Missing Mass in AutoPIPE Nuclear vs. RG 1.92, Rev. 3

The AutoPIPE methodology for calculating the missing mass effect for higher, rigid modes in a dynamic analysis is discussed in help topic “Missing Mass and ZPA Correction.” The equations presented are not the same as the Kennedy missing mass method of U.S. NRC Regulatory Guide 1.92, Revision 3. Would Bentley please demonstrate whether or not the “missing mass” methodology and equations used by AutoPIPE Nuclear are equivalent to Appendix A of RG 1.92, Revision 3?

  • Hi Tim

    In terms of the combination methods for AP Nuclear we are providing those from Rev 1 and 2 of RG 1.92.  We have not added any changes as a result of Rev 3, although I notice that there are no technical changes only editorial; so it is safe to assume we comply also to Rev 3 in this regard.  The method described in Appendix A is certainly the process we follow for determining the amount of missing mass that can then be addition in using one of 3 methods: Modal, SRSS or Abs Sum. 

    Missing Mass Correction

    • The amount of mass captured by all extracted modes is subtracted from the total mass.
    • The structure is subjected to acceleration equal to the cut-off frequency. Only the uncaptured mass is considered.
    • The static response is obtained for the structure.
    • The static response is combined with the dynamic response using the specified combination method.

    Our modal solution is finding the participating mass and thus the missing and dealing with it as described above.  The equations in the help show what we do with the missing mass, not how we find it.  If you need more, we can dig into the archive and program for the exact mathematic process and provide this. 

    Phil Senior

    Director or Product Management - Piping

    Bentley Systems

    Answer Verified By: Tim Barta 

  • Thanks, Phil. The help file equations looked similar to what I have seen referred to as the "left-out force" method. I wanted to make sure that was the same as "missing mass" from the regulatory guide because the equations are somewhat different. In addition to your confirmation above, one of our dynamic analysis engineers told me that the two can be shown to be equivalent for modal superposition analysis.