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ADDIS ABABA UNIVERSITY SCHOOL OF GRADUATE STUDIES This is to certify that the thesis prepared by Tsion Fekadu Mekonnen, entitled: Assessment of Seismic Design Practice and Overall Structural Design Process of Buildings Designed in Addis Ababa and submitted in partial fulfillment of the requirements for the degree of Degree of Master of Science (Structural Engineering) complies with the regulations of the University and meets the accepted standards with respect to originality and quality. Signed by the Examining Committee: Examiner_________________________________Signature__________Date_______ Examiner________________________________ Signature __________Date_______ Advisor__________________________________Signature__________Date_______ Advisor__________________________________Signature__________Date_______

___________________________________________ Chair of Department or Graduate Program Coordinator

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ABSTRACT Assessment of Seismic Design Practice and Overall Structural Design Process of Buildings Designed in Addis Ababa.

Tsion Fekadu Mekonnen Addis Ababa University, 2015

This thesis is conducted to assess seismic design practice and overall structural design process of buildings designed in Addis Ababa. Design models were collected from Addis Ababa City Administration and randomly selected ten models were assessed. Eurocode, which is similar to Ethiopian Building Code and Standard, is used to evaluate the models. The main objective of this thesis is assessing seismic design practice at different consulting offices, consideration of dynamic analysis for irregular structure, fulfillment of fundamental requirements for earthquake design, structural configuration of structures for earthquake design, stiffness reduction, slab and column design. The findings of this research shows that in all the models the slab has been designed as part of the lateral force resisting system, majority of models did not make stiffness reduction for the slab section and reduce flexural and shear stiffness properties of concrete and masonry element for sway frame. It is also found that fundamental requirement and structural configuration of structures for earthquake design are not fulfilled. In addition some of the models did not consider accidental eccentricity, P-∆ analysis and dynamic analysis as per code requirement. After making modification to iii   

   

these models as per Eurocode, it is found that there is a significant increase in column and beam shear force and bending moment. This would imply that seismic design practice and overall structural design of building designed in Addis Ababa are not up to the code requirement. Keywords: Seismic Design; Dynamic Analysis; P-∆ Analysis; Stiffness Reduction; Fundamental Requirement of Earthquake Design

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ACKNOWLEDGEMENT Above all I want to thank God for giving me the strength to complete this thesis work. My deepest gratitude then extends to my beloved families for always standing by my side and their support. I would like to thank Female Scholarship for providing me the opportunity to study MSc in Structural Engineering and sponsoring me throughout my study. I would like to deeply express my gratitude to Eyasu Ashenafi for his help in selection of this thesis topic and support for completing this thesis work. My deepest gratitude extends to my advisor Dr-Ing.Adil Zekaria for his encouragement and guidance throughout this thesis work. I would like to thank my colleges at MH Engineering and Daniel Taye for providing me information and data which brought this thesis in to picture. I would like to thank Elias Tsga for his help in editing and finalizing the paper work. Finally I would like to thank Addis Ababa City Administration for providing me designed models. Without their help this work could have not been real.

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TABLE OF CONTENTS 1

2

INTRODUCTION .................................................................................................. 1 1.1

General ............................................................................................................ 1

1.2

Objective ......................................................................................................... 4

1.3

Materials and Methods .................................................................................... 4

1.4

Limitations ...................................................................................................... 5

LITERATURE REVIEW ....................................................................................... 6 2.1

Review of Historical Records of Earthquake in Ethiopia ............................... 6

2.2

Review of Seismic Mechanisms and Seismicity in Ethiopia .......................... 7

2.3

Review of Response of Built-up Structures to Seismic Events in Ethiopia .... 7

2.4

Background and current state of code-required seismic design in Ethiopia . 12

2.4.1

Ethiopian Building Standard Codes ....................................................... 12

2.4.2

Seismic Zoning ...................................................................................... 13

2.5

Deficiencies in current code and proposed revisions .................................... 16

2.5.1 2.6

Seismic Zoning and Peak ground acceleration (PGA) .......................... 16

Characteristics of earthquake resistant buildings .......................................... 19

2.6.1

Basic principle of conceptual of design ................................................. 19

2.6.2

Structural regularity ............................................................................... 20

2.6.2.1 General ............................................................................................... 20 2.6.2.2 Criteria for regularity in plan according to EBCS 8-1995 ................. 21 2.6.2.3 Criteria for regularity in Elevation according to EBCS 8-1995 ......... 22

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2.7

Structural analysis according to EBCS 8-1995 ............................................. 23

2.7.1

Modeling ................................................................................................ 23

2.7.2

Accidental eccentricity........................................................................... 24

2.7.3

Method of analysis ................................................................................. 24

2.7.3.1 Equivalent static analysis according to EBCS 8-1995 ....................... 25 2.8

Safety verifications ........................................................................................ 29

2.8.1

Ultimate limit state ................................................................................. 29

2.8.2

Seviceability limit state .......................................................................... 30

2.8.3

Combination of actions .......................................................................... 30

2.9

Frame member stiffness ................................................................................ 30

2.10 Slab modeling ................................................................................................ 32 2.11 Number of modes considered ........................................................................ 33 2.12 Scaling factor for base shear ......................................................................... 33 3

RESULT ............................................................................................................... 34 3.1

Model 1:-Two basements + Ground + 10 story +Roof + Top Roof ............. 34

3.1.1

Structural configuration ......................................................................... 34

3.1.1.1 Plan regularity .................................................................................... 35 3.1.1.2 Elevation regularity ............................................................................ 36 3.1.2

Seismic design ....................................................................................... 39

3.1.3

Summary ................................................................................................ 44

3.2

Model 2:- Ground +Mezzanine + 4 story+ Roof +Top Roof ........................ 56

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3.2.1

Structural configuration ......................................................................... 56

3.2.1.1 Plan regularity .................................................................................... 57 3.2.1.2 Elevation regularity ............................................................................ 57 3.2.2

Seismic design ....................................................................................... 57

3.2.3

Summary ................................................................................................ 58

3.3

Model 3:- Three Basement + Ground + 16 story + Roof + Top Roof .......... 61

3.3.1

Structural configuration ......................................................................... 61

3.3.1.1 Plan regularity .................................................................................... 62 3.3.1.2 Elevation regularity ............................................................................ 62 3.3.2

Seismic design ....................................................................................... 62

3.3.3

Summary ................................................................................................ 63

3.4

Model 4:- Two Basement + Ground +Mezzanine + 11 story ....................... 65

3.4.1

Structural configuration ......................................................................... 65

3.4.1.1 Plan regularity .................................................................................... 66 3.4.1.2 Elevation regularity ............................................................................ 66 3.4.2

Seismic design ....................................................................................... 67

3.4.3

Summary ................................................................................................ 67

3.5

Model 5:-Two basements + Ground + 10 story +Roof + Top Roof ............. 69

3.5.1

Structural configuration ......................................................................... 69

3.5.1.1 Plan regularity .................................................................................... 70 3.5.1.2 Elevation regularity ............................................................................ 70

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3.5.2

Seismic design ....................................................................................... 70

3.5.3

Summary ................................................................................................ 71

3.6

Model 6:- Basements + Ground + 7 story +Mezzanine floor + Roof ........... 72

3.6.1

Structural configuration ......................................................................... 72

3.6.1.1 Plan regularity .................................................................................... 73 3.6.1.2 Elevation regularity ............................................................................ 73 3.6.2

Seismic design ....................................................................................... 73

3.6.3

Summary ................................................................................................ 74

3.7

Model 7:- Two Basement + Ground + 10 story + Roof + Roof Top ............ 75

3.7.1

Structural configuration ......................................................................... 75

3.7.1.1 Plan regularity .................................................................................... 76 3.7.1.2 Elevation regularity ............................................................................ 76 3.7.2

Seismic design ....................................................................................... 76

3.7.3

Summary ................................................................................................ 77

3.8

Model 8:- Ground + 7 story + Roof + Roof Top........................................... 78

3.8.1

Structural configuration ......................................................................... 78

3.8.1.1 Plan regularity .................................................................................... 79 3.8.1.2 Elevation regularity ............................................................................ 79 3.8.2

Seismic design ....................................................................................... 79

3.8.3

Summary ................................................................................................ 80

3.9

Model 9:- Ground + 3 story + Roof + Top roof ............................................ 81

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3.9.1

Structural configuration ......................................................................... 81

3.9.1.1 Plan regularity .................................................................................... 82 3.9.1.2 Elevation regularity ............................................................................ 82 3.9.2

Seismic design ....................................................................................... 82

3.9.3

Summary ................................................................................................ 83

3.10 Model 10:- Two basement + Ground + Mezzanine + 13 story + Top roof ... 85 3.10.1

Structural configuration ......................................................................... 85

3.10.1.1

Plan regularity................................................................................. 86

3.10.1.2

Elevation regularity ........................................................................ 86

3.10.2

Summary ................................................................................................ 87

4

DISCUSSION....................................................................................................... 88

5

CONCLUSION AND RECOMMENDATION ................................................... 95

6

REFERENCE ....................................................................................................... 97

APPENDICES ............................................................................................................. 98 Appendix A- Detail Calculation for Each Model..................................................... 99 Appendix-B Sample Architectural Drawings ........................................................ 147

DECLARATION

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LIST OF FIGURES Figure 2.1 (a) Seismic zoning of Ethiopia as per Gouin (1976) which was also used by CPI-78 (b) Seismic zoning of Ethiopia as per ESCP-1:1983 (c) Seismic Zoning of Ethiopia as per EBCS-8:1995. [1] ............................................................................... 15 Figure 2.2 Criteria for regularity of setbacks ............................................................... 23 Figure 3.1 Three dimensional view of model one ....................................................... 35 Figure 3.2 Three dimensional view of model two ....................................................... 57 Figure 3.3 Three dimensional view of model three ..................................................... 62 Figure 3.4 Three dimensional view of model four ...................................................... 66 Figure 3.5 Three dimensional view of model five ....................................................... 70 Figure 3.6 Three dimensional view of model six ........................................................ 73 Figure 3.7 Three dimensional view of model seven .................................................... 76 Figure 3.8 Three dimensional view of model eight ..................................................... 79 Figure 3.9 Three dimensional view of model nine ...................................................... 82 Figure 3.10 Three dimensional view of model ten ...................................................... 86 

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LIST OF TABLES Table 2-1 List of earthquakes and reported damages between 1979 to 2011 ................ 9 Table 2-2 Optimal structural configuration [5] ............................................................ 20 Table 2-3 Consequences of structural Regularity on Seismic Design ......................... 21 Table 2-4 Bedrock acceleration ration αo .................................................................... 25 Table 2-5 Importance categories and important factors for buildings ......................... 26 Table 2-6 Site coefficient ............................................................................................. 26 Table 2-7 Basic value γo of behaviour factor ............................................................... 28 Table 3-1 Limit of re-entrant corners in meter ............................................................ 35 Table 3-2 Percentage exceedance of re-entrant corners from the limit ....................... 36 Table 3-3 Setback on elevation of the building in meter ............................................. 37 Table 3-4 Percentage exceedance in setback on elevation of the building from the limit .............................................................................................................................. 37 Table 3-5 Summery of structural configuration of model one .................................... 38 Table 3-6 Stiffness reduction factor ............................................................................. 38 Table 3-7 Column design ............................................................................................. 38 Table 3-8 Load combination ........................................................................................ 39 Table 3-9 Auto seismic input data using UBC 94 ....................................................... 40 Table 3-10 Design interstory drift for serviceability limit state in meter. .................. 42 Table 3-11 Interstory drift sensativity coefficient for ultimate limit state in meter..... 43 Table 3-12 Total reactive force at the origin. .............................................................. 45 Table 3-13 Design interstory drift for servicibility limit state in meters. ................... 47 Table 3-14 Interstory drift sensativity coefficient for ultimate limit state in meters ... 48 Table 3-15 Comparison of Column moment and shear force for Envelope X and Y in KN-m for C106 ............................................................................................................ 49 xii   

   

Table 3-16 Comparison of Column moment and shear force for Envelope X and Y in KN-m for C42 .............................................................................................................. 50 Table 3-17 Comparison for beam moment and Shear force for envelope X and Y in KN-m ........................................................................................................................... 52 Table 3-18 Comparison for beam moment and shear force for envelope X and Y in KN-m ........................................................................................................................... 54 Table 3-19 Total reactive force at the origin ............................................................... 59 Table 3-20 Seismic loads ............................................................................................. 80 Table 4-1 Summary of stiffness modification factor ................................................... 89 Table 4-2 Summery of regularity, dynamic analysis, Accidental eccentricity and diaphragm .................................................................................................................... 90 Table 4-3 Summery of fundamental requirement for earthquake design .................... 91 Table 4-4 Response spectrum function used ............................................................... 92 Table 4-5 Summary of P-∆ analysis ............................................................................ 93 Table 4-6 Modified S.L.S and U.L.S ........................................................................... 94 

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DESCRIPTION OF SYMBOLS Symbols eij

Description Accidental eccentricity of story mass i from its nominal location, applied in the same direction at all floors

Li

Floor-dimension perpendicular to the direction of the seismic action

Fb

Base shear force

Sd (T1)

Design spectrum normalized by the acceleration of gravity

T1

Fundamental period of vibration

W

Seismic dead load

α

Ratio of the design bedrock acceleration to the acceleration of gravity

αo

Bedrock acceleration ratio from the site and depends on the seismic zone

I

Importance factor

β

Response factor

Vs

Shear wave velocity

H

Height of the building above the base in meter

γo

Basic value of the behaviour factor

KD

Factor reflecting the ductility class xiv 

 

   

KR

Factor reflecting the structural regularity in elevation

KW

Factor reflecting the prevailing failure mode in strucutral system with wall.

θ

Interstory drift sensitivity coefficient

θ limit

Design interstory drift sensativity coefficient

Ptot

Total gravity load at and above the story considered

dr

Design interstory drift, evaluated as the difference of the average lateral displacement at the top and bottun of the story

ds

Displacement of a point induced by the design seismic action

γd

Displacement behavior factor, assumed equal to γ

de

Displacement of the same point of the structural system, as determined by a linear analysis based on the design response spectrum

Vtot

Total seismic story shear

h

Interstory height

E

East

N

North

W

West

s

Second

S.L.S

Service limit state

U.L.S

Ultimate limit state xv 

 

   

ht

Height

EQXP

Earthquake in the x- direction along positive eccentricity

EQYP

Earthquake in the y- direction along positive eccentricity

EQXN

Earthquake in the x- direction along negative eccentricity

EQYN

Earthquake in the y- direction along negative eccentricity

DL

Dead Load

LL

Live load

Combo

Combination

αo

The prevailing aspect ratio of the wall i of a structural system

Z

Zone factor

S

Soil parameter

DC”H”

Ductility class high

DC”M”

Ductility class medium

DC”L”

Ductility class low

E

Modulus of Elasticity

Ec

Modulus of Elasticity of concrete

Ig

Gross moment of inertia

Ib

Moment of inertia of beam

Ic

Moment of inertia of column xvi 

 

   

UBC

Uniform building code

ACI

America Code Institute

EBCS

Ethiopian building code and standard

mm

millimeter

RSEQXP

Earthquake response spectrum in the X-direction along positive eccentricity

RSEQYP

Earthquake response spectrum in the Y-direction along positive eccentricity

RSEQXN

Earthquake response spectrum in the X-direction along negative eccentricity

RSEQYN

Earthquake response spectrum in the Y-direction along negative eccentricity

Bzmnt

Basement

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1

INTRODUCTION 1.1 General

The current economic expansion in Ethiopia which seems to be driven by a number of enabling factors has had substantial impact in the transportation, energy, and water supply sectors with a growing number of large-scale infrastructure projects such as dams, power-plants, highway roads, water reservoirs, and expansion of railways either coming online or entering construction phase. Furthermore, pressure from other natural developments, the staggering population growth of the country being a primary one, continue to force rapid implementation of large-scale engineering infrastructure works such as mass-housing, water supply reservoirs, power-plants, dams, and new cities. As things stand, the country's population is projected to reach a staggering 120 million by 2025 positioning Ethiopia to be among the top 10 to 15 populous countries on the planet. In addition to a multitude of other threats that this population growth could bring, the issue of housing these additional 30 to 40 million Ethiopians in the next few decades will pose a huge risk factor. In a recent paper, it has been argued that 25 new cities with size equivalent to present Dire Dawa are needed or the current 10 cities such as Addis Ababa and Dire Dawa will have to become mega cities of 10 million or more to accommodate this growth. While these projections regarding urbanization may be a little bit on the high-side, there is no denying regarding the need for housing these additional millions of citizens in the next several decades. [1] Interestingly, however, a substantial amount of these large infrastructure works already lie or will be in or in close proximity to the some of the most seismically active regions of the country such as Afar Triangle, the Main Ethiopian Rift (MER), 1   

   

and the Southern Most Rift (SMR) where well documented damage causing earthquakes are common. A review of the engineering reports associated with some of the largest and most expensive infrastructure projects in the country suggest that despite the presence of a substantial amount of published literature on the significant seismicity of the region the severity of threats posed by seismic hazards on the safety and serviceability of these structures is not well understood by the main stake-holders such as policy-makers, insurance companies, real-estate developers, capital investors, building design checkers and, not infrequently, the engineering community itself as well.[1] Against this background, therefore, the need for preparing for this real and substantial threat of seismic hazards in the country is pressing and requires attention at all levels. Several fundamental problems still main before rationale seismic design is practiced well in the country. These are: (i)

There is growing evidence that the current building codes themselves are inadequate, out-dated, and not stringent enough when compared to the level of seismic risks associated with the country,

(ii)

Ambiguities that exist in this first legislation attempt that do not explicitly address the seismicity of the country (Part Three-Design, Item 34 that reads "buildings may not exhibit signs of structural failure during their life span under normal loading") may give a ground for stakeholders to ignore seismic effects because 'normal loadings' may arguably not include seismic loads, and

(iii)

The mechanism for enforcing strict adherence through design checks at the municipality offices (as opposed to external peer review system) is inadequate because it relies on design checkers who are neither well aware of the seismicity of the country nor well-trained in seismic design to start with.

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Further, the legally mandated requirements and design review process do not apply to public and government large-scale infrastructures (dams, railway structures, electrical transmission structures) which actually are the sources of some of the major concerns. [1] Therefore, ambiguities of the new building construction law coupled with the lack of awareness and mechanism for truly enforcing code requirements continue to introduce a significant risk of endangering the useful life of these expensive projects as well as human life. [1] Most of the designers working at consulting offices are Bachelor degree holders and seismic design is not given as independent course at this level. Instead it is given at Masters Level as earthquake engineering course explicitly. This might be taken as a problem in seismic design practice of Ethiopia since it is hard to get enough awareness about seismic design without knowing in depth about earthquake engineering. Thus having mentioned the above problem it is pressing issue to assess the seismic design practice of Ethiopia. There are many design office in Ethiopia and it is difficult to get all data so the research will focus on assessing seismic design practice of building structures at selected design offices of Addis Ababa.

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1.2 Objective The general objective of this thesis is to assess seismic design practice and the overall structural design process of building designed in Addis Ababa. The specific objectives are  Assessing seismic design practice of building structures  Assessing consideration of dynamic analysis for irregular structure  Assessing fulfillment of fundamental requirements for earthquake design (ultimate limit state and serviceability limit state).  Assessing structural configuration of structures for earthquake design  Assessing stiffness reduction for structural elements  Slab and column design 1.3 Materials and Methods The objective of the research was achieved in accordance with the method outlined below. I . Literature review: Ethiopia building codes and standards, Eurocode and literatures on history of earthquake in Ethiopia shall be reviewed. II . Data analysis: Structural models from randomly selected design offices will be collected and analyzed. These structural models are obtained from Addis Ababa City Administration. Since design models are not submitted to City Administration anymore, randomly selected design offices are assessed. In this thesis ten models are analyzed.

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1.4 Limitations This thesis is limited to building structures. Since almost all design offices are not willing to give design models, the selection criteria is random and only data’s available from Addis Ababa City Administration is used as input data. Due to time constrain only 10 models are analyzed. The code used to assess the case studies is Eurocode 8:2004 which is similar to EBCS8:1995.

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2 2.1

LITERATURE REVIEW Review of Historical Records of Earthquake in Ethiopia

It is well established now that, due to its location right on some of the major tectonic plates in the world, that is, the African and Arabian plates, earthquakes have been a fact of life in Ethiopia for a very long time. The earliest record of such earthquake dates as far back as A.D.1431 during the reign of Emperor Zara Yaqob. In the 20thcentury alone, a study done by Pierre Gouin suggests that as many as 15,000 tremors, strong enough to be felt by humans, had occurred in Ethiopia proper and the Horn of Africa. A similar study by Fekadu Kebede indicated that there were a total of 16 recorded earthquakes of magnitude 6.5 and higher in some of Ethiopia’s seismic active areas in the 20th century alone. The most significant earthquakes of the 20thand 21st centuries like the 1906 Langano earthquake, the1961 Kara Kore earthquake, the 1983 Wondo Genet earthquake, the 1985 Langano earthquake, the 1989 Dobigraben earthquake in central Afar, the 1993 Adama earthquake, and the 2011 Hosanna earthquake were all felt in some of the major cities in the country such as Addis Ababa, Jimma, Adama and Hawassa. In addition to Gouin's book that describes the earthquakes of 1906 and 1961 that shook Addis Ababa and caused wide spread panic, a recently published Amharic biography of Bladen GetaMersie Hazen Wolde Qirqos vividly describes the effect of the 1906 Langano earthquake in Addis Ababa and Intoto. In addition to these well documented seismic events starting from the 15th century, a number of earthquakes have shaken the Main Ethiopian Rift (MER), and the Southern Rift Valley of the country recently between 2005 and now bringing the danger of seismic hazard to the forefront. As built up environments and human development activities increase in areas close and within the MER, the Afar Triangle and the Southern Rift Valley of the country, it is expected that the damage on property 6   

   

and loss of human life due to seismic hazard will increase very significantly. One of the important observations is that newer buildings are experiencing damages under these relatively moderate earthquakes of magnitude around 5.0. [1] 2.2

Review of Seismic Mechanisms and Seismicity in Ethiopia

In terms of the mechanism that gives rise to seismic hazard, the well accepted theory suggests a simplified model that typically considers three distinct seismic zones in Ethiopia proper. These are: the Afar Triangle seismic zone (which further consists of the junction between Red Sea, Gulf of Aden, and the Main Ethiopian Rift), the Escarpment seismic zone (characterized by north south running faults associated with some of the devastating earthquakes such as the 1961 magnitude 6.7 Kara Kore earthquake) and the Ethiopia Rift System seismic zone (which links the Red Sea, Gulf of Aden with East African Rift system through the Afar Triangle). [1] 2.3

Review of Response of Built-up Structures to Seismic Events in Ethiopia

As discussed above, while an extensive amount of earthquake records on Ethiopia exist, the structural damage to infrastructures in the vast part of this period was obviously very low due to the extreme limitation of built-up environments in the country. It is only, perhaps, starting from the 1950s and 1960s that one sees what could be characterized as noticeable building and infrastructure activity in the country, particularly in the seismic-prone areas. For the period between 1960 and 1978, Gouin’s work provides a wealth of information on the response of built-up structures like buildings and bridges to some of the large and damaging earthquakes such as Kara kore (1961) and Serdo (1969). With regard to infrastructural damages from 1978 onward, there have been isolated reports of which some are unpublished. Interestingly, this period coincides with a growth in built-up areas and infrastructure in some of the seismically active areas, particularly MER and the Afar Triangle. 7   

   

Areas where there were no infrastructure damages even under strong ground motions such as the 6.3 intensity Chabbi Volcano earthquake of 1960 near the present day Hawassa have now seen encroachment of built-up areas which have suffered damages under recent but much less strong ground motions. Therefore, it has increasingly become clear that structural damages to buildings and infrastructure due to earthquakes are on the rise in the country. A catalogue of these damages presented in Table 2-1 particularly for the time period after 1978 is a first attempt in understanding the pattern of damages observed so far and preparing the groundwork for predicting the potential structural damages that could occur in the years to come. [1]

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Table 2-1 List of earthquakes and reported damages between 1979 to 2011 Earthquake

Intensity

Year

STRUCTURAL DAMAGE

Akaki

Magnitude 4.1

1979(28 July)

Cracks in poorly built masonry structures.

8.85N 38.7E

Intensity

1981

Cracks in masonry buildings in AwaraMelka town,

(February 7)

north of the Fentale volcanic center

VII

near epicenter 8.9N 39.9E 7.03N 38.6E

5.1 5.1

1983

Rock slides and damage and destruction of masonry buildings in Wendogenet, east of Lake Hawassa. Well-built single-story building cracked at the Forestry Institute. Large boulders dislodged, plaster fallen off walls, electric poles thrown down.

Hawassa

5.3

1983

Damage to steel frames in Hawassa. Damage to Wetera Abo Church in Wondo Genet (1983 earthquake, masonry building with irregular vertical and horizontal stiffness. Damage seems

to

occur

where

there

is

stiffness

discontinuity). 11.37N

1984

High-rise buildings shaken. Mortgage Bank

38.7E

(April 10)

Building in Kazanchis.

8.95N

1984

Concrete building in Piazza shaken

39.95E

(August24)

Near

Lake

Hayk.

8.3N 38.52E

5.1

1985

Strongly felt in Lake Langano camp, central MER.

Oitu Bay

Cracks in buildings in resort area hotels

(Langano) 9.47N

(4.8), 105 Km

1985

39.61E

away

(October )

5.4

1987(October

Panic in high-rise buildings in Addis Ababa.

Langano Already 9   

weakened

hollow

block

building

   

28)

collapsed, strongly felt – Arba Minch. Panic – No damage in Jimma. Students knocked against one another in classroom, poorly built house collapsed in Sawla.

Hamer and

5.3

–

Gofa

magnitude.

6.2 1987 (October7-28)

Earthquake

Details given separately for Hawassa, Jima and Arba Minch.

Swarm 5.3

1987 (October 7)

Light-sleepers woken. No structural damage in Hawassa. Poorly built structures cracked, many woken up, birds Shaken-off trees.

8.9N 40E

4.9

1989

Cracks in buildings in the town of Metehara, northern MER. Felt like passing truck by many, shaking beds.

DobiGraben

1989

Several bridges damaged.

1989(April

Felt by many causing some panic.

(afar) Mekelle

5.3

13) Dichotto

5.8

1989(August

Dining people thrown-off table, masonry house

20)

collapsed, landslides killed 4 people and 300 cattle, 6 bridges destroyed in Dichotto.

Soddo

5.0

6.84N

1989

Widespread panic, broken windows and some

(June 8)

injured in

37.88E

Soddo.

8.1N 38.7E

5.1

1990

Minor damage in towns at the western escarpment, i.e., at Silti and Butajira, West of Zway town.

8.3N 39.3E

5.0

1993

Nazareth

Collapse of several adobe buildings in Nazareth town northern MER. Felt as far as DebreZeit and Addis Ababa.

7.2N 38.4W

5.0

1995

Cracks in flour factory building at Hawassa town.

Mekelle

5.2

2002

Buildings shaken in the city of Mekelle

(August 10) 10   

   

Afar

2005

Fumes as hot as 400 oC shoot up from some of

Triangle

September 26

them; the sound of bubbling magma and the smell of sulfur rise from others. The larger crevices are dozens of meters deep and several hundred meters long. Traces of recent volcanic eruptions are also visible. This was followed by a week-long series of earthquakes. During the months that followed, hundreds of further crevices opened up in the ground, spreading across an area of 345 square miles.

Ankober

5.0

2009

Earthquake strikes near Ankober Town and was

September 19

widely felt in Addis especially by residents who live on multistory buildings.

Hosanna

5.3

2010

Damage sustained by reinforced concrete frame

(December

dormitory building at Jimma University with in-

20)

filled walls at where as many as 26 students were injured. Structural damage to slab and column joint. Damage to many building in Hosanna

Ethio-Somali

6.1

border Abosto/

5.0

YirgaAlem

2011

Buildings shaken in Dire Dawa, Jijiga, and

(March 3)

Somalian towns.

2011

Damage to unreinforced cinder-block cladded

(March 9)

timber building. 100 houses were destroyed and 2 people were injured in this earthquake

Hosanna

5.2

November

Damage of infill walls in medium rise buildings.

24,2011

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2.4

Background and current state of code-required seismic design in Ethiopia

2.4.1

Ethiopian Building Standard Codes

The first seismic code for building in Ethiopia was introduced in 1980 (CP1-78). This code defined four seismic codes regions (that is 0, 1, 2 and 4) with a return period of 100 years and 90% probability of not being exceeded. To each zone, a danger rating was assigned with no, min, moderate and major corresponding to zones 0, 1, 2 and 4.The CP1-78 code dealt primarily with seismic zoning and determination of equivalent static loads on structures and left actual a seismic design of structural members (beams, columns, and shear walls) to the judgment of the engineer with other established international building codes, primarily UBC, serving as a basis for a seismic design. ESCP1-83 has a separate code (ESCP-2:1983 - Ethiopian Standard Code of Practice for the Structural use of Concrete) for guidelines for concrete design. These were followed by a substantial change introduced in 1995 as EBCS-1995 by the Ministry of Works and Urban Development. The seismic zoning was an improvement over previous codes based on additional data obtained from newer earthquake records inside Ethiopia as well as neighboring countries. However, the whole Ethiopian Building Code Standard (EBCS) that consisted of 10 volumes was predominantly based on the European Pre-Standard (experimental) code (ENV 1998) which was drafted by CEN (European Committee for Standardization). The seismic provisions code, EBCS-8: 1995 (Design of Structures for Earthquake Resistance), was also predominantly based on ENV 1998:1994 Euro code8 Design Provisions for Earthquake Resistance of Structures except the equivalent static load procedure which still had the UBC influence. The use of the draft Euro code as a model was a significant departure from earlier codes which used UBC as a model to a large extent. It appears that there was no overriding technical basis for this departure. Further, the 12   

   

adaptation of this 'draft' code before the Europeans themselves commented on it and approved an improved version as a standing code causes a number of significant inconsistencies and controversies. [1] A commonality between all the three codes introduced in the country over the past 30 years is the choice of 100 years return-period in contrast with a 475 years returnperiod which is adopted by most codes around the world. The main argument in favor of this choice has been the relatively economical construction of structures designed for a less powerful earthquake. In general, PGA (peak ground acceleration) values corresponding to a return-period of 475 years are about twice those of 100 years return-period. While the existence of history of three generation of seismic codes in the country is a commendable effort, its legal enforcement was never codified by the country's legal systems until 2009 when the Ethiopian Building Proclamation 624/2009 was introduced as a legal document that outlines the building regulations and requirements, for use by local authorities to ensure building standards are maintained in their jurisdiction. [1] 2.4.2

Seismic Zoning

Gouin who used probabilistic approach is credited for the initial attempts in producing the first seismic hazard map of Ethiopia as shown in Figure 2.1. Gouin's work also served as a basis for the seismic zoning adopted by the ESCP-1:1983 building code of Ethiopia (see Figure 2.1b). Since the production of Gouin’s seismic zoning maps, quite a large number of destructive earthquakes have occurred in the country causing damages both to property and human life. Further, destructive earthquakes that occurred in the neighboring countries were not included in the production of the first map in 1976. Subsequently, Kebede produced a new seismic hazard map of Ethiopia and its northern neighboring countries to account for these additional earthquake 13   

   

records. Unlike previous works, the seismic zoning of Ethiopia and the Horn of Africa reported by Kebede, Kebede and Asfaw also account for ground motion attenuation in addition to newer data obtained from such sources as the US National Earthquake Information Service (NEIS). The works of Kebede and Kebede and Asfaw served as a basis for the seismic zoning adopted by the current Ethiopian building seismic code EBCS-8:1995 as shown in Figure 2.1c. Further, there have been other attempts on seismic zoning of some of the country’s important economic regions such as the city of Addis Ababa. The work of the RADIUS project is a notable example. There have been additional studies that are continually shaping understanding of seismicity in Ethiopia. [1] A summary of the seismic zonings corresponding to each of these three codes are given in Figure 2.1. Seismic Zoning of Ethiopia as per CP1-78, ESCP1- 83 and EBCS-8:1985 all considered 4 seismic zones. The availability of relatively newer data was credited for the changes in seismic zoning of Ethiopia as per EBCS-8: 1995 which considers some areas in MER to have the same zoning as the severest of the Afar region. The nature and location of recent damage-causing earthquakes such as the December 2010 Hosanna and March 2011 Aboso/YirgaAlem earthquakes is expected to add further support for the need for further improving the current seismic zoning to account for previously unknown and less understood faults as well as local site conditions. [1]

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Figure 2.1 (a)) Seismic zooning of Ethiiopia as per G Gouin (19766) which wass also used bby C CPI-78 (b) Seismic S zoniing of Ethioopia as per ESCP-1:198 E 83 (c) Seism mic Zoning oof E Ethiopia as peer EBCS-8:11995. [1] 15

   

2.5

Deficiencies in current code and proposed revisions

Substantial amount of new data has been accumulated from earthquakes that have occurred in Ethiopia in the 90s as well as early parts of the current century that suggest that the current seismic zonings adopted in the codes are incomplete, inadequate, and non-cognizant of local site effects that could amplify earthquake effects. Further, the inherent weakness and flaws of basing the country's code on a 'draft' European code that was not even reviewed and critiqued by the Europeans themselves at that time add a lot of urgency on the call for the substantial review of the current building code, EBCS-1995. In fact, the European code has not been accepted 'as is' even by its member states like Italy who have added not insignificant modifications for national uses. [1] 2.5.1

Seismic Zoning and Peak ground acceleration (PGA)

As stated earlier, the works of Fekadu Kebede and L.M. Asfaw served as a basis for the seismic zoning adopted by the current Ethiopian building seismic code - EBCS8:1995 (with a return-period of 100 years which corresponds to 0.01 annual probability of exceedance). Associated with this, there are at least three areas that offer an opportunity to improve the usefulness as well as address some of the inadequacies of the current seismic zoning. [1] 1. The effects of local site-conditions such as local fault lines and soil conditions for at least the major population areas need to be considered. While preparing a detailed one may be too prohibitive of an expense and beyond the means of the country, doing so for major cities like Addis Ababa, Jimma, Adama, Hawassa, Mekelle, and Dire Dawa may be a reasonable approach. Even in current practices, there have been isolated attempts in performing such local site effects for some infrastructure projects around the country. The inconsistencies of the current seismic zoning devoid of local 16   

   

site-conditions becomes more apparent when considering the case of Addis Ababa where areas such as Nefas Silk which is only 20 to 25 kilometers away from DebreZeit (zone 4, α0=0.1) has the same seismic zone 2(α0=0.05) classification as Intoto and its mountainous surroundings. Interestingly, Akaki which is only 5 or so kilometers away from Nefas Silk and has no overriding geological dissimilarities with the latter is classified as zone 3 with α0=0.07. Against this background, the work of L.M. Asfaw's where he showed that there is significant geological and topographic variation in different parts of Addis Ababa that had resulted in variations in the felt intensities in past earthquakes adds another dimension to the argument .In general, L.M. Asfaw's work suggests that the southwestern part of Addis Ababa mainly consists of thick alluvium deposits whereas the northern part of the city has prominent topographies (mountains) with thin soil cover. Both types of topographies are known to increase felt intensities. Interestingly, L.M. Asfaw shows that, due to local site effects, the felt intensities in Intoto area (seismic zone 2 according to EBCS-8:1995) were higher than those in the southeast of the city towards Bole field (seismic zone 3). Therefore, until a complete site specific zoning is available sometime in the future, it is suggested that for consistency purposes as well as conservative designs the city of Addis Ababa and its industrial surroundings adopt similar seismic zoning of at least zone 3. This could be addressed, for example, by establishing the contour lines of seismic zones near major metropolitan areas to be continuous with no jump in zones giving continuity in seismic zoning. [1] 2. The current code considers a return-period of 100 years only which effectively reduces peak ground acceleration by almost half as compared to the commonly used 475 years return-period (10% probability of exceedance in 50 years). As discussed before, economic considerations were often cited as the main argument in favor of

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this choice. However, this view needs a revisit in light of the current significant boom in construction activities across the country which is expected to continue in the foreseeable future despite some hiccups along the way as well as with regard to continuity and compatibility of risk levels in the region and beyond. Does the costsaving in designing for lower seismic loads offset the risk of losing large investments in these infrastructures due to large earthquakes with return periods of 200 to 475 years? While it may be argued that a return-period of 475 years may introduce a sudden substantial jump in cost, that the level of investment going to these structures is substantially high enough to warrant consideration of 475 year return period. Further, it is suggested that for large infrastructure projects such as dams, bridges, power plants, railway structures these structures should be mandated by specialized codes as is done elsewhere, the tendency to use existing practice of 100 year return period should also be discouraged and disallowed and the proposed use of 475 years of return period should also be extended to these specialized codes. [1] 3. While the catalogue of earthquakes used for the current zoning extended up until 1990 only, the earthquakes that have occurred since then in the past 20 years have some interesting aspects that could have a bearing on the current seismic zonings. A good example is the 5.3 magnitude Sunday December 19, 2010 Hosanna earthquake that injured as many as 26 students in Jimma and damaged buildings. While the current seismic zoning puts Jimma in seismic one 1(with α0=0.03) and the city is at least 100 kilometers away from the epicenter, the damage caused is surprising. Interestingly, the city of Jimma had always felt the effect of past earthquakes in the MER (Main Ethiopian Rift) and SMR (Southern Most Rift) including the Woito earthquake swarm of October -December 1987 that rattled the city and its residents. As development in the Jimma area expands, the damage from earthquakes centered in

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the MER, SMR and beyond could cause more damages and this current classification of this city of increasing commercial importance as seismic zone 1 and α0=0.03 is non-conservative and hard to support. [1] 2.6

Characteristics of earthquake resistant buildings

Buildings are designed by architects and engineers. Architects are responsible for the architectural configuration of buildings at the start. Configuration has to do with the size, shape and proportion of the 3D form of the building. Architectural configuration determines the location, shape and approximate size of structural and nonstructural elements of the building. Any architectural design should incorporate effective seismic design to minimize earthquake hazards. While the provision of earthquake resistance is accomplished through structural means, the architectural design and the decision that create it, play a major role in determining the building’s seismic performance. [5] 2.6.1

Basic principle of conceptual of design

1. The aspect of seismic hazard shall be taken into consideration in the early stages of the conceptual design of the building. [2] 2. The guiding principles governing this conceptual design hazard are → Structural simplicity → Uniformity and symmetry → Bidirectional resistance and stiffness → Torsional resistance and stiffness → Diaphragmatic action at story level → Adequate foundation .[2]

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Table 2-2 Optimal structural configuration [5] Attributes

Benefits

Low width-to-depth ratio

Low torsional effects

Low height-to-base width/depth ratio

Low overturning effects

Similar story heights

Elimination of weak/soft story

Short spans

Low unit stress and deformation

Symmetrical plan shape

Elimination/reduction of torsion

Identical resistance on both axes

Balanced resistance in all directions

Uniform plan/elevation stiffness

Elimination of stress concentrations

Uniform plan/elevation resistance

Elimination of stress concentrations

Uniform plan/elevation ductility

High energy dissipation

Perimeter lateral resisting systems

High torsional resistance potential

Redundancy

High plastic redistribution

Direct load path, no cantilevers

Elimination of stress concentration

2.6.2

Structural regularity

2.6.2.1 General For the purpose of seismic design, building structures are distinguished as regular and non-regular. This distinction has implication on the following aspect of seismic design: → The structural model, which can be either a simplified planar or a spatial one, → The method of analysis, which can be either a static or dynamic → The value of the behavior factor which can be either increased or decreased depending on the type of non-regularity in elevation → Geometric non- regularity exceeding the limits → Non-regular distribution of over strength in elevation exceeding the limits

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With regard to the implications of structural regularity on the design, separate consideration is given to the regularity characteristics of the building in plan and elevation, according to table 2-3. [2] Table 2-3 Consequences of structural Regularity on Seismic Design Regularity

Allowed simplification

Behavior factor

plan

Elevation

Modal

Analysis

Yes

Yes

Planar

Static*

Basic

Yes

No

Planar

Static*

Increased

No

Yes

Spatial

Static*

Basic

No

No

Spatial

Dynamic

Increased

*For T1