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earthquake load can be reduced with increase of the age of bridges. As the result many exiting bridges
can be exempted from seismic retrofit. But if the owner wants to extend the lifetime of bridges under
consideration, the appropriate level of seismic design earthquake should be applied.
(The yearly occurrence probability of earthquake for seismic retrofit x target lifetime of existing bridge)
= (The yearly occurrence probability of seismic design earthquake x design lifetime) (1)
In fact the above idea was introduced on the background to secure urgently the seismic performance of
a lot of existing bridges with the limited economic resource. Therefore it is still controversial due to the
lack of validity backed up by sufficient studies. But we hope this idea can be embodied as a realistic
and reasonable alternative to determine the necessity of retrofit by the further intensive studies.
Examples of Seismic Retrofit
The manual for the seismic retrofit bridges has not been completed yet. But many analytical and
experimental studies have been performed on performance of retrofitted bridges. The dominant failure
modes of existing bridges are the failure of bearings and the premature bond failure of lap spliced
longitudinal bars in the fixed piers. Therefore, research efforts are concentrated on the enhancement of
ductility by wrapping plastic hinge zone of piers with steel plates, carbon sheet, and glass fiber sheet
etc.. In parallel the implementation of the various earthquake protection devices has been studied.
Based on the results of these primary studies quite a few importance bridges have been retrofitted
already prior to completion of the manual. Once the manual for retrofit is completed the retrofit of
existing bridges will be carried out actively.
The Figure 4 and 5 present the example of seismic retrofit of existing 4-span continuous bridge. The
seismic assessment showed that it was very vulnerable at the bearings and the fixed pier. The bearings
were protected by shear keys; steel brackets in the transverse direction and concrete blocks in the
longitudinal one respectively. And the fixed pier was retrofitted by wrapping with the steel plate to
enhance ductility.
a) STEEL BRACKETS b) CONCRETE BLOCKS
FIGURE 4.
PROTECTION OF BEARINGS BY SHEAR KEY