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?
* * * 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
RE: ETABS UNSTABLE MODEL UNDER P-DELTA 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 !