ETABS UNSTABLE MODEL UNDER P-DELTA ANALYSIS

[pdub18]

I receive a stability error when running P-Delta analysis:
* * * 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 UNDER P-DELTA LOADS (IF ANY), OR

- AN EIGEN SHIFT (IF ANY) ONTO A NATURAL FREQUENCY



The analysis run log gives a negative stiffness warning here:



* * * W A R N I N G * * *

NEGATIVE STIFFNESS FOUND DURING SOLUTION FOR DOF RY OF JOINT 11897

LOCATED AT X = 2319.000, Y = 1428.000, Z = 180.000000,

STIFFNESS MATRIX DIAGONAL VALUE = -412.270688,

I stiffened the structure in this area and still get areas. Any suggestions?

 

[asixth]

After you run the analysis, go to File>Last Analysis Run Log, and see the force and moment error at the origin. If the number is 10^-6 or less than I normally proceed.

 

[PicoStruc]

Are the same warning with linear analysis ?

P-Delta is a geometric stiffness that is equivalent to negative stiffness for compression member. With P-Delta for compression member, the structure become more flexible.

Thus,

1) Check the axial load in members, you might have negative NET stiffness (Axial/Bending stiffness+ geometric stiffness)

2) Check your design. Maybe, the structure is too flexible and the displacement is too big to be stable in non-linear analysis using small displacment theory !