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anti-symmetnc mode excited by the horizontal ground motion. The large displacement responses of
the non-isolated arch are found to be reduced drastically by introducing the base-isolation system,
demonstrating the effectiveness of the proposed base-isolation system. The seismic isolation system
possesses only a mechanism to isolate the horizontal ground motion. In the present case the vertical
displacement of the arch is governed by the contribution from the anti-symmetric mode excited by the
horizontal ground motion for which the base-isolation system is effective. This is the reason why the
seismic isolation system is effective in reducing the displacement responses not only in the horizontal
but also in the vertical directions.
In order to investigate the effectiveness of
additional springs in reducing the residual
displacement of the base-isolation system at the
termination of the earthquake, tests were also
performed for the isolated model without the
springs. Fig. 2.3 shows the comparison between
the displacement responses of the base-isolation
system with and without the springs. It is FIG. 2.3
obvious that the additional springs are very MEASURED DISPLACEMENT RESPONSE
effective in reducing the residual displacements
OF BASE-ISOLATION SYSTEM
SIMULATION ANALYSIS FOR EXPERIMENTAL ARCH MODEL
Simulation Method
The earthquake response analysis is based on the sub-structure method which divides the whole system
into the arch and the base isolation system. Assuming that it remains within the elastic range, the
response of the arch is expressed as a superposition of free vibration modes in the absence of the base
isolation system. The base isolation system is idealized as a single d. o. f. system with bi-linear
hysteresis characteristics which is movable only in the horizontal direction. This makes it easy to
implement the parametric analysis of the coupled arch-base isolation system and to treat the different
damping mechanisms (non-proportional damping) for the arch and the base isolation system. In the
simulation below the measured viscous damping ratios were adopted for the arch, while no viscous
damping was assumed for the base isolation system. The reduction of the stiffness of the arch due to
the axial compression introduced in the process of forming the arch by bending a flat PVC sheet was
taken into account as geometric stiffness matrices. Time-domain simulation was performed by
employing the linear acceleration method with a time interval of 0.01 seconds.
Comparison between Model Test and Simulation
Prior to earthquake response analysis the free vibration analysis was performed for the non-isolated
pin-supported arch. The predicted natural periods for the lowest two modes are shown in Table 2.2
together with the measured values. Satisfactory agreement is observed between the prediction and
measurement. Figs. 3.1 and 3.2 show the comparison of the measured and simulated time histories of
displacement responses at the point A. Agreement is seen to be satisfactory between the simulation and
model tests except the large discrepancies in the peak values of vertical displacement of the nonisolated
model. These discrepancies may be attributed to the geometrically non-linear effect (biharmonic
response) that is not considered in the simulation.