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STAAD Missing Mass Correction

DAR5052 (Structural) (OP) 
4 Sep 12 10:29 
Im doing a spectrum analysis on a structure that is highly irregular so it is very difficult to get the 90% mass participation that the code requires. I was going to use the missing mass correction command to boost the mass participation. Does anyone know what exactly this function does? I tried reading the help section in STAAD but it doesn't really say much. I'm not comfortable just applying factors without knowing what they are actually doing. Any input would be appreciated. Thanks 

As I understand it (from Dynamics text books, not from STAAD), the process is the following:
1) Sum up the modal response for the known / solved modes.
2) If the left over modes can be assumed to be "rigid" then you can combine their response together into an assumed rigid response mode. You know the mass distribution of this mode based on whatever mass is left over when you subtract the solved modal response (sum of mode shape * modal participation factor) from the unity vector.
3) You know the acceleration of this mode is the rigid response acceleration of your spectra. With all of this known, you can use your stiffness matrix to back out a residual model shape.
Be cautious though. There are a lot of ways this calculation could go wrong. If you don't have the ability to independently confirm the results then I'm not sure I would trust STAAD to get it right. Though my experience with STAAD is mostly from the old REI days (10~15 years ago). Perhaps their QA/QC has improved since they were acquired by Bentley.
Disclaimer: I work for one of STAAD's competitors. Though most of my skepticism is based on my time with another engineering firm where I worked on my firms NQA testing / validation for STAAD, SAP, and RISA. That time frame means that I was working with some of the last versions of STAADIII and maybe the early versions of STAADPro. That could have just been a difficult transition for them.... But, it drove me bonkers at the time and instilled in me a deep seated skepticism of computer output in general and STAAD in particular. 

DAR5052 (Structural) (OP) 
5 Sep 12 14:24 
Is there certain criteria required to assume the left over modes to be rigid?
Thanks! 

Excellent question, though we are venturing into relatively esoteric territory. If the program does an orthogonality check on the calculated missing mass mode then that should be sufficient. If it comes up as an orthogonal mode then all the modes were essentially rigid. If not, then the assumption may not have been good. I have not seen this explicitly confirmed in any of the dynamics texts that I have looked at. But, it makes sense to me.
That being said, I doubt STAAD performs that type of check. They may not even output the missing mode that they calculate. Therefore, you may be left to use old rules of thumb. Something like 33Hz and above may be considered rigid. Though I cannot remember precisely where I got that number. 

Sye123 (Structural) 
12 Sep 12 21:37 



