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>I've always heard that including 90%-95% of the modal mass
>is the "acceptable" criterion (for academic purposes).  I'm
>inclined to say that would be a good target in general, although
>I'm not sure what the typical standard is for professional
>modeling.  Anybody?

The rule I use with FEA is 75%. It'll be different for various manual 
approximations. The 75% rule is about what you'll get for an equivalent 
(dynamic) mass if you do a simple cantilever beam. That's because not all 
the mass is moving at the same rate so the product of mass and 
acceleration for each element will vary from a maximum at the free end to 
zero at the fixed end. 

Again, this is the rule for FEA use and it gives accurate enough results 
for seismic engineering analysis. Depending on the structure you may have 
directions where the structure is extremely stiff (like the spanwise 
direction in a beam, where the equivalent mass will be less than 75% of 
the theoretical mass, and re-meshing or choosing more DOF won't help. 
Normall the acceleration of such mass equals the ZPA, and the load won't 
amount to much, but there are ways of 'correcting' the analysis if you 
feel the need.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)

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