Looking for a HO fish Belly bridge

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J

jacklight

Jack
Hello,
I am looking for a single track HO fish belly or reverse arch bridge. The problem with my search is that the bridge must be 29 to 31 inches long. I am willing to kit bash a bridge kit that comes close to this, however I have not found a close match. Does anyone know of a manufacturer of this type of bridge or any other suggestions?

Thanks,
Jack
 


Fishbelly Bridge

Hello Jack!

Neither the Faller or Vollmer Fishbellies are near long enough for your requirements, albeit they are nice kits.

For a bridge to meet your needs, one might be tempted to take a Pengram or Bowstring Truss bridge and flip it over (upside down) as a basis for a kitbash. I think that Atlas and Vollmer both made one or the other, but perhaps not as long as you require.

The problem with the fishbelly design was, of course, that the interaction of forces on spans which crossed multiple supports was difficult to calculate. Therefore, this type of bridge was not used much before the advent of computers, and did not take advantage of the cantilever principles which allows structures to achieve spans longer than simple spans of the same superstructure type.

In other words, fishbellies were not found in extant in the lengths that you are suggesting. Would it be feasible to use a pair of them in tandem to cross your proposed span?

David Martin, Ph.D.
 
Fishbelly Bridge reference

A lenticular, "bow-string" or "fish-belly" through truss named for the famous German engineer Friedrich August von Pauli (1802-1883). For Pauli see Zeitschrift fŸr Baukunde, VII: 6 (1884), pp.379-396; Allegemeine Deutsch Biographi, (Leipsig, 1887), 25, pp. 251-258; and Zeitschrift des Vereins Deutscher Ingenieuren (1865) -- "den Artikel von Gerber uber die Berechnung des BrŸckentrager nach Paulischen Systems".
 
Thank you...David I could run two fish belly bridges next to each other. The problem I have is the span is over a five foot valley and I can not have any pillars or supports in the middle of the span. Is it realistic to have two fish belly bridges together without a center support?
 
Fishbelly Spans

Hello Jack!

No, that would defy gravity! :") Fishbellies were designed to be anchored on or between large piers or abutments. One couldn't realistically place them end-to-end without a supporting pier. They simply don't have the structural support in the axial position. Von Pauli's design was intended for short spans or use in viaducts (many short spans over low valleys with multiple piers).

I believe that the largest domestic span was - and still is - in Pittsburgh. And it is far short of your needs.

They certainly are unique looking, and make an impressive addition to a layout, but one should try, I believe, to at least to remain somewhat faithful to prototype use. Your span requirement is rather large, but not impossible to fulfill. The fishbelly just isn't the solution.

Have you thought about a lift bridge? CMR makes a doozy of a kit, single or double track. It's not functional out of the box, but I am building one right now that will be motorized. (Think of the Dock Bridges at Newark's Penn Station.)

david
 
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Your five-foot valley

Hello Jack;

Never mind my suggestion concerning the lift bridge; I just reread your clearance requirements and the geography - a valley. A lift bridge would look silly.

This sounds like a case for a viaduct. I am assuming that there are sloping sides to the valley floor. Faller, Kibri, and Vollmer make stone arch bridges - that combined with one or more approaches - might do the trick. You would still have to drop down a couple of tall piers, but they could be as far apart as, say 18" - 24".

Noch and Heki both have stone sheets that would allow you to construct a larger stone arch span. In prototype, these were constructed in Europe with very large spans - well over 30" in HO.

There is also a modeler, in Belgium I think, that sells a beautiful plaster stone arch bridge. Spans are avaialble to over 36" in HO.

David
 
Thanks David....I may have to revist my vision of this span and valley. I have not constructed the lower valley walls or the floor. So I have some flexability. I may have to construct a terrain that supports a large piers as you suggested.

I see you are from Hunterdon County. I live in Flemington off of 31 by the hospital.

Thanks again
 


I know the area well; I live in Alexandria Township, right off of County Road 614. What era and country do you model? I do European in Epochs II - V.

David
 
Fish Belly - Truss Details

Hello Mikey;

Notice how the bracing is crossed, that's the hallmark of both a Fish Belly Truss and a Howe truss (with counter braces).

The difference maker between a Howe Truss and a Fish Belly is the lens shape of the overall truss.

Friedrich August von Pauli (1802-1883) published details of his truss design in 1865. Probably the most famous Pauli truss, better known as the lenticular truss - named because of the aforementioned lens shape, is Pittsburgh's Smithfield Street Bridge.

Its opposing arches combine the benefits of a suspension bridge with those of an arch bridge. But like the willow tree, some of its strength is expressed in its flexibility which is often noticeable to bridge traffic.

One will also see fish bellies with curved trusses going both up and down. Then the proper name is the Lenticular truss, as the 'fish belly' shape no longer really applies.

David
 
bridge_smithfield.jpg


"The historic Pittsburgh Smithfield Street bridge, built in 1881 by engineer Gustav Lindenthal, is probably the oldest extant steel truss in the United States, and has been designated a National Historic Civil Engineering Landmark. The Smithfield Street Bridge is considered by many to be the prettiest bridge in Pittsburgh and is still used by thousands of cars and pedestrians each day." Mikey has been over this bridge more than a few times.
 
I think people are confusing a fishbelly truss with a lenticular truss. As far as I can see, the Faller model is a fishbelly--really just an inverted arch below the track. The Smithfield St Bridge in Pittsburgh is indeed a lenticular truss, and it's clear that it's totally different. Of course I.K. Brunel's Royal Albert Bridge in England (opened 1859) is also a lenticular truss, though the top and bottom chords aren't the same type of structure:

http://en.wikipedia.org/wiki/Royal_Albert_Bridge

The point of a lenticular truss is that it combines an arch bridge (which tends to spread its anchorages apart) with a suspension bridge (which pulls them together) to neutralize the horizontal forces. But it's complicated to build, and never became very common.
 
Fish Belly vs. Lenticular Truss

Hello John,

No confusion is present, except perhaps in your post. A Fish Belly Truss is a Lenticular Truss. Read Von Pauli's description is his German Patent (1857) or the reference I provided in an earlier post.

The Smithfield Street Bridge is a complete Lenticular Truss, with both upper and lower (the belly) forms. The Faller kit is a Lenticular Truss with only the lower (the belly) form.

In either case, the first Lenticular Truss design was patented by von Pauli in Germany. In a lenticular truss, either the top or bottom chords or both are curved in such a way that the top chord functions as an arch, and the bottom chord functions as a suspension cable. Vertical and diagonal web members are necessary to divide the top and bottom chords into manageable lengths and to transfer deck loads to the top chord and the bottom chord. They also redistribute nonuniform loading, which cannot be resisted by arch and cable action alone. The deck can be placed above the top chord, supported by posts; below the bottom chord, supported by hangers; or in between the two chords; or above only the bottom chord (i.e., fish belly).

The Royal Albert Bridge is not, based on von Pauli's description, a lenticular truss, but a design by Isambard Kingdom Brunel (1859). Brunel didn't complete the design nor the construction; due to declining health his chief engineer Robert Pearson Brereton was primarily responsible. Brunel in fact died in 1859.

The Royal Albert is designed with two spans which are lenticular trusses with the top chord of each truss comprising a heavy tubular arch in compression, while the bottom chord comprises a pair chains. Each of the trusses is simply supported and therefore no horizontal thrust is exerted on the piers. Between these two chords are supporting cross-bracing members and suspension standards which hang beneath the bottom chord to carry the railway deck which is a continuous plate beam. Therefore, while there are elements of a lenticular truss in this bridge, it is NOT, strictly speaking, a lenticular truss or fish belly bridge. Although to the untrained eye, it might appear to be. Similar, yes. The same, no.

Couple of points. Lenticular trusses are and were very prevalent in Europe. Nearly every viaduct not constructed of stone has them as an integral part of their design. The relative complexity of the construction of the lenticular truss had very little to do with their perceived scarcity in the US. American engineers in the 19th century, by European standards, were computationally lazy. The interaction of forces on spans which crossed multiple supports was difficult to calculate. Therefore, this type of bridge was not used much in the US before the advent of computers, and did not take advantage of the cantilever principles which allows structures to achieve spans longer than simple spans of the same superstructure type.

To a European-trained mechanical engineer, there is nothing 'clear' about the so-called differences between the Faller 'model' and the Smithfield Street Bridge. The inherent design elements that make both of these bridges lenticular trusses literally screams out to be recognized. One simply has to know what to look for. To an engineer, the alignment and placement of structural steel; the tensile and compressive load carrying elements, are clearly identifiable. To an engineer, the differential equations describing the behavior of the truss almost automatically appear on the chalkboard behind our foreheads. (If they didn't, then during the recitation period of the lecture, we would be most humbled in front of our audience.)

Finally, as stated previously, like the willow tree, some of the strength of a lenticular truss is expressed in its flexibility which is often noticeable to bridge traffic. In other words, long spans tend to be 'bouncy', which is somewhat disconcerting to passengers - be it vehicular or rail. Hence, we have another reason for the relative lack of popularity of this bridge type in the US.

Although the Wikipedia contains a cornucopia of information, be advised that it can be edited by anyone. Many of the technical and engineering observations contain therein are so wrong they're not even bad.

David Martin, Ph.D.


John P said:
I think people are confusing a fishbelly truss with a lenticular truss. As far as I can see, the Faller model is a fishbelly--really just an inverted arch below the track. The Smithfield St Bridge in Pittsburgh is indeed a lenticular truss, and it's clear that it's totally different. Of course I.K. Brunel's Royal Albert Bridge in England (opened 1859) is also a lenticular truss, though the top and bottom chords aren't the same type of structure:

http://en.wikipedia.org/wiki/Royal_Albert_Bridge

The point of a lenticular truss is that it combines an arch bridge (which tends to spread its anchorages apart) with a suspension bridge (which pulls them together) to neutralize the horizontal forces. But it's complicated to build, and never became very common.
Click to expand...

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