Free Nodes / Beam joints ?

Replies to This Discussion

Permalink Reply by Karamba3D on August 25, 2014 at 11:30am

Hello Yousef,

if you use ropes for connecting the members of your structure you should model the connections in such a way that only shear forces are transmitted.

On option would be to artificially decrease the torsional stiffness of your members - as you have done by using slender rectangular cross sections. There will be a - probably slight - error because if the members are not at right angle they exchange bending moments which makes the structure too stiff.

A more complicated solution would be to use the felting-component of Karamba: This component lets you introduce short members between neighboring elements. These should have bending hinges (My, My, Mz) on one end and a rigid connection at the other end. For this to work you have to offset your main structural elements from the reference geometry so that they do not intersect - which is the tricky part.

Best,

Clemens 

 

Permalink Reply by yousef anastas on August 25, 2014 at 12:13pm

Hi Clemens,

Thank you for your answer. the slender rectangular cross sections are connected through notched joints.

In the actual model, elements are linked together through short infinitely stiff elements between mid axes of elements ( being perpendicular to both). On the attached image, the links are the small elements connectiong nodes 4 to 5, 6 to 7 and 8 to 9. All nodes (1 to 9 including 1', 2', 3') are defined in Karamba as fixed supports but nodes 1,1', 2, 2', 3, and 3' have hinges added with the beam joint component. The freed rotations are shown on the figure.

I wondered if that was the correct way of defining such a structure in Karamba bearing in mind that nodes 1', 2' and3' are free nodes in the reality.

Thanks again for your help !

Yousef

Attachments:

• kARAMBA FORUM.jpg, 682 KB

Permalink Reply by Karamba3D on August 25, 2014 at 1:48pm

Hi Yousef,

I do not understand why you attach hinges to the free nodes 1', 2' and 3'. Did you try to attach hinges to one end of the short elements? You should not make those too stiff because this might lead to numerical errors (the stiffness matrix gets ill-conditioned). You could try springs instead of short beams for the connectors.

Clemens

Permalink Reply by yousef anastas on August 25, 2014 at 2:32pm

Hi clemens,

If I don't the analyze component fails. This is why I wanted to know two things :

-how do you define a free node ?

-on the intersections of the small links with the elements, do I have to define two points ( one on the element and on on the link ?)

For the time being, if I do not define the nodes 1', 2', and 3' as fixed and then attach hinges, the model won't work.

yousef

Permalink Reply by Karamba3D on August 25, 2014 at 10:46pm

You could try to use the Eigenmodes component in order to detect rigid body modes. Enable the display of the local coordinate systems so that you see also beams rotating about their axis.

A node can not be free - it has to be connected to an element. Otherwise the node will not be part of the structure.

You do not have to define points on the element and the link. One is sufficient. Introduce a hinge in order to decouple the cross section forces.

In Karamba 1.0.5 a hinge is implemented as a zero length spring between the endpoint of an element and an internal, invisible node.

Clemens

Permalink Reply by yousef anastas on August 26, 2014 at 8:52am

Hi clemens,

Thanks you for your help.

Like you said a node can not be free and disconnected from the structure. But do they all have to be supports ?

What I do not understand is that I have to assign fixed supports to all nodes and then add hinges using the beam joint component ? ( this is how I defined the model so far ).

Yousef

Permalink Reply by Karamba3D on August 27, 2014 at 1:18pm

Hi Yousef, 

nodes that are connected to an element do not need to be supported.

Did you check the Eigenmodes of your structure when you eliminate the surplus supports and hinges? 

Best,

Clemens

Permalink Reply by yousef anastas on August 27, 2014 at 2:16pm

Hi clemens,

I removed all supports except for the foundation and removed all hinges. I checked the eigenvalue and I obtained -9.16e-10

Yousef

Permalink Reply by Karamba3D on August 27, 2014 at 11:44pm

Hi Yousef,

this means your system is kinematic. Feed the model that you get from the Eigenmode-component into a ModelView-component and limit the display of load-cases to the first one (i.e. eigenmode zero). Then enable the display of local coordinate systems and change the slider for scaling the displacements. Do you see a movement?

Clemens

Permalink Reply by yousef anastas on August 28, 2014 at 8:15am

Hi clemens,

When I connect the Model from the Eigenmodes to a Model view and turn on the Deformation I can see one link that is moving.

Best,

Yousef

Permalink Reply by yousef anastas on August 28, 2014 at 8:33am

And if i replace the links with  springs, I get a movement of an element.

Permalink Reply by Karamba3D on August 28, 2014 at 12:24pm

Probable your structure has more than one rigid body mode. Calculate the first view Eigenmodes and have a look at the eigenvalues. When the eigenvalues are small the corresponding Eigenmodes are rigid body modes. You can visualize them individually by selecting them in the ModelView and move the slider for scaling the displacements.

Best,

Clemens