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Examples of Seismic Design
In Korea, as in other moderate seismic regions, using the strength and ductility of columns is sufficient
to meet performance criteria in many types of bridge system. But earthquake protection devices such as
LRB (Lead Rubber Bearing), STU (Shock Transmission Unit) and viscous damper are indispensable in
multi-span continuous bridge, cable-stayed bridge and cable-suspended bridges. As an example of
seismic design Seohae Bridge can be supposed, which consists of 3 different types of bridges: cablestayed
bridge (990m), FCM bridge (500m), and PSM bridges (5820m). It was designed with
earthquake load of only 6% of self-weight and constructed. According to the current code, the
earthquake force can be as high as 35% of self-weight. So, Korea Highway Corporation decided to
retrofit these bridges. Strictly speaking the methods introduced here was applied as retrofit methods.
But the same methods can be implemented to new bridge system as alternative seismic design concept.
The 97 spans of PSM bridges are grouped as 16 bridges: 3 to 10 continuous span bridges. If the bridges
with flexible piers can withstand the earthquake force by increasing the number of fixed piers concrete
block and/or restrainers was implemented as shown in Fig 1. On the other hand, this method was not
applicable to the bridge with stiff piers. The viscous fluid dampers were used, which placed between
the superstructure and the pier as shown in Fig. 2. In 5-span continuous cables-stayed bridge, of which
center span length is 470m and side span length is 60m, large earthquake load would concentrate on
the fixed pylon by the seismic analysis. So, LUD (lock-up device; it is called Shock Transmission Unit
also) was adopted to distribute this force to the other pylon as shown in Fig. 3. It can provide a
temporary rigid link between bridge structural members under seismic, braking, or other dynamic load,
while permitting slow thermal movements. It was the first application in Korea.
FIGURE 1.
CONCRETE BLOCK AND RESTRINER
FIGURE 2.
VISCOUS DAMPER