Glenfinnan Viaduct query

[Hilux5972]

Hi all

 

A little question I have for my recreation of this iconic viaduct. The diagram below shows (I hope clearly) what I need to know. Are the viaducts piers wider at the back than the front to account for the curvature or are the arches wider? 

 

Thanks in advance 

[JinglingGeordie]

After a brief look at a few photographs it looks more like the piers are perpendicular to the curve. They appear to the same at each end.

[Hilux5972]

Yes but looking at photos the arches also look the same from both sides too. Does anyone have any plans or know where I could find some? Being in New Zealand, I can’t just pop up to Scotland and measure it myself lol

[Nick Holliday]

1 hour ago, Hilux5972 said:

Yes but looking at photos the arches also look the same from both sides too. Does anyone have any plans or know where I could find some? Being in New Zealand, I can’t just pop up to Scotland and measure it myself lol

What is confusing things is that, although the curve on the real viaduct looks outrageous, in modelling terms it is shallow.  According to Wikipedia, the radius is 792 feet, (11 foot in 4mm) and the width of the viaduct is 18 feet, and the arches are 50 feet long.  Using good old pi, the length of the viaduct, if we assume it describes 180°, would be 2,488 feet on the inside curve, and 2,544 on the outside (depending on where you take the first radius).  This means that the longer side is only 57 feet longer than the inner side.  With each arch roughly 50 feet wide, this means there would be 50 arches, and, on each arch, one face is just over 1 foot longer than the other.  This 2% difference would be impossible to detect, especially given the height above ground.

I realise this doen't answer the original question, and things are further complicated in that the viaduct is built from concrete, and probably the same sets of shuttering would be used several times during the work, and it doesn't really matter which way they designed the piers, as there was no brickwork coursing and bonding to be accommodated.  Again, the same calculation would show that, if they had gone for a tapering pier footprint, the outside face would only be 2% wider on the real thing, virtually undetectable.  I'll leave you to work out what would be required with your first scenario, which would depend upon the radius you have available for your model.  I think the second option would offer the best visual solution, though.

[AJCT]

Way back in the 1960s, my Dad built a viaduct for his (never finished) 00 layout.  I think it was influenced by Glenfinnan, but it was a much scaled-down version with 8 main arches and a smaller one at each end, on a curve radius of about 3', and also on a gradient.  I still have said viaduct stashed away in my attic and a cursory check says that the arch spans are 4" on both sides, which would confirm that the piers are tapered as per your first diagram.  Given that my Dad had an engineering degree and wartime experience as an army engineer, I would assume he'd got this right....

 

HTH.

 

Alasdair

[Hilux5972]

Well that’s a good explanation Nick. If it’s only going to be a 2% difference in width then it will hardly be noticeable at all. 

[Argos]

3 hours ago, Nick Holliday said:

What is confusing things is that, although the curve on the real viaduct looks outrageous, in modelling terms it is shallow.  According to Wikipedia, the radius is 792 feet, (11 foot in 4mm) and the width of the viaduct is 18 feet, and the arches are 50 feet long.  Using good old pi, the length of the viaduct, if we assume it describes 180°, would be 2,488 feet on the inside curve, and 2,544 on the outside (depending on where you take the first radius).  This means that the longer side is only 57 feet longer than the inner side.  With each arch roughly 50 feet wide, this means there would be 50 arches, and, on each arch, one face is just over 1 foot longer than the other.  This 2% difference would be impossible to detect, especially given the height above ground.

I realise this doen't answer the original question, and things are further complicated in that the viaduct is built from concrete, and probably the same sets of shuttering would be used several times during the work, and it doesn't really matter which way they designed the piers, as there was no brickwork coursing and bonding to be accommodated.  Again, the same calculation would show that, if they had gone for a tapering pier footprint, the outside face would only be 2% wider on the real thing, virtually undetectable.  I'll leave you to work out what would be required with your first scenario, which would depend upon the radius you have available for your model.  I think the second option would offer the best visual solution, though.

 

I'm not sure shuttering as we know it would have been used. Concrete technology was very much in its infancy and most structures at this time were built of mass concrete with multiple small layers built up over time. I must confess that I don't know whether this technique was used here.

 

This is as opposed to the modern method of a steel re-unforced cage shuttered up with a large concrete pour needing batching plant, pumps, cranes etc.

 

I would imagine any shuttering would be bespoke to the location.

 

Labour was cheap enough to warrant this approach.

[Hilux5972]

Well that’s a good explanation Nick. If it’s only going to be a 2% difference in width then it will hardly be noticeable at all. 

[ejstubbs]

13 hours ago, Nick Holliday said:

What is confusing things is that, although the curve on the real viaduct looks outrageous, in modelling terms it is shallow.  According to Wikipedia, the radius is 792 feet, (11 foot in 4mm) and the width of the viaduct is 18 feet, and the arches are 50 feet long.  Using good old pi, the length of the viaduct, if we assume it describes 180°, would be 2,488 feet on the inside curve, and 2,544 on the outside (depending on where you take the first radius).  This means that the longer side is only 57 feet longer than the inner side.  With each arch roughly 50 feet wide, this means there would be 50 arches, and, on each arch, one face is just over 1 foot longer than the other.  This 2% difference would be impossible to detect, especially given the height above ground.

 

Wikipedia also says that the viaduct is 416 yards long.  That's pretty much half your estimate, and suggests that the curve is only about 90°, not 180° as you assumed.  However, it also says that there are 21 arches, which is again roughly half your estimate, so your result is probably not that far off after all (though you could have got there quicker if you'd used all the information available).

[Hilux5972]

Ok these are the actual dimensions I have found out from online research. 

 

Overall length 1248ft

Overall width over manhole spaces 22ft

Overall width 18ft

Curve radius 800ft

Height 100ft

Arch span 50ft

Arch height 85ft

 

which equates to the following in a full scale 1:76 replica

 

Overall Length 4992mm

Overall Width over manhole spaces 120mm

Overall Width 88mm

Centreline Radius 3200mm

Inner Radius 3164mm

Outer Radius 3236mm

Arch Span 200mm

Arch Height 340mm

 

Ive double and triple checked these measurements so I’m positive they are correct. It seems that the difference between the front and back of each pier is going to be so minimal that it’s not going to matter so I’m happy with that. 

 

[Graham R]

17 hours ago, Hilux5972 said:

Yes but looking at photos the arches also look the same from both sides too. Does anyone have any plans or know where I could find some? Being in New Zealand, I can’t just pop up to Scotland and measure it myself lol

 

It sounds like your online research has given you all you need! But if you're interested, there are 1921 plans and elevations of the viaduct in the National Records of Scotland, catalogue entry here. NRS will send you a digital copy for £15 (a minimum payment "search fee" which, if successful, gets you copies at £2.50 a plan) plus £6 or so for a memory stick. Or something like that .... the exact teerms are on their website. If you explain you are in NZ, they may be willing to e-mail what you want rather than insisting on transfer by memory stick although they will probably insist on charging you (the search fees are laid down by statute so they cannot waive them; the stick fee however is not).

 

I have seen one of these drawings (a friend has a copy) and can pass on a specific enquiry to him if you want to check something. The NRS is a great resource once you learn their ways, and i think it's reasonable to get full copies direct from them if required.

 

Hope that's useful and good luck with the model.

Graham

[Hilux5972]

That’s great news Graham. I’ll get in touch with them and see what they can come up with for me. 

[Nick Holliday]

16 hours ago, Argos said:

 

I'm not sure shuttering as we know it would have been used. Concrete technology was very much in its infancy and most structures at this time were built of mass concrete with multiple small layers built up over time. I must confess that I don't know whether this technique was used here.

 

This is as opposed to the modern method of a steel re-unforced cage shuttered up with a large concrete pour needing batching plant, pumps, cranes etc.

 

I would imagine any shuttering would be bespoke to the location.

 

Labour was cheap enough to warrant this approach.

The concrete viaduct at Compbyne on the Lyme Regis branch was built a couple of years later, and was of similar mass concrete construction. A contemporary account describes them as being built in six foot stages, using shutter boxes prefabricated, as they were using a Blondin crane to lift them into position, and I'm not sure if MacAlpine had the services of anything quite so versatile. Even using modern shuttering techniques, each lift would probably be restricted to around twelve feet, as the pressures of the wet concrete become significant, unless being slip formed in a continuous process. Each stage would require preparation of the top surface, to ensure a good bond at each joint.

5 hours ago, ejstubbs said:

 

Wikipedia also says that the viaduct is 416 yards long.  That's pretty much half your estimate, and suggests that the curve is only about 90°, not 180° as you assumed.  However, it also says that there are 21 arches, which is again roughly half your estimate, so your result is probably not that far off after all (though you could have got there quicker if you'd used all the information available).

I used the term "assumed" in a mathematical sense, as it made the calculations very simple. I would like to see your quicker solution!