Buckling results

[ianrhale]

Guys, just a quick check on something.

I have done a buckling analysis on the frame shown below, 100 kN applied at the top of each column. Up to now I have been reading the critical co-eff and applying this to the load to get the buckling load, so in this case 33.95 kN. I have just discovered (!) that you can get lots of additional info on the results table, Critical Force being one such value. In this case this is 41.8 kN so higher than my value.

I assume this difference then is down to self weight? And that the buckling force is the EXTRA load required to cause buckling? 

Thanks in advance!

[Rafacascudo]

Critical Factor being 0.3395 means that it will buckle  if you multiply all the loads on this load case by this factor .

You can do a test and run a combination which will be your buckling load case multiplied by your critical force factor. Run buckling analysis for it .Critical factor then should be 1

[ianrhale]

Ok, sorry i have left this so long to reply!

I have just run a case where i have deleted the self weight out of the structure, and got the critical co-efficient. I then multiplied this by my loading and applied this new load to the model. Critical buckling factor is now practically 1.0, as expected, on an applied load of 34.5 kN. The table though says critical force is 41.17 kN - it is this difference which I am confused about? Maybe critical force is not what I think it is?

[Rafacascudo]

It has nothing to do with the force that is applied.

Not sure , but it is probably related with the bar which is buckling locally on the 1st buckling mode.

Can you send the model?

[Artur.Kosakowski · Accepted solution]

Hi @ianrhale

The value of the force for the element that doesn't buckle for the selected mode will by the principle of the buckling analysis implementation in Robot incorrect. 

See: 

https://forums.autodesk.com/t5/robot-structural-analysis-forum/critical-buckling-length/m-p/3841687

https://forums.autodesk.com/t5/robot-structural-analysis-forum/robot-buckling-length-critical-load-c...

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[ianrhale]

Ok, so i think I understand. What we are saying is that the buckling mode can be caused by EITHER local or global instability, and that the coefficient refers to whichever is critical? And that the critical force is not necessarily related so I am best to ignore it 

[Artur.Kosakowski]

Hi @Rafacascudo

Correct.