the particular isolation device. The primary objective
Calculate the design displacement, DD,
(4)
of the design is to obtain a structure such that the
using Equation 8-1. Compare the calculated value
isolated period of the building is sufficiently longer
with the assumed value, and if necessary, reiterate the
than both the fixed-base period of the building (i.e.,
process with revised values of KDmax, TD, and BD until
the period of the superstructure), and the predominant
isolator properties provide the desired base shear, VS,
period of the soil at the site. In this way, the
in the building.
superstructure can be decoupled from the maximum
earthquake input energy. The spectral accelerations
Calculate maximum displacement, DM,
(5)
at the isolated period of the building are significantly
using Equation 8-3 and total maximum displacement,
reduced from those at the fixed-base period. The
DTM, using Equation 8-6. The isolated building and
resultant forces on structural and nonstructural
all connecting utilities and appurtenances must be
elements of the superstructure will be significantly
able to accommodate these displacements without
reduced when compared with conventional fixed-
base design.
The benefits resulting from base
isolation are attributed primarily to a reduction in
b.
Device Description. A number of seismic
spectral demand due to a longer period, as discussed
isolation devices are currently in use or proposed for
in this Paragraph.
Additional benefits may come
use in the U.S. Although the specific properties vary,
from a further reduction in the spectral demand
they are all designed to support vertical dead loads
attained by supplemental damping provided by high-
and to undergo large lateral deformations during a
damped rubber components or lead cores in the
major earthquake.
Some of these systems use
isolation units.
A preliminary evaluation of these
elastomeric bearings; others use sliding systems that
benefits requires the following considerations:
rely on frictional resistance.
Select a target base shear, VS, and an
(1)
appropriate response modification factor, RI, for the
isolated building. Calculate KDmax DD from Equation
8-8.
(2) From test data supplied by the isolation
manufacturer, select units with effective stiffnesses
KDmin and KDmax that approximately satisfy the
calculated value of KDmax DD.
(3) From the isolator damping characteristics
provided by the manufacturers, assume an effective
damping coefficient, $D, and obtain the appropriate
value of BD from Table 8-1.
8-6
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