Grasshopper
Kangaroo
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Kangaroo is a Live Physics engine for interactive simulation, optimization and form-finding directly within Grasshopper.
Gridshell / Formfinding
Hi !
I am trying to implement in Grasshopper the propose method by Kuijvenhoven and Hoogenboom in this article :
http://repository.tudelft.nl/assets/uuid:f957bbad-db57-4b57-8087-a5...
In order to validate the implementation, I am first building this really simple example you can find in the appendix of the article. The *.gh file is attached.
The idea is to inject a horizontal force in the member and checking his deflection. I have several problems.
1. Rhino is configured in meter. I am using Newton and Meter as inputs of the parameters. The value of 95000 Nm² force the Kangaroo simulation to "oscillates wildly" as the kangaroo manual say ! Is it normal in this simple situation ?
2. I have inject a small vertical force in order to leave the equilibrium of the perfectly horizontal member. However, the horizontal forces don't bend the member. Am I missing something ?
If I can manage to make it work, the next step would be to introduce the "shaping springs" proposed in the article, then to analyze the stresses and finally to adjust the stiffness of the shaping springs with Galapagos. I'd like then to study the final geometry in a conventional structural analysis software.
Best
Jean-Baptiste
Daniel Piker
Hi Jean Baptiste,
From a quick look, the first thing I notice is that in your definition you are applying forces to the anchorpoints, which will have no effect. Anchored points will remain completely fixed, regardless of any force applied. If you want to have them free to slide horizontally, try using the AnchorXYZ component.
Also I see in that paper they are not concerned with the extensional stiffness of the springs, only that it is high relative to the bending stiffness, so the resulting geometry will not be dependent on the actual numerical values chosen for flexural stiffness vs applied load - only their ratio.
The shaping springs are an interesting idea - the CurvePull would give the same effect, though to adjust their strength based on the bending would require some further iteration...
Jun 25, 2014
jjsolly
Hi Jean Baptiste,
I'm also currently looking at simple methods of implementing this sort of active bending behaviour within Grasshopper. That paper has helped out already.
Looking quickly over your definition I'm not sure you've managed to input properties that accurately represent physical properties.
For example. The spring stiffness is the k of F=kx. For real properties you'd need to consider the length of the member such that k=EA/L. Attached snapshot shows the issue. For the red spots vs green spots all I have done in your script is modify the number of points that form the polyline. As you can see the difference in final form is significant and this should not occur if you have physical properties in place. There might be a small difference due to increased accuracy with more points but if you use the current definition then adding more points will increase the deflection seen.
Actually - a question I'd been meaning to ask Daniel - within Kangaroo what does "Bending Strength" relate to? I.e. is it a linear spring stiffness related to the angle between the points? Or is it a strength of snap of one point back to a straight line formed by two others? Any advice would be ace.
Jun 26, 2014