DESIGN SPECIFICATIONS FOR HIGHWAY BRIDGES - IISEE

Chapter 9 Verification of Seismic Performance of Seismically
Isolated (Menshin) Bridges
9.1 General
(1) Introduction of seismic isolation design shall be considered in view of the effects of
increase in natural period and energy absorption capacity of a bridge for both
normal and seismic conditions. In particular, for a bridge meeting the following
conditions, seismic isolation shall not normally be adopted.
1) A bridge on a soil layer for which the seismic geotechnical parameter determined
from Section 8.2.4 is zero.
2) A bridge with a fairly flexible substructure and long fundamental natural period.
3) A bridge on a soft soil layer of long natural period that may cause resonance with
that of the bridge if seismic isolation is introduced.
4) A bridge with uplift at bearing supports.
(2) When seismic isolation is adopted, the bridge should be designed with emphasis on
enhancement of damping properties using a high energy absorption system, and on
dispersion of seismic forces. Excessive increase in the natural period of the bridge
shall be avoided.
(3) Natural period of a seismically-isolated bridge shall be determined in such a
manner that energy is absorbed by the isolation bearings and that an increase in
seismic displacement of the superstructure does not cause any adverse effects on
the bridge functions.
(4) Isolation bearings having good performance by simple mechanisms shall be selected
and used within a range of accurate mechanical behavior. Isolation bearings shall
be firmly fixed to the superstructure and substructure by anchor bolts and be
capable of replacement.
(5) When seismic isolation is adopted, gap allowances shall be kept between main
structures such as abutments, piers and girders so that the displacement assumed
in the design of an isolation system is possible.
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