Dear DSANU
I assume you are refereeing to FEMA356-2000 as FEMA 256 is a "Flood Management & Development" Standard. Please note that FEMA 356-2000 which Staad Pushover is based on is a "Pre-Standard". It was never published as a Standard. It was partly replaced by FEMA440, which is a Standard. With regards to participation please refer to ASCE 41-06 clause C3.3.2.4 (referencing recent research in FEMA-440-2005). Furthermore the actual code to which a push over should have been written is ASCE 41-06. Staad Development team are way behind the latest approved Standard (to be exact 8 years(2014-2006)). Staad Pushover Program should have a warning for the user saying the approach used here is based on a superseded code.
Furthermore what does the user has to do when they get the error "MASS PARTICIPATION IS LESS THAN 75 PERCENT IN PUSH LOAD DIRECTION. FURTHER ANALYSIS IS STOPPED". It is not clear on any of on line document or the Help Menu of Staad Advance Analysis package.