second-period structures, and reduced by a smaller
(1) Elastomeric Systems.
Applications. While base isolation is an ideal
amount for the more flexible building with the 1.2-
second period. It is important to remember that using
solution for some building structures, it may be
conventional design principles, all three of these
entirely inappropriate for others. Since the objective
structures would soften during a major earthquake,
of isolation design is to separate the response of the
and the forces would consequently be reduced, even
fixed-base structure from the predominant period of
without the addition of isolators. Nonetheless, these
the underlying soil, it is most effective when these
structures would be damaged, and if damage control
two periods coincide.
In cases where they are
and post-earthquake functionality are important
already widely separated, base isolation may increase
issues, then isolation may still be useful even for the
the response of the structure rather than reducing it.
more flexible steel frame structure.
For instance, a very stiff structure on very soft soil
would be a poor candidate, as would a very soft
(2) Soft soil example. The same three fixed-
structure on very stiff soil. This is shown in Figures
base structures are considered as potential candidates
8-2, 8-3, and 8-4 using three representative building
for isolation. The period of the isolated structure for
types and three different soil types, represented by
all three cases is assumed to be 2.5 seconds. From
earthquake response spectra.
The damping of the
Figure 8-3, it may appear that none of the three
isolation devices may serve to further reduce the
buildings are good candidates for base isolation. The
response of the building, but for the sake of
simplicity, the effect of damping is not included in
structures are reduced at a period of 2.5 seconds, but
the following examples.
not dramatically. The response of the 0.3-second-
period building would increase; nevertheless, the 0.3-
Hard soil example.
second fixed-base structure would soften during a
structures are considered as potential candidates for
large earthquake, resulting in higher seismic forces
isolation. The period of the isolated structure for all
and additional damage.
Thus, if post-earthquake
three cases is assumed to be 2.5 seconds. The three
functionality is important, all of these structures
buildings, and fixed-base periods without isolators,
might benefit from an appropriate isolation system.
are as follows:
Concrete shear wall or steel braced frame
(3) Very-soft-soil example. In this case, all
three structures shown in Figure 8-4 would be
building; T = 0.3 seconds;
subjected to higher seismic forces at the isolated
period than at the fixed-base period, and no
Concrete frame building; T = 0.7 seconds;
advantage would be gained from base isolation.
Steel frame building; T = 1.2 seconds;
From Figure 8-2, it is evident that the seismic forces
would be significantly reduced for the 0.3- and 0.7-