Synopses of Master Papers

Bulletin of IISEE, 47, 97-102, 2013

SEISMIC EVALUATION AND RETROFIT OF
EXISTING REINFORCED CONCRETE BUILDINGSNYI Nyi Zaw ∗MEE11617Supervisor:

Masanori TANI ∗∗ABSTRACT
Seismic evaluation and retrofitting of existing RC buildings are very important for earthquake disastermitigation for Myanmar because almost all RC buildings in Myanmar have been built without nationalbuilding codes and seismic designs. In this study, the Japanese seismic evaluation method is studied
and applied to one Japanese building and one Myanmar building in order to introduce these evaluation
methods into Myanmar in the future. To verify the seismic evaluation methods, the responses of twobuildings are performed for the nonlinear frame analysis with STERA 3D program again. After
analyzing data by STERA 3D program, the results are also coincided with the seismic evaluation
methods. After evaluation, both target buildings are found to be weak in seismic performance. Simple
retrofit methods such as wing walls, RC shear walls and structural slits are chosen to increase theirseismic capacity. After retrofit, enough seismic capacity is obtained in both target buildings.
Keywords: Seismic Evaluation, Nonlinear Frame Analysis, Retrofit, Existing RC Building.

1. INTRODUCTION
Although Myanmar had many disasters in the past, disaster education has been very weak and all thesedisasters were totally forgotten. Then, the Tarly’s earthquake (M W =6.8) hit Myanmar on 24th March2011 and people’s awareness has risen again now. Most of buildings in Myanmar are moment
resisting RC frame structures and they were built without any seismic consideration. In such asituation, their seismic performance is uncertain and it is also difficult to judge which buildings aresafe or not. So, seismic performance of all existing RC buildings needs to be evaluated and checked.
Seismic evaluation and retrofit for existing RC buildings are useful and effective measure for
mitigation of earthquake disasters. If these buildings do not have enough seismic capacity, proper
retrofit and strengthening are needed for them to prepare for the future earthquakes.
In this study, the Japanese seismic evaluation method is studied and used. First, seismic
evaluation is conducted with a Japanese building damaged by the 2011 Off the Pacific Cost of Tohoku
Earthquake. Then, this method is applied again to evaluate seismic performance of a Myanmar RCschool building so that seismic capacity of existing RC buildings in Myanmar will be able to bechecked in future by these methods. To verify the two screening methods, the responses of twobuildings are performed for the nonlinear frame analysis with STERA 3D program. If the target
building does not have enough seismic capacity and needs to increase strengths, it is retrofitted bysimple methods such as wing walls, RC shear walls and structural slits to increase its seismic capacity.

2. METHODOLOGY
There are three levels of seismic evaluation methods in current Japanese standard guidelines. In thefirst method, material strength and contribution of cross sectional areas of vertical members are∗Public Works, Ministry of Construction, Nay Pyi Taw, Myanmar
Building Research Institute (BRI), Tsukuba, Japan∗∗considered. In the second level method, ductility or deformation capacity and strength of the verticalmembers are considered. To know the seismic capacity of buildings, it is necessary to calculate theseismic index of the structure (I S ) first. The seismic index (I S ) is an index which represents the seismic
performance of the structure. The larger value of I S means higher seismic performance of thestructures. The seismic index of the target structures (I S ) is calculated with following basic Eq.(1).
Basic seismic index of the structures (E 0 ) can be obtained by product of strength index C and ductility
index F. Strength index is expressed in term of story shear coefficient. The ductility index F iscalculated from the ultimate deformation capacity normalized by the story drift of 1/250 in which mostcolumns fail in shear.
Other reduction factors such as structural irregularity (S D ) and time index (T) are also used toreduce the seismic index of the target structure. But main influencing factor in seismic evaluations isthe basic seismic index (E 0 ) which is simply the product of F and C in all three methods.

I S = E 0 .S D .T

(1)

2.1. First Level Screening Method of Seismic Evaluation
In this level, seismic capacity is calculated based upon the cross sectional area of vertical elements,
their shapes and the concrete compressive strength. The vertical elements are divided into three groups
depending upon h 0 /D values of different column and wall types. The basic seismic index (E 0 ) of thetarget structure is obtained by using following two equations. They depend upon the target buildings
which have extremely short column or not. The larger value would be taken from the Eq.(2) and (3).

n +1
(CW + α 1 .CC )FWn+i(2)n +1(C SC + α 2 .CW + α 3 .CC )FSC
n+i(3)E0 =
E0 =Then, irregularity index (S D ) and time index (T) are calculated with the standard guidelines. After that,the seismic index of the target structure (I S ) is compared with seismic demand index of the target
building (I S0 ). If Is value is larger, the seismic performance of the building is ok. If not, we need tocheck it with the second screening method.
2.2. Second Level Screening Method of Seismic Evaluation
In the second level, the axial force and reinforcement are considered to calculate the strengths andductility of the vertical members. The vertical elements are classified into five different categories.
The ductility indexes F of all vertical members are grouped into three different groups in maximum. In
the second level seismic evaluation, shear force at ultimate flexural capacity and ultimate shear
capacity of columns and walls are calculated, and then their results were compared. The effective
strength factor α can be taken from the standard according to their different ductility F. The
cumulative strength index C T of each story can also be calculated by the sum of the strength C withmultiplication of the story shear modification factor (n+1)/(n+i).There are two basic equations in thesecond level screening. Eq.(4) is the ductility- dominant basic seismic index of the structure and Eq.(5)is strength-dominant basic seismic index of the structure.

E0 =

n +1
E12 + E 22 + E32
n+i(4)

E0 ={}n +1
C1 + ∑ j α j C j F1n+i(5)The strength index C in the second screening method is the ratio of the ultimate load-carrying
capacity of the vertical members in the story concerned to the weight of the building including live
load for seismic calculation supported by the story concerned. For ultimate lateral load-carrying
capacity of the vertical members (Q u ), minimum value must be chosen from Q mu and Q su .

C=QU
∑W(6)

2.3. Nonlinear frame analysis by STERA 3D
After the analysis of the target building with Japanese seismic standard guidelines, the calculated
results are verified again with nonlinear frame analysis program STERA 3D. First, the pushover
analysis is carried out in longitudinal and transverse directions for both target buildings. Then,earthquake response analysis is conducted with the 2011 earthquake ground motion to T-City Hall andthe El Centro earthquake ground motion to Myanmar School building.

3. CASE STUDY OF THE JAPANESE AND MYANMAR BUILDINGS
3.1. Detail of the Target Buildings in Japan and Myanmar
The Japanese target building is the T-City Hall in Ibaraki prefecture. It is a two-story RC building
constructed in 1966 and had been damaged by the 2011 Off the Pacific Cost of Tohoku Earthquake.
The first floor plan and the appearance are shown in Fig. 1 and Photo 1. This building has 3 spans inlongitudinal direction and 1 span in transverse. The first story height is 3.6 m and the second story
height is 2.9 m. The unit floor weight is calculated as 5.98 kN/m2 for the second floor and 11.03
kN/m2 for the first floor by assuming the unit weight of concrete is 21.42 kN/m3. The hoop spacing of
the columns is 300 mm. The compressive strength is obtained as 25.44 N/mm2. Yield strength of steel
is assumed as 294 N/mm2. Two columns with spandrel walls in the first floor were severely damagedby shear failure as shown in Photo 1. But, no damage of columns is observed in the second story. In
the first story, due to a RC shear wall out of frame which caused eccentricity, some damages occurredin non-structural hollow block in-filled walls between columns in the east frame by torsional response.

Figure 1. First floor plan of T-City Hall

Photo 1. Appearance of T-City Hall

Myanmar School Building is two-story RC building built after the 2008 Cyclone Nargis. The
first floor plan and the elevation are shown in Figs. 2 and 3. This building has 7 spans in longitudinaldirection and 3 span in transverse. All columns are long without shear walls, standing walls or hanging
walls. Both the first and second story heights are 3.657 m. Structural and elevation plans are regular inboth the first and second story. The unit weights of the second and first floor are calculated as 5.11
kN/m2 and 10.27 kN/m2 respectively. The compressive strength of concrete is taken as 17.24
N/mm2 (2,500 psi) and yield strength of steel is taken as 275.79 N/mm2 (40,000 psi). The hoop spacingof columns is 152 mm (6 inches).

Figure 2. First floor plan of Myanmar School

Figure 3. Front elevation of Myanmar school

3.2. Results of Seismic Evaluation
The calculated results of seismic evaluation for T-City Hall and Myanmar school building are shownin Table 1 and 2, respectively. For T-City Hall, the seismic performance is weak in first story by thefirst screening. The seismic performance is weak in both first and second story by the secondscreening. The main reason is due to the effect of short columns with spandrel walls and insufficientlateral reinforcement in both stories.
After seismic evaluation for Myanmar School Building, seismic performance is weak in thefirst story in both longitudinal and transverse directions. The main reason is due to larger longitudinal
reinforcement ratio (P t > 1%) in some of columns and this effect reduced the ductility of thesecolumns in the first story. Flexural failures of these columns are also observed in the dynamic analysis.

Story
212
1

Story

Screening
LevelFirst
screening
Secondscreening

Table 1 Results of seismic evaluation for T-City Hall
Before retrofit
After retrofit

ISC TU S DEvaluation

RISC TU S DEvaluation

1.10
0.50
0.58
0.30

0.72
0.37

OKNG
NGNG

1.80
1.29
1.41
0.86

0.78
0.63

OKOK
OKOK

Table 2 Results of seismic evaluation for Myanmar School Building
After retrofit
After retrofit
Before retrofit(Longitudinal)(Transverse)
Screening
Level
C TU S Evalua
C TU S Evalua
C TU S Evalua
ISRIS
RIS
tion
tion
tion
D2121First
Screening
Secondscreening

1.52
0.5
0.91
0.36

0.70
0.36

DOKNG
OKNG3.3. Results of structural analysis by STERA 3D

1.52
1.49
0.91
0.75

0.70
0.75

DOKOKOKOK

1.52
1.39
0.91
0.76

0.70
0.76

OKOKOKOK

The contribution of the non- structural concrete hollow block walls is neglected in this analysis. Thefloor is also assumed as flexible in this analysis. The results of nonlinear response analysis for T-CityHall are shown in Figs. 4 and 5. The strong motion records of the 2011 Off the Pacific Cost of Tohoku
Earthquake observed at K-NET station near T-City Hall are used for the dynamic analysis of T-CityHall. In both static and dynamic analysis, shear failure of columns are observed due to spandrel
walls in T-City Hall. These results are coincided with seismic evaluation methods and also with actualdamages.
The results of response nonlinear analysis for Myanmar school building are shown in Figs. 6and 7. Flexural failures of columns are observed in both static and dynamic analysis with El Centro
ground motion. These results are also the same as the seismic evaluation methods. In this building, wecannot verify the results with actual damages because this building does not have any earthquake
experiences.

Figure 4. Static analysis in T-City Hall

Figure 5. Dynamic analysis in T-City Hall

Figure 6. Static analysis in Myanmar School

Figure 7. Dynamic analysis in Myanmar School

3.4. Retrofitting
It is important and necessary to consider upgrading the strength or ductility before retrofit. The I S
values of 0.6 or less are recommended for retrofit. For both buildings, I S is less than 0.6 in the firststory as shown in Tables 1 and 2. For this paper, only RC shear walls, wing walls and structural slits
are used for retrofitting for both cases because these methods are easy to apply with available workforce and materials in Myanmar. For T-City Hall, wing walls are used. For Myanmar School Building,
RC shear walls are used. The calculated results of seismic evaluation after retrofit are shown in Tables
1 and 2.
For T-City hall, the seismic performance is increased by using wing walls and structural slits
in both story. After retrofit with 8 wing walls in the first story and with 4 wing walls in the secondstory, enough seismic capacity is obtained. For Myanmar school building, enough seismic capacity isobtained after retrofit with two RC shear walls in longitudinal direction and with two RC shear walls
in transverse direction.

4. CONCLUSION AND RECOMMENDATIONS
4.1. Conclusion
The objective of this study is to learn the current Japanese seismic evaluation methods and verify theireffectiveness through actual damages and response analysis. Then, these evaluation methods would beadopted for seismic evaluation methods for Myanmar after some modification with local seismicity
and geological conditions. After study and analysis of the Japanese seismic evaluation methods andresponses by STERA 3D, the coincidence and correlation of the evaluation methods and analysis arewell proofed.


For T-City Hall, the seismic performance is weak in first story by the first screening. The seismic
performance is weak in both first and second story by the second screening. The required seismic
performance is increased by using wing walls and structural slits in both story. After retrofit with8 wing walls in the first story and with 4 wing walls in the second story, enough seismic capacity
is obtained.
After seismic evaluation for Myanmar School Building, seismic performance is weak in the firststory in both longitudinal and transverse directions. After retrofit with two RC shear walls in
longitudinal direction and with two RC shear walls in transverse direction, enough seismic
capacity is obtained.
It is also observed that the results of the base shear coefficient (which is the ratio of lateral load
capacity to total weight of the building) of both buildings with seismic evaluation methods aredecreased by about 25 % to 35 % when compared with the results of non-linear static analysis.

4.2. Recommendation for Further StudyIt is recommended to consider the contribution of non-structural concrete hollow block or burnt
brick masonry in-filled walls that are neglected in the standard seismic guidelines.
In the Myanmar School Building, assumption of the seismic demand index of the structure
I S0 =0.6 for the second screening may be a little large compared with low seismicity of Myanmar.
Appropriate seismic demand index should be reinvestigated through further study in order toapply to Myanmar which has different characteristics of buildings as well as soil parameters.
The ductility of columns becomes smaller when large longitudinal reinforcement ratio (in whichp t is greater than 1%) is observed in the Myanmar School Building. So, appropriate longitudinal
reinforcement ratio is needed to use in designs.
ACKNOWLEDGEMENT

I first greatly thank to my supervisor Dr.Masanori TANI, research engineer from BRI for accepting
and guiding me through the whole period of my studying for this paper. I also thank Dr.Taiki Saito,
Dr.Koichi Morita, Dr.Toshihide Kashima and all our professors from BRI and other universities or
institutions for great contribution of their knowledge to me.
REFERENCE
G.A MacRae.,et al.( 2010) Steps in Earthquake Proofing a Country-A Case Study of Myanmar.
NZSEE Conference.
Hazard Profile of Myanmar- July 2009
Saito. : STERA_3D Technical Manual Ver.3.2, Building Research Institute, JapanThe Japan Building Disaster Prevention Association (JBDPA) (1st English Version 2001).” JBDPA,
NIED: Strong-motion Seismograph Networks (K-NET, KiK-net).