have only a marginal effect on the response.
It
should be noted that the reduction of the response is
(6) Perform the analysis of the revised model
most dramatic when the frequency of the structural
with the modified spectrum and compare the story
system (including the effects of the yielding device)
displacements with the assumed target displacements.
coincides with the frequency at the peak of the input
If necessary, revise the target displacements and
acceleration spectrum. This is shown in Figures 8-5
reiterate the analysis.
and 8-6 using four representative building types and
two different soil types, represented by earthquake
(7)
Optimize the design by using several
response spectra. These examples are constructed to
assumed values of the effective stiffness of the
demonstrate the effect of the supplemental damping.
devices and the target displacements.
For the sake of simplicity, the effect of the added
stiffness has been included with the building period
Evaluation of the benefits of velocity-displacement
cited below.
energy-dissipation devices is much more complex
and beyond the scope of this document. Guidance
for such an evaluation can be obtained from the
design examples in FEMA 274 (Commentary to
FEMA 273).
b.
Device Description. A number of energy-
dissipation devices are currently in use or proposed
for use in the U.S.
The specific properties vary
widely. Some of these systems use viscous fluids or
viscoelastic materials; some rely on the hysteretic
behavior of metallic elements; and others use sliding
systems that rely on frictional resistance.
The
systems that use viscous and viscoelastic materials
are rate-dependent (i.e., the hysteretic response of the
device depends upon the rate of loading), and also
may be temperature sensitive. The other systems are
generally rate-independent.
c.
Applications. Supplemental damping may
significantly reduce the seismic input where the
structural period is in resonance with the predominant
period of the site. If the structural period and site
period are widely separated, added damping may
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