Viaducts and Railway Arches

Having half-built a large curved viaduct in N gauge I think the prototype approach as suggested by 34C will look wrong on a model due to the tighter than prototype cuves that we use.

My effort was built with stripwood and will eventually be clad in embossed styrene. In the meantime I have built some Plastikard sides to stop the trains falling off, and these also have the shape of the main arches. My viaduct is on a 1m radius and is about 65mm wide, the arches are at about 120mm centres on the inside and about 8mm more on the outside IIRC. I thought piers tapering from maybe 10mm on the inside to 18mm on the outside would look strange, so I have made the piers parallel and the arches are circular on the inside and slightly elliptical outside. Also I have made the sides on a continuous curve because making each arch straight in plan, as I think 34C is suggesting, would just look wrong. Either the parapets would overhang by a large amount or the whole thing would resemble a giant 50p piece!

***You mean like this? masters made with plaster and details on a layer of DAS (separate master for Arch, Piers and two masters for the parapet, one for each side of it. cast in polyester Resin. Scale 1/4 mile long in 4mm scale (about 18 feet) and on a 1/2 scale mile radius.

View attachment 968

View attachment 967

Doug, the same techniques work well for all forms of bridges or items that need reliable repeats - I also have a master for Inverness roundhouse done the same way.

DAS is a great material to work with for the masters - give it a try. Resin is expensive though - Ribblehead used about $A1500 worth of it as their is a lot of volume in each item

Richard

PS: Edwin. I agree with 34C... this viaduct, like all of them, is a series of straight sections.

Thats the way it is in reality and making it a constant curve would simply be wrong! The trick is to NOT build it on a tight curve.... if built on a tighter curve, each arch is made much smaller in arc diameter to compensate for the curvature... we should do the same in model form rather than try to make larger arches turn corners - Visually the "engineering" will simply be somewhat dubious if the arches themselves are curved, a dead giveaway that it is a model!

Or.... Use concrete not stone. Thats at least almost possible....

REJ

Hi Doug,

Interesting problem but not, I think, unsurmountable.

Although I work in N Gauge I have taken some scale 'license' and am building a viaduct using the 00 Linka system. Although my viaduct is on the straight I did have a problem with casting the curved undersides of the arches in plaster of paris until, in a flash of inspiration, I tried using modelling clay, well tamped into the Linka moulds, then removed while still supple and fixed to the plywood former (which actually provides the structural strength and support.) Et Voila !!

It occurs to me that you could use a similar method by firstly constructing a plywood viaduct, curved to suit, and then, using the Linka moulds and modelling clay, make panels of stone (or brick - Linka do both) and fixing them to the plywood while they are still flexible.

Food for thought anyway.

QUOTE (neil_s_wood @ 29 Nov 2008, 06:31) <{POST_SNAPBACK}>Gosh, so it is. I suppose this makes it easier to design and plan.

***No Neil, its because it will fall down if made curved.

The physics of lateral/horixontal curved sections are all wrong so the engineering is simply not possible without hugely adding to mass (and therefore cost) as the forces are not properly balanced.

Especially true if its a stone structure.

Remember its not just the weight it has to carry, but the huge side thrust of hundreds of moving tonnes of train which would prefer to go in a straight line ALL of the time! Thats also the reason why stone viaduct on curves almost always have significant speed restrictions.

We do have a couple of qualified Engineers on the forum who can probably explain it more clearly. (Cue to you, Paul/Trevor)

What SHOULD happen on any curved viaduct is that the arches are 100% straight and the piers are wider on the outside (stronger) than on the inside to compensate for the thrust of the trains weight... The internal structure will be thicker on the outer edge too to make the whole thing stronger.

On my Ribblehead viaduct, each Pier is actually about 2mm wider on the outside of the curve than the inside.

Where such a real world structure looks to be curved in any way, its really cosmetic curves - perhaps at parapet level which is above the primary force vectos, added to a plain straight primary geometry, and railways don't worry about beauty when stability is their primary concern.

Richard

QUOTE (Doug @ 29 Nov 2008, 05:47) <{POST_SNAPBACK}>Thanks for the ideas guys. Richard, If you can spare a 4' section, please send it. It seems you have too much anyway

For the viaduct, I have some of those Skaledale arches, but they don't even line up straight. I'll be building something then I suppose.

I'll try and cast my city arches flat and see how they turn out/ Perhaps I could remove the parapet wall and add a curved one afterwards.

*** More possible than you think Doug: I have been discussing making it available as a sort of "bespoke-to-order" kit. As big as Ribblehead is (24 arches and 3 King Piers in assition to the normal piers) would be exxy though - about $A2500. A six arch section would be about $A650, and still need about 12 hours assembly time. Dead accurate to prototype though, as the scaling of stone courses and most details is exactly right to the prototype!

It will be released when we have completed high quality instructions for assembly.... and when I've found an easierway of making the very detailed drainage pipes, which currently take about 1.5 hours each as they have brass collars soldered onto brass rod every scale 6 feet to represent the cast pipe sections used when it was made.

We COULD also assemble one and ship complete... but freight may be scary cost wise....

EVERY stone or brick viaduct needs drainage detail done properly by the way... without it, it would collapse in very few years as the core become eaten away and the mortar joints and stone split due to freezing and thawing cycles!

Richard