July 21, 2020
|
Seismic,
Dynamics,
RISA-3D,
New
In
structural
engineering,
a
modal
analysis
uses
the
overall
mass
and
stiffness
of
a
structure
to
find
the
various
periods
at
which
the
structure
will
naturally
resonate,
along
with
the
frequencies
of
vibration
of
the
structure.
In
RISA-3D,
the
modal
analysis
is
a
prerequisite
to
the
response
spectra
analysis,
which
uses
these
frequencies
to
calculate
forces,
stresses
and
deflections
in
the
model.
When
conducting
a
modal
response
spectrum
analysis,
building
codes
(including
ASCE
7)
require
that
a
sufficient
number
of
modes
be
used
to
understand
the
overall
response
of
the
structure.
While
it
is
not
always
possible
to
achieve
full
participation
(a
case
where
100%
of
the
modal
mass
is
accounted
for)
the
code
does
at
minimum
require
that
a
modal
analysis
shall
include
a
minimum
number
of
modes
in
order
to
obtain
a
combined
mass
participation
of
at
least
90%
of
the
actual
mass
in
each
orthogonal
horizontal
direction
(X
&
Z
directions
in
RISA-3D).
In
order
to
determine
the
appropriate
number
of
modes
for
a
given
structure
(while
also
achieving
the
90%
mass
participation
required
by
code),
RISA-3D
users
must
first
pick
an
arbitrary
number
of
modes
(5
to
10
is
usually
a
good
starting
point)
and
solve…
