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. 

These warnings are telling us that the model is not properly done and hence these give us an opportunity to correct the model. For example in a model the numerical issues may be developing due to multiple releases at a joint which make a few DOF of the joint orphaned (means without a stiffness). This can be checked easily by the user as ETABS will also be reporting location of such numerical issues noticed in the model.