Non-Linear Non-Convergence Troubleshooting

[rushthedj]

Hi all,

I have a complex model which I'm trying to run in a non-linear analysis, but am having convergence problems. (This is an updated version of the same large model I sent a few months ago with different queries - still in Robot 2010 I'm afraid.)

At the moment I get realistic looking results up to about 15% of the ULS loads before non-convergent iterations, although the greater the number of load increments generally the worse this is. For example if I use 10,000 load increments I can only get to 1.4% loading before non-convergence.

The Newton-Raphson analysis has given the best convergence, as expected, and I have tried various permutations of tolerance for residual forces/displacements but these don't help much. Here's a screenshot of some sample settings:

If I delete all of the releases from the model I can get it up to around 60% ULS load before non-convergence (with the settings in the screenshot above), and it wouldn't surprise me if the problems were related to instabilities in the analysis model due to the way elements like stiff links have been modelled. I've run a linear buckling analysis on the load combination in question, and with and without releases and the lowest mode has a critical factor of 7 in the model without releases.

Any general advice would be welcome!

I also have two specific questions:

1. Can anyone explain what the "Relative code tolerance for residual forces" (and displacements) is? I'm familiar with the concept of residual force tolerance from other software, but this seems to be more complex than the absolute maximum residual force value at any node that I've seen before.
2. Does Robot provide any further information to troubleshoot non-convergence? Again previous software I've used provides details of which particular node numbers were giving the critical residual force at each stage of iteration, so the user can find see which area of the structure is unstable.

Thanks!

Rory

[Tuctas]

So, the question is addressed to the Support (in order to be formally and officially answered)... 🙂

[rushthedj]

Bit of both! Happy to hear from anyone who can confirm what Robot's doing 🙂 Would just expect any professional software to clearly explain what it's doing behind the scenes... Otherwise how can we rely on it do decide if a building stands up or not?