...
Why
...
do
...
I
...
get
...
negative
...
stiffness
...
during
...
P-Delta
...
analysis?
...
Extended
...
Question:
...
When
...
I
...
get
...
negative
...
stiffness,
...
and
...
P-Delta
...
analysis
...
does
...
not
...
converge,
...
does
...
this
...
indicate
...
instability
...
and
...
buckling?
...
Answer:
...
Convergence
...
problems
...
may
...
correlate
...
with
...
...
behavior.
...
Inadequate
...
member
...
capacity
...
and
...
insufficient
...
structural
...
support
...
may
...
lead
...
to
...
instability
...
and
...
...
.
...
Excessivley
...
large
...
stiffness
...
values
...
may
...
also
...
disrupt
...
convergence
...
by
...
causing
...
...
...
which
...
adversely
...
affect
...
computation.
...
When
...
a
...
model
...
fails
...
under
...
P-Delta,
...
increase
...
the
...
size
...
and/or
...
quantity
...
of
...
columns
...
and
...
bracing
...
to
...
improve
...
lateral
...
capacity,
...
and
...
thereby
...
avoid
...
buckling.
...
The
...
Analysis
...
Run
...
Log
...
will
...
indicate
...
which
...
members
...
have
...
experienced
...
instability.
...
As
...
an
...
alternative,
...
you
...
may
...
also
...
refine
...
the
...
factors
...
and/or
...
loads
...
applied
...
during
...
analysis.
...
It
...
may
...
be
...
best
...
to
...
troubleshoot
...
the
...
model
...
without
...
P-Delta
...
until
...
you
...
are
...
confident
...
that
...
behavior
...
is
...
appropriate.
...
If
...
problems
...
persist
...
with
...
geometric
...
nonlinearity,
...
it
...
may
...
be
...
helpful
...
to
...
run,
...
after
...
the
...
P-Delta
...
...
...
,
...
a
...
...
analysis
...
which
...
uses
...
Stiffness
...
At
...
The
...
End
...
Of
...
This
...
Stage.
...
Modal
...
analysis
...
may
...
then
...
reveal
...
the
...
source
...
and
...
location
...
of
...
instability.
Additional suggestions which may resolve negative-stiffness errors include:
- Remove auto line constraints. Select all deck/shell objects in the model, then select Assign > Shell Elements > Auto Line Constraint and uncheck the Apply to Full Structure option. This will remove auto line constraints and possibly resolve instabilities.