And now for something completely different.
As usual, there is a pdf version posted here.
And a complete index here.
Many years ago now, I was introduced by Mike Chrimes to the special collections in the ICE library. They include Telford’s library; the gift which was the foundation. It has been my habit to browse there whenever I had the time. One of the delights I found was Paul Séjourné's book of Grandes Voûtes. Actually, there are seven volumes, bigger than A4 and about 240 pages per volume. I only ever had chance to browse quickly and it never occurred to me to question who he was. Obviously, surely a historian of a particular bent.
No, indeed, he was the last great arch bridge builder, and collecting this volume was a labour of love. Some, at least, is available scanned on Google.
Anyway, I was browsing in my enormous stack of pictures and came across this.
The first thing to consider is the scale. You could be forgiven for thinking there is a man in a white jacket close to the middle, but the only absolutely clear person is standing with a big stone at about one thirty on the arch. The text says this is Pont du Sornin. It isn’t Constantine because that is over a deep gorge and Luxemburg is bigger. Still haven’t found the location of this. But it does say 35m span.
And since this was first posted, it has been found. A railway viaduct with one big span here.
I was planning to work this BoM around a wonderful sketch by Mark Hipwell who is well worth following on twitter: @markhipwell1990.
But then realised I had to build something myself to do the breakdown.
Things to notice before we break this apart and try to re-assemble. The bottom of the inverted pylon is level with the springings. The radial timbers all radiate from that point, but they don’t go through in a straight line. The outer ends of the radials aren’t equally spaced. The flatter the arch, the less the stone is supported by friction on the one below and the more load the beam must carry so they get shorter.
The bottom part of the arch, up to the top of where there is stone behind it, is built as a corbel without support. Now here is a more schematic version.
All the pieces are labelled. The black bits are pairs of wedges to release the centre when the arch is complete. They carry only vertical force. All the other members must meet either directly above or directly below the wedges. If we lift the centre and the wedges up, that might be clearer.
So, there are two separate elements with wedges between. The joints between the big arch pieces below are complex and I will leave those till last.
The first pieces to go in are A, and they need support from below. That little strut is really handy.
The big arch members are almost certainly pairs of 500mm square (19in) timbers. They weigh about 1.4 tonnes each. Double that and add the weight of the steel connectors for the whole lift. It is a matter of conjecture whether they were lowered in one at a time and then braced, or the whole braced frame went in together.
Once those pieces are tilted and tied in position, the B pieces can go in. They require two wires from the far side and one from the near.
Notice that I have sent the F pieces out with them. If that isn’t right, it isn’t a big issue.
Next, piece C gets slung across and lowered.
Once they are fastened together, there will be modest moment capacity in the joints, but it would be well to keep one pair of guys in place.
Now the struts from the string course to support the horizontal beam must be lowered over, stepped on the string and pulled back against the little spacer struts.
Once they are in place, the horizontal table can be completed with lighter timbers, perhaps single 300mm sq pieces E go in next.
Then the F struts are lifted and clamped.
Now the wedges are laid out, all but the middle three directly supported from below.
And the deck placed for the centre.
Once the first sections of the arch support are in place, building the stonework can begin again, but I will leave that off the sketch.
The next bay goes up.
Then the pylon and guys go in to take the weight of the middle section off the beam. The guys must not be tightened until some weight comes on.
Once all the radials are in place, those long struts need some bracing.
And when the arch is complete. The wedges can be knocked back progressively to lower the centre.
That’s the process complete, but it seems necessary to look at a couple of details that provided clues to how this works.
First, The fabricated shoe at the top of piece A. It is a massive thing but must hold together five pieces of wood (the main members are paired, plus F) and the tie wire. But it sticks out the back too and I think that is a place to anchor a temporary tie to hold it in its initial position. Piece F needs to rotate (possibly) but A and B must be held firmly to create a moment connection. Basically, to stop the arch folding up.
The other shoes are very similar except at the bottom of the pylon.
Here, I have taken one face off to show saddles inside. The cables may have been rods, and may have terminated at this shoe too, but this shows an alternative. Terminating cables is expensive.
The whole Pylon was probably lifted with lighter stuff and then held in place with big strands.
And, as I said at the start. That is very different from the usual fare, but I found the picture fascinating. Perhaps it is worth looking back at it, blurred as it is.
The photo has been cropped for publication so we cannot see what the big stone or box top left is hanging from. Probably a mini gantry on the platform above. At the right, there is something similar further down that looks as though it might have been slid down some timbers. These are possibly boxes of rubble or mortar to build the core of the arch.
This picture of an arch being built shows that Séjourné put in book ends to allow the arch to progress on many fronts. That carries the cost, of course, of also needing many keystones. The propped ladders are pole ladders such as were common on site in my youth. A round straight pole split down the middle with rungs drilled in. They came in considerable lengths but were no doubt more expensive than rungs nailed to battens which are being used to get up the slope of the arch.
And that is another month gone. I hope you are all keeping well in this lockdown.
Oh, and even if we can’t find this one, we should see one of his bridges.
This Picture by Denise Hastert of the Adolphe bridge in Luxembourg (84m span) is kindly provided on a Creative Commons Licence. https://creativecommons.org/licenses/by-sa/4.0/
And here is the man himself: https://en.wikipedia.org/wiki/Paul_S%C3%A9journ%C3%A9