posted
I disagree. Would you want to be standing on the road with golfball sized rocks falling on your head from a few hundred feet up? Bad call on Amtrak's part for allowing it, and bad call on emergency services for not responding quicker.
As far as the guy mentioning money being wasted, I'll agree with you there....it's nothing of concern. That is, until someone gets bonked on the head, then it will cost them $$$$$$ in civil damages.
Posts: 2355 | From: Pleasanton, CA | Registered: Apr 2007
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posted
I've spoken with a few people in the industry about this and apparently "it happens". Maybe not to the extent that one of those pictures shows but certainly the odd rock falling from a bridge is common worldwide.
-------------------- Geoff M. Posts: 2426 | From: Apple Valley, CA | Registered: Sep 2000
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quote:Originally posted by Mike Smith: Why is there ballast on a bridge?
According to AREMA (pdf), three main reasons: 1. Better ride quality 2. Less issues with track height due to grading/levelling either side of the bridge if the track is reballasted/levelled, since it can be done on the bridge as well. 3. Cost and life of bridge.
On a side note I happened to watch a documentary the other day about the Medway bridge (in the UK) on the High Speed line (186mph). Firstly, the bridge sits on a low point so trains hit the level of the bridge with some considerable downward force that has to be countered. Secondly, if a train went into emergency braking whilst on the bridge, the momentum involved could topple the bridge in the direction of travel! Scary stuff.
-------------------- Geoff M. Posts: 2426 | From: Apple Valley, CA | Registered: Sep 2000
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posted
There are lots of bridges that have ballast on them. If anyone gets a look at the Northeast Corridor through Elizabeth NJ, all of the bridges save the one over the former Jersey Central main line are ballasted.
Posts: 566 | Registered: Mar 2002
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posted
Actually, you'll see more and more ballasted-deck bridges every year if you look carefully, for all the reasons mentioned above. Amtrak, in fact, has an ongoing program to replace open-deck bridges with ballasted decks. You can frequently see flatcars with precast concrete bridge deck segments outside the north end of Wilmington Shop. That's what those segments are for.
Most ballasted bridge decks have high sides to keep the ballst from falling off. It is possible, though, over time as more ballast is added, for the ballast to build up to the point that some spills over. That's when you come through and undercut, removing the old, fouled ballast and lowering the track. This is a standard track maintenance procedure. The fouled ballast is put through a screen, the "fines" (dirt) are discarded to the side of the ROW, and the cleaned ballast is put back on the track.
Posts: 614 | From: Merchantville, NJ. USA | Registered: Aug 2000
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posted
I'm baaaaak. Found my password, never got it from the moderator.
Yes, Amtrak, and others, are turning open deck to ballasted deck bridges. But, what they are converting are old style open deck bridges where the ties are directly attached to the beams. Generally, on new high speed lines, the choice is to go to a non-ballasted track from. That is, rail attached to some form of fastening system set on or in a concrete slab of some form. There are several of these:
The direct fixation fastener system commonly used in underground and aerial sections of transit systems in the US. Stand in many of the stations in WMATA, MARTA, BART, DART, etc. and you will see this form.
The system used on large portions of the newer lines of the Japanese Shinkansen, consisting of multipiece fasteners functioning much like the common US transit fasteners on precisely set precast concrete slabs.
Sonneville's LVT (Low Vibration Track) consisting of precast concrete tie blocks in rubber boots located precisely and then surrounded by cast in place concrete.
A couple of German systems, Rheda being the most common, consisting of concrete ties or portions thereof with exposed reinforcing cast into concrete. The sole cushioning there is at the rail seat.
There are undoubably others. Some no longer in use.
The idea is not new. There were trial sections of concrete based track in the US back to just after World War I. It did not take off until the development of simple cushioned fasteners in the early 1970's or thereabouts.
For example: All but a few kilometers of the Taiwan High Speed Railway is on non-ballasted track including track in tunnels, on bridges, and on the ground. The few km at the south end that are on ballast are on very poor ground in a location where the speed is under 80 mph for unrelated reasons.
Posts: 2808 | From: Olive Branch MS | Registered: Nov 2002
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posted
George, You're back! (7 months we've not heard from you, too long!) I keep my passwords in a file on my computer; not recommended by the pros, but it works for me.
If you look at Geoff Mayo's entry above, he mentions that AREMA says that bridge life factors into the equation. I'm not an engineer and I don't see the connection. Can you explain?
posted
Remember, what we are comparing here is a ballasted deck bridge on one hand and an open deck bridge where the ties are directly on, and of necessity attached to the bridge structure.
Ballast offers some cushioning so that the impact of the the train on the bridge, both in theory and in reality, is reduced. Reduced impact equals reduced load on the bridge, which results in a longer life of the structure, particularly if the load on bridge members approachs the yield stress
Also, the elevation of the rails on the open deck is by necessity fixed. The ballasted track in the approach gradually settles. If the ground is poor, the settling can be more than gradual. The track on the open deck is stiffer than that on ballast. All these work together so that there is usually a distinct bounce at each end of an open deck bridge. If the bounce gets bad enough, there is risk of a rail break. The use of a ballasted deck reduces, but does not completely eliminate this effect.
It is sometimes seen on open deck steel bridges, but I have never seen it on open deck wood brides, that a 1/4 inch or so thick rubber pad is put under the tie plate. This somewhat reduces the stiffness of the track on the bridge, making it closer to that of the ballast track on the approach.
This entire issue of change in stiffness, is reduced significantly by use of a fastening system on the bridge that incorporates cushioning.
One curiosity that I saw in looking in some of the European typical track sections: (I think these were German, but I really do not remember whose, but pretty sure they were not British.) Normally in US practice, the designed thickness of ballast on a ballasted deck bridge is somewhat less than that used on ordinary track. In these European sections, the design ballast thickness on the ballasted deck bridge was thicker than that on ordinary track.
A little thought says this makes good sense. In ordinary track the underlying subballast and soil also provides some of the cushioning and load spreading. On the bridge only the ballast provides cushioning. Therefore, unless the ballast on the bridge is thicker, the track on the bridge will still be stiffer than the ordinary track.
Presumably the use of a thinner ballast section is to reduce the dead load on the structure, which also part of the logic behind the use of open deck bridges. However, once a little thought is applied, the desirability of using a thicker ballast section on the bridge becomes obvious.
Posts: 2808 | From: Olive Branch MS | Registered: Nov 2002
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OK; now for a little "Beulah, peel me a grape" time.
The ICRR should have deck ballast on their Ohio River bridge.
From my very first journey to New Orleans during Semester Break 1965 on the Panama Limited, it seems as if I have waken up every time I've made that X-ing. The joyride to Jackson last June was no exception.
Posts: 9976 | From: Clarendon Hills, IL USA (BNSF Chicago Sub MP 18.71) | Registered: Apr 2002
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posted
I'm glad you're back, too, George but I never received a request from you for a password. The only way to reach me is by way of a private message, something you couldn't do because you couldn't sign in. The only solution I can suggest, for members having problems like this, is to sign on at the Silver Rails forum and post your concerns there. Kind of a back-door approach but you will get results.
Posts: 169 | From: Northwest Wisconsin | Registered: Dec 2003
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