Arch Bridge Like Arley

Peaky 556 · May 07 2015 23:05

Hello all,
My bridge musings in the other thread have reached a conclusion in that I'd like to model an 'arch' bridge, and in particular the one taking the SVR across the River Severn. It is variously called the 'Victoria'or 'Arley' Bridge. I cannot post any photos of it as they all have copyrights! If you Google the Arley or Victoria bridge images you will see exactly how lovely this early cast iron and wrought iron structure is.
My book tells me that an 'arch' bridge is one that uses an arch to carry gravity loads, and that bridge abutments are required to prevent the arch from 'speading'. This makes it distinct from a girder or truss design in which the various stiffening members are inter-connected such that the bridge will just "sit"on the top of abutments without any spreading force.
In brief my bridge has a span of 5 metres, an arch radius of approx 6 metres, and will be required to carry my weight in the centre, on the odd occasion that emergency retrieval is required! I am adding a little to make a design load of 100 kg.
I will describe progress in this thread, and the design is developed already to use readily available steel lengths, in combination with some laser cut pieces. Before I finalise the design there is the not inconsiderable matter of stress analysis! If it breaks I will be dropped into the pond, and very red-faced too! My immediate problem is not finding any convenient formulae for deflection and bending moments in an arch member. Is anyone out there a civil engineer? I will continue looking for one.
Bye for now,Tim

AllWight · May 08 2015 08:09

Very ornate bridge. Was used in the 1970 remake of "the 39 steps" with Robert Powell. Point of interest it was of similar construction to Ironbridge from the village of the same name. May be worth a look on the web for scamatic details and drawings. All this talk of bridges reminds me of my school history lessons on the black country and particularly Coalbrookdale.

Mark

Peaky 556 · May 11 2015 20:56

I shall watch the film with interest next time Mark.
I have been a little worried about my ability to stress the bridge, so searched for some suitable learned papers. I found one by Dym and Williams, in the Journal of Structural Engineering. Maybe your Dad knows of them Andy?
The outcome is that my arch is far too highly stressed, so much compressive axial load that it would certainly cripple under my weight. It appears that I need to adjust the proportions, in particular making the arch 'less shallow', and thicken it up where possible. I'll post again when I have a design that works!
Regards, Tim

Peaky 556 · May 17 2015 23:36

Hi all,
Just a quick post to let you know the project is progressing, as do all of my projects albeit some move incredibly slowly!
I have revised the design to have a smaller radius arch, meaning the arch is 'less flat' and instead the ends of the arch come down more or less to ground level. This looks more prototypical, and gives lower compressive loadings.
Using two technical sources for loadings, deflections and stresses on arches (Roark 4th Edn 'Formulas for Stress and Strain' and Dym/Williams), and applying traditional Euler crippling formulae, it all seems to work. It is difficult to rationalise the two methods for loading, as one is based on a point load of 100kg in the middle, and the other assumes the 100kg is spread evenly across the span, however the worst case is OK. I will try and get some independent checks done by work colleagues, as its all 'back of the fag packet' hand scrawls at present!
Cheers, Tim

Peaky 556 · May 27 2015 21:45

Hello all,
My calcs have been checked and I spotted a few mistakes. The outcome is very positive, strength should be very adequate for a 100 kg load and I am busily drawing up the arch segments, stiffeners and handrail stanchions on CAD for laser cutting. As I will soon be ready for quotation, I think in the best interests of value for money I should go out for competitive tender. I have an uneasy feeling it will all be very expensive!

My question to you all is "are there any laser cutting contractors out there, used to handling small quantities and sheet steel used by the model maker, in addition to MEL?" I seem to remember that you have used someone else Mike? Just having two to quote, and knowing they are in competition, should give me a more realistic price. Of course it may all be too much and I may resort to a couple of planks!!

Regards, Tim

MikeWilliams · May 27 2015 23:26

Tim,

I would not go to a model supplier because they are probably too expensive and probably just pass it to another firm to do the actual cutting anyway. I normally use B&Z Metalcraft at Leighton Buzzard, but have used a couple of places in Milton Keynes, one near Coventry (Coventry Laser Cutters) and even one at Keighley which alas has closed down.

I would do a local search for steel fabrication people and call them. If they are using the metal you need they might just slip yours in at the end of their bugger jobs, especially for cash.

Can you do all the CAD yourself? Might tip the balance in your favour to have that with you when you make contact as so often these professionals despair when they see artwork by modellers which needs a lot of work before they can use it - work they can't charge for. I have however found that most places are very busy at the moment - seems that for them the recession is over and they can't keep up with demand.

If you have any problems drop me a line and I'll try.

Mike

MikeWilliams · May 27 2015 23:33

This is who I normally use
http://www.bandzmetalcraft.co.uk/

but this one sounds possible
http://www.stannssheetmetal.co.uk/index.htm

Mike

Peaky 556 · May 28 2015 22:50

Mike,
Thanks very much for the leads. I have sent files off to Saint Ann's Sheet Metal Co for quotation. They would be very convenient for us East Midlands lot if they come good...
Fingers crossed,
Tim

Peaky 556 · May 30 2015 12:28

If this bridge structure is affordable, and I now have a second invitation out there to quote for the laser cut steel bits, I will need to think about how to "tin bash" the curved segments of the arch to make them into channel section, to simulate the original castings. I am thinking along the lines of a piece of half-inch ply, shaped to the "pocket" size, then I clamp the steel panels to it one at a time and bash the edges over so I have a curved steel "tray" with the four edges about 12 mm high.The corners are then MIG welded to make the segments rigid. Each segment is then "pop" rivetted together end to end to make the arch length required.
Basically I need this piece of ply shaped to have edges of around 14.3 m radius, with the ends radially disposed.
Unless there are any other good ideas for putting large radii onto bits of wood, and I don't think the long bit of string and pencil method will be accurate enough, maybe I need to get some ply blanks laser cut too?
Is this feasible?

Tim

Andy B · May 30 2015 13:15

Plywood is easily laser cut or CNC routed.

Might be worth trying the following first, if you only need one former:
Plot it on paper (with a grid to check plotter accuracy), stick paper to plywood, use saw / plane / spokeshave to cut to shape?
If you need >A3 size plots I know someone who could get that done f.o.c.

Andy

Peaky 556 · May 30 2015 15:44

Thanks Andy, hadn't thought of that!
So if I draw them out in BiiCADoPro (with thruppenny-bit edges), send it to work and then open it on an Autocad machine, it should plot out fine?
One is a wee bit bigger than A3, but the CAD guys at work will help me out.
Regards, Tim

Peaky 556 · Jun 01 2015 23:29

Well my first quote has arrived for the lasercut parts,and very reasonable I think too. The viability is on! There are over 150 of these parts, with quite a lot of duplication, there only being four designs. This must have enabled the unit cost to be very low.
The most numerous item is a humble handrail stanchion. I have modelled it on simple prom hand railing of approx 2 inches diameter, so am looking for piano wire or similar of around 2mm or 14 swg diameter. So far I have drawn a blank regarding decent lengths.
The span is around 270 feet scaled, so if you think of handrail stanchions about every six feet, this is why there are over 50 down each side!
I'm awaiting two more quotes.
Regards, Tim

Peaky 556 · Jun 11 2015 12:38

A little bit of progress to report.
Three invitations to tender for the laser cut parts were sent out. One has replied they are too busy to give me a quote yet, and two have offered prices. I must say that it is worthwhile going out for tender as the price differences I have are very significant... I am about to place the order.
There is still one supposedly 'standard' channel section that is being elusive. This is basically 1.5" x 0.5" x 0.030" (nowadays its called 38x12.7x0.8 mm!). One drywall sections supplier who lists it has reported no stocks and no prospect of re-stocking. I have a couple of enquiries out for this to be folded up from sheet steel.
For handrailing I have bought in some 14 swg tinned copper wire, which is 2 mm dia. It will look a little 'kinky' when installed but that will just give a care-worn appearance!
One challenge has been to keep the mass of the bridge down to something reasonable, such that two people can lift it away from the pond when it needs repainting etc. I'm quite pleased that the predicted mass is around 52 kg, which should be manageable.
I have plotted out the templates for the two formers for the arches, as suggested by Andy (thanks Andy!).
Errata from my previous posts:
The arch basic radius is 4.3 m not 14.3 m.
The scale span is 370 feet, not 270.
Regards, Tim

Peaky 556 · Jun 25 2015 22:53

This is a real bridge that looks most like the one I am building, except that the span is probably double at over 700 feet, and that it links two countries...
The arch is very well braced to keep it from any sideways collapse, and the columns are simple, straight and unbraced.
I'm gradually buying in the bits, having now all of the square hollow section, bracing wires and ferrules, handrails, bolts and rivets. I have even used some Meccano brackets! At some point soon I dig out my MIG welder and try and remember what I learnt when welding up a Series II Landrover about 15 yrs ago!
Regards, Tim
Ps having trouble with a photo, will try later.

Peaky 556 · Jul 29 2015 12:35

Update to explain the silence!
Two of the three laser cutting suppliers contacted have quoted what I would call 'value for money' prices, but I have made a modification to one component (the diagonal brace between the two arches at multiple positions) to incorporate the ATR (Apple Tree Railway) logo, and asked these two for a re-quote. They currently seem so busy that my request is a long way down their in-trays!

Still no joy with finding a light-gauge steel channel.

Regards, Tim

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