missing mass correction in staad
 

missing mass correction in staad
i am using staadpro 2002 to do a shock (response) spectrum analysis of rc building. they have an option for missing mass. does anybody know what this is? they describe it as
"the sum of the modal weights for the computed modes may be compared to the total weight of the structure (that has not been lumped at supports). the difference is the amount of mass missing from a dynamic, base excitation, modal response analysis. if too much is missing, then rerun the eigensolution asking for a greater number of modes or read about how to employ the missing mass correction method."
you will need the 'stardyne' solver that staad offers.