Earthquake vulnerability assessment of RCC buildings in Kolkata

International Journal of Current Trends in Engineering & Research (IJCTER) e-ISSN 2455–1392 Volume 2 Issue 5, May 2016 pp. 560 - 567 Scientific Journal Impact Factor : 3.468 http://www.ijcter.com

Earthquake Vulnerability Assessment of RCC Buildings in Kolakata Kaustav Sengupta1, Ajit Kamath2, Indrani Gogoi3 1

Department of Civil Engineering, International Institute of Information Technology, Hyderabad, [email protected] 2 Practising Structural Engineer, New Delhi, [email protected] 3 Department of Civil Engineering, Assam Institute of Technolohy,[email protected]

Abstract— Kolkata is one of the most densely populated cities in India that is susceptible to earthquakes due to the Eocene Hinge subsurface fault running right beneath the city. Kolkata, although falls in zone 3 of the seismic zonation map of IS 1893:2002, is highly susceptible to earthquakes due to the recent earthquake in Nepal in 2015. Rapid Visual Screening has been carried out for buildings in Sreerampur Goria locality of the East Kolkata township, and a statistical analysis has been carried out of the performance scores. Keywords—Seismicity, Vulnerability, Rapid Visual Screening, Performance Score, Cut-off Score, Normal Distribution Function I.

INTRODUCTION

Kolkata is one of the most densely populated cities in India situated on the banks of Hooghly river, with an approximate population of 4.6 million as per UN Data, 2001. The whole region of Kolkata is comprised of medium to soft soil. Tremors were felt in Kolkata during the earthquake that had its epicenter 36 km east of Khudi in Nepal. This earthquake caused widespread devastation in Nepal. Kolkata is around 760 km South-South East from Khudi. Experts all over the world are of the opinion that another major earthquake of comparable magnitude may occur due to the continuous collision of Eurasian & Indian sub-continental tectonic plate. It is a well known fact that the Eocene Hinge Zone, which has been inactive for years, lies right beneath the city of Kolkata. The continuous energy buildup along the fault line may trigger an earthquake, in future, making the medium to soft soil of Kolkata vulnerable to Liquefaction. The East Kolkata township is one of the densest residential pocket of Kolkata. In the township, Sreerampur Goria house the maximum number of people. The quality of the building stock is apparently moderate to poor. Therefore, an initiative was carried out for Earthquake Vulnerability Analysis, and a total of 77 Reinforced Concrete Buildings have been screened in concordance with the Rapid Visual Screening procedure. II.

RAPID VISUAL SCREENING APPROACH

The process of Rapid Visual Screening may be broken down into 6 steps: 2.1. Selection of the region based on Seismicity As explained earlier, the region of Kolkata is selected for survey based on its proximity to the epicenter of the Nepal Earthquake caused in May 2015. The inactive Eocene Hinge Subsurface Fault runs right beneath the city of Kolkata, and the medium to soft soil with a high water table makes Kolkata highly vulnerable to Liquefaction in the event of an earthquake . The following table, obtained from the US Geological Survey, summarizes the details of earthquake having Magnitude >= 4.5 in the last 100 in a square area of 300 km2 with Kolkata at its center. Kolkata is located at latitude 22.5726 & longitude 88.3639.

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International Journal of Current Trends in Engineering & Research (IJCTER) Volume 2, Issue 5; May – 2016 [Online ISSN 2455–1392] TABLE 1: EARTHQUAKE DATA IN A 300 KM2 AREA AROUND KOLKATA FOR THE LAST 100 YEARS

TIME LATITUDE LONGITUDE DEPTH MAGNITUDE MAGNITUDETYPE 2015-12-15 T02:35:15.500Z 23.8301 86.5783 14.79 4.5 mb 2015-04-08 T01:43:47.840Z 21.8203 89.5318 17.68 4.6 mb 2013-08-06 T18:36:30.500Z 23.3104 88.4902 35.88 4.5 mb 2012-03-18 T02:56:13.390Z 23.662 90.259 44 4.5 mb 2009-02-27 T10:42:38.010Z 20.473 89.107 10 4.8 mb 2008-07-26 T18:51:50.970Z 24.788 90.536 17.5 4.8 mb 2007-08-31 T12:06:32.770Z 23.063 90.611 42.5 4.5 mb 2005-11-28 T16:57:13.040Z 21.015 89.158 10 4.7 mb 2001-12-19 T07:54:07.960Z 23.632 90.376 10 4.5 mb 1996-09-24 T03:28:01.860Z 23.342 88.586 36.4 4.6 mb 1994-09-28 T16:46:06.410Z 23.337 88.547 33 4.6 mb 1993-05-16 T09:05:58.650Z 23.101 86.982 33 4.6 mb 1992-07-09 T21:34:02.010Z 21.005 89.973 29.3 5.3 mw 1991-04-26T09:15:51.190Z 20.733 89.563 33 4.7 mb 1989-06-12 T00:04:09.760Z 21.861 89.763 5.8 5.8 mw 1976-06-23 T15:38:39.400Z 21.415 88.79 23 5.3 mb FIGURE 1: FIGURE SHOWING 300 KM2 AREA AROUND KOLKATA


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International Journal of Current Trends in Engineering & Research (IJCTER) Volume 2, Issue 5; May – 2016 [Online ISSN 2455–1392]

2.2. Selection of the locality/sub-region for Survey The Sreerampur Goria locality selected for Rapid Visual Screening has a high density of population with the buildings closely packed together. Therefore, even though the seismicity of the region is low, the probable event of an earthquake makes Sreerampur Goria highly susceptible to losses of life and property rendering it necessary to carry out a vulnerability assessment of the aforementioned region to earthquakes. 2.3. Selection of Buildings to be surveyed The average age of most of the buildings in the Sreerampur Goria locality is approximately 15 years. The apparent quality of buildings is good to moderate with a small percentage of buildings displaying conspicuous cracks. Therefore, to obtain the distribution of scores of buildings, a total of 77 buildings have been screened. 2.4. Rapid Visual Screening of Buildings based on Type of Building & Seismic Zones Kolkata falls into Zone 3 according to the seismic zonation map of IS1893:2002. Considering the high density of population, recent seismic events in Nepal, vulnerability assessment has been carried out for a seismic zone higher i.e. using RVS forms for Seismic Zone 4. 2.5. Calculation of Vulnerability Scores The calculation of Vulnerability scores is based on parameters noted during the screening procedure. The RVS forms have the recorded data of buildings that elucidates the sound structural features like symmetry in mass distribution, good apparent quality, presence of basement that reduce the earthquake vulnerability as well as architectural & structural features like heavy overhangs, falling hazards, plan and vertical irregularities, reentrant corners, soft storeys, short columns, pounding effect, soil conditions large proportion of buildings that prove detrimental in the event of an earthquake. The RVS score can be calculated from the mathematical expression (Jain, 2010): PS= (BS) – Σ[(VSM) x (VS)] Where: PS – Performance Score BS – Base Score VSM – Vulnerability Score Modifier VS – Vulnerability Score The RVS form is as shown below: TABLE 2: PERFORMA FOR EVALUATION OF PERFORMANCE SCORE FOR RCC BUILDINGS

RAPID VISUAL SCREENING OF EARTHQUAKE SAFETY

CALCULATION SHEET

RC FRAME

Falling hazards identifier 'F'

Seismic Zone

Base Score

Stories IV

Marquees/hoardings/roof signs


1 or 2

130

562


AC units/grillwork

3

120

Elaborate parapets

4

100

Heavy elevation features

5

85

Heavy canopies

>5

80

Substantial balconies

Heavy cladding

Structural glazing

Vulnerability Score (VS)

NUMBER OF STOREYS

Soft storey

Vertical irregularities, setbacks, building on slopes

1or2

3

4

5

> 5

Vulnerability Score Modifier (VSM)

0

-15

-20

-25

-30

Absent =0 Present=1

-10

-10

-10

-10

-10

Absent =0 Present=1

Plan irregularities

VS×VSM

None=0

-5

-5

-5

-5

-5

Moderate=1

Extreme=2

Heavy overhangs

-5

-10

-10

-15

-15

Apparent quality

Absent =0 Present=1

Good=0

-5

-10

-10

-15

-15

Moderate=1

Poor=2

Short columns

Poundings

-5

-5

-5

-5

-5

Absent = 0 Present=1

0

-2

-3

-3

-3

Absent = 0


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Unaligned floors= 2

Poor apparent quality of adjacent building = 2

Soil conditions

Medium = 0

10

10

10

10

10

Hard =1

Soft = -1

Frame action

Absent = 0

10

10

10

10

10

Present =1

Not sure = 0

Water tank at roof capacity

Absent = 0

0

-3

-4

-5

-5

Capacity < 5000l= 0.5

Capacity > 5000l=1

Location of water tank

symmetric=0 0

-3

-4

-5

-5 unsymmetrical=1

Basement-full or partial

Absent = 0 0

3

4

5

5 Present =1

∑ (VSMXVS) =

Performance Score = Base Score + ∑ (VSMXVS) =

2.6. Detailed Assessment of Buildings below Cut-off Scores Performance scores for each building is obtained from Rapid Visual screening that may be considered as a Random Variable.. So, a Normal (Gaussian) distribution function is used to find the frequency of different scores. The nature of the graph (RVS score vs. frequency of building having the score) has a peak value for a given RVS score. With this information we can summarize the general performance/vulnerability of the building stock of the given area. FEMA 155 suggests that the Base Score of a building corresponds to 60% damage of the structure. The performance scores @IJCTER-2016, All rights Reserved

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obtained is compared to a “Cut-off” score that is solemnly a score below which the building performance may be deemed unsatisfactory. These buildings may then be subjected to a detailed analysis that would envisage the requirement of retrofitting or suitable repair schemes. Cut-off score is highly dependent on the amount of funds available for retrofitting (FEMA 155), which implies that the the Cut-off score mainly depends on the funds that are available with the local community to repair the seismically vulnerable buildings. FIGURE 2: NORMAL DISTRIBUTION FUNCTION

FIGURE 3: PROBABILITY DISTRIBUTION FUNCTION FOR RVS SCORES (PROBABILITY VS NUMBER OF BUILDINGS)


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III.

OBSERVATIONS

A total of 77 buildings were screened as a part of the Rapid Visual Screening. The Probability Distribution function resembles a Normal Distribution Curve, with a mean value of 102.86 and a standard deviation of 26.34. The distribution table can be shown as: TABLE 3: TABLE SHOWING PROBABILITY DISTRIBUTION VALUES.

Range of buildings μ-3σ

μ-2σ

23.86 0

50.20544 4

0.000 0.000

μ-σ

μ+σ

μ+2σ

76.55 129.23 155.5738 9 51 13 Probability density 0.052 0.117 0.662 0.169 Cumulative Probability density 0.052 0.169 0.831 1.000

μ+3σ 181.9159 0 0.000 1.000

Where, μ: Mean σ: Standard Deviation. In majority of the buildings, Water tank has been located unsymmetrically, which may lead to torsional loads in the event of an earthquake. IV.

CONCLUSIONS

Rapid Visual Screening of buildings in Kolkata suggests a good average performance score of RCC buildings. However, a few buildings have unsatisfactory Performance score which requires details assessment to envisage suitable repair and retrofit schemes. A large number of openings were observed & recorded in the buildings surveyed. However, the RVS form for RCC buildings does not have any provision for recording the Orientation & percentage of openings. The same needs to be incorporated in the RVS forms in the future. Unsymmetrical location of the Water tanks in almost all of the buildings suggests lack of knowledge on part of the planners or residents of the society.

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International Journal of Current Trends in Engineering & Research (IJCTER) Volume 2, Issue 5; May – 2016 [Online ISSN 2455–1392] [9] [10] [11] [12] [13] [14]

SP 6: Part 6: 1972, Handbook for structural engineers - Application of plastic theory in design of steel structures. IS 13920: 1993, Ductile detailing of reinforced concrete structures subjected to seismic forces - Code of practice, Bureau of Indian Standards, New Delhi. FEMA 154: Rapid Visual Screening of Building for Potential Building Hazards, 2002 FEMA 155: Rapid Visual Screening of Building for Potential Building Hazards – Supporting Documentation, 1988 U.S. Geological Survey, https://www.usgs.gov/ Amateur Seismic Centre, Pune, India, http://www.asc-india.org/


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