Uses of Plate Girder Bridges

[gd891]

My understanding is that a prototype would prefer to use a deck girder bridge instead of a through-plate girder bridge because the latter can interfere with wide loads. However the through bridge also provides better clearance to anything below.

Thus, is the only place the prototype going to use a through girder bridge in situations where they need the clearance? I.E. over a automobile roadway or over another railroad.

Or, are there other reasons they would use a through girder bridge?

TIA, Greg

[dehusman]

They "recycled" an old bridge and that's the type they had available.

Dave H.

[Anonymous]

For tracks over a roadway, even when clearance is not a problem, the railroad would prefer to use a through girder bridge. This type is better able to keep a derailed car on the bridge, and provides protection (for the vehicles on the roadway) in the event that anything falls off of a train car while on the bridge. It also allows better access for maintenance of the track and roadbed without interfering with traffic below.
Regards- Richard W.

[Anonymous]

Most of the girder bridges I have worked on or seen were filled almost to the top with ballast leaving only about a foot above grade. That is also how I modeled them on my layout .

[ndbprr]

That is the way they are on the old PRR corridor in Philly. You can barely see the curve on the ends from the horizontal to the vertical members. Wonder what kind of condition things are under all that rock and dirt and how they check them?

[mvlandsw]

There is one on the NS ex PRR line that crosses over the CSX ex B&O line in Ravenna, Ohio with holes rusted through the girder.

[Anonymous]

It is important to understand that even a thru plate girder has as much as a foot or so of steel structure under the roadbed that connects the girders and carries the weight of the roadbed and trains while it also braces the girders so that they remain vertical under the rolling loads. If it is a ballasted deck, there is also normally a layer of concrete on top of the steel that acts like a "tray" to hold the ballast. Thus, even on a thru truss, the girders still project a couple of feet below the track, not a fraction of an inch like in the typically available plastic models.

CJ Riley

[Anonymous]

Most railrods now use concrete girders where they can, say up to 100-ft spans, beyond that if headroom is enough, they want deck girders. All new bridges will have either a steel pan or concrete pan to hold the ballast, normally 0.5 feet deep below the tie. Through girders have to be braced from the transverse floor beams up to the top of the through girder, thus adding width. The through girgers, as mentioned above, have transverse floor beams 24 to 36-inches in depth placed 3 to 4 feet on centers.

The concrete girders used are "I" shaped like the ones used on highways. With either, the concrete pan is "Composite", meaning that it helps the girders carry the load. Don't forget that many bridges will have a walkway too. Especially if they are long, turn outs close, or for any other reason that the crewmay have to walk the bridge. Also, increase width if bridge is in a curve.

[ndbprr]

The railroad engineering departments will always use the state of the art but in the case of the PRR that was steel in 1900 when they were built so the bridges are now nearly 100 years old on many parts of the corridor. Plus over the years as is typical the ballast has built up to where 6" to 1' of the bridge is all that is visible from the track level. Now a steel bridge could be immersed in water and show no ill effects after all that time but add oxygen and the rusting could be severe. Just speculation but at some time every one of those bridges will need to be replaced. Maybe soon or maybe in another 100 years.

[Anonymous]

Since nobody has given "my" answer yet, here it is: Assuming you are asking about [older] steel bridges, the railoads generally prefer bridges with the supporting structure below the tracks, as it is cheaper. The weight of the train is transferred via the rails to the ties, so the structure thoretically has to be no wider than the ties - the rest, i.e. walkways etc. is "cosmetic". If clearance below is a problem, they go to a steel structure above the rails, but this neccessitates cross girders to take the weight to the outside of the train, i.e. 12 - 15 ft wide, more if there are walkways etc. These structures are normally not designed to keep derailed equipment on the roadway, as well maintained trains on well maintained roadbeds proceeding at the prescribed speed do not derail.

[Sorry, no decorations, this is my first comment]

PAC in Canada.

[underworld]

Like dehusman said.....many cases they are recycled....it's what they had on hand. Just a question of transportation and installation.

underworld

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