Hi,
some of you might have seen this model already in the forum. We are working on kind of truss like structure although i’m not sure if that’s the right term.
At the moment the goal is to figure out which parts of the structure are under tension and which under compression to design them accordingly. Before that we kind of need to figure out a stable configuration.
The requirements we got from our supervisor were to create a structure which is in equilibrium based on the structure we designed last semester. For this structure we should assume the joints are free to move and rotate cause if the structure is in equilibrium it should still be stable.
Anyway we are struggling. Thanks to the forum here we managed to create the joints but now we have another Problem. Well maybe we defined the joints wrong or our goal just isn’t possible to create like that.
There are 306 rigid body modes in the system. This means some parts can move freely without causing deformation.
Try to use the ‘Eigen Modes’-component and activate the display of local coordinate axes: The first eigen-mode will be the rigid body motion.
If this does not help, check whether you have a pinned support directly attached to a hinge. A hinge introduces an extra node which may cause the problem.
When analyzing a flat shell structure one has to lock the rotation perpendicular to the plate in at least one node.
I already checked and there shouldn’t be any duplicate lines and as far as i understand from the connected parts they should all be connected to each other.
the rigid body modes come from the fact that there are too many hinges in the structure.
In Karamba3D joints at elements act like additional beam pieces added at their ends. If for example there are too many hinges around a node that node my freely rotate and thus give rise to a rigid body mode. Another example are beams with rotational hinges at their ends which give them the ability to rotate about their longitudinal axes.
I have a truss modelled and also have the same error and from the connected parts component, the structure is connected as 1.
I have plugged the eigen modes component into my model and I am looking at the eigen load cases with the local axes turned on but I am not sure what I am looking for to solve the problem?
I have an image attached below of the rhino model with an eigen load case turned on, I am not sure if that will be of any help to you.
Thanks for your answer. I changed how we defined the joints so that we work with the “at” and “to” elements part and that already helped somehow to reduce the rigid body modes. So i guess there were some doubled.
What really helped was to give the rotational and translational spring stiffness a value.
Just another question because we also had it before but now i guess it doesn’t matter anyway cause we don’t have rigid body modes anymore. What is the meaning of it when it kind of disassembles.
Hi @Clarissa_Harte,
guessing from the image you sent, it looks like you used truss elements for the members of your truss. The orange circles indicate hinges which ensure that the truss elements take normal forces only - and no bending moments.
When a node in a finite element model is connected to truss elements only, these elements need to be arranged such that they do not lie in a plane - otherwise the node is movable perpendicular to that plane (like in your case).
Try the following: either enable bending stiffness for the elements by setting ‘Bending’ to true or add out-of-plane supports at the nodes of your structure.
– Clemens
Hi @julia.geisler,
when the model’s Eigenmodes show gaps in the deformed shape this indicates insufficient connection - so no or very little stiffness between parts of the model.
– Clemens
ok that makes sense, we are working on finding a stable construction.
I think we have a decent definition know with spherical joints but we have a few new questions.
We are wondering if there is a right or wrong with if we change the bending stiffness to true or false cause we get way different results.
(we changed the colors for compression and tension since we learned it that way)
With the display scales of the deformation 1 is the deformation it would have in real live right?
So if we set it to 100 it’s just scaled up to see better where the results are?
Edit: I just saw that if the bending stiffness is set to false all moment are 0. Could this be right?
Is this because karamba doesn’t define movable joints when set to false and calculates it with rigid joints or how are the joints defined when set to false?
With Bending Stiffness true we get rotational as well as bending moments. If I’m not completely wrong there shouldn’t be bending moments with a spherical joint but still we get bending moments for them. Really small values but they are still there. I understand the rotational moments since a rotation is possible with a spherical joint.
Hi,
when you deactivate bending, it converts your beams to truss elements. The results would of course be different (refer to 3.1.10: Modify Element - Karamba3D 2.2.0). From looking at the images it seems as though truss elements would work, but you need to check if your model is not kinematic as Clemens mentioned. Setting up your model with trusses usually means that the connection design would be generally easier.
