Fabric

Hi,

I am trying to simulate fabric as a shell on a grid shell, where the grid shell is wood and the 'skin' is fabric. The analysis fails citing that the structure buckles under the given loads which is only under gravitational load. The grid shell analysis works on its own without the fabric shell applied on it - only works with barely any loads applied on the assembly.

1. I am running the grid shell as shell element in Karamba as well as the 'skin'. What I would like to test is the impact of the loads from 'skin' and its own weight on the structural behaviour of assembly. Does Karamba understand shell on shell transfer of loads?

2. The material properties I assigned for the fabric are the following:

E= 40 KN/cm^2

G = 0.7 KN/cm^2

gamma = 13 kN/m^3

Fy (yield strength) = 6 kN/cm^2

The material widget turns orange with the following warning: "1. Material #0 : For isotropic materials G must be larger than E/3 and smaller than E/2. If this condition is not fulfilled the material behaves very strange. It may lead to an unstable structure."

I'm not sure if I should push up the shear value as I suspect that fabric does not experience that much shear anyways.

Thanks,

-Y

Replies to This Discussion

Permalink Reply by Karamba3D on November 19, 2015 at 12:11am

Hi Yassin,

the buckling of your structure is probably due to the fact that the fabric wrinkles somewhere. You could use the 'AnalysisThI'-component to see where this happens by looking for compressive stresses.

The element property 'NII' governs the second order theory effects. It can be set manually via the ModifyElement-component. In that case set the number of iterations in the 'AnalysisThII'-component to one so that the preset values for 'NII' are not overwritten.

Karamba 1.1.0 does not differentiate between transverse and in-plane shear stiffness. This causes the warning when G is not within the boundaries that apply to an isotropic material. For thin plates the influence of shear deformations can be neglected. You can therefore push up the value of G without compromising the accuracy of your results.

Best,

Clemens

Permalink Reply by Yassin Ashour on November 20, 2015 at 2:07pm

Dear Clemens,

Thanks for the reply! Yes, that makes sense as the fabric was extending beyond the structure, hence the failure.

I was wondering if the tensile strength of the fabric can be computed in Karamba, i.e. the pulling force of fabric under tension between two beams (see attached)? And if so how can it be modeled/defined for Karamba?

Thanks,

-Y

Attachments:

beam.png, 6 KB

Permalink Reply by Karamba3D on November 30, 2015 at 12:28am

Dear Yassin,

you could pretension the fabric by e.g. moving apart the supports at the beams and use 'AnalyzeThII' to calculate the system. The application of a pretension load on the fabric would also be a possibility. However this leads to an isotropic prestress in the fabric.

Best,

Clemens

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