Grasshopper

algorithmic modeling for Rhino

Hi everyone,

I have a trouble with the truss joints. I have drawn a V-truss in Grasshopper and analyzed with karamba. As default the joints are fixed but i want to change them into hinges. I did this with the Beam Joints-component and fixed them at Ry. The truss is seperated in top chord, bottom chord and web and when i put all three of them in the input ElementIds, then i get a warning about rigid body modes. So i disconnect the web from the component to avoid multiple nodes and this warning disappears. But still all nodes are included (because of the top and bottom chord). This creates shear forces and moments in the beams of the web which is not the intention. If i only put the web in ElementIds, then these multiple nodes also disappear (and still all nodes are included), but this time it creates moments en shearforces in the top and bottom chord. (In the top chord there are already shear forces and moments because of the line load, but it get’s extra moments in the endpoints because of the fixed joints). So, despite that all nodes are still included, the beams connected to those nodes are not calculated as beams with hinges if you don’t put them in the ElementIds input of the beam joint component. (I also did this with a N-truss and then it worked perfectly)

To summarize: I want to include all beams (from the web, top- and bottomchord) tot the joints but I dont know how to avoid these extra nodes because of the hinges. I have attached the gh file and hope you guys can help me :) Thank you in advance!

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Dear Cansu,

there are three ways to solve the problem:

1.) add small rotational stiffness values at the hinges (see attached definition, alternative 1)

2.) turn the beams into trusses and add suports at the nodes in the direction perpenducular to the truss system (see attached definition, alternative 2).

3.) ignore the fact that there are kinematic modes in your system. The solver of Karamba 1.2.2. will return a precise result in most cases.

Best,

Clemens

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Hi!

Thank you so much! I've tried possibility 1 and it worked! The shear forces and moments are nearly 0 and also the axial forces are correct. Thank you again!

Kind regards,

Cansu 

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