| Applies To | |||
| Product(s): | STAAD.Pro | ||
| Version(s): | All | ||
| Environment: | N/A | ||
| Area: | Analysis | ||
| Subarea: | Instability and Zero Stiffness | ||
| Original Author: | Bentley Technical Support Group | ||
I have analyzed a structure and find that there are instability messages in the .anl (output) file, as follows :
***WARNING - INSTABILITY AT JOINT 26 DIRECTION = FX
PROBABLE CAUSE SINGULAR-ADDING WEAK SPRING
K-MATRIX DIAG= 5.3274384E+03 L-MATRIX DIAG= 0.0000000E+00 EQN NO 127
***NOTE - VERY WEAK SPRING ADDED FOR STABILITY
**NOTE** STAAD DETECTS INSTABILITIES AS EXCESSIVE LOSS OF SIGNIFICANT DIGITS
DURING DECOMPOSITION. WHEN A DECOMPOSED DIAGONAL IS LESS THAN THE
BUILT-IN REDUCTION FACTOR TIMES THE ORIGINAL STIFFNESS MATRIX DIAGONAL,
STAAD PRINTS A SINGULARITY NOTICE. THE BUILT-IN REDUCTION FACTOR
IS 1.000E-09
THE ABOVE CONDITIONS COULD ALSO BE CAUSED BY VERY STIFF OR VERY WEAK
ELEMENTS AS WELL AS TRUE SINGULARITIES.
What is the significance of such messages?
An instability is a condition where a load applied on the structure is not able to make its way into the supports because no paths exist for the load to flow through, and may result in a lack of equilibrium between the applied load and the support reaction.
There is some explanation available in Section 1.18.1 of the STAAD.Pro Technical Reference Manual for the typical cause of instabilities. You will find it under the heading "Modeling and Numerical Instability Problems".