Respnse Spectrum Analysis oddities...

[Backcheckrage]

Hello community.
When I run the deformed shape (or animation) for my Response Spectrum Load Cases in the X-only (U1) or Y-only (U2) loading, the animation shows the rigid diaphragm distorting significantly! also, it shows the building moving diagonally in plan, as if it is loaded simultaneously in the X and Y directions!... animation/deformed shape for my Automatically load generated equivalent static loading shows distortions as I expect, meaning I see no diaphragm distortion and mostly movement of diaphram is the direction i expect.

any ideas? ETABS model is attached.

 

[inggelu]

Hi, I check your model and I received some warning, please check and confirm (on your log analysis file) if you have this warnings.
For lots of points this is the warning:

* * * W A R N I N G * * *
NEGATIVE STIFFNESS FOUND DURING SOLUTION FOR DOF UZ OF JOINT 45
LOCATED AT X = 640.236220, Y = 23.622047, Z = 660.000000,
STIFFNESS MATRIX DIAGONAL VALUE = -37.238835,

Also an important warning is this one:

* * * 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


This warning are important and should not apear when you run an analysis.
Confirm or infirm if you have this problem and we will try to find a solution.

To check the log file after you run the analysis go to menu File > Last Analysis Run Log ...
scroll down until we see the warnings or not if is the case.

Good luck.

 

[Backcheckrage]

inggelu: thanks, when I turned off p-delta analysis, those errors went away -which is OK with me because I am only interested in the dynamic analysis and results of this model for now. I can troubleshoot the model at a later date.

Any ideas on why the diaphragms are deforming in such a way?