Scalefour Forum

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



thanks

David

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?

May be overly detailed but might be of interest.

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

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.

time for a mock up..

David

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

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)



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

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

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

5-centre arch shown in red.



that's a new skill learnt..

David

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



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

Scalefour, Getting it, Alright?

David

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?

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.



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



David

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 ...