Grasshopper

algorithmic modeling for Rhino

# Information of load/stress values for mesh points

Hey everyone!

I have a pretty simple question:

If I have my mesh which I analyse as a shell in karamba, is there a way of obtaining the information of how much stress forces act upon each seperate point/vertex of the analysed mesh?

Best,

Cristina

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### Replies to This Discussion

In case the problem persists please send me a short definition which reproduces the issue.

Best,

Clemens

Hi Marinella,

the stress results are nearly the same: From the length of the stress vectors one gets a maximum of 0.039kN/cm². These vectors are evaluated at the center of each element. The maximum stress at the nodes is 4.41e-02kN/cm² which translates to 0.044kN/cm².

The difference of 10% is due to the fact that the latter values are calculated by interpolating the element results from the centers to the nodes.

Best,

Clemens

Hello Clemens,

this reply is for the comment below from jan 29th (for some reason there is no reply button for that one).

A friend and i are currently starting up on our Bachelor thesis and we encounter a similar issue as above. As we are starting to test some functions in Karamba we have stumbled apon an issue when it comes to the principal stresses and moments on a simple volume. When we use the "Shell Forces" vs the "Line Results on Shells" function and extract the principal stresses and moments from the two, they differ. It is stated that the "Shell Forces" retrieves the Shell forces from the center of the faces, hence we used the "Face normals" function to create Points on each face-center and use that as input values. The reason for trying out the two methods is that we would prefer to be able to extract Shell forces at chosen Points, however we are unsure about the use of the second functions functionality. Is it wrong to assume that inputing the centerpoints of each face should yield the exact same result as the "Shell Forces" function?

The end goal is to create a Shell-element-based calculation of a simple retain wall with variable lineloads, selfload etc and optimise the dimention of said wall.

Please keep in mind that we have only worked with Karamba for a couple of days now and might require some guidance to some of the steps you might suggest :)

/Wesam and Markus

Hello Wesam and Markus,

the calculation of the line results on shells is based on values at the vertices. To get these values per vertex, the results at the neighboring face centers are weighted and averaged. This is probably the reason for the difference that you observe. The difference should decrease with incresing mesh refinement.

Best,

Clemens

Thanks a lot for the answer Clemens.

For the purpose of the structural analysis, which one of the values is more reliable?

Thanks,

Marinella

They are both reliable. When you refine the mesh the two values should converge in most cases. Further information on this topic can be found in the excellent text book 'Plates and FEM: Surprises and Pitfalls' by Johan Blaauwendraad.

Best,

Clemens

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