The few plate girder deck bridges that I’ve seen up close, the girders are spaced about the same as the rails of the track. That is, the rails appeared to be directly over each girder. This was on a standard gauge line. I’m planning to build some of these bridges for a narrow gauge line, and I need to know how wide the bridge should be. I.E., should the girders be positioned under the rails, or should they be farther apart?
TIA.
I would think there would be variations in design. Readily at hand is the plan on page 214 of Georgetown and the Loop on the Colorado and Southern. For the 30-foot deck girder sections, there are four girders. The outside girders are 3 feet tall and are 3 feet, 4 inches outside the center of the rails. (The center of the rails are also 3 feet, 4 inches apart, so the outside girders were 10 feet apart from each other.) The inside girders look about 1 foot to 15 inches tall and are directly under the rails (3-foot gauge). The ties are 12 feet long, overhanging the outside girders by 1 foot.
The girders do the job of stopping the train falling through into the river (or whatever) so you either want longitudinal girders (same way as the rail) under the rails or sufficient suitable cross members to distribute the load from the rails out to longitudinals set wider apart. Most times on a bridge there will be load bearing/distributing timbers between the rails/track and the bridge structure.
The best thing would be to go to somewhere like http://www.rrpicturearchives.net/ and hunt through for cab views of bridges. there are many variants… pick one that you like.
Ray
I recommend Model Railroad Bridges & Trestles: A Guide to Designing and Building Bridges for Your Layout by Bob Hayden
Also Bridge & Trestle Handbook by Paul Mallery
For a pure deck girder (no load-bearing floor beams) the girders are directly beneath the rails to prevent placing shear stress on the ties. On the other hand, a bridge that is more than 10-12 times as long as its width will be very unstable, especially if there are side thrusts from curved tracks or wind load.
The Georgetown Loop bridge sections mentioned above are on a sharp curve. They are actually more like the norm for a through girder, only narrower and entirely below the clearance line of the rolling stock used.
If the underside cannot be seen, a quick and dirty way to model a deck plate girder is to detail the visible side of a piece of solid wood (or use a styrene face on a piece of thick foam), paint the top and bottom black and install closely-spaced bridge ties. This also has the advantage of giving spikes too long for the ties a place to hide.
Chuck (who models a prototype that was a big user of deck plate girder bridges)
Deck girders bridges need the beams under the rails. It eliminates the shear loading of the ties. If your prototype uses 4 beams instead of the more normal two, each pair of beams is centered under the rail. On curves, the railroad sort-of averages the curve out (ie 6 inches to the inside on the ends and 6 inches to the outside in the middle)
Problem with models is we have a lot more curves than the prototype and a lot sharper but to keep them looking right, average the misalingment. And if it gets too far off, pick another type of bridge for thay part… the prototype was not shy about mixing two types of construction on a single bridge site.
… Former bridge engineer, Bob