Instability type 2

[james.schembri]

I am getting a type 2 instability on the part structure attached. And I cannot understand why. Could you identify how the releases are causing such an instability and why?

Thanks,
JS

[Simau]

Hi @james.schembri 

Change parameters for these releases

[james.schembri]

Shouldn't it only be restrained on one end in the RX? could you explain?

[Simau]

@james.schembri 

There is no rule of thumb. When you have too many releases at one node you'll get instabilities.
The most easy release to uncheck is Rx at one end.

[DonBAE]

If you release a beam in torsion at both ends then the beam itself is free to revolve around it's local X axis, regardless of if any load condition will initiate this rotation the underlying math will indicate that the member is locally unstable.

For a full general 3D analysis program like Robot beams should always have Rx fixed on at least one end of a member, this can be tricky to track through models to maintain the torsional stability. I tend to fix Rx rotation at both ends for all members then review the results for any members that are accumulating significant torsion and work backwards from there with releases as appropriate for the material and end connections.