Which is better: using shells or beams for ribbed slabs?

For my master thesis I am developing a new structural system for floor slabs, using stress-aligned ribs and Karamba:

In the Karamba TidBits tutorials #26 and #32, beams are used to simulate the ribs that support a slab. However, for my first attempt I decided to use shells to simulate the ribs, because of the following reasons:

  • The beams in the tutorials are linear elements that are interrupted in each of the vertices of the stress lines, since they cannot follow curved paths. On the other hand, shells are continuous elements that can follow the curved path of a rib from start to end, and thus I believe they can reproduce in a more accurate way the load-bearing behavior of the ribs.
  • Since (in my opinion) ribs are very thin and thus they are not exactly the same as beams (which are thicker), Karamba shells could be more appropriate for this than Karamba beams. Also, they could be more suitable for simulating ribs made of thin materials such as metal sheets, or reinforced concrete ribs less than 10 cm thick, for example.
  • The cross section of both shells and beams can be optimized in Karamba, so I think that if I want to optimize the thickness (for example, making ribs thicker where the stresses are more concentrated, like the ribs of Pier Luigi Nervi’s Gatti Wool Factory) both shells and beams are fit for this purpose.

Are these assumptions correct? Or you would still recommend me to use Karamba beams for the ribs as shown in the tutorials? If so, why?

The attached images illustrate what I am trying to achieve with the ribs as shell elements. For now I just want to know which is the best approach to start with. I will share the Grasshopper file at a later moment for more technical questions.

In advance, thanks for your attention!

Hi, both options should work, however you do need to consider that your curved shells also need to share the same vertices as your plate, which is exactly the same effect as what you would do with the beams.

1 Like

Thanks for your quick reply @matttam! And indeed, to create line joints so that the ribs and the slab become connected to each other, I made the vertices of the shell have the vertices of the ribs as inclusion points, using the “Mesh Breps” Karamba component.