Paper No. TS-4-3

BEHAVIOR OF RC STRUCTURES UNDER TSUNAMI TRIGGEREDBY THE GREAT EAST JAPAN EARTHQUAKECarlos CUADRA1SUMMARY
The present report describes the characteristics of damages and behavior of reinforced concrete buildings
during tsunami action. The discussion is based on the field damage survey in selected cities located on thecoast of the zone affected by the Great East Japan Earthquake on March 11, 2011. This earthquake is themost powerful know earthquake that has hit Japan with a magnitude 9.0 and with epicenter located at 129
km of Sendai city (off the coast). The earthquake triggered a destructive tsunami with run up height of up
to 40 meters that mainly affect cities located on the Pacific Ocean coast of the Tohoku region (north-eastregion of Japan). Reinforced concrete buildings in general resist the tsunami without collapse however thenon-structural elements like panels and ceilings were severely damaged. The analysis of damages haspermitted to understand the behavior of RC buildings under tsunami attack, and has also permitted toestablish recommendations for their use to take refuge from tsunami in places where natural topography
makes impossible to reach hilltops or other safer places.

INTRODUCTION
An earthquake with a magnitude of Mw 9.0 struck the north-east part of Japan (Tohoku region), on 11
March 2011 at 14:46 local time. This earthquake is the most powerful know earthquake that has hit Japan
with its epicenter located at 129 km of Sendai city. The earthquake triggered a destructive tsunami withrun up height of up to 40 meters. The tsunami affects mainly cities located on the Pacific Ocean coast ofIwate, Miyagi and Fukushima prefectures the Tohoku region, and also Ibaraki prefecture in Kanto region.
Wooden structures were destroyed by the tsunami action when the water depth reaches or covers at least
the first floor of the building. These wooden structures were the most vulnerable constructions that were
washed up by the tsunami. Steel frame structures and steel trusses that are used mainly for industrial
constructions suffered heavy damages of walls, ceilings, finishing panels and non-structural elements. In
some cases the failure of these non-structural components produced the failure of structural elements andeven the collapse of the structure.
In the case of reinforced concrete buildings in general resists the tsunami without collapse, however thenon-structural elements like panels and ceilings were severely damaged. However, in some cases low rise
RC buildings were tilted by the tsunami action. In this report, the characteristics of the damages andbehavior of RC buildings during the tsunami action is discussed based on the field survey of the damages
in selected cities located on the coast of the affected zone.
1

Associate Professor, Akita Prefectural University, Japan. Email: carlos@akita-pu.ac.jp

The analysis of the damages characteristics has permitted to understand the behavior of this kind ofbuilding and has also permitted to establish recommendations for their use to take refuge from tsunami inplaces where natural topography makes impossible to reach hilltops or other safer places.
SELECTED AREA AND DAMAGES
The damages on building structures that were produced by the tsunami are described in this section and inspecial the damages that occurred on reinforced concrete structures. However it is necessary to mention
that damages are not limited only to building structures. Tsunami affected also many infrastructures likeports, embankments, roads, railroads, oil tanks, and in its more dramatic damage affected the nuclear
power plant of Fukushima. Environmental damages were also reported like the chlorination of agriculturalsoils, sedimentation of debris near ports, transportation of old industrial and mine residues from the sea
bottom to the ground surface, etc.
The sites that were selected for the present survey are shown in Fig. 1. Cities of Ofunato, Rikuzentakata,
Kesennuma, Ishinomaki and Onagawa were visited to perform the corresponding survey. The Fig. 1 showsthat the shape of the coast line is intricate with coast lines that converge forming shapes like river deltas
that facilitate the run-up of the tsunami. In the case of Ishinomaki city that is located in the lower left
corner of the figure the coast line is straight however the large portion of the city is located parallel to thiscoast line and therefore the inundation area was large in comparison with other cities. On the other handhigh ground level in Ishinomaki city is located a little far away of the coast line.

Figure 1 : Zone of survey (Google map)

Table 1 shows the extension of the inundated area and the number of family units in selected cities of thissurvey. The number of affected families and the extension of the inundated area are larger for Ishinomaki
city. The extension of inundation area was large due to the location of ports on lower ground along thecoast line. On the other hand, Onagawa city is the zone with the smallest inundated area however thenumber of victims (deaths and disappeared) was of the order of 1000 persons (1/4 of the number ofaffected families) and in the case of Ishinomaki victims reached 4000 persons (1/10 of the number ofaffected families). The high rate of victims of Onagawa city was due to the topographical condition of thelocation of the city with a very narrow area between the coast line and the surrounding hill. This condition
originated that the tsunami reached high inundation depths and it is also supposed that the successive
waves of the tsunami produced a violent flows of the sea water.
Table 1: Inundated area and affected families in the investigated cities
Inundated
NumberofPrefecture
CityArea
affected families
Ofunato
8 km2
6,957
Iwate
Rikuzentakata
13 km2
5,592
Kesennuma
18 km2
13,974
MiyagiOnagawa
3 km2
3,155Ishinomaki
73 km242,157DAMAGES ON BUILDINGS
To compare the damages of reinforced concrete, also damages on wooden structures and steel structures
are described as references. In general the wooden structures collapsed when were attacked by the tsunami.
The steel structures remain stand up however the finishing walls, ceilings and other non-structuralelements fails and in some cases these failures originate the failure of the main structure. The reinforced
concrete structures presented better behaviour and were the structures that in general remain in their
original location without structural damages. This can be explained by the high lateral stiffness of thereinforced concrete buildings in comparison to steel structures and wooden structures. The woodenstructures have the smaller lateral stiffness and in general were washed up when the tsunami reached orcovered the first floor. The relation between the damages and the earthquake resistant characteristics ofthese three types of structures are summarized in Table 2.
Table 2: Relation of Earthquake resistant forces and damages due to Tsunami
Earthquake resistantLevel ofType of structure
force for design
Tsunami damages
Wooden structure
Low
High
Steel structure
MediumMediumReinforced concreteHigh
Low
Damages on wooden houses
From damage surveys reported by other authors, it is recognized that in general the wooden houses
collapse or present damages due to the tsunami action when the inundation depth is larger than 2 meters.
As is presented in Figure 2, the damages on wooden houses are divided into three zones according to theinundation depth. Zone A correspond to a places where the inundation depth is smaller than 2 meters andstructural damages are not observed, that is the wooden houses are safe in this case. Zone B correspond toa zone where the inundation depth ranges from 2 to 4 meters, and depending on the structural shape andcondition of the structural elements, the houses can suffer from light damages to severe damages. From
the field observation it can be said that when the water level cover the first floor of the wooden house thestructures collapse not only due to the lateral force of the water but also due to the water pressure on theceiling of the first floor causing floating of the upper floors. Zone C correspond to a zone where theinundation depth is large that 4 meters and total collapse of the structure is expected to occur.

Figure 2: Inundation depth and damages on wooden houses
Figure 3 shows some typical damages on wooden houses that were produced by the tsunami. In Figure3(a) it can be observed that wooden houses were completely destroyed by the tsunami action. It can bealso observed that houses located on high sites are not affected. In Figures 3(b) and 3(c) the first floor wasdestroyed by the tsunami and the upper parts of the houses were transported and then left by the tsunami
on a different place from their original location. In the case of Figure 3(d) the tsunami destroyed the firstfloor and also produced the overturning of the remaining upper floor.
(a)

(b)

(c)

Figure 3: Damages on wooden houses(d)Damages on steel structures
Steel structures are used in general for industrial facilities and office buildings. Most of the buildings areframed structures and structural elements like beams and columns are slender elements. Floors and walls
are made of light panels and the lateral stiffness is appropriately designed to resist the earthquake force.
However during the tsunami attack the water pressure acting on panels or in general in elements of large
area generated large lateral forces that destroyed that non-structural elements and in some cases the failureof these elements lead to the failure and even to the collapse of the main structure.
Damages on Steel structures can be observed in Figure 4. When the inundation depth reaches only the firstfloor the structure remains almost intact however the wall panels of the first floor suffered some damages
as can be observed in Figure 4(a). In Figures 4(b) and 4(c) the inundation depth reached the second floor
and wall panels and ceiling are destroyed. In this case the structures remain stand up however some local
failure of the structural elements were observed. Figure 4(d) shows a total collapse of steel structures. In
this case the building was completely covered by the water.
(a)

(b)

(c)

(d)

Figure 4: Damages on steel structures
Damages on RC structures
Many reinforced concrete structures resisted the tsunami action without collapse as can be observed in
Figure 5. Figure 5(a) is the building of a local bank located at Ofunato city, and it can be observed fromthe damages of the windows and the damage of the advertisement panel of the left corner of the buildingthat the tsunami reach the third floor. In Figure 5(b) it is also inferred that the tsunami covered the 2-story
building by observing that the windows glasses were destroyed and now are replaced by wood panels.
Figure 5(c) is an apartment building located at Rikusentakata city and from the damages of the balconies it
is inferred that the tsunami reaches the fourth floor. Detailed of the damages of the balconies can beobserved in Figure 5(d), where the panels of the balconies of the fifth floor are intact while the panels of
the lower floors are completed destroyed.

(a)(c)(b)(d)Figure 5: RC buildings after tsunami attack
Figure 6 shows the condition of a hotel building located very near the shoreline in Rikuzentakata city. Thebuilding resists the tsunami attack however the lower floors suffered the destruction of the non-structuralelements and also, as it is observed in the Figure 6(a), the damage of a reinforced concrete wall due to thelateral water pressure. In this building it was also observed that the cover concrete of structural elements
like columns was spall-out, probably due to a combination of earthquake vibration action and posterior
tsunami action as can be observed in Figure 6(b).
(a)

Figure 6: Condition of RC Building (hotel) near shoreline(b)

In general reinforced concrete buildings did not collapse however under certain conditions the tsunami
action caused the overturning and even the translation of building from its original location. This wasreported by a survey team of the Tohoku Branch of the Architectural Institute of Japan (ref. 5) andverification survey was carried out by the author. These damages occurred at Onagawa town and theaffected buildings were buildings with weak foundation, and with shape like boxes that do not permit thetransit of the water and facilitate the action of the floating force. In Figure 7 damages of these buildings
are shown.
(b)

(a)

(d)

(c)

Figure 7: Damages on Reinforced Concrete buildings
Figures 7(a) shows an overturned 3-story building with a shape of box and few openings. This building
was completely covered by the tsunami water and as it is shown in Figure 7(b) it has a sallow slab
foundation that together with the shape of the building facilitates the action of the floating forces. Figure7(c) shows the overturning of a 2-story building with exposition of the pile foundation. The use of piles
for lower rise buildings indicates the poor quality of the foundation ground. On the other hand in theFigure 7(d) it can be observed that the building has suffered the impact of some body (probably a ship)that could originate the overturning of the building.
CONCLUSIONS
Behaviour of reinforced concrete building during the tsunami action originated by the Great East Japan
Earthquake on March 11, 2011 was discussed by comparing their damages with those of other type ofconstructions. Reinforced concrete buildings in general resist the tsunami without collapse however whenconstructions present shallow or weak foundation and building shapes induce the action of the floating
force, overturning of the building was observed. In the case of not collapsed buildings severe damages on
non-structural elements like panels and ceilings were observed.
If reinforced concrete buildings are intended to be used as refuge, buildings of more than four floors ormore than 15 meters are recommend. It is also important to check the condition of the foundation of thestructures that are designated as refuge since the failure of the foundation could originate the overturningof the building. Additionally, this selected building must be verified to resist some impact forces or in any
case must be located in places where the impact of displaced ships or other building can be avoided.
The survey has permitted to understand the behaviour of reinforced concrete buildings under tsunami
attack and it can be state that these kind of buildings could be used to take refuge from tsunami in places
where natural topography make impossible to reach hilltops or other safer places.
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EarthquakeofMarch11,
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