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If numerical instability occur within an ETABS model, we recommend considering the basic troubleshooting techniques which follow:

  • Stability. Under the File menu, check the last analysis run log to ensure that the model is stable. Results from an unstable model are not reliable.
  • Releases. Do not release moment for objects adjoined at the same joint, such as with beam-column connections. Also, do not release moments at the base of a column that has  a pinned restraint (i.e rotations allowed).  Multiple coincident releases will cause numerical problems. If necessary, one object may be released.
  • Self weight. Ensure that deflected configuration under self-weight is as expected.

What does it mean when numerical instability warnings are reported, but there is no joint at the location indicated?

Answer: Numerical instability is reported according to numerical formulation and the sequence in which equations are solved. The point indicated is not necessarily a discrete point at which instability occurs, but rather a location related to the assemblage of objects with significantly different stiffness.

For example, if all or part of a structure is unsupported, no single point can be identified as missing support, so the software indicates that numerical instability occurs, and the point shown relates to the structural systems and solution convergence.

Refer to page below  for further recommendations:

Lost digit of accuracy and Ill-Conditions