Instability of Nodes on pinned-pinned members

[PaulGrimes2282]

Hi,

I have a model where i have one final instability that i cannot figure out - a node sits on a pinned-pinned member (Ry, Rz release one end, Ry Rx the other) and it says that it has an insability in Ry direction (which i think is Ry global? - i.e. Rx in the member axes).

I have attached a screenshot, and the model - would someone be able to explain to me why the node is still unstable?

thanks!

[Rafal.Gaweda · Accepted solution]

Possible reasons :  http://forums.autodesk.com/t5/forums/searchpage/tab/message?location=forum-board%3A351&q=instability...

Try this addin

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/API-addin-for-short-calculation-elements-det...

The reason:

[PaulGrimes2282]

That looks like a really useful plugin - thank you i will have a look at it.

So the instability isn't actually anything to do with the node it is saying - it is just a result of the way that the matrix is set up? Is this the case for any type 3 instability? Sorry just trying to understand so that i can correct my own models in the future!

[Rafal.Gaweda]

 Is this the case for any type 3 instability? 

In case of instability type 3 it may not point directly to node with so called "problem".

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/instability-type-3-RX-direction/m-p/3364875/...

In such case you should look for short elements in your model first.

[PaulGrimes2282]

thanks - will do!