Bridge arch question

Outside the fence.
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barhamd
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Bridge arch question

Postby barhamd » Mon Nov 12, 2018 3:54 pm

Hi folks

Can anyone advice on how to work out (or whether there was a standard radius) what the arch of an overbridge might have been? I can see that there is obviously a different radius for the top of the arch than for the 'corners'. Would this have been an ellipse or two different radii?

Prototype is Clare, Suffolk, ex. GER ~ 1865

IMG_0258.jpg


thanks

David

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Mark Tatlow
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Re: Bridge arch question

Postby Mark Tatlow » Mon Nov 12, 2018 4:19 pm

That is an ellipse.

The change in radius that would be inherent in having two different arcs would be clumsy from a structural point of view.

You can probably prove this by loading the photograph onto CAD and tracing an ellipse over it.

If you know the overall dimension of the span and the height of the span, the characteristics of the ellipse ought to be capable of being worked out quite easily. Just plug them into your favoured CAD program which I envisage has draw ellipse functions based on the minor and major axis of the ellipse?
Mark Tatlow

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Le Corbusier
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Re: Bridge arch question

Postby Le Corbusier » Mon Nov 12, 2018 5:11 pm

May be overly detailed but might be of interest.

Sundry Arches.jpeg
Tim Lee

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dcockling
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Re: Bridge arch question

Postby dcockling » Mon Nov 12, 2018 5:49 pm

Try Digest 62, The Permanent Way in Miniature—Prototype Considerations by Derek Genzel, page 13, Appendix 1. Setting out an Elliptical Arch: https://www.scalefour.org/members/diges ... -0v1-2.pdf

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barhamd
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Re: Bridge arch question

Postby barhamd » Tue Nov 13, 2018 3:13 pm

thanks everybody

I imported the picture into TurboCAD, reducing the colour depth to make it quick and easy but so I could still see the outline of the bridge. A combination of the digest pages, brick counting etc and I think I've got the right outline.
bridge.jpg

time for a mock up..

David

Alan Turner
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Re: Bridge arch question

Postby Alan Turner » Wed Nov 14, 2018 8:24 am

Actually it is NOT an ellipse it is either a 3 point arch or a 5 point arch.

Any decent practical geometry book will tell you haw to set it out.

The reason it's not an ellipse is because it would be a nightmare for the carpenters to make the false work.

regards

Alan

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barhamd
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Re: Bridge arch question

Postby barhamd » Wed Nov 14, 2018 12:29 pm

In the absence of a 'decent practical geometry book' (like I suspect 99.9% of British households) I resorted to the internet. I tried drawing a 3-centre arch and that fits reasonably over the picture (green line)

bridge1.jpg
bridge1.jpg (181.5 KiB) Viewed 1394 times


I'm afraid I got completely lost trying to follow the instructions for constructing a 5-centre arch.
If there is anyone who fancies drawing a 5-centre arch with a width of 106mm and a rise of 27.5mm please have a go!

Ultimately I very much doubt anyone will be able to tell the difference once the drawing is turned into a model.

David

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Le Corbusier
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Re: Bridge arch question

Postby Le Corbusier » Wed Nov 14, 2018 12:48 pm

The semi elliptical arch in the drawing I posted above shows the methodology for setting out a 5 centred arch. :thumb

5 centred arch.jpg
Tim Lee

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barhamd
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Re: Bridge arch question

Postby barhamd » Wed Nov 14, 2018 1:30 pm

Yep, managed to follow a better description online...

5-centre arch shown in red.

bridge2.jpg
bridge2.jpg (181.5 KiB) Viewed 1377 times


that's a new skill learnt..

David

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barhamd
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Re: Bridge arch question

Postby barhamd » Wed Nov 14, 2018 3:42 pm

And just to show the difference here are the two together with the brickwork drawn on.

bridge3.jpg
bridge3.jpg (228.41 KiB) Viewed 1346 times


And obviously any fool can spot the difference from a typically viewing distance.

Scalefour, Getting it, Alright?

David

dal-t
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Re: Bridge arch question

Postby dal-t » Thu Nov 15, 2018 10:01 am

At the risk of spreading confusion where there should be none, am I right in thinking the photo shows a bridge with a (slight) skew, but the drawing is square-on?
David L-T

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barhamd
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Re: Bridge arch question

Postby barhamd » Thu Nov 15, 2018 2:19 pm

Hi David,

No, it might look like that but the bridge isn't skew, there is just a slight gradient on the road over it.

It is easier to see if you have the 'full colour' version at a decent size.

IMG_0258a.jpg


You still have the white sighting painted area behind the advanced starter.

IMG_0259a.jpg


David

dal-t
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Re: Bridge arch question

Postby dal-t » Thu Nov 15, 2018 9:27 pm

Ah, fooled by a slope - thanks for clearing that up! I do love sighting panels (guilty as charged, m'lud, of blatantly over-using them whenever the possibility presents). Obviously that bridge was custom-made for one of my static dioramas ...
David L-T


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