Many years ago, Peter Ross, then at Arup R&D challenged me to produce a thrust line analysis of a rose window that showed it would work. I never quite got to that point. It is a difficult three dimensional problem. I will get there one day though, because they do work so it MUST be possible to find the load paths. I do have some ideas and they will be expounded below.
A couple of days ago, while working on the cathedral bridge, I found myself talking to Bruce Duncan (Lazenby Chaplain at the University of Exeter) about other things I do and that got me back to thinking about roses and about the Dean's Eye at Lincoln in particular.
The Dean's Eye is huge. I have a photocopy of a photogrametric survey which shows the outer diameter as about 8.5m and the main diameter of the tracery as 7.3m. Converting those to feet gives 28 and 24. So, there is a 2ft chamfer on the walls, reducing the thickness to that of the tracery which is only 8in (200mm). We have an open network of stone only 200mm thick and 7.3m diameter. It is high in the north wall of a cathedral which in turn is on top of a hill. There is nothing between it and the north pole to soften a northerly wind. It has stood up for the better part of 700 years, albeit with some repairs along the way.
What is very strange about the layout of the window, though, is the use of iron bars within the stone and bronze bars outside it in some places. The drawing I have done of the window as it is now shows the bronze bars crossing at the centre. The iron bars are in the straight members which bear on the hollow faces of the stone surrounding the centre quatrefoil.
I was uncomfortable with the bronze bars from the moment I saw them, but when I saw the photogrammetric survey, I realised that the window could not, originally, have been built like that. If we take out the spurious, relatively modern, parts (we know the stone isn't original because it comes from a different place) the picture looks like this.
It is, surely, obvious that the original window was built like the final drawing. Here, there is at least a relatively direct path through the stonework across the cente of the window.
I think I will stop there and leave further thoughts on Roses for a separate post.
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