When a bridge sits on the lower shelf of an abutment, the area that the tracks go is too low for the actual level needed to traverse the area with rails. What do you use to raise the level of the roadbed? Cork, plastic, wood, levitation, combination of the above?
While levitation would be neat I always raise abutment to join with the track. If you are only talking about a 1/32 inch or so put some 'bridge feet/pads' under the bridge to raise the track. This is prototypical and solves the problem. I use small plastic shims for this purpose. I don't know what kind of bridge you are using, but virtually all steel bridges have 'shoes.' Jim
Sounds like the bridge toolowitis syndrome which can be corrected by properly applied levitational methodology covered in the Zen Master Seven Series. Having not completed that level I would suggest going with nscaller44's idea of shimming. Evergreen Styrene strips come in thicknesses from .010 to .100 and widths of .010 to .250 and can be cut to length and layered for the right fit.
Unless you have ballasted deck, you raise the bridge. Now mind you, a lot of model bridges have nowhere near the proper 'undercarriage' of the prototype. There have to be stringers and crossmembers under the ties of any bridge, including a ballasted deck. You may not be as far off as you think. The typical mistake you see time and time again is the Atlas-style snap-track truss bridge with no visible means of support under the track. That bridge could not support itself in real life. The stringers go under the track, and the truss structure ties into those stringers and crossmembers, not the end of the ties. So the track is always higher through the structure. Same with girder bridges; any through structure. The big deal with a ballasted deck is that you can align the bridge track for superelevation and cross-level, so that usually means there are no speed restrictions on a ballasted structure.
I am about to start construction of a ballasted deck girder bridge. From looking at prototype photos, the dec of the bridge is level with the top of the roadbed leading up to the bridge. Therefore, I will continue the cork roadbed right across the bridge uninterupted and then lay the track on that. Ballast across the bridge will also be the same as on the track leading up to the bridge. Jamie
As others have suggested, raise the bridge. If the bridge looks too flimsy to support itself, glue some Plastruct I-beams longitudinally under each rail, as in the prototype. Ok, the prototype didn't actually glue them. The size of the I-beams depends on the length of the span, and whatever vertical structure supports the span. The I-beams could be as deep as 6 feet for a single span, or as small as 2.5 feet deep for a long span truss with an overhead structure such as an arch, or semi-arch. I thought tracks were ballasted only on bridges with very short spans between supports, such as viaducts built with stone or concrete or, rarely, trestles that were not very high and slated for fill.
It's a nasty trade-off. If you put ballast on a span, you greatly increase the dead load, needing a bigger bridge. But because you have that 'cushion' of ballast to spread the load impacts, and can tweak the track for alignment, your load impacts are lessened. So you end up with a beefier, faster bridge. My own favorite is on the upper end of that scale - Canyon Diablo. Massive, massive steel cantilever, two tracks, 640' span.... 90mph speed limit. Beautiful trackside shots here. http://www.chasingsteel.com/blog/2010/8/25/i-finally-met-the-devil.html I never thought there was such a thing as a ballasted deck truss until I found the plans for our own local bridge and discovered the reason it is so beefy is that it had been originally designed as a ballasted span. If there's some other reason that you don't get the 'bang for the buck' and get the speed out of it, it's probably a normal bridge. Far more common. I think, but don't know, that it's a lot more common in the west than northeast. I think it may be related to corrosion problems, maybe not. Neither PRR nor Erie in our region had any of them on the local lines.
I raise the level of the bridge to the level of the subroadbed, then lay the cork straight across the bridge. Here's my "5th crossing of Tehachapi Creek" bridge: compared to the prototype: I need to re-take that picture w/ my Kato SD75M, Athearn FP45, and at lower camera angle now that I have my backdrop painted. My handrails are the old SP-style, when I took the proto photo UP had unfortunately taken over and replaced them w/ all steel....
When installing any single span N-scale bridge, either ballasted or un-ballasted, the last thing to go in (other than the scenery) are the abutments. I've got 19 bridges to install (two of which are now installed) on my modular layout of Echo and Weber Canyons in Utah on the U.P.'s Ogden to Wahsatch helper district. Multiple span bridges are girder-Warren Truss-girder, or girder-girder combinations, and none of them are ballasted in my 1951 time slot. What I've done with my two little girder spans over the Lincoln Highway at Echo was to build the bridges first, glue the bridge feet onto them, and make sure the pads on the feet were even (by sliding them back and forth on a sheet of 220 grid sandpaper taped to my workbench...gently). I then installed the bridges, sans abutments by installing rail-joiners and soldering them to the existing mainline on either side of the cut. After running a few trains on them and watching to make sure there was no excessive unevenness (even the prototype bridges are slightly uneven), I slid the back portion of my "concrete" abutments (made from 1/4" tempered hardboard) behind the bridge feet up to the bottoms of the ties on the mainline trackage, and glued them to my carefully placed track risers supporting the mainline over the bridge cut. I then slid my pre-shaped and carved abutment sections that the bridge feet sit on up to and just touching the bridge feet to check the "look" and function of the parts, then I applied a thin layer of 5 minute epoxy to the backs of these parts and slide them up, one at a time, until they are again, just touching the bridges feet, and I held each one in place until the epoxy had cured enough to hold it in place. After I was done, the bridges are rock-solid, with the feet actually supporting the span. Now I just need to finish adding the rest of the convoluted concrete abutment work on these bridges, paint and weather the concrete...and I'm done with these two little scratch built bridges. What assisted me greatly were my trips to the actual location to photograph, in detail, how U.P. installs their bridges and include such details as how long the guard rails are, what type of tip is on them, what the tie lengths are at the ends of the bridges and how do the ties sit on the top part of the abutments. Of course, many of the details I'm interested in may not be applicable for ballasted deck bridges, but a trip to document a prototype bridge similar to the one you're installing would be advantageous to you, and answer additional questions I am sure you will have. Cheers! Bob Gilmore
Dave! I love it when a modeler actually models a specific prototype. You've even got the dead tree in the right spot in the cut! I thought I was the only one who modeled specific trees along my mainline! Very nice indeed! Bob Gilmore
Dave, That looks like heavy steel girders topped by a thick layer of concrete (probably post-stressed) with ballast on top. Interesting!
