Q: While running a model in ETABS, it is reporting several numerical instability warnings. What these warnings are telling us?
A: The program performs most arithmetic with 16 digits of accuracy. When elements with significantly different stiffnesses are connected to a common joint, some digits of accuracy may be lost (for the element with smaller stiffness) during the arithmetic operations. Another example would be using rigid diaphragm assumption together with flexible elements, local or global instabilities, etc. The program provides the following messages based on the number lost digits of accuracy:
- Less than 6 digits lost: When the program suspects that less than 6 digits of accuracy will be lost during solution it does not give any messages.
- Between 6 to 11 digits lost is reported as a warning: The results of the analysis may still be acceptable but the user should carefully check the results, especially the global sums of loads.
- Above 11 digits lost results in the analysis being terminated with an error message because the results may not be sufficient accurate. The model should be checked and revised.
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h2. What do numerical instability warnings indicate? *Answer:* Numerical instability warnings indicate that digits of accuracy were lost during computation. {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software performs computation with 16 digits of accuracy, some of which may be lost when analysis involves systems of significantly different stiffness, where lower-order values are truncated to suit operations with higher-order values. When this warning occurs, [ETABS|etabs:Home] 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 [frame|kb:Frame] objects of significantly different stiffness connect at a common [joint|kb:Joint]. * 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 analysis to terminate because results may not be sufficiently accurate. ETABS indicates the location of the numerical problem, and presents a warning message which prompts the user to check and revise the model. |