no convergence of nonlinear problem

[Anonymous]

Hi, 

Can someone help me with this, I'm fairly new to Autodesk Robot and I am trying to check a design given to us by another engineer. I was originally getting "matrix is not positive definite - the critical load may have been exceeded" error and I have fixed this by adding more bracing. Now I am getting "no convergence of nonlinear problem. I have tweaked the tolerance as per https://forums.autodesk.com/t5/robot-structural-analysis-forum/no-convergence-of-nonlinear-problem/t... and still no luck. If I fix all the bases the problem doesn't arise but I would like no moments in the base.

Any help would be appreciated.

Regards

Kerril

[Rafacascudo · Accepted solution]

too many bar releases . Block torsion rotation on all bars and the model will beautifully converge for all load cases

Other than that ,You can save a lot of analysis time by setting simple  load cases as live loads , wind, temperature ,etc as "auxiliary" as there´s no point in results for them acting alone on a nonlinear analysis. At least , structure self weight load case must be on the combination.

Having a closer look I found enormous node displacements in your model.

There are combinations with all 4 main wind load cases in the same load combination. It doesn´t make sense to me.

But anyway , probably still too many releases combined with pinned supports although no instabilities warnings reported.

[Artur.Kosakowski]

Thank you Rafael for all your help for all of us on this forum. Much appreciated. 

[Anonymous]

Many Thanks. I noticed something wrong with my load combinations alright. I generated the load combinations using the automation selection/manual/ selecting all and generate. I feel I may have gone about this the wrong way and as you point out there are cases where all four wind loads are included. I have since removed all load combinations, re-labelled the 4 wind load directions and assigned their nature as wind. Now the automatic generation doesn't include any wind loads in any load combination. Shall I just input each loads case manual?

Regards

Kerril

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

Try these steps:

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.

[Anonymous]

Many Thanks