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How to build stone arches on a railway bridge.


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Hoping to tap your brains, so grateful for any guidance. I'm in the process of building a three arch railway bridge on my own layout. I've cut the trackbed and sides and there are six arches, each of which is a different radius as the bridge is both on a curve and skewed across the four track main line.

 

DSCF0111.jpg.fb03215ae4829a38070d11cd90efb459.jpg

 

I shall probably use either Wills or Slater stone sheet to clad the bridge and have no problem with the pillars or string courses etc. Where I really need some input is how to create the stone arch that supports the bridge over the tracks.

 

Here's a typical bridge.

 

420308098_Screenshot2019-12-05at17_32_14.png.69fd29b7e71c7c439e102c2519adfbd3.png

 

Is there an easy way of creating the stones that form the arch? Are they parallel stones or is each stone tapered?  They appear parallel in this pic but others I have seen are definitely tapered.

 

I can cut the overall shape, once I understand how to create the arch itself. I've played around with chord formulae and have created some dummy shapes in Templot, but it's really how to model the three dimensional shape of the stones and the angles to scribe them.

 

Any chance you can give me some inspiration on a method that works for you?

 

Thanks guys....

 

 

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Gordon,

 

First you need to create the elipse, to do this you need the magor axis (a horizontal line where the arch is horizontal) and then the minor axis (this will be measured as half of the axis, using a straight edge on the magor axis measure vertical to the midpoint of the arch and double it to get the minor axis. There are various ways to draw the elipse or oval, draw a 2nd oval parallel to the first,  the distance apart of your arch stones.

 

The arch stones should be slightly tapered in the centre and increasing in taper towards the ends. I would start drawing the arch stones from the centre key stone and work out from there. The joints should be more or less 'square' to the elipse line.

 

If you hit a snag, post the magor and minor axis on here and your desired arch stone size and I will draw it up on cad for you.

 

 

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Apologies SS, as I may have misled you. The arches are not elliptical as per the pic, but are of a constant radius. I was able to measure the width of each arch and knowing the height of the chord, that gave me the radius. I took that into Templot and drew a Template to that radius which I was able to cut along the centre line and the outer rail. That gave me a paper template arch that fitted the shape.

 

So far so good...

 

It's the next bit where I'm looking for ideas. Having cut the paper shape, I could tape that to a sheet of Plasticard and cut out the shape. I'm OK to that point. It's the stones themselves and raised texture where I'm looking for ideas.

 

If the stones are not parallel, then in theory I can take them back to a centre point and scribe lines or if parallel, mark out some pencil lines and then scribe them by hand. I did wonder about using plaster or DAS clay, but I haven't done that before, so it would be a learning curve.

 

Couple more pics I've found. A brick arch beneath the stonework...

 

121182023_Screenshot2019-12-05at18_51_03.png.ebbfadd83e0acb7e2fc063c1a15b43d5.png

 

...or here, the arch stones have shaped ends.

 

187058664_Screenshot2019-12-05at18_52_49.png.32fc0e30f1733b441d5fe7b83e5e1559.png

 

No problem cutting the shapes of the stones, but cutting the stone sheet to match may be a little fiddly to get a good fit.

 

I'm sure there are expert modellers on here that can do it in their sleep, but as a 'scenic and structural' beginner, I'm hoping they will share their experience.

 

I really appreciate your offer of mapping them out on CAD and I may well take you up on that.

 

I'm guessing laser etch material will be expensive for six arches, but maybe not if it looks good and saves a lot of time.

 

 

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From an engineering viewpoint, I would be surprised if they weren't tapered, Gordon.

 

From a modeling viewpoint, if you don't want to use plain plasticard - the route I took for the approach spans to the big bridge on Stockrington - I would suggest cutting yourself individual stones from the Slaters 7mm sheet.   Cut them to the exact size you need, and then use filler to make good any gaps or stones not proud enough compared to their neighbours.

 

Cheers

 

Scott

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Hi Gordon,

 

Your design are skew arches, which has a significant effect on the design. Lots of pictures and explanation on this page: https://en.wikipedia.org/wiki/Skew_arch

 

A few of them:

 

Stephensonbridge1.jpg.242e37cae598ef7955adf7f2625bc975.jpg

Public Domain, https://commons.wikimedia.org/w/index.php?curid=16134839

 

Hereford_Road_bridge,_Ledbury_(2).jpg.e1345fa1d3270989744adc2735dcc445.jpg

MegaPedant CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7854624

 

1024px-Boxmoor_Skew_Bridge_detail.jpg.43ab1efa0daf8bac85c96427d3c90840.jpg

Creative Commons Attribution-Share Alike 3.0 Unported

 

Skew_Bridge_LLC-74A_(1).jpg.80b09f6c677744e360b1d2ef11917d5c.jpg

Peter Robinson Creative Commons Attribution-Share Alike 3.0 Unported

 

Martin.

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Thanks guys for taking the time to respond. The pics are very useful Martin and the link you provided back to Stockrington, Scott, was a great reminder of your excellent work. I started reading it early this morning and will revisit it again once I get started today. Friday is always a day I have to myself, so I have several hours to digest your ideas and fabricate a solution. I still have some Wills and Slaters sheets kicking around, so it will be great to have a play around and see what materialises.

 

Ernie was kind to me this month, so I've invested in a Proxxon scroll saw, which should prove invaluable for cutting intricate shapes around brickwork.

 

Where would we be without this resource and it's contributors....:good:

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One last thought as my day ends and your's starts, Gordon; Martin's photos are very useful and he has a valid point about your bridge being prominently skewed. 

 

You know my mantra is that even though I'm not modelling a specific location, I will always be guided by real life examples.  With that in mind, when you do create your stone veneer, think about representing the walling above the deck in a different material to the arch itself, or at least having a representation of the deck line in the outer brickwork. 

 

I'm a civil engineer by trade, and one of my pet modelling peeves is getting the "balance" between the span and the thickness of the arch wrong; in all the photos above, there's very little thickness between the keystone and the deck at the top of the arch - make the arch too thick, and the span will not look delicate enough.  You'll work it out, I'm sure.

 

Cheers,

 

Scott

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That's a very welcome intervention, Scott, as I was just pencilling in where the string course was going to be and was concerned there was room for the arch stones, but not much space above to continue the walling between the centre of the arch and the string course above. Your reminder and Martin's pics show I needn't worry as there is only a few stone courses above the arch stones. None at all in the first pic and just one in the last.

 

It was great to revisit your thread and see again the very graceful towers of your own bridge.

 

Off to fiddle with a few bits of wood and plastic.....

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Hi Gordon,

 

You asked about stone masonry bridges, but I think brick would be more in keeping with your site. Probably blue engineering brick.

 

Railway-over-railway bridges do tend to be brick, by definition coming later than the original laying down of the great main lines. Also locality plays a part. Where is Eastwood Town? How far to the nearest stone quarry? Brick is much easier to transport and bricks were used in their millions for Victorian infrastructure such as this. Also the significant skew would make any engineer choose brick given half a chance. 

 

cheers,

 

Martin.

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I agree with Martin that the masonry work would most likely be in brickwork, rather than stone. What we really need is a brief history of your scenario, as this would impact on the solution adopted. Introducing a substantial brick or stone skew arch crossing an existing line would close the line for quite a period of time, and if the bridge is a foreign interloper this would be totally unacceptable. Similarly, the extent of foundations permitted within the ownership of the lower line would also be severely restricted, if not totally disallowed, and construction would require access to the lower line's property.

I would also suggest that, given the rather restricted height that seems to be available there is insufficient room to accommodate the depth required for the arch itself, together with the depth of construction required for the rail bed above, and the length of the middle arch and the extreme skew of it would make the stability of the whole thing rather suspect. I would therefor suggest that probable solution would involve, at least for the middle arch, the provision of a steel structure, probably a fairly simple girder type, although the extreme skew might require a grid-iron solution, as per the Metropilitan Widened Lines. This would maximise the depth of road bed above, and also reduce the track occupation period to a minimum.

There is a similar situation near Stewart's Lane depot, and a photo of a similar bridge on the M&GN being erected shows how this could be done by lifting from the high level tracks, without impinging much on the line below.

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Martin has a valid point about the type of masonry used in the bridge construction,  it's pretty likely that in the South East of England brick would be the predominant building material, due to the extensive clay layers prevailing there and ISTR that your station building is brick,  so it would follow that brick would be used for other structures, or brick piers and steel girders, with possibly brick parapets,  just to relieve the monotony,  especially in later structures.

 

Any circle on the skew is an elipse or oval.

 

 

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Concerning the drawing of ellipses (if anybody needs to do this, and yes I understand that skew arches are not the same): almost all computer drawing-packages have a circle tool that can stretch the circle into a true ellipse. Just print out a template to the size you need.

 

Concerning skew arches: I would start with a 3D drawing-package (one of the free ones would do), draw the bridge as a cuboid, then bore through it a circular elliptical section, on the desired skew, using the package's "intersection" function. I am ignorant of civil-engineering practice, so would be guided by whatever looked right in comparison with photos.

 

Concerning a dressed-stone edge to an arch in a rough-stone bridge, I'd be inclined to use the moulded, rough stone, then fill along the line of the edging and script the dressed stones.

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This will probably explain why geometry wasn’t my strongest suit back in the early 60’s, but trying to get my head around circles and ellipses and whether this a real situation or an optical illusion depending on the view.

 

Looking down the track at a skew bridge, I can understand a circle becomes an ellipse, but if you are looking at right angles to the front face, is the arch elliptical or is it still circular?

 

My wife has been wondering what I’m doing waving a plate in the air whilst we’re enjoying coffee and lemon curd tarts.
 

Stop laughing at the back.....:D

 

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2 hours ago, Siberian Snooper said:

Looking along the track it will look circular until you get close enough to see the skew.

 

 

 

Are you sure? My understanding is the opposite. Looking down the track with the bridge on the skew going away from you from left to right, it gives the appearance of an ellipse with the apex? nearer the front left. Standing at right angles to the bridge itself, the arch will appear circular.

 

At least that's what my experiment with my morning coffee plate showed.....:D

 

Anyway, enough of the Christmas jollity, I have a railway to build.  Thanks for your input guys. It really is much appreciated as I'd hate to create a real howler, even though none of my family and friends would know or care.....;)

 

I have ordered a few brick papers to play around with and even spent an hour or so last night relaying the four lines of Templot to 2+2 instead of 1+2+1 to give me another option.

 

In the old days, you just reached in the box, pulled out a few bits of track on the carpet and ran trains rounds and round. Now we make life hard for ourselves trying to create perfection.

 

Life was simple then....:D

 

 

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37 minutes ago, gordon s said:

Are you sure? My understanding is the opposite. Looking down the track with the bridge on the skew going away from you from left to right, it gives the appearance of an ellipse with the apex? nearer the front left. Standing at right angles to the bridge itself, the arch will appear circular.

 

Hi Gordon,

 

It could be either or neither -- it depends how the bridge was designed. There are several different methods for skew arches.

 

The principle is that the force supporting the load should travel perpendicular from the face of one stone/brick into the face of the adjacent stone/brick. If the force is at an angle it will tend in time to make the stones/bricks slide across one another and the bridge will fall apart. In a skew arch everything is at an angle, so laying out the rows of stones/bricks is tricky. To keep the forces perpendicular the faces of each stone must be cut at complex angles. That needs skilled masons and some means of drawing out the plans of each stone, hence the number of different skew designs. With rectangular bricks it is easier, in that they are small enough in relation to the size of the arch that you can fudge it with the mortar. That's why there was a great preference for brick as soon as anyone mentioned "skew".

 

cheers,

 

Martin.

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Hi Gordon,

 

When I built the bridge for my Bakewell layout I copied the prototype construction, which was a skew bridge of stone with a brick lining inside the arch.

 

This is the prototype:

 

bridge-underside.jpg.64702c893f6dc9060b39561d1068e298.jpg

 

You will notice that the brick courses are set on a diagonal with reference to the structure.

 

I modelled it in similar fashion:

 

bak-bridge0033.jpg.8f24cbda9a42520f56255b6f03d66f79.jpg

 

 

bak-bridge0034.jpg.23f242e4f51e57e58d9a50d7c14f3583.jpg

 

With regard to your question regarding the arch stones, if the arch is of constant curve, then I use a pair of dividers to cut an arc out of plain styrene sheet, and use the dividers set to the same radius from a known origin to scribe the stone sheet. I then cut up the arc into seperate stones, starting from the centre to make sure the key stone is in the right place, and then stick them to the structure.

 

the-bridge-build067.jpg.361b4169f1552ee93fc6f23f7e2c00d7.jpg

 

For a different structure I used a silhouette cutter to cut out a set of "pointed" arch stones to fit a tunnel mouth, and then painstakingly cut around the resulting shape in the stone sheet. Then as before I seperated the individual stones:

 

the-tunnel-build009.jpg.2a72d0ca56768d10696be5e81aeb22a8.jpg

 

 

the-tunnel-build011.jpg.fea7e2b2e7f0c30e72aa466516154640.jpg

 

The resulting structure:

 

the-tunnel-build043.jpg.490a294afddfb6c6cb20cf1a2fd51c36.jpg

 

Hope this helps.

 

Al

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6 hours ago, Alister_G said:

Hi Gordon,

 

When I built the bridge for my Bakewell layout I copied the prototype construction, which was a skew bridge of stone with a brick lining inside the arch.

 

This is the prototype:

 

bridge-underside.jpg.64702c893f6dc9060b39561d1068e298.jpg

 

You will notice that the brick courses are set on a diagonal with reference to the structure.

 

I modelled it in similar fashion:

 

bak-bridge0033.jpg.8f24cbda9a42520f56255b6f03d66f79.jpg

 

 

I was lucky to have a former architect help me with my arches. He asked me if I intended to 'rifle' the arch & he had to explain what that meant. Your photos sho it nicely though.

 

 

I bought a laser cutter to get my arch faces right. Mine are brick, which is much more uniform & lends itself to something like laser cutting. Stone has a bit more depth & detail.

While I may be ok at controlling the laser cutter, I needed help to create the artwork so I am unable to volunteer to help with that part.

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Phew! Love the family dearly, but they've all gone home now.....:D

 

Thanks you for your response Alister. You've made a lovely job of that bridge. Good to see Slaters 7mm dressed stone making another appearance.  I did wander away from it as it can look overscale in certain situations, but I've yet to find a decent alternative. It really looks the part on your bridge....

 

I first saw it put to good use on Tetley Mill's. It all looked so good together and Dave had a wonderful eye to capture the atmosphere of a much wider area. I still have some as I used it on the early versions of ET and I'm sure it will appear again on retaining walls and bridges.

 

I experimented with scribing stones, but cutting them out and dealing with them separately, is certainly more effective.

 

Can you tell me more about laser cutting, Pete? As an ex draughtsman all those years ago, I feel reasonably confident about generating an artwork (I can do that!) but suspect there is far more to it.

 

I'll take a look at your Bakewell thread, Alister, as I'm not sure I've seen it before. Just noticed it's 143 pages, so I'm sure there will be loads of useful information.

 

 

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@gordon s There is another thread (it was on here a few days ago) imprinting onto foam board/thin extruded polystyrene using ferrules from small paintbrushes bent into rectangular shapes. The extruded polystyrene seemed to give a slightly more realistic finish as the board was harder and gave the edges a slightly cracked finish making it look like worked stone rather than a cut finish (rubble rather than ashlar?). The front face will normally be tapered and underneath parallel, the tapering can be achieved by bending the ferrule to shape - if that's the way you should want to go.

 

Interesting selection of photos from @martin_wynne . I recognised midway the one bridge that is in Ledbury. The multi-brick arch supporting the Ledbury to Gloucester branch was not a success. Yes, easy to erect at the time on the skew, but because the soffit was discontinuous, in rain it leaked like a sieve (still does!).

 

Cheers,

 

Philip

 

BTW - good luck with Eastwood. I keep an eye on your progress.

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Just now, gordon s said:

Can you tell me more about laser cutting, Pete? As an ex draughtsman all those years ago, I feel reasonably confident about generating an artwork (I can do that!) but suspect there is far more to it.

 

Probably easiest to describe things in my layout thread. Don't worry, it is one of the shorter ones, being only 2 pages long so far. Someone asked me about the laser stuff so I described it a little the last time I added to the thread.

Laser cut stuff bought at shows tends to be woof (MDF I think) but I have used Acrylic (Perspex). Thinner is easier to cut but the laser's heat warps it. Thicker requires several passes to cut it.

After trying thicker & thinner sheets, I settled on 2mm acrylic. This curls a bit so needs good gluing onto something nice & flat like plywood. I could only find 2mm acrylic in clear, but I was not bothered by this because anything would have needed painting anyway.

I tried a cheap cutter (Chinese type based on an engine called a K40) & it does what I want it to. 'Cheap' for a laser was £400. Better ones cost many times this.

I found the included software completely unusable. A nice, big design for squashed into a small space the size of an SD card & I could find no way to control this, so I sought out an alternative & found K40 Whisperer, which worked for me. This also requires some freeware called Inkscape to format it in 3 colours: black for raster engraving, blue for vector engraving & red for cutting. Inkscape seems to re-size anything imported from Illustrator. It is easy to re-size but you just need to be aware that it needs to be done.

The raster engraving was suitable for cutting brick courses. I am not sure it wold be controllable enough to do the contours of a stone. Raster engraving is a similar process to dot matrix printing, but being finer, can take 2-3 hours to do a wall. The arches over the DC lines on my layout took about 2 hours per side & the double small tunnel portal took a little longer.

Cutting usually requires several passes.

 

Laser cut items on my layout are:

Large tunnel portals (including the surrounding walls), the capping on top & their base stones. The circular capping around the portals was a bead of plastruct fixed in place.

Small tunnel portals (including the surrounding walls)

DC line bridge arches (including the surrounding walls)

Arches alongside the fast lines (There are some images with clear laser cut parts & red Slaters brick card, so you can see which is which).

Tunnel arch for the GC line which passes over the top. You won't find a photo of this because I have never taken one & have yet to fit it on the layout.

 

Vector engraving is quicker of you want to engrave fine lines. I think I have only ever used this for the stones at the base of the large tunnel portals.

The artwork was done in Adobe Illustrator. I have searched for free alternatives, but the best I could find was Inkscape. I had read it was 'almost as good as Illustrator', but I found this to be inaccurate.

 

The laser cutter itself is another issue. It can cut 300x200mm, (virtually A4). The cutter itself is a big beast though. It has a laser tube along the back of the unit & a power supply on the right with a control panel on top. This makes its footprint about A1 size.

The burnt material creates fumes, so it has an extraction fan with a large hose, so needs to be placed by a window. The laser get hot, so it needs to be cooled. A water pump is supplied for this, but it needs a water supply. I use a bucket.

So on my kitchen table is an A1 sized machine & siting beside it is a bucket of water sitting on a chair. I am not sure how I could get away with that if I did not live alone.

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Alan Downs captured many scenic structures which were visually stunning, were they architecturally  correct?   I doubt it but like back scenes in shows they look the part. Sometimes things which are dimensionally accurate look wrong when scaled down

 

Thinking back to your previous efforts they looked the part and did not detract from the stars of the show being the rolling stock 

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