Learning Forum

[anhnguyentuong]

Hello everyone,

Actually, I study the behavior of a prestressed cable-strut structure (photo below) and I have a problem of modeling pretension in cables. The structure is prestressed with tension on cables (thin elements) and compression on struts (thick elements). All values of tension and compression are known, the problem is how to put them into the structure and let ANSYS know.

 

 

All elements are linked by spherical joints (DOF=3, free RX RY RZ). The type of elements are Beam188 for strut and Link180 for cable (with Keyopt(3)=1 for the cable, it means they work on "tension only"). I actually used "Bold Pretension" for modeling the tension of the cable but it's not properly correct. In reality there are no bolt here and the cable and the strut linked by a spherial joint.

 

  

So the question is how can I put the pretension in the cables correctly ? 

 

Thank you in advance.

   

[jj77]

Use the inistate command snippet for this - see this post:

/forum/forums/topic/cable-driven-parallel-robot-modal-analysis/

/forum/forums/topic/modeling-of-a-prestressed-concrete-sleeper/

/forum/forums/topic/transmission-line-simulation/

/forum/forums/topic/modeling-of-cable-in-a-bridge/

 

Further more one does not need joints - just put all line bodies in a multi-body part, or if you do not have that and have them a separate line bodies use the merge nodes to connect all nodes shared by two or more elements - since links and beams are used we will have pinned connection between them. 

 

[anhnguyentuong]

Hello @jj77,

Thank you so much for your reply and your advice. I will try the multi-body part and will see if it works. On the other hand, it seems that the actual structure is really unstable. I got many different results every time I solved and they are really sensitive to the value of prestressed and the external force that I put on. It makes me so confused and I have many doubts about the modelling especially the joint. 

That lead me to the second question. Since I use a spherical joint (DOF=3 RX RY RZ) to connect two different types of element BEAM188 (DOF=6) and LINK180 (DOF=3), the question is : Does it cause any conflicts when we connect the two elements with the different DOF ?

I hope my question is clear for you.

Thank you in advance.   

[jj77]

Frame structure I never use joints - most frame software and in beam modelling one does not use that it is not a best practice and not standard in anyway.

 

1 D elements are connected as needed and dof are released or kept depending on their interaction at joints.

 

When we connect a beam and a truss they are automatically pinned at their connection since trusses do not have any rotations.

 

if you want to use joints then double check your results and if they are Ok I assume it is OK then.