Seismic retrofitting of a garments factory building in Bangladesh

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1 Seismic retrofitting of a garments factory building in Bangladesh Anup Kumar Halder 1 1 B.Sc.Engg., Executive Engineer, Public Works Department, Government of the People s Republic of Bangladesh. akhalder2000@yahoo.com Akira Inoue 2 2 M.Engg., OYO International Corporation, Japan inoue@oyointer.com Yosuke Nakajima 3 3 B.Sc.Engg., ERS Corporation, Japan nakajima@ers-co.jp Md. Rafiqul Islam 4 4 M.Engg., Executive Engineer, Public Works Department, Government of the People s Republic of Bangladesh rafiq89bd@gmail.com 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 Seismic Retrofitting of a garments factory buildings in Bangladesh

2 October 2015, Kathmandu, Nepal 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 m 2. 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 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 I s 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 I s shall be calculated by following equation which was developed by The Japan Building Disaster Prevention Association in, It is calculated at each story and in each principal horizontal direction of a building. I S = E 0 S D T (1) Where: E o = Basic seismic index of structure. S D = Irregularity index. T = Time index. Seismic demand index of structure I so, for a building is defined as a product of E S, Z, G and U. Where E S, 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 I s is compared with seismic demand index of structure I so. If I s I so then the seismic performance of the building is satisfactory. New Technologies for Urban Safety of Mega Cities in Asia

3 Strength Index, C C F = constant Storey shear coefficient ( Story shear force / building weight) S B A X X Strength oriented retrofit D Existing buildings X Seismic target zone Both Strength and Ductility oriented retrofit X Ductility oriented retrofit Ductility Index, F Storydeflection angle (storey deflection/ storey height) or ductility factor Figure 1: Strength index and Ductility index Figure 2: Load and Deflection Curves and Concept of 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 I s after retrofit exceeds I so. I s 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 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 Seismic Retrofitting of a garments factory buildings in Bangladesh

4 October 2015, Kathmandu, Nepal 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 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 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 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 1,811m 2 Total floor area 6,038.7m 2 Year of design 2002 (approved) Figure 3: Front view of D.K. Knit Wear Ltd Figure 4: Framing plan of GF and Typical floor. Figure 5: Framing plan of 1 st Floor of the building. New Technologies for Urban Safety of Mega Cities in Asia

5 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, I so = 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 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 S D 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 I s value at level 1 and level 2 are lower than I so (= 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 Table 2: Result of Seismic Evaluation Story (n+1) / X- direction Y- direction (n+i) C F E o I s C F E o I s 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. Story ΣW, Weight (kn), Un factored load Table 3: Required numbers of Steel Framed Brace n + i n +1 Design shear coefficient, n + i 0.30 n + 1 F I so = 0.30, S D = 0.95 (after retrofit) Design shear strength Q, after retrofit, n + i 0.30 W n + 1 F i (1) Original strength, C (at F) ΣW i, (2) Required additional strength, Q (1)- (2) (kn) Q (kn) C Q(kN) 4 15, (F =1.5) 5,077 in case F =1.5 x 9, Seismic Retrofitting of a garments factory buildings in Bangladesh

6 October 2015, Kathmandu, Nepal y 8, , (F =1.5) 9,788 in case F = , (F =1.27) 15, , (F =1.27) 16,531 x 10, y 10, x 9, ,925 y 8, ,831 x 7, ,622 y 7, ,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 In X-direction four numbers of steel frame bracing at GF and four numbers at 1 st floor is required. Similarly, in Y-direction four numbers at GF and four numbers at 1 st floor is suggested. Figure 6: Retrofitting plan at GF Figure 7: Retrofitting plan at 1 st Foor Figure 8: Typical sectional elevation of a retrofitted frame. New Technologies for Urban Safety of Mega Cities in Asia

7 4. RESULT OF SEISMIC RETROFIT DESIGN Seismic index of structure, I s at level 1 and level 2 are more than I so (= 0.30) and are satisfactory. Irregularity Index, S D2, is 0.95 after retrofit Table 4: Result after Retrofit n +1 X-direction Y-direction Story n + i C F E o I s C F E o I s = = = = 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 E o or I so expressed by, n + i I So C F = (2) n + 1 S D T Where I so =0.3, S D =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 Seismic Retrofitting of a garments factory buildings in Bangladesh

8 October 2015, Kathmandu, Nepal 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), Bangladesh National Building Code (BNBC), Dhaka, Bangladesh. Housing and Building Research Institute (HBRI) & Bangladesh Standards and Testing Institution (BSTl), 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 visa vis the International Seismic Code, Journal of Earthquake Science and Engineering, Publisher ISES Otani, S.,2000, Seismic Vulnerability Assessment Methods for Buildings in Japan, Earthquake Engineering Seismology Volume 2, Number 2, September 2000, pp New Technologies for Urban Safety of Mega Cities in Asia