evaluation of the strength and stiffness characteristics
used so that the accidental torsion increases the total
of the diaphragm. If the diaphragm is providing out-
design force for all walls. In computer calculations,
of-plane lateral support to the top of a relatively short
the accidental torsion may be handled by running one
or stiff concrete or masonry wall, it should be noted
analysis, using for eccentricity the calculated
that wall will experience the diaphragm deflections
eccentricity plus the accidental eccentricity, then
plus the in-plane deflection of the vertical lateral-
running a second analysis, using the calculated minus
load-resisting system. For use of Table 7-24, the
the accidental eccentricity, and finally, selecting the
flexibility category in the first column of the table
larger forces from the two cases.
can be determined with little or no calculation:
concrete diaphragms are rigid; bare metal deck
(f)
Dynamic amplifications of torsion.
diaphragms can be stiff or flexible; plywood
Section 5.3.5.3 of FEMA 302 specifies dynamic
diaphragms can be considered to be rigid when used
amplifications for Type 1 torsional irregularities in
in light wood framing, but should be considered to be
Seismic Design Category C, D, E, and F structures
stiff or flexible with other framing systems; special
analyzed by the ELF procedure.
diaphragms of diagonal wood sheathing are flexible;
and conventional diaphragms of diagonal wood
(5)
Flexibility limitations.
The deflecting
sheathing and diaphragms of straight wood sheathing
diaphragm imposes out-of-plane distortions on the
are very flexible (very flexible diaphragms are
walls that are perpendicular to the direction of lateral
seldom used in new construction because of their
force.
These distortions are controlled by proper
small capacities).
attention to the flexibility of the diaphragm.
A
diaphragm will be designed to provide such stiffness
(b)
Diaphragm deflections.
When a
that walls and other vertical elements laterally
deflection calculation is needed, the following
supported by the diaphragm can safely sustain the
procedure will be used.
stresses induced by the response of the diaphragm to
seismic motion.
1.
Deflection criterion.
The total
deflection of the diaphragm under the prescribed
(a) Empirical rules. Direct design is not
static forces will be used as the criterion for the
feasible because of the difficulty of making reliable
adequacy of the stiffness of a diaphragm.
The
calculations of the diaphragm deflections; instead,
limitation on the allowable amount relative to out-of-
diaphragms are usually proportioned by empirical
plane deflection (drift) of the walls, between the level
rules. The design requirement is considered to be
of the diaphragm and the floor below, is equal to the
met if the diaphragm conforms to the span and
deflection of the orthogonal walls at the ends of the
span/depth limitations of Table 7-24.
These
diaphragm, plus the deflection of the diaphragm, as
limitations are intended as a guide for ordinary
buildings. Buildings with unusual features should be
treated with caution. The limits of Table 7-24 may
be exceeded, but only when justified by a reliable
7 -120
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