A proposal on the simplified structural evaluation method for existing reinforced concrete buildings in Bangladesh

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1 A proposal on the simplified structural evaluation method for existing reinforced concrete buildings in Bangladesh Matsutaro Seki 1 1 Dr. Eng., Visiting Research Fellow, Building Research Institute, International Institute of Seismology and Earthquake Engineering, Tsukuba, Japan sekimatsutaro@yahoo.co.jp Md. Rafiqul Islam 2 2 M.Eng., Executive Engineer, Public Works Department Government of the People s Republic of Bangladesh, Dhaka, Bangladesh rafiq89bd@gmail.com ABSTRACT In the 1995 Hyogoken Nanbu (Kobe) Earthquake of Japan, a large number of buildings, which were designed by the old Japanese seismic code, suffered severe damage. Based on this lesson, the Japanese government has encouraged promoting of the seismic evaluation and retrofitting work for existing buildings, especially constructed before the latest code of After 1995, several big earthquakes occurred in Japan but these retrofitted buildings survived safely. Now buildings all over Japan are becoming better seismic resistant and remarkably decreasing damage even to higher level hazard by earthquakes. On the other hand, the developing countries in the earthquake prone regions in the world are still suffering a lot of casualties as well as building damage. These damages might be caused by inadequate structural design by engineers and/or poor quality control of construction works. In order to contribute to disaster mitigation for existing reinforced concrete buildings in developing countries, the simplified structural evaluation method based on the philosophy of Japanese evaluation standard vis-a-vis the international seismic code was developed by Seki (2015). In this paper, the developed simplified evaluation method is introduced and structural evaluation of existing reinforced concrete buildings of readymade garment factories in Bangladesh is performed using it. Keywords: structural evaluation, reinforced concrete, existing building, seismic capacity, service load capacity 1. INTRODUCTION Many of countries in the world, especially the developing countries which usually suffer severe damage by reason of poor quality buildings, are yet on the way to the goal of achievement for disaster mitigation for the strong earthquakes. Generally these countries tend not to have the seismic evaluation provision and retrofitting technology in their national seismic code and/or other related regulations. The developed simplified structural evaluation method can be utilized as the first stage screening of the structural capacity, especially, for the lots of vulnerable buildings in the developing countries which induced the international seismic code for their own

2 October 2015, Kathmandu, Nepal building national code. In this paper, a case study applying to Bangladesh using the data from readymade garment factories project by JICA is discussed. Table 1 shows the three kinds of structural evaluation such as simplified structural evaluation (SE), advanced simplified structural evaluation (ASE) and detail seismic evaluation (DE). This paper s evaluation method (SE) can be useful for the first stage screening for lots of buildings as above mentioned. Table 1 Structural evaluation procedure for existing reinforced concrete buildings Title SE ASE DE Evaluation Method Simplified structural evaluation Advanced simplified structural evaluation Detail seismic evaluation Objective Average ultimate Accurate ultimate Between SE and DE capacity for lots of capacity for (Screening) buildings (Screening) individual building Resource data Structural drawing Structural drawing & brief site investigation (Non-destructive tests) Structural drawing & detail site investigation (destructive tests) 2. BASIC PRINCIPLE FOR EVALUATION The flow diagram of the proposed simplified structural evaluation is shown Figure 1. In this figure, various index for Bangladesh such as seismic index (I BS ), Judgement seismic index (I BS0 ), service load index (I BD ) and judgement service load index (I BD0 ) are used in this figure. The proposed simplified structural evaluation method is based on the following basic principles and the detail content of evaluation method can be referred in Seki(2015). (i) Seismic evaluation is basically based on the philosophy of the Japanese Seismic Evaluation Standard for existing reinforced concrete buildings issued by JBDPA2001 and IBC2000. (ii) The target building is the reinforced concrete moment resisting (beam-column) frame building. (iii) Evaluation is done by only original structural drawings and architectural drawing. This evaluation is performed on the condition that the building was constructed faithfully due to the approved original drawings. (iv) Evaluation is basically performed at the ground floor (level-1) which may be usually the weakest floor of the whole building floors. (v) If the necessary information such as material strength, profile of rebar is lack in the structural drawings, these may be assumed with construction year and/or the experience of engineer, etc. (vi) As for the final judgment after simplified structural evaluation, the vulnerability evaluation on two items; (1) seismic capacity by horizontal seismic load, (2) service load capacity by vertical service load is carried out. Final structural rank based on combination of seismic capacity and service load capacity can be obtained. If the evaluation result doesn t satisfy the target capacity values, the detail evaluation method will be recommended. New Technologies for Urban Safety of Mega Cities in Asia

3 Start Calculate Seismic Index: I BS Calculate Service Load Index: I BD Judgment : I BS I BS0? NO YES Judgment : I BD I BD0? NO End YES Detail Evaluation Figure 1 Flow diagram of simplified structural evaluation for the existing reinforced concrete buildings in Bangladesh 3. EVALUATION METHOD 3.1 Seismic Index (I BS ) I BS =E BS *S D *T (1) Where, E BS : Basic Structural Index E BS =C BS *F B (2) C BS : Strength Capacity of Building C BS =τ*σac/w (3) τ:average shear strength of Column (N/mm 2, JBDPA 2001) h 0 /D >6 : τ=0.7 N/mm 2 (4) h 0 /D 6 : τ=1.0 N/mm 2 h 0 : Clear height of column (mm) D: Depth of column section (mm) ΣAc : Total area of columns (mm 2 ) W: Total weight of building (N) F B : Ductility Index F B = R/Ω 0 (5) R: Response modification factor based on structure type in BNBC2015 Final Draft Ω 0 : Over strength factor (FEMA &IBC2000) S D : Irregularity Index (here, S D =1.0) T : Time deterioration Index (here, T=1.0) A proposal on the simplified structural evaluation method for existing reinforced concrete buildings in Bangladesh

4 October 2015, Kathmandu, Nepal 3.2 Service Load Index (I BD ) I BD = W/ΣAc (6) Where, W: Total weight of building (N) ΣA : Total area of columns (mm 2 ) 4. JUDGMENT 4.1 Definition of Judgment Index I BSO : Seismic Index I BSO = V (7) V: Total design base shear coefficient ( BNBC2015 Final Draft ) V=2/3*Z *I*C S (8) Where, Z: Seismic zone coefficient (Table 2, Figure 2) I : Structural importance coefficient (here, I=1.0 ) C S : Normalized acceleration response spectrum, which is a function of structure (building) period and soil type (site class) (Figure 3) Table 2 Seismic zone coefficients, Z (BNBC2015 Final Draft) Zone Coefficient Zone (unit: g) Figure 2 Seismic zoning map of Bangladesh (BNBC 2015 Final Draft) New Technologies for Urban Safety of Mega Cities in Asia

5 Figure 3 Normalized design acceleration response spectrum for different site classes (BNBC2015 Final Draft) I BD0 : Service Load Index I BD01 = 0.4*Fc (9) I BD02 = 0.7*Fc Where, Fc: Designed concrete strength (N/mm 2 ) 4.2 Judgment Seismic Capacity I BS I BSO : Higher than seismic demand (Rank SA) (10) 0.5I BSO I BS <I BSO : Lower than seismic demand (Rank SB) I BS <0.5I BSO : Remarkably lower than seismic demand (Rank SC) Service Load Capacity I BD <I BD01 : Higher than service load demand (Rank DA) (11) I BD01 I BD I BD0 : Lower than service load demand (Rank DB) I BD02 <I BD : Remarkably lower than service load demand (Rank DC) Final Rank based on Combination of Seismic Capacity and Service Load Capacity Final structural rank based on combination of seismic capacity and service load capacity can be defined as following Table 3. A proposal on the simplified structural evaluation method for existing reinforced concrete buildings in Bangladesh

6 October 2015, Kathmandu, Nepal Table 3 Final capacity rank of simplified structural evaluation Final Capacity Rank Seismic Capacity Combination Service Load Capacity Recommendation A SA DA Safe B SB DA, DB C SC DA, DB, DC Detail Evaluation Recommended Immediately Detail Evaluation Recommended 5. EXAMPLE OF STRUCTURAL EVALUATION IN BANGLADESH 5.1 Target buildings and method of structural evaluation Target buildings for structural evaluation are reinforced concrete moment resisting buildings of readymade garment factories in Dhaka city, Bangladesh by JICA project. Total number of buildings is fifty one. For decision of ductility index; F B, response modification factor; R is 5 defined as ordinary moment resisting frame in BNBC1993 and over strength factor; Ω 0 is 2.0 in FEMA because this value is not defined in BNBC1993. If the information from structural and architectural drawings is deficient in evaluation, the following assumption was done; if there is no description on aggregate, concrete strength; Fc= 17 Mpa, and if there is, for stone chip Fc=21 Mpa, for masonry chip Fc=17 Mpa. Story height is 3600 mm for ground floor and 3000 mm for other floor. Story average unit weight is 7 KN/m 2 for top floor and 10 KN/m 2 for other floor, etc. 5.2 Results of structural evaluation (1) I BS -I BSO Relationship (Figure 4); Two dashed lines, I BS =I BSO and I BS =0.5*I BSO, are shown. The buildings with I BS above the I BSO, which satisfy the target earthquake resistance, represent approximately 25% of all the buildings. (2)I BD -I BD01 Relationship (Figure 5); One dashed line, I BD =I BD01 is shown. The buildings with I BD above the I BD01, which don t satisfy the target service load resistance, represent approximately 35% of all the buildings. (3)I BS /I BS0 -I BD /I BD01, I BD /I BD02 relationship (Figure 6); This figure shows the earthquake resistance index and the service load index. Approximately 20% of the buildings are higher in both indices; I BS0 and I BD01. Namely 75% of buildings are recommended to perform the detail seismic evaluation. Approximately 35% of buildings are above I BD01 and two buildings above I BD02. These buildings show the remarkably low vulnerability in vertical resistant capacity. New Technologies for Urban Safety of Mega Cities in Asia

7 IBD (=W/ΣAc) (N/mm 2 ) Figure 4 IBS-IBSO relationship IBD-IBD01 Relationship IBD=IBD01 Figure 5 I BD -I BD01 relationship IBD01 (=0.4Fc) (N/mm 2 ) A proposal on the simplified structural evaluation method for existing reinforced concrete buildings in Bangladesh

8 IBS/IBSO October 2015, Kathmandu, Nepal 3.00 IBS/IBSO-IBD/IBD01 & IBD/IBD02 Relationship 2.50 IBD=IBD01 IBD=IBD IBS=IBSO IBD/IBD01, IBD/IBD02=1.0 Figure 6 I BS / I BS0 I BD /I BD01 & I BD /I BD02 relationship 6. CONCLUSIONS A simplified structural evaluation method developed by Seki(2015) was able to be applied to the vulnerable buildings in Bangladesh and was verified to be suitable for the preliminary structural screening evaluation for the developing countries. This will also help to fix the priority among lots of vulnerable structures. ACKNOWLEGEMENT We would like to express our deepest appreciation for valuable advice and information by the JICA Expert members as well as for permitting and encouraging this paper presentation by the JICA Bangladesh Office. And we also appreciate the cooperation for execution of structural evaluation by the structural engineers, Public Works Department, Government of the People s Republic of Bangladesh, Dhaka, Bangladesh. REFERENCES Seki, M., A proposal on the Simplified Structural Evaluation Method for Existing Reinforced Concrete Buildings Based on the Japanese Seismic Evaluation Standard visà-vis the International Seismic Code, Journal of Earthquake Science and Engineering, Publisher ISES JBDPA2001. The Japan Building Disaster Prevention Association "Standard for Seismic Evaluation of Existing Reinforced Concrete Buildings (Version 2001) (in Japanese)." BNBC2015 Final Draft. Bangladesh Building Code 2015, Final Draft, July 2015, Housing and Building Research Institute (HBRI) IBC2000. International Code Council, Inc., "International Building Code 2000" March FEMA NEHRP Recommended Provision, New Technologies for Urban Safety of Mega Cities in Asia