SEIMIC DESIGN IN URBAN AREA

1. Introduction

In taking the course Geology for Engineers we are going to dig out the consideration in design and planning of any engineering projects in reference to the seismic zonation of our country. In doing so we have tried to see areas which needs to be in question of the seismic hazard and what should be the design and planning consideration for those zones. Earth quake loadis one of the loads that is considered in load analysis which are dead load, liveload, wind load and dynamic loadwhich comes from vibrating machineries.

Earthquake destruction has provided the motivation for detailed investigations of earthquake effects upon buildings and civil engineering structures as well as infrastructure systems with the basic aim of undertaking corresponding protective measures, depending on the economic and technical power of the country involved. The implementation of technical regulations for design and construction in seismic areas is the basic protective measure which provides the required resistance of the structure against earthquake.

The implementation of protective measures against destructive earthquake effects upon structures is much more effective than mitigation of other direct or indirect consequences of earthquake. The present level of knowledge and techniques enables effective implementation of protective measure against the destructive effect of earthquakes upon structures through the methodology of physical planning and urban design in earthquake prone areas. The physical and urban planning for seismic areas is young discipline.

One of the basic elements in urban planning and design in seismic areas is the seismic urban microzoning map with its appendices. This map is developed on the bases of previously performed detailed investigation of the urban area including seismological, seismotectonics, seismological- engineering, geophysical, hydrological and other studies.

The seismic microzoning map of an urban area should show zones of maximal expected seismic intensity and even sub-zones. The map should also show all characteristics from performed seismotectoincs, geological, geotechnical, hydrological and other investigations, i.e. the characteristic elements of the soil, which show the suitability of the terrain for construction.

The seismic microzoning map of the urban area shows clearly the possibilities for rational utilization of the terrain for developing the physical concept of the urban plan. The zones with most favorable conditions, i.e. where lowest maximal seismic intensities are expected are clearly defined. Such seismically favorable terrains should be used as construction sites of the most important basic urban activities as well as for future urban development. However, the dispersion of industry in to several industrial zones within the urban area of larger towns should also be considered, so that disastrous earthquakes will not have the same effects in all industrial zones.

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2. Review of related literatures

1.Habtom G/Medhin (June 2006) Earthquake hazarded assessment with the help of remote sensing and GIS technology:According to his study the earthquake hazard in an area depends mainly on three factors: 1) The Regional earthquake sources and seismic wave propagation characteristics, 2) The local geology’s response to, and modification of, Earth- quake ground shaking and 3) Type and use of buildings and life lines constructed in the area, which includes the:- the number of people living under the roof, the time the earthquake happened ,night, rush hour, day time etc.

In this study the study area is assessed in terms of the above controlling factors, it is found to be the most earthquake hazard prone area in terms of earthquake frequency in Ethiopia is around Semera.The city of Semera in Afar Region is located in one of the most active seismic zones inEthiopia and east Africa. An earthquake intensity level of VIII and IX are expected in the area, these earthquake intensities have a power that can damage the buildings that are constructed in the area, the Ethiopian building code standard uses the same building regulations for all towns in the rift(Arbaminch, Ziway,Nazret, Semera, Dessie, michew, Mekelle, Adigrat, etc.), hence the building code needs updating, because the intensity levels expected in semera area is higher than the other areas in the rift valley system.

Figure1: the seismic hazard map of Ethiopia based on the GSHAP data for a return Period of 475 years.

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On the basis of this study there are two methods of analysis for the assessment of seismic hazard at any given site, which are the probabilistic and the deterministic approaches. Problemastic method:-seismic hazard is defined as the probability of certain level of ground motion being exceeded at given place and within a given time period. This is the most widely used probabilistic approach and the main input is the activity rate. While, deterministic approach requires inputs of discrete, single-valued events(maximum earthquakes) and models to arrive at a description of earthquake hazard, input parameters are seismic sources(source parameters), size of damaging earthquake(controlling earthquake) and an earthquake ground motion attenuation relationship.

After assessing the study area he has recommended that theconstruction that are built in semera must be classified in to land use categories for engineering purpose,as an example in areas of very high hazard we can built buildings from wood, villas from highly reinforced massonery, and prohibit from building dams, oil reservoirs, nuclear plants and septic tanks, across the axis of the red sea rift.

2, Samuel Kinde (march 2002) ; Earthquake Risks in Addis Ababa and other major Ethiopian cities:-

As the study tells us due to its location right on one of the major tectonic plates in the world, earthquakes have been a fact of life in Ethiopia for a very long time. As a result numerous earthquakes had happen. The most significant earthquakes of the 20 th century like the 1906 Langano earthquake, the 1961 Kara Kore earthquake, the 1983 Wondo Genet earthquake, the 1985 Langano earthquake, the 1989 Dobigraben earthquake in central Afar, and the 1993 Nazret earthquake were all felt in Addis Ababa, and the other major cities of Nazret and Awassa. 

The increase in the urban population results a strong growth in the number of high-rise buildings, residential houses, schools, bridges, water supply pipes, and other infrastructure constructions. Therefore, the proximity of these significant earthquakes to the major population centers such as AddisAbaba, Awassa and Nazret, obviously leads to the question of how much damage will be sustained by these buildings, bridges, dams, water supply pipes and other so-called life-lines. And it willdestruct not only the buildings and infrastructures available but also the economic performance of the country.

With the continuing realization of the potential risks of earthquake hazards to the country's major urban centers, the study has mentioned some discussion on preventive as well as retrofitting measures along with contingency plans has already begun in the country even though there is no clear guidance if the momentum could be kept and the country’s pertinent professionals are involved.

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1. From pre-disaster planning point of view, the following action items are proposed:

Implement aggressive enforcement of the country's building code standard requirements for newer buildings.

Improve quality control procedures in construction of new buildings. Start sustained retrofitting effort on some of the key hospital and school

buildings. Particular emphasis should be placed on industrial structures in the Nifas Silk area that house heavy chemical and petroleum-product factories. The retrofitting work recently completed on the privately owned Selam Hospital in the Mekanisa area can serve as a good example.

Implement retrofitting of major lifelines like water pipelines and electrical sub-stations. The major dams also need to be strengthened.

Provide incentives such as low-interest rate loans to owners of buildings to finance retrofitting projects.

2.From "during and post" disaster contingency plan point-of-view, the following recommendations should be adopted for the desired results:

Prepare mobile bridges to be used in key locations. Provide emergency water supply, and emergency maintenance of water

pipelines.  Provide stand-by tents to house emergency medical units Provide stand-by emergency generators and mobile sub-stations. Provide alternate emergency airfields.

3.Sallehunae kefyale (December 2010 ):- avoiding seismic risk to health center buildings.

According to his paper, The horn of Africa has significant seismic hazard associated with the East Africa Rift Valley system. In the region a number of destructive earthquakes, some deadly, have been reported this century. Similarly in Ethiopia several earthquakes have been occurred. Notable events include the 1961(M=6.1in Richter magnitude) Kara Kore earthquake destroyed the town of majete, the 1969(M=6.3) Serdo event resulted in four deaths, and the 1989(M=6.5) Dobi graben earthquake destroyed several bridges between Assab and Addis Ababa.

Ethiopia is placed 42nd out of 153 states with earthquake risks. The rift valley covers a substantial portion of the century with relatively moderate seismic hazards. Located in the great valley, there are hundreds of health facilities serving a large number of the Ethiopian people.

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CODE OF PRACTICE FOR BUILDING IN SEISMIC REGIONS

Almost all code of practices including the Ethiopian codes of practices, before outlining the basic requirements and compliance criteria applicable to buildings in seismic regions, make the following assumption;

A. structures are designed by appropriate qualified and experienced personnel. B. adequate supervision and quality system are provided during execution of the work,i.e.,in design offices, factories, plants and on site. C. construction is carried out by personnel having the appropriate skill and experience. D. the construction material and products are used as specified in the relevant material or product specification. E. the building will be adequately maintained. F. the building will be used in accordance with the design brief.

Buildings should be designed to resist minor earthquakes without damage, because they may occur almost everywhere. For major earthquakes, it may not be economical to prevent all damage but collapse should be precluded.

PRINCIPLE AND STRATEGIES

The principle and strategies of seismic design and construction follow three steps;

1. Analyze site condition; the location and physical properties of the site primary influences of the entire design process.

2. Establish seismic design objective; a performance based approach to establish seismic design objective is recommended. This determines a level of predictable building behavior by responding to the maximum considered earthquake.

3. Select/Design appropriate Structural System; seismic design objectives can greatly influence the selection of the most appropriate structural system and related buildings system for the project.

Structural and architectural detailing and construction quality control are very important to insure ductility and damping and to keep damages to a limited and repairable range.

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3. Analysis

3.1. Earthquake Hazard History In Ethiopia

3.1.1. Where do earthquakes happen in Ethiopia?

Epicenters of earthquakes which could have produced damage within the borders of Ethiopia, it is apparent that most of the country is subject to seismic activity. The greatest concentration of epicentres is found along the western escarpment of Afar and along the Ethiopian rift. This seismic belt of about 200 km in width runs parallel to the Asmara-Addis Ababa-Moyale highway. During the last century, centers of activity have moved along that zone: in 1842, the activity was located near Ankober and the then-capital of Shoa was completely destroyed (1); in 1853-1854, the epicentres had moved 3° to the north and left substantial fissures in the ground near lake Ashangui (2); in 1884, they were located off the coast of Massawa and the city was heavily damaged (3,4,5,6); in 1906, swarms of earthquakes shook the Shoa province at an epicentral distance of 75-100 km from Addis Ababa; (7,8,9); during June-July 1913, the epicenters were located North of Asmara; in 1921, Massawa was completely destroyed (10,11) and the people of Eritrea remember these years as zemenedebleklek in 1960, an earthquake of magnitude 6.3 hit a few kilometers west of Sheshamane near the dormant volcano Chabbi; in 1961, the seismic activity was centred in Wollo where Majete was 10010 destroyed and Kara Kore heavily damaged; in 1964, Dessie was hit. When compared to the rest of Ethiopia, it is estimated that since 1906, about 75 of the total energy has been released along this seismic zone.

The south-eastern region of Ethiopia, referred to in geokgy as the Somali plateau, is much less seismically active. Apart from the series of shocks which occurred in Rararghe in 1953 (12) and caused damage both in Rarar and Dire Dawa, no other seismic event has ever been reported. Ogaden seems to be practically inactive.

3.1.2. How deep are the foci in Ethiopia?

In estimating the seismic risks, the focal depth of a shock is important since it is a primary factor controlling the energy radiation pattern. A good example of the importance of focal depth to the degree of damage for shocks of similar magnitudes is the result of earthquakes of Kara Kore and Sardo. During the Kara Kore quakes of 1961 damage was observed as far as 250 km from the epicentral region: among other instances, in Addis Ababa partition walls of a well-designed modern building were detached from their reinforced concrete frame.

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The focal depth of the Kara Kore quakes reached 57 kilometres. In Sardo, this year, damage was extremely heavy but restricted to very small areas: the village of Sardo and a particular fault-line. The only damage to houses outside of Sardo was the collapse of an adobe brick house in Dubti, about 40 km away. Most of the shocks had a focal depth of 4-5 kilometres only.

The smaller the focal depth, the higher the absorption of radiated energy with distance, the higher the damage in a restricted zone near the epicentre. All earthquakes happening in Ethiopia are classified as "shallow"; the focal depths range between 4 and 60 kilometres.

3.1.3. How often do earthquakes happen in Ethiopia?

During seismically quiet years, the Geophysical Observatory has recorded since 1959 an average of 1.5-1.6 seismic events per day originating within a radius of 1000 km from Addis Ababa. Since there is only one seismic station in the country, events happening beyond a maximum epicentral distance corresponding to their magnitude are naturally eliminated from these statistics.

In the years of higher seismic activity, the daily count reached 350 at Addis Ababa in May 1961 (13).In Asmara, between June 6 and July 16, 1913, 141 shocks were recorded on the very low sensitivity equipment temporarily installed in the city (14); 457 shocks were also reported felt at the same place between May and June 1921 when swarms of earthquakes destroyed Massawa (11). Historical documents often report that tremors were felt for months and months and even years, especially in Tigre, after a large earthquake. All the recorded earth tremors are not necessarily reported felt or destructive.

3.2. Basic Criteria The following three basic criteria for Seismoresistant Structural Design are valid in the Seismoresistant architecture (SRA) as well, not only to prevent the seismoresistance stepping, but also to optimize the design. 1- The seismic coefficient for the various stories of a building increases according to the building height. Consequently, during the architectural design, it is very important to place archives, swimming pools, or rooms containing heavy equipment in lower levels, in order to minimize seismic effects. 2- The resistant elements may be placed with a certain degree of independence from the vertical load. This feature remarkably facilitates both the structural and the architectural designs. In fact, it allows us to place the main resistant elements in the most suitable fashion to minimize the torsional effects and to attend to architectural project requirements. 3- Seismic forces are proportional to the building weight. With seismic forces proportional to the building weight, it is advisable to reduce the latter. In practice, this goal can be attained in the following items: Partition walls Floor slabs Floors Wall plastering and/or covering resistant structure of the building.

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3.3. Effectiveness and optimization of the seismoresistant response in building

Essentially this objective may be achieved either decreasing the seismic forces or increasing the efficiency of the seismic capacity of buildings. Reduction of values of seismic forces may be achieved in various ways, i.e.:1- By using lightweight materials or avoiding unnecessary fillings and finishing’s.2- By relocating heavier weights, that is, trying to place those rooms that will bear heavier weights (e.g. Archives, swimming-pools, meeting rooms, etc.) in lower levels. Seismic bending moments and shearing acting on the structure are thus reduced and consequently, the size of the resistant elements. These are very important facts for the Architectural Design.3- By avoiding the pseudo-resonance. This means to prevent the fundamental period of the building from coinciding with the main one of the foundation soil. The optimization of the seismoresistant capacity of the building must be done by using spatial shapes that may lead to a building with a clear and simple structure, having its torsion stress centre coincident with its mass centre.

3.4. Reason which cause the stepping of theSeismoresistant capacity of a building

Reasons for which the maximum earthquake- resistant capacity of a building is not equal to the sum of the capacities of each element.

1-Inherent causes to the structural design: The process for matching both the resistance and the stiffness of structural elements undergoes trials with difficult solutions in the earthquake structural design practice, which introduces an uncertainty factor that may give rise to a decrease in earthquake resistant capacity characteristics, and a later collapse of the structure.

2-Earthquake torsion: The fact that the seismic shear in a column caused by a torsion moment is proportional to the distance to the centre of torsion or stiffness (CT) results in, for columns which are dimensionally equal, but placed at different distances from CT, their seismic stresses are different. This may be the fundamental cause for the decrease in resistant capacity, especially if the torsional effects have not been provided for. This may be the cause of the stepping up of the seismic resistance, especially if the torsional effect has not been adequately provided for. There are some other cases where the development of a moment is due to an unpredicted strain of floors. Finally, even when torsion is taken into account into the structural analysis, it is still difficult to keep the stiffness- resistance ratio for columns.

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3-Flexible floor: A flexible floor greatly decreases the seismic shear on the remainingstiffer floors, but, at the same time, the shear increases on the flexible floor itself. Here, again, we have a stepping up of seismic resistance. Taking this fact into account during the structural analysis calls for ductility and resistance characteristics which are very difficult to achieve in the practice. It would be better to assign the necessary stiffness-resistance ratio to such a floor instead of using it to improve the seismoresistant performance of the whole building.

4- Short beams: With rigidly framed structures, when one of the beams on a certain floor or level is of a notably lesser length than the remaining ones, the so-called short beam case occurs. Here the problem arises because angular stiffness is inversely proportional to its length. Like in the previous cases, the difficulty here is to achieve the required resistance-stiffness relationship. The concentration of bending moments may break the beam, facilitating the decrease of the seismoresistant capacity of the structure.

5- Non-structural elements: It is known that non-structural elements such as walls, partition walls, installations, etc. interfere with the expected behaviour of the resistant structure. This interference may be either of a positive or a negative sense. This has also been a frequent reason for decreasing the building total seismic capacity.

6- Constructive defects: A localized constructive defect on the resistant structure, besides decreasing the seismoresistant capacity, may originate stepping. In fact, it may give rise to an unexpected torsion moment.

7- Like constructive defects, this may be the cause of a decrease in the seismoresistance capacity and its unexpected stepping.

3.5. METHODOLOGICAL PROPOSAL

3.5.1. Structural Design Requirementsa) To avoid short columns. b) To exclude any unnecessary weight, and to use lightweight materials. c) To use rigidly framed structures of high hyperstaticity in order to attain ductility. d) To prevent seismic torsion.e) To promote symmetric plans.f) To prevent building collision. g) To avoid flexible floors. h) To avoid liquefiable soils. i) To consider epicentre distance. j) To design either flexible or stiff buildings according to the predominant period of the foundation soil. k) To prevent pseudo-resonance.

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Each of the above constraints interacts with the following subsystems or building components; Constructive system, Structural system, spatial configuration, Installations, Economical and Esthetical.

3.6. STANDARDS OF COMPATIBILIZATION OF ARCHITECTURAL AND STRUCTURAL DESIGNS

The referential variables which are to be morphologically compatibilized that are considered at the very beginning are.Flexible Floors.This situation arises when, at a certain floor, the stiffness of a tall building is considerably reduced in relation to the contiguous floors. This situation causes a strong concentration of seismic forces on the site, giving rise to a dangerous stepping mechanism of the building resistance. The morphological answer is to avoid this feature in the architectural design. Whenever a floor with large separations between columns is required, it should be the last one or it should be placed outside the tower site, preferably designed as a single level.

Building Collision This phenomenon takes place when there are no joints between contiguous buildings and the collision is produced when the oscillations are not synchronized. This is a completely abnormal situation which must be definitively avoided. The morphological answer is building separation, as current rules specify. It is recommended to take into consideration into the design the various functions of the completely separated bodies for the case of the same building, in order to prevent building collision, to provide a uniform structure and also to avoid sudden stiffness changes in plan and elevation.

Seismic TorsionThis effect is produced whenever the Stiffness Centre (SC) and the Torsion Centre (TC) do not coincide, thus causing additional constraints especially in those elements which are far removed from the SC, which might lead to the stepping of the seismoresistant capacity of the building. Although this problem is considered in the structural analysis, it is completely undesirable since it generates large additional and misbalanced seismic forces in the set of columns, giving rise to the stepping of the seismoresistant capacity of the building. The morphological solution is met by designing buildings with a symmetrical plan and elevation. In addition, the structural and non-structural interacting elements symmetry is required, as well as the functional symmetry of the architectural site.

Pseudo-ResonanceThis phenomenon arises whenever the period of the building matches the predominant period of the foundation soil. This condition remarkably increases the seismic effects. On the other hand, if the fundamental vibration period of the building depends on its dimensions and structure stiffness, then, the morphological solution is to manipulate these parameters.

Sudden Stiffness Changes in Plan and ElevationThis situation can be prevented by using compact, homogeneous spatial shapes in the architectural design.

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Stiffness-FlexibilityWhenever a rigid or a flexible building is required, i.e. one that can be strained to a certain low or high degree respectively, the common practice is to use for the first case rigid structures such as partition walls made up of reinforced concrete and/or high density and high resistance masonry walls, 0.20m. Thick, and for the second case, the selection is for materials which are adequate for flexible buildings. Both cases will influence the spatial morphology of such buildings.

Concentrated WeightIn most of the current seismoresistant standards, the seismic coefficient increases almost proportionally to the floor level with respect to the ground level. Consequently, in Architectural Design, this principle must be borne in mind, not only to avoid using heavy materials, subfloors, partition walls, coverings,etc., at higher levels, but also to place the sites designed for archives, swimming-pools, or heavy equipment at lower levels. In so doing, two purposes are achieved: firstly, a reduction of the seismic forces, since the seismic coefficient increases at higher levels and secondly, a logical reduction of the seismic shear and moments.

Short Columns:Another aspect related to the resistance-stiffness problem is the so-called “Short Column”. In this case, the seismic shear increases inversely proportional to the cube of its height for columns of equal cross-sectional area. In addition, this situation worsens for short columns because concrete is unsuitable for resisting strong tangential stresses, thus notably decreasing its ductility. These instances are originated by a particular feature of masonry which reduces the columns height and consequently, their stiffness becomes greatly increased. This causes the seismic shear to concentrate on the column, which logically cannot resist.

The rupture of the resistant elements could make the rest of the elements yield as well, which could in turn bring about the total collapse. This situation can be easily avoided by appropriately designing the shape and location of spaces and openings. On the other hand, when this problem results from differences in elevation between medium-height mezzanines, its elimination is practically impossible. Therefore, these elevation differences must be removed from the seismoresistant architectural design.

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Table 1 sesimic zonition of ethiohpa from EBCS.

Zone 0 Zone 1 Zone 2 Zone 3 Zone 4

Agaro Ambo Adiss Ababa Akaki Adigrat Arjo Axuim Adewa Aleta Wendo Ankober Asosa Harar Hagere Mariyam Asebe Teferi Arba MinchAzezo Jijiga Alem Gena Asela Asayta Bahir Dar Jima Alem ketema Dila Awash StationBako Kibre Mengist Dire Dewa Felge Neway Awassa Bedele Maji Fiche Jinka Aysha Bonga Weliso Moleta Mekdela BatiBure Yabelo welikite Sekota Debre BrehanChagni Sire(Arsi) Deber SinaDebark Tensae Berhan Debre ZeitDebre Markos Yergalem Dese Debre Tabor Durame Degen Gawani Dembi Dolo Gidole Gambela Hosaina Goba Kara koreGembi Kobo Goha Tsion Koka Gonder Kombolicha Gure Maychew Inda silasie Mehal Meda Metema Mekele Mezan Tepi Meki Negele borena Metehara Nejo Mojo Nekemte Molale Setit humera Naztirt

Serdo Shashemene Sodo Tendaho Weldiya Welenchite Wendo GenetZiway

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4. Conclusion

This technical project paper is developed with the purpose of planning and design in the effective results of earthquake mitigation efforts at national levels. It is sincerely hoped that the ability of the individual to use the information presented in this report will accommodate incentives to explore alternatives, examine the effectiveness of new initiatives in earthquake hazard reduction plans and policies, and undertake innovative studies to reduce the damage caused by major seismic events.

5. Recommendations

In planning and designing all structures and various types of infrastructure in large urban areas with high seismic activity special attention should be given to protective measures so that they can remain effective in case of catastrophic earthquakes. Basic recommendations concerning planning and design of infrastructure in urban areas which are in seismic zone are as follows;

Buildings shall be protected from earthquake-induced pounding with adjacent structures or between structurally independent units of the same building. This is deemed to be satisfied:(a) For buildings, or structurally independent units, that do not belong to the same property, if the distance from the property line to the potential points of impact is not less than the maximum horizontal displacement of the building at the corresponding level(b) For buildings, or structurally independent units, belonging to the same property, if the distance between them is not less than the square-root-of-the-sum-of-the-squares (SRSS) of the maximum horizontal displacements of the two buildings or units at the corresponding level.

The urban plan should avoid organization of a single main traffic centre, since that can result in complete disruption of normal traffic after a disastrous earthquake.

In principle, gas systems should be avoided, if possible, since they can cause secondary damage by fire in large urban areas with high seismic activity. In case such systems are planned, it is necessary to provide for automatic disconnection of the system in the event of catastrophic earthquake as well as for devices which will enable disconnecting damaged sectors from the system.

The outline of a building should be symmetric to the main orthogonal axis. It is best if buildings have square or rectangular shapes.

In case of structures with complex bases and structures where certain parts have different numbers of stories, they should be divided by seismic joints in such a way that the bases of the divided parts should have simple geometrical shapes.

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In case where constructions are in a row, such buildings should be separated by seismic partitions at selected intervals.

Structures which vibrate with the same resonance as the soil are subjected to greater damage than those which have a different resonance. Because of this , when determining the number of stories of a building complex in the frame work of urban plans, the map of predominant periods of the urban area should be taken in to consideration along with other factors. This map shows the distribution of zones with the same predominant periods. Thus, in area with very small predominant periods, usually from 0.15-0.25 sec., it is recommended construction of low buildings should be avoided, i.e. buildings of one to three stories. On terrain with small predominant periods low buildings can be constructed, as well as high ones.

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6. REFFERENCES

HUGO GIULIANI, “seismic resistant architecture” (2000) HABTOM G/MEDHIN, “earthquake hazard assessment with the help of remote

sensing and GIS techniques” (2006) TIBERIJE KIRIJAS, “physical planning in seismic regions” Prof. EZIO FACCIOLI & Prof. GAIAN MICHELE CALVI, “model building code

for earthquakes” (2003) SAARC Disaster Management Centre , New Delhi , India, “seismic micro

zonation” (2011) Journal of Ethiopian Engineers & Architect “vol.28” (2011) Journal of Ethiopian Engineers & Architect “vol.4” (1970) SALLEHUNAE KEFYALEW, “avoiding seismic risks to health centre

buildings”(2010)

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