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stiffness of the springs added to give restoring
forces after sliding was tuned such that the
natural period of the isolation system after
sliding is 3.0 seconds. Young's modulus of PVC
material and the friction coefficient of the roller
were evaluated by measurement. The damping
ratios were evaluated by the random decrement
method from the records of forced oscillation
tests (Vandier, 1982). The principal dimensions
and parameters of the model are listed in Table
2.1.
Method of Measurement
Tests were performed using the bi-axial shaking
table at Nagoya University which can generate
horizontal and vertical motions simultaneously.
The NS and UD components of El Centro 1940
acceleration records which were normalized such
TABLE 2.1
PRINCIPAL PARAMETERS OF TEST MODEL
Span
Rise
Radius of curvature
Width
Thickness
Half subtended angle
Total mass of arch
Total mass of sub-structure
Young's modulus of PVC
Damping ratio
non-isolated model
isolated model
Stiffness of a spring
Friction coefficient of rollers
780mm
212.5 mm
480.6 mm
300mm
I mm
54.2°
0.409 kg
0.329 kg
2.90 kN/mnf
0.809 N/m
0.03
that the maximum horizontal acceleration is 300 gal were adopted as input horizontal and vertical
ground motions, respectively. The accelerations of the shaking table were measured by means of
strain-gauge type acceleration meters, and the displacement responses were measured at 7 points along
the arch section and a point on the shaking table by means of an optical image tracking system.
Test Results
Prior to forced oscillation tests using earthquake records, sweep oscillation tests were performed to
measure the natural frequencies of the non-isolated arch model, of which the results are shown in
Table 2.2.
Fig. 2.2 shows the measured time histories of
displacement responses at the point A of the nonisolated
and isolated models. These are the
displacements relative to the base-isolation system
for the isolated model and to the shaking table for
the non-isolated model. It is observed that large
displacement responses are excited not only in the
horizontal but also in the vertical directions of the
non-isolated arch. This is due to the fundamental
TABLE 2.2
MEASURED AND PREDICTED NATURAL
PERIODS OF NON-ISOLATED MODEL
Mode
Measured
Predicted
1 st mode
(antisymmetric)
0.632 s
0.637 s
2 nd mode
(symmetric)
0.172s
0.180s
(s)
Horizontal disi
Vertical displacanent
Mon-isolated
Non-isolated
— Isolated
FIG. 2.2
MEASURED DISPLACEMENT RESPONSES
AT POINT A OF ARCH