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

If there are instability messages, are there any simple checks to verify whether my analysis results are satisfactory?

There are 2 important checks that should be carried out if instability messages are present.

a) A static equilibrium check. This check will tell us whether all the applied loading flowed through the model into the supports. A satisfactory result would require that the applied loading be in equilibrium with the support reactions.

b) The joint displacement check. This check will tell us whether the displacements in the model are within reasonable limits. If a load passes through a corresponding unstable degree of freedom, the structure will undergo excessive deflections at that degree of freedom.

One may use the PRINT STATICS CHECK option in conjunction with the PERFORM ANALYSIS command to obtain a report of both the results mentioned in the above checks. The STAAD output file will contain a report similar to the following, for every primary load case that has been solved for :

***TOTAL APPLIED LOAD ( KG METE ) SUMMARY (LOADING 1 )
SUMMATION FORCE-X = 0.00
SUMMATION FORCE-Y = -817.84
SUMMATION FORCE-Z = 0.00

SUMMATION OF MOMENTS AROUND THE ORIGIN-
MX= 291.23 MY= 0.00 MZ= -3598.50


***TOTAL REACTION LOAD( KG METE ) SUMMARY (LOADING 1 )
SUMMATION FORCE-X = 0.00
SUMMATION FORCE-Y = 817.84
SUMMATION FORCE-Z = 0.00

SUMMATION OF MOMENTS AROUND THE ORIGIN-
MX= -291.23 MY= 0.00 MZ= 3598.50


MAXIMUM DISPLACEMENTS ( CM /RADIANS) (LOADING 1)
MAXIMUMS AT NODE
X = 1.00499E-04 25
Y = -3.18980E-01 12
Z = 1.18670E-02 23
RX= 1.52966E-04 5
RY= 1.22373E-04 23
RZ= 1.07535E-03 8

Go through these numbers to ensure that 

i) The "TOTAL APPLIED LOAD" values and "TOTAL REACTION LOAD" values are equal and opposite.
ii) The "MAXIMUM DISPLACEMENTS" are within reasonable limits.