If the local congressman is earmarking it anyway, your shortline would probably spring for a precast concrete ballasted approach with a prestressed beam ballasted deck for the long span(s), assuming you aren't in seismic country, which is a whole other animal and the bridge might be cast-in place. This is current technology and there's lots of info out there. If you know someone with the AREMA or individual company bridge book, that would be a start.
Have fun.
There are two mainlines in the area (Beaver County, PA : The former P&LE and the former PRR/PFtW&C. All the bridges I know of ballasted deck, and they are are the same ones that were installed in ~1909 and ~1926 respectively. The only ones that may be open deck are P&LE's huge Ohio river bridge in Beaver and the PRR's bridge from Rochester to Bridgewater - and I emphasize may.
So, in this neck of the woods on two pretty major roads, ballasted decks are used almost exclusively. I should add that the PRR used mainly deck girder bridges (little above grade except parapets, even with four tracks) while the P&LE used multiple plate girders (for example, a four track line would run through five girders. Exceptions to both cases exist.
KL
tomikawaTT wrote: Shawnee,Quick story to justify a nice, new ballasted deck bridge on a not-too-wealthy short line.The town just widened the road, and sprung for the new bridge to replace the old deck girder on wood trestle bents. If you leave some remnants of th old bridge, it should work.Using ballast former sectional track is easier, but would give you a girder-to-railhead thickness that's a bit excessive. I'd go for a faux 'concrete' deck to cover up about half of the built-in ballast.Chuck (modeling Central Japan in September, 1964)
Shawnee,
Quick story to justify a nice, new ballasted deck bridge on a not-too-wealthy short line.
The town just widened the road, and sprung for the new bridge to replace the old deck girder on wood trestle bents. If you leave some remnants of th old bridge, it should work.
Using ballast former sectional track is easier, but would give you a girder-to-railhead thickness that's a bit excessive. I'd go for a faux 'concrete' deck to cover up about half of the built-in ballast.
Chuck (modeling Central Japan in September, 1964)
I also like the story...
I once planned a layout for a pretty small space which was an old line from end to end cut in the middle by a new highway with median strip and a new line just opened on a bridge clear of the new road.
I went for an old low clearance bridge having been taken out but I could have gone for a grade crossing.
Similarly the new highway could be completely new build or an upgrade/widening of an existing road. The thing that I liked about both scenarios was that it gave all sorts of excuses for construction equipment on and around the new road.
I guess that (if you like lts of bridges) you could have an existing road bridged (low clearance) by the old line next to a newly cut new highway. IIRC one of my ideas was to use such an old bridge for rail hoppers to drop aggregate into dump trucks for the ongoing work on the highway. This assumed that the new line and bridge were carrying rail traffic and that the old bridge had ended up on the end of a stub spur.
Something else that was going on in these ideas was that all the local industries that had been served by the old lower level line were still being served. Now though they were being served via secondary track instead of the main. In some cases this caused access to be by a switchback route.
There has recently been a thread about "thinking about operations before building". One of the things with a scheme like the one I was working on is that (if you have the space) you can work the layout for ops with movements into/out of the industries in detail - keeping out of the way of through movements on the main - or you can sit back and watch the trains roll around the main / over the new bridge --- or a mix.
Of course, if you want, you can drop the industries below the level of the old main... raise them above it... or both.
You can - if you want to be crazy - add a river to bridge...
These ideas are really a modern working of the much older idea of a grade seperation.
If I had gone with a grade crossing between an older road and the old rail route I guess that I could have used the track that had become a dead end to deliver concrete sections (like median strip barriers) to the construction site...
There would also be interesting issues about how the new routes (both road and rail) cut through existing properties and what was done to provide cuts and fills in (possibly) limited/cluttered spaces...
Great answers Chuck
Being really "detailed"... one thing that people don't look at so much with regard to open or decked bridges is what the track is doing.
Elsewhere I've posted about the whole mix of rail, ties and ballast being involved with drainage and supporting/spreading the loads. More significantly here is the movement of ballast which provides both an element of cushioning and facilitates traction.
Trying to put this briefly...
Well, just think of the weight of a loco (followed by train) rolling over you. If you are rigid [with fright maybe?] it will be like a hammer blow smashing into something solid. This impacts on both the solid and the hammer...and back through the shaft etc. If there is some flex both elemets suffer less. In open deck bridges this is why there is usually a timber longitudinally or cross ties between the rail and the girders. (Yes a steel brisge does flex... but the timber softens out and spreads the load hitting the bridge... it also makes a great place for water to hold between the wood and the steel potentially giving more oportunity for the bridge to rot).
Ballast helps traction in the same way that it's easier to walk on ordinary (slighlty giving) ground than hard ice. If the rail dips by a tiny fraction the wheels get something (however slight) to push off against. If the rail doesn't shift at all the steel wheel can spin on it more easily.
The reverse happens when a train is braking. Without slight movement in the rail/track it is easier for the wheels to slide (like a dog trying to stop on a rug on a polished floor). The small element of movement in ballasted track gives enough "dig in" to help stop the train.
If we put these ideas together into a bridge we can rapidly see that a ballasted deck bridge will have huge advantages over an open bridge.
For a start the bridge becomes much less of an interruption in what is going on in the track as a whole. There will be a difference between the track on the ballasted bridge and other track but probably little more than other variations not caused by bridges. Against this an open bridge will be very different and the transition on and off the bridge will be significant. The transitions will also be confined in area.
This goes to the issues of first cost and lower maintenance costs already mentioned. It also goes to the question of longevity of a bridge - and the trains.
Where train speeds are high maintaining consistancy/continuity in track conditions also becomes important (more significant). I would guess that you will find more ballasted bridges on routes that have been used for fast passnger traffic.
I would guess (as I have no experience of open deck bridges) that the noises of train/bridge would vary significantly. This would be indicative of some of the forces being transmitted. In the late 80s / early 90s it might also be a factor in the county stumping up some if not all of the cash in a bridge repalcement. A quieter bridge would improve a neighbourhood whether it was residential, commercial or industrial.
So, some of the factors are:
When replacing a bridge a significant improvement also shows as a level of confidence/commitment to the RR's future. This may be significant for both the RR and the local community... so there is an element of politics and economics which goes way beyond the simple cost of the bridge.
I guess that the late 80s/early 90s would be a bit of a weird time for this "regeneration". A community would have to be getting into improvement pretty early to be thinking this way in the late 80s but, once you move into the 90s, you increasingly get "civic amenities", "quality of life" and then "green" factors working into the equation.
While this wanders pretty far from a simple %age of type of bridge asnwer I hope that it helps.
Chuck, thanks. Your reasons for the ballasted deck sum up the stuff I've read about their long-term advantages.
I'm modeling at the late 1980's/early 90's, the new "transition era" after the Staggers act, when railroad dereg began to take results (as an aside, an interesting transition time indeed...end of caboose era, mergers and acquisitions, reemergence of shortline enterprises, the time when a lot of shifting was happening in diesel motive power and in freight car types).
I'm using Unitrack as a base, and life would be simpler if i used a ballasted deck bridge, but am not sure it would be accurate for a backcountry shortline. I plan to have the the bridge over a mountain town road, a town of some commercial size of regional importance. The bridge is over a road connecting two halves of the town which, by necessity of track elevation and associated topography, are split. Trying to get some sense of whether a ballasted deck bridge would be out of place in this setting, whether a prototype shortline or class one branch would bother with one in a town such as this.
This is another of those, "When are you modeling?" questions.
Before WWII, the most common reason to go to a ballasted deck was to prevent things falling onto (or into) whatever was under the bridge. They became more common in new construction as their inherent advantages began to outweigh their first-cost disadvantage:
As a direct result, ballasted deck girders became the first choice for new construction. Now, as you noted, at least one Class 1 is retrofitting their old open decks, presumably to benefit from the list above.
(The original reason, keeping things from falling on whatever is under the bridge, is still valid.)
Just a quick question...how common are steel ballasted deck grider bridges, vs. steel open deck girder bridges? I know that the open deck variation is cheaper to build, and thus is more common on many prototypes. But I'm wondering just how common it is for the ballasted deck variety, what the percentages may be...are 30% of deck bridges ballasted, for instance?
And is there any general rules as to when a ballasted deck is used vs. and open deck?
It's also my understanding that ballasted deck bridges, while more expensive to build, are usually sturdier and require less maintenance, more cost-effective in the long-run than short run, and therefore there are more examples of them these days in class ones. For instance, I was reading how NS is converting a lot of their open deck bridges to ballasted deck. I just don't know the prevalence of use of ballasted deck bridges, and whether they are as relatively common as other types.