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

In the output file of a response spectrum analysis, there is the section that shows the mass participation factors in the x, y, and z directions. Then it shows the 'base shear' in all 3 directions. What is the reference point for this? I mean, does Staad select a 'base' or is the value just the sum of all forces in that particular direction?

Each mode has a base shear that comes from the modal displacement at each joint with mass in the direction being excited by the base acceleration and the input spectral acceleration and the modal frequency. These modal base shears are combined by SRSS or any other method in STAAD that you select. In effect, all supported joint directions form the base where the displacement of every mode is zero.