# Comparison "beam forces" and "shell forces"

I have two flat structures of the same size, 5x5 m. The first structure is a concrete shell, the second is a wooden gridshell. It is essentially the same structure, but one of the two is discretized into horizontal, vertical, and oblique rods. The two structures are subject to the same load and the anchor points are in the same position. My goal is to understand the different reactions of the two structures and compare them. Let me explain: I would like to know at every point the values of the normal effort N, cut V, bending moment M, obviously both for the shell and for the gridshell. I can get them with the commands “beam forces” and “shell force”. The problem is that the points where N, V, M are calculated are different in the shell and in the gridshell. At the extremes and midpoint of each beam in the gridshell, and at two points of each square (mesh division) in the shell. (I attach a photo) So the points in the shell and in the gridshell don’t coincide so I can’t make the comparison I want. So what I would like is to get the values of N, V, M in the shell and in the gridshell in the same points. Is it possible?

THANKS SO MUCH

Hello @fallarinomonica,
be aware of the fact that the physical units of shell and beam resultants are different. In case of shells you get distributed moments and forces (kNm/m, kN/m) whereas for beams the cross section forces are given at points (kNm, kN).
When refining the mesh the difference of location between shell and beam results should get smaller and smaller.
Clemens-

what do you mean refine? How could I get my goal of comparing the M, N and V values ​​of the shell and the gridshell at each point?
Thanks

@fallarinomonica: by ‘refine’ I mean decrease the size of the mesh faces. The tricky thing in your case is to chose the beam cross sections such that they add up to be mechanically equivalent to the plate. I think it would be a good idea to do a literature research on this.
– Clemens

If I didn’t want to change the size of the mesh faces, could I multiply the value by the area where the force exists? In this case they are triangles

@fallarinomonica,
in case of shells you get distributed moments and forces (kNm/m, kN/m) whereas for beams the cross section forces are given at points (kNm, kN).
You have to find equivalent cross sections for the beams which result in a behavior similar to that of the shell. Once this is achieved one can divide the cross section forces of the beams by the cross section widths, combine them and compare the results to the distributed shell forces. This whole procedure is not trivial.
– Clemens

When you say that the behavior of the beams must be similar to that of the shell, are you referring to “nodal displacements” and “utilization”??
Thanks so much

@fallarinomonica, to achieve similarity in all mechanical respects at once will not be possible. Try to find a paper on the subject like e.g. https://www.sciencedirect.com/science/article/abs/pii/S0045794903003419 which covers your area of application.
– Clemens

This article has been very useful to me, could you indicate others?

Hi @fallarinomonica,
I am not an expert on that subject. Try to follow the references listed in the article or search the internet for publications where the paper has been cited.
– Clemens