Cable structure working as a truss


I am trying to model a tensegrity roof in Karamba. In this tensegrity roof all elements should be in tension (BLUE) apart from the vertical struts (RED). Additionally, some elements should be prestressed (Orange).

However, when looking at the internal forces, most elements seem to be in compression (below a visualisation of the elements in compression); Thus the structure seems to be modelled as a truss instead of the cable structure it should be.(normal force<0)

My question, therefore, is how to model this as the cable structure it is supposed to be instead of this truss. I looked at several fora how to model cables and do not see how to solve this problem.

Thank you in advance!

(I tried ‘cleaning’ the model as much as possible by clustering unnecessary components) (107.0 KB)

Well … given the opportunity : If you are after a self supporting tensegrity truss go for a classic double tetrahedron like this:

or the so called XFrames:

BTW: This (cubic) is not stable and requires external cables/masts and the likes:

Anyway … if you are familiar with K2 you can “test” your structure for stability. I have similar things but are 100% C# code (as the thing that does the above trusses as well). If memory serves well Daniel posted some time ago an indicative K2 script for that matter (search this or the old (dead) Forum for “tensegrity”).

BTW: Hope that you are aware that tensegrity trusses (most notably on double curvature guide surfaces) are the most expensive structures known to man (for not very obvious reasons). A tensile membrane is by far the rational thing to do.

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Might have been worthwhile to specify that this roof is part of a larger (fictive) structure. Therefore, the roof does not have to be self-supporting (it, therefore, is indeed no true tensegrity and costs don’t matter). Nevertheless, thank you for your reply and it is indeed a good idea to try K2. However, I specifically wanted to try this in Karamba (if that is even possible).

Hi @dennishollanders17, your structure currently is kinematic - is there a reason why you split the vertical elements into two, and defined them both as trusses?

Hi Matthew, this indeed should not be the case. I am however unable to replicate what you did, when I try, with the eigenmode component, the vertical elements remain functioning as 1. I also looked back into what I inserted in Karamba and see no reason why they could be functioning as split elements.

Hi, this is actually a new feature in Karamba 2.2.0 where the vertical elements were split into separate lines. Not a problem in the old version.

To troubleshoot this issue, can you simplify the model into just one segment. You have a symmetrical system so it should be quite easy to just calculate one module before applying it to the global system.

Hi, I tried to model the same structure in section before I did the 3d model, everything worked in that script. I then used exactly the same script to model the 3D version.

tensegrity in (69.7 KB)

Is this sufficient to troubleshoot or would it require a 3D segment?

Hi @dennishollanders17,
the above structure is unstable in two modes (see
tensegrity in (130.9 KB)). Apart from this it seems to work fine.
– Clemens