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Page 1: Earthquake Risk Assessment of Old Dhaka (Puran Dhaka)

ABSTRACT

Page 2: Earthquake Risk Assessment of Old Dhaka (Puran Dhaka)

ACKNOWLEDGMENT

I desire to express our heartiest gratitude to Dr. Md. Mahmudur Rahman, Assistant Professor,Department of Civil Engineering of Ahsanullah University of Science and Technology and Supervisor ofthis Thesis. We faced a lot of problems while going through this assignment. We should also express ourconvivial gratitude to him, for all support and encouragement. He also guided us all the way and helpedus to accomplish our aspiration. His unstinting efforts on our behalf are worth mentioning. We arecertainly indebted for his precious insights. Those proved more than enough to overcome thedifficulties.

In this regards, we should express our regards and warm gratefulness to all of my teachers ofAhsanullah University of Science and Technology, for their valuable advice and facilitate.

At last we affectionately appreciate all of friends for their cordial helps thorough out the year.

Arman Zaman (060203010)

Shamsul Arefin Khan (060203014)

Prosenjit Paul (060203037)

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CHAPTER1

INTRODUCTION

1.1 General

Earthquakes originate due to various reasons, which fall into two major categories viz non-tectonic and tectonic. The origin of tectonic earthquakes is explained with the help of ‘elastic reboundtheory'. Earthquakes are distributed unevenly on the globe. However, it has been observed that mostof the destructive earthquakes originate within two well-defined zones or belts namely, 'the circum-Pacific belt' and 'the Mediterranean-Himalayan seismic belt'.

Although Bangladesh is extremely vulnerable to seismic activity, the nature and the level of thisactivity is yet to be defined. In Bangladesh complete earthquake monitoring facilities are not available.The Meteorological Department of Bangladesh established a seismic observatory at Chittagong in 1954.This remains the only observatory in the country.

The classical engineering approach for providing seismic safety in building structures is to ensuretheir conformance to the current seismic design codes. This is indeed a valid approach for new buildings.However, the majority of the existing buildings in seismic regions do not satisfy modern coderequirements. Yet, the ratio of severely dam- aged or collapsed buildings observed after a severeearthquake is much less than the ratio of substandard buildings. The difference is usually significant.

An effective step for seismic risk mitigation in large urban areas under high seismic risk is toidentify the most vulnerable buildings that may sustain significant damage during a future earthquake.Once they are identified properly, existing seismic risks may be reduced either by retrofitting suchbuildings, or by replacing them with new buildings in view of a particular risk-mitigation planningstrategy.

It is basically a sidewalk survey procedure based on observing selected building parametersfrom the street side, and calculating a performance score for determining the risk priorities forbuildings.

Several studies have been made on buildings in a small town in Israel in a mountainous area.Most of its residential buildings, as is quite typical all over Israel, were built during the last 60 years.There are some 1600 low to moderate height (up to 8-9 stories) residential buildings in this town. In thefirst stage of the research a group of buildings was arbitrarily selected in order to implement themethodology's procedures and then conduct site visits in order to document and compare the real datawith the predicted data. Comparisons were done between the estimated values established according tothe present methodology and the real values of the examined buildings. The following parameters werecompared: the number of dwelling units per typical floor, the number of expansion joints and thenumber of stories in the building. The comparison shows good predictions, with a limited number of

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discrepancies, that are related to several reasons among which are: uncommon distance betweenexpansion joints in one building, mistaken data in the basic GIS database regarding the height of thebuilding in another building, and in another building we found out that retrofit of the building wascarried out long after its construction and added a new wing thus adding significantly to the dwellingunit area. These discrepancies cannot be predicted, however they are exceptional compared to a verygood correspondence of all other examined buildings.

1.2 Status Of Earthquakes In Bangladesh:

Bangladesh is surrounded by the regions of high seismicity which include the Himalayan Arcand SHILLONG PLATEAU in the north, the Burmese Arc, Arakan Yoma anticlinorium in the east andcomplex Naga-Disang-Jaflong thrust zones in the northeast. It is also the site of the Dauki Faultsystem along with numerous subsurface active faults and a flexure zone called Hinge Zone. These weakregions are believed to provide the necessary zones for movements within the basin area.

Figure 1

In the generalized tectonic map of Bangladesh the distribution of epicenters is found to belinear along the Dauki Fault system and random in other regions of Bangladesh. The investigation of themap demonstrates that the epicenters are lying in the weak zones comprising surface or subsurfacefaults. Most of the events are of moderate rank (magnitude 4-6) and lie at a shallow depth, whichsuggests that the recent movements occurred in the SEDIMENTs overlying the basement rocks. In thenortheastern region (SURMA BASIN), major events are controlled by the Dauki Fault system. The events

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located in and around the MADHUPUR TRACT also indicate shallow displacement in the faults separatingthe block from the ALLUVIUM.

The first seismic zoning map of the subcontinent was compiled by the Geological Survey ofIndia in 1935. The Bangladesh Meteorological Department adopted a seismic zoning map in1972. In 1977, the Government of Bangladesh constituted a Committee of Experts to examine theseismic problem and make appropriate recommendations. The Committee proposed a zoning mapof Bangladesh in the same year.

In the zoning map, Bangladesh has been divided into three generalised seismic zones: zone-I, zone-II and zone-III. Zone-I compri sing the northern and eastern regions of Bangladesh with thepresence of the Dauki Fault system of eastern Sylhet and the deep seated Sylhet Fault, andproximity to the highly disturbed southeastern Assam region with the Jaflong thrust, Naga thrust andDisang thrust, is a zone of high seismic risk with a basic seismic co-efficient of 0.08. NorthernBangladesh comprising greater Rangpur and Dinajpur districts is also a region of high seismicitybecause of the presence of the Jamuna Fault and the proximity to the active east-west running fault andthe Main Boundary Fault to the north in India. The Chittagong-Tripura Folded Belt experiencesfrequent earthquakes, as just to its east is the Burmese Arc where a large number of shallow depthearthquakes originate. Zone-II comprising the central part of Bangladesh represents the regions ofrecent uplifted Pleistocene blocks of the Barind and Madhupur Tracts, and the western extension of thefolded belt. The Zone-III comprising the southwestern part of Bangladesh is seismically quiet, with anestimated basic seismic co-efficient of 0.04.

1.3 Objective& Scope Of Study

The scopes are expanding, fragility functions pertain to a group of buildings in a given area (cell)rather than a specific building. The scope of the study presented herein extends one step further:several selected parameters are evaluated simultaneously to obtain a performance score for eachbuilding. This score separates each building from the other buildings in the inventory in riskclassification.

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CHAPTER 2

CONCEPT OF EARTHQUAKE ASSESSMENT & METHODOLOGY

2.1 Introduction

Recent earthquakes in urban environments revealed that building damage increases with thenumber of stories when the building lacks basic seismic-resistant design fea- tures. Other factors thathave significant contribution to damage are also well estab- lished. These are: the presence of severeirregularities such as soft stories and heavy overhangs; other discontinuities in load paths; poor materialquality, detailing, and work- manship. It is usually difficult to quantify the sensitivity of damage to eachparameter analytically; however, statistics help. Fragility functions may be developed for determin- ingdamage probabilities, hence for estimating losses in certain building types under given ground-motionintensities

The proposed approach aims at developing of a rapid GIS based technique for assessing thestructural systems of a large inventory of residential buildings, where only limited data is available.There is need for much more data in order to come up with the "most likely" structural scheme of abuilding that will enable its analysis, and this data is derived from logical procedures that are based onseveral data bases. The proposed methodology makes an attempt to produce the information from a"distance", namely without the need to search for the buildings documents, or conduct site visits tocheck and document the buildings, or perform any measurements or tests whatsoever. The entire workis done in the office by a computerized set of algorithms, with automatic decisions based on pre-definedrules, at a very short time and with minimal time resources compared to all other alternatives.

Some of the important stated parameters that influence damage significantly can bedetermined quite easily by visual observation. The simplest ones are the number of sto- ries, softstories, heavy overhangs, and the overall apparent quality of the building re- flecting the quality ofconstruction. These are discussed separately below.

2.2 Parameters

Number of stories Presence of soft story Presence of heavy overhangs Apparent building quality Presence of short columns Pounding between adjacent buildings Local soil conditions Topographic effects

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Number Of Stories

Field observations after the 1999 Kocaeli and DUzce earthquakes revealed a very sig- nificantcorrelation between the number of unrestrained stories and the severity of build- ing damage. If allbuildings conformed to modern seismic design codes, then such a distribution would not occur and auniform distribution of damage would be expected regardless of the number of stories. The increase inseismic demand with the number of stories is not balanced with the increase in seismic capacity insubstandard buildings. After the 1999 DUzce earthquake, damage distribution for all 9,685 buildings inDUzce was obtained by official damage assessors. These data were then sorted with respect to thenumber of stories (Sucuog˘ lu and Yilmaz 2001). The results are shown in Figure 1, where the number ofdamaged buildings is normalized with the total number of build- ings at a given story number.

It can be observed that damage grades shift almost linearly with the number of sto- ries.However, the objectivity of the assigned damage grades is questionable since the distributions indicatehigher damage than that observed by the field survey teams de- ployed by the Middle East TechnicalUniversity. According to the Turkish Natural Di- saster Law, the owners of the damaged buildings areentitled to state compensation, which increases with the damage grade. This practice places a publicpressure on the official damage assessors. Particularly, assignment of moderate and higher damagegrades to all five- and six-story buildings is misleading. There were many undamaged or lightly damagedfive- to six-story buildings in DUzce after the earthquake, yet there is a clear indication that the numberof stories is a very significant or perhaps the most domi- nant parameter in determining the seismicvulnerability of typical multistory concrete buildings in Turkey. The number of freestanding stories in abuilding is identified as the number of “seismic” stories in this study.

Presence Of A Soft Story

Soft stories usually exist in buildings when the ground story has less stiffness and strengthcompared to upper stories. This situation mostly arises in buildings located along the side of a mainstreet. Ground stories that have level access from the street are reserved as commercial space whereasresidences occupy the upper stories. These upper stories benefit from the additional stiffness andstrength provided by many partition walls, but the commercial space at the bottom is mostly left openbetween the frame members for customer circulation. Besides, the ground stories may have tallerclear- ances and different axis systems, causing further irregularity. The compound effect of all thesenegative features from the earthquake-engineering perspective is identified as a soft story. Manybuildings with soft stories were observed to collapse due to a pancaked soft story in past earthquakesworldwide.

During street surveys, the presence of a soft story is evaluated on an observational basis, wherethe answer is either yes or no.

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Presence Of Heavy Overhangs

Heavy balconies and overhanging floors in multistory reinforced concrete buildings shift themass center upwards; accordingly increase seismic lateral forces and overturning moments duringearthquakes. Buildings having balconies with large overhanging cantilever spans enclosed with heavyconcrete parapets sustained heavier damages during the recent earthquakes in Turkey compared toregular buildings in elevation. Since this building feature can easily be observed during a walk-downsurvey, it is included in the parameter set.

Apparent Building Quality

The material and workmanship quality, and the care given to its maintenance reflect theapparent quality of a building. A well-trained observer can classify a buildings apparent quality roughlyas good, moderate or poor. A close relationship had been observed between the apparent quality andthe 4 Haluk Sucuoglu and Ufuk Yazgan experienced damage during the recent earthquakes in Turkey. Abuilding with poor apparent quality can be expected to possess weak material strengths and inadequatedetailing.

Presence Of Short Columns

Semi-in filled frames, band windows at the semi-buried basements or mid-story beams aroundstairway shafts lead to the formation of short columns in concrete buildings. These captive columnsusually sustain heavy damage during strong earthquakes since they are not originally designed toreceive the high shear forces relevant to their shortened lengths. Short columns can be identified fromoutside because they usually form along the exterior axes.

Pounding Between Adjacent Buildings

When there is no sufficient clearance between adjacent buildings, they pound each other duringan earthquake as a result of different vibration periods and consequent non-synchronized vibrationamplitudes. Uneven floor levels aggravate the effect of pounding. Buildings subjected to poundingreceive heavier damages at the higher stories.

Local Soil Conditions

Site amplification is one of the major factors that increase the intensity of ground motions.Although it is difficult to obtain precise data during a street survey, an expert observer can be able to

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classify the local soils as stiff or soft. In urban environments, geotechnical data provided by localauthorities is a reliable source for classifying the local soil conditions.

Topographic Effects

Topographic amplification is another factor that may increase the ground motion intensity ontop of hills. Besides, buildings located on steep slopes (steeper than 30 degrees) usually have stoppedfoundations, which are incapable of distributing the ground distortions evenly to structural membersabove. Therefore these two factors must be taken into account in seismic risk assessment. Both factorscan be observed easily during a street survey.

2.3 Assessment Of Available Methods

Most of the existing evaluation methods refer to a single building, among which we may find:methods that a rebased on statistics of past EQ damage records (Whitman, 1974), methods that arebased on experts subjective opinion (ATC-13, 1985. FEMA 178, 1992. EMS 1998) methods that are basedon score assignments of predefined checklists exposing structural deficiencies that do not contain evenelementary engineering calculations (FEMA 154/5, 1998. NRC-CNRC, 1996. NZSEE, 1996. I. S 2413,2003), simple analytical methods to simulate buildings response that are essentially simple approximatesolutions that must rely on a few parameters (ATC-14, 1987. Calvi, 1999. Priestley, 2003) and detailedanalytical procedures (ASCE 41-06, 2007) which are more accurate but require much data and are time-consuming.

The reliability of these methods differ considerably, from limited reliability of the simplestatistical and rapid screening methods, to the most reliable methods that are based on detailedanalytical procedures that may evaluate the mechanical behavior of the structural system underconsideration, but require an enormous amount of data, that is commonly not available, and take muchtime in their processing.

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Chapter 3

Background Literature

3.1 Introduction

Earthquake is trembling or shaking movement of the earth's surface. Most earthquakes areminor tremors, while larger earthquakes usually begin with slight tremors, rapidly take the form of oneor more violent shocks, and end in vibrations of gradually diminishing force called aftershocks.Earthquake is a form of energy of wave motion, which originates in a limited region and thenspreads out in all directions from the source of disturbance. It usually lasts for a few seconds to aminute. The point within the earth where earthquake waves originate is called the focus, fromwhere the vibrations spread in all directions. They reach the surface first at the point immediatelyabove the focus and this point is called the epicenter. It is at the epicenter where the shock of theearthquake is first experienced. On the basis of the depth of focus, an earthquake may be termed asshallow focus (0-70 km), intermediate focus (70-300 km), and deep focus (> 300 km). The most commonmeasure of earthquake size is the Richter’s magnitude (M). The Richter scale uses the maximumsurface wave amplitude in the seismogram and the difference in the arrival times of primary (P) andsecondary (S) waves for determining magnitude (M). The magnitude is related to roughly logarithm ofenergy, E in ergs.

Accurate historical information on earthquakes is very important in evaluating theseismicity of Bangladesh in close coincidences with the geotectonic elements. Information onearthquakes in and around Bangladesh is available for the last 250 years. The earthquake recordsuggests that since 1900 more than 100 moderate to large earthquakes occurred in Bangladesh, outof which more than 65 events occurred after 1960. This brings to light an increased frequencyof earthquakes in the last 30 years. This increase in earthquake activity is an indication of fresh tectonicactivity or propagation of fractures from the adjacent SEISMIC ZONEs.

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3.2 Chronology

Before the coming of the Europeans, there was no definite record of earthquakes. Followingis a chronology of important earthquakes from 1548.

1548 The first recorded earthquake was a terrible one. Sylhet and Chittagong were violently

shaken, the earth opened in many places and threw up water and mud of a sulphurous smell.1642 More severe damage occurred in Sylhet district. Buildings were cracked but there was no

loss of life.1663 Severe earthquake in ASSAM, which continued for half an hour and Sylhet district was not

free from its shock.1762 The great earthquake of April 2, which raised the coast of Foul island by 2.74m and the

northwest coast of Chedua island by 6.71m above sea level and also caused a permanentsubmergence of 155.40 sq km near Chittagong. The earthquake proved very violent in Dhakaand along the eastern bank of the MEGHNA as far as Chittagong. In Dhaka 500 persons losttheir lives, the RIVERs and JHEELs were agitated and rose high above their usual levels andwhen they receded their banks were strewn with dead fish. A large river dried up, a tract of landsank and 200 people with all their CATTLE were lost. Two volcanoes were said to have openedin the Sitakunda hills.

1775 Severe earthquake in Dhaka around April 10, but no loss of life.1812 Severe earthquake in many places of Bangladesh around May 11. The earthquake proved

violent in Sylhet1865 Terrible shock was felt, during the second earthquake occurred in the winter of 1865,

although no serious damage occurred.1869 Known as Cachar Earthquake. Severely felt in Sylhet but no loss of life. The steeple of the

church was shattered, the walls of the courthouse and the circuit bungalow cracked and in theeastern part of the district the banks of many rivers caved in.

1885 Known as the Bengal Earthquake. Occurred on 14 July with 7.0 magnitude and the

epicentre was at Manikganj. This event was generally associated with the deep-seated

Jamuna Fault.

1889 Occurred on 10 January with 7.5 magnitude and the epicentre at Jaintia Hills. It fected Sylhettown and surrounding areas.

1897 Known as the Great India Earthquake with a magnitude of 8.7 and epicentre at Shillong

Plateau. The great earthquake occurred on 12 June at 5.15 pm, caused serious damage tomasonry buildings in Sylhet town where the death toll rose to 545. This was due to the collapseof the masonry buildings. The tremor was felt throughout Bengal, from the south Lushai Hillson the east to Shahbad on the west. In Mymensingh, many public buildings of the districttown, including the Justice House, were wrecked and very few of the two-storied brick-builthouses belonging to ZAMINDARs survived. Heavy damage was done to the bridges on theDhaka-Mymensingh railway and traffic was suspended for about a fortnight. The

river communication of the district was seriously affected (BRAHMAPUTRA). Loss of life was notgreat, but loss of property was estimated at five million Rupees. Rajshahi suffered severeshocks, especially on the eastern side, and 15 persons died. In Dhaka damage to property washeavy. In Tippera masonry buildings and old temples suffered a lot and the total damage wasestimated at Rs 9,000.

1918 Known as the Srimangal Earthquake. Occurred on 18 July with a magnitude of 7.6 and

epicentre at Srimangal, Maulvi Bazar. Intense damage occurred in Srimangal, but in Dhaka onlyminor effects were observed.

1930 Known as the Dhubri Earthquake. Occurred on 3 July with a magnitude of 7.1 and the

epicentre at Dhubri, Assam. The earthquake caused major damage in the eastern parts of

Rangpur district.

1934 Known as the Bihar-Nepal Earthquake. Occurred on 15 January with a magnitude of 8.3 and

the epicentre at Darbhanga of Bihar, India. The earthquake caused great damage in Bihar,Nepal and Uttar Pradesh but did not affect any part of Bangladesh.Another earhquake occured on 3 July with a magnitude of 7.1 and the epicentre at Dhubri

of Assam, India. The earthquake caused considerable damages in greater Rangpur district ofBangladesh.

1950 Known as the Assam Earthquake. Occurred on 15 August with a magnitude of 8.4 with the

epicentre in Assam, India. The tremor was felt throughout Bangladesh but no damage wasreported.

1997 Occurred on 22 November in Chittagong with a magnitude of 6.0. It caused minor damage

around Chittagong town.1999 Occurred on 22 July at Maheshkhali Island with the epicentre in the same place, a

magnitude of 5.2. Severely felt around Maheshkhali island and the adjoining SEA. Housescracked and in some cases collapsed.

2003 Occurred on 27 July at Kolabunia union of Barkal upazila, Rangamati district with

magnitude 5.1. The time was at 05:17:26.8 hours.

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3.3 Review of previous works

Saidur Rahman (Director of Bangladesh Disaster Preparedness Centre)

A world famous seismologist Professor Billham said in 2001 that in the Himalayan region, atleast seven earthquakes of the strength 8.1 and above on the Richter scale are overdue. A team ofexperts led by him did a survey and they identified seven to eight risk prone countries andBangladesh is obviously one of them because of its geographical location. Secondly a study by a UNsponsored programme called International Decade for Natural Disaster Reduction in the period from1991 till 2000 surveyed at least 30 different cities. And the findings of the survey are very threateningto us. They are saying that the two most vulnerable cities to earthquake are Tehran and Dhaka.There were several factors to come to this conclusion. For example situation in an earthquake zone,physical infrastructure, socio-economic condition of the people living there and most importantlyresponse management.

Dr M Shahidul Islam( Professor, Department of Geography, University of Chittagong)

Potential earthquake threat and our coping strategies

Although earthquake in Bangladesh has not yet been recognised as a case of serious naturaldisaster, but recent occurrences and assumptions have already generated a potential threat. Theincidents of recent repeated earthquakes on 27 July in Chittagong have raised a great concernamong the people of the country, particularly among those around Chittagong region.

What is an earthquake? It is a shock or a series of shocks on the earth surface resulted fromrelease of pressure due to sudden movement of crystal rocks along active fault lines or plate boundariesof the earth surface or in areas of volcanic activities. Some parts of the world are earthquake pronemore than others, although such event may happen at any place, any time and that of any magnitude.Japan, the Philippines, Southeast Asia and North America are particularly vulnerable to earthquake.

Geographically Bangladesh is located close to the boundary of two active plates: the Indian platein the west and the Eurasian plate in the east and north. As a result the country is always under apotential threat of earthquake of any magnitude at any time, which might cause catastrophicdevastation in less than a minute. In the seismic zoning map of Bangladesh, Chittagong region has beenshown under Zone II with basic seismic coefficient of 0.05, but recent repeated jerk around this regionindicate the possibilities of potential threat of even much higher intensity than projected.

A total of about six lackh incidents of quakes of different magnitudes occur annually throughoutthe world of which that of magnitudes 6-7, 7-8 and above 8 are 120, 18 and 1, respectively. The recordsin Bangladesh during the last 175 years shows total number of 25, 18 and 4 incidents of earthquakeshaving intensity more than 6, 7 and 8 on Richter scale, respectively. Among such incidents Bengal Eq of14 July 1885 (R-7), Great Indian Eq of 12 June 1897 (R-8.7), Srimangal Eq of 8 July 1918 (7.6) and AssamEq of 15 August 1950 (R-8.5) are well known. However, people's awareness regarding earthquakes in

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Bangladesh began to generate after the tragic death of Sadia (a little girl) in a quake of only R-5.6magnitude on 21 November, 1997. Moreover, the incidents of repeated shocks between 22 July and 2August, 1999 at Moheskhali and the damages to lives and properties could draw the attention of thenation considerably. Since then earthquake in Bangladesh has been considered as a potential naturalkiller to human lives. The last major earthquake in Bangladesh occurred about 30 years back.Statistically the threat of such a high magnitude tremor has the highest possibly to happen at any time,which might cause devastations particularly in Dhaka and Chittagong cities.

The occurrence of earthquakes is part of the natural process in the earth's geophysical system.Under the present stage of scientific development it is not possible to stop such natural events, andeven if it was possible to do so, we should not intervene such internal system of the earth. However,understanding the characteristics of internal geophysical process of the earth and possibility of itsforecasting can reduce the casualties from such incident considerably. Developed countries are doingcontinuous research in this field. Rather it is better to accommodate this event and develop technologyto live with such incident, as we are living with cyclones, storm surges and floods. However, locatingthe epicenters and monitoring the characteristics of each shock may improve our understandingconsiderably and lead us to develop some preventive measure to live with earthquakes. It is thusimmediate necessity to upgrade the existing earthquake measurement station at Ambagan inChittagong and complete the two other proposed stations at Dinajpur and Sylhet.

Bangladesh has improved tremendously to mitigate and manage many of its natural disasters,although the mitigation strategies regarding earthquake has remained nearly in its infant stage. At thisstage the country does not need to take any radical measures to mitigate the earthquake incident,rather the concept of earthquake mitigation and management issues can be incorporated within theexisting disaster management programme of the government, ranging from National DisasterManagement Council to Union Disaster Management Committee. Proper training to voluntaryorganisations and NGOs, and procurement of instruments required for rescue operation must get toppriority in the management agenda. Moreover, motivation programme and increasing of people'sawareness can reduce the casualties from any earthquake incident considerably.

It is not the earthquake rather it is the building that kills people. If the collapse of even a singlebuilding can become possible to stop, it can save many lives residing in that building. It is not possible toabandon all old buildings, under the potential threat of earthquake. However, it is quite possible that allnewly constructed buildings and structures must be brought under strict building code that resistsearthquake damage.

Bangladesh is possibly one of the countries most vulnerable to potential earthquake threat anddamage. An earthquake of even medium magnitude on Richter scale can produce a mass graveyard inmajor cities of the country, particularly Dhaka and Chittagong, without any notice. Construction of newbuildings strictly following building code or development of future controls on building construction arethe activities which will be functional in future. However, under the present stage of human occupancy,buildings, infrastructures and other physical structures of different areas of a city will not be equallyvulnerable to any such shock. Earthquake vulnerability of any place largely depends on its geology and

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topography, population density, building density and quality, and finally the coping strategy of itspeople, and it shows clear spatial variations. It is thus necessary to identify the scale of such variationsand take necessary measurements to cope with that.

Although the earthquake tremors cannot be stopped or reduced, the human casualties and lossof properties can be reduced with the help of an earthquake vulnerable assessment atlas. Anearthquake atlas is the presentation of facts relating to earthquakes and the guideline for earthquakemitigation measurements at regional scale in the form of map, graphs, pictures and text. Such an atlasprovides clear guidelines to post disaster rescue operation, regional scale mitigation strategies andstepwise disaster management activities. We do not have any such atlas neither at national level nor atregional level. However, it is the timely demand to prepare an earthquake vulnerability assessment atlasof Bangladesh in general, and for the major cities in particular.

Large scale mitigation measurement needs huge initial investment; however, to save humanlives and properties, we should not hesitate to do so. Particularly strict control of building codes,enforcement of laws and orders, and development of people awareness has no alternatives. However,some immediate measures are suggested below:

- Make an inventory of all old buildings which are vulnerable to earthquake, and either repair orevacuate occupants from those buildings.

- Make an inventory of houses, which are constructed at the foot of steep hillsides, particularlywhere hill slopes have been cut, even ten years back. Relocate those families to suitable places.

- Make earthquake vulnerability atlas of major cities, which will show in detail the list ofvulnerable sites, their possible consequences and possible measurements of mitigation at differentscales of earthquake events.

- Strict application of building codes for all newly constructed buildings, particularly all high risesbuildings.

- Development of awareness programme to educate people regarding the causes andconsequences of earthquakes. And also to disseminate knowledge to them regarding theirresponsibilities before, during and after the earthquake through seminar, symposium and workshop,and also through non-formal education by GO and NGOs.

During the 20s and 30s of the last century Japan lost 1.5 lackh human lives only in fiveearthquake incidents. But that society has faced this challenge successfully over the last 50 years.During the last 80s and 90s a total of 30 events hit the country causing loss of less than six thousandlives. Japan has not succeeded to stop earthquakes but has reduced the human casualties and loss ofproperties dramatically. At the present stage of our society and current level of development we mayseem helpless but through our sincerity, honesty and commitment we may even do better than theJapanese society. We should therefore be optimistic and thus active.

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MIR FAZLUL KARIM, (Director, Geological Survey of Bangladesh)

Vulnerability to earthquake for Bangladesh:

There are some valid questions: Is Bangladesh vulnerable to earthquakes? Should we beconcerned about an earthquake when occurrences of earthquake damages are not so significant? Thecountry faces so many day-to-day problems related to environment, industrial pollution, traffic, waterand power shortage, and annual calamities such as flood, drought, cyclone and tidal bore. Can weafford to ignore earthquake hazards?

Earthquakes are the detectable shaking of the earth's surface resulting from seismicwaves generated by a sudden release of energy from inside the earth. Any landmass whichhas experienced natural ground shaking in the past is vulnerable to earthquake risk and thus liableto earthquake hazard. A severe earthquake can bring devastation to the economy of the country and wecannot ignore potential danger of earthquakes.

Bangladesh: A geological location for earthquakes

The geological structures in and around Bangladesh are capable of accumulating tectonicstrain. These structures have released enough energy to produce destructive shakes in the past.

Fortunately, the frequency of large earthquakes in and around the country is less than inother earthquake-prone regions of the world, though sometimes the lone national seismicobservatory station at Chittagong measures a relatively high frequency of low magnitude shakes.

Bangladesh, along with its neighboring counties, shared the experience of extraordinaryground shaking due to an earthquake of magnitude 8.7 which is widely known as "The Great IndianEarthquake." The earthquake occurred due to a vertical displacement along the Dauki Fault locatednear the north-east international boundary between Bangladesh and India. The earthquake causedabout 20m of pop-up of the Shillong Massive within a few seconds, and debris were blown evenmiles away from the epicenter area.

A similar strong and extraordinary earthquake of magnitude 7.5 occurred in Bhuj on January26, 2001, damaging many urban areas of Gujarat and killing an estimated 25,000 people. Scientistsconsider these as rare earthquakes, but this type of earthquake could be extremely devastating in theperipheries of the Indian peninsula.

Bangladesh occupies a greater part of the Bengal basin. It is located in the eastern extremity ofthe peninsula and the Kutch basin in the western extremity is a mirror image of the Bengal basin. Theregional geological structures from south to north at both the eastern and western extremitiespostulate a geometrical symmetry that would be receptive to similar tectonic behaviour in terms ofstress distribution (except for some local differential characteristics). Considering such a geologicalsetting, Bangladesh could be a receptive place for extraordinary earthquakes.

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The rapidly growing urban centers increase the susceptibility of earthquake damage Generally,unplanned and populous townships are always vulnerable to earthquake hazard or damages.Bangladesh is a densely populated country. At the beginning of the twentieth century there were only48 urban centers in the country and at present there are 491 including the densely populated cities andgrowth centers. A rapid change in infrastructure development has resulted in significant changes inhousing pattern and transportation, sewerage, water supply, waste disposal system andcommunication network. All development has taken place in a very short time. The planners and citymanagers could not keep pace for regulating the government's planned efforts in the face of suchrapid development. The lack of planned development puts the cities and growth centers in avulnerable situation for larger earthquake damages. The experts foresee the most deadly future forDhaka mega-city in the event of an earthquake here.

Prediction of ground conditions

The geology of Bangladesh is complex due to the presence of about 100m to 1000m(30,000ft) of sedimentary deposits over the basement rock of Indian plate. More than 80% of thecountry is covered by soft sediments (soil) or holocene deposits with unpredictable changes in theupper 100m of deposits, having considerable variations in the constituent geological materialsand geotechnical properties. The geological map of the country indicates that the upper 10m ofsediments in about 60% of the land area is susceptible to liquefaction during earthquake, making theground vulnerable to immediate shear failure.

More effort is needed for building up earthquake hazard awareness

As the frequency of earthquakes is low in Bangladesh, the people and government are notclearly aware of earthquake devastation and we can not afford any experiment with it. Building upof public awareness could be the first and essential step towards preparedness for reduction ofearthquake damages. It is necessary to remember the alarming Dhaka Earthquake 2001, when strongtremors were felt in the city and many people rushed out of their homes and offices in panic. 100prison inmates were hurt in a stampede at the Dhaka Central Jail.

What shall we do?

The country has had many damaging earthquakes in the past and is placed in a high seismiczone in the Global Seismic Hazard Map. We have not investigated the source structures, but due to itscomplex geological setting, Bangladesh is not capable of sustaining the strong shaking produced in theHimalaya and Meghalaya source area. Unfortunately, many of the infrastructures and buildings inBangladesh may not meet BNBC standards and may be considered vulnerable from seismic safetyviewpoint. Generally earthquake damages are irreparable. If we consider the potentiality ofearthquake disaster, we may not be able to ignore this extraordinary geological hazard.

We are at the early stage of possible earthquake hazard assessment and cannot expect anyovernight understanding of earthquake vulnerability of the country. But steps can be taken toreduce the losses and damages by implementation of Bangladesh National Building Code (BNBC) in the

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construction practice, identification of appropriate subsurface geology, determining the right type ofarchitectural setting and engineering design of both foundation and superstructures,development of fire safety options, keeping open spaces for rescue operations, and other suchmeasures.

At least we need an plan of action. There is an urgent need for reasonable seismic riskassessment of the country. It is a multidisciplinary task and includes technical training,institutional development, development of technical manuals, legal and enforcement aspects,and public awareness programmes.

Dr. Aftab Alam Khan (Professor, Geology Department, Dhaka University )

Earthquake hazard : Dhaka city perspective

A sudden, transient motion or trembling in the earth's crust, resulting from the propagationof seismic waves caused by faulting of the rocks either at shallow and/or deeper depths is known asearthquake. The motion is caused by the quick release of slowly accumulated energy in the form ofseismic waves. The release of accumulated energy may occur at any depth and time but theintensity of damage is directly proportional to the movement on a fault, which is a thin zone, both atvertical and horizontal plains, of crushed rock between two blocks of rock. A fault can range in lengthfrom a few centimeters to hundreds of kilometers. The larger the fault length, the larger the energyrelease by fault movements. The ground shaking and the radiated seismic energy are caused mostcommonly by sudden slip on a fault, or other sudden stress changes in the Earth. Sudden breakwithin the upper layers of the earth, sometimes breaking the surface, resulting in the vibration of theground, where strong enough will cause the collapse of buildings and destruction of life and property.Based on long term historical records, about 18 major earthquakes (7.0 - 7.9 on the Richter scale) andone great earthquake (8.0 or above) are expected in any given year globally.

Any physical phenomenon associated with an earthquake that may produce adverse effectson human activities is termed as earthquake hazard. This includes surface faulting, ground shaking,landslides, liquefaction, tectonic deformation, tsunami, and their effects on land use, man-madestructures, and socio-economic systems. A commonly used restricted definition of earthquakehazard is the probability of occurrence of a specified level of ground shaking in a specified period oftime. Similarly, earthquake risk is the expected (or probable) life loss, injury, or building damagethat will happen, given the probability of earthquake hazard. Earthquake risk and earthquakehazard are occasionally used interchangeably.

Bangladesh, by and large, is seismically active. The occurrence of earthquakes withmagnitude averaging around 5 in Richter scale is quite frequent especially in its eastern region.Although, Dhaka has not been experienced with any moderate to large earthquake in historical past,even then the earthquake of December 19, 2001 with magnitude of 4.5 and focal depth of 10 kmlocated very close to Dhaka is certainly an indication of its earthquake source and vulnerability. In

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addition, micro-seismicity data also supports the existence of at least four earthquake source pointsin and around Dhaka. The earthquake disaster risk index has placed Dhaka among the 20 mostvulnerable cities in the world. Dhaka with its population of around 13 million and enormous poorlyconstructed and dilapidated structures signifies extremely vulnerable conditions for massive loss oflives and property in the event of a moderately large earthquake.

The recently measured plate motions at six different sites of Bangladesh including Dhaka;(the research being jointly conducted by Lamont-Doherty Earth Observatory, Columbia University, USAand the Department of Geology, Dhaka University) clearly demonstrate that Dhaka is moving 30.6mm/year in the direction northeast. Further, the rate of strain accumulation is relatively high in andaround Dhaka. It may precipitate in an earthquake of magnitude 6.8 in the event of the release ofaccumulated strain. The shallow subsurface of Dhaka is also characterized by number of faults ofvariable dimensions. These faults are vulnerable to motion where these coincide with the zones of highparticle velocity.

The coincidence of the zones of high particle velocity with the location of faults suggests thatthe western part of Dhaka city from Mirpur-Kalyanpur to Pagla along Buriganga river and the easternpart of Dhaka city from Uttar Khan-Badda to Demra along Balu river has emerged as high risk zone. Thepeak ground acceleration in these areas has been calculated ranging between 0.3 to 0.35 if anearthquake of magnitude 5.6 occurs in and around Dhaka city. The resonant length in these areassuggests an optimal height beyond five stories; additional seismic factor needs to be introduced inaddition to general seismic factor which is introduced based on seismic factors of the sitespecifically for earthquake resistant building code. The entire Dhaka megacity has been looked uponfrom earthquake hazard point of view. It has been divided into four zones of earthquake hazardvulnerability ranging between very high risks and low risk.

Earthquake cannot be prevented. But certainly it is high time to be much more concernedabout the probable impending earthquake in order to minimise the loss of lives and property innational interest. On the basis of the above facts, we should develop earthquake monitoringnetwork in Bangladesh immediately. It is of prime importance to set a national institute of earthquakeresearch to develop high skilled manpower that can perform the task for earthquake risk assessmentand management. We should remember that one earthquake of moderate intensity would killthousands of people and destroy enormous national property. Death is certain for all human beings butpainful death is not desirable.

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Figure2: Earthquake Hazard Zoning Map of Dhaka Megacity

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CHAPTER 4

STATISTICAL ANALYSIS

4.1 Statistical Analysis

The objective of statistical analysis is to develop a performance score for prioritizing thebuildings in an urban area, based on a set of vulnerability indicators that can be ob¬served visuallythrough a street survey. Multiple linear regression analysis is employed for developing a mean-valuefunction that returns the expected value of the performance score. This function can be established byusing the Duzce database presented above.

Figure 3. The effect of heavy overhangs on damage distribution.

4.2MULTIPLE LINEAR REGRESSION ANALYSIS

A linear function is fit to the Duzce damage database for calculating the expected performancescores (EPS) based on the presence of soft story (SS), apparent building quality (AQ), and the presence ofheavy overhangs (HO) for groups of buildings with the same number of stories. In developing the linearregression functions, a numerical value, namely, an observed performance score (OPS), was assigned toeach building ac¬cording to its observed performance during the 1999 Duzce earthquake as given inTable 2. The assigned OPS values for the corresponding damage states are rather subjective, indicating aperformance ranking on a normalized scale.

The mean-value function for the multi-linear regression analysis is:

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E(PS \ K) is the EPS of the building with a given set of vulnerability indicators ss, aq, and ho; and β0, βSS,βAq, and βHO are the set of coefficients that minimize the weighted least squares error, ∆2,

in which OPSi is the observed performance score and EPSi is the expected performance score of the ithbuilding, respectively, and n is the total number of buildings in the group.

The physical effect of the multi-linear regression of EPS on vulnerability indicators SS, AQ, andHO can be measured by reduction of the variance of OPS, by taking into account the general trend withthe vulnerability indicators. This reduction is represented by Ang and Tang (1975).

In Equation 3, S2EPS is an unbiased estimate of the conditional variance of OPS around the mean-value

function E(PS| K), and calculated as:

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where n is the total number of buildings in the group and m is the number of parameters taken intoaccount in the regression (Equation 1). S2

OPS is an estimate of sample vari- ance of OPS, which iscalculated from

where OPS is the mean of observed OPS values for the particular building group.

A set of MATLAB routines has been programmed to perform the calculations listed in Equations1 through 5. The set of regression coefficients calculated according to the procedure explained aboveand the associated R values from Equation 3 are presented in Table 3.

The EPS for a building is then calculated from Equation 1, where β0, βSS, βAQ, and βHO are givenin Table 3 for different number of seismic stories. Note that β0 is an initial performance score for abuilding with no observed vulnerabilities, and the remaining terms in Equation 1 reduce the initial scorefor each indicated vulnerability: SS=ss, AQ=aq, and HO=ho. The value taken by ss is either −1 (so stor ypresent) or 0 (no soft story); the value taken by aq is −1 (poor quality), 0 (moderate quality), or +1 (goodquality); and the value taken by ho is either −1 (heavy overhangs present) or 0 (no heavy overhangs).

The developed method bears some similarities with the seismic evaluation procedure proposedin FEMA-154 (2002). However, this method provides a broader description of seismic risk for medium-rise reinforced concrete buildings that do not conform to the requirements of modern seismic designand construction codes. In FEMA-154, the num-ber of stories is not taken into consideration for low- tomid-rise buildings, contrary to the experience in Turkey. Additionally, the relative scores presented hereare based on statistical analysis of field data, and thus are believed to represent the actual behaviormore accurately.

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4.3 VARIATION OF BUILDING PERFORMANCE WITH PGV

The seismic performance of a structure subjected to severe ground motion can be measured bythe observed structural damage. The maximum post-yield deformation (plastic deformation, Ap)experienced by a structure during severe earthquake ground motion can be accepted as one of themajor contributors to structural damage. Hence it can be accepted as a suitable performance parameterin quantifying the damage, as it is zero when the structure behaves in its elastic limits and takes largervalues as the struc¬ture deforms beyond its yielding level.

Nonlinear response history analyses of SDOF systems are performed using the strong ground-motion data described in the preceding section. The inelastic behavior is simulated by the elastoplastichysteretic model. At a given period of vibration, the maxi¬mum plastic deformation, Ap, of an SDOFsystem is computed for a lateral elastic strength demand that is normalized by the corresponding lateralyield strength value.

Figure 4. Spectral variation of mean plastic deformations in Groups I and IV with R.

This normalized lateral strength parameter is known as the strength reduction factor R. Themaximum plastic SDOF deformations computed in this way correspond to plastic deformation spectrafor constant strength. A total of six R values (1.5, 2.0, 3.0, 4.0, 5.0, and 6.0) is used in thesecomputations.

Figure 4 presents the variation in mean Ap values with respect to the period of vi¬bration and Rfactor for ground-motion data Groups I and IV. Comparison of curves for Groups I and IV indicates thesensitivity of plastic deformations to PGV. The curves in Figure 9 also show the changes in mean plasticdeformation trend with respect to the strength reduction factor R. The mean plastic deformation valuesobtained for the ground motions with larger PGV exhibit a stronger sensitivity to the R factor.

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Figure 5 shows a close-up view of mean plastic deformation variation in Group II groundmotions for periods of vibration between 0.1 and 1.0 s. The mean plastic defor¬mation values follow analmost well defined, linear trend with respect to the R factors. The first-order polynomial fits computedfor each R value are also shown in Figure 10. Similar to the fits presented in Figure 10, mean plasticdeformation curves of other ground-motion groups are represented by linear straight lines for periodsof vibration between 0.1 and 1.0 s, and these fits yielded very high correlation coefficients with re¬spectto the actual data trend. It should be noted that the period interval from 0.1 to 1.0 s contains asignificantly large percentage of existing building stock.

Observation of strong correlation between PGV and plastic deformation demands on structuralsystems, together with the observed linear trend in mean plastic deformations with period can becombined to derive a simplified approach for performance modifi¬cation. Taking Group I mean plasticdeformations as a base, one can compute the mean structural performance modification factors (PM)for the other ground-motion groups. Figure 11 shows the results of such computations for Groups III andIV by using the linear curves fitted on the exact mean plastic deformation data for periods of vibration

Figure 5. Variation in mean plastic deformation of Group II ground motions for different Rvalues and the first-order polynomial fits.

between 0.1 and 1.0 s. These graphics exhibit weaker strength dependency and stronger perioddependency of the plastic deformation (damage) ratios. This dependency in¬creases with increasingPGV

Performance modification factors are calculated from Figure 11 and from similar graphicalinformation for Group II, for representative buildings in the Duzce database with three, four, five, and sixseismic stories. The fundamental period estimations are based on the effective period concept definedin FEMA-356 (BSSC 2000). This refer-

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Figure 6. Mean damage amplification factors for Group III and IV ground-motion data setsfor various R values.

ence period approximately corresponds to the secant stiffness at 60% of the yielding strength of thestructure and is recommended for seismic performance-assessment pro¬cedures based on structuraldeformation. The strength reduction factors are selected as three for all representative buildings, whichis thought to be reasonable for low- and medium-rise substandard concrete buildings. It also has to benoted that the variation of PM with R is very slow for periods longer than 0.4 seconds. Effective periodsfor three-to six-story concrete buildings fall into this range. The calculated performance-modificationfactors are presented in Table 4.

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4.4 INTENSITY-DEPENDENT EXPECTED PERFORMANCE SCORES

The β values given in Table 3 were calculated for the 1999 Düzce earthquake ground motion,where the geometric mean value of PGV for the horizontal components was 70.6 cm/ s. Therefore, thevalues in Table 3 represent the 60<PGV<80 cm/ s intensity range in Table 4. It is decided to keep theregression coefficients βSS, βAQ, and βHO for the vulnerability indicators in Table 3 the same, but applythe intensity scaling to the initial performance scores β0 of three-, four-, five-, and six-story buildings indifferent PGV zones. A building with an “average” vulnerability is selected first from each story group.Such an average building has a moderate apparent quality (aq=0) and a median vulnerability concerningthe presence of a soft story (ss=−1/2) and heavy overhangs (ho=−1/2).Assuming that the buildingvulnerability score is correlated with the maxi-mum plastic deformation p, and using the relationship ofPGV with p presented ear-lier, the variation of vulnerability score with ground-motion intensity can bedetermined. Accordingly, the expected performance score is calculated for each average building fromTable 3 and Equation 1, then this score is modified by using the proportions of performance-modification factors in Table 4 with reference to the 60<PGV<80 cm/ s intensity range. Finally, the initialperformance scores are recalculated from the modified expected-performance scores. The results arepresented in Table 5. The co-efficients are rounded to integers for simplicity.

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CHAPTER 5

RESULT AND DISCUSSION

We choose the value of peck ground velocity (PGV) within 60< PGV<80 for this area.

For this range of PGV values, we have the value of the coefficient as below-

Initial Performance Score Vulnerability Co-efficient

Numbers of Stories 60<PGV<80Soft Story

(βSS)Apparent Quality

(βAQ)Heavy overhangs

(βHO)

3 80 23 9 23

4 73 22 15 30

5&6 64 24 23 33

We also assign the following value of indicated vulnerability in the equation of expected performancescore(EPS)

* No soft story (ss) –(0) * No heavy overhangs –(0)

* Apparent quality :

Good – (+1) ;

Moderate – (0)

Poor – (-1)

* Presence of (ss) –(-1) * Presence of (ho) –(-1)

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho) ………………….(1)

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Now for the area of BUET teacher`s quarter

Building no. – 06

As this building is a 4 storied building we get the values from the above table are

β0 = 73 βSS = 22 βAQ = 15 βHO = 30

ss =0, aq = 0, ho =-1

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 73+22(0)+15(0)+30(-1)

= 43

Comment : The building`s performance is at moderate (from table 2)

Building no. - 08

As this building is a 5 storied building we get the values from the above table are

β0 = 64; βSS = 24; βAQ = 23; βHO = 33

ss =0; aq = 0; ho =0

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 64+24(0)+23(0)+33(0)

= 64

Comment : The building`s performance is at moderate (from table 2)

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Building no. - 09

As this building is a5 storied building we get the values from the above table are

β0 = 64; βSS = 24; βAQ = 23; βHO = 33

ss =0; aq = 0; ho =-1

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 64+24(0)+23(0)+33(-1)

= 31

Comment : The building is at severe risk(from table 2)

Building no. - 10

As this building is a 6 storied building we get the values from the above table are

β0 = 64; βSS = 24; βAQ = 23; βHO = 33

ss =0; aq = 0; ho =-1

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 64+24(0)+23(0)+33(-1)

= 31

Comment : The building is at severe risk(from table 2)

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Building no. - 30

As this building is a 4 storied building we get the values from the above table are

β0 = 73; βSS = 22; βAQ = 15; βHO = 30

ss =0; aq = 1; ho =-1

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 73+22(0)+15(1)+30(-1)

= 58

Comment : The building`s performance is at moderate (from table 2)

For the chalk bazaar area

Building no. - 11

As this building is a 3 storied building we get the values from the above table are

β0 = 80; βSS = 23; βAQ = 9; βHO = 23

ss = 0; aq = -1; ho = 0

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 80+23(0)+9(-1)+23(0)

= 71

Comment : The building`s performance is at moderate (from table 2)

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Building no. - 18

As this building is a 4 storied building we get the values from the above table are

β0 = 73; βSS = 22; βAQ = 15; βHO = 30

ss =-1; aq = 0; ho =0

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 73+22(-1)+15(0)+30(0)

= 51

Comment : The building`s performance is at moderate (from table 2)

Building no. - 22

As this building is a 3 storied building we get the values from the above table are

β0 = 80; βSS = 23; βAQ = 9; βHO = 23

ss =-1; aq = -1; ho =0

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 80+23(-1)+9(-1)+23(0)

= 48

Comment : The building is at severe risk (from table 2)

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Bakshi Bazaar area

Building no. - 12

As this building is a 6 storied building we get the values from the above table are

β0 = 64; βSS = 24; βAQ = 23; βHO = 33

ss =0; aq = 0; ho =-1

so from eq 1, we get

EPS = β0 + βSS(SS) + βAQ(aq) + βHO(ho)

= 64+24(0)+23(0)+33(-1)

= 31

Comment : The building is at moderate risk(from table 2)

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CHAPTER 5

CONCLUSION

A simple screening procedure for three- to six-story substandard concrete buildings, based on a sidewalksurvey of the vulnerable building stock in an urban environment, is developed in this study. Theproposed procedure is calibrated with field data collected in the region of Old Dhaka CITY. The basicobjective is to accelerate the vulnerability-assessment studies in large urban regions populated with avery high number of vulner¬able buildings.

It has to be noted that the proposed procedure is intended to serve as an initial step for the treatmentof a large-scale epidemic, but not for detailed treatment of each indi¬vidual patient in the population atrisk.

Summary and Conclusions

This paper presents a seismic vulnerability assessment application on a regional scale. In theintroductory parts of this paper assessment methodology was summarized and in the second part thedetails of the field applications were introduced and the findings were presented.

It was concluded that the sidewalk survey procedure should be complemented by PAM (preliminaryassessment methodology). In doing this, those buildings with lower performance scores should be givenpriority and in the long run, the entire building stock in the region should be screened by the use ofPAM.