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442
Base Isolation System
The base isolation system
considered herein is a combination
of elastic sliding devices and
commonly used laminated rubber
isolators to give restoring forces
after sliding, for which bi-linear
hysteresis characteristics may
reasonably be assumed In Tables
42 and 4.3 are shown the principal
parameters and specifications of
the base isolation devices which
were designed according to ALT
recommendation (ALT, 2001). In
order to prevent the sliding occur
under ordinary wind condition the
yield displacement was set larger
than the maximum displacement
2.24cm of the base isolation
system, predicted by the spectral
modal analysis under the design
wind loads in the return period of
100 years (Matsui et al, 2001).
Free Vibration Analysis
TABLE 4.2
PARAMETERS OF BASE-ISOLATION SYSTEM
Elastic natural period
Natural period after yielding
Initial horizontal stiffness
Secondary horizontal stiffness
Yield displacement
Yield shearing force coefficient
Friction coefficient
Item
1.0s
4.0s
303 kN/mm
18.9kN/mm
0.025 m
0.10
0.16
TABLE 4.3
SPECIFICATIONS OF BASE-ISOLATION SYSTEM
Number of devices
Diameter
Rubber thickness
Shear modulus of rubber
Shape coefficient S l
Shape coefficient S 2
Horizontal stiffness
Vertical stiffness
Surface pressure
Elastic sliding
device
40
600mm
5mmx5
0.628 N/mm 2
30.0
24.0
7. 10 kN/mm
12.0 MN/mm
4.43 N/mnr
Laminated
rubber isolator
20
600mm
6 mmx22
0.441 N/mm 2
25.0
4.6
0.95 kN/mm
2.06 MN/mm
4.43 N/mm 2
Prior to earthquake response
analysis the free vibration analysis was performed for the non-isolated pin-supported dome. Symmetry
with respect to the x-axis was exploited to analyze only a half of the dome. The natural periods and
corresponding free vibration modes for the lowest few modes are shown in Fig. 4.2. It is noted that the
natural periods with significant participation factors are closely spaced over the period range between
0.58-0.25 seconds and that not only horizontal but also vertical motions are excited by the horizontal
ground motion (anti-symmetric modes). In the earthquake response analysis below the lowest 64
modes with significant participation factors were adopted.
1st mode/0.580s
(anti-symmetric)
3rd mode / 0.528 s
(anti-symmetric)
4th mode / 0.524 s
(symmetric)
FIG. 4.2
NATURAL PERIODS AND FREE VIBRATION MODES OF NON-ISOLATED DOME