Non-Convergence Is Not Structural Collapse

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.

11 thoughts on “Non-Convergence Is Not Structural Collapse

  1. Dear Professor Scott
    Thank you for your post. You discussed the nuances of non- convergence topics. Your observation of the issues and narrative on them is awesome.

    Liked by 1 person

  2. Thanks Prof Scott for highlighting this important issue.

    The same ‘belief’ of non-convergence as a sign of failure is also popular among Geotechnical FEA colleagues, particularly while analysing static slope stability using the Strength Reduction Method. May be, the same was true or a meaningful conclusion during the days of FEA implemented in Computers of late ’80s and early ’90s. However, for today’s sophisticated non-linear solvers (both for commercial and more so for OSS like OpenSees, Code-aster, RealESSI etc.), coupled with decent computing powers of today’s common PCs/laptops (let alone be the cloud computing platforms), such a conclusion is no longer valid. For example, in a slope analysis problem, instead of relying on non-convergence to identify failure, one needs to monitor the (static) load-settlement response and also many a times the shear strain contours showing failure bands/surfaces within the slope. In a nutshell, your observations are true, not only specifically for OpenSees or Dynamic Analysis, but also for a wide range of FEA applications.

    Liked by 1 person

  3. Can no convergence be solved with using more digits in calculations. Maybe you can use in c++ bignum library – arbitrary precision arithmetic and check whether using more digits in a calculation can improve convergence.

    Liked by 1 person

      1. OpenSees uses all double precision variables, at least on the C++ side. If you think it may make a difference, then come up with a MWE in OpenSees that demonstrates the issue.

        Like

  4. Could you please send me your email address, so I can send you matlab files. In 32 digits, calculations do not converge, but in 64 it converges. A very simple example.

    Liked by 1 person

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