Hi,
The structure consists of plate elements and beam elements. Plate elements represent equipments and are not designed in STAAD Pro. They are modeled so that wind load can be applied as area load on plates. Actual stiffness of equipments is larger than stiffness of modeled plates.
When I use linear elastic analysis, the structure is stable. I am interested in global frame/ main frame displacements. For this purpose, a group named "DISPLACEMENT" is used which is made up of nodes in global frames.
However, when I use direct analysis, analysis results I get don't make any sense(displacement of the order of 10^24 in). I am using version 20.07.11.70 of STAAD Pro V8i.
I get this warning message for many nodes:
***WARNING - INSTABILITY AT JOINT 157 DIRECTION = MY
PROBABLE CAUSE SINGULAR-ADDING WEAK SPRING
K-MATRIX DIAG= 5.5370948E+04 L-MATRIX DIAG= 1.0186341E-10 EQN NO 3887
***NOTE - VERY WEAK SPRING ADDED FOR STABILITY
I understand that this message means there is instability in the structure. But structure is perfectly stable in linear elastic analysis !
Could anyone please help me understand this behavior and possible reasons for it?
Thanks,
Madhav