interpretation, these combinations could also
various structural systems are provided in Table
include different systems in the same vertical
7-1.
plane (e.g., a two-story building with steel
(2) Modification factors, m, for enhanced
moment frames in the second story and a
performance objectives for beams, columns, and
concrete shear wall system in the first story). In
connections in fully restrained moment frames
the first case above, FEMA 302 permits the use
are provided in Table 7-12, and for partially
of the appropriate R factor pertaining to the
restrained moment frames in Table 7-13.
structural system in each orthogonal direction.
For the second case, the FEMA provision states:
(3) Modeling parameters and numerical
"The response modification coefficient, R, in the
acceptance criteria for nonlinear procedures are
direction under consideration at any story shall
provided in Table 7-22 for fully restrained
not exceed the lowest response modification
moment frames, and in Table 7-23 for partially
factor, R, for the seismic-force-resisting system
restrained moment frames.
in the same direction considered above that
story, excluding penthouses. For other than dual
(4) Expected strength of columns, beams,
systems where a combination of different
and other deformation-controlled components
structural systems is utilized to resist lateral
shall be determined using the expected yield
forces in the same direction, the value of R used
strength, Fye, as defined in the AISC Seismic
Provisions and the plastic section modulus, Z,
in that direction shall not be greater than the least
value of any of the systems utilized in the same
where applicable.
direction. If a system other than a dual system
(5)
The lower bound strength of
with a response modification coefficient, R, with
connections
and
other
force-controlled
a value of less than 5 is used as part of the
components shall be determined in accordance
seismic-force-resisting system in any direction of
with the nominal strength and N factors
the structure, the lowest such value shall be used
prescribed by AISC Seismic Provisions.
for the entire structure. The system overstrength
factor, Ω o , in the direction under consideration
7-6.
Dual Systems.
at any story, shall not be less than the largest
value of this factor for the seismic-force-resisting
a.
General.
system in the same direction considered above
that story."
(1) Combinations of structural systems.
The connotation of dual systems is sometimes
Exceptions:
erroneously
interpreted
to
mean
different
systems in each orthogonal direction of structural
(a) Supported structural systems with a
To clarify this point, FEMA 302
weight equal to or less than 10 percent of the
describes that condition as "combinations of
weight of the structure.
structural systems."
In addition to the above
7-105
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