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ETABS UNSTABLE MODEL UNDER P-DELTA ANALYSIS

ETABS UNSTABLE MODEL UNDER P-DELTA ANALYSIS

ETABS UNSTABLE MODEL UNDER P-DELTA ANALYSIS

(OP)
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?

RE: ETABS UNSTABLE MODEL UNDER P-DELTA ANALYSIS

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.

RE: ETABS UNSTABLE MODEL UNDER P-DELTA ANALYSIS

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 !

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