Cables in Karamba

Dear experts,

I´m trying to obtain the “exact” shape of a hanging cable under wind, ice, and self-weight loads: (56.3 KB)

First, I tried to deform a straight line between the two fixed end points but the Analysis Th.II component crashes (you can check it by setting “Straight” in the input value list). However, I only obtain “feasible” results using catenary (or nurb) initial curves instead of a straight line (set “Catenary” or “Nurb” in the input value list).

Note that I´ve disabled Bending and Buckling using the corresponding inputs in Line to Beam and in Modify Element components as indicated elsewhere for modeling cables in Karamba.

Is this the adequate way to model a hanging cable in Karamba? Should I use some other kind of analysis type, e.g. Large Deformations?

Thanks in advance!

Dear @Vigardo,
the 'AnalysisThII '-component performs a calculation which takes into account the impact of axial forces on the system stiffness. It is based on the assumption of small displacements though. This means that transversal displacements do not lead to a change of axial or in-plane forces.

In order to make your definition work for straight lines I added a pre-strain load to avoid buckling (see here: (63.2 KB) ).

For a more realistic calculation on needs to use the geometric non-linear analysis option (see also attached definition). The latter is still work in progress and sometimes does not converge when the displacements are to large.
– Clemens

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Thank you very much Clemens for your kind help!

I have a few questions about your additions to the definition.

  1. Adding 10 kN to NII forces in Modify Element component seems to do nothing. Is it really necessary or am I missing something?

  2. Whereas the Analyze Nonlinear WIP component works fine for Straight cable, it does not for curved lines as Catenary or Nurb (using any of the three non-linear algorithms within). I only can make the latter work using the Analyze Th.II component. Is there any way to make my definition more robust?

  3. I want to apply pretension in kN (Fpre). To this end I´ve used an Expression component with the following formula. Is it correct?

  • Fpre = pi·((D/2)^2)·E·Eps0/1000
    *where Eps0 is the initial axial strain [mm/m] provided to the Forces component, D is the cable diameter, and E is Young’s modulus. The 1000 factor is used to convert mm/m to m/m units.

You can check the updated definition here: (78.7 KB)

Hello @Vigardo,

ad 1. The 10kN in ‘Modify Element’ help to stabilize the system in the first iteration step. In case of a straight cable made up of truss elements without bending stiffness the system would otherwise be kinematic. Sometimes though the solver can deal with the situation and nevertheless comes up with a result.

ad 2. It seems like the system is unstable. By adding supports it is possible stabilize it (see here (73.5 KB) ). And as I have said: the geometric non-linear algorithm is still work in progress.

ad 3. The formula is correct. In case of a curved cable the pre-stress will however vanish due to the deformation of the system.
– Clemens

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Hi Clemens,

  1. Then I understand that the 10 kN NII input in Modify Element component does not change the final forces upon analyses, does it?

  2. Ok, I see that you´ve restrained x and y displacements to stabilize the system. Unfortunately, this prevents the wind lateral deflections I´m trying to model :slight_smile:

  3. Ok.

Thanks a lot!

Hi @Vigardo,

The 10 kN NII induce geometric stiffness only, no stresses, strains or cross section forces.
– Clemens

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If I may, you really need to make some videos that make all this clear, cos without knowing if these modules can solve structural analysis in deigns I make, I am not going to invest long painful hours to understand your approach and implementation. (RTFM is not the answer)

Thanks Clemens for your kind help and patience :slight_smile: