the sikkim earthquake of 14 th february, 2006

A RECONNAISSANCE BASED VULNERABILITY & DAMAGE SURVEY REPORT

THE SIKKIM EARTHQUAKE OF 14TH FEBRUARY, 2006

SCHOOL OF COMMUNITY SCIENCE AND TECHNOLOGY BENGAL ENGINEERING AND SCIENCE UNIVERSITY, SHIBPUR

A RECONNAISSANCE BASED VULNERABILITY & DAMAGE SURVEY REPORT

THE SIKKIM EARTHQUAKE OF 14TH FEBRUARY, 2006

TECHNICAL VISIT UNDERTAKEN TO

EAST & SOUTH SIKKIM AND DARJEELING 26th March, 2006 — 1st April, 2006

DR. S. C. DUTTA, Professor, Dept. of Civil Engineering

([email protected])

DR. A. L. GUHA, Professor, Dept. of Civil Engineering DR. J. GUHA NIOGI, Asst. Professor, Dept. of Arch. Town & Regional Planning P. MUKHOPADHYAY, Sr. Lecturer, Dept. of Arch. Town & Regional Planning

SCHOOL OF COMMUNITY SCIENCE AND TECHNOLOGY BENGAL ENGINEERING AND SCIENCE UNIVERSITY, SHIBPUR

A RECONNAISSANCE DAMAGE SURVEY REPORT — THE SIKKIM EARTHQUAKE OF 14TH FEBRUARY 2006

FOREWORD Earthquakes are responsible globally for 47% of casualties and 35% of economic losses caused by various natural disasters. Almost no time is available during the event of an earthquake causing massive havoc in less than even a minute. At least two districts of West Bengal may experience earthquakes of intensity more than X in the MMI Scale, and almost all the districts can experience the same of intensity more than VII. This clearly demonstrates the vulnerability level of our State. Furthermore, non-adherence to code-recommended seismic design guidelines is established by the damages caused by the Sikkim Earthquake, which is a warning for thorough preparedness in North Bengal and North-East India. The occurrence of such hazards cannot be prevented, rather all necessary measures are to be taken to reduce their effects, and keep all options of mitigation open to tackle any such eventualities. The principle underlying is a balanced compromise between economy and safety that will be relied for a seismic design. Thus, long term preparedness is the only way to combat this natural hazard. Thorough reconnaissance survey, to identify the reasons of weakness of the damaged structures followed by examination of each structure to identify their weaknesses and to fix up the strategy for their retrofitting, is the need of the hour. Subsequently training programmes are to be organized for percolating adequate technical know-how in the practicing level; adequate techno-legal measures should also be devised for incorporating earthquake-resistant architectural and planning features. In this context, the Bengal Engineering and Science University, Shibpur with the help of a group of distinguished faculty members, wholeheartedly desires to extend their expertise to cater to the need of minimizing the devastating effect of such calamities. I welcome this well-timed endeavour of the School of Community Science and Technology of the University in conducting this Technical Visit, and congratulate Prof. S. C. Dutta and his colleagues for preparing this very useful reconnaissance based vulnerability and damage survey report. I hope this will be a proper prelude for augmenting R & D work in this area under the School of Disaster Mitigation created at this University. The University will provide all necessary support for developing research and academic activities at the School.

(DR. N. R. BANERJEA) VICE - CHANCELLOR BENGAL ENGINEERING AND SCIENCE UNIVERSITY, SHIBPUR

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PRELUDE After the Sikkim Earthquake of 14th February 2006, the School of Community Science and Technology, Bengal Engineering and Science University, Shibpur decided to organise an One-Week Technical Visit from 26th March 2006 to 1st April 2006 in Gangtok and Darjeeling. The objective of the Technical Visit was to assess the nature and extent of damage suffered by the engineered as well as the non-engineered buildings of earthquake affected areas. The investigation, being a multidisciplinary one, was carried out by a team of faculties with different backgrounds — (i) Dr. Sekhar Chandra Dutta, Professor, Dept. of Civil Engg.; (ii) Dr. Ajit Lal Guha, Professor, Dept. of Civil Engg.; (iii) Dr. Jayita Guha Niogi, Assistant Professor, Dept. of Arch. Town & Regional Planning; and, (iv) Sri Parthasarathi Mukhopadhyay, Senior Lecturer, Dept. of Arch. Town & Regional Planning. The reconnaissance based Vulnerability and Damage Survey Report, suggesting the measures to be adopted for retrofitting the different types of damaged buildings, is a follow up of the Technical Visit. It is prepared by Dr. Sekhar Chandra Dutta, Professor, Dept. of Civil Engg. and Shri Parthasarathi Mukhopadhyay, Sr. Lecturer, Dept. of Arch. Town & Reg. Plng., with assistance from Dr. Prithwish Kumar Das, Asst. Professor, and Shri Rana Roy, Lecturer, both of the Dept. of Applied Mechanics. The Technical Visit and the Vulnerability & Damage Survey Report are supported by the Technical Efficiency and Quality Improvement Programme. The submission of the Report is intended to be followed up by a State Programme for Capacity Building of Civil Engineers and Architects in Earthquake Risk Management with support from the Ministry of Home Affairs, Government of India, for which six faculties of the University have already been identified as Resource Persons trained at National Resource Institutions through NPCBEERM and NPCBAERM. In pursuant with its objective, the School of Community Science and Technology is committed to promote similar activities in future interfacing technology with society.

(DR. N. R. BANDOPADHYAY) PROFESSOR & DIRECTOR SCHOOL OF COMMUNITY SCIENCE AND TECHNOLOGY BENGAL ENGINEERING AND SCIENCE UNIVERSITY, SHIBPUR

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PREFACE An earthquake of magnitude 5.7 violently rocked Sikkim and North Bengal on 14th February 2006, which though did not cause major casualties, but had resulted in considerable structural damage. The intensity of the earthquake as experienced by various parts of Gangtok city as well as its surrounding region in East and South Sikkim was in the range of 5 to 6 as per MSK. On the other hand, this area comes under the earthquake Zone IV where a severe seismic intensity is generally expected. In this context, the present earthquake can be treated as a warning only, in view of the widespread building construction undertaken in different hilly centres of Sikkim and North Bengal, many of which are not properly designed. The extensive structural damage due to the earthquake this time clearly exhibits the level of vulnerability of the existing structures. This report makes an attempt to assess the vulnerability of the existing structures constructed in the hilly regions of Sikkim as well as Darjeeling through a reconnaissance survey of the damaged as well as undamaged structures. Certain recommendations to reduce the vulnerability of the structures are also included. We are grateful to the School of Community Science and Technology, Bengal Engineering and Science University, Shibpur for supporting the investigation. We would like to thank Mr. J. N. Dhakal, Additional Secretary, Department of Land Revenue and Disaster Management, Govt. of Sikkim; ADIG, Police Head Quarter, Gangtok; Mr. Sonam Daju Bhutia, Deputy Chief Architect, Building and Housing Department, Govt. of Sikkim; and, Mr. R. C. Rangre, Executive Engineer, Central Public Works Department, Gangtok for extending their co-operation during the technical visit to Sikkim. I, on behalf of the team, would especially thank Mr. A. K. Bal, Asst. Engineer and Mr. S. Kundu, Jr. Engineer of the Gangtok Central Sub-Division I, Central Public Works Department, without whose help the Visits at South and East Sikkim would not have become viable. We would also like to thank Mr. D. Chakraborty, Executive Engineer, Public Works Department, Darjeeling, Govt. of West Bengal, for extending his co-operation, especially in tracing the history of major earthquakes at Darjeeling.

(DR. S. C. DUTTA) PROFESSOR DEPARTMENT OF CIVIL ENGINEERING BENGAL ENGINEERING AND SCIENCE UNIVERSITY, SHIBPUR

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CONTENTS 1.

INTRODUCTION … … … … … … … … … … … … … … … … … … … … … … … …

1

2.

TERMINOLOGY … … … … … … … … … … … … … … … … … … … … … … … …

3

3.

TECTONIC FEATURES … … … … … … … … … … … … … … … … … … … … … …

7

4.

DAMAGE PROFILE … … … … … … … … … … … … … … … … … … … … … … …

10

5.

DAMAGE ANALYSIS & RECOMMENDATIONS … … … … … … … … … … … … … …

24

ANNEXURE 1 — PHOTOMONTAGE OF THE STRUCTURAL DAMAGES … … … …… … … … …

32

ANNEXURE 2 — PREVIOUS MAJOR

……. …. ….. … … …

41

REFERENCES … … … … … … … … … … … … … … … … … … … … … … … … … … …

43

DARJEELING EARTHQUAKES

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1.0 I N T R O D U C T I O N A moderate earthquake of magnitude 5.7 measured on Richter scale rocked Sikkim on 14th February, 2006. The tremor also shakes North Bengal, however, no loss of life or property was reported. Two army havildars, Rajinder Kumar and Virender Singh, got succumbed to injury as their vehicle got struck by a boulder set loose during the tremors at the Sherathang international trade mart below Nathu-la. The earthquake, which had its epicentre in Chungthang, 75 km north of Gangtok, lasted 22 seconds and caused extensive damage to property. The old Raj Bhavan structure has been declared unfit. “Cracks have developed in the ceiling above the main staircase and there are fractured columns all around,” said N. T. Bhutia, the Raj Bhavan Public Relations Officer. Government staff primarily refused to enter the Tashiling Secretariat when they saw the extensive base-level damages. Enchey Monastery, Sikkim Assembly building and the Police Head Quarter at Gangtok have reportedly developed cracks. State Bank of India’s branch at Zero Point had got forced to close for the day due to the large in the ceilings and walls. Chief Secretary N.D. Chingapa declared forming “a task force to assess the damage and take precautionary measures. We have also decided not to allow any new building to be constructed above four storeys”. Despite no major casualties, Mr. K. N. Sharma, Relief Commissioner, Department of Land Revenue and Disaster Management, Government of Sikkim, in his Revised Memorandum for

Restoration Works of Earthquake Damages of 14.02.2006, Sikkim, has claimed that more than 500 buildings have been damaged by the earthquake. In view of the same, he has sought to the Government of India a Central Assistance of Rs. 3,016.15 lakhs for different Government Sectors including Building, UDHD, W. S. & PHED, Health, HRD, Energy & Power and DLRC East. According to the opinion of the Geological Survey of India, Gangtok, the epicentre was 46 km away along North East from the observatory of Indian Meteorological Department at Tadong near Gangtok. UNDP Bubaneswar reports the same earthquake to have the epicentre at 25 km west-north of Gangtok, 11.1 km north east of Kewzing, 35.5 km from north west of Kalimpong, 38.6 km north-east of Darjeeling, 76 km north of Siliguri, 138 km of Thimpu and 67.5 km north-east of Elam (Nepal). Though Darjeeling and Kalimpong were not far away from the epicentre, but hardly any structural damage has been reported at these two places while most of the structural damages were concentrated in Gangtok and its surrounding regions. The people in Gangtok felt very severe shaking. The furniture was displaced from their positions. Books and other loose objects were thrown from shelves. Hanging lights and fans started swinging. On the other hand, New Jalpaiguri which was about 120 km away from

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Gangtok seems to have experienced almost an intensity of 5 as understood from the awakening of sleeping persons and displacements of small unstable objects. So, less change in intensity over a large distance indicates geological strata has a better transmissibility of the seismic waves and increases the concern about the seismic vulnerability of entire Sikkim as well as North Bengal region. Such an earthquake has also another specialty of being followed by a large number of aftershocks which approximately have their epicentre about 27 km away from the observatory of the Indian Meteorological Department. Tremors from about 10 such earthquake aftershocks were experienced. The objective of the present report is to identify the various forms of damages experienced by the structures and thereby pointing out the inadequacy of the design and construction practices. Some suggestions to appropriately take care of such structural weaknesses have also been put forward.

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2.0 T E R M I N O L O G Y 2.1 CRACKS, FLEXURAL Flexure or bending characterizes the behaviour of a structural element subjected to a lateral load, viz. beam. Flexure produces reactive forces inside a beam as the beam attempts to accommodate the flexural load. This leads to cause compression at the top and stretching at the bottom of the member in case of sagging curvature of the member. On the contrary, in case of hogging curvature, tension is developed at the top and compression at the bottom. The resulting deformation of the structural element undergoing flexure may produce cracks known as flexural cracks. 2.2 CRACKS, SHEAR When a force system consisting of two equal and opposite parallel forces not in the same line, acts on any element of a structural component, there is a tendency for one part of the element to slide over or shear from the other part across the section. This phenomenon of sliding that may lead to, depending on the relative intensity of the applied loading, tearing across the section of an element of a structure is called shear, and, the resulting crack is called as shear crack. 2.3 DAMAGE, NON-STRUCTURAL / ARCHITECTURAL The non-structural damage refers to the damage that does not affect the strength and stability of the structure on the whole. Such damage, viz., cracking and overturning of masonry parapets, large cantilever cornices; falling of plaster from walls and ceilings; cracking and overturning of partition walls, filler walls; cracking of glass panes; falling of loosely placed objects, overturning of cupboards, etc. occurs quite frequently even under moderate intensities of earthquake. In fact, the same is permitted in the existing seismic design philosophy. 2.4 DAMAGE, STRUCTURAL A structural damage is caused by the collapse of a structure, that is, when vertical and / or lateral load carrying elements, such as beams, columns or shear walls fail. Structural failure refers to loss of the load-carrying capacity of a component or member within the structure or of the structure itself. Structural failure is initiated when the system, in part or whole, is stressed to its strength limit, thus causing fracture or excessive deformations. The ultimate failure strength of the component or system is its maximum load-carrying capacity. In a well-

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designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure. 2.5 EARTHQUAKE, INTENSITY — MODIFIED MERCALLI INTENSITY SCALE The intensity of an earthquake at a place is a measure of the strength of shaking during the earthquake at that particular location. The intensity scale consists of a series of certain key perceptions and responses such as people awakening, movement of furniture, damage to chimneys, total destruction etc. The currently used intensity scale is the Modified Mercalli Scale or MSK Scale of seismic intensities. The scale is composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction. 2.6 EARTHQUAKE, MAGNITUDE — RICHTER SCALE Magnitude of an earthquake is usually expressed in terms of the Richter Magnitude Scale, developed by a seismologist named Conrad Richter, which involves measuring the amplitude of the largest recorded wave at a specific distance from the earthquake. The magnitude of earthquake is a number, which is a measure of energy released in an earthquake and is defined as logarithm to the base 10 of the maximum trace amplitude, expressed in microns, which the standard short-period torsion seismometer (with a period of 0.8 s, magnification 2800 and damping nearly critical) would register due to the earthquake at an epicentral distance of 100 km (IS: 1893: 2002). It may be worth mentioning that the magnitude of Alaska Earthquake in 1964 was about 8.6 whereas the energy released in the catastrophic Hiroshima bomb blast is equivalent to about 5.5 only. This parameter does not, however, indicate the expected damage at a particular place. 2.7 EARTHQUAKE RESISTANT DESIGN, PHILOSOPHY Earthquake resistant design philosophy, accepted worldwide, allows the structures to respond elastically only under minor to moderate earthquakes, while the same is supposed to withstand an earthquake of large magnitude through vibration in the post-elastic range without undergoing collapse. Such design principle, terminologically known as duel-design philosophy, is perceived to make a balance between safety and economy. 2.8 FRICTION PILES Pile foundations are parts of a structure used to carry and transfer the load of the structure to the bearing ground located at some depth below ground surface. Friction pile is a type of pile foundation which transfers this load, principally through skin friction. Load carrying capacity of the piles is derived mainly from the adhesion or friction of the soil in contact with the shaft of the pile. Friction piles resist vertical, lateral and uplift load.

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2.9 MICROZONATION For the purpose of determining seismic forces, India is classified into four seismic zones. But different separate areas in a given zone may have their particularity in nature of soil, slope etc. that may effect its performance under earthquake. The identification of these separate individual areas within a zone having different potentials for hazardous earthquake effects is known as microzonation. 2.10 POUNDING The phenomenon of collision between two adjacent buildings, constructed in close proximity to each other, during strong shaking of an earthquake is known as pounding; and this may cause substantial damage. Pounding may result due to irregular response of buildings of different heights, local damage to columns as the floor of one building collides with columns of another, collapse of damaged floors, and in many cases collapse of entire structures. Again, several examples can be cited where buildings in close proximity apparently supported one- another rather than resulting in damage. In most cases, the buildings are of similar story and total height, of similar stiffness, and located sufficiently close that pounding impacts are of relatively low energy. 2.11 REPAIR The main purpose of repairs is to bring back the architectural shape of the building so that all services start working and the functioning of building is resumed quickly. Repairs do nor pretend to improve the structural strength of the building and can be very deceptive for meeting the strength requirements of the next earthquake. 2.12 RESTORATION The main purpose of restoration is to carry out structural repairs to load bearing elements of structures for restitution of the strength that they had before the damage occurred. This type of action must be undertaken when there is evidence that the structural damage can be attributed to exceptional phenomena that are not likely to happen again and that the original strength provides an adequate level of safety. 2.13 SEISMIC RETROFITTING OR STRENGTHENING OF EXISTING BUILDING Many existing buildings do not meet the seismic strength requirements of present earthquake codes due to original structural inadequacies and material degradation due to time or alterations carried out during use over the years. Their earthquake resistance can be upgraded to the level of the present day codes by appropriate seismic retrofitting techniques.

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2.14 SEISMICITY The frequency or magnitude of earthquake activity in a given area is termed as its seismicity. 2.15 TECTONICS FEATURES The nature of geological formation of the bed rock in the earth’s crust revealing regions characterized by structural features, such as dislocation, distortion, faults, folding, thrusts, volcanoes with their age of formation, which are directly involved in the earth movement or quake resulting in the above consequences.

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3.0 T E C T O N I C F E A T U R E S 3.1 THE REGIONAL SETTING — GEOGRAPHICAL FEATURES Sikkim is a small beautiful state of India in the Eastern Himalayas with steep mountains and deep valleys. It lies between latitudes 27° 5' N to 20° 9' N and longitudes 87° 59' E to 88° 56' E. It is wedged between Nepal in the west and Bhutan in the east and China in the north and northeast. In the south it shares its Indian border with the state of West Bengal. The longest north-south distance is about a hundred kilometres and the east-west breadth ranges between 60 and 70 kilometres. Its total area is 7, 299

square

kilometres.

Encircled

by

three

international boundaries, this strategically located

FIG. 3.1: DISTRICTS OF SIKKIM

state consists of for districts, viz. East Sikkim, West Sikkim, North Sikkim and South Sikkim. Spanning Sikkim’s western borders are the Khangchendzonga and the Singalila Range, a northsouth spur of the Great Himalaya. The northern limit, which reaches out to the Tibetan Plateau, is straddled by the Donkia Range while the eastern flank is bounded by the Chola Range. The average steepness is about 45 degrees. Sikkim encompasses the Lesser Himalaya, Central Himalaya and the Teethes Himalaya. Although the trend of Great Himalaya is to run across in an east-west direction, the two ridges demarcating Sikkim’s eastern and western sides, the Chola and the Singalila, follow a north-south pattern. Across the middle, another north-south ridge of lesser elevation separates the Rangeet Valley from the Teesta Valley. The major mountain peaks of Sikkim are: (i) Khangchendzonga – 8,846 m., (ii) Jonsang – 7,444 m., (iii) Talung – 7,351 m., (iv) Kabru – 7,338 m., (v) Siniolchu – 6,887 m., (vi) Pandim – 6,691 m., (vii) Rathong – 6,680 m., (viii) Koktang – 6,148 m, and, (ix) Simvo – 6,811 m. Sikkim’s two major rivers are Teesta and Rangeet. The turbulent Teesta, which has its source at the Chho Lhamu Lake in the Tibetan Plateau is an unseeming little stream at first, but gradually swells into a raging river as more tributaries converge into its path as it snakes through deep mountain valleys into the plains of Bengal. The gentler Rangeet has its source at the Rathong Glacier south of the Khangchendzonga massif. It meets with the Teesta at the valley dividing Sikkim and Bengal.

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3.2 GEOLOGICAL SET UP The Himalayan Mountains were created by the collision of India with Asia which began 60 million years ago. The range is presently characterized by high rates of seismicity and deformation, and widespread exposure of post-collisional, high-grade metamorphic rocks implying long-term, large-scale vertical transport. Over the past 30 years, numerous models have been proposed to explain the key tectonic features of the Himalayas. According to these models, in the Sikkim region of north-east India, the Main Central Thrust (MCT) juxtaposes high-grade gneisses of the Greater Himalayan Crystallines over lower-grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large-scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. Although the MCT has long been thought to be an early Miocene structure, rocks within the broad associated shear zone were recently reported to have experienced metamorphism later than previously thought.

FIG. 3.2: SAMPLE GEOLOGICAL TRAVERSE MAP OF SIKKIM HIMALAYAS SHOWING PLACEMENT OF THE BOUNDS OF MCT SHEAR ZONE (MCT & MCT-1) BASED ON STRUCTURAL OBSERVATIONS AND MONAZITE AGES

Source: Catlos et al, JMG 2004

All these factors contribute to the fact that the slopes are highly susceptible to weathering and prone to erosion, which along with intense rain, causes extensive soil erosion and heavy loss of soil nutrients through leaching. These, combined with the high rates of seismicity, cause frequent landslides, isolating the numerous small towns and villages.

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3.3 SIKKIM MICROZONATION The Sikkim and adjoining region forming a part of the Himalayan mobile belt exhibit considerable seismic activity. The region is located in Zone IV according to seismic zoning map of India where maximum intensity of VIII is expected. Significant earthquakes that have occurred here in the past: — (a) 1934 Bihar-Nepal Earthquake of magnitude 8.4, (b) earthquake of 1965 of magnitude 5.9, (c) earthquake of 1980 of magnitude 6.0, (d) earthquake of 27th September, 1988 of magnitude 6.7. The first three have occurred in the Sikkim Himalayas and the one in 1988 occurred in the FIG. 3.3: MICROZONATION MAP OF SIKKIM

foothills.

Source: DST, GoI

The following table shows the recorded history of earthquake of intensity more than 5 in the Richter Magnitude Scale: — SL. NO.

1.

DATE

EPICENTRE

MAGNITUDE

LATITUDE

LONGITUDE

th

27.60

88.30

5.2

th

30 August, 1964

2.

12 January, 1965

27.60

88.00

5.9

3.

21st August, 1972

27.20

88.00

5.1

th

4.

19 November, 1980

27.39

88.75

6.0

5.

5th April, 1982

27.42

88.86

5.1

6.

27th September, 1988

27.17

88.29

6.7

th

7.

25 September, 1996

27.43

88.55

5.0

8.

2nd December, 2001

27.15

88.17

5.1

27.35

88.36

5.3

9.

th

14 February, 2006

This context prompted for the preparation of a microzonation map of Sikkim. With support from the Seismology Division, Ministry of Science & Technology, Department of Science & Technology, Government of India, a seismic hazard assessment of Sikkim Himalaya was carried out with the help of site response studies, factor analysis and computing response characteristics by IIT, Kharagpur. This resulted into a Seismic Microzonation map of Sikkim integrating various data like site factor, geology, soil types, slope, peak ground acceleration and resonant frequency.

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4.0 D A M A G E P R O F I L E A large number of structures in the Gangtok city and in the surrounding regions have undergone damage in both structural and in non-structural damages in government as well as private buildings. Details of damage of some of the significantly damaged buildings are described below. 4.1 RAJ BHAWAN BUILDING This building is an old two-storeyed building. The walls as well as columns of the building are constructed with chisel dressed stone and mud mortar. The columns of the building connected by reinforced concrete beams are observed to be severely damaged. One such damaged column is shown in figure 4.1. The cracks clearly show that there is no reinforcement within the column section. Further, the infill walls are found to be separated from the frame, primarily due to out-of-plane rotation. This implies a lack of adequate bond between the infill wall and the frame. Shear cracks are also found at various places of the wall.

FIG. 4.1: DAMAGED COLUMN IN THE RAJBHABAN COMPLEX — DEEP PERIPHERAL CRACK SHOWS THAT THERE IS NO REINFORCEMENT IN THE COLUMN

While retrofitting, the columns should be re-constructed by concrete with adequate reinforcements. The joints of such reinforced concrete members with beam should be adequately made with proper confining reinforcements as suggested in IS 13920. The damaged infill wall should be strengthened by providing vertical reinforcements at the sides of the openings. Such reinforcements should be adequately anchored in beam or slab to provide a monolithic behaviour with the walls. Further, they will also provide a confining action to the

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infill materials and improve the performance in future earthquake. Some other types of failure observed in various parts of this building are also indicated in figures A -1.1 and A -1. 2 of Annexure 1. 4.2 LEGISLATIVE ASSEMBLY The assembly house is a Reinforced Concrete frame-building in a hexagonal shape. The main hall of the building is consisting of eight large sized columns arranged in circular pattern in the centre of the building. The columns are having overhanging beams at the top supporting the domical roof. The roof of the central hall is sloping in all the directions with a rectangular configuration in the centre. The roof of the rooms around the central hall is flat Reinforced Concrete slab. The building is situated on a hill slope with perimeter columns having isolated foundations at different levels. The down-slope columns, on the rear side of the building, are four-storey tall while the up-slope columns, towards the front side of the building, are twostorey tall. The building has thick partition wall in brick masonry, which is resting directly over

FIG. 4.2.1: HORIZONTAL CRACKS IN ROOF SUPPORTING BEAMS OF LEGISLATIVE ASSEMBLY

the floor slab at the first floor level on the rear side of the building. The building experienced damage earlier and was repaired. However, such cracks are observed to be aggravated in this occasion. These cracks are generally in the vicinity of the perimeter columns and confined in the infill wall. The floor of the building on the rear side has experienced a settlement by about a few inches. View of some cracks in roof supporting beams due to such settlement are presented in figures 4.2.1 and 4.2.2 Some horizontal cracks in infill walls of the Legislative

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FIG. 4.2.2: VERTICAL CRACKS IN ROOF SUPPORTING BEAMS OF LEGISLATIVE ASSEMBLY

Assembly building are also presented in figures A -1.3 and A -1.4 to have a better insight of damage. 4.3 TASHLING SECRETARIAT BUILDING This building is a five storey Reinforced Concrete framed structure. The infill walls are made of chisel dressed stone masonry at ground floor. The infill walls at the upper floors consist of masonry made with mortar blocks. These masonry walls have two layers with a hollow portion in between, probably for the purpose of insulation.

FIG. 4.3.1: TASHLING SECRETARIAT BUILDING

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FIG. 4.3.2: SHEAR CRACK ON THE WALL OF TASHLING SECRETARIAT BUILDING

The primary nature of the damage is of non-structural type. The infill wall has been separated from the actual frame of the building. In addition to that they have undergone considerable shear cracking (refer to figure 4.3.2) accompanied by local failure at places as shown in figure 4.3.3. The VIP block side of the building is found to have more non-structural damage, including out of plane action of the infill walls. In fact, considerable pounding observed at the separation joint indicates that the relative movement of one side is higher than the other.

FIG. 4.3.3: LOCAL FAILURE OF THE HOLLOW WALLS MADE OF MORTAR BLOCKS IN THE TASHLING SECRETARIAT BUILDING

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The damage of the infill wall should be retrofitted by providing reinforcements to the extent possible apart from the patch repairing through mortar grouting. On the other hand, separation joint should be made clear enough by removing any mortar or stiff structural material. Different types of failure as observed in the various parts of Tashling Secretariat building are presented in figures A -1.5 and A -1:6 of Annexure 1. 4.4 ENCHAY MONASTERY The Enchay Monastery is an old Buddhist monastery situated just above the township of Gangtok. An important seat of the Nyingma order, this 200 year old monastery has in its premises images of Gods, Goddesses and other religious objects. Lama Drupthob Karpa is supposed to have built a small hermitage at the spot he reached after he flew from Maenam Hill in South Sikkim. Later during the reign of Sikyong Tulku (1909-1910), the present monastery was built in the shape of a Chinese Pagoda. This heritage structure was primarily constructed by stone masonry in 1840.

FIG. 4.4.1: ENCHAY MONASTERY: A WELL KNOWN HERITAGE STRUCTURE IN GANGTOK

Photo Courtesy: Sikkim State PWD

The walls of the monastery structure are about 500mm thick. Inside the masonry walls of this two storey structure, the timber frame is provided. Such frame consists of four timber columns connected by timber beams in both of the principal directions. The floor consists of wooden beams and planks, while the roof is made of Galvanised Cast Iron sheeting supported by timber trusses. The masonry wall of this structure has been severely undergone shear cracks (refer to figure 4.4.2). These cracks have been generally propagated from the openings and primarily created due to diagonal tension generated for the action of lateral

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seismic shear. While repairing, the diagonal lintel band and vertical reinforcements should be provided adequately to avoid such shear as well as out of plane failure of masonry walls. The decorative plaster inside the monastery has cracked and spilled at several locations primarily because of failure of the masonry walls. This has spoilt rich heritage of the structure.

FIG. 4.4.2: SHEAR CRACKS DEVELOPED IN THE MASONRY WALLS OF ENCHAY MONASTERY

FIG. 4.4.3: DAMAGES OCCURRED THE MASONRY WALLS OF ENCHEY MONASTERY

Before repairing such decorative plaster and other decorative and ornamental works, extensive cement grouting should be done to improve the local strength of the different cracked regions. Some other types of failures, observed in various parts of this building, are indicated in figures A -1.7 to A -1.13 of Annexure 1.

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FIG. 4.4.4: DAMAGES OCCURRED IN DECORATIVE AND ORNAMENTAL PLASTER MAINLY DUE TO SHEAR CRACKS DEVELOPED IN THE MASONRY WALLS OF THE ENCHAY MONASTERY

4.5 POLICE HEAD QUARTER BUILDING This three-storeyed Reinforced Concrete framed building has partly open ground storey provided for parking while the other portion is having basement. The portion adjacent to the basement has undergone considerable damage. Primarily, the infill walls are damaged. The natures and reasons of damage are very similar to those observed in earlier cases as may be understood from figure 4.5.1. Partly open ground storey has not only made this storey weaker than the other

storeys,

it

has

also

introduced

asymmetry endangering the structure in the event

of

an

earthquake.

Some

other

structural damage features in various parts of this building are presented in figures A 1.14, A -1.15 and A -1.16 of Annexure 1.

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FIG. 4.5.1 : POLICE HEAD QUARTER BUILDING WITH PARTIALLY OPEN GROUND STOREY


FIG. 4.5.2 : VERTICAL CRACKS IN THE INFILL WALLS OF THE POLICE HEAD QUARTER BUILDING

FIG. 4.5.3 : HORIZONTAL CRACKS IN INFILL WALLS OF THE POLICE HEAD QUARTER BUILDING

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4.6 OFFICE BUILDING OF THE GEOLOGICAL SURVEY OF INDIA AT DEORALI This office is housed in a four-storeyed building at Deorali. The building has a regular beamcolumn grid at the front side; whereas, some of these frames are not continuous at the backside. This has introduced irregularity in plan as well as elevation of the building, which causes severe damage at its backside (refer to figure 4.6.2). This comprises of damage of beam-column joints leading to distortion of the frame, which in turn, has caused crushing of the brick walls at many places. Attempts have been made to repair the cracks by mortar. However, such repairing did not involve any attempt for structural strengthening. The partially open ground storey behaving as soft storey, the asymmetry in plan and elevation, as well as the FIG. 4.6.1: OFFICE BUILDING OF THE GSI AT

discontinuous

DEORALI WITH PARTIALLY OPEN GROUND STOREY

backside

frames

are

major

reasons of vulnerability of the building. The

damages of the Office Building of the Geological Survey of India are further presented through figure A -1.17 of Annexure 1.

FIG. 4.6.2: DAMAGED INFILL WALLS AT THE BACKSIDE OF OFFICE BUILDING OF THE GSI AT DEORALI

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4.7 GOVERNMENT SECONDARY SCHOOL, SICHEY, EAST SIKKIM The Government Secondary School building at Sichey, East Sikkim was having an open corridor at the ground storey which introduced asymmetry to the structure. Figure 4.7.1 clearly shows that columns at this flexible or weak side have been damaged severely. The reason seems to be simple that the effect of asymmetry was probably not considered in the design through adequate structural design.

FIG. 4.7.1: DAMAGED STRUCTURE OF GOVERNMENT SECONDARY SCHOOL BUILDING AT SICHEY, EAST SIKKIM HAVING ASYMMETRY AT GROUND STOREY LEVEL

FIG. 4.7.2: DAMAGED INFILL WALL IN SICHEY SCHOOL BUILDING SHOWING OUT-OF-PLANE ROTATION

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The nature as well as reason of non-structural damage is similar to all other structures. Figure 4.7.2 illustrates how primarily out-of-plane rotation has caused their separation from the main structural frames and the shear cracking has caused further damage among them. The provision of vertical reinforcements and horizontal reinforced concrete bands could have solved the problems. 4.8 GOVERNMENT OF INDIA PRESS OFFICE The main building of this office is a newly constructed three-storeyed building and no significant damage is noticed. However, the canteen is housed in an old one storeyed building which was constructed by mortar block masonry. These mortar wall as well as floor has undergone damage. The architectural repairing may be adequate to retrofit the damage of such buildings. 4.9 AUDITOR GENERAL QUARTER COMPLEX, DEORALI This is a four-storeyed residential complex with Reinforced Concrete framed structures. The infill walls consist of hollow walls made up of mortar block masonry. In addition to shear cracks in infill walls likewise the other structures, there are flexural cracks developed in the columns (refer to figure 4.9). Perhaps the adequate seismic load was not considered in the design of such Reinforced Concrete framed structures or the adequate section or reinforcements with detailing were not provided. One of the other type of structural damage as observed in this building is shown in figure A -1.18 of Annexure 1.

FIG. 4.9 : FLEXURAL CRACK IN COLUMNS AT AUDITOR GENERAL QUARTER COMPLEX, DEORALI

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4.10 GENERAL POOL QUARTERS These are four-storeyed buildings constructed for central government employees near Government of India Press. These buildings seem to have undergone sliding as a whole perhaps due to the lack of adequate gripping of the foundations with the rock bed or the prepared base with stone masonry. 4.11 A PRIVATE BUILDING AT DEORALI, NH 31A Generally, the private buildings are designed with less financial investment and that is why the damages in the private buildings are found to be much more than government buildings. This building can be considered as a typical example of the same. While the damage in most of the government buildings were limited to the non-structural elements, both structural and nonstructural elements are found to be considerably damaged in private or semi-private buildings. This private building is having four storeys on the backside while two storeys at the front. The overall asymmetry as well as irregular placement of columns, which are alike features of many buildings at hilly regions, are the prime structural deficiencies. This building, which is situated at one side of the National Highway 31A, is severely damaged due to earthquake and the damage was primarily concentrated in the backside of the building (refer to figure 4.11.1), which seems to be structurally deficient due to lack of adequate framing action. The columns are severely damaged and, of course, the infill walls. In fact, this building structure may be considered as a typical

representative

of

the

severe

damage that many private buildings have undergone

almost

due

structural

deficiencies

to

the

same

as

mentioned

above. The columns are found to be devoid of adequate lateral ties which has caused the concrete to be easily bulged out from the core (refer to figure 4.11.2). The damage of the infill walls includes outof-plane action of the walls resulting in its separation from the frame, and shear

FIG. 4.11.1: THE DAMAGED PRIVATE BUILDING OF

cracking from the openings. This nature of

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MR. SHARMA AT DEORALI, NH 31A


damage

is

structures

very and

common hence,

with

the

other

remedial

measure should also be similar. However, in some places, the wall has been very severely damaged exposing the mortar blocks (refer to figure 4.11.3). These figures also point out to the fact that the infill walls are not only made by mortar blocks of inadequate strength, there is also a lack of adequate cementing material because of poor constructions. The figure shows clear gaps between the mortar blocks. Such inadequate quality has been the major reason of failure of many infill walls including this particular building. Furthermore, early failure of such infill FIG. 4.11.2 : DAMAGED COLUMNS AND INFILL WALLS IN THE PRIVATE BUILDING AT DEORALI

walls may be due to transfer of more lateral load to the columns leading to

severe damage of the same. This has been a typical feature in many other private buildings. Some other types of failure observed in various parts of this building are indicated in figures A-1.19 to A-1.21 of Annexure 1.

FIG. 4.11.3: INFILL WALL WITH VERY LESS CEMENTING MATERIAL & MORTAR BLOCK

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4.12 NON-ENGINEERED BUILDINGS AT THE HILLS It was observed at both Gangtok and Darjeeling that many-a-time buildings constructed with bamboo and other ‘non-engineered materials’ sustained the aftermath of earthquakes rather better than those built with the ‘engineered’ materials. The point is best illustrated through figures 4.12.1 – 4.12.3, in which one can find an undamaged private residence built with non-engineered materials situated at the rear side of the damaged private building at Deorali, NH 31. The light weight of bamboo and its high tensile strength may explain the reason. However, this warrants further investigation. The undamaged private residence built with non-engineered materials

Damaged Private Building at Deorali, NH 31

The undamaged private residence viewed

Further close-up of the

through the damaged private building

undamaged private residence

FIG. 4.12.1 – 4.12.3: VIEWS OF THE UNDAMAGED PRIVATE RESIDENCE BUILT WITH NON-ENGINEERED MATERIAL, SITUATED JUST BEHIND THE DAMAGED PRIVATE BUILDING AT DEORALI

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5.0 D A M A G E A N A L Y S I S & R E C O M M E N D A T I O N S — The general nature of vulnerability of the structures in Sikkim region can be attributed to the following common reasons valid for many structures constructed in the region: — 5.1 The foundations of many buildings are just resting on hard rock (refer figure 5.1). No further excavation within the rock bed is made because the surface of the hard rock layer generally has sufficient bearing capacity. Generally filled-up by soil is observed up to about 1.0 to 1.5 metre above the rock level. This soil layer is generally retained by constructing masonry wall at the edge of the hill bed. These masonry walls are made of stone masonry using local stones with adequate chiselling. The slope of such retaining wall is maintained as – one horizontal : four vertical. Such kind of foundations may not be adequate to provide resistance against sliding and overturning. To stop sliding, shear keys may be provided. Overturning may be avoided by provision of friction piles (as shown in figure 5.2). Lateral Force

Strong foundation

Strong foundation

No gripping

Rock Bed

No gripping

Rock Bed v

FIG. 5.1: BUILDING RESTING ON ROCK BED

FIG. 5.2: PROVISION OF FRICTION PILES TO AVOID SLIDING AND OVERTURNING

5.2 In most of the buildings, the masonry made by mortar block is used. The material used as cementing mortar seems to be of very poor quality and at the same time the mortar blocks seem not to have adequate strength. Infill panel made by this kind of masonry is performing very poorly undergoing diagonal shear cracks. Furthermore, the strength may be poorer due to use of hollow mortar blocks. This problem of using poor quality masonry made of mortar block for structures is in common at Darjeeling as well as Sikkim. These different inadequacies may be avoided by following the following methodologies :— (a) The infill walls are very frequently found to get separated from the rest of the frames because of out-of-plane rotations. These tendencies may be checked by providing

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adequate horizontal reinforced concrete bands and vertical reinforcements in places as shown in the figure 5.3, including the boundary of the openings.

FIG. 5.3: VERTICAL REINFORCEMENT IN MASONRY WALLS — WALL BEHAVIOR MODIFIED

Murty, 2002: Earthquake Tips

(b) Out-of-plane collapse often observed in masonry system may be arrested if the walls in mutually orthogonal directions are properly bonded with each other. To this end, in addition to vertical reinforcement, alternating toothed joint in walls at corners and Tjunctions as shown typically in figure 5.4 may be adopted.

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all dimensions are in mm.

a, b, c toothed joints in walls A, B and C

Alternating toothed joints in walls at corner and T-Junction FIG. 5.4: DETAIL OF MASONRY AT CORNERS AND JOINTS

(Source: Seismic Code, IAEE, 2004)

(c) The possibility of the generation of shear cracks in the walls may also be reduced significantly by adopting appropriate structural configuration. Some important guidelines suggested in relevant Indian standard in this regard are pictorially presented in figures 5.5 and 5.6.

Notes: b1+b2+b3