December 16, 2010
|
RISA-3D,
New,
Instability
When
it
comes
to
trusses
in
RISA-3D,
boundary
condition
definitions
(pin
vs.
roller)
can
make
a
huge
difference.
Let’s
take
an
example
of
a
typical
roof
truss.
Note
that
a
pin-pin
boundary
condition
has
been
applied
to
the
ends.
First
we
must
make
sure
to
give
the
truss
out
of
plane
stability
by
setting
this
up
as
a
2D
Model
within
RISA-3D.
Then,
after
solving,
you
will
find
that
this
is
not
at
all
behaving
like
a
truss.
There
are
huge
axial
compression
forces
in
the
top
chords,
and
there
is
no
tension
in
the
bottom
chord.
The
reason
is
that
the
pin-pin
boundary
condition
prevents
the
bottom
chord
from
elongating.
If
it
cannot
elongate
then
there
cannot
be
axial
strain
in
the
member.
You
will
recall
from
your
Mechanics
of
Materials
class
that
if
there
is
no
strain
there
cannot
be
stress,
and
therefore
there
can
be
no
axial
tension
in
the
bottom
chord.
The
program
works
out
statics
the
only
way
it
knows
how
by
using
arching
action
with
the
top
chords
instead
of
looking
at
truss
behavior.
Now
instead
try
a
roller
boundary
condition
on
one
end
of
the
truss:
Now
the
bottom
chord
is
experiencing
a…
