Analysis Unit

Hi Kiwi 3D Team,

I have a question regarding the unit in kiwi 3D, is the plug-in using the metric system by the default? I assumed it is using the metric system, but I will like to confirm with you guys.



Kiwi is without predetermined units. You have to define your own unit for length and force, as e.g. m and kN. But also other units as e.g. inches are valid. However, you have to be aware that the units of other variables such as the Young’s modulus are defined by your choice of units:
L[m], F[kN] --> E[kN/m²], Moments [kNm], …
So you have to take care that all of them are matching.

Okay a lot. That makes sense. Can I assume the kiwi unit is associated with the Rhino unit? So I don’t need to define the unit in grasshopper, is that correct?

Using the units from Rhino is probably the best but it is not related. So even if Rhino is in inches, a curve with the length 1 can be 1m in the Kiwimodel. It is up to you. You have to define it for yourself but not in the plugin or Rhino or Grasshopper.

Hi, but in the case that I had some results from the model, how can I know ehich are the units of those results, in case of stresses.


You have two options:

  • you just define the two units for force and length and then the result is automatically in these units. So if the geometry is in meters (note that this is independent of the Rhino units even though it is recommended to use the same units) and you decide that the force is kN, then the result is in kN for forces, kNm for moment (1d elements) and kN/m for forces, kNm/m for moment (2d elements). But this affects also the input. The value of the Young’s Modulus is interpreted as kN/m², same holds for prestresses, loads, etc. if you decide to use N, all values have to be interpreted with N instead of kN. This holds also for e.g. for the default material component. Even if another unit is chosen there, the value is still interpreted with your chosen global units.
  • You can also use the material and the Rhino unit to define the global units for Kiwi3d. Then it is just the same as before. These units define the units of the results and also the units of the load, prestresses, etc…

If you see some units in the descriptions, they should only show the style of the unit. [kN/m] e.g. shows only that the stress is already preintegrated over the thickness but it might be as well [N/mm].

Hi Anna, How do I define my own units?


Just for yourself. If you say that you want everything to be in [kN] and [m] then all values (input and output!) have to be interpreted with [kN] and [m].

It might help to write it into a textcomponent in the definition in order to remind yourself what you assumed while generating the input.

Hi anna, amazing plugin by the way. I’m very new to Kiwi and have a question. If the setting of Load Units is somewhat subjective, how then do we simulate the realisitc deformation of trusses etc? How do we know if the analysis diagram is showing the actual amount of deflection/deviation in a member etc? Thanks

Hey, it is basically the same as any other FE-program regarding the interpretation of the results. By deciding for the two units of force and length, every input unit is defined just as if there would be a unit stated by the program. The results have also the same units. And if e.g. another FE-program wants the Young’s modulus in N/mm² but you input the value in kN/cm² you have the same problem in the result…
Regarding the deflection, this is the real deflection if the scaling factor is set to one. However, you have to ensure that the analysis is in the scope of the assumptions, e.g. small displacements for linear analysis.