Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanDAMAGE TO EIGHT-STORY RC BUILDINGRETROFITTED TO HAVE BASE SHEAR
COEFFICIENT OF 0.65
Isaac VILLANOVA1, Atsushi NAGAYA2, Masaki MAEDA3, Santiago PUJOL4, Susumu
TAKAHASHI5, Toshikatsu ICHINOSE6
1Graduate Student, Nagoya Institute of Tech., Nagoya, Japan, ishigamy@gmail.com
2Graduate Student, Nagoya Institute of Tech., Nagoya, Japan, nagoyan_nitech@yahoo.co.jp
3Professor, Tohoku University, Sendai, Japan, maeda@archi.tohoku.ac.jp
4Associate professor, Purdue University, West Lafayette, Indiana, USA, spujol@purdue.edu
5Assistant professor, Nagoya Institute of Tech, Nagoya, Japan, takahashi.susumu@nitech.ac.jp
6Professor, Nagoya Institute of Technology, Nagoya, Japan, ich@nitech.ac.jp

ABSTRACT: An eight-story reinforced concrete building retrofitted according to the1990 Japanese Standard was severely damaged during the 2011 Tohoku earthquake. The
building was constructed in 1966 at Tohoku University. In 1996 the building was
retrofitted using structural walls and carbon fiber. The base shear coefficients of thebuilding in the longitudinal and transverse directions after the retrofit were estimated tobe 0.67 and 0.65, respectively. This paper discusses the reasons for the severe damage
that took place despite the retrofit.
Key words: RC structure, structural walls, carbon fiber, seismic retrofit, penthouse, baseshear coefficient

INTRODUCTION
This paper deals with a reinforced concrete building in Tohoku University built in 1966 and retrofitted
in 1996.The retrofit was expected to bring the building the capacity of withstand a big earthquake.
However, it experienced great damage in some of its members.
The observed failure modes in this building were different from the predicted ones. Structural walls,
which were expected to fail in shear, seemed to have yielded in flexure. Compressive failure
associated with bending was also observed in a wall without boundary column. In addition, the carbon
fiber used to prevent shear failure of coupling beams did not work properly because of anchorage
failures. The penthouse, was also greatly damaged, though the areas of the walls in X and Y directions
were 2.8% and 1.3%, respectively, of the total floor area. The penthouse was tentatively strengthened
after the earthquake as shown in Photo 1.

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Photo 1 General view of the building after tentative strengthening of the damaged penthouse
This paper discusses hypotheses about the discrepancies between the predicted and observed failuremodes, as well as the reasons why the building retrofitted to have such high base shear coefficient was
so damaged during the 2011 Tohoku Earthquake. Data from the buildings have been drawn based on
the original and reformation plans. Observed damage patterns, analysis models and results are alsopresented in this paper.

CONFIGURATION OF THE BUILDINGThe building under analysis was constructed in 1966 in Tohoku University, Sendai, locatedapproximately 130 km from the epicenter and it is dedicated to Electrical Engineering. It is areinforced concrete building with eight stories and a two-story penthouse. The first story is 4.3 m, each
upper story is 3.6 m and the two stories of the penthouse are 5.4 m and 3 m high respectively. It has
seven spans in the East-West direction of 8m and three spans in the North-South direction of 6.6 m, 6
m and 6.6 m respectively, defining the structural grid of the building (Fig. 1). The upper stories havethe same distribution in plan. Storage, baths and vertical communication nucleus are in the middle
longitudinal axis. The rooms are around this nucleus, giving a configuration quite symmetrical to thebuilding. The configuration of the building becomes different when reaching the penthouse level. It ischaracterized by having three bays of 8.0 m x 6.0 m in the western part of the roof (Fig. 2). Also, fromthird to eighth stories the West and East facades have cantilevers of 3 m (Fig. 3).

Fig. 1 Sixth story plan1271

Fig. 2 First story of the penthouse planDue to this configuration the walls are concentrated in the middle bay, having totally free frames inboth south and north facade. The building has also suffered several modifications. Some of the holes
in some structural walls have been filled after the first completion of the building but before theretrofit, like in the penthouse or in the first story, giving continuity to the structural system.
The building was retrofitted in 1996 according to the 1990 Japanese Standard (Building ResearchInstitute 2001). The main changes during the retrofit were addition of new walls, change concrete
walls for new and thicker ones and reinforcing some of the beams of the building with carbon fiber.
According to the documents for the retrofit design, the base shear coefficients of the building in thelongitudinal and transverse directions after the retrofit were estimated to be 0.67 and 0.65 respectively.
While the wall changes were concentrated in lower stories, from first to fifth stories, the carbon fiber
reinforcement has been located mostly in the upper ones. About the location of the carbon fiber
reinforcement, since there was no documentation of where was applied, a deep field investigation wasneeded. From the total of 159 short beams in the building only 109 were visible. The fiber was used in
24 of them, specifically in those in the third to eight stories. However, the fiber was not only used in
short beams, but this reinforcement was also found in 7 additional beams. There were three kinds offiber reinforcement: the first one is tape-shaped (Photo 2), the second one was a continuous fiber
wrapping all beams (Photo 3) and the third one is similar to the previous one but with anchorages
inside the concrete (Photos 4). Regarding to the penthouse no retrofit was done.

Photo 2 Stuck tape-shaped
carbon fiber

Photo 3 Stuck carbon fiber
without anchors

Photo 4 Stuck carbon fiber
reinforcement with anchors

Referring to the constructive aspect, the building has slabs of 120 mm thickness in all stories. They
have also a thickening of 60 mm near the beams (Figs. 8 and 10). The size of the beams varies from
350 mm x 800 mm to 300 mm x 1400 mm, but the most common section is 400 mm x 1000 mm. The
size of the columns decreases around every 2 stories about 100 mm each side, varying the size of1272

columns from 1100 mm x 1000 mm in the first story to 400 mm x 700 mm in the penthouse. About thefoundation, the building has reinforced concrete piles. They are 400 Ø, 10 m long and they have aresistance of 392 kN per pile.
The used stirrups are plain and 9 Ø or 13 Ø, distributed a distance between 150-250 mm in beams andevery 150 mm in columns; satisfying the limit of 300 mm of the Building Standard Law Enforcement
Order of 1950 (Architectural Institute of Japan 1951). The used materials are concrete of 18 MPa,
longitudinal bars of 324 MPa (SD35) for girders and columns, and 235 MPa (SR24) for other elements.
These values were also adopted in the retrofit design.
The ratio of the area of longitudinal reinforcement to the concrete section area is between 0.86% 0.45% in columns and between 0.57% - 1.60% in beams. The transverse reinforcement ratio is
between 0.09% - 0.88% in beams and between 0.08% - 0.21% in columns. The ratio of 0.88% is usedin the beams connecting walls. The reinforcement ratio of wall panel is between 0.23% - 1.09%.

OBSERVED DAMAGES AND ANTICIPATED CAUSESThe heaviest damages in the building are located mainly in beams and in the penthouse. Damages in
the first story were minor. Figures 3-6 shows the observed cracks in selected frames. The grey shadeshows the parts that were not visible, the green one represents carbon fiber reinforcement, the brown
one is tape-shaped reinforcement, the red one is detached concrete and the blue one is completely
destroyed concrete. Also the thickness of cracks represents cracks between 0.2-1.00 mm, 1.10-2.00
mm and above 2.00 mm. The blue cracks were not measured. The most damaged stories in decreasingorder were third, fourth and fifth stories. Regarding to the frames in X direction, damages were more
prominent in frames I and J with walls (Fig. 3) than open frames H and K. In Y direction, damages in
the penthouse were large as shown in Figs. 4 and 5. Damages in the beams with openings were also
prominent as shown in Figs. 5 and 6.

Fig. 3 Frame I cracks schedule1273

Fig. 4 Frame 10 cracks
schedule

Fig. 5 Frame 11 cracks
schedule

Fig. 6 Frame 14 cracks
schedule

Beams
Carbon fiber reinforcement has been used in short and normal beams. The short beams took a lot ofdamage and failed in shear, while the other beams, even the not reinforced ones, had less damage.
Different failures in the carbon fiber of the short beams can be identified such as unstuck tapes (Photo5), peeled-off tapes (Photo 6), broken fibers (Photo 7), torn fibers (Photo 8) and failure of anchorages(Photos 9 and 10).

Photo 5 Unstuck tapes
Floor 5 Frame 12 Beam I-J

Photo 6 Peeled-off tapes
Floor 5 Frame 11 Beam I-J

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Photo 7 Broken fibers
Floor 7 Frame 11 Beam I-J

Photo 8 Torn with anchors
Floor 7 Frame 11 Beam I-J

Photo 9 Anchorage failed
Floor 6 Frame 12 Beam I-J

Photo 10 Anchorage failed
Floor 7 Frame 11 Beam I-J

The most severe damage in this group is the failure of the anchorage of two of the carbon fiber
reinforcements that were visible. In the last photo the damage was so great that a rebar was visible. For
clarifying the photo of the failure a zoom and a drawing are provided in Photo 11 and Fig. 7:

Photo 11 Zoom in failed anchorage
Floor 7 Frame 11 Beam I-J

Fig. 7 Drawing of the failure in Photo 10

Figures 8, 9 and 10 show the reinforcement detail of the 400 mm x 1000 mm beam in Photos 10 and
11. It is between a column on the left side and a wall in the right one. Its longitudinal reinforcement is25 Ø, the diagonal reinforcement is 16 Ø and the stirrups are 13Ø. The only data obtained about theanchorage is that, according to photo measurement, it is around 12 Ø.

Fig. 8 Section near column

Fig. 9 Bars and anchors

Fig. 10 Section at midspan

In addition, stirrups were visible in 26% of the beams that were not covered with carbon fiber.
Therefore, the damage was so great that a high number of members had exposed bars. Also, in somecases has been detected bad concrete casting problems, especially in the beams of the fourth story.

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Severe damages were also detected in the coupling beams of the frame number 14. They have two 300
Ø openings at their midspans and were highly damaged around this area (Photo 12). These openings
have been detected also in all the coupling beams of the frame 11 and in some of the frame 9.

Photo 12 Coupling beams damaged around openings
Structural walls
Figures 3-6 indicate that the cracks were prominent in the walls in the 2nd and upper stories.
Furthermore, most of them were narrower than 1 mm. Because flexural cracks tend to close after
earthquake while the shear cracks usually remain, we conclude that flexural deformations occurred in
the 2nd story and were more dominant than shear deformations.
On the other hand, the seismic retrofit documents indicate that 0% and 64% of the walls in the 2ndstory were predicted to yield in flexure in X and Y directions, respectively. We conclude that theprediction was not correct both in X and Y directions.
From fifth to sixth stories the number of walls decreases in the X axis, specifically in the frame I.
Their thickness has also decreased in the fifth floor. This may have caused a weakening in the sixth
story and the reason of the failure of walls in the upper stories. It should be noted that in the retrofit
documents, 0% of the walls in the 6th story were predicted to yield in flexure in X direction, but noshear failure was observed in the story.
Compressive failure associated with flexural failure was observed in walls without boundary column
as shown in Photo 13. The detail of the wall is provided in Fig. 11. The wall has double 9 Ø 150@
reinforcement. Some bars were not found in the plans but were visible at the place (Photo 14), the oneon the left side may be a reinforcement at the end of the wall. According to the recent study(Takahashi et al, in press), the flexural drift capacity of RC wall can be computed as sum of elastic andplastic drifts and plastic, where the plastic drift is estimated by using the neutral axis depth, plastic
hinge length and ultimate compressive strain of concrete. The calculated neutral axis depth is 1250
mm from the border of the wall and is shown in Fig. 11. The height of the observed compressive
failure zone was almost 2.5 times of the wall thickness (Photo 13) as predicted by Takahashi et al (inpress). From Photo 13, it is supposed that the compressive strain of concrete was very large. If weassume that the compressive strain was from 1% to 2%, the flexural drift is estimated from 0.6% to
1% by the methodology proposed by Takahashi et al. Therefore, actual lateral drift of this building isassumed to be comparable to these values.

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1250

2.5t

Photo 13 Compressive failure
Floor 6 Frame I between frames 11-12

Photo 14 Detail of bars from the other side ofphoto 13

Fig. 11 Detail of the wall at 6th story, between frames 11-12 of the frame I
However, some mistakes in the retrofit documents have been found. During the retrofit the walls in the
2nd story in frame 8 between frames J-K and in frame 10 between frames H-I, and also in 3rd story inframe 10 between frames H-I were not taken into account. This negligence caused underestimation of
the seismic capacity in Y direction in the 2nd and 3rd stories. In addition, the following walls that
actually are not structural were taken into account during the calculations of the retrofit: 6th story inframe I between frames 7-8, 9-10 and 10-11, and 7th story in frame I between frames 7-8. These wrong
suppositions caused overestimation of the seismic capacity in X direction in the 6th story.
Penthouse
The penthouse of the building was so severely damaged that required reinforcement just after theearthquake in order to prevent collapse (Photos 15, 16 and 17). Both columns and walls werecompletely destroyed at the first story of the penthouse, while the second story was in a similar
condition to the rest of the building.

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Photo 15 Column of thepenthouse after theearthquake

Photo 16 Column of thepenthouse after theearthquake

Photo 17 Later reinforced
column of the penthouse

In the penthouse, the reinforcement bars used are 9 mm @200 double in 200 mm thickness walls andsingle in 120 mm thickness walls (Fig. 2). According to the pictures the used bars are all plain in thepenthouse walls. In the columns the reinforcement bars are 14-D25 for the first story and 10-D25 forthe second story and their stirrups are 9 Ø @150. It is also noteworthy that the penthouse of thisbuilding is quite slender. Its geometric slenderness is 1.40 and it may have affected the damaged
inflicted to the walls and columns of the penthouse.

Fig. 12 Penthouse elevations

ANALYSIS
For the analysis of each story of the building the C/Ai coefficient has been calculated. In the current
Japanese seismic design code (Building Research Institute 2001), Ai factor represents vertical
distribution of a seismic story coefficient relative to that at the first story. Figure 13 shows the
distribution of Ai factor along the height of the building; note that Ai is 1 at the first story and verylarge at the penthouse. Figure 14 shows the weight above each story, W. Multiplying Ai and W, we
get the story shear force that corresponds to base shear coefficient of 1.

Fig. 13 Distribution of Ai

Fig. 14 Weight above each
story

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Fig. 15 Story shear force
corresponding to base shear
coefficient of 1

In the seismic retrofit document, strength of each column and wall is described. Adding the strengths
in each story, we get the strength of each story, Qi. Dividing the strength by AiW in Fig. 15, we get
equivalent base-shear-coefficient as shown in Figs. 16 and 17, where the contributions of the columns
and walls are also indicated. In the seismic retrofit document, it is also reported that the drift capacityof each story is larger than 0.4 %. Based on the Japanese standard (Building Research Institute 2001),
this building is expected to withstand strong earthquake.

Fig. 17 Y direction

Fig. 16 X direction

According to this graphs in X direction the second and eighth stories are the strongest ones and thesecond story of the penthouse is the weakest one. Referring to the Y direction the second story of thepenthouse is by far the strongest, and the fifth and first story of the penthouse are the weakest.
Equivalent base-shear-coefficients are not small. On the other hand, the damage of this building wasserious especially in beams and penthouse. To investigate this reason, another analysis is conducted.
For the analysis of the beams and walls the model of calculation of the frame number 11 has beenchosen as representative. The reasons are that it has walls in all the stories and is around the averagecontribution of the rest of the walls at the 2nd story level (Table 1), which is the story from which thewalls begin having significant damages. This table has been calculated according to results of theseismic evaluation documents, adding the contribution of each element per frame in the 2nd story.
Table 1 Contribution of each frame of the second story
Frame
MN

7
18.7

8
8.3

9
12.2

10
4.8

11
10.7

12
4.9

13
11.0

14
18.7

Average11.2

Due to the damages seen in frame 11 (Fig. 5) the short beams between frames I-J failed in shear.
Therefore, the failure mode as shown in Fig. 18 is assumed to estimate the lateral capacity of thisframe. In this model the walls of frames H-I and J-K are strong enough while the coupling beams fail.
For the analysis the first story is considered as rigid, because it does not have significant damages.
These walls rotate as shown in Fig. 18 and vertical reinforcement bars are assumed to be yielded at thebottom of the second story, where at is the cross section area of the longitudinal reinforcement on
boundary column and σy is the yield strength of the steel. The values N1 and N2 are the long termloading axial forces through the boundary columns of the walls. The double circle in this figure
represents the plastic hinge of the joint and its plastic moment is computed as sum of the plastic
moment of the column and that of the beam.

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Fig. 18 Model of calculation for the frame number 11
During the analysis, two models have been proposed. In the first one the short beams of the span LB1of the model are completely destroyed, and in the second one the beams yield in flexure at both ends.
According to the documents of the seismic evaluation, the lateral capacity of the frame number 11 is
10.7 MN; considering the walls from the second, third and fourth story as flexural walls and the rest as
shear walls. According to the model of the analysis, all the walls have been considered as flexuralwalls because of the hinge in the base of the walls of the model. This assumption is admissible since
the small width of the cracks found in the walls of the building indicates that almost every wall yieldin flexure rather than fail in shear. As to the results of the analysis, the flexural capacity of the frame
number 11 is 8.7 MN, in the case of considering the contribution of the coupling beams. In the case ofneglecting those beams the flexural capacity is 6.5 MN. Thus, the real value is between these two
results and is a fact that the beams make an important contribution to the capacity of the frame, beingaround the 25% of the total capacity.
The results of the analysis are around 81% for the first case and 61% for the second one of the total
capacity specified in the documents of the seismic evaluation. This suggests that the base shear
coefficient of the building may be smaller than the one provided in these documents. In addition, somemistakes have been found in the calculations of the documents of the seismic evaluation. They
considered some walls that actually do not exist in both directions of the building.
Regarding to the analysis of the penthouse, the results of the analysis in Figures 16 and 17 show how
the damages at the first level of the penthouse could be predicted when comparing the coefficientswith the ones of the other stories. However, the C/Ai coefficient of the penthouse is still 0.653, whichshould be enough.
The huge damage in the penthouse happened also despite the fact that the areas of the walls in X andY directions were 2.5% and 1.3%, respectively, of the total floor area. These values are high if the1280

penthouse is considered as a building itself. But in this case, it cannot be said these values are highbecause it is located on the top of the building and the amplification must be considered.
To study the penthouse the contribution of walls in each frame has been analyzed. The walls are very
similar and all have columns in both sides. But due to the openings that have the frames 9, 10 and 11
reduction factors of their capacity must be taken into account (Building Research Institute 2001). Thus,
they are actually around 44% and 67% of their capacity (Table 2). These differences of capacity
produce an eccentricity in the transverse direction of the penthouse.
Table 2 Capacity of walls in the first story of the penthouse
Frame
8
9
10
11

PH1
Qsu (MN) γ reduction factor γ Qsu (MN)
4.3
1.00
4.3
4.3
0.44
1.9
4.3
0.67
2.9
4.3
0.67
2.9

CONCLUSIONS(1) In the transverse direction, one half of the frames had coupling beams. More than 50% of
these beams had 300 Ø openings and failed in shear, including those retrofitted using carbon
fiber. One of those frames was analyzed assuming rigid plasticity of each element andneglecting the contribution of the coupling beams. The computed strength was approximately
61% of that obtained in the document of the seismic retrofit.
(2) In the longitudinal direction, a wall in the sixth floor yielded in flexure with compressive
failure of wall panel. The wall panel was not provided with a boundary column. In the
document of the seismic retrofit, it was predicted to fail in shear.
(3) The penthouse had some eccentricity in the transverse direction. This may have caused a
torsional response of the penthouse which damaged more in the transverse direction than in
the longitudinal one.
More research needs to be done to understand the behavior of this structure at the Tohoku earthquake.

ACKNOWLEDGEMENTS
This work was financially supported by the Japan Science and Technology Agency and the U.S.
National Science Foundation. They are gratefully appreciated.

REFERENCES
Architectural Institute of Japan (1951), “AIJ Standard for Structural Calculation of Reinforced
Concrete Structures (revised 1949),” Gihodo, Tokyo
Building Research Institute (2001), “Seismic Evaluation and Retrofit,” The Japan Building Disaster
Prevention Association
Takahashi, S. et al. (in press) “Flexural Drift Capacity of Reinforced Concrete Wall with Limited
Confinement”, ACI Structural Journal1281