1638
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
4 EFFECTIVENESS OF IN-SOIL SEISMIC ISOLATION
SYSTEM SUBJECTED TO REAL RECORDS
The model is subjected to a seismic scenario significantly
exceeding the design. The Takatori record (Kobe, Japan 1995)
is used as seismic excitation. As seen in Figure 5, the Takatori
record is a quite adverse case seismic event: the maximum
recorded acceleration was 0.61g, while their spectral values
substantially exceed the design accelerations of the pier
throughout the entire period range.
In Figure 6 a comparison between the response of the
isolated pier using the in-soil isolation system and the response
of the conventionally designed pier subjected to the Takatori
record is presented. Figure 6a compares the acceleration time
histories at the base of the pier for each of the two alternatives.
Notice that without the proposed seismic isolation, the pier is
subjected to a maximum acceleration of almost 1 g. On the
other hand, the favorable effect of the application of the in-soil
isolation system becomes apparent, since in that case the pier is
subjected to maximum acceleration of only 0.35 g at its base.
This decrease in the maximum acceleration may not be
adequate to reduce the required reinforcement of the pier, yet it
proves to be salutary for the survival of the pier.
As depicted in Figure 6b, where the bending moment–
curvature response at the base of the pier is presented, plastic
hinging quickly forms at the base of the pier, leading to intense
accumulation of curvature, than in turn causes the pier to
exhaust its ductility capacity and ultimately to collapse. In stark
contrast, the seismically isolated pier may reach the moment
capacity, yet there is no significant inelastic response, indicating
that the pier remains almost intact after the end of the excitation.
Finally, in Figure 6c the time histories of deck drift
Δ
are
presented. The conventionally designed pier accumulates
horizontal offset towards the one direction and ultimately
collapses. On the other hand, the pier founded on the in-soil
seismic isolation system survives this extremely strong seismic
scenario with maximum drift during the excitation Δ = 0.1 m,
and consequently with limited if any damage. In summary, the
in-soil seismic isolation system proves to be an effective
measure of fuse mechanism, in case of an extreme seismic
loading, preventing pier collapse.
t
(sec)
Δd
(m)
‐0.4
‐0.3
‐0.2
‐0.1
0
0.1
0.2
0.3
0.4
0
5
10
15
20
25
Figure 7. Time history of the relative displacement of the embankment
top compared to the non isolated free field.
The beneficial function of the in-soil isolation system comes,
however, with a drawback. The system is designed to impose a
cut-off at the acceleration that is transmitted to the
superstructure, materialized through embankment sliding. This
means that excessive slip displacement may occur at the
synthetic liner layer that translates to significant relative
displacement of the structure compared to the non isolated free-
field soil surface. This may be of importance, especially for
long structures, such as bridges, where the superstructure is
founded on several supports that cannot be isolated at the
exactly the same manner. In Figure 7, the time history of the
relative diplacement of the empbankment surface compared to
that of free field is presented. During this admittedly
excessively strong seismic shaking, the embankment is
subjected to a significant relative dispalcment compared to the
non isolated free-field, with a maximum diplacement
Δd
= 0.3 m. Although such a diffrential displacemnet may be
tolerable, it has to be carefully taken into account during design.
5 CONCLUSIONS
The main conclusions of this study can be summarized as
follows:
The application of the in-soil isolation system proves to
have a rather beneficial effect on the seismic perfromance of
the superstructure (at least for the idelaized bridge pier
examined herein). Although the decrease of the maximum
acceleration that is transmitted to the superstructure is not
adequate to allow the design of the pier for reduced seismic
loads, it proves to quite effective in ensuring its
survivability.
The effectiveness of the isolation system depends on the
presence of the superstructure. The sliding surface is curved
due to the pier imposed additional stresses, demanding in
this case from the isolated embankment to slide on an
inclined surface rather than a horizontal one. As a result, the
acceleration needed for the slip displacement to occur
increases, rendering the isolation system less effective
Since this isolation system relies on slip displacement at the
base of the isolated embankment to impose a cut-off at the
transmitting onto thesuperstructure accelerations, significant
relative to the non isolated free field should be expected and
taken into account during design.
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