Having roughly devised a strategy for bracing the bamboo against the tension of the fabric and the weight of the cast material, based on my own investigations and discussions with Adam Atraktzi and Chris Matthews, I wanted to test how this would actually work with the physical reality of bamboo culms. Working with an irregular, imperfect medium such as living bamboo, I felt that physical testing was the only way that I would be able to see any flaws in the strategy and test and refine it.
I tried two different methods, using the same bamboo – one of threading rope through drilled holes in the bamboo and one of wrapping around it. These are 1:1 studies. For both, they aim to create a mini-system of a tension ring and then guy ropes attached to the tent in the ground. Also, in each example, natural rope is used and the tensioning method used is that of a trucker's hitch knot. These keeps the strategy low-tech and with natural materials, in contrast with the other options that I have been considering – webbing with ratchet straps, or metal cables with turnbuckles. The process of tying the trucker's hitch knot and tensioning it is shown as part of the video below.
Film showing Strategy B tying, tensioning, and untying process
Strategy A: drilling and threading through
Process and finished strategy photos
One drawback of the drilling method is that it weakens and damages the bamboo culms. This is because it splits the fibres which run axially along the culm, the fibres which give it its strength. Related to this, when the rope is threaded through and then tensioned, there are large forces concentrated around where these split fibres are (show in the diagram below), making it liable to breaking. This would not only compromise the structure for the fabric formwork process, but also render the culms useless for any second-life function as structural members.
Section and plan forces diagram
Strategy B: wrapping around
Process and finished strategy photos
The second strategy appears to be an improvement on all of the negatives identified with the first strategy. Firstly, no time is lost drilling holes. Secondly, the tension in the rope loads the culms evenly, with no high stress points, making use of the naturally strong cylindrical section (shown in the diagram below). Finally, the culms are not in any way damaged or compromised for their first or second uses, with none of the fibres being split or weakened.
It is proposed, as shown in the elevation, that by wrapping the rope around just above nodes in the bamboo, the rope will not slip downwards due to the nodes’ thicker section, and therefore guy-ropes pulling downwards at 45° will not pose issues. Where required, some small pins could be used, as in some places in the physical model.
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