# Pre-stressed cables bridge issue

Hello

I’m having a problem with the model of a structure, which is to represent a little bridge. As shown in the picture taken from the attached file, the horizontal line at the top is to represent the deck of the bridge, which is connected to struts under it.

Model of the structure

Model of the structure with loads

The struts are then “pulled-up” by two pre-tensioned cables which are represented by the two curves adjacent to the struts.

I’m encountering two problems (the two probably being related)

• The pre-tensioning strain is too high (6 mm/m)
• The “Colour as list” component is not outputting anything

Does anyone know how to deal with these?

Thank you

karamba_forum.gh (52.7 KB)

Hi,

Firstly you need to use at least 2nd order analysis. Maybe even large deformations analysis. Correct me if I am wrong.

EDIT: Also, Im used to define prestress as negative temperature.
EDIT_2: You need to define correct materials for your members.

Check the definiton below:

Dan

Hi Daniel,

Why do you think I would need to use 2nd order/large deformation analysis?

I’ve seen the script and I’m guessing I have a newer version of Karamba since some components of the script are missing. Would you mind sharing a high-res picture of the script?

Also, I tried rebuilding the script but when I analyze with 2nd order analysis I keep getting this error: 1. Displacement increments did not converge: The Euclidean norm of the difference of the nodal displacements for the last two iterations was 7.01355457018337E-07[m]. The target tolerance is 1E-07[m]. .

It is certainly mi intention to set the materials, though it seems there is something not working at the macroscopic (the script in general) level.

Regarding the prestress, it says on the component that the elongation is positive, so I’m reckoning I should put a positive value for the pre-stress. What would be your opinion on this?

Hello parametricalb,

you need to use second order theory (ThII) in order to take account of the favorable effect of the tension forces in the cables: tensile normal forces increase the stiffness of a structure. In your case the tension forces from the cables go directly into the supports (like in an externally anchored cable stayed bridge). Would the supports be sliders in axial direction, the cable forces would cause compression in the box cross section and the total effect of ThII would be zero.
ThII analysis still assumes small displacements. With a geometrically non-linear analysis one can include the change of the member’s normal forces during the deformation process.

Your system is rather soft which I think causes the convergence problems. Sometimes it helps to increase the maximum number of iterations (see attached definition: karamba_forum_edit_cp.gh (54.0 KB) ). The message that the ThII analysis did not converge is a warning, not an error: It signals that one should have a critical look on the structure.

The sign convention for prestress has changed from old Karamba3D versions to newer ones. In order to be sure, you could enable the display of the normal forces in the BeamView-component.

Best,
Clemens

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