On my layout in progess (HO), I am installing 2 plate girder bridges. I’ve seen them with ballast and without, what dictates whether they should be ballasted or not?
Hi!
This question has come up before, and there (IMHO) is no definitive answer. I suspect that each RR had their own guidelines on this, but even those that didn’t normally ballast the solid bridge decks ended up with ballast as the years went by. Those I recall seeing close up had ballast - not supporting the track/ties, but somewhat surrounding them.
In other words, the rails/ties were laid on the metal bridge, and ballast - either intentionally or not - ended up around the ties.
“Twer I were you”, I would lightly add some ballast on the bridge, and you won’t be off base.
I suspect ballast is only added to the decks of any kind of construction where the ballast can be rid of water below it. This means the ballast must be kept in place, but also allowed to ‘breath’. I don’t know if a steel mesh is used for this, or wood ties spaced appropriately,…but the road in question would have engineers figuring out the cost benefit for ballast vs. no ballast around steel.
Crandell
Normally, a plate girder bridge has the plates running about 1/2 way up the side of the track, and have large members running under the track. These are not ballasted. There are versions that have a ‘track pan’ that holds ballast and the track is laid on top of the ballast like normal track. The Atlas plate girder is sort of like this design.
The current preferred bridge construction is a deck or box girder design with a ballast pan. Even the newer segmental concrete designs use ballast. It makes fastening the track easier and the ‘feel’ is similar to running on normal grade. This also reduces the chance of a shifted load from hitting a bridge member.
Jim
You are referrring to two designs called open deck (bridge ties but no ballast) and closed deck (ballasted with regular ties). On the open deck you can see between the ties down to ground level (therefore, you can’t retain ballast).
Open decks with plate girder whether thru plate or deck plate are much more common, but there are some that are closed deck as you refer to. A closed deck has a couple of adavantages: It is easier to get a smooth transition on and off the bridge by leveling the ballast and it is more fireproof. With a closed deck design you do need some kind of trough to retain the ballast and this can be made with steel, concrete or timbers. Often some kind of drainage system is used for the water that accumulates. I have seen some new designs of thru plate and deck plate girder bridges that are closed deck. I suspect this is the trend though they shy away from thru plate girders as contemporary practice does not want anything to obstruct side clearance. Some modelers may be suprised to know that there are closed deck truss bridges out there too. Whether it be closed deck or open deck has nothing to do with the era. It is more of an engineering preference.
I am currently building a bridge that is a combination open and closed deck. There are three steel beam girders, (one of which spans a roadway) that are open deck and a closed deck concrete span over a creek. So therefore, this bridge has both open deck and closed deck. It is interesting to note that the concrete span is 100 years old but built as a closed deck. The trough is made from timbers. The steel beam girders date to 1930. However, the bridge is still a part of the NS mainline btw Detroit and St. Louis and holding up well.
Jeff Wilson’s recent book on bridge and tunnels published by Kalmbach gives a lot of great info and photos including how to modify an open deck kit to a closed deck design.
As you have noticed, there are two general types of decks, the open deck with bridge ties, and the ballasted deck where normal track ties and ballast are carried in a “pan” The pan can be either concrete of steel. For new construction, a ballasted deck is often the choice since subsequent track maintenance easier when the surfacing gang can simply work on through. But it costs a bit more up front, so sometimes the bean counters go only for the short term saving.
Except for highway grade separations, many of the railroad bridges are old with 100 year-old spans quite common. Sometimes an existing open deck bridge will get converted to a ballasted deck. Whether this can be done depends on the specific bridge, since a ballasted deck adds considerable dead load (weight) to the bridge. Some can handle it easily, others can if certain components are upgraded, and of course some can’t safely handle the extra weight. I should emphasize here that safety is relative and engineers calculate a huge factor of safety.
Where a bridge crosses a road there is now a very strong preference for a ballasted deck. It nearly eliminates the possibility of debris from passing trains or maintenance activity falling through the ties and hitting cars or pedestrians below.
There is a third type of closed deck, where the rails get bolted directly to the steel deck plate. No ballast required! It was probably never very common, and replacement was forced anywhere track circuits were installed.
John
I too studied this some time ago, when I scratchbuilt a small wooden timber bridge to span a creek. Seems that there were two factors that came into play years ago, at least, when considering construction and/or engineering practices for RR bridges: 1. cost and, 2. simplicity of construction. At least early on (pre- WW II probably) the preferred design for most railroads was open deck, plate deck bridges. Both considerations above came into play in that preference. These bridges had a set of cross members that connected and supported both sides pf the plate deck, then stringers (usually large timbers) that the ties rested on. The ties were bolted onto the stringers and no ballast. I can remember walking over these bridges as a kid. It was a bit scary looking down between the ties!
One consideration not yet mentioned - exactly what does the individual bridge span? In some cases, where the span is over a public street. a pedestrian crossing or an open channel carrying potentially potable water, closed deck construction may be mandated by local authorities who want anything dripping off or falling from the passing trains to stay up on the bridge floor.
To elaborate slightly - track construction on open-floor bridges is not the same as that on the approaches. Bridge ties are frequently longer and of larger cross-section than ‘laid in ballast’ ties, and are placed closer together. Rail laid on open deck bridges does not lend itself to easy realignment. Frequently, as the earthwork of the approach settled, there would be a noticeable bump at the bridge abutment.
OTOH, track laid on closed-deck bridges is no different from that laid on the approach fills, or through nearby cuts, with the possible exception of having guard rails laid between the running rails. It is treated exactly the same when track is being realigned, ballast is being cleaned, etc. Finally, the closed, ballasted deck is safer for track workers.
Closed deck bridges are designed to carry the somewhat higher static load of the ballast pan and ballast, but this is a difference that would be hard to notice in anything smaller than G scale.
Chuck (Modeling Central Japan in September, 1964 - with open-deck girder bridges)
Thank you for the great answers. But I forgot one other question. If it is ballasted, are guard rails required?
yep – the guard rails are there to keep a derailed car’s truck(s) facing the “right” way so as to not tear up the bridge.
Actually, whether or not there are guard rails, and how heavy they are, is somewhat railroad-specific. Some roads required guard rails on all bridges, some omitted them on bridges without above-track structures, some don’t require them at all.
Also, guard rails come in many weights. Close to running rail standard is common on many prototype bridges, but I can point to one example (a tall deck girder-on-masonry-pier viaduct) where the guard rails under the tripleheaded 2-8-2s appear barely suitable to support mine carts. Not only are they very light, they’re only spiked every fifth tie…
To approach that look, I’m going to have to use Code 40 guard rails between the Code 83 running rails on my HOj bridges.
Interestingly, the same prototype (JNR) used much heavier rail for guard rails through tunnels…
Chuck (Modeling Central Japan in September, 1964)
Note that in modeling there is another good reason to use smaller code rail for gurard rails. Track cleaning can rub the paint off on the guard rails over time if the rails are the same height of the track rails. Yes, if you paint your track, you should paint the entire guard rail–often a rustier color then the sides of your main track rails.
Reference prototype guard rails, Tomikawa is correct. This was railroad specfic–and I might add, sometimes era specific. Railroads today don’t use them as much as they used to as there is some disagreement among bridge engineers whether they are helpful or not. One thing for sure is that if they are used, they are used on bridges that have side members like truss and thru plates girders that could be damaged by cars that come off the track. Even in the classic era, some railroads that used them did not use them on lower height timber trestles and deck plate bridges less than a given length. Incidentally, there were a series of artcles in Railroad Model Craftsman a couple of years ago written by a genleman by the last name of Lambert that discussed this issue with photos and even referenced engineering material from different eras and railroads. He also discussed the length of guard rails on the approaches to bridges. I got the impression that guard rails are often made from hand-me-down rail (relay rail) and therefore usually of smaller pounds per yard than mainline rail on a bridge. I use code 70 for guard rails on my bridges equipped with code 83 rail and code 55 for bridges with code 70 rail.
A few more points about guard rails. CPR requires them only on bridges with a total length of more than 30 feet. If it is a ballasted deck with more than two tracks, only the two outer tracks require them. Generally the guard rails extend another 20-30 feet beyond the abutments, with various designs of taper. Mostly a pair of rails is used but I have seen places where three or more rails were used. Guard rails are also sometimes used in tunnels and at high retaining walls as well. Usually they are made from used rail that is no longer suitable for use in track.
Some railroads did not use guardrails, believing that the timber tie spacer (4x6"?) used on open deck bridges woul