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FIGURE 3.
LOCK-UP DEVICE
SEISMIC RETROFIT
Progress of Seismic Retrofit in Korea
When the necessity of seismic design began to get support from the public after Kobe earthquake
almost all resources have been concentrated on the research related to the development of new seismic
design concept. Therefore the retrofit of existing structures did not receive due attention. But the
seismic retrofit should be also treated as an urgent task in order to ensure uniform social safety against
seismic hazard. In Korea only less than 1% of bridges are designed seismically. Fortunately it is
realized by the preliminary studies that the seismic performance of existing bridges is adequate even if
the ductility is not so satisfactory. But multi-span bridges were widely constructed in Korea entering
1990's. This type of bridge has only one fixed bearing pier in the longitudinal direction in general.
Therefore it is very vulnerable to the earthquake ground motion since a great earthquake load is
concentrated on the single pier. The enhancement of ductility of fixed pier or the anti-seismic devices
can be considered as a seismic retrofit method. Realizing the urgency for retrofit the MOCT has
initiated a serious of project to develop the method for seismic assessment and seismic retrofit of
existing bridges. As a result the manual for seismic assessment for existing bridges has been drafted in
this year. The development of the seismic retrofit manual is on progress.
Basic Concept of Seismic Assessment
The manual for seismic assessment is to assist the engineers for the easy and efficient assessment and
retrofit design. The check of capacity/demand ratio is adopted as the basic idea. Once the earthquake
load is determined seismic performance is checked at each member/component level such as pier,
bearing, abutment, foundation and so on. Considering the assessment efficiency it is performed through
two- stage progress. In the first stage, the bridge considered is grouped to decide its priority by
seismictiy, vulnerability and impact into 4 categories. The detailed assessment is carried out in the
second stage for the bridges dropped into the critical category, the important and the observed grade. In
this detailed assessment new philosophy is introduced in the determination of design earthquake for
retrofit of bridges. As shown in Eq. (1) it is determined by the idea that the possibility of existing
bridges to experience the seismic design earthquake equates to that of new ones. Therefore the