How to Locate and Fix the Negative Eigenvalues
Causing ETABS Model Instability.
The previous video linked in the description
below shows us how to perform an ETABS standard
solver and found negative eigenvalues that
cause the model instability.
Stay tuned as this video will discuss how
to locate and fix those joints with values
more than eleven digits loss of accuracy.
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bell to continue.
In the same model which you performed the
Standard Solver. Go to File. Show input/output
text files.
Open. Scroll down and locate the joints with
negative stiffness or with more than 11 digits
lost of accuracy. Notice that these joints
had x, y and z coordinate which we will use
to locate. The x- coordinate is the location
of this joint in the x-direction, the Y- coordinate
in the y-direction and the Z-coordinate as
the height or elevation.
To locate using its coordinates. Go to the
ETABS model. Set the plan view that corresponds
to Z-coordinate. And draw a temporary line
in a vacant space. Right-click on the tip
of this line and the "Joint Object Information"
Window will display. Edit the Global x and
global Y under the "Geometry" list as per
the coordinates of the joints having a negative
stiffness error. You can copy-paste it to
save your time.
The tip of the drawn temporary line is the
location of the joint that causes the instability
error. Investigate this joint for a possible
loose joint to joint connections. You can
also align its shells to make sure that every
joint are coinciding.
Do this procedure on the other joints that
cause error messages.
Once you completed this exercise. Delete the
temporary drawn line. Set the P-delta to "Iterative".
And run the model as "advanced solver". Investigate
the analysis log once more. If the "negative
stiffness eigenvalues" is equal to zero, then
the model is stable and you successfully fix
it.
