Mark - thanks for the Grangeville link - now you have made me homesick
dd
BRF…I’ve been over the Blue Ridge Parkway but probably not that part…It looks rather new and it’s been years since I did that…In fact we were driving a fairly new 59’ Chevy Impla convertable when we did that…and had the top down and the farther up we went the cooler it got and also got dripped on in a tunnel…That was a great drive. Have crossed the area real close…{to the Parkway}, recently on I believe it was I-64. Wonder how they fastened those cement sections together…?
According to one of my civil engineering texts trestles or viaducts are plate-girder spans supported by either towers or bents; . A bent is two columns braced perpendicular to the direction of the track or the roadway on the span. Trestles or viaducts can be up to 100’ long, and they are supported on towers, which are two bents braced together by bracing parallel to the direction of travel. Bridges are often spans that are over 100’ long, and they are
also supported
…What is the sicence that controls the tensioning of the cables that ensures they will always maintain proper tension during different heat ranges, etc…and perhaps a degree of stetching over time…? These cables are not embedded in the concrete such as prestressed beams have and are subjected to temp. changes…
Hey, we’re starting to mix technical terms here…
Prestressed = tension is pulled on cables in a mold, the concrete is poured in and allowed to come to high early strength, and the tension on the cable ends is relieved.
Post-tensioned: the cables, or ‘tendons’ as the French named them, are tensioned and anchored after the sections have been positioned. This has the advantage that the precise tension can be adjusted after the assembly is together, and precast units (which themselves can incorporate both reinforcing steel and prestressed elements) can be put together into a longer structure with full tension integrity. It is possible to ‘tremie’ the joints and tendons when assembled; it is always good practice to ‘seal’ every exposed or potentially-exposed (e.g. via inevitable stress-distributing cracking) joint where water, dissolved salts, etc. can get to the tendons or their anchors.
You are sadly mistaken if you think that post-tensioning cables are not “embedded in the concrete…” The only difference is that the PT cables are tensioned in the absence of grout – they are very much contained within concrete, and whether or not the tendons are grouted fully, the ends will be. (When I worked in Louisiana, a fairly large number of our apartment complexes in the Pine Bluff, Arkansas area were built with post-tensioned foundations. The heads of the tendons were never grouted in – you can imagine my reaction when I saw all the corrosion, rust stains running down the exposed face, etc…)
The prestressing is usually adjusted to give elements the correct ‘camber’ so they’ll be straight (or otherwise properly aligned) when the added weight of decking, live load, or other structure is imposed on them. (If you’ve read Steinman’s book on bridges, or have seen pix of suspension bridges under construction, you’ll know the principle).
Post-tensioning is done with due attention to temperature, anticipated loads, etc. – but the factor
A viaduct never was intended to carry water. An aqueduct does that. Via = road.
overmod – technically you are right on causeway being a shallow fill crossing water. However… we always called the thing we shared with the Rutland crossing the norh end of Lake Champlain to Rouse’s Point a causeway, even though a good bit of it was trestle (wood – and the amount that’s open got less every year, as ballast and fill was built out from the shores until the channel got too restricted, then they stopped.). It’s an interesting bit of railroading – gantlet track, which you don’t see all that often, and a 10 mph permanent slow order, and a royal pain on the whole.
Unless you live in Chicago. Then the steel, deck girder bridges carrying the tracks over all the streets in the city are called “viaducts” by the native locals.
Most Class 1’s used standard bent spacings for their trestles so that they could simplify their stock of replacement beams, braces, etc. On SP it was 15ft as I recall.
dd
Overmod: The thought that post -tensioning cables weren’t embedded in concrete was indicated to me when I read Mark’s piece…that said cables run through conduits in the precast sections…and hence I wondered if they are in conduits they are not sealed within the concrete as it hardens and permently attached to the cables and hence not necessarly against the concrete mass to control temp like it would be if it was cast into the mass as in Pre-stressed beams…Whew, what a mouthful…Hope you get what I’m saying.
On causeways…I’m on the side you indicate…I have been under the impression they generally are strips of land…{man Made}, to carry a roadway or railroad, etc…
As for the sections such as on the Blue Ridge Viaduct…Are there several sections held in place while assembling the structure and then after so many are installed, install the post-tensioning cables and tighten to secure them in place…? As opposed to installing cables after each one is installed.
True, I typed before thinking, fingers faster than brain cells…
Although they both, for the most part, use the same basic construction, arches.
Ed
…When one drives into Disney World in Florida, and on into the Contemporary Hotel you drive under a “bridge” that carries water connecting the lake behind the hotel and the lake that fronts the edge of the Magic Kindom…The water is deep enough that sizable boats go over the “bridge”…I suppose that really has to be an aqueduct. It appears to be constructed probably of prestressed concrete beams. When one drives under it the underside of the “bridge” is flat…And I might say, dry…
…Thanks Mark, for the additional info…Understand what you are saying.
The Linn Cove Viaduct was opened in September 1987, so whether or not you consider that old or not is entirely up to you. It’s on the backside of Grandfather Mountain, near Blowing Rock, N.C.
…Well, William…as I mentioned, I was driving a rather new 59’ Chevy convertible when we did it so in that context I’d have to consider it rather new…{The bridge, that is…}.
Question- what is “gantlet track”?[?]
All of this talk about definitions — what I am going to say next is not accurate engineering terminology, but how we used them where I used to work.
Trestle - a series of short bridges set upon piling and bents, are open deck, and may or may not have ballested decks (open deck here means no type of enclosure above the tie height).
…wood trestles usually have a 15 foot bridge set on the bents
…concrete trestles usually have a 30 foot bridge set on the bents - piles are usually steel I-beams driven into the ground or set on a poured concrete footing.
…steel trestles usually (but not always) have 30 foot spans and are completely steel in construction.
Viaducts are 1) very long trestles over a body of open water or over a land area that can not be filled in 2) a very long fill over a body of open water (and its land approaches), or a combination of the above. Clearspan bridges can also be included in the trestle, or 3) - massive stone or concrete structure designed to carry a road, path or railroad.
Bridge – a clear-span self supporting structure for crossing an abstruction overhead. Bridge segments may be placed end to end and supported on piers. Types would include thru-truss, girder deck, thru-girder. They are usually 60 feet or more in length, although many older ones are shorter, except for the next definition…
A single-span trestle that, say, crosses a road, is concrete and sits on pile or cast-in-place abutments is a bridge.
Like I said, these are not engineering definitions, but the definitions we used to identify structures by the way they were constructed.
It’s a borderline-weird way of “solving” problems with maximum loading or weight distribution, usually on bridges. (It can also be used in unavoidably narrow places.)
Suppose you have a double-track main line that runs over a structure (like Erie’s Starrucca Viaduct) that you’re worried can’t bear the weight of two trains passing. For ‘safety’, you cross one of the two lines partially over the other one, so you have what are essentially two overlapping lines. This looks a bit like broad gauge with a guard rail, except that trains continue to operate on the ‘appropriate’ pair of rails – it’s as if you’ve interleaved the two tracks. Obviously you can’t get two trains on this at the same time, as it is not hard to figure out a good common term for ‘interleaved trains.’
Note that since there are no switches involved, there is no possibility of a point-related derailment – there are just a couple of crossover frogs to allow the inside rail of one line to cross over the inside rail of the other. There are safety conventions to ensure that signaling is appropriately ‘interlocked’, but I don’t have electrical diagrams to indicate how that is best done – of course, there are serious potential problems from anything that can put equipment on that overlapping trackage by mistake, and there really is no good solution to prevent, say, loose equipment from fouling the gantlet (derails can do it but are not particularly conducive to subsequent safe or nondestructive handling of whatever they derail…)
(As it turned out, Starrucca proved quite capable of handling full double-track operations with heavy modern locomotives – both steam and diesel – and had its gantlet removed.)
…Did South Shore do away with their Gantlet since their tragic accident back in 93’…?
You’ve seen them “called” trestles, bridges, etc. in publications because the cutline writers are usually writers or graphic artists, not railroad civil engineers and don’t know a cowcatcher from a Federal Rear End Device.
Jock Ellis