UL/NFPRF rack storage cases fail with Numerical Instability

[mcgratta]

During the most recent round of validation runs, UL_NFPRF_P-3.fds, UL_NFPRF_P-4.fds, and UL_NFPRF_P-5.fds failed with numerical instabilities. Over the past few weeks, I've done a manual bisect and it appears that the problem started in February.

This version runs successfully, and this version does not. I suspect it is this line

Given the sensitivity of these cases, let's do anything immediately. I just want to start the issue and investigate more the potential problems.

BTW, the problem starts with the version I posted above, and there were changes to the suspect line of code in the days following, but these changes did not stop the instabilities from occurring.