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Potential earthquake risk of buildings in Sri Lanka.

In this paper, seismic hazard and hazard mitigation in Greater Colombo, Sri Lanka are studied. The objectives of the study are to evaluate the characteristics of local site response against different levels of seismic action and to identify the areas vulnerable to liquefaction damage, and thereby to make a seismic evaluation and hazard level distribution of existing buildings located in the area. The methodology included a series of investigations involving the exploration of the geotechnical structure of the study area, and structural evaluation of existing building stocks against earthquake loading. The analysis demonstrated the possibility of up to twenty times variation in the level of ground shaking at two distinct locations for a particular magnitude of earthquake. A structural evaluation of buildings in the area confirmed that un-reinforced masonry buildings built before 1960 are vulnerable against seismic loading. For the case of moderate to high level of shaking more than 10% of the buildings are exposed to major damage.

INTRODUCTION

Sri Lanka is considered to be in an aseismic zone away from major plate boundaries or any active faults. However, historically there had been many seismic events within Sri Lanka and in the neighbouring areas, which had considerable influence on the island as shown in Abaykoon (1983). Even though some of these are very small in magnitude and some had epicenters far away from the country, the necessity to study the seismic effects on Greater Colombo area has been realized by engineering professionals.

Colombo is the commercial capital of Sri Lanka with having more than 2 million population and further expansion of the city is expected with country's growing economy. Initial seismic risk analysis in Sri Lanka and demonstrated that Colombo is more vulnerable to seismic events than any other part of the country: Abaykoon (1998). Considering the importance of Colombo, a seismic status analysis of Greater Colombo area is contemporary issue.

AIM AND OBJECTIVE

The aim of the paper is to study the seismic hazard and hazard mitigation of Greater Colombo area of Sri Lanka. The main outcome of the study would be an indicator of the seismic status of Greater Colombo and it will provide necessary knowledge to the geotechnical and structural engineers in assessing the local site conditions against seismic risk. It is further expected to help local engineers in introducing appropriate allowances in structural design practice within the area concerned and to adopt preventive or remedial measures to reduce the seismic risk and hazard in critical areas of interest.

The objectives of the study are as follows;

i. Identification of the geotechnical condition,

ii. Identification of locations susceptible to soil liquefaction, and

iii. Seismic evaluation of existing building stocks

METHODOLOGY

Geotechnical condition & Dynamic soil parameters About 300 boreholes at 200 locations situated within Greater Colombo area have been used to determine the stratigraphy of the study area. This data have been collected from the local consultancy firms and government departments which provide geotechnical investigation services for construction purposes.

The determination of the dynamic soil parameters involves a series of successive calculations for valuating [N.sub.60], corrected SPT, effective stress, total stress, relative density, untrained shear strength, void ratio, shear modulus and shear wave velocity for each of the soil layers considered. This is a time consuming process involving a series of repetitive calculations with several interpolations. Therefore, it has been decided to formulate all these calculations in a spreadsheet program. This program requires only the entry of soil type, SPT values, and the depth of water table just as it appears in a borehole log sheet. The programme processes all the necessary calculations corresponding to each soil type and displays the soil type, unit weight, shear wave velocity, and thickness along with the depth of ground water table and location details.

Seismic evaluation of existing buildings

There are many types of buildings in Colombo with distinct structural settings. These buildings can be classified upon many aspects such as, the number of stories, construction techniques, materials of construction, period of construction, building configuration and etc. During the current study, many organizations including National Building Research Organization, National Housing Development Authority, Colombo Municipal Council, Department of Census and Statistics were contacted to explore the possibility of getting useful technical information on the building stocks of Colombo.

However, other than a report listing details of 202 historical buildings in Colombo Urban area no data were available as shown by Lewcock (1980). Considering the general construction type of the buildings and the period where there are major changes occur in the construction practices, they can be grouped into two different categories; namely, those constructed after 1960 and prior to 1960.

Most of the buildings in Colombo fall into the first category. Many of these buildings have been constructed using reinforced concrete and therefore, naturally have some shear capacity to tolerate low level of seismic shaking. The buildings in the second category, i.e. those constructed prior to 1960, lack reinforcement. According to Lewcock (1980), a few hundreds of such buildings are found in the Greater Colombo area. Most of these buildings are in poor condition having no capacity to resist any seismic loading. Some of them are situated in Colombo Fort and its neighbouring areas which are thickly populated. Therefore, these buildings are the most vulnerable category in Greater Colombo. It was decided to focus on assessing this type of buildings in the current study.

Forty old buildings (pre 1960 era) identified as weak, and ten buildings constructed after 1960s' were considered for the seismic evaluation. These fifty buildings include twenty seven un-reinforced masonry buildings, twenty two concrete frame buildings and one light metal building.

Evaluation was carried out in the following sequence.

i. Sketch plan showing the location of the building was drawn

ii. Details such as the year of construction, number of stories, usage of building, numbers of occupants, any special identifiers, etc. were obtained.

iii. Evaluation of the building type / facility class

iv. Efforts were made to obtain any other relevant information.

The seismic vulnerability of the structures was determined based on the evaluation procedures recommended by the FEMA of USA (Federal Emergency Management Agency of United States of America). In this procedure each building is given a set of basic and modifying scores to account for various structural conditions affecting the seismic stability. The structural weaknesses were visually inspected from outside the building. The corresponding score sets have been developed based on the expert's opinion obtained to assess the seismic vulnerability of California buildings for different levels of ground shaking and adjusted to evaluate the seismic performance of buildings in the regions out of California where low level of shaking is expected. The seismic hazard score for a particular building is the algebraic sum of the basic and modifying scores corresponding to the level of ground shaking considered. The final score typically range from 0 to 6, with higher scores corresponding to better seismic performance. These hazard scores have been formulated in such a way that the antilogarithm of the negative of the hazard score of a particular building will give the probability against a major structural damage. The analysis was carried out on all the structures against the three levels of shaking namely low, moderate and high level as reported by FEMA (1988). The performance modification factor to account for the effect of local soil profiles is another aspect to be accounted in the assessment procedure. The soil conditions within the study area generally comes into SL1 representing stable deposits of sands, gravels and stiff clays and into SL3 representing the and short to medium stiff clays and sands, exceeding 30 ft in thickness: FEMA (1988). The corresponding modification factors are 0.0 for SL1 and--0.6 and--0.8 for SL3 in respect of low rise buildings and 8 to 20 story buildings respectively. In most of the locations considered for the structural assessment there is not enough soil data available in the vicinity of the building assessed. Therefore, it was decided to take up the worst case of the presence of soil profile SL3 in all the locations as recommended by FEMA (1988). The sum of the hazardous scores of each of the buildings indicates the vulnerability of the structure against the level of shaking considered.

OBSERVATION AND DISCUSSION

Soil/rock profiles in the project area and their properties From the analysis of borehole logs it was found that the depth of the top soil layers is in the range of 5 m to 25 m. Sand profiles were observed in most part of the Colombo Municipal areas and some other adjoining urban areas along the Galle Road up to Moratuwa. Clay deposits were observed as isolated pockets within Colombo Municipality except near Borella where sand as well as clay layers have been observed. Rajagiriya, Malabe, Nugegoda, Nawala, Athurugiriya, Orugodawatta, Negombo, Ja-ela, Etul-Kotte, Boralesgamuwa, Wattala, Kelaniya are also some other locations where sand and clay profiles were observed. The peat deposits were generally observed in the low lying areas of Greater Colombo (some of these areas are now reclaimed) namely Peliyagoda, Orugodawatta, Blomendhal, Yakbedda, Nawala, Battramulla, Maligawatta etc: Senanayake, 1996. In most of the locations the soil profile ends up with the weathered rock.

Seismic evaluation of buildings in Colombo

The final seismic hazard scores corresponding to each of the building assessed are given in Table 1.

CONCLUSIONS

For the case of high level of shaking, all the buildings considered except the light metal structure, registered with poor seismic performance with more than 10% of the buildings exposed to major damage. About 75% of the buildings considered recorded less than 1.0 structural hazard score for moderate level of shaking. All the 27 un-reinforced masonry buildings have been registered with critical hazard scores for all the three levels of shaking. Out of the 22 cases corresponding to concrete framed buildings, 15 satisfied the cut-off score (2.0) for the low level of shaking. The light metal building exhibited best performance with high structural hazard scores. However, as a global measure the basic structural hazard scores can be used to get the overall picture assuming that the average building has no adverse effect due to building configuration or any supplementary strength due to the adoption of any seismic provisions.

It is possible to predict the vulnerability of a building against a given magnitude of earthquake. It is clear that an earthquake of magnitude 6 in Richter scale and epicentral distance grater than 160 km may cause no major damage in any type of the structures present within the study area.

ACKNOWLEDGEMENT

This work was supported by National Science Foundation of Sri Lanka under Research Grants RG/2001/E/03 and RG/2002/E/01, and these grants are gratefully acknowledged.

REFERENCES

Abaykoon S.B.S (1983) "Seismic Risk Analysis of Sri Lanka," Journal of the Geological Society of Sri Lanka, Vol. 6, pp 65-72.

Abaykoon S.B.S. (1998), "Seismic Response of Low Lying Areas in Colombo Sri Lanka," Engineer, Journal of Institution of Engineers, Sri Lanka, Vol. xxviii, Journal No-02, 29-36.

Federal Emergency Management Agency, (1988), "Rapid Visual Screening of buildings for Potential Seismic Hazards; A Handbook", FEMA 154, July 1988, Earthquake Hazard Reduction Series 41, pp 185.

Lewcock, R. (1980) "Report on the listing and presentation of historic buildings and zones in the Colombo Urban Area", United Nations Centre for Human Settlements (Habitat), August-September.

Senanayake, K.S., (1996) "Geotechnical Mapping of Low-lying Areas in and around Colombo City", Asian regional Symposium on Geotechnical Problems and Practices in Foundation Engineering, Colombo, Sri Lanka.

P.B.R. DISSANAYAKE

Department of Civil Engineering, University of Peradeniya Peradeniya, 20400, Sri Lanka
Table 1 Structural hazard scores of the buildings assessed in
Colombo area

 Final Score

Building Low Level of Moderate Level of High Level of
ID Shaking Shaking Shaking

A1 1.9 1.4 0.4
A2 0.9 0.4 -0.6
A3 1.4 0.9 -0.1
A4 2.4 1.4 0.9
A5 1.4 0.9 -0.1
A6 0.9 0.4 -0.6
A7 0.9 0.4 -0.6
A8 0.4 -0.1 -1.1
A9 1.9 1.4 0.4
A10 0.9 0.4 -0.6
A11 0.9 0.4 -0.6
A12 1.4 0.9 -0.1
A13 1.4 0.4 -0.1
A14 1.9 1.4 0.4
A15 1.9 1.4 0.4
A16 5.9 5.4 4.9
A17 1.9 0.9 0.4
A18 1.4 0.4 0.4
A19 1.4 0.4 0.4
A20 0.9 0.4 -0.1
A21 0.9 0.4 -0.1
A22 1.4 0.9 -0.1
A23 2.4 1.4 0.9
A24 1.9 0.9 0.4
A25 1.9 0.9 0.4
A26 2.4 1.4 0.9
A27 1.9 0.9 0.4
A28 1.4 0.9 -0.1
A29 1.4 0.4 -0.1
A30 0.9 0.4 -0.6
A31 1.9 0.9 0.4
A32 0.9 0.4 -0.6
A33 0.9 0.4 -0.6
A34 1.4 0.4 0.4
A35 0.4 -0.1 -0.6
A36 -0.6 -1.1 -1.6
A37 1.4 0.4 0.4
A38 0.9 0.4 -0.6
A39 1.9 1.4 0.4
A40 1.9 0.4 0.4
A41 -0.1 -0.6 -1.1
A42 0.9 0.4 -0.6
A43 0.9 -0.1 -0.1
A44 2.4 1.4 0.9
A45 1.9 0.9 0.4
A46 2.4 1.4 0.9
A47 1.9 1.4 0.4
A48 1.9 0.9 0.4
A49 1.9 0.9 0.4
A50 1.9 0.9 0.4
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Author:Dissanayake, P.B.R.
Publication:Geotechnical Engineering for Disaster Mitigation and Rehabilitation
Article Type:Conference news
Geographic Code:9SRIL
Date:Jan 1, 2005
Words:2313
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