I have been analyzing water hammer using force time histories. With a cutoff frequency of several hundred Hz I have seen time history captured modal mass greater than 100%. When the cutoff frequency is not specified and the system is analyzed up to 999 modes, one transient extracts 26 modes and gives me Captured modal mass (%) : 104.19. The Frequencies sub-report does not show a captured mass total over 100% even when the Time History analysis summary does have a value above 100%. This doesn't make physical sense to me - why is the Time History analysis capturing more mass than exists in the model? For what it's worth, all supports in my model have rigid stiffness.
Are you including Zero Period Acceleration in your Time History Analysis case? Please note that at the bottom of the Frequency report, there is a note:
Note: Participation factors and captured modal mass apply only to Response Spectrum and Earthquake Time History load cases. Captured modal mass for other load types can be found in the analysis summary sub-report.
This note tells you that the captured modal mass shown in the Frequency report is not what is being used for your water hammer time history case. The captured modal mass that is being used in this case is what is shown in the Analysis Summary Report. This value can be higher than 100% and that is explained in the program help. Please go to Help > Contents > Contents Tab > Reference Information > Analysis Considerations > Modal Analysis > Theory of Missing Mass and ZPA Correction. Review the Time history analysis and Captured modal mass report sections, which explain how the total captured modal mass is calculated.
Bentley Systems - Mechanical Application Engineer
No, I am not using ZPA in the Time History case. I checked out the help section you referenced and I do see that it says the captured modal mass "could exceed 100% in certain cases." It doesn't explain why it's possible, however, and since I don't have a background in dynamic analysis theory I haven't been able to deduce it from the help section. If you have examples of the "certain cases" where the captured mass can be over 100%, I'd be interested. It is helpful to know that there is some discussion of the possibility in the program documentation, which shows that it's not necessarily an anomaly, so thank you.
Would you be kind enough to post an example model for review?
Option 1: Post file on Forum page here.
Option 2: Upload your file to our Bentley Secured Server, here. Be sure to mention your file uploaded.
Option 3: Log a Bentley Service Request and send your model for review.
Mike DattilioBentley Design AnalystAutoPIPE Product Line Bentley Systems, Inc.===================================================
I can't share the actual model because release of our design info is tightly controlled. Can you give examples of the "certain cases" in which time history captured mass can be over 100%, as the help file states?
It could happen in certain cases such as when the applied forces coincides with modal high points. The missing mass (or a better term could be missing force) correction could result in higher included force than the applied force.