Applies To
	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	Analysis Solutions
	Subarea:	Response Spectrum Analysis
	Original Author:	Bentley Technical Support Group

I need to know how to input the missing mass correction command as too much mass is missing even after increasing the number of modes.

The answer to your question is available in Section TR.32.10.1 Response Spectrum Analysis of the STAAD.Pro Technical Reference manual. Just use the keyword MIS along with the SPECTRUM command. For example, 

SPECTRUM CQC X 1.0 ACC DAMP 0.05 SCALE 32.2 MIS 

Please refer to example 11 in the Examples manual (File > Help > Contents > Application Examples > EX. American Design Examples > EX. US-11 Response Spectrum Analysis of a Frame) for information on the commands required for doing a response spectrum analysis.