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The lattice truss is unusual and it is difficult to establish its purpose. Constructed of (possibly) the equivalent of 4 x 4 and 6 x 6 inch timber members, the structure (calculated on a six inch wide beam) is about 90 feet wide. It has been catalogued in the past as 'Section a Large Roof or Centre for an Arch'. Its external profile is eight-sided, internally it has five sides and nothing indicates an arch form.
David Yeomans, structural engineer and historian wrote (letter with drawings, 8 June 2006) 'It cannot be a bridge. Where is the deck? And if it's the many layers of horizontal members, where does it go to? I doubt if it can be an arch centre as the angles of the outside shape do not lie on a circular curve - much too pointed towards the top.
I start thinking about this, wondering how it was made. I drew out the longest members (page 1) but curiously these do not go through the principal points on the circumference, A - D, except for B. So were these long members the starting point, or simply stiffeners put on at the end? I decided that was a red herring.
If I were building I would first set out the long members 1 (see page 2). This is a four-bar chain (the ground is the fourth bar) and has to be stiffened to keep it stable so I must add in the stiffening frame 2. This provides two of the horizontal timbers. On top of these I put the third horizontal and the supporting braces at the ends 3. This defines the point B. Note also that these horizontals are all continuous, which is not true of the one above them - the top horizontal.
Having got the framework I can now erect the rectangular frame 4 to define C, and the king post 5 to define D. Horizontals are put between these posts and the other two verticals stood on those when required.
But first I want to define point A - see page 3 - which I can do from the point B and the framing of 1 and 2 (page 2) that I have already built and of course the point on the 'ground'. All the rest is just a lot of extra timber to make it all stiff enough.
But what is all for? Go back to my page 2. What is the purpose of the lower member of frame 2? - the timber that my leader goes to. This member could more easily be fixed to frame 1. Either the point at the bottom, marked X, had to be defined or it was where the support came from.
At first I wondered if this was the framing of a dome, so that the inside shape was as important as the outside - an internal dome to suit the space below and an external one for sufficient grandeur. But that does not make sense because we would need to have several intersecting at the centre, and there is no sign of that. At the moment my best guess is that is it some form of 'scaffolding.' DY may re-write his notes (4.1.07) and re-draw?
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Sir John Soane's collection includes some 30,000 architectural, design and topographical drawings which is a very important resource for scholars worldwide. His was the first architect’s collection to attempt to preserve the best in design for the architectural profession in the future, and it did so by assembling as exemplars surviving drawings by great Renaissance masters and by the leading architects in Britain in the 17th and 18th centuries and his near contemporaries such as Sir William Chambers, Robert Adam and George Dance the Younger. These drawings sit side by side with 9,000 drawings in Soane’s own hand or those of the pupils in his office, covering his early work as a student, his time in Italy and the drawings produced in the course of his architectural practice from 1780 until the 1830s.
Browse (via the vertical menu to the left) and search results for Drawings include a mixture of Concise catalogue records – drawn from an outline list of the collection – and fuller records where drawings have been catalogued in more detail (an ongoing process).