What do numerical instability (ill-condition) warnings indicate?
Answer: Numerical instability warnings indicate that digits of accuracy were lost during computation.
Software performs computation with 16 digits of accuracy, some of which may be lost when analysis involves systems of significantly different stiffness, in which lower-order values are truncated to suit operations with higher-order values. When this warning occurs, ETABS reports the location of the occurrence such that the user may edit the model if necessary.
Some situations where numerical instability may occur include:
- When the rigid-diaphragm assumption is applied to a slab with flexible interconnecting objects.
- When multiple releases are assigned to a joint, causing joint DOF to become orphaned (without stiffness). In this instance, numerical instability warnings indicate a modeling problem which should be corrected.
Depending on the number of digits lost, actions which result include the following:
- Less than 6 does not generate a response in that values and solution are sufficiently accurate.
- Between 6 and 11 lost digits of accuracy may or may not create numerical problems, therefore warning message is reported which reads: The results of the analysis may still be acceptable but the user should carefully check the results, especially the global sums of loads.
- Over 11 digits lost causes an Ill-condition warning at the end of the analysis and results may not be sufficiently accurate. ETABS indicates the location of the numerical problem, in this case the user needs to check and revise the model.
ETABS 2013 offers different options for equation solvers, when getting an ill-condition message while running Advanced or Multi-threaded solver, the user must re-run analysis using standard solver (Analyze>Advanced Sap fire), to get complete information about numerical issues. Also refer to page below:
P-Delta FAQ and troubleshooting
Another source of numerical issues including ill-conditions is an auto-merge tolerance that is too small, the program default's is 0.1in and rarely needs to be changed you can review this via Options>Tolerances