Warning in Non-linearly Static Analysis in Etabs 2016

[SPR Baker]

Dear Alls,
I have faced this warning in Etabs2016, How can I solve it?

For Slab Combination and Wall Combination
"
----------------------------------------
BASIC STABILITY CHECK FOR FORCE-CONTROLLED NONLINEAR STATIC LOAD CASES:
NUMBER OF NEGATIVE STIFFNESS EIGENVALUES SHOULD BE ZERO FOR STABILITY.
(NOTE: FURTHER CHECKS SHOULD BE CONSIDERED AS DEEMED NECESSARY)

NUMBER FOUND AT FINAL CONVERGED STATE = 2

* * * W A R N I N G * * *
THE STRUCTURE IS UNSTABLE OR ILL-CONDITIONED !!
CHECK THE STRUCTURE CAREFULLY FOR:
- INADEQUATE SUPPORT CONDITIONS, OR
- ONE OR MORE INTERNAL MECHANISMS, OR
- ZERO OR NEGATIVE STIFFNESS PROPERTIES, OR
- EXTREMELY LARGE STIFFNESS PROPERTIES, OR
- BUCKLING DUE TO P-DELTA OR GEOMETRIC NONLINEARITY, OR
- A FREQUENCY SHIFT (IF ANY) ONTO A NATURAL FREQUENCY

TO OBTAIN FURTHER INFORMATION, RUN AN EIGEN ANALYSIS FROM
THE END OF THIS NONLINEAR CASE USING AUTO FREQUENCY SHIFTING
(WITH ADDITIONAL MASS IF NEEDED) AND INVESTIGATE THE MODE SHAPES "

For Concrete Frame Combination
"* * * W A R N I N G * * *
MAXIMUM NUMBER OF NULL STEPS REACHED FOR
CASE: DCON28-NL
SUBSEQUENT RESULTS WILL NOT BE AVAILABLE"

Many Thank for Replying

 

[JasonMcKee71]

Make sure that the structure is stable corresntly supported.

Jason McKee
proud R&D Manager of
Cross Section Analysis & Design
Software for the structural design of cross sections
Moment Curvature Analysis
Interaction Diagrams
Reinforcement Design etc.

 

[serrojo]

Hello,

I would follow exactly the Analysis Log advice:

CHECK THE STRUCTURE CAREFULLY FOR:
- INADEQUATE SUPPORT CONDITIONS, OR
- ONE OR MORE INTERNAL MECHANISMS, OR
- ZERO OR NEGATIVE STIFFNESS PROPERTIES, OR
- EXTREMELY LARGE STIFFNESS PROPERTIES, OR
- BUCKLING DUE TO P-DELTA OR GEOMETRIC NONLINEARITY, OR
- A FREQUENCY SHIFT (IF ANY) ONTO A NATURAL FREQUENCY

In other words: unconnected joints, incorrect material properties, exaggerated load values, incorrect supports. If you fail finding any of this you can upload the model file and I could give it a look.

Regards.

MSc. Eng. Serguei Joa
Structural Engineer
Bouygues Batiment International, Cuba.

 

[SPR Baker]

Many thank you both for sharing,
I solved it : the solution are,
the main cause is meshing just remove Restraint between slab and wall when we assign mesh.

 

[SPR Baker]

Hi Serrojo !
I have questions
1.What your technique to take account P-Delta affect in Etabs or tool in Etabs ? And make sure it really takes account?
2.When we run Non-linearly Geometry Analysis , When we let Etabs run design should we set design override (e.g change k buckling length as =1, non/sway factor as =1) or use Etabs design as nonsway (Etabs will calculates as Moment Magnifiers)?
Many thank