SEISMIC RESPONSE EVALUATION AND RETROFIT
OF A FIVE-STORIED RC BUILDING DAMAGED DUE
TO THE 2017 TRIPURA EARTHQUAKESk Toufiqur RAHMAN
MEE17715

Supervisor:

Matsutaro SEKIABSTRACTThe 2017 Tripura earthquake caused wide spread structural and nonstructural damage to some buildingsin the North-Eastern part of Bangladesh. To investigate the dynamic behavior of a building, seismic
response simulation was carried out. For time history analysis, synthetic ground motions were used since
no ground motion was available for the Tripura earthquake. When actual damage was compared withthe analytical result from the nonlinear time history analysis, reasonable correlation was obtained. It was
found that low strength concrete, bare frames in the ground floor, insufficient shear reinforcement,
inadequate column sizes and weak beam-column joints were mainly responsible for the damage due tothis earthquake. Both strength oriented and ductility oriented retrofitting approach were considered forrehabilitation. From the nonlinear analysis, it can be concluded that strength oriented retrofitting is thebest option for strengthening for local design and construction conditions in Bangladesh.
Keywords: Tripura earthquake, Seismic evaluation, Seismic rehabilitation, Non-linear analysis.

1. INTRODUCTION
On January 3, 2017, an earthquake of Magnitude (Mw) approximately 5.7 on the Richter scale hit IndiaBangladesh Border region with a focal depth of 30 km. This shallow strike-slip earthquake hasoriginated from the Indian Plate which lies beneath the Burmese Plate. While this moderate intensity
earthquake occurred in Dhalai district in India, it caused some damage in some parts of Bangladeshclose to India-Bangladesh Border region. At least seven buildings in this region experienced structuraland nonstructural damage. One building named “Sarkari Shishu Paribar” situated in Sreemangal
Upazila in Maulvibazar District in the Sylhet Division of Bangladesh suffered severe damage due tothis earthquake. Damaged to this building due to moderate earthquake (MMI intensity V, according toUSGS) is causing real concern. Most of the buildings in Bangladesh are constructed without following
the Building Code. These buildings are very vulnerable to moderate to strong earthquakes.
Bangladesh has experienced eight major earthquakes over the last 250 years. Bangladesh issituated in the place where the Indian Plate, the Burmese Plate & the Eurasian Plate meet. Owing to thefact, Bangladesh is one of the tectonically active seismic regions in the world, probability of occurrenceof moderate to strong earthquake is very high. The Bangladesh National Building Code (BNBC) was
prepared in 1993 and enacted in 2006. For this reason, a large number of buildings in Bangladesh whichwere constructed before 2006 have not complied with the seismic provisions of BNBC. Under thiscircumstances, it is necessary to assess seismic vulnerability and then rehabilitate buildings according
to the seismic provisions of the building code.

Public Works Department, Bangladesh.
Visiting Research Fellow, International Institute of Seismology and Earthquake Engineering, BRI, Japan.

1

2. TARGET BUILDING
2.1. Outline
The target building of this individual study which was subjected to the Tripura Earthquake, January 3,
2017 experienced some damage in the ground floor. This building is a five-storied reinforced concrete
residential building named as “Sarkari Shishu Paribar”. It is located in Sreemangal Upazila ofBangladesh. According to BNBC 2015 Draft, this area is in Zone IV (Z = 0.36) and the site class is SD
̅𝑠 < 180 m/s). This building is located 40 km away from the epicenter of the 2017 Tripura earthquake.
(𝑉
The isometric view and location of the target building are shown in Figure 1 and Figure 2 respectively.

Figure 1. Isometric view of the target building.

Figure 2. Location of the target building andthe epicenter in the seismic zoning map.

2.2. Description of the damage
One column in the ground floor damaged severely. Big shear cracks and concrete spalling were foundat the top of the column (Figure 3). Flexural Cracks were found just below beam-column Joints for mostof the ground floor columns. One masonry wall in the ground floor suffered heavy damage (Figure 4).
At first, diagonal cracks initiated and then sliding failure occurred. Lots of horizontal cracks were foundin the masonry walls of the staircase in the ground floor.

Figure 3. Big shear crack in column.

Figure 4. Damaged masonry infill.

2.3. Damage ranking of the structural members
Damage classes based on “the Japanese Damage Evaluation Guideline” (JBDPA 2001a) were assigned
to structural members of the target building according to the post-earthquake field survey after the
Tripura Earthquake, 2017. Figure 5 shows the damage ranking of each structural vertical member andassociated average ductility factor (µ) proposed by Maeda et al (2012).

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Table 1. Ductility Factors based on
Damage Class (Maeda et al, 2014).
Damage class
0
I
II
III
IV
V

Average ductility factor (µ)
0.05
0.5
1.5
2.5
4
5.5

4625

3875

μ=0.05μ=0.5 μ=4

0

I

IV

3875

3875

3875

1950

3875

μ=1.5

μ=1.5

μ=1.5

I

II

II

II

μ=0.5

μ=0.5

μ=0.5

μ=1.5

μ=1.5

I

I

I

II

II

μ=0.5

μ=0.5

I

5975
μ=0.5

I

2150μ=0.5

μ=0.5 μ=0.5

I

μ=0.05I

μ=0.05

I

0

0

μ=0.5

μ=0.5

μ=0.5

μ=1.5 μ=1.5

IIII

μ=1.5 μ=1.5

II

μ=1.5

II

μ=1.5

II

II

μ=1.5

μ=1.5

II

II

5975

I

I

3250

I

μ=0.5

μ=0.5

I

I

3875

1950

Figure 5. Damage distribution in the ground floor
and associated average ductility factors.
3. SEISMIC RESPONSE SIMULATION
3.1. Input ground motion generation
No real ground motion acceleration data is available for the Tripura Earthquake, 2017. Hence, synthetic
ground motion data is generated to perform simulation of seismic responses. For generation of synthetic
ground motion, Kanno et al. (2006) attenuation relation has been used. The empirical attenuation
formulas used are presented in Eq. (1) and Eq. (2):log 𝑝𝑟𝑒 = 𝑎1 𝑀𝑤 + 𝑏1 𝑋 − log(𝑋 + 𝑑1 10𝑒1 𝑀𝑤 ) + 𝑐1 + 𝜀1 𝑤ℎ𝑒𝑛 𝐷 ≤ 30𝑘𝑚
log 𝑝𝑟𝑒 = 𝑎2 𝑀𝑤 + 𝑏2 𝑋 − log(𝑋) + 𝑐2 + 𝜀2 𝑤ℎ𝑒𝑛 𝐷 > 30𝑘𝑚

(1)
(2)

where, pre is the PGA, PGV or 5% damped response spectral acceleration in cm/sec2, Mw is Moment
magnitude, X is source distance (km), D: Focal depth (km) and 𝜀2 , 𝜀2 are errors. To take into accountthe local soil condition of the site, soil amplification factor is used based on BNBC 2015.

Acceleration (cm/sec2)

200
150
100
50
0
-50 0

10

20

30

-100
-150
Time, sec.

Figure 6. Synthetic ground motion.

Figure 7. Pseudo velocity response
spectrum (h = 5%).

3.2. Ductility factors obtained from nonlinear time history analysis
Non-linear dynamic analysis is performed using STERA 3D, a finite element method (FEM) based
software developed by Dr. T. Saito. Synthetic ground motion is applied for this analysis. From the nonlinear time history analysis, ductility factors for each vertical element (µ) are obtained. After that these3

ductility factors are compared with the average ductility factor assigned in Figure 5 to verify simulation
result in terms of ductility factors.
The comparison between average
ductility factors based on damage class (Figure5) and the ductility factors (Figure 8) acquired
from non-linear time history analysis usingsynthetic ground motion shows reasonable
correlation. The values of ductility factors forvertical structural elements obtained from time
history analysis are close to the average value ofductility factors depending on damage rankings
Figure 8. Ductility factors in the ground floorof structural members.
through nonlinear time history analysis.
4. SEISMIC EVALUATION AND REHABILITATION
4.1. Seismic evaluation by JBDPA 2nd Level Screening
For seismic evaluation of the target building, shear strength of masonry infill is considered only for thetransverse direction. In the longitudinal direction, Contribution of masonry infill walls is minor in thelongitudinal since most of the walls are out of the frames and the rest of the masonry walls have the big
openings.
4.1.1. Seismic demand Index (ISO) and minimum strength requirement
According to the seismic evaluation manual of Bangladesh, for the 2nd level and the 3rd level, the seismic
demand index (ISO) and the minimum strength requirement of the target building irrespective of thestories are calculated as follows:22

𝐼𝑆𝑂 = 0.8 × 𝑍. 𝐼. 𝐶𝑠 (= 0.8 × × 0.36 × 1.0 × 3.37 = 0.65)
3

3

2

2

𝐶𝑇𝑈 . 𝑆𝐷 ≥ 0.4. 𝑍. 𝐼. 𝐶𝑠 (= 0.4× × 0.36 × 1.0 × 3.37 = 0.325)
3

3

(3)
(4)

where, 𝑍: Seismic Zone Coefficient, 𝐼: Structure Importance Factor, 𝐶𝑠 : Normalized Acceleration
Response Spectrum which is a function of building natural period and site class.
4.1.2. Seismic evaluation results
The results demonstrated in Table 2 indicate seismic
rehabilitation needs to be provided for all stories along
the longitudinal direction. The results of seismic
evaluation along the transverse direction reflect all thestories except the top story require strengthening. In caseof seismic evaluation considering masonry infill for the
4th story seismic index (IS) value is more than seismic
demand index (ISO). Still this story along the transverse
direction requires strengthening since CTUSD value isless than the minimum strength requirement.

Table 2. Seismic evaluation results.
Story

5
4
3
2
1

Evaluation along
Evaluation along
longitudinal
Transverse direction
direction
Is =E0*SD*T CTU SD Is =E0*SD*T CTU SD
0.51
0.19
1.11
0.38
0.34
0.13
0.67
0.20
0.21
0.11
0.47
0.47
0.11
0.10
0.43
0.43
0.10
0.10
0.13
0.13

4.2. Seismic Rehabilitation
Required seismic performance can be achieved through improvement of strength or ductility or both.
For retrofit design strength dominant seismic rehabilitation is given a priority. Three retrofitting methods
have been applied. Chosen retrofitting methods are steel framed brace insertion, RC column jacketing
and aramid fiber sheet wrapping.

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4

0.34

Story

3

0.21

2

0.11

1
0

ISO = 0.65

0.51

5

0.95

4

0.85
0.65

 
Unsafe Safe

Story

5

0.84

0.88
0.1
0.2
0.4
0.6
0.8
1
Seismic index (Is)Before retrofit
After Retrofit

0.47

3
2

1.21

1.11
0.67

1.1
0.99

0.43

1
0

Figure 9. Seismic Index (Is) along longitudinal
direction before and after the strength oriented
retrofit.

ISO = 0.65

4.2.1. Seismic evaluation result after retrofitting
Seismic evaluation after the retrofit is conducted taking into account the strength of masonry infill.
Seismic indices (Is) of the stories of the target building along longitudinal direction and transverse
direction before and after the retrofit are depicted in Figure 9 and Figure 10 respectively.

1.06

0.13
0.77
0.2 0.4 0.6 0.81
1.2 1.4
Seismic index (Is)Before retrofit
After Retrofit

Figure 10. Seismic Index (Is) along transverse
direction before and after the strength oriented
retrofit.

4.3. Non-linear analysis of the target building
4.3.1. Performance objectives
Table 3. Structural performance levels and associated drift
According to FEMA 356 and ATClimits according to FEMA 356 and ATC-40.
40, immediate occupancy (IO), life Type
Immediate
Life Safety
Collapsesafety (LS), and collapse prevention
Occupancy
Prevention(CP) are three discrete structural Primary Very limitedSignificant post- Verge of partial or
performance levels. Structural
post-earthquake earthquake
total collapse,
damage state,damage state, still however, structure
performance levels and associatedbuilding is safe to structure isn’t
can still bear gravity
drifts for concrete frames are shown
occupy.
collapsed.
load.
in Table 3.
Drift
limit

4% transient orpermanent

2% transient or
1% permanent

1% transient or
negligible permanent

4.2.2. Inter-story drift
When strength oriented retrofit was opted, inter-story drift ratio of all the stories were (shown in Figure11) below the immediate occupancy level. In the case of ductility dominant retrofit inter-story drift ratio(depicted in Figure 12) exceeded the immediate occupancy level. From Capacity Spectrum Method(CSM), inter-story drift ratios for all the stories for both directions are higher than those from theaccurate method time history analysis.
Immediate Occupancy

5

Immediate Occupancy

4Life Safety

4Life Safety

3

CSM

3

CSM

2

El Centro phase2

Christchurch (2011) phase

1

Story

Story

5

Kobe phase

1

Christchurch (2011) phase

0
0.00

0.01
Inter-Story Drift

Kobe phase

0
0.00

0.02

Figure 11. Inter-story drift ratio after thestrength oriented retrofit along thelongitudinal direction for THA and CSM.

El Centro phase

0.01
Inter-Story Drift

0.02

Figure 12. Inter-story drift ratio after the ductility
oriented retrofit along the longitudinal direction for
THA and CSM.

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5. CONCLUSIONSThe target building of this study suffered damage in the ground floor due to the 2017 Tripura earthquake.
To get an analytical explanation of the damage, seismic response simulation was performed usingsynthetic ground motion. It shows reasonable correlation between actual behavior and analyticalbehavior. Low strength concrete, bare frames in the ground floor, inadequate column sizes, inadequate
shear reinforcement were responsible for the damage. Kanno et al. (2006) Attenuation relation proved
to a good option to generate artificial ground motion when the earthquake is shallow.
Both strength oriented and ductility oriented retrofit approach were studied to choose thefeasible retrofit methods. Strength dominant retrofit was found to be the best option in case of no damageis allowed. However, ductility dominant retrofit can be opted, if some damage is allowed in the structure.
RC column jacketing and steel framed braces have reduced large, concentrated deformation (due to softstory) in the ground floor. For inadequate shear reinforcement and weak beam column joints, RC
jacketing and aramid sheet wrapping provided effective retrofitting options. FRP wrapping increased
the shear strength thus increased the ductility of the brittle columns.

6. RECOMMENDATIONThe strength of columns surrounding the masonry infill, and the mortar strength of the masonry, cangreatly impact the ductility and strength of the infilled frame. Research works need to be conductedvarying these parameters especially to bear in mind the construction conditions of the concernedcountry.
Weak beam-column joint areas and inadequate or no joint-shear reinforcement inside joints
exist in most of the RC framed buildings in Bangladesh. These beam-column joint areas will fail withina small deflection of the concerned columns. Especial attention should be paid when seismic evaluationis conducted if a weak-beam column joint subsists. Further research works can be carried out to findfeasible retrofit methods for weak beam-column joint area.

ACKNOWLEDGEMENTS
I would first like to express my deepest gratitude to my supervisor Dr. Eng. Matsutaro Seki and Dr.
Toshihide Kashima for their guidance, constant support and instruction during my study.

REFERENCES
Al-Chaar, G., Lamb, G. E., and Issa, M. A., 2003, ACI Special Publications, 211-212, 247-261.
BNBC, 2015, Final Draft Version Bangladesh National Building Code 2015.
Federal Emergency Management Agency (FEMA) 356, 2000.
Hossein, M., and Toshimi, K., 2004, Bulletin of the Earthquake Research Institute, 133-156.
Japan Building Disaster Prevention Association (JBDPA), 2001, Standard for Seismic Evaluation of
Existing Reinforced Concrete Buildings.
Japan Building Disaster Prevention Association(JBDPA), 2001a, Standard for Post-Earthquake
Damage Level Classification of Reinforced Concrete Building.
Kanno, T., Narita, A., Morikawa, N., Fujiwara, H., and Fukushima, Y., 2006, Bulletin of the
Seismological Society of America, 96(3), 879-897.
Saito, T., 2015, STERA 3D Technical Manual, Version 5.8.
Sugano, S., 2017, IISEE Lecture Note 2017-2018, IISEE, BRI.

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