Prototype question...

"Dan Merkel"

Inclined, usually.

-- Cheers

Roger T.

Home of the Great Eastern Railway

The followed the grade of the rest of the line.

__________ Mark Mathu bridge engineer

"Mark Mathu" wrote in news:4B40h.40824$ snipped-for-privacy@tornado.rdc-kc.rr.com:

Makes sense to me. I can imagine a 20' bridge crossing a creek or ditch being level and putting all sorts of stress on the couplers. It would be worse as you got closer to the size of the car too, wouldn't it?

Puckdropper

My understanding is that probably a majority of bridges on grades are built level and the track continues on the grade across the bridges. On steel truss bridges the main truss would be level, and the stringers supporting the bridge ties would be built on the slope of the grade. For ballasted deck bridges, I would expect the bridge to be level and the ballast to be a bit deeper under the ties at the upper end of the grade. Diagrams of wooden trestles in the NMRA Data Sheets show that shims were added above or below the stringers on curved bridges to achieve superelevation; I imagine the same technique would be used on a shorter trestles on a grade. Most RR bridges were designed before the advent of computers, and from what I remember from school, designing bridges is a whole lot easier if gravity is perpendicular to the bridge structure. Longer trestles and truss bridges were built to the needed grade, as in the case of the Rio Grande Southern's bridge 9A - 836' total length, consisting of 388' and 246' pile trestle approaches on either side of a 202' wooden deck truss all built on a 4% grade. Geezer

"Geezer"

This is contradictory. :-)

The if the bridge is "level", then the grade will stop at the bridge, go level, and continue after the bridge.

Anyway, that's not how it's (generally) done. The grade and the bridge deck are all considered as one. A track on a one percent grade will continue as a one percent grade as it crosses the bridge and a one hundred foot bridge will be one foot higher at one end than at the other.

-- Cheers

Roger T.

Home of the Great Eastern Railway

That would require an extra 33.44' depth of ballast at the upper end if the structure was built level! =8^O

Regards, Greg.P. NZ

Longer trestles and truss bridges

Otto Mears could not afford ballasted deck bridges. That's why he built it on a grade. But he paid a price. Had he built the deck truss level, he would only have had to design for the dynamic horizontal loads of a train accelerating or braking while on the bridge (in addition to the usual considerations for wind loads, etc.). But because he built the bridge on a

4% grade, about 4% of the static weight of the entire deck truss structure, plus about 4% of the static weight of a train on the bridge, is always trying to push the deck truss bridge laterally down hill, and these extra horizontal loads had to be accounted for in the design of the bridge abutments. Geezer

Rog:

Not at all. I understand him to mean that the bridge per se is level, possibly even the deck, and the track is blocked up as it climbs. I'll have to defer to others as to whether this was actually done, but I could certainly see it happening for relatively short bridges, especially in the old days, when bridge loading was incompletely understood and the engineers may have hoped to avoid complicating everything. A hundred-foot bridge, then, would need the ties raised 1' more at one end than the other.* A long trestle would, I guess, follow the grade.

*(Edit: For a 1% grade, that is.)

Cordially yours: Gerard P.

Yes, I kinda gather that's what he meant but he's still wrong anyway.

Even brick or stone arched bridges and viaducts were built with the grade built into the structure. A 100 foot long brick viaduct on a one percent grade would have the "deck" one foot higher at the upgrade end than at the downgrade end.

You can find photos, if you care to look, where it's obvious that the whole bridge/viaduct/trestle is built on a grade.

-- Cheers

Roger T.

Home of the Great Eastern Railway

Look more closely. You're correct about the top surface of the arch bridge, but each arch is built symmetrically about a true vertical line, not about a perpendicular from the deck surface when its on a grade. Geezer

Absolutly correct.

Even tressell on a grade will have all the verticals, vertical amd everything but the deck will be horizontal (in most cases).

Howard

"Geezer"

Of course, you couldn't build it any other way.

Some of those funicular and rack and pinion railways there are brick built single and multi-arch bridges/viaducts where one end of the bridge is several feet higher than the other, on every single arch.

-- Cheers

Roger T.

Home of the Great Eastern Railway

Well, of course; that seems obvious. I don't think Roger implied that the arches would be tilted off-vertical just because the deck was gradually elevated.

I've never heard the reason stated in terms of stressing the couplers (and I've designed a couple of railroad bridges in my career). The bridges are built to the same grade as the rest of the rail line because 1) it is easily accomplished, and 2) if the bridges were level on the railroad then the remaining tracks on grade would need an _even steeper_ grade to get the necessary change in elevation from one end of the line to the other.

On a side note, railroad bridges are of two types: "open" deck or "ballasted" deck. Open deck bridges have the rails sitting on the ties, which are then connected directly to bridge structure itself. Ballasted deck bridges have the rails and ties sitting on a layer on ballast placed on top of the bridge structure. Ballasted deck bridges allow the railroad to make slight adjustments to the grade and elevation of the railroad tracks on the bridge as needed, while open deck bridges introduce a "hard point" into the railroad's profile which can't be easily adjusted in the futue without alterations to bridge itself.

__________ Mark Mathu

The simple solution would have been to build the railway entirely level

- or only run weightless trains!

Regards, Greg.P.

If you tilt a circular arch around it's central radius point and mount it on vertical piers it will be exactly the same as if it hadn't been tilted!

Regards, Greg.P.

Correct Steve. Though one leg of the arch may be slightly higher (longer?) than the other, all the piers are vertical and the arch describes and even arch.

-- Cheers

Roger T.

Home of the Great Eastern Railway