# Karamba analysis of Tension Fabrics

(Lzerbe) #1

Hi, I’m trying to analyze a tension-only membrane structure supported by beams on all sides. It seems that I’m getting compression forces within the mesh and as a result, the reactions don’t make sense (some are pushing upwards). I’ve tried varying the shell thickness and modulus of elasticity.

Is there any way to force a shell to be tension only?

TensionFabric.gh (50.2 KB)

#2

Dear Lzerbe,
I think the reason why you get compression forces is, that your supports are not correctly defined. In order to avoid moments at the shell boundaries the supports need to be oriented parallel to the middle surface of the shell - there should be no support reactions perpendicular to the shell.
The fact that you get support reactions in different z-directions is due to the corners. For example a rectangular plate under dead weight which is vertically supported along its boundariy shows uplift forces at the corners. The reason for this is that it would like to deform into a paraboloid but gets constrained by the supports.
When you increase the number of edges this effect fades away (see TensionFabric_cp.gh (52.1 KB)
) .
Best,
Clemens

(Lzerbe) #3

Thanks for the reply! I see that has pretty much mitigated the problem for the simple shape. I’m wondering what to do for more complex shapes that will have less than perfect catenary shapes?

I’m analyzing suspended cable netting and it will occur over a large and irregular shape, so getting perfect paraboloids isn’t possible.

The physical material has negligible compressive and flexural strength, but I can’t seem to recreate that in the karamba module. Modifying the in-plane shear modulus helps, but doesn’t get it quite there. Any other suggestions you can think of? Should I be breaking it up into tension-only segments rather than a shell surface?

cheers,
LZ

#4

Dear Izerbe,
when you set G=0.5E a material results with a lateral expansion factor of zero. This corresponds to the behavior of an orthogonal cable net.
You could try to cut out small regions at the kinks of the boundaries - much like they do in case of membrane structures to avoid wrinkles.
Also try to reduce the shell thickness so that bending effects get neglectable.
Did you take a look at Karamba3D’s ‘LaDeform’-component?
Best,
Clemens