A blogging Carnival; Grand Challenges for Archaeology; reverse engineering Stonehenge

#blogarch

In response to the latest blog Carnival organised by Doug Rocks-Macqueen, the champion of archaeological blogging, over at Doug’sArchaeology, I am posting about the challenges of modelling a prehistoric roof structure in 3D.

The story so far…

My work is based on the idea that archaeological buildings are mathematical structures which can be detected and understood using the same principles that underpin the engineering of the built environment.

As regular readers will know, I have had the misfortune to have discovered how, in theory, the large Neolithic / EBA structures represented by postholes known as Class Ei buildings [1] worked, at least in plan and section.   The next stage is to model the structure in 3D to understand its assembly; the initial challenge is finding an appropriate starting point, since the value of everything else, and many man hours is dependent on this decision.  What is also challenging, at least in an abstract sense, is that the Ei building I am currently modelling at moment is Stonehenge, the well-known ritual monument and mystery at heart of British faith-based archaeology.

While I have established, by what might be called conventional means, that these arrangements of postholes represented the footprint of buildings, listing 12 fairly straightforward observations that demonstrate this, it was necessary to develop a more complex understanding to explain precisely how these posts might support a roof.

I called this structural system Interlace Theory; just how complex it is, may be judged by the fact that I first grasped the principle almost exactly 6 years ago, [although to be fair on myself I have been otherwise engaged during most of the intervening period].  Woodhenge has been my principle case study because it is by far the best data set, however, Stonehenge is a much simpler structure and it has an elevation which can serve as the necessary starting point.  

A tale of two analogies

While I am happy to discuss a myriad of minor matters arising from interlace theory, it would be a somewhat monotonous monologue, - you know the bit after you tell the taxi driver that you are not aware of a secret room under the sphinx - so I will find a different way of explaining this stage of the research.
As an archaeologist, I might find pieces of kitted fabric, and while it is possible to reverse engineer the process of knitting, how you construct garments out of a fabric is a very different question.  Just like buildings, garments vary in pattern, size and style, which cannot be fully extrapolated from the basic technique.  Interlace theory explains how the structural might fabric work, but not necessarily how it was used to  construct a particular building; as with garments, this  involves different and more flexible rules.
However, if we find a piece of a tailored cloth then it might be possible to reconstruct the type of  garment,  or,  a better analogy might be finding a piece of metal, which luckily it is recognised as the base plate from a watch.   Each of the holes in the plate correspond to the location of a component, which, with a knowledge of watch-making, you could reverse engineer the mechanism.  However, while you could demonstrate it was part of a watch by modelling how it worked, what it actually looked like would not necessarily be apparent.   Regrettably, If you do not understand horology it will remain a ritual object.  

Doin’ the math

While watches are complicated, they are small compared to a building, which may have simpler engineering, but a lot more components, creating its own complexities for modeller and builder alike.  While a modern building might get away with a hundred standard mass produced components, most of the components in a prehistoric structure are bespoke and made to fit, which is time consuming and resource intensive.  
It is also worth noting that most drawing systems are set up of assumption you will be modelling rectilinear structures, in which for example, the rafter pairs are at right angles or parallel to the main axis.  By contrast, in a circular building the rafters are radial and all at different angles relative to whatever serves as the principle axis.  We can do a simple guestimation to demonstrate how complex roofing a structure the size of Stonehenge might become;  just like the builder you have to work up from the lowest components, which in terms of the roof is the outer edge represented by the 30 Y posts.   
The footprint of a building is was not circular, neat or regular, it is 30 sided, an irregular Triacontagon;  to cover it would take the equivalent of 4 roofs with a combined width of 180’, which, if we want a rafter every 6’ along the edge need about 90 rafter pairs or 3 supported per Y post.  Each pair has a tie so that’s 3 per post along with the 3 horizontal plates or elements, which is actually 6 because they run between 2  Y posts.  So that is 9 horizontal components per post, which we could double if a clerestory or widows are indicated, which is why with a guestimated 270 or 540 components to be attached to the 30 Y posts and fitted together, before we reach the 90 or 180 pairs of rafters which form this outer part of roof, which is why where you start is so important. 
However you figure it, it’s is going to be a complex challenge to assemble such a structure successfully so that components don’t pass through each other, which is why Interlace Theory was necessary.   It is also why I am working on Stonehenge, since structures are supported at their lowest point, the stone components particularly the stanchions [trilithons]   give us a significant clue to the position of the lowest parts of the structure and a starting point. 
In the model  I have created and positioned 90 individual ties, now the challenge is too fit them together systematically, this could take some time. 

Snapshot from a 3D model of a ritual monument showing 90 roof ties.

The Challenges of reverse engineering
Strange as it might seem, it is important not to think about what the structure looked like, but while it is actually hard not to speculate, and if pressed I could tell you all sorts of things about this type of roof, the problem is that I would almost certainly turn out to be wrong, and this is not about guess work. Reverse engineering an archaeological structure is an evidence based deductive process, so, in an ideal world, it is not; 

• how I would have done it;
• the best way of doing it;
• the simplest way of doing it;
• how someone else does it, or did it, whether locally, or on the other side of the world.

Problematically, the latter is a basic building block of much scholarship which considers plan “shape” to be of primary importance regardless of form, function, scale, technology and cultural or environmental  context.
This in turn has resulted in a visual culture of the past for which there is no real evidence.  This has given rise to the biggest challenge faced by structural archaeology - peoples' imagined “pictures” of the past; being told that Stonehenge was a largely wooden building is a bit like trying to imagine a different coloured Jesus.  

Post University Challenge.
It is often said that employers or institutions want people who can think for themselves, while what they actually want is people think like themselves.  Universities invest in and monetise accepted ideas, thus new ones are not welcome, as they disrupt the market, and so research that does not support the existing narrative is of no inherent value.
In later periods people dug postholes for buildings, and pits to shit in, but  apparently, in the British Neolithic they did both to communicate complex ideas to supernatural forces and post-processualists.  Thus, while research into “...how postholes served as focus for structured deposition in a wider ritual landscape..” could be funded and even peer reviewed, evidence based reverse engineering will remain way too challenging for Universities, but freely available in the blogosphere.

. . . and on a lighter note

As a gesture to the Gods indicating we intend to try and keep on going, TSA has rescued a new member of staff; Tiny is in charge of destructive testing, spatial redistribution, disciplining the printer, and pest control.  This year, I was trying to teach him structural archaeology, but his responses are a bit incoherent, and he is still coming to terms with the concept of gravity; however, he is young, so we have decided it would be best if he concentrated on his master’s degree in post-processual thinking, at least until his brain develops further.


Source
[1] Neolithic houses in northwest Europe and beyond’ (1996) (Oxbow monograph 57) [Paperback]. T.C. Darvill (Editor), Julian Thomas (Editor) [[Fig 6.9]].