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

When the PDELTA ANALYSIS command is specified with the CONVERGENCE (m) parameter, the output file contains a message such as:

Load Case = 1 Convergence Error (SRSS) = 2.790E-02 INCH

What does this number represent?

The SRSS is the square root of the sum of the squares of the change in displacement (from one iteration to the next) of every dof. If after an iteration, the SRSS falls below the SET DISP tol or default tolerance then
that PDELTA load case is converged.
Mathematically this is the Euclidean norm of the vector of displacement changes.

|| {DISPi} - {DISPi-1} ||

SRSS=0.00
do iDOF=1,nj6
SRSS=SRSS+(dj(iDOF,ipdload)-olddj(iDOF,ipdload))**2
enddo
SRSS=sqrt(SRSS)

The SRSS printed is for the last iteration performed for that load case and tends to be an upper bound on the amount of error in any one displacement.