October 17, 2025
|
Featured,
RISA-3D
Rigid
diaphragms
in
RISA-3D
are
a
powerful
way
to
model
how
floor
systems
distribute
lateral
loads.
By
forcing
all
connected
nodes
to
move
together
in-plane,
they
effectively
capture
the
stiffness
of
a
concrete
slab
or
diaphragm
deck—often
simplifying
analysis
without
sacrificing
accuracy.
However,
when
rigid
diaphragms
are
combined
with
sloping
members,
they
can
introduce
unexpected
behavior
that
changes
how
the
structure
resists
loads—sometimes
creating
a
hidden
“tension
tie”
that
doesn’t
exist
in
the
real
system.
When
Rigid
Diaphragms
Alter
the
Model’s
Behavior
Consider
a
simple
moment
frame
with
sloped
beams
under
gravity
loads—common
in
pre-engineered
metal
buildings.
Model
1:
No
rigid
diaphragm
applied
Model
2:
Identical
frame,
but
with
a
rigid
diaphragm
located
at
the
eaves
When
reviewing
the
strong-axis
bending
moments,
column
base
reactions,
and
thrust
forces:
The
first
frame
behaves
as
expected.
The
second
frame
(with
the
rigid
diaphragm)
shows
reduced
bending
moments
and
smaller
thrust
reactions
at
the
column
bases.
At
first
glance,
this
might
seem
like
an
improvement—but
it’s
actually
unrealistic
behavior
caused
by
the
diaphragm.
Why
It
Happens:
The
“Hidden
Tension
Tie”
In
the
model
with
the
rigid
diaphragm,
the
diaphragm
prevents
the
eaves
from
moving
apart
under
load.
This
effectively
turns
the
diaphragm
into…