Instabilities in Model (After Latest Update)

RE: Instabilities in Model (After Latest Update)

Nulukkizdin — Tue, 29 Mar 2022 22:14:32 GMT

Gotcha - makes sense. For that reason I guess it would be nice to see if we can figure it out and still use the default (sparse solver). That said, we can play around with some of our models over here and see if changing that to just Direct fixes the issue in other situations or just for this particular one.

RE: Instabilities in Model (After Latest Update)

Seth Guthrie — Tue, 29 Mar 2022 22:06:44 GMT

The sparse solver uses some Intel technology to optimize the matrix and generally offers better performance for large models, but it's a little more sensitive to a 0 diagonal issues.

RE: Instabilities in Model (After Latest Update)

Nulukkizdin — Tue, 29 Mar 2022 21:57:19 GMT

It does look like changing from the default Direct Sparse solver to the standard Direct solver does make a difference. Although both return instabilities, the Direct Sparse solver always returns (Node 1 at DOF UX) while the Direct Solver returns the actual location of the instability.

Being unfamiliar with teh differences in solvers, is the Direct Sparse better/more accurate (besides allowing you to use all available CPU cores)?

RE: Instabilities in Model (After Latest Update)

Nulukkizdin — Tue, 29 Mar 2022 21:53:06 GMT

Seth, thanks for testing. In our tests over here we have tried with and without the P-Delta analysis (per your suggestion) with no change. I'll check and see if the sparse solver has any affect. In the past one workaround was restricting the DOF in the model and re-running. Without changing any releases/fixities, this sometimes changed the error output to reference a different node (other than Node 1) which sometimes helped the user locate the source of the model instability. None of the models that we were running had tension only braces that I know of.

RE: Instabilities in Model (After Latest Update)

Seth Guthrie — Tue, 29 Mar 2022 21:48:27 GMT

I ran a bunch of test using various models. Some have an instability, some only when performing P-Delta analysis, and some only when using the sparse solver. I also tested some where the model DOF are restricted (i.e a 2D frame, vs 3D frame). The results can all be accounted for and are expected. The only thing that stands out is the P-Delta analysis of a model with tension-only braces and using the Sparse solver, this always fails.

RE: Instabilities in Model (After Latest Update)

Nulukkizdin — Tue, 29 Mar 2022 16:10:18 GMT

Seth - looks like I got the first question from a coworker on it around the beginning of October 2021, so I'm thinking it was maybe the 16.5 update that did it? Not 100% sure on that, though.

RE: Instabilities in Model (After Latest Update)

Seth Guthrie — Tue, 29 Mar 2022 15:42:56 GMT

I'll run some tests, but having one of your old models where the behavior changed would be useful. Did you notice the version going from version 16.05 to 16.06 or were you on something older before?