Legend has it that some published research results based on nonlinear dynamic analysis–incremental dynamic analyses, seismic fragility curves, Monte Carlo simulations, etc.–considered a non-convergent OpenSees model to indicate structural collapse or failure.
Let’s think about this for a minute.
Here is the displacement response in two orthogonal directions at the top of a nearly 50 m tall structural model subjected to two components of earthquake ground motion. The analysis fails to converge at about 11 seconds into a 60 second simulation.
Has this structural model collapsed? At 0.4 m, or less than 1% drift, probably not.
If the performance criteria is that the displacement remain under 0.5 m, did this structural model perform acceptably? Maybe, but it sure looks like the displacement is headed to 0.5 m in a hurry.
Oops, I forgot to define damping! Rayleigh–1.5% at periods 0.5 sec and 5.0 sec.
Let’s run that again.
Much better. There’s some residual displacement, but no collapse, no failure.
For this model, a small amount of damping makes a difference, killing off the higher modes of response; however, there’s many other ways to remedy non-convergence, e.g., by using variable time steps and algorithm switching. But first, check your model.