topic Re: Continuous purlins and girts with rigid links in Robot Structural Analysis Forum

Continuous purlins and girts with rigid links

ptica1cro — Sun, 13 Dec 2015 23:24:56 GMT

Hi,

I have a problem with my 3d model. I wanted to model purlins and girts as continuous beams and to make this happen I used rigid links.

Also, another reason for using rigid links is independent roof bracing system which should not be mixed with purlin static system.

To achieve the static system of continuous beam, I used end releases on bars and rigid link releases:

And RZ instability appears always in the end:

So I tried many variations...changing bar releases, support types...

If I remove girts, with purlins only remaining, there isn't any instability, it works fine.

But when I add girts, RZ instability appears somewhere.

Ultimately I tried removing all bar releases (all fixed connections) and adding all fixed supports, but RZ instability still appears. That's why I suspect that there's something wrong with this modeling method 😕

But despite this instability, results seem ok.

I attached my model, so if you could please check it out.

Or is there any other way to model continuous purlins and girts?

Re: Continuous purlins and girts with rigid links

Artur.Kosakowski — Mon, 14 Dec 2015 08:12:38 GMT

You may want to consider use of compatible nodes:

http://forums.autodesk.com/t5/robot-structural-analysis/compatible-nodes/m-p/3689964/

http://forums.autodesk.com/t5/robot-structural-analysis/compatible-nodes/m-p/3797147/

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

Re: Continuous purlins and girts with rigid links

ptica1cro — Mon, 21 Dec 2015 08:21:09 GMT

Hi,

I solved this problem by replacing rigid links with dummy elements (manually created new cross-section with relatively high Ax,Ix,Iy,Iz, i.e. rigid) and modeling releases like in previous case with rigid links.

If someone stumbles upon a problem like this, don't forget to remove self-weight from dummy elements. 😉