addition, seismic isolation systems are currently not
should be considered. Proximity to an active fault
designed for use at locations where the site period is
may be another important consideration.
Seismic
in the range of 2 to 3 seconds, since this is also the
isolation and energy-dissipation systems can both be
range of most current isolators.
effectively used to reduce earthquake damage when
compared with conventional construction, but each
(a) Sites where seismic isolation systems
type of system is most effective for a different range
are not recommended. During recent earthquakes,
of dynamic characteristics. In addition, the selection
near-fault spectral displacements of approximately 40
of one or other system may depend on whether
inches have been measured for periods in the range of
acceleration control, displacement control, or both,
2 to 3 seconds.
Current isolators typically have
are required to reduce the earthquake damage at a
periods of approximately 2.5 seconds.
These
particular facility.
isolators have not been designed to accommodate
such large spectral displacements, and may fail and
(1) System comparison. Table 8-3 provides a
develop vertical instabilities. Deep soil sites with 2-
comparison of building behavior for these three
to 3- second periods also would not be appropriate
systems conventional design, seismic isolation, and
for seismic isolation.
At such sites, the isolators
energy dissipation.
Generally, seismic isolation
could be in resonance with the ground motion,
systems are most effective in reducing damage to
resulting in the undesired amplification of the
buildings that are already very flexible.
Base
structural response. In the future, isolation systems
isolation is most effective when the original building
may he developed for these sites, but current seismic
period is significantly shorter than the isolated
isolation
techniques
and
hardware
are
not
building period, typically about 2.5 seconds. Energy
recommended for either the near-fault site, or the
dissipation systems are almost the reverse. They are
deep soil site with a 2- to 3- second period.
most effective in reducing damage to flexible
structures, and much less effective in reducing
(b) Sites where energy dissipation systems
damage to rigid structures.
are not recommended. During recent earthquakes,
including both Northridge, California and Kobe,
(2)
Site selection - inappropriate sites.
Japan, very large energy pulses have been recorded
Particular care must be used in selecting a structural
within the first few earthquake cycles at some near-
system for a building site located very close to an
fault sites. Very close to a fault, the majority of the
active fault or in an unmapped area that may be
total input energy at the site may be contained in an
underlain by blind thrust faults.
Recent seismic
initial large pulse.
Currently available energy
recording from near-fault sites include measurements
dissipators are generally designed to dissipate a
of very large spectral displacements at some stations,
portion of the energy input during each of several
and very large, one-cycle, energy pulses at other
cycles in order to obtain the maximum benefit.
stations.
Typical seismic isolation and energy
Current dissipators are not designed to dissipate the
dissipation systems are currently not designed to
total input energy from a major earthquake in one or
accommodate these extreme near-fault motions. In
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