topic Re: Instability of Nodes on pinned-pinned members in Robot Structural Analysis Forum

Instability of Nodes on pinned-pinned members

PaulGrimes2282 — Fri, 26 Apr 2013 10:24:31 GMT

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!

Re: Instability of Nodes on pinned-pinned members

Rafal.Gaweda — Fri, 26 Apr 2013 10:46:26 GMT

Possible reasons : http://forums.autodesk.com/t5/forums/searchpage/tab/message?location=forum-board%3A351&q=instability+type+3&filter=labels%2Clocation

Try this addin

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/API-addin-for-short-calculation-elements-detection/m-p/3807860/highlight/true#M11621

The reason:

Re: Instability of Nodes on pinned-pinned members

PaulGrimes2282 — Fri, 26 Apr 2013 13:28:59 GMT

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!

Re: Instability of Nodes on pinned-pinned members

Rafal.Gaweda — Fri, 26 Apr 2013 13:34:44 GMT

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/highlight/true#M3270

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

Re: Instability of Nodes on pinned-pinned members

PaulGrimes2282 — Fri, 26 Apr 2013 14:34:15 GMT

thanks - will do!