seismic retrofitting of a garments factory building in ... retrofitting of a garments factory...

Seismic Retrofitting of a garments factory buildings in Bangladesh

Seismic retrofitting of a garments factory building in Bangladesh

Anup Kumar Halder1

1B.Sc.Engg., Executive Engineer, Public Works Department,

Government of the People’s Republic of Bangladesh.

[email protected]

Akira Inoue2

2M.Engg., OYO International Corporation, Japan

[email protected]

Yosuke Nakajima3

3B.Sc.Engg., ERS Corporation, Japan

[email protected]

Md. Rafiqul Islam4

4M.Engg., Executive Engineer, Public Works Department,

Government of the People’s Republic of Bangladesh

[email protected]

ABSTRACT

Seismic assessment and retrofitting methods are not properly addressed in Bangladesh

National Building Code (BNBC 1993 and draft of BNBC 2015), although they are

strongly needed. After the “Rana Plaza” collapse, owners of many garments factory

wanted technical assistance for structural evaluation of their factory buildings. Existing

Japanese seismic assessment and retrofitting method was applied to one such factory

building, DK Knitwear Ltd with necessary modifications considering local building

construction conditions and codal requirements. This Japanese method was selected for

simplicity of calculation. After seismic assessment, the selected four storied garments

building revealed deficiency in seismic performance in both directions at ground and

first floor level. Later retrofitting was done using steel frame bracing at Ground and

first floor level. The steel bracings were supported over R.C.C. shear wall at foundation

level. Application of the method showed improvement of the seismic behavior. This

paper shares the Bangladesh experience in retrofitting works.

Keywords: seismic assessment, reinforced concrete, existing building, retrofitting

design.

1. INTRODUCTION AND BACKGROUND

After the “Rana Plaza” collapse in 2013, JICA was keen to help garment factory owners

in Bangladesh by sanctioning very soft loans for retrofitting vulnerable R.C.C. factory

buildings. The two ongoing JICA funded projects with Government of Bangladesh

1)CNCRP- a technical co-operation project concerning seismic retrofitting techniques

with Public Works Department and 2) FSPDSME-a financial co-operation project with

Bangladesh Bank were engaged to support the factory owners. A lot of factory owners

showed interest to take the technical and financial help. To make a priority list for

October 2015, Kathmandu, Nepal

New Technologies for Urban Safety of Mega Cities in Asia

immediate attention a simplified evaluation method proposed by Seki (2015) was used.

The simplified method was derived from existing Japanese seismic assessment and

retrofitting method taking into considerations of local construction conditions and

characteristics of buildings. The selected building is a four storied R.C.C frame

structure garments building with a floor area of 1811 m2. Its performance against

vertical loading was acceptable but performance under seismic loading was

questionable. After detailed evaluation it was found that, concrete core strength was

considerably lower than the design strength. As a result retrofitting was the ultimate

solution to ensure BNBC requirement.

2. JAPANESE SEISMIC EVALUATION METHOD

Japanese seismic evaluation method was used to evaluate a few existing R.C.C.

buildings in Bangladesh with some modification considering local seismicity and

building characteristics according to Manual for Seismic Retrofit Design of Existing

Reinforced Concrete Buildings draft version 2015. The method recognizes the strength

and ductility of a building, sequence of failure of less ductile to more ductile members.

The earthquake resisting capacity must be compared with an index to characterize the

earthquake damaging power (Otani, 2000). Generally, seismic Index of structure Is

shows its seismic performance level. There are three levels of seismic screening method

for seismic evaluation of a building. First level is preliminary method used only

geometric section ignoring the reinforcement contribution. Second level screening uses

detailed investigation where strong beam and weak column failure is considered. Third

level is more rigorous and requires tedious calculations. Seismic Evaluation using

second level is more appropriate for Bangladesh buildings where narrow column and

small volume of beam-column space is common. This will eventually result in failure of

column before beam failure which resembles fundamental assumption of second level

screening method and is appropriate for Bangladeshi buildings.

2.1 Methodology

The seismic index of structure Is shall be calculated by following equation which was

developed by The Japan Building Disaster Prevention Association in, 1997. It is

calculated at each story and in each principal horizontal direction of a building.

IS = E0 ⋅ SD ⋅T (1)

Where: Eo= Basic seismic index of structure.

SD= Irregularity index.

T = Time index.

Seismic demand index of structure Iso, for a building is defined as a product of ES, Z, G

and U. Where ES, stands for basic seismic demand index of structure, Z for zone index,

G for ground index of soil and U is for usage pattern. Seismic index of structure Is is

compared with seismic demand index of structure Iso. If Is ≥ Iso then the seismic

performance of the building is satisfactory.


X

X

X

Existing buildings

Strength

oriented

retrofit

S

Sto

rey

sh

ea

r co

eff

icie

nt

( S

tory

she

ar

forc

e /

bu

ild

ing

we

igh

t)

Ductility oriented

retrofit

Story deflection angle (storey deflection/

storey height) or ductility factor

Seismic

target zone

X

D

A

B Both Strength and Ductility

oriented retrofit

When it is not satisfied structural strengthening elements such as column

jacketing, R.C.C wing wall, R.C.C shear wall, steel brace frame and others are provided

so that Is after retrofit exceeds Iso. Is is proportional to C⋅F [strength index (C) × ductility

index (F)]. Strength and ductility is evaluated for each vertical member. Then C⋅F

relation expressed by multi-linear lines a floor in each direction can be prepared through

the summation of all vertical members of that floor. In case of seismic evaluation and

retrofit design, simplified multi-linear lines express the performance of a building

shown in Figure 1. Vertical axis C and horizontal axis F is non-dimensional.

2.2 Seismic Retrofitting

The concept of retrofit design of an existing RC building is shown in Figure 2. Vertical

axis is horizontal strength at ground floor divided by building weight, which is base

shear coefficient. Horizontal axis is story deflection angle (ductility factor), which is

story deflection divided by story height. The curve A of the Figure 2 is a typical existing

R.C.C. building where strength and ductility is not enough. There are three retrofit

methods strength oriented (curve S), ductility oriented (curve D) and both strength and

ductility retrofit method (curve B). Right upper side of hyperbolic curve of the Figure is

expressed as “Seismic target Zone”. In case the curve of a building reaches the target

zone, it is judged that the building is acceptable.

2.3 Comparison of shear strength between BNBC and Japanese standard

In CNCRP project relationship between the codes was studied before applying the

Japanese standard for seismic evaluation and retrofitting for Bangladesh buildings. In

the study it was found that BNBC formula can be used to calculate shear strength where

following points need to be considered according to Manual for Seismic Assessment of

Existing Reinforced Concrete Buildings draft version 2015.

BNBC formula provides safer results compared to both experimental and

evaluation formula of Japan. On the contrary, in case of high axial force ration and low

strength of concrete it need to be careful for using BNBC formula because in some

Figure 1: Strength index and Ductility index Figure 2: Load and Deflection Curves and

Concept of Retrofit

Ductility Index, F

Strength

Index, C

C⋅F = constant



cases it provides lesser value than the Japanese evaluation formula. Similarly, for high

shear reinforcement ration same cautions should be considered for applying BNBC.

3. EXAMPLE OF DK GARMENTS BUILDING RETROFITTING

The selected building was capable of carrying vertical load only according to the BNBC

1993. However, considering earthquake loading its performance was questionable.

Therefore, seismic performance was assessed following seismic assessment of Japanese

method and eventually retrofitting was done following Japanese method.

3.1 Description of the factory building

The building is a four story R.C.C garment factory constructed in 2002. One of the

characteristics of the building is the presence of a double height space at one side of the

building. The height of building at ground floor is large compared to other floors.

Table 1: Building Data

Figure 3: Front view of D.K. Knit Wear Ltd

Name D.K Knit Wear Ltd.

Usage Garments Factory

Story 4

Building height 15,292mm

Story height 3,658mm(Typical)

4,878mm (GF to 1F)

Structural type R.C.C Framed

Structure

Foundation Individual footing

Building area 1,811m2

Total floor area 6,038.7m2

Year of design 2002 (approved)

Figure 4: Framing plan of GF and Typical floor. Figure 5: Framing plan of 1st Floor of the building.


3.2 Structural Assessment

Concrete strength, Fc=10.7N/mm

2 and re-bar yield strength 400kN/mm

2 was found

according to test report of cores and sample re-bars which were collected from site.

Proposed seismic demand index of structure, Iso= 0.30 is selected for buildings of Dhaka

considering importance factor =1, according to Manual for Seismic Retrofit Design of

Existing Reinforced Concrete Buildings draft version 2015.

The strength and deformation capacities of structural members are calculated on the

basis of structural dimensions and material properties investigated at site. Ductility

index, F=1.50(=1/100) is calculated at ground floor level for most of the columns

manually considering structural data. In this evaluation, F = 1.27(=1/150) at ground

floor level was used due to high axial force ratio. This conservative adjustment will

reduce the damage of brick walls and non-structural elements. During the assessment

irregularity index SD is found 0.76 at GF for both direction (X and Y direction), which

is relatively large. Time Index, T is estimated following standard table, and T = 1.0 is

used for further calculation. Result of seismic evaluation shows Is value at level 1 and

level 2 are lower than Iso (= 0.30) which suggests for retrofitting. Calculated F is higher

than 1.5 at level 3 and level 4, but F =1.5 was used for assessment considering “low

strength concrete” according to Japan Concrete Institute in February 2009.

Table 2: Result of Seismic Evaluation

Story (n+1) / (n+i)

X- direction Y- direction

C F Eo Is C F Eo Is

4 0.63 0.63 1.50 0.59 0.56 0.59 1.50 0.56 0.53

3 0.71 0.33 1.50 0.35 0.33 0.32 1.50 0.34 0.33

2 0.83 0.18 1.27 0.19 0.18 0.16 1.27 0.17 0.15

1 1.00 0.12 1.27 0.15 0.11 0.10 1.27 0.16 0.12

3.3 Retrofit design

Retrofit elements are provided at outside of perimeter column. This will reduce

disturbances during execution of construction work and production will go on smoothly.

Among the different retrofitting options steel framed brace is preferred which will allow

windows and other openings of perimeter walls to function properly. However, in-filled

R.C.C. walls are provided under the steel framed brace up to the existing foundation

footing to transfer the strength of steel framed brace at GF. It contributed for the

improvement irregularity both in plan and vertical direction. Required number of steel

framed brace can be calculated by following standard equations.

Table 3: Required numbers of Steel Framed Brace

Story

ΣW,

Weight

(kN), Un

factored

load

1+

+

n

in

Design shear

coefficient,

0.195.0

30.0

1 ××

×

+

+

Fn

in

Iso = 0.30, SD = 0.95

(after retrofit)

Design shear strength Q,

after retrofit,

iW

Fn

in∑×

××

×

+

+

0.195.0

30.0

1

(1)

Original strength,

C (at F)× ΣWi, (2)

Required

additional

strength,

Q

(1)- (2)

(kN)

Q (kN) C Q(kN)

4 15,065 1.6 0.33 (F =1.5) 5,077 in case F =1.5 x 9,481 0.63 ---



y 8,955 0.59 ---

3 33,178 1.4 0.29 (F =1.5) 9,788 in case F =1.5 x 10,892 0.32 ---

y 10,646 0.32 ---

2 51,803 1.2 0.29 (F =1.27) 15,437 x 9,512 0.18 5,925

y 8,606 0.16 6,831

1 66,391 1.0 0.24 (F =1.27) 16,531 x 7,909 0.12 8,622

y 7,013 0.11 9,518

Following combination of steel framed brace is proposed. Well balanced layout of steel

brace is planned to improve the irregularity. Irregularity index of each floor is 0.95. In

X-direction four numbers of steel frame bracing at GF and four numbers at 1st floor is

required. Similarly, in Y-direction four numbers at GF and four numbers at 1st floor is

suggested.

Figure 6: Retrofitting plan at GF Figure 7: Retrofitting plan at 1st Foor

Figure 8: Typical sectional elevation of a retrofitted frame.


4. RESULT OF SEISMIC RETROFIT DESIGN

Seismic index of structure, Is at level 1 and level 2 are more than Iso (= 0.30) and are

satisfactory. Irregularity Index, SD2, is 0.95 after retrofit

Table 4: Result after Retrofit

Story in

n

+

+1

X-direction Y-direction

C F Eo Is C F Eo Is

4 0.63 0.62 1.50 0.59 0.56 0.57 1.50 0.56 0.53

3 0.71 0.32 1.50 0.34 0.32 0.32 1.50 0.34 0.32

2 0.83 0.14 + 0.17 = 0.32 1.27 0.34 0.32 0.16 + 0.14 = 0.30 1.27 0.32 0.30

1 1.0 0.12 + 0.16 = 0.28 1.27 0.35 0.33 0.11 + 0.14 = 0.25 1.27 0.32 0.30

Figure 9 indicates the performance after retrofitting 1st floor X-direction. The X axis

indicates the F (ductility index) and the Y axis C (strength index). C (strength index) F

(ductility index) relation at each floor before and after retrofit can be shown in similar

way. Right upper side of the hyperbolic curve shows the target area of seismic

performance. Before retrofit the performance remains below the line and after retrofit

the performance line matches with the line ensuring building performance. This

hyperbolic curve shows target Eo or Iso expressed by,

TS

I

n

inFC

D

So

×

×

+

+

=⋅

1 (2)

Where Iso=0.3, SD=0.95 and T=1.0.

Figure 9: Performance of Building after retrofit (1

st floor X direction)

5. CONCLUSION

The application of Japanese method can be used for the buildings of Bangladesh.

However, reliability of the procedure needs to be examined with respect to the damage

in buildings (Otani, 2000). This can be achieved by scale down model test in simulator



or other suitable methods as Bangladesh does not have ample earthquake damage data

of buildings. In this regard new research should be encouraged to come up with

appropriate solutions considering Bangladesh building characteristics and availability of

local construction materials.

6. ACKNOWLEDGEMENT

The authors gratefully acknowledge JICA Bangladesh for their continuous support to

build safer cities in Bangladesh through technology transfer and fund for retrofitting

projects. All the CNCRP members involved in this project, Bangladesh Bank and DK

authority deserve heartiest thanks for their co-operation during the execution of the

work.

REFERENCES

The Japan Building Disaster Prevention Association, 1997, Standard for Seismic

capacity Assessment of Existing Reinforced Concrete Buildings(in Japanese).

The Japan Building Disaster Prevention Association, 2001, Standard for Seismic

Evaluation of Existing Reinforced Concrete Buildings, Japan.

The Japan Building Disaster Prevention Association, 2001, Guidelines for Seismic

Retrofit of Existing Reinforced Concrete Buildings, Japan.

The Japan Building Disaster Prevention Association, 2001, Technical Manual for

Seismic Evaluation and Seismic Retrofit of Existing Reinforced Concrete Building.

Housing and Building Research Institute (HBRI) & Bangladesh Standards and Testing

Institution (BSTl), 1993. Bangladesh National Building Code (BNBC), Dhaka,

Bangladesh.

Housing and Building Research Institute (HBRI) & Bangladesh Standards and Testing

Institution (BSTl), 2015. Draft final of Bangladesh National Building (BNBC), Dhaka,

Bangladesh.

Public Works Department, 2015, Draft version Public Works Department, Manual for

Seismic Retrofit Design of Existing Reinforced Concrete Buildings unpublished, Dhaka,

Bangladesh.

Public Works Department, 2015, Draft version Public Works Department, Manual for

Seismic Assessment of Existing Reinforced Concrete Buildings unpublished, Dhaka,

Bangladesh.

Seki, M., 2015, Proposal on the Simplified Structural Evaluation Method for Existing

Reinforced Concrete Buildings based on the Japanese Seismic Evaluation Standard vis-

a vis the International Seismic Code, Journal of Earthquake Science and Engineering,

Publisher ISES 2015.http://www.joes.org.in.

Otani, S.,2000, Seismic Vulnerability Assessment Methods for Buildings in Japan,

Earthquake Engineering Seismology Volume 2, Number 2, September 2000, pp.47-56.

seismic retrofitting of a garments factory building in ... retrofitting of a garments factory...

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