale, they say 5.32". Worst case center to center for same is Smin, 14ā / 20.3 = 8.28. So, 5.32+8.28+5.32 = ~19".
Which seems a bit much!
FWIW, and this is from a discussion on another forum,
I too have 10 foot radius curves and model in Fn3. ā¦ I found the overhang with a Connie even on these curves to be wide enough that I use 4.5 inches (11 cm) clearance from track centrelineā¦ This clearance is more than is really required on straight sections. Brian quoted 7 inches which I believe is sufficient for all F scale equipment even standard gauge, but it is a bare minimum as it corresponds to just under 12 feet in prototypical width.
This 12ā proto separation corresponds with the NMRAās number for āOld-Time/Narrow Gauge.ā If youāre good with that, and have no curves involved (as Rooster pointed out), then youāre back to 14" total witch I think.
If you use a deck girder bridge approach, where the girders are beneath the ties, the channel could be much narrower since its walls support the rails (and the ties extend beyond).
You could then use two parallel ~2x2 channels, perhaps screwed to each other with 5" wide spacers of some sort. I got the below figure from here.
Hereās another drawing from here.
Jerry B and others have used aluminum channel, and bonded styrene or brass angles to it for the ābracingā.
I was thinking of sectional track
Understood, but that leads to this gap I was referring to.
I suppose you could put in 1/2" x ~1/4" planks between the ties, to span the center and get the outer walkway back.
[edit to show the planks]
Recognize the fact that no railroad in north America, and perhaps the world ever had a double track wooden trestle. I shake my head every time someone builds one. the dynamics of a normal train passing over trestles causes them to shake, now imagine another train going the other way. the dynamics would cause the trestle to collapse IMHO.
In 1:20 i have been using 7.5"
AL P.
I concur, Iāve tried a number of times to find a prototype for one, and you bring up a good reason why we donāt see them. Other bridge types sure; but not trestles.
However, for Billās modelling purposes, a two-track bridge is clearly in order. As to type selection, itās his layoutā¦
FWIW I like the deck-style plate girder, because in selecting the thickness you can make it much stronger than a stick-built trestle, as long as the piers are stout. Like, concrete pylons, which is what many of them are / were. This one was in our neck of the woods:
Interesting that one end of the main span was on rollers, I presume for differential expansion/contraction.
This and two detail sheets are here.
Interesting that you brought this up As I noted years back that rollers are used. And you are right on the movement issue.
While rollers are used to allow for movement due to thermal expansion and contraction, they are typically only used on one end of a bridge span. One end is usually fixed to prevent longitudinal movement, while the other end is supported by rollers to accommodate expansion and contraction. In multi-span bridges, rollers may be used at both abutments, with fixed supports at the piers
Hereās a more detailed explanation:
- Fixed End:
One end of the bridge is typically fixed to the abutment. This provides stability and prevents the bridge from shifting due to forces like wind or traffic
Roller End:
The other end of the bridge is supported by rollers, allowing it to move horizontally. This movement is crucial to accommodate thermal expansion and contraction, preventing stress buildup in the bridge structure
Multi-Span Bridges:
In longer bridges with multiple spans, the piers supporting the spans in the middle of the bridge may also have roller supports, especially if the spans are long and subject to significant thermal movement
Integral Abutments:
Some modern bridges utilize integral abutments, which are designed to allow for movement without the use of bearings or rollers. This approach can reduce maintenance costs
search using the following for pics and the unwanted need to view every bridge you go under to see what type they used
types of expansion bearing in bridge
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Fascinating, thanks Hollywood.
So you make me wonder if the issue might affect a long aluminum channel for a garden RR bridge.
According to this site, a 10ā channel, experiencing an 80F temperature swing, with 0.0000128 being the coefficent of thermal expansion for aluminum, would be about 1/8".
At our scale, rather than rollers, a piece of UHMW sheet on one end would let it slide some. Maybe combined with slotted holes + screws, or some other guide. Otherwise, if both ends were firmly attached to a pier, said piers would be rocked slightly back and forth.
Or attach the channel to the piers with well nuts.
Thank you Al, I wasnāt gonna say it cause Iām usually incorrect. However there are steel versions out there and even on curves that go into a tunnel.
Since everyone is google expert here I will just suggest looking into the Garwood Trestle in WV.
However if you want to see it on you tube look here
I will also say that Iām very surprised Bill didnāt dig this up as heās good at resurrections!
Then you could really send Bill down a rabbit hole down the old Enola Low grade line in the Safe Harbor/Martic Forge area.
Now for the detail oriented guys here is one of my own videos.
Nice citation, Rooster. However, those look like deck plate girder bridges that land on steel piers.
Not a trestle.
If you say so governor!
Wellā¦ maybe Iām misunderstanding ātrestle.ā A series of piers, with bridges between them, might be considered a trestle, but that would surprise me. Iām no bridge expert though!