On 11th March, 2011 East Japan was hit by the huge earthquake of
Magnitude 9.0 which might be never experienced in our history. The North
American Plate has moved in the area of 500 km long and 200 km wide,
resulting in the record breaking scale of Tsunami in Japan. The casualties due
to the Tsunami were more than 20,000 and lost houses were more than 180,000.
Since the Great Hanshin Earthquake in 1995, a lot of works have been done to
develop the technology on how to improve the seismic resistance of concrete
structures. What we have done so far might be proved effective because
concrete structures strengthened by the seismic design recommendations in
Japan have scarcely damaged. Except the damage by liquefaction of foundation,
even wooden houses suffered less damage than the damages due to recent
earthquakes in Japan.
Figure 1 shows the railway viaduct near Sendai Station. It was built by the
current seismic design code. There recognizes no damage on concrete columns
and girders. Electric poles made of precast prestressd concrete on the top of
viaduct in Figure 2 were tilted or broken by the Earthquake. Shaking of viaduct
could produce resonance of poles. Damage of poles looked minor but it took
about 50 days to replace all damaged poles while the retrofit of viaduct in the
whole Tohoku Shinkansen Line was completed only in ten days.
To the contrary of the minor damage of structures by the ground motion, the
damage by the Tsunami was extraordinary. At least 280 bridges were confirmed
to be washed away and down to the ground. Figure 3 shows PC girders fallen
upside down. Girders fixed to piers could be washed away after broken out of
piers as shown in Figure 4.
Figure 5 shows broken deck plates by falling water overflowing a water gate.
Awful scene of destruction of town by the Tsunami can be seen in Figure 6.
Anything inside buildings up to the fifth floor was washed away and a couple of
buildings were overturned. The height of tsunami must be over 15m.