January 18, 2011
|
RISA-3D,
New,
Instability
Before
reading
this,
be
sure
to
check
out
the
article
linked
below
on
X-Bracing.
In
RISA-3D,
you
can
set
a
member
to
be
tension-only
by
selecting
it
and
selecting
Tension
Only
or
Euler
Buckling
from
the
Additional
Properties
section
of
the
Member
Properties.
Link:
Tips
&
Tricks:
X-Bracing
In
the
example
below,
if
the
braces
are
set
to
Tension-Only,
the
model
will
register
as
unstable:
Under
gravity
loading
the
elastic
shortening
of
the
columns
will
cause
some
elastic
shortening
of
the
braces.
This
means
that
each
brace
is
slightly
compressed
(with
about
50
lbs
of
axial
force).
When
the
braces
are
set
to
Tension
Only
they
are
not
allowed
to
take
any
compression
at
all,
so
they
are
both
ignored
during
the
solution,
resulting
in
an
unstable
frame.
By
setting
the
braces
to
be
Euler
Buckling
instead
of
Tension
Only,
the
program
will
allow
these
braces
to
take
compression
only
up
to
their
Euler
Buckling
limit,
which
is
enough
to
compensate
for
the
elastic
shortening.
If
any
significant
compression
enters
the
brace
it
will
still
be
ignored
during
solution…
