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Structural Engineering Thesis

2020-03-17

ASSESSMENT OF SEISMIC

SEPARATION REQUIREMENT ON

ADJACENT REINFORCED

CONCRETE BUILDINGS

Dessie, Yilak

http://hdl.handle.net/123456789/10527

Downloaded from DSpace Repository, DSpace Institution's institutional repository

I

BAHIR DAR UNIVERSITY

BAHIR DAR INSTITUTE OF TECHNOLOGY

SCHOOL OF RESEARCH AND POSTGRADUATE STUDIES

FACULTY OF CIVIL AND WATER RESOURCE ENGINEERING

ASSESSMENT OF SEISMIC SEPARATION REQUIREMENT ON

ADJACENT REINFORCED CONCRETE BUILDINGS

Yilak Dessie

Bahir Dar, Ethiopia

June, 2018

II

ASSESSMENT OF SEISMIC SEPARATION REQUIREMENT ON

ADJACENT REINFORCED CONCRETE BUILDINGS

By

Yilak Dessie

Advisor: - Temesgen Wondimu (PhD)

A Thesis submitted to the School of Research and Graduate Studies of Bahir Dar

Institute of Technology, BDU in partial fulfillment of the requirements for the

Degree of Master of Science in Civil Engineering (Structures)

Bahir Dar, Ethiopia

June, 2018

I

II

III

ABSTRACT

Seismic evaluations of adjacent regular and irregular reinforced concrete buildings are

commonly adopted in Ethiopian design philosophy; this evaluation is performed to proportion

cross sectional elements of buildings like beam, column and slab. However, during design of

adjacent buildings in seismic zone designers do not consider the minimum gap that should be

provided between adjacent buildings, which may cause pounding effect during earth quake

excitation.

Several new buildings located in Ethiopian major towns are designed and constructed without

considering the effect of existing buildings behavior in the surrounding under earth quake

excitation. Different seismic zones may have different site conditions; in most cases a simple

copy of building design from one seismic zone and site condition to another with or without

modification of cross sectional element is a common practice in major towns of our country.

However, in both cases there is no modification that is made on the gap between adjacent

buildings. Even if the gap between adjacent buildings is sufficient in one seismic zone and site

condition, it may not be sufficient for the other.

The aim of this research was to assess the seismic gap requirement of adjacent buildings by

considering all seismic zones and site conditions to prevent pounding. For evaluation purpose

four independent buildings (two of them are regular and the other two are irregular) buildings

were selected. The buildings were assumed to be constructed adjacently in three arrangement as

a case study (i.e. regular vs. regular, irregular vs. regular and irregular vs. irregular) to compute a

minimum gap required for preventing of pounding damage in different seismic zone and site

conditions. Minimum gap was calculated by assuming zones verses site conditions as (zone 1

have site condition-A, zone 2 have site condition-B, zone 3 have site condition-C, zone 4 have

site condition-D and zone 5 have site condition-E).

All buildings were been analyzed by using the finite element software program, SAP2000, with

response spectrum function. The analysis was conducted for elastic structures under different

seismic zones.

IV

The results of the analysis shows that in all three cases the separation distances requirements of

adjacent buildings in different seismic zones and site conditions to withstand the pounding

damage is absolutely different, according to ES-EN-1998-1-1,2015. Providing of passive

structural control systems specially water tank as tuned mass damper is used to minimize the

separation distance between adjacent buildings and use the land effectively by reducing the

dynamic response of buildings under earthquake excitation. This innovation method of building

mitigation shows the significant change in lateral displacement and the period of the buildings.

V

ACKNOWLEDGEMENTS

First I thank God for giving me the abilities, courage and drive to complete this thesis work.

I am grateful to express my deepest gratitude to my advisor Temesgen Wondimu (PhD) for his

encouragement and guidance throughout the thesis work starting from the very beginning. I

would like to thank him for his suggestion of this thesis topic.

I would like to thank the people who provide the construction drawings of the selected case study

buildings. Without their help this work could have not been real.

Finally I would like to give special thanks to my family (especially my Mother) for their love

and continuous encouragement during the thesis work.

VI

CONTENTS DECLARATION ........................................................................................ Error! Bookmark not defined.

ABSTRACT ................................................................................................................................................. II

ACKNOWLEDGEMENTS ......................................................................................................................... V

LIST OF FIGURES ................................................................................................................................. VIII

LIST OF TABLES ....................................................................................................................................... X

CHAPTER ONE ........................................................................................................................................... 1

1. INTRODUCTION .................................................................................................................................... 1

1.1Background ............................................................................................................................................ 1

1.2 Problem statement ................................................................................................................................. 2

1.3 Objectives ............................................................................................................................................. 3

1.4 Scope of the study ................................................................................................................................. 4

1.5 Report organization ............................................................................................................................... 4

CHAPTER TWO .......................................................................................................................................... 5

2. REVIEW OF LITERATURE ................................................................................................................... 5

2.1 Overview of seismic pounding ............................................................................................................. 5

2.2 Building configuration effect on pounding phenomena ...................................................................... 11

2.2.1 Regularity in plan ........................................................................................................................... 11

2.2.2 Regularity in elevation ................................................................................................................... 12

2.2.3 Other building irregularity ............................................................................................................. 14

2.3 Characteristics of pounding ................................................................................................................ 16

2.4 Causes of pounding ............................................................................................................................. 18

2.5 Code provisions regarding to pounding .............................................................................................. 21

2.6 Seismic Pounding Damage Classification .......................................................................................... 25

2.7 Modeling of Pounding ........................................................................................................................ 28

2.7.1 Contact Element Approach ............................................................................................................ 29

2.7.1.1 Linear Spring Model ................................................................................................................. 30

2.7.1.2 Kelvin Model ............................................................................................................................ 31

2.7.1.3 Hertz Model .............................................................................................................................. 32

2.7.2 Stereo Mechanical Model .............................................................................................................. 34

2.8 Methods of Seismic Analysis for Structures to compute seismic separation ...................................... 36

2.8.1 Linear Static Analysis (Seismic Coefficient Analysis) .................................................................. 37

2.8.2 Linear Dynamic Analysis (Response Spectrum Analysis) ............................................................ 37

2.8.3 Non-linear Statics Analysis (Pushover Analysis) .......................................................................... 39

2.8.4 Non-Linear Dynamic Analysis (Time History Analysis) .............................................................. 41

2.9 Application of TMDs on Seismic Gap Requirement .......................................................................... 42

VII

2.9.1 Tuned Mass Dampers on Dynamic Properties of Structures ......................................................... 44

2.9.2 Principle of Tuned Mass Damper .................................................................................................. 46

2.9.3 Modeling of Water tank as TMD ................................................................................................... 48

CHAPTER THREE .................................................................................................................................... 50

3. BUILDING DESCRIPTIONS, MODELING AND ANALYSIS .......................................................... 50

3.1 Building Descriptions ......................................................................................................................... 50

3.1.1 Case study-I (Both adjacent buildings have regular arrangement) ................................................ 54

3.1.2 Case study –II (Both adjacent buildings have irregular arrangement) ........................................... 56

3.1.3 Case study-III (Adjacent buildings have different arrangement) ................................................... 57

3.2 Building Modeling and Analysis ........................................................................................................ 59

3.2.1 General ........................................................................................................................................... 59

3.2.2 Assigning loads .............................................................................................................................. 59

3.2.3 Response-Spectrum Function ........................................................................................................ 60

3.2.4 Response Spectrum Analysis in SAP 2000 .................................................................................... 63

3.2.5 Time History Analysis in SAP 2000 .............................................................................................. 63

CHAPTER FOUR ....................................................................................................................................... 66

4. RESULTS AND DISCUSSIONS ........................................................................................................... 66

4.1 General ................................................................................................................................................ 66

4.2 Result and Discussion of all Cases ..................................................................................................... 67

4.2.1 Case study -I (B1 vs. B4):-Both adjacent buildings have regular arrangement ............................. 68

4.2.2 Case study-II (B2 vs.B3): Both adjacent buildings have irregular arrangement............................ 71

4.2.3 Case study -III (B1 vs. B3):-Adjacent buildings have different arrangement................................ 74

4.2.4 Time History Analysis ................................................................................................................... 77

4.2.5 Summary on the discussion of all three cases ................................................................................ 78

4.3 Method of Mitigation using passive structural controls ...................................................................... 80

4.3.1 Reduce the Dynamic Response of Adjacent Buildings for all Cases ............................................. 80

4.3.2 Water Tank dimension and Capacity Calculation .......................................................................... 80

4.3.3 Reduce the Dynamic Response for case-I ...................................................................................... 83

4.3.4 Reduce the Dynamic Response for case-II .................................................................................... 85

4.3.5 Reduce the Dynamic Response for case-III ................................................................................... 88

CHAPTER FIVE ........................................................................................................................................ 91

5.CONCLUSION AND RECOMMENDATIONS ..................................................................................... 91

5.1 Conclusions ......................................................................................................................................... 91

5.2 Recommendations ............................................................................................................................... 92

REFERENCES ........................................................................................................................................... 93

APPENDIX ................................................................................................................................................................. 97

VIII

LIST OF FIGURES

Figure 2.1:Adjacent buildings constructed up to the property line …………………….…............8

Figure 2.2:New buildings are designed and constructed without the information of existing

neighboring building behavior under earth quake excitation.........................................................9

Figure 2.3:Similar adjacent buildings which are constructed in different places haven

approximately the same seismic gap and frame structure cross sections……..............................10

Figure 2.4: Regular and irregular vertical configurations…………..............................................13

Figure 2.5: Regular and Irregular horizontal configurations.........................................................13

Figure 2.6: weak story (irregularity in strength) …………………………...................................14

Figure 2.7: soft stories buildings…………………………............................................................15

Figure 2.8: Mass irregularity…………………………..................................................................15

Figure 2.9: seismic separation on adjacent buildings…………………………............................16

Figure 2.10: seismic behavior of adjacent buildings……………………….................................16

Figure 2.11: pounding of regular and irregular building………………………...........................17

Figure 2.12: pounding of building due to liquefaction………….…………….............................17

Figure 2.13: Various types of impact (pounding) ………………….……....................................20

Figure 2.14: separation distance of adjacent buildings…..............................................................23

Figure 2.15: pounding effect………………………......................................................................26

Figure 2.16: pounding damage effect between buildings; 2008 Sichuan (China) Earthquake......27

Figure 2.17: Pounding damage observed in the 2011 Christchurch earthquake............................27

Figure 2.18: Analytical model………….……………..................................................................29

Figure 2.19: Liner spring model and contact force relation……….………………….................30

Figure 2.20: Kelvin model and its contact force relation………………………….......................32

Figure 2.21: Hertz model and its contact force relation…………….……………........................34

Figure 2.22: stereo-mechanical model...........................................................................................35

Figure 2.23: Different stages of plastic hinge................................................................................39

Figure 2.24: Structure with Tuned Mass Damper, Act Tower, India...........................................46

Figure: 2.25 Viscous and Tuned mass damper on a structure.......................................................47

Figure: 2.26 Two-DOF modeling of main structure and tuned mass damper system...................48

Figure: 2.27 Optimization of idealized structure with and without TMD.....................................49

IX

Figure 3.1: Regular sample building (B-1) G+7............................................................................51

Figure 3.2: Irregular (in plan) sample building (B-2) G+9............................................................52

Figure 3.3: Irregular (in elevation) sample building (B-3) G+7....................................................53

Figure 3.4: Regular sample building (B-4) G+6 ...........................................................................54

Figure 3.5: (a) Building 1, a total of 8 stories (G+7) (b) Building 4, a total of 7 stories (G+6) ...55

Figure 3.6: Top view of adjacent building modeling for pounding (case-1) ................................55

Figure 3.7: (a) Building 2, a total of 10 stories (G+9) (b) Building 3, a total of 8 stories (G+7)..56

Figure 3.8: Top view of adjacent building modeling for pounding (case-2) ................................57

Figure 3.9: (a) Building 1, a total of 8 stories (G+7) (b) Building 3, a total of 8 stories (G+7)....58

Figure 3.10: Top view of adjacent building modeling for pounding (case-3) ..............................58

Figure 3.11: Defining response spectrum function for different soils and seismic zones.............63

Figure 3.12: Time history plot of Elcentro earthquake..................................................................64

Figure 3.13: Defining time history function (Elcentro, 1940) in SAP2000...................................65

Figure 4.1: The deformed shape of building 1(after analysis of 10th

mode) ................................69

Figure 4.2: The deformed shape of building 4(after analysis of 10th

mode) ................................69

Figure 4.3: The deformed shape of building 2(after analysis of 9th

mode) ..................................72

Figure 4.4: The deformed shape of building 3(after analysis of 9th

mode) ..................................72

Figure 4.5: The deformed shape of building 1(after analysis of 8th

mode) ..................................75

Figure 4.6: The deformed shape of building 3(after analysis of 8th

mode) ..................................75

Figure 4.7: The effect of different adjacent building configurations on seismic gap value..........79

Figure 4.8:3D view deformed shape of building 1 with TMDs.....................................................83

Figure 4.9: 3D view deformed shape of building 4 with TMDs....................................................84

Figure 4.10: 3D view deformed shape of building 2 with TMDs..................................................86

Figure 4.11: 3D view deformed shape of building 3 with TMDs.............................................86

Figure 4.12: Sample adjacent buildings seismic gap requirement with and without TMDs in

different seismic zones and site conditions....................................................................................90

X

LIST OF TABLES

Table 2.1 basic value γo of behavioral factors (table 3.2 in EBCS-8) ........................................24

Table 2.2 Value of the damping ratio in function of coefficient of restitution ...........................31

Table 3.1 Ground acceleration coefficient of seismic zone of Ethiopia .....................................62

Table 4.1: Maximum story displacement of buildings in different seismic zones......................67

Table 4.2: Response Spectrum Modal Information of building 1 & 4 .......................................68

Table 4.3: Joint Displacements for building 1.............................................................................69

Table 4.4: Joint Displacements for building 4.............................................................................70

Table 4.5: Minimum required separation distance.......................................................................70

Table 4.6: Recommended minimum seismic separation (d min) ..................................................71

Table 4.7: Response Spectrum Modal Information of building 2 & 3.........................................71

Table 4.8: Joint Displacements for building 2..............................................................................73

Table 4.9: Joint Displacements for building 3..............................................................................73

Table 4.10: Minimum required separation distance ....................................................................73

Table 4.11: Recommended minimum seismic separation (d min) ................................................74

Table 4.12: Response Spectrum Modal Information of building 1 & 3.......................................74

Table 4.13: Joint Displacements for building 1............................................................................76

Table 4.14: Joint Displacements for building 3............................................................................76

Table 4.15: Minimum required separation distance .....................................................................76

Table 4.16: Recommended minimum seismic separation (d min) .................................................77

Table 4.17: Top floor displacements of sample buildings (d e) ...................................................77

Table 4.18: Minimum required separation distance of all three cases..........................................78

Table 4.19: Recommended minimum seismic gap provide for the three cases ...........................79

Table 4.20: Determination of Water Tank Size for all sample buildings.....................................82

Table 4.21: Minimum required separation distance of buildings with TMDs .............................84

Table 4.22: Recommended minimum seismic gap of buildings with and without TMDs............85

Table 4.23: Minimum required separation distance of buildings with TMDs .............................87

Table 4.24: Recommended minimum seismic gap of buildings with and without TMDs...........87

Table 4.25: Minimum required separation distance of buildings with TMDs ............................89

Table 4.26: Recommended minimum seismic gap of buildings with and without TMDs...........89

XI

LIST OF SYMBOLS AND ABBREVIATIONS

Cm Center of mass

Cr Center of rigidity

ds Displacement of a point of the structural system induced by the design seismic action

E Modules of elasticity

eox , eoy Distances between the center of mass and center of stiffness in x and y direction

g Acceleration due to gravity

I Moment of inertia

Lx The shortest length of the span

Ly The longest length of the span

ls Radius of gyration

M Mass

W Weight

rx Torsional radius in x direction

ry Torsional radius in y direction

q Behavioral factor

βvx The load transfer coefficient along the shorter direction

βvy The load transfer coefficient along the longer direction

αo Ground acceleration coefficient

ξ Damping ratio

DOF Degree Of Freedom

XII

TMD Tuned Mass Damper

MDOF Multiple Degree Of Freedom

PGA Peak Ground Acceleration

SDOF Single Degree Of Freedom

SRSS Square Root of the Sum of the Square

`1

CHAPTER ONE

1. INTRODUCTION

1.1 Background

Currently our country, Ethiopia, allocates much of its annual budget for the construction of

infrastructures. Among these the construction of buildings, roads, dams and bridges require

significant amount of capital and skilled man powers.

Construction of buildings needs careful attention starting from modeling, analysis and design

activity up to finalizing their construction. Buildings are the major structures which are

exposed to earthquake loading. Therefore, careful attention should be employed in the

modeling and assessment of building behavior under such loadings.

The behavior of building structures under the influence of seismic load has been a major

point of interest for engineers over a long period of time. Although significant advances have

been achieved in the design and construction of earthquake resistant buildings, gaps still

remain in the understanding of adjacent reinforced concrete building design consequences

under different seismic zones and site condition.

Different design codes state that in the event of earthquakes, human lives are protected,

damage is limited and structures important for civil protection remain operational. Although

buildings designed according to our code are expected to perform sufficiently well to prevent

the loss of life under seismic loads that are equal to those they were designed for, how they

will actually perform in the event of an earthquake is largely unknown due to most buildings

are expected to deform beyond the limit of linearly elastic behavior when subjected to strong

ground shaking and the ensuing dynamic instability can cause excessive structural damage,

and even overall collapse. But almost in all building codes the effect of pounding is not

critically considered.

Seismic vulnerability is the expected degree of damage to a given structure at risk resulting

from a given level of seismic hazard. Vulnerability assessment can be observed vulnerability

or predicted vulnerability. Observed vulnerability is based on statistics of past earthquake

`2

damage and predicted vulnerability refers to assessment of expected performance of

buildings based on engineering computations and design specifications or, if no other method

is available, on engineering judgment. Predicted vulnerability is more suitable for area of low

and moderate seismicity. Since no major damaging earthquake has occurred in Ethiopia in

recent times, vulnerability assessment from observed damage patterns are not available.

In our country major towns some independent adjacent buildings are constructed without gap

up to the property line in order to use the land effectively. This situation may lead to non-

structural and structural damages to the buildings and may also give rise to total collapse of

buildings during seismic pounding.

Several new buildings located in Ethiopia major towns are designed and constructed without

considering the effect of existing buildings behavior in the surrounding under earth quake

excitation. Different seismic zone may have different site condition; in most cases a simple

copy of building design from one seismic zone and site condition to another with or without

modification of cross sectional elements is a common practice in major towns of our country.

However, in both cases there is no modification that is made on the gap between adjacent

buildings. Even if the gap between adjacent buildings is sufficient in one seismic zone and

site condition, it may not be sufficient for the other.

Even if the predicted vulnerability is more suitable for area of low and moderate seismicity

like our country, the adjacent building separation distance effect on vulnerability is not

considered by designers. Thus the expected result of predicted vulnerability value during

earth quake shaking becomes high. Therefore, pounding effect is a severe phenomenon in

seismicity area that should be considered by designers during design of adjacent buildings.

1.2 Problem statement

The Ethiopian and other Building Code Standards give basic guide lines to prevent pounding

effect of adjacent buildings during earthquakes. But there are lots of adjacent buildings,

which are constructed before and still under construction, with no separation distance. This is

because the excessive need of land, as cost of land is very expensive in major towns of

Ethiopia and also designers have limited knowledge about pounding effects.

`3

Since the seismic pounding vulnerability of a particular adjacent building arrangement that

was constructed and designed in different seismic zones and site conditions were not similar.

So, it is better to provide sufficient seismic separation distance between adjacent buildings to

the respective seismic zones and site conditions to prevent seismic pounding between

adjacent buildings.

1.3 Objectives

a. General objective

The main objective of the study was to assess the required sufficient seismic gap between

adjacent reinforced concrete buildings to the respective seismic zone and site conditions for

the selected sample buildings, in addition to this how to minimize the seismic gap

requirement of adjacent high rise buildings by using passive structural controls specially

water tanks modeled as tuned mass dampers (TMDs), which are assumed to be constructed

on the top story of the adjacent buildings for the selected regular and irregular building

configurations.

b. Specific objectives

The following tasks are the specific objectives of the research work;

1. How to classify the buildings as regular and irregular based on the configuration

behaviors

2. Finite element modeling of the regular and irregular buildings

3. Perform response spectrum analysis for each buildings in different seismic zone and

site conditions

4. Determine the point at which adjacent buildings can possibly impact during earth

quake excitation

5. Determine buildings mode shape effect on the period of adjacent buildings to know

that a building vibrates out-of-phase or not

6. Compute the minimum separation distance between adjacent buildings in different

seismic zones and site conditions by using different building codes

7. Provide passive structural controls specially water tanks modeled as tuned mass

dampers

8. Compare the seismic gap required by adjacent buildings with and without passive

dampers

`4

1.4 Scope of the study

The paper will present the effect of seismic separation between adjacent regular and irregular

building configurations. This work is limited only to evaluate the minimum required separation

distance between adjacent buildings to avoid pounding effect. In addition to this it provides

passive structural controls in buildings which are used to minimize the required seismic gap

between adjacent buildings. This can be achieved by taking four independent buildings (two

of them are regular and the other two are irregular) buildings are selected. The buildings are

constructed adjacently in three arrangements as a case study, in different seismic zones and

site conditions of the country.

1.5 Report organization

This research has been achieved in five stages.

The first is literature review, which is related to the topic, has been exhaustively undertaken.

In particular, Ethiopian Building Code and Euro-Codes, International Building Codes and

Federal Emergency Management Agency, and various International edited books and

scientific journals in Engineering served as important documents to develop the basic

formulation pertinent to modeling and analysis of adjacent buildings.

In the second step collecting of the sample buildings design data. The data have been

collected from different consulting architects and engineering plc.

The third step is modeling and analysis of these collected designs of sample buildings by

using finite element software SAP2000. During modeling and analysis, the 3D model has

been made for the adjacent buildings separately. The analysis has been done with response

spectrum function.

The fourth step is providing of water tank as innovative mitigation measures to reduce the

required seismic gap between adjacent buildings and use of land effectively.

Finally, Draw an overall comparison and conclusion for the study, In addition to this give

some recommendations for future research works which was the limitations in this study.

`5

CHAPTER TWO

2. REVIEW OF LITERATURE

2.1 Overview of seismic pounding

Structural design of buildings for seismic loading is primarily concerned with structural

safety during major earthquakes, but serviceability and the potential for economic loss are

also of concern. Seismic loading requires an understanding of the structural behavior under

large inelastic, cyclic deformations. Some structural damage can be expected when the

building experiences design ground motions because almost all building codes allow inelastic

energy dissipation in structural systems.

Most seismic codes specify criteria for the design and construction of new structures

subjected to earthquake ground motions with three goals: 1) minimize the hazard to life for

all structures; 2) increase the expected performance of structures having a substantial public

hazard due to occupancy or use; and 3) improve the capability of essential facilities to

function after an earthquake. It is important to see the hazard level caused by the improper

design of seismic separation distance between adjacent buildings in addition to poor design of

cross-sectional member like beam, column, and slab; is the critical issue that briefly

explained in the research. [11]

Hammering of adjacent structures due to out-of-phase relative motion of the independent

structures is called pounding. This occurs during an earthquake, due to different dynamic

characteristics of adjacent buildings and due to insufficient lateral seismic separation of

buildings.

When buildings are erected without sufficient seismic separation between them and their

interaction has not been considered, the buildings may impact each other, or pound, during an

earthquake. Building pounding can alter the dynamic response of both buildings and impart

additional inertial loads on both structures.

Buildings of the same height with matching floors will exhibit similar dynamic behavior.

When the buildings pound, floors will impact with other floors, which means that the damage

`6

due to pounding usually will be limited to non-structural elements. Contrary, when the floors

of adjacent buildings are at different levels, the floors will impact with the columns of the

adjacent building, causing serious structural damage. [4]

Pounding did not cause any major damage when floor levels coincided and further, that most

of the damage occurred in the center of Mexico City, Where there is a concentration of

engineered buildings practically in contact to each other and having different height, different

structural system or different configuration. [18]

Similarly, analytical studies have always assumed regular buildings in plan. Since adjacent

buildings with little or no separation will generally be found in the older sections of down

town, building plans are often very irregular, leading to torsional effects under ground

motion. [9]

Pounding can seriously damage perimeter force-bearing elements like columns and walls,

especially if the floor slabs of adjacent buildings do not align or when the adjacent buildings

are highly irregular configuration. Designers should at least provide alternative force paths,

such as supplementary columns or props located away from the potential damage zone so that

in the event of pounding damage gravity loads continue to be safely supported. [7]

The actual displacement that should be used for building separation is the displacement at

critical locations (contact point) with consideration of both the translational and torsional

displacements. These values can be significantly different. The purpose of seismic separation

is to permit adjoining buildings, or parts thereof, to respond to earthquake ground motions

independently and thus preclude possible structural and nonstructural damage caused by

pounding between buildings or other structures. [5]

Pounding as characterized in Codes and Guide lines and in most analytical research studies

takes the form of in plane displacements of two adjacent buildings, as in the investigation of a

row of adjacent buildings. [9, 21]

The seismic joint between individual independent structural systems should be sufficiently

wide to prevent pounding under the design seismic action, or, in the event that such pounding

`7

occurs, limit its consequences. The seismic joint may be filled, locally or fully, with a soft

non-structural material. Some codes give prescribes the necessary width of the seismic joints.

The principles underlying best retrofit method of existing building (pounding effect

minimization), which should be applied to the specific structural forms utilized for a given

project. The various structural forms in use around the world all have their strong and weak

points, the main structural forms suitable for earthquake resistance and pounding effect

minimization are: conventional methods including global retrofit methods like adding new

structural walls (shear walls); adding new steel braced and also local retrofit method like

beam or column jacketing in addition to this innovation methods are used. [1, 2, 7]

Empirical observation shows that building separations are complex in their basic conditions

and in their effects, and lack of separation is not necessarily detrimental. Observation has

shown that the end buildings of a row of adjacent buildings tend to suffer more damage than

interior buildings. Analytical pounding studies consider regular buildings in elevation. In fact,

the sway characteristics of buildings are much influenced by irregularities, particular that of

soft first stories, that can lead to extreme displacements or even collapse. [9]

For this thesis work some sample adjacent building photos was taken in different area of the

country during observation we have seen different cases some independent adjacent buildings

are constructed without gap up to the property line in order to use the land effectively.

Sometimes buildings adjoin to public way without sufficient gap up to the property line. This

situation may lead to non-structural and structural damages to the buildings and may also

give rise to total collapse of buildings during seismic excitation and also this is the main case

of collision phenomenon. (Figure 2.1) shows example of adjacent buildings that are

constructed in different area of the country without calculating appropriate gap up to the

property line.

`8

(a) (b)

Figure 2.1: Adjacent buildings constructed up to the property line

(a) Addis Ababa around Adisu gebya (b) Dessie around piazza

Several new buildings located in Ethiopia major towns are designed and constructed without

considering the effect of existing buildings behavior in the surrounding under earth quake

excitation, See (Figure 2.2). Different seismic zone may have different site condition, but in

most cases a simple copy of building design from one seismic zone and site condition to

another with or without modification of cross sectional elements is a common practice in

major towns of our country. e.g. Condominium apartment houses, buildings constructed in

different universities like student dormitories and class rooms and buildings that constructed

by the agency for the development of houses are commonly a simple copy of building

designs. In (Figure 2.3): (a) and (b) shows that G+3 adjacent buildings that are constructed by

the agency for the development of houses in Bahir dar around Kebele 13 near to Police

commission and Dessie around piazza respectively haven almost the same frame structure

cross sections and seismic gap. However, in both cases there is no modification that is made

on the gap between adjacent buildings. Even if the gap between adjacent buildings is

sufficient in one seismic zone and site condition, it may not be sufficient for the other.

`9

(a)

(b)

Figure 2.2:- New buildings that are designed and constructed without the information of

existing neighboring building behavior under earthquake excitation. (a) Addis Ababa around

Adisu gebya (b) Dessie around piazza

`10

(a)

(b)

Figure 2.3:-Similar adjacent buildings which are constructed in different places haven

approximately the same seismic gap and frame structure cross sections (a) Bahir dar around

kebele 13 near to Police commission (b) Dessie around piazza in front of Dessie tower

`11

Generally, the seismic assessment was conducted in order to compare the seismic separation

distance between adjacent buildings in different seismic and site condition of the country.

The analysis is carried out using response spectrum loading function from the design

spectrum data. The study must focuses on identifying realistic analytical model that can

predict and assess the actual behavior of pounding or impact phenomena between adjacent

buildings. Furthermore, the difference in seismic response behavior of regular and irregular

structures and the phenomena of pounding have been studied and presented by several

authors. Consequently the research work employs both of these building types for seismic

assessment in different seismic and site conditions. During the past few years, several

researchers have found new assessment, evaluation and modeling method of pounding effect

between adjacent structures, as a result some discussions are made here after

.

2.2 Building configuration effect on pounding phenomena

Most codes and journals list and define five types of horizontal irregularities in order to

classify a building either regular or irregular: torsional and extreme torsional, re-entrant

corner, diaphragm discontinuity, out-of-plan offsets, and non-parallel systems. And also

define five types of vertical irregularities in order to classify a building either regular or

irregular: a floor significantly heavier than an adjacent floor (mass irregularity), vertical

structure of one story more flexible and/or weaker than that above it (stiffness uniformity),

short columns, discontinuous and off-set structural walls, and an abrupt change of floor plan

dimension up the height of a building (set-back), in fact it is rear to design and construct

regular buildings in the real world. [7]

2.2.1 Regularity in plan

Classification as regular in plan requires the following

1. „Approximately ‟symmetrical distribution of mass and stiffness in plan

2. A „compact‟ shape, i.e. one in which the perimeter line is always convex, or at least

enclose not more than 5 percent re-entrant area

3. The floor diaphragms shall be sufficiently stiff in plane not to affect the distribution of

lateral loads between vertical elements. EC-8 warns that this should be carefully

examined in the branches of branched system such as L, C, H, I and X plan shapes.

(Figure 2.5)

4. the ratio of longer side to shorter side in plan does not exceed 4 in other word the

slenderness λ = Lmax/Lmin of the building in plan shall be not higher than 4, where

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Lmax and Lmin are respectively the larger and smaller in plan dimension of the

building, measured in orthogonal directions.

5. In single story buildings the center of stiffness is defined as the center of the lateral

stiffness of all primary seismic members. The torsional radius r is defined as the

square root of the ratio of the global torsional stiffness with respect to the center of

lateral stiffness, and the global lateral stiffness, in one direction, taking into account

all of the primary seismic members in this direction.

6. In multi-story buildings only approximate definitions of the center of stiffness and of

the torsional radius are possible. A simplified definition, for the classification of

structural regularity in plan and for the approximate analysis of torsional effects, is

possible if the following two conditions are satisfied:

All lateral load resisting systems, such as cores, structural walls, or frames, run

without interruption from the foundations to the top of the building;

The deflected shapes of the individual systems under horizontal loads are not very

different. This condition may be considered satisfied in the case of frame systems and

wall systems. In general, this condition is not satisfied in dual systems.

7. In frames and in systems of slender walls with prevailing flexural deformations, the

position of the centers of stiffness and the torsional radius of all stores may be

calculated as those of the moments of inertia of the cross-sections of the vertical

elements. If, in addition to flexural deformations, shear deformations are also

significant, they may be accounted for by using an equivalent moment of inertia of the

cross-section.

2.2.2 Regularity in elevation

A building must satisfy all the following requirements to be classified as regular in

elevation

1. All the vertical load resisting elements must continue uninterrupted from foundation

level to the top of building or (where set backs are present) to the top of the setback.

(Figure 2.4)

2. Mass and stiffness must either remain constant with height or reduce only gradually

without abrupt changes. EC-8 recommended that buildings where the mass or

stiffness of any story is less than 70 per cent of that of story above or less than 80 per

cent of the average of the three stories above should be classified as irregular in

elevation.

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3. In buildings with moment resisting frames, the lateral resistance of each story (i.e. the

seismic shear initiating failure with in that story, for the code-specified distribution of

seismic loads) should not vary „disproportionality‟ between stories. Generally, no

quantified limits are stated by EC-8, although special rules are given where the

variation in lateral resistance is due to masonry infill with in the frames. EC-8

recommends that buildings where the strength of any story is less than 80 per cent of

that of the story above should be classified as irregular in elevation.

4. Buildings with setbacks (i.e. where the plan area suddenly reduces between

successive story‟s) are generally irregular, but may be classified as regular if less than

limits defined in the code. The limits broadly speaking are a total reduction in width

from top to bottom on any face not exceeding 30 percent, with not more than 10 per

cent at any level compared to the level below. However, an overall reduction in width

of up to half is permissible with in the lowest 15 per cent of height of the building. [8,

14, 15]

Figure 2.4: Regular and irregular vertical configurations: (a) compact shape: (b) set-back

configuration: (c) off-set configuration

Figure 2.5: Regular and Irregular horizontal configurations: (a) symmetric lay out; (b)

irregular lay out

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2.2.3 Other building irregularity

a. Weak story (irregularity in strength): (Figure 2.6) the story strength is the total

strength of all lateral force-resisting elements in a given story for the direction under

Consideration. For the shear walls, the strength is the shear capacity of walls. In the framed

structures, the strength is given by the shear capacity corresponding to the flexural strength of

columns. Weak stories are usually found where vertical discontinuities exist, or where

member size or reinforcement has been reduced. [4]

Figure 2.6: weak story (irregularity in strength)

b. Soft story (irregularity in stiffness) (Figure 2.7) the increasing of story stiffness is

given by the non-structural infill panels (masonry, concrete or metal). The soft story is the

one where these infill panels are missing due to functional demands.

A very widespread structural configuration in existing buildings is characterized by the

absence of these infill panels at the ground floor, while they are present at the elevation

stories. This configuration is called pilotis configuration, giving rise to a soft first story,

which allows a good use and distribution of the space at the ground floor, but it is very

dangerous from the seismic point of view. The lateral response of these buildings is

characterized by a large rotational ductility demand, which is concentrated at the end sections

of the columns of the first story, while the superstructure behaves like a quasi-rigid body.

All the inelastic deformations are concentrated in the soft story. This structural type was the

cause of many collapsed buildings during the 1999 Kocaeli earthquake (Turkey). [4]

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(a) (b)

Figure 2.7: soft stories (a) soft ground story (pilotis story); (b) soft story

c. Mass irregularity can be detected by comparing the story weights (Figure 2.8). The

effective mass is given by the dead load of the structure at each level plus the actual weight of

partitions and permanent equipment‟s on each floor.

When these shapes are proved to be unavoidable, it is desirable to use seismic separations to

divide the building into independent parts, which provide a regular seismic behavior (Figure

2.9). [4]

Figure 2.8: Mass irregularity

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Figure 2.9: seismic separation on adjacent buildings

2.3 Characteristics of pounding

It is nearly impossible to construct a building which has similar seismic behavior to another

building. Structural poundings happen because of swaying of adjacent buildings with

different mode shapes and periods under seismic loads which are not separated from each

other properly (Figure 2.10). During earthquakes, structure‟s mass and rigidity affect seismic

behavior.

Figure 2.10: seismic behavior of adjacent buildings

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Poundings may occur because of structural irregularities. For example, eccentricity between

mass and rigidity centers cause torsion in the structure (Figure 2.11a). If pounding building is

regular, an impact surface can be formed between two adjacent buildings (Figure 2.11b). In

some cases this situation is better than the situation that is seen in Figure 2.11a.

Figure 2.11: pounding of regular and irregular building

Also buildings may collide to each other because of liquefaction. This case is not considered

while calculating separation distances. Otherwise, separation distance between two adjacent

buildings must be equal to the height of the highest one (Figure 2.12). [19, 20, 34]

Figure 2.12: pounding of building due to liquefaction

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2.4 Causes of pounding

During strong earthquakes, adjacent structures that do not have appropriate distance and hit

each other, that is called pounding. The difference between dynamic properties (mass,

hardness and height) of adjacent structures results different-phase oscillations which is the

main cause to pounding and the more different in shape of vibration causes stronger pounding

and vice versa, if the dynamic properties are the same, even if the distance is zero, there is no

impact. Impact phenomenon has been reported in the strong earthquakes, for example Alaska

(1964), San Fernando (1971), Romania (1977), Greece (1981), Northbridge (1994) and Kobe

(1995). In Lumaprita earthquake (1989), most damages were caused by the impact of

structure. In Mexico City earthquake (1985), more than 15% of the 330 structure that

damaged or totally destroyed were result of impact phenomenon. Because of the important of

this object, many researchers have reconsidered on it.

Different modal behavior of two adjacent buildings causes different pounding types. This

makes pounding more complex. In dynamical analysis of structures, buildings are modeled

by forming lumped masses at floor levels. Various types of impact (pounding) seen in the

recent earthquakes can be categorized into 5 main groups

I. Pounding of the structure on the column of an adjacent building: This type of

impact occurs in some adjacent buildings in which the floors levels are not in the same

heights. Therefore, when shaking with different phases occurs, the floor of one building hits

the column of another and causes serious damages which can lead to the fracture of the

columns of the story. This type is the most dangerous impact that can result in sudden

destruction of the structure (Figure 2.13a).

II. Pounding of a heavier building on a lighter one: Since adjacent buildings may differ

in the structural system of floors and/or in their applications, they have different masses, this

can cause different phase oscillations, since the lighter building tolerates more intensive

response (Figure 2.13b).

III. Pounding of a shorter building on a taller one: When two structures with different

heights are adjacent, because of different dynamic properties, the shorter structure hits the

adjacent one, which results in floor shearing in higher levels of impact part. It is important to

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know that the higher in the impact part level, the greater impact is tolerated more intensive

response (Figure 2.13c).

IV. Pounding of two adjacent buildings with non-coaxial mass centers: In building

with non-coaxial mass centers, the structure may pound on the edge of the adjacent structure

and cause strong tensional torques, which can lead to seriously damage to the column on the

edges and corners of the pounded building (Figure 2.13d).

V. Adjacent buildings with equal height and with aligned floor levels: Buildings of

similar height and with similar structural systems tend to suffer less damage than buildings of

different height and with different structural systems. This is due to the fact that buildings

with the same height will have similar natural frequencies and will tend to move in-phase

relative to one another. Buildings that are the same height and have matching floors are likely

to exhibit similar dynamic behavior. If the buildings pound, floors will impact other floors, so

damage usually will be limited to non-structural components. (Figure 2.13e) [8, 19, 24, 25]

(a) Pounding of the structure on the column of an adjacent building

(b) Pounding of a heavier building on a lighter one

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(c) Pounding of a shorter building on a taller one

(d) Pounding of two adjacent buildings with non-coaxial mass centers

(e) Adjacent buildings with equal height and with aligned floor levels

Figure 2.13: Various types of impact (pounding)

However, floor highest of adjacent buildings are not always the same. So, collisions may

occur at different levels. Structural behavior can change due to impact point in the pounding

case. For analyzing structural pounding effects for different contact points a solid frame

model is generated by Structural Analysis Program software. [34]

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2.5 Code provisions regarding to pounding

Past seismic codes did not give definite guide line to preclude pounding. Because of this and

due to economic considerations including maximum land usage requirements, there are many

buildings world-wide which are already built in contact or extremely close to one another that

will suffer pounding damage in future earth quakes. [27]

Most of the world regulations for seismic design do not take into account the pounding

phenomenon. Among the exceptions are the codes of Argentina, Australia, Canada, France,

India, Indonesia, Mexico, Taiwan and USA. These codes specify a minimum separation

distance between adjacent buildings. However, the procedure to determine the separation

distance varies from country to country. In UBC-1997, it depends on the maximum

displacements of each building. In Canada and Israel, it is simple sum of the displacements of

each building. In France it is a quadratic combination of the maximum displacements. In

Taiwan it is depends on the building height and in Argentina minimum gap is 2.5 cm. The

provisions on separation distance are very similar in the 2000 and 2003 International

Building Code (IBC, 2003). In 2006 version there is no code provision on building

separation. [21]

1. International Building Code (IBC-2009) the separation distance between two adjacent

buildings (δM) is computed as

….......................................2.1

Where, δmax is the maximum elastic displacement that occurs anywhere in a floor from the

application of the design base shear to the structure. Cd is the deflection amplification factor

and ‟I‟ is the importance factor for seismic loading. [16, 21]

2. Ethiopian Building Code Standard (ES-EN-1998-1-1: 2015) and Euro Code-8:2004

Stated as follows, Buildings shall be protected from earthquake-induced pounding from

adjacent structures or between structurally independent units of the same building. This

principle is considered to be satisfied.

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

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horizontal displacement of the building at the corresponding level, calculated in accordance

with expression (part 4.23);

dsT = ds1+ ds2 ………………………….….2.2

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, calculated in accordance with expression (part 4.23).

dsT =√ ………………...….. 2. 3

dsT is the total separation distance of adjacent building

ds1is the displacement of a point of the structural system induced by the design seismic

action of building 1.

ds2 is the displacement of a point of the structural system induced by the design seismic

action of building 2.

A specification is made: If the floor elevations of the building or independent unit under

design are the same as those of the adjacent building or unit, the above referred minimum

distance may be reduced by a factor of 0.7.

Critical Parameters used in structural analysis program software are The Behavior factors (q

) for horizontal seismic actions which are determined based on the formula provided in the

code section 5.2.2.2 and also the lower bound factor for the horizontal design spectrum (β).

The value to be ascribed to β for use in a country can be found in its National Annex. The

recommended value for β is 0.2. [14, 15]

3. Uniform Building Code (UBC 1997) also specifies spacing between the adjacent

buildings equal to the square root of the sum of squares (SRSS) of the individual building

displacements.

All structures shall be separated from adjoining structures. Building Separations shall allow

for the displacement ΔM. Adjacent buildings on the same property shall be separated by at

least, ΔMT,

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(a) (b)

Figure 2.14: separation distance of adjacent buildings (a); on different property line (b); on

the same property line

Absolute sum method (ABS)

ΔMT = ΔMA+ΔMB ……………………….2.4 (for Figure 2.14 a)

Square root of the sum of the squares method (SRSS)

ΔMT = √ ……………….2.5 (for Figure 2.14 b)

ΔMA and ΔMB are the displacements of the adjacent buildings. When a structure adjoins a

property line not common to a public way, the structure shall be set back from the property

line by at least the displacement, ΔMT, of that structure. In other words this can be

determined by the ABS (absolute sum) expression. Adjacent buildings separations on the

same property line shall be determined by the square root of the sum of the squares (SRSS)

expression. [17, 21, 30]

4. Federal Emergency Management Agency (FEMA: 273-1997)

The separation distance between adjacent structures shall be less than 4% of the building

height and above to avoid pounding. FEMA states that buildings intended to meet enhanced

objectives shall be adequately separated from adjacent structures to prevent pounding during

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response to the design earthquakes. Pounding may be presumed not to occur whenever the

buildings are separated at any level i by a distance greater than or equal to si. The value of si

need not exceed 0.04 times the height of the buildings above grade at the zone of potential

impacts. [21]

5. Ethiopian Building Code Standard (EBCS 8: 1995)

Specifies about the separation distance of adjacent buildings as follows:-

Seismic Joint Condition (2.4.2.7)

1. Building shall be protected from collisions with adjacent structures induced by earthquake.

2. The requirement of (1) above is deemed to be satisfied if, the distance from the boundary line

to the potential point of impact is not less than the maximum horizontal displacement

according to equation (2.6) given as:

........................................................2.6

Where γd is displacement behavior factor equal to γ and can be expressed as follow.

γ = γo KD KR KW………………………...2.7

γo is basic value of the behavior factor, depending on the structural type. The values is given

in the Table 2.1 below

Table 2.1 basic value γo of behavioral factors (table 3.2 in EBCS-8, 1995)

Structural type Γo

Frame system 0.2

Frame equivalent 0.2

Dual system Wall equivalent, with coupled walls 0.2

Wall equivalent, with uncoupled walls 0.2

Wall system With coupled walls 0.2

With uncoupled walls 0.25

Core system 0.3

Inverted pendulum system 0.5

KD is factor reflecting the ductility class and given as:-

1.00 for DC”H”

KD = 1.50 for DC”M”

2.00 for DC”L”

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KR is factors reflecting the structural regularity in elevation and shall be given as follows.

KR = 1.00 for regular structures

1.25 For non-regular structures

KW is factors reflecting the prevailing failure made in structural system with walls shall be

taken as follows

1.00 for frame, frame equivalent dual system.

KW = (2.5-0.5αo) for wall, wall equivalent system

≥ 1 for core system

3. If the floor elevations of a building under design are the same as those of the adjacent

building, the distance referred in (2) above may be reduced by a factor of 0.7.

4. Alternatively separation distance is not required if, appropriate shear walls are provided on

the primate of the building to act as collision walls (“bumpers”). At least two such walls must

be placed at each side subject to pounding and must extend over the total height of the

building. Then must be perpendicular to the side subject to collisions and they can end on the

boundary line. [28]

Even though, the building code puts a guide line as stated above, due to the maximum land

use often required because of high population density and economic considerations there are

many buildings which are already constructed and being constructing in contact with or

extremely close to one another. It is noted that out-of-phase vibrations may be induced when

adjacent buildings are subjected to earthquake loading and pounding may occur if the

separation distance is inadequate. As a result, the seismic pounding of adjacent buildings may

pose a potentially serious problem in Ethiopia major cities.

2.6 Seismic Pounding Damage Classification

Pounding is one of the main causes of severe building damages in earthquake. The non-

structural damage involves pounding or movement across separation joints between adjacent

structures. Seismic pounding between two adjacent buildings occur

· During an earthquake

· Due to Different dynamic characteristics of adjacent buildings

· Adjacent buildings vibrate out of phase

· At-rest separation of adjacent building is insufficient

`26

Separation joint is the distance between two different building structures - often two wings of

the same facility - that allows the structures to move independently of one another.

A seismic gap is a separation joint provided to accommodate relative lateral movement

during an earthquake. In order to provide functional continuity between separate wings,

building utilities must often extend across these building separations, and architectural

finishes must be detailed to terminate on either side. The separation joint may be only an inch

or two in older constructions or as much as a foot in some newer buildings, depending on the

expected horizontal movement, or seismic drift. Flashing, piping, fire sprinkler lines,

partitions, and flooring all have to be detailed to accommodate the seismic movement

expected at these locations when the two structures move closer together or further apart.

Damage to items crossing seismic gaps is a common type of earthquake damage. If the size

of the gap is insufficient, pounding between adjacent buildings may result in damage to

structural components of the buildings. [4, 19, 26, 34]

(a) (b)

Figure 2.15: pounding effect

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Pounding damage pattern is classified as follows: - [23, 29]

Type-1 Major structural damage; (Figure 2.16a)

Type-2 Failure and falling of building non-structural elements creating life-safety hazards;

(Figure 2.16 b)

Type-3 Loss of building functions due to failure of mechanical, electrical or fire protection

Systems;

Type-4 Architectural, non-structural and/or minor structural damage; (Figure 2.17)

(a) (b)

Figure 2.16: pounding damage effect between buildings; 2008 Sichuan (China) Earthquake

Figure 2.17: Pounding damage observed in the 2011 Christchurch earthquake

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2.7 Modeling of Pounding

Pounding is a highly nonlinear phenomenon and two types of modeling techniques are

adopted. The methods used in the dynamic analyses of non-linear multi-degree-of-freedom

systems under random excitations can be classified into two categories: the theoretical

approach and the numerical simulation approach. The main advantage of the theoretical

approach is that an exact solution may be obtained. However, its use is frequently limited

because of the assumptions made on the analytical procedures. For the past few decades, the

numerical simulation approach has gained a wide range of applications in solving responses

of non- linear multi degree-of-freedom systems. [30]

Numerical modeling has been employed frequently to predict pounding forces and structural

responses. Such models define a constant “gap” between the buildings and if the relative

closing displacement between the two buildings is more than the “gap”, the contact force is

activated. The most common form of contact model assumes the presence of an elastic spring

with or without a viscous damper to model energy loss. These models have an advantage that

they can be implemented in most existing numerical time history analysis software without

significant programming modification. Other approaches like Lagrange and Laplace methods

have also been employed in some studies, but their application is limited as they need special

programming by the users or the modification to the current commercial software. In this

study, the theoretical approach method is adopted. [23]

Two analytical techniques are available for modeling – the contact element method and the

stereo mechanical approach. In the former approach, a contact element is activated when the

structures come into contact. A spring with high stiffness is used to avoid overlapping

between adjacent segments, sometimes in conjunction with a damper. The contact elements

used in the past include the linear spring, the energy-dissipating Kelvin-Voigt and the non-

linear Hertz contact element.

The contact element approach has its limitations, with the exact value of spring stiffness to be

used being unclear. Moreover, using a spring of very high stiffness can result in

unrealistically high impact forces and also lead to numerical convergence problems. The

stereo mechanical approach assumes instantaneous impact and uses momentum balance and

the coefficient of restitution to modify velocities of the colliding bodies after impact.

`29

However, the stereo mechanical approach is no longer valid if the impact duration is large

enough so that significant changes occur in the configuration of the system. Furthermore, it

cannot be easily implemented into existing commercial software. See (Figure 2.18)

Typically, pounding is modeled using either a continuous force model (contact element

approach) or via a stereo mechanical (coefficient of restitution) approach. Several of the

existing impact models are considered in this study. In addition, a contact model based on the

linear spring model; Hertz model and Kelvin model is also introduced. The analytical

formulations of the various impact models are outlined below: [12, 31]

2.7.1 Contact Element Approach

Contact Element is one of the simple but effective approaches to overcome the Pounding

Effect. During the Earthquake the impact forces that get generated due to the pounding effect

can be readily be thought as being provided by contact element and these forces are

continuous in nature. The contact element approach uses super suspension materials like

Dampers, springs with high stiffness between the adjacent Structures to absorb the impact

forces thus minimizing the forces directly acting on the structures. [12, 32]

Figure 2.18: Analytical model

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2.7.1.1 Linear Spring Model

A linear spring of high stiffness (Kl) can be used to simulate impact once the gap between

adjacent bodies closes, as shown in (Figure 2.19). A linear contact element model is best

suited for the simulation of seismic pounding between multi-story buildings. The contact

force during impact is taken as: [12]

Fc = Kl (U1-U2-gp) ; U1-U2- gp ≥ 0…………....2.8a (contact-approach)

Fc = 0 ; U1-U2- gp < 0 ……….....2.8b (no contact)

Where U1 and U2are the displacements of the impacting bodies, Kl is the spring constant of

the element and gp is the static separation between the structures.

This contact force-based approach is relatively straight forward, and can be easily

implemented in commercial software. However, energy loss during impact cannot be

modeled.

Whenever two mechanical systems collide there is an exchange of momentum and energy is

dissipated in the high stress region of contact. This energy dissipation is the work involved in

damped elastic behavior and also plastic deformation and fracture.

Figure 2.19: Linear spring model and its contact force relation

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2.7.1.2 Kelvin Model

The most fundamental linear contact element model, which can take into account the energy

dissipated during impact is the Kelvin-Voigt model

A linear spring of stiffness (Kk) is used in conjunction with a damper element (Ck), as shown

in (Figure 2.20).

This model is capable of modeling energy dissipation during impact and the impact force

representation is: [12]

Fc=Kk(U1-U2-gp) + Ck(U1*-U2*)

; U1-U2- gp ≥ 0……………….2.9 a

Fc=0 ; U1-U2- gp < 0…………….....2.9 b

Where Kk and Ck are the spring and dashpot constant of the element, U1 and U2 are the

displacement of the impacting bodies U1* and U2* are the velocities of impacting bodies and

gp is the static separation between the structures.

In this model the constant of the dashpot determines the amount of energy dissipated but the

viscous element remains activated when the structures tend to separate.

The damping coefficient Ck can be related to the coefficient of restitution (e), by equating the

energy losses during impact.

Ck =2 ξ

; ξ =

√ ………………….…2.10

Where m1 and m2 are the mass of colliding bodies and ξ is the damping ratio.

Values of ξ i and corresponding values of the coefficient of restitution (e) are listed in Table

2.2. The value of (e) ranges from 0 (for perfectly plastic impact) to 1.0 (for elastic impact).

Table 2.2 Value of the damping ratio in function of coefficient of restitution

ξ i 0 0.02 0.05 0.1 0.2 0.5 1.0

e 1 0.94 0.85 0.73 0.53 0.16 0

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The disadvantage of the traditional Kelvin element is that the viscous element remains

activated when the structures tend to separate, that is, the dashpot in the element opposes the

motion of the structure when they come together, but also opposes the motion of the structure

when they bounce back, which does not have a physical explanation.

A variation of the Kelvin model may include a non-linear spring and a contact element that

only contributes for positive loading; see Figure 2.20. This model is called the Impact Kelvin

element.

Figure 2.20: Kelvin model and its contact force relation

2.7.1.3 Hertz Model

Another popular contact force model for representing pounding is the Hertz model, which

uses a nonlinear spring of stiffness (Kh), as illustrated in Figure 2.21. The impact force

representation is: [12]

Fc=Kh (U1-U2- gp) 3/2

; U1-U2- gp ≥ 0…….……..2.11 a

Fc=0 ; U1-U2- gp < 0……….…..2.11 b

Where Kh is the spring constants of the element, U1 and U2 are the displacements of the

impacting bodies and gp is the static separation between the structures. The coefficient Kh

depends on material properties and geometry of colliding bodies.

`33

This model is more realistic but Energy loss cannot be modeled and not easily implemented

in software. Hertz impact model has been adopted by various authors.

The Hertz damped model suffers from the limitation that it cannot represent the energy

dissipated during impact. Hence, an improved version of the Hertz damped model is

considered here in, where by a non-linear damper is used in conjunction with the Hertz

spring. Similar models have been used in other areas such as robotics, and multi-body

systems and the constant of the dashpot determines the amount of energy dissipated.

However, its efficacy in structural engineering has not been considered. The contact force can

be expressed as

Fc=Kh (U1-U2- gp) 3/2

+ Ch(U1*-U2*) ; U1-U2- gp ≥ 0…………...2.12 a

Fc=0 ; U1-U2- gp < 0…….…......2.12 b

Where Ch is the damping coefficient, U1-U2–gp is the relative penetration and U1*-U2* is the

penetration velocity. A nonlinear damping coefficient (Ch) is proposed so that the hysteresis

loop matches the expected loop due to a compressive load that is applied to and removed

from a body within its elastic range at a slow rate.

Ch = ζ δn….........................................................2.13

Where δ is the damping constant, and δ is the relative penetration (U1-U2– gp).

Equating the energy loss during stereo mechanical impact to the energy dissipated by the

damper, the value of δ can be related to the spring constant, Kh, the coefficient of restitution,

e, and the relative velocity of the bodies at the instant of impact, V1 – V2, as shown below.

ζ =

………………….………..….………2.14

The contact element approach has its limitations, with the exact value of spring stiffness to be

used, being unclear. Uncertainty in the impact stiffness arises from the unknown geometry of

the impact surfaces, uncertain material properties under loading and variable impact

velocities.

`34

Hence, the force during contact in (2.12a) can be expressed as:

Fc = kh(u1-u2-gp)3/2

[1 +

(u

*1-u

*2)] ;u1-u2-gp ≥0…………..…2.15

Figure 2.21: Hertz model and its contact force relation

2.7.2 Stereo Mechanical Model

This approach uses the momentum conservation principle and the coefficient of restitution to

model impact. The duration of impact is neglected. The coefficient of restitution (e) is

defined as the ratio of separation velocities of the bodies after impact to their approaching

velocities before impact. Since this is not a force-based approach the impact force under this

approach is zero (Fc = 0). However, the velocities of the colliding bodies are adjusted after

impact, as shown in Equation 2.17. [12]

v’1 = v1 - (1+e)

……………….……….2.16a

v’2 = v2 + (1+e)

……………………….2.16b

where v1’, v2 ’ are the velocities of the colliding masses (m1, m2) after impact, v1, v2 are the

velocities before impact and e is the coefficient of restitution. See Figure 2.22

`35

Traditionally the value of the coefficient of restitution was assumed to depend only on the

material properties. However, the influence of the mass, the shapes, and the relative velocities

has been recognized. It should be noted that at (e) equal 1, the Kelvin model reduces to the

linear spring and the Hertz damp model reduces to the Hertz model.

The contact between two structures by assuming it instantaneous and the values of velocities

to be used at the new time step are calculated using Newton's law in combination with the

principle of conservation of momentum. They concluded that the effect of the coefficient of

restitution in the realistic range of 0.2 to 0.8 is relatively minor.

It has been shown that the variation in the coefficient of restitution (e) has a relatively minor

effect on the structural response due to pounding. Classical formulation of the problem and

Permanent deformation is accounted by the coefficient of restitution but it is not valid if the

impact duration is large and difficult to implement in software.

Some literatures proposed an algorithm, which can be incorporated into computer program.

The method is based on the Lagrange multiplier approach by which the geometric

compatibility conditions due to contact are enforced.

(a) (b)

Figure 2.22: stereo-mechanical model :( a) pre-impact state; (b) post-impact state

In general the contact element approach (piece-wise model) can provide a better

approximation than stereo-mechanical to the real problem, under the condition that

appropriate values of the impact element Properties are used.

`36

2.8 Methods of Seismic Analysis for Structures to compute seismic

separation

The model of the building shall adequately represent the distribution of stiffness and mass in

it so that all significant deformation shapes and inertia forces are properly accounted for

under the seismic action considered. In the case of non-linear analysis, the model shall also

adequately represent the distribution of strength.

The model should also account for the contribution of joint regions to the deformability of the

building, e.g. the end zones in beams or columns of frame type structures. Non-structural

elements, which may influence the response of the primary seismic structure, should also be

accounted for.

In general the structure may be considered to consist of a number of vertical and lateral load

resisting systems, connected by horizontal diaphragms.

When the floor diaphragms of the building may be taken as being rigid in their planes, the

masses and the moments of inertia of each floor may be lumped at the center of gravity.

In concrete buildings, in composite steel-concrete buildings and in masonry buildings the

stiffness of the load bearing elements should, in general, be evaluated taking into account the

effect of cracking. Such stiffness should correspond to the initiation of yielding of the

reinforcement. Unless a more accurate analysis of the cracked elements is performed, the

elastic flexural and shear stiffness properties of concrete and masonry elements may be taken

to be equal to one-half of the corresponding stiffness of the un cracked elements. [15, 28]

Various methods of differing complexity have been developed for the seismic analysis of

structures. The two main techniques currently used for this analysis are:

1. Linear Procedures.

- Linear Static Analysis (Seismic Coefficient Analysis).

- Linear Dynamic Analysis (Response Spectrum Analysis).

2. Non-linear Procedures.

-Non-linear Statics Analysis (Pushover Analysis).

- Non-Linear Dynamic Analysis (Time History Analysis). [20, 26]

`37

2.8.1 Linear Static Analysis (Seismic Coefficient Analysis)

Linear-elastic analysis may be performed using two planar models, one for each main

horizontal direction, if the criteria for regularity in plan are satisfied.

Depending on the importance class of the building, linear-elastic analysis may be performed

using two planar models, one for each main horizontal direction, even if the criteria for

regularity in plan are not satisfied, provided that all of the following special regularity

conditions are meet.

a) The building shall have well-distributed and relatively rigid cladding and partitions

b) The building height shall not exceed 10 m;

c) The in-plane stiffness of the floors shall be large enough in comparison with the

lateral stiffness of the vertical structural elements, so that a rigid diaphragm behavior

may be assumed.

d) The centers of lateral stiffness and mass shall be each approximately on a vertical

line and, in the two horizontal directions of analysis.

Whenever a spatial model is used, the design seismic action shall be applied along all

relevant horizontal directions (with regard to the structural layout of the building) and their

orthogonal horizontal directions. For buildings with resisting elements in two perpendicular

directions these two directions shall be considered as the relevant directions.

This type of analysis may be applied to buildings whose response is not significantly affected

by contributions from modes of vibration higher than the fundamental mode in each principal

direction.

The fundamental mode shapes in the horizontal directions of analysis of the building may be

calculated using methods of structural dynamics or may be approximated by horizontal

displacements increasing linearly along the height of the building. [15, 28]

2.8.2 Linear Dynamic Analysis (Response Spectrum Analysis)

All real physical structures, when subjected to loads or displacements, behave dynamically.

The additional inertia force from, Newton‟s second law, are equal to the mass times the

acceleration. If the loads or displacements are applied very slowly then the inertia forces can

be neglected and astatic load analysis can be justified. Hence, linear-dynamic analysis is a

simple extension of static analysis.

`38

The response spectrum technique is really a simplified special case of modal analysis. The

modes of vibration are determined in period and shape in the usual way and the maximum

response magnitudes corresponding to each mode are found by reference to a response

spectrum.

The response spectrum method has the great virtues of speed and cheapness. The basic mode

superposition method, which is restricted to linearly elastic analysis, produces the complete

time history response of joint displacements and member forces due to a specific ground

motion loading. There are two major disadvantages of using this approach. First, the method

produces a large amount of output information that can require an enormous amount of

computational effort to conduct all possible design checks as a function of time. Second, the

analysis must be repeated for several different earthquake motions in order to assure that all

the significant modes are excited, since a response spectrum for one earthquake, in a

specified direction, is not a smooth function.

There are significant computational advantages in using the response spectra method of

seismic analysis for prediction of displacements and member forces in structural systems.

The method involves the calculation of only the maximum values of the displacements and

member forces in each mode using smooth design spectra that are the average of several

earthquake motions. In this analysis, the CQC method to combine these maximum modal

response values to obtain the most probable peak value of displacement or force is used. In

addition, it will be shown that the SRSS methods of combining results from orthogonal

earthquake motions will allow one dynamic analysis to produce design forces for all

members of the structure.

In elastic response spectrum it is the 5% damped response spectrum for the seismic hazard

level of interest, representing the maximum response of the structure, in terms of spectral

acceleration Sa, at any time during an earthquake as a function of period of vibration T. [26]

This type of analysis shall be applied to buildings which do not satisfy the conditions given in

the lateral force method of analysis. The response of all modes of vibration contributing

significantly to the global response shall be taken into account.

The requirements specified in paragraph above may be deemed to be satisfied if either of the

following can be demonstrated:

− The sum of the effective modal masses for the modes taken into account amounts to at

`39

least 90% of the total mass of the structure;

− All modes with effective modal masses greater than 5% of the total mass are taken into

account. [15, 28]

2.8.3 Non-linear Statics Analysis (Pushover Analysis)

The non-linear static procedure or simply push over analysis is a simple option for estimating

the strength capacity in the post-elastic range. This procedure involves applying a predefined

lateral load pattern which is distributed along the building height.

The lateral forces are then monotonically increased in constant proportion with a

displacement control node of the building until a certain level of deformation is reached.

The applied base shear and the associated lateral displacement at each load increment are

plotted. Based on the capacity curve, a target displacement which is an estimate of the

displacement that the design earthquake will produce on the building is determined. The

extent of damage experienced by the building at this target displacement is considered

representative of the damage experienced by the building when subjected to design level

ground shaking.

The most frequently used terms in pushover analysis are discussed below

Capacity-curve

It is the plot of the lateral force V on a structure, against the lateral deflection d, of the roof of

the structure. This is often referred to as the „push over‟ curve. Performance point and

location of hinges in various stages can be obtained from pushover curves as shown in Figure

2.23. The range AB is elastic range, B to IO is the range of immediate occupancy IO to LS is

the range of life safety and LS to CP is the range of collapse prevention.

Figure 2.23: Different stages of plastic hinge

`40

Demand

It is a representation of the earthquake ground motion or shaking that the building is

subjected to. In nonlinear static analysis procedures, demand is represented by an estimation

of the displacements or deformations that the structure is expected to undergo.

This is in contrast to conventional, linear elastic analysis procedures in which demand is

represented by prescribed lateral forces applied to the structure.

Elastic response spectrum

It is the 5% damped response spectrum for each seismic hazard level of interest, representing

the maximum response of the structure, in terms of spectral acceleration Sa, at any time

during an earthquake as a function of period of vibration T.

Performance level

A limiting damage state or condition described by the physical damage within the building,

the threat to life safety of the building‟s occupants due to the damage, and the post-

earthquake serviceability of the building. A building performance level is that combination of

a structural performance level and a nonstructural performance level

Displacement-based analysis

It refers to analysis procedures, such as the nonlinear static analysis procedures, whose basis

lies in estimating the realistic, and generally inelastic, lateral displacements or deformations

expected due to actual earthquake ground motion. Component forces are then determined

based on the deformations.

Yield (effective yield) point

The point along the capacity spectrum where the ultimate capacity is reached and the initial

linear elastic force-deformation relationship ends and effective stiffness begins to decrease.

Building Performance levels

A performance level describes a limiting damage condition which may be considered

satisfactory for a given building and a given ground motion. The limiting condition is

described by the physical damage within the building, the threat to life safety of the

building‟s occupants created by the damage, and the post-earthquake serviceability of the

building.

Immediate occupancy

The earthquake damage state in which only very limited structural damage has occurred. The

basic vertical and lateral forces resisting systems of the building retain nearly all of their pre-

earthquake characteristics and capacities. The risk of life threatening injury from structural

failure is negligible.

`41

Life safety

The post-earthquake damage state in which significant damage to the structure may have

occurred but in which some margin against either total or partial collapse remains. Major

structural components have not become dislodged and fallen, threatening life safety either

within or outside the building. While injuries during the earthquake may occur, the risk of life

threatening injury from structural damage is very low. It should be expected that extensive

structural repairs will likely be necessary prior to reoccupation of the building, although the

damage may not always be economically repairable.

Collapse prevention level

This building performance level consists of the structural collapse prevention level with no

consideration of nonstructural vulnerabilities, except that parapets and heavy appendages are

rehabilitated.

Primary elements

Refer to those structural components or elements that provide a significant portion of the

structure‟s lateral force resisting stiffness and strength at the performance point.

These are the elements that are needed to resist lateral loads after several cycles of inelastic

response to the earthquake ground motion.

Secondary elements

Refer to those structural components or elements that are not, or are not needed to be,

primary elements of the lateral load resisting system. However, secondary elements may be

needed to support vertical gravity loads and may resist some lateral loads. [15, 26, 28]

2.8.4 Non-Linear Dynamic Analysis (Time History Analysis)

Nonlinear Dynamic analysis can be done by direct integration of the equations of motion by

step by step procedures. Direct integration provides the most powerful and informative

analysis for any given earthquake motion. A time dependent forcing function (earthquake

accelerogram) is applied and the corresponding response–history of the structure during the

earthquake is computed. That is, the moment and force diagrams at each of a series of

prescribed intervals throughout the applied motion can be found.

`42

The mathematical model used for elastic analysis shall be extended to include the strength of

structural elements and their post-elastic behavior.

As a minimum, a bilinear force –deformation relationship should be used at the element level.

In reinforced concrete and masonry buildings, the elastic stiffness of a bilinear force-

deformation relation should correspond to that of cracked sections. In ductile elements,

expected to exhibit post-yield excursions during the response, the elastic stiffness of a

bilinear relation should be the secant stiffness to the yield-point. Trainer force–deformation

relationships, which take into account pre-crack and post-crack stiffness‟s, are allowed.

Zero post-yield stiffness may be assumed. If strength degradation is expected, e.g. for

masonry walls or other brittle elements, it has to be included in the force– deformation

relationships of those elements. Unless otherwise specified, element properties should be

based on mean values of the properties of the materials. For new structures, mean values of

material properties may be estimated from the corresponding characteristic values.

Gravity loads in accordance with ES-EN-1998-1-1, 2015 section 3.2.4 shall be applied to

appropriate elements of the mathematical model.

Axial forces due to gravity loads should be taken into account when determining force –

deformation relations for structural elements. Bending moments in vertical structural

elements due to gravity loads may be neglected, unless they substantially influence the global

structural behavior. [15, 26, 28]

The seismic action shall be applied in both positive and negative directions and the maximum

seismic effects as a result of this shall be used. Computer programs have been written for

both linear elastic and non-linear inelastic material behavior using step-by-step integration

procedures. One such program is SAP2000 in which three–dimensional non-linear analyses

can be carried out taking as input the three orthogonal accelerogram components from a

given earthquake, and applying them simultaneously to the structure.

2.9 Application of TMDs on Seismic Gap Requirement

In large metropolitan areas several adjacent buildings, which are constructed before and still

under constructions without sufficient seismic separations distance. If moderate and major

earthquakes takes place it induced severe pounding damage in some cases, the additional

`43

forces generated by the impact interactions have led to the minor local damages. In other

cases, it presented total collapse of structure. There for to resist this pounding damage

supplemental mitigation measures like Conventional method or Innovative method are used.

The traditional / conventional /approach to seismic hazard mitigation is to design structures

with sufficient strength capacity / stiffening of structures / and ability to deform in a ductile

manner. It is generally accomplished through the selection of an appropriate structural

configuration and the carefully detailing of structural members, such as beams and columns,

and the connections between them. Increasing the stiffness of the building reduces its period

and decreases its displacement. An important thing to recall is that it is not possible to

increase stiffness without increasing strength. To increase the stiffness of the buildings, there

are different techniques such as:-Installation of RC shear wall, Installation of steel bracing

and jacketing of structural elements. [5, 10]

Today, one of the main challenges in structural engineering is to develop innovative /

mechanical / design concepts to better protect civil engineering structures, including their

material contents and human occupants from these hazards. The basic approach underlying

more advanced techniques for earthquake resistance is not to strengthen the building, but to

reduce the earthquake-generated forces acting up on structures. By de-coupling the structure

from seismic ground motion it is possible to reduce the earthquake induced forces in it. This

can be done in two ways:-Base isolation and Energy dissipating devices (Structural Controls).

Newer concepts of structural control, including both passive and active control systems have

been growing in acceptance and may preclude the necessity of allowing for inelastic

deformations in the structural system. A passive control system does not require an external

power for operation and utilizes the motion of the structure to develop the control forces.

Another advantage of such devices is their low maintenance requirements. Examples of

passive systems are visco-elastic dampers, tuned liquid column dampers, tuned liquid and

tuned mass dampers, metallic yield dampers and friction dampers. An active control system

requires external power for operation and has the ability to adapt to different loading

conditions and to control different vibration modes of the structures. Active Tuned Mass

Dampers (ATMD), active tendon systems and actuators/ controllers are examples of active

systems. Active and passive control systems may be combined to form hybrid systems;

operating both systems together enhances the robustness of the passive system and reduces

`44

the energy requirements of the active system. There are two main approaches for the

implementation of hybrid systems: the Hybrid Mass Damper (HMD) and the hybrid seismic

isolation system. A compromise between passive and active control systems has been

developed in the form of semi-active control systems, which are based on semi-active

devices. A semi active control device has properties that can be adjusted in real time but

cannot inject energy into the controlled system. Frequently, such devices are referred to as

controllable passive dampers. Of all these control devices passive control systems in the form

of TMD‟s, base isolation and frictional dampers have been implemented in many building

across the world. [5, 10, 37]

In this paper also going to be discussed only one passive structural control method: - reduce

the dynamic response of the structure by tuned mass dampers (TMDs).

2.9.1 Tuned Mass Dampers on Dynamic Properties of Structures

A need for new and better means of designing new structures and retrofitting existing ones

from the damaging effects of severe dynamic loadings specially pounding effect has

motivated civil engineer to embark on rather unfamiliar but innovative concepts of structural

control. The use of lightweight, high strength materials, and advanced construction

techniques have led to increasingly flexible and lightly damped structures, which is prone to

cause human discomfort, structural damage and even failure in extreme dynamic loadings.

The means to suppress undesirable levels of vibration have then become essential and

integral aspect of structural system in tall buildings.

New generation high rise building is equipped with artificial damping device for vibration

control through energy dissipation. The various vibration control methods include passive,

active, semi-active, hybrid. Various factors that affect the selection of a particular type of

vibration control device are efficiency, compactness and weight, capital cost, operating cost,

maintenance requirements and safety. [36, 40]

A tuned mass damper (TMD) is a device consisting of a mass, a spring, and a damper that is

attached to a structure in order to reduce the dynamic response of the structure. The

frequency of the damper is tuned to a particular structural frequency so that when that

frequency is excited, the damper will resonate out of phase with the structural motion. Energy

is dissipated by the damper inertia force acting on the structure.

`45

The passive mass dampers have endured longer and rigorous validation, and have reached the

stage of full implementation. In fact passive mass dampers, e.g., tuned mass dampers

(TMDs), tuned liquid dampers (TLDs) and tuned liquid column dampers (TLCDs) have been

implemented with some degree of success.

Many scientists have conducted numerous works in related with Tuned Mass Dampers in the

history. Earlier it was having mechanical applications. Later its effectiveness in the

construction field was found out and numerous experiments were conducted on it. First,

experiments with single Tuned Mass Damper were done and slowly scientists identified the

dominancy of Multiple Tuned Mass Dampers (MTMD) over single TMD. Then various and

varieties of experiments were done with MTMDs to find out various characteristics and

behavior of these MTMDs by constructing the building models. It was little bit difficult to

construct the models of various structures. [39]

Current trends in construction industry demands taller and lighter structures. These structures

are flexible and constructed as light as possible (as seismic load acts on a structure is a

function of self-weight), which have low value of damping, makes them vulnerable to

unwanted vibration. This vibration creates problem to serviceability requirement of the

structure and also reduce structural integrity with possibilities of failure. Now-a-days several

techniques are available to reduce wind and earthquake induced structural vibration. Shear

wall is an already existing technique and commonly used. Passive tuned mass damper (TMD)

is widely used to control structural vibration under wind load but its effectiveness to reduce

earthquake induced vibration is an emerging technique. Tuned Mass Damper (TMD) is the

most effective for controlling the structural responses for harmonic and wind excitations.

The first implementation of water tank to resist natures force like wind and earthquake was

the 304m high Sydney center point tower, Austria. This building is considered as one of the

safest buildings in the world. Water tank in the top serves to act as tuned mass dampers to

resist wind and earthquake induced motion. There are two buildings in the United States

equipped with TMDs; one is the John Hancock Tower in Boston and the other is the Citicorp

Centre in New York City. The Citicorp Centre building is 279 m high with a damping ratio of

1% along each axis. The Citicorp TMD is located on the sixty-third floor in the crown of the

structure. The damper is expected to reduce the building sway amplitude by about 50%. Two

dampers were added to the 60-storey John Hancock Tower in Boston to reduce the response

`46

to wind loading. The dampers are placed at opposite ends of the fifty-eighth story, 67 m

apart, and move to counteract sway as well as twisting due to the shape of the building. [36,

40]

Later a Finite Element based software SAP 2000 was introduced in to the field. In the various

structural models can be created very easily and various alterations also can be made easily.

So the problem of constructing the model was solved and analysis of various structural

models can be done by using the software SAP 2000.This paper focuses on the effective

method of construction of high rise structures for resisting pounding damage and to reduce

the response of these structures by using response spectrum analysis. Here a suitable TMD

system (water tank as a lumped mass without sloshing effect) is modeled on top story of

buildings.

Figure 2.24: Structure with Tuned Mass Damper, Act Tower, India

2.9.2 Principle of Tuned Mass Damper

With recent development in computer-based structural design and high-strength materials,

structures are becoming more flexible and lightly damped. When subjected to dynamic loads

such as traffic load, wind, earthquake, wave, vibration blasting for long duration may be

easily induced in this type of structures. To increase comfort of working people, function of

installed machineries and equipment‟s, and reliability of structures, damping capacity of

structures in the elastic region should be increased. If we have a fixed reaction wall adjacent

to the top of a structure as shown in Figure 2.25 (a) viscous or frictional damper can be

`47

installed effectively to increase damping capacity of the structure. However, this is usually

impossible because flexible structures are very tall and no fixed point is available. TMD is a

vibration system with mass mT, spring kT and viscous cT usually installed on the top of

structures as shown in Figure 2.25 (b). When the structure starts to vibrate, TMD is excited

by the movement of the structure. Hence, kinetic energy of the structure goes into TMD

system to be absorbed by the viscous damper of TMD. To achieve the most efficient energy

absorbing capacity of TMD, natural period of TMD by itself is tuned with the natural period

of the structure by itself, from which the system is called "Tuned Mass Damper". The viscous

damper of TMD shall also be adjusted to the optimum value to maximize the absorbed

energy. TMD is a mechanically simple system which does not need any external energy

supply for operation. Because of easy maintenance and high reliability, TMD is used in many

flexible and lightly-damped towers and buildings. [40]

(a) (b)

Figure: 2.25 (a) Viscous damper with fixed reaction wall (b) Tuned mass damper on a

structure

One TMD is effective in reducing dynamic response of only a single vibration mode of the

structure. Although a structure has many vibration modes in reality, basic properties of TMD

can be clearly discussed using a simplified 2-DOF model consisting of the main structure and

the TMD system (Figure 2.26). [40]

`48

Figure: 2.26 Two-DOF modeling of main structure and tuned mass damper system

2.9.3 Modeling of Water tank as TMD

Water tanks are integral part of all buildings and they impart large dead load on the structure.

This additional mass can be utilized as TMD to absorb the extra energy imparted on the

structure during earthquakes. In the present work the water tank was placed at the center of

mass of structure or on symmetrical corner of the building to avoid undesirable twisting or

torsional effects due to unsymmetrical lay out of water tank location. The tank plan

dimension is calculated based on mass ratio and the amount of water required on that

particular building (based on tank capacity calculation). It is placed above a top floor about

two meter height columns. The beam-column supports for the tank were concrete sections

and the walls and roof were also modeled as concrete sections.

The dimensions of the tank were taken to suit the condition that the tank have full of water

condition coincided with the optimized parametric values, i) mass ratio i.e., ratio of mass of

water tank (water + tank + beams and column) to the mass of the structure and ii) frequency

ratio i.e., ratio of frequency of water tank (water + tank + beams and columns) to the

frequency of the structure coincided.

`49

Mass of the secondary system (TMDs) varies from 1-10% of the structural mass. As a

particular earthquake contains a large number of frequency content now a day‟s multiple

tuned mass dampers has been used to control earthquake induced motion of high rise

structure where more than one TMD is tuned to different unfavorable structural frequency.

[39]

The effectiveness of the TMD depends on the proper tuning of the characteristics of TMD to

that of the structure. In the present work the mass ratio µ (Mass of TMD to Mass of the

Structure) and frequency ratio α (Frequency of TMD to the frequency of the structure) are

optimized and the objective function is to reduce the peak structural response subjected to

seismic excitation. For optimization the structure was modeled as lumped single degree of

freedom spring-mass system as shown in Figure 2.27(a), with mass Equal to that of the unit

modal mass and stiffness adjusted to the natural frequency of the structure. The TMD was

attached to the idealized system as spring mass system as shown in Figure 2.27(b). [37]

The TMD is evolved based on the concept of creating ancillary structure on the primary

structure such that the frequency of the ancillary structure is as close as possible to the

primary structure so as to achieve the fundamental frequency of the primary structure. Hence,

6% mass ratio is used in this study. The frequency of ancillary structure, which is a SDOF

system, shall have the same frequency as that of the primary structure. Method of dynamic

analysis used is: Response spectrum analysis. The elevated RCC water tank on the top of the

building story which have hinged supports are found to be effective TMD mechanism for

reduction of the time period and story drift.

Figure: 2.27 Optimization of idealized structure with and without TMD

For in this thesis work, the adjacent building has been modeled separately and the required

separation distance between the adjacent buildings has been calculated according to different

building codes by taking the displacement at the top floor of both buildings in the possible

facing direction.

`50

CHAPTER THREE

3. BUILDING DESCRIPTIONS, MODELING AND ANALYSIS

3.1 Building Descriptions

The codes define an “irregular structure” as one that has a certain geometric shape or in

which stiffness and/or mass discontinuities exist. While a “regular structure” is one in which

there is a minimum coupling between the lateral displacements and the torsional rotations for

the mode shapes associated with the lower frequencies of the system.

In our cases four independent buildings are selected for analysis purpose, all data

specifications and detail drawing information are based on the document provided by

different consulting architects and engineering plc. But in this thesis work the selected sample

buildings are assumed to be constructed adjacently in different arrangements to show the

seismic gap requirements in different seismic zone and site condition of the country. The un-

deformed frame structures of the buildings are given below:

The first building B-1 (Figure 3.1) shows that Floor plan and 3D view of the building. The

building is proposed to construct in Finote Selam city. The main purpose of the building is

given access for West Gojam zone administration office; in this study for modeling of the

building some logical assumptions are made on the number of story and frame structure cross

sections, because it is very difficult to get regular buildings in the real world.

(a)

`51

(b)

Figure 3.1: Regular sample building (B-1), G+7: (a) Floor plan of the building (b) 3D view of

the building

The second building B-2 (Figure 3.2) shows that Floor plan and 3D view of the building. The

building is proposed to construct in Dessie city. The main purpose of the building is given

access for South Wollo zone administration office, in this study for modeling of the building

some logical assumptions are made on the number of story and frame structure cross sections,

because in a real case a pair of buildings haven almost the same floor plan are constructs and

connects by staircase but in our case take only a single building which satisfies the criteria of

irregularity in plan.

`52

(b)

Figure 3.2: Irregular (in plan) sample building (B-2) G+9: (a) Floor plan of the building (b)

3D view of the building

The third building B-3 (Figure 3.3) shows that elevation and 3D view of the building. The

building is proposed to construct in Dessie city. The main purpose of the building is given

access for South Wollo zone high court & justice office, in this study for modeling of the

building some logical assumptions are made on the number and height of story and also on

frame structure cross sections, because for satisfying the criteria of buildings irregularity in

elevation.

(a)

`53

(b)

Figure 3.3: Irregular (in elevation) sample building (B-3) G+7: (a) elevation of the building

(b) 3D view of the building

The fourth building B-4 (Figure 3.4) shows that Floor plan and 3D view of the building. In

this study for modeling of the building some logical assumptions are made on the number of

story, shape of floor plan and frame structure cross sections, because it is very difficult to get

regular buildings in the real world.

(a)

`54

(b)

Figure 3.4: Regular sample building (B-4) G+6: (a) Floor plan of the building (b) 3D view of

the building

As mentioned above there are three cases and will be discussed one by one as follow:-

3.1.1 Case study-I (Both adjacent buildings have regular arrangement)

In the previous section the sample buildings usage, location, floor plan, elevation, 3D view

and assumptions made on the sample buildings are shown and discussed briefly. This all

descriptions about the sample buildings are bases for modeling and analysis of buildings. In

the first case it will be discussed the modeling and analysis of the two adjacent buildings

which are both of the sample buildings have regular configurations. For the purpose of

discussion, as shown in Figure 3.5 below, the two buildings are designated by the symbols

1&4 shows the 3D modeling of adjacent buildings for pounding.

These adjacent buildings are assumed to be modeled as aligned floor-to-floor; But in this case

taller building pound to the shorter one. Building 1 have a total of 8 stories (G+7) with the

total building height of 24m and story height of 3m (in average), Similarly building 4 have a

total of 7 stories (G+6) with the total building height of 21m and story height of 3m (in

average).. In this very common case the slabs of the diaphragms of one structure hit the slabs

of the other diaphragms of structure. In this phenomenon especially the taller building shears

`55

off and loaded on shorter one. The aim of this paper is to calculate the appropriate separation

distance between sample adjacent buildings in different seismic zone and site conditions.

(a) (b)

Figure: 3.5 (a) Building 1, a total of 8 stories (G+7) (b) Building 4, a total of 7 stories (G+6)

Figure: 3.6 Top view of adjacent building modeling for pounding (case-1)

Modeling shown on (Figure 3.6) is the top floor plan of sample buildings. As shown from the

above figure the model consists of only the frame system. The floor slab is designed and

calculated manually according to Ethiopian Building Code Standard two (EBCS-2-1995) and

the loads that transfer to the beam also according to the code. In addition to this the load of

the non-structural element of the building has been calculated and loaded as uniformly

distributed load.

`56

3.1.2 Case study –II (Both adjacent buildings have irregular arrangement)

In the second case it will be discussed the modeling and analysis of the two adjacent

buildings which are both of the sample buildings have irregular configurations. For the

purpose of discussion, as shown in Figure 3.7 below, the two buildings are designated by the

symbols 2&3 shows the 3D modeling of adjacent buildings for pounding.

These adjacent buildings are assumed to be modeled as not aligned floor-to-floor; that means,

the heights of the story levels of the two structures are not equal in this case also taller

building pound to the shorter one. Building 2 have a total of 10 stories (G+9) with the total

building height of 30m and story height of 3m (in average). Similarly building 3 have a total

of 8 stories (G+7) with the total building height of 25m and story height of 3m (in average),

But it have soft story at third floor level of the building. In this very common case the slabs of

the diaphragms of one structure hit the columns of the other structure at a point with in the

deformable height. This phenomenon is especially intense at the contact point of the upper

story level of the short stiffer structure with the corresponding column of the tall building for

adjacent buildings with different total building height. The aim of this paper is to calculate

the appropriate separation distance between sample adjacent buildings in different seismic

zone and site conditions.

(a) (b)

Figure: 3.7 (a) Building 2, a total of 10 stories (G+9) (b) Building 3, a total of 8 stories (G+7)

`57

Figure: 3.8 Top view of adjacent building modeling for pounding (case-2)

Building floor plan, elevation and 3D view are discussed earlier. Modeling shown on (Figure

3.8) is the top floor plan of sample buildings. As shown from the above figure the model

consists of only the frame system. The floor slab is designed and calculated manually

according to Ethiopian Building Code Standard two (EBCS-2-1995) and the loads that

transfer to the beam also according to the code. In addition to this the load of the non-

structural element of the building has been calculated and loaded as uniformly distributed

load.

3.1.3 Case study-III (Adjacent buildings have different arrangement)

In the third case it will be discussed the modeling and analysis of the two adjacent buildings

which one of the sample building have regular configuration and the other sample building

have irregular configurations. For the purpose of discussion, as shown in Figure 3.9 below,

the two buildings are designated by the symbols 1& 3 shows the 3D modeling of adjacent

buildings for pounding.

These adjacent buildings are assumed to be modeled as aligned floor-to-floor; that means, the

heights of the story levels of the two structures are almost equal, Building 1 have a total of 8

stories (G+7) with the total building height of 24m and story height of 3m (in average).

Similarly building 3 have a total of 8 stories (G+7) with the total building height of 25m and

story height of 3m (in average), But it have soft story at the third floor level of the building.

In this very common case the slabs of the diaphragms of one structure hit the slabs of the

`58

other diaphragms of structure. The aim of this paper is to calculate the appropriate separation

distance between sample adjacent buildings in different seismic zone and site conditions.

(a) (b)

Figure: 3.9 (a) Building 1, a total of 8 stories (G+7) (b) Building 3, a total of 8 stories (G+7)

Figure: 3.10 Top view of adjacent building modeling for pounding (case-3)

Building floor plan, elevation and 3D view are discussed earlier. Modeling shown on (Figure

3.10) is the top floor plan of sample buildings. As shown from the above figure the model

consists of only the frame system. The floor slab is designed and calculated manually

according to Ethiopian Building Code Standard two (EBCS-2-1995) and the loads that

transfer to the beam also according to the code. In addition to this the load of the non-

structural element of the building has been calculated and loaded as uniformly distributed

load.

`59

3.2 Building Modeling and Analysis

3.2.1 General

Now day‟s engineers use computer software for design and analysis of structures. There are

lots of engineering soft wares that can be used in civil engineering field. Using computers

engineers can design and analyze the structure with less error and time particularly for

complex structures. In order to evaluate the Seismic gap between buildings; it is batter to

model the structure in 3D with a help of engineering- software. Under the previous topic,

there are three different cases to be studied with detailed discussion presented under the sub-

topic of the unit. All cases are modeled and analyzed using SAP2000 software.

The finite element analysis software SAP2000 is utilized to create 3D model and condition of

run all analyses. The software is able to predict the geometric nonlinear behavior of space

frames under static or dynamic loadings, taking into account both geometric nonlinearity and

material inelasticity. The software accepts static loads (either forces or displacements) as well

as dynamic (accelerations) actions and has the ability to perform Eigen values, Ritz-vector,

nonlinear static pushover and nonlinear dynamic analyses.

3.2.2 Assigning loads

After having modeled the structural components, all possible load cases are assigned. These

are as follows:

Gravity loads

Gravity loads on the structure include the self-weight of beams, columns, slabs, walls

and other permanent members. The self-weight of beams and columns (frame

members) is automatically considered by the program itself.

i. The slabs (area sections) and wall loads have been calculated manually.

ii. Live loads have been assigned as uniform area loads on the slab elements as

per (EBCS-2 1995)

iii. Live load on roof 2 KN/m2

iv. Live load on all other floors depends on the functional use of the floor

`60

Earthquake lateral loads

Earthquake loads can be considered in the design in two different ways. These are;-

i. By calculating the design lateral loads at different floor levels corresponding

to fundamental time period and are applied to the model.

ii. Program calculation method by SAP2000.

In our case the second alternative approach is used. Since the software is capable to consider

earthquake load automatically by giving the necessary data‟s.

3.2.3 Response-Spectrum Function

The seismic assessment or design of buildings is generally based on extreme or maximum

dynamic response quantities and doesn‟t necessarily require a complete time history

response. Analysis models used for modal spectral analysis are linear elastic models based on

effective stiffness properties and on assumed equivalent viscous damping ratios. With these

requirements response spectrum analysis can be performed for building systems which are

expected to perform essentially in linear elastic range. Hence, it is typically sufficient and

more convenient to determine maximum modal response quantities by means of response

spectra, once the dynamic response characteristics in the form of natural periods of vibration

and mode shapes have been determined. Therefore, response spectrum analysis is concerned

with procedures to compute the peak response of a structure during an earthquake directly

from design spectrum without the need for response history analysis of the structure. [10]

The ground motion hazard (response spectrum analysis) could be carried out by using

different analysis programs. SAP2000 structural analysis software is the most appropriate

program that has been utilized by several engineers. The analysis might be performed by

defining the building location, the user can input any user defined response spectrum file

from seismic hazard map of the country. The ground motion hazard and the soil site

classification are the main parts of the user input data.

Modal analysis which is a pre-requisite to response spectrum analysis is the analysis

procedure employed in this study. As provided in various research outputs, the goal of modal

analysis in structural mechanics is to determine the natural mode shapes and natural periods

of the building or structure during vibration.

`61

In this case there are four independent buildings under three cases, which are constructed in

different seismic zone and site condition of Ethiopia. Buildings are modeled as elastic

moment resisting frames with a 5% damping ratio using SAP2000 with Response-spectrum

analysis.

Response-spectrum analysis is a statistical type of analysis for the determination of the likely

response of a structure to seismic loading. Response- spectrum analysis seeks the likely maxi

mum response to these equations rather than the full time history. The earthquake ground

acceleration in each direction is given as a digitized response- spectrum curve of pseudo-

spectral acceleration Response versus period of the structure. [10]

The response-spectrum curve for a given direction is defined by digitized points of pseudo-

spectral acceleration response versus period of the structure. The shape of the curve is given

by specifying the name of a function. All values for the abscissa and ordinate of this function

must be zero or positive.

For the Seismic pounding effect between adjacent buildings, response spectrum analysis is

carried out using the spectra for all soil conditions and seismic zone of Ethiopia as per ES-

EN-1998-1-1, 2015.

The spectral acceleration coefficient (Sa/g) values are calculated as follows.

Assumed soil type A in Seismic zone-1,

{

Assumed soil type B in Seismic zone-2,

{

`62

Assumed soil type C in Seismic zone-3,

{

Assumed soil type D in Seismic zone-4,

{

Assumed soil type E in Seismic zone-5,

{

When T is Time period in second

Scale factors used for each seismic zone during modeling of buildings in SAP 2000 are based

on ES-EN-1998-1-1, 2015.

Table 3.1 Ground acceleration coefficient of seismic zones in Ethiopia

The values of the spectral acceleration coefficient (Sa/g) for the time periods of 0.00 to 4.00

seconds calculated as per the above equations and plotting of spectral acceleration coefficient

(Sa/g) vs. Period is as shown below;

Seismic Zone Ground acceleration coefficient (αo = ag/g)

1 0.04

2 0.07

3 0.1

4 0.15

5 0.2

`63

Figure 3.11: Defining response spectrum function (Sa/g) vs. Period for different soil types

3.2.4 Response Spectrum Analysis in SAP 2000

The procedures are mentioned as the following:-

a) Defining quake loads under the load type „quake‟ and naming it appropriately.

b) Defining response spectrum function as per required building standards. The values

of Sa/g vs. T can be linked in the program in the form of a data file or use Euro code-

8, 2004 on SAP 2000 program which are the same as new ES-EN-1998-1-1; 2015

except the national annex. Therefore, in this case use our code bed rock acceleration

value for each seismic zone as an input data.

c) Modifying the quake analysis case with the appropriate analysis case type, applied

loads and scale factors.

d) Running the analysis

3.2.5 Time History Analysis in SAP 2000

Time History analysis has been carried out using the Imperial Valley Earthquake record of

May 18, 1940 also known as the Elcentro earthquake for obtaining the various floor

responses.

The peak ground acceleration is 0.319g. Newmark‟s direct integration method has been

adopted and the mass and stiffness proportional coefficients have been calculated taking into

account the frequency of the structure in two consecutive modes in the same direction.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Spec

tra

l A

ccel

era

tio

n C

oef

fici

ent

(Sa

/g)

Period in sec...

RS-1A

RS-2B

RS-3C

RS-4D

RS-5E

`64

Elcentro earthquake

Peak ground acceleration (PGA)-0.319g

Figure 3.12: Time history plot of Elcentro earthquake.

The step by step procedure is as follows

a) Defining a time history function by adding a function from file. In our case, the

Elcentro earthquake record of 1940 has been linked to the program.

b) Defining a separate analysis case under the load type „quake‟ with the appropriate

analysis case type i.e. linear direct integration time history.

c) Applying earthquake acceleration values from the defined time history function.

d) Specifying the damping coefficients by calculating the mass and stiffness proportional

coefficients as per the equations mentioned on the literatures or inputting the

frequency or time periods of two consecutive modes of the structure in the same

direction whereby the program itself calculates the required damping coefficients.

e) Specifying a direct integration method in the program. In our case, we have adopted

Newmark‟s direct integration method.

f) Running the analysis.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30 35

Acc

eler

ati

on

(g

)

Period (sec)

Acr (m/s2)

`65

Figure 3.13: Defining time history function (Elcentro, 1940) in SAP2000

For all cases, the required material properties like mass, weight density, modulus of elasticity,

shear modulus and design values of the material used can be modified as per requirements.

Beams and column members have been defined as „frame elements‟ with the appropriate

dimensions and reinforcement.

The slabs are designed manually and the loads which transfer to the beam are calculated

according to EBCS-2 1995. This calculated load is loaded as uniformly distributed force for

the solid slab. During the modeling process the necessary data and the whole dimension of

the structural element (frame, slab) and non- structural element (wall) has taken as per built

or as per design.

`66

CHAPTER FOUR

4. RESULTS AND DISCUSSIONS

4.1 General

Different seismic zones may have different site conditions; in most cases a simple copy of a

building design from one seismic zone and site condition to another with or without

modification of important frame cross section elements is a common practice in major towns

of our country. In this thesis work we have to compute a seismic separation of a given sample

adjacent buildings that are constructed in different area of the country, but it assumed to be a

simple copy of building designs (i.e. constructed without modification of frame cross sections

and other important parameters). Even if the frame structure cross sections are assumed to be

unchanged for all seismic zones and soil conditions the seismic separation is a critical

parameter in seismic zones, therefore it should be modified. Based on the assumption all

important frame cross section parameters are assumed to be constant and the seismic

separation is calculated for all case studies in each seismic zone and the corresponding soil

conditions.

SAP2000 was used to compute the response of buildings (B1-B4) which are mentioned above

with response spectrum analysis. Results from Response Spectrum analysis are observed for

the period and Displacements of the joints to determine the seismic pounding gap (separation

distance) between adjacent structures of two models.

The two results (period and displacement of the joint) have been taken to identify that

whether the two buildings oscillate (vibrate) in-phase or out- of-phase and, if the building are

vibrate out-of –phase the separation distance between the adjacent buildings could be

calculated. The separation distance between the adjacent buildings could be calculated

according to different Building Code Standards.

The detail discussion will be present below for each case.

`67

4.2 Result and Discussion of all Cases

The maximum top story displacement was noted for each sample buildings and used for gap

calculation. Table 4.1 shown below is the maximum story displacement of various sample

buildings for the respective seismic zone and soil condition data.

Table 4.1: Maximum story displacement of buildings in different seismic zones

Zone vs. Soil condition

Displacement(cm)

S. Buildings Story 1-A 2-B 3-C 4-D 5-E

0 0.06 0.14 0.22 0.5 0.44

1 0.42 1.06 1.72 3.99 3.5

2 0.9 2.34 3.82 8.9 7.74

3 1.39 3.6 5.9 13.63 11.98

B-1 4 1.86 4.83 7.93 18.62 16.1

5 2.29 5.95 9.77 22.92 19.82

6 2.7 6.99 11.47 26.89 23.25

7 3.05 7.88 12.92 30.27 26.17

8 3.36 8.61 14.06 32.87 28.48

0 0.09 0.24 0.38 0.87 0.78

1 0.66 1.71 2.78 6.37 5.7

2 1.42 3.69 6.02 13.91 12.23

3 2.13 5.5 8.98 20.86 18.17

B-2 4 2.88 7.4 12.09 28.2 24.45

5 3.57 9.1 14.84 34.6 29.97

6 4.33 11.03 17.98 41.89 36.29

7 5.03 12.73 20.73 48.19 41.76

8 5.85 14.78 24.05 55.95 48.45

9 6.5 16.35 26.57 61.77 53.48

10 7.04 17.74 28.78 66.64 57.88

0 0.08 0.21 0.34 0.76 0.69

1 0.69 1.79 2.9 6.57 5.94

2 1.45 3.78 6.17 14.1 12.58

3 2.2 5.77 9.46 21.86 19.23

B-3 4 3.52 9.2 15.09 35.26 30.62

5 4.19 10.97 18 42.19 36.54

6 4.76 12.49 20.5 48.09 41.61

7 5.24 13.75 22.57 52.85 45.81

8 5.76 14.94 24.42 56.78 49.54

0 0.05 0.13 0.21 0.46 0.42

1 0.4 1.02 1.66 3.75 3.4

2 0.85 2.23 3.63 8.31 7.42

B-4 3 1.37 3.58 5.87 13.57 11.92

4 1.83 4.79 7.87 18.35 15.98

5 2.24 5.87 9.64 22.56 19.57

6 2.64 6.92 11.35 26.56 23.04

7 2.95 7.66 12.54 29.18 25.42

`68

4.2.1 Case study -I (B1 vs. B4):-Both adjacent buildings have regular

arrangement

It has been discussed the modeling and analysis of the two adjacent buildings which both of

the sample buildings have regular configurations in the previous topic; but here it will be

interpret and discussed the result of this analysis. See modal information versus period of the

adjacent buildings to show the buildings are vibrating in-phase or out-of –phase which causes

pounding.

Table 4.2: Response Spectrum Modal Information of building 1 & 4

Based on the results in table 4.2 and chart above the two buildings (building 1 and 4 see

figure 4.1 and 4.2 below) have different period as it is observed from the first twelve mode of

the SAP output; as a result of this, it is possible to say that, the vibrations of these adjacent

buildings are out-of-phase. This indicates that the calculation of the separation distance

between the two buildings is necessary and it should be modified for the respective seismic

zones and site conditions.

Step type

Mode B-1 B-4

1 2.68 2.09

2 2.47 1.9

3 2.26 1.84

4 2.08 1.48

5 1.85 1.43

6 1.6 1.05

7 1.32 0.97

8 1.16 0.71

9 0.85 0.6

10 0.82 0.56

11 0.68 0.48

12 0.65 0.45

Period(se)

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20

Per

iod

(se

c..)

Mode

RS modal information of buildings

B-1

B-4

`69

Figure 4.1: The deformed shape of building 1(after analysis of 10th

mode)

Figure 4.2: The deformed shape of building 4(after analysis of 10th

mode)

The joint displacement at the top floor of building 1 and building 4 in all seismic zones and

soil conditions (in the possible facing side) are given below in the table (table 4.3 and table

4.4) respectively.

Table 4.3: Joint Displacements of building 1

Top floor displacement (de )at possible impact point (cm)

Joints

Zone Soil type Output case a b c d e

1 A EQX-Spectrum 3.36 3.19 3.14 3.19 3.36

2 B EQX-Spectrum 8.61 8.20 8.08 8.20 8.61

3 C EQX-Spectrum 14.06 13.42 13.23 13.42 14.06

4 D EQX-Spectrum 32.88 31.39 30.97 31.39 32.88

5 E EQX-Spectrum 28.48 27.17 26.80 27.17 28.48

`70

Table 4.4: Joint Displacements of building 4

Using these results, the minimum required separation distance (d) between the two buildings

calculated and summarized in the table 4.5 below (according to ES-EN-1998-1-1: 2015/EC-8,

FEMA-1997 and UBC-1997).

Table 4.5: Minimum required separation distance

These adjacent buildings are assumed to be aligned floor-to-floor; But in this case taller

building pound to the shorter one. Therefore the result that calculated by ES-EN: 2015/EC-8

is factored with reduction factor of 0.7. As a result the minimum required separation distance

between the adjacent buildings (building 1 and building 4) based on ES-EN: 2015/EC-8 and

UBC-1997 which are widely used codes in Ethiopia, take the greater of the two.

Top floor displacement (de ) at possible impact point (cm)

Joints

Zone Soil type Output case a b c d e f g

1 A EQX-Spectrum 2.22 2.58 2.82 2.95 2.82 2.58 2.22

2 B EQX-Spectrum 5.79 6.71 7.34 7.66 7.34 6.71 5.79

3 C EQX-Spectrum 9.49 10.98 12.01 12.54 12.01 10.98 9.49

4 D EQX-Spectrum 22.19 25.57 27.96 29.18 27.96 25.57 22.19

5 E EQX-Spectrum 19.26 22.26 24.36 25.42 24.36 22.26 19.26

Max. top floor displacement

in cm

Min- Seismic Separation (d) in (cm) according to

Zone Soil condition ds1 ds4 ES-EN:2015/EC-8

FEMA-1997 UBC-1997

1 A 3.36 2.95 3.13 96 4.47

2 B 8.61 7.66 8.07 96 11.52

3 C 14.06 12.54 13.19 96 18.84

4 D 32.88 29.18 30.77 96 43.96

5 E 28.48 25.42 26.72 96 38.17

`71

Table 4.6: Recommended minimum seismic separation (d min)

Location of adjacent building construction

Recommended min. seismic separation (d min) in cm Seismic zone Site condition

1 A 4.47

2 B 11.52

3 C 18.84

4 D 43.96

5 E 38.17

4.2.2 Case study-II (B2 vs.B3): Both adjacent buildings have irregular

arrangement

It has been discussed the modeling and analysis of the two adjacent buildings which both of

the sample buildings have irregular configurations in the previous topic; but here it will be

interpret and discussed the result of this analysis. See modal information versus period of the

adjacent buildings to show the buildings are vibrating in-phase or out-of –phase which causes

pounding.

Table 4.7: Response Spectrum Modal Information of building 2 & 3

Based on the results in table 4.7 and chart above the two buildings (building 2 and 3 see

figure 4.3 and 4.4 below) have almost different periods as it is observed from the first twelve

mode of the SAP output; as a result of this, it is possible to say that, the vibrations of these

adjacent buildings are out-of-phase. This indicates that the calculation of the separation

distance between the two buildings is necessary and it should be modified for the respective

seismic zones and site conditions.

Step type

Mode B-2 B-3

1 3.21 2.74

2 2.96 2.29

3 2.83 1.94

4 1.47 1.47

5 1.22 1.36

6 1.14 0.94

7 1.13 0.69

8 1.11 0.59

9 0.93 0.52

10 0.89 0.47

11 0.82 0.45

12 0.74 0.4

Period(se)

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20

Per

iod

(se

c..)

Mode

RS modal information of buildings

B-2

B-3

`72

Figure 4.3: The deformed shape of building 2(after analysis of 9th

mode)

Figure 4.4: The deformed shape of building 3(after analysis of 9th

mode)

The joint displacement at the top floor of building 2 and building 3 in all seismic zones and

soil conditions (in the possible facing side) are given below in the table (table 4.8 and table

4.9) respectively.

`73

Table 4.8: Joint Displacements of building 2

Table 4.9: Joint Displacements of building 3

Using these results, the minimum required separation distance (d) between the two buildings

calculated and summarized in the table 4.10 below (according to ES-EN-1998-1-1, 2015/EC-

8, FEMA-1997 and UBC-1997).

Table 4.10: Minimum required separation distance

Top floor displacement (de )at possible impact point (cm)

Joints

Zone Soil type Output case a a‟ b b‟ c c‟

1 A EQX-Spectrum 6.96 6.68 6.87 6.75 6.87 7.09

2 B EQX-Spectrum 17.26 16.48 17.10 16.75 17.18 17.74

3 C EQX-Spectrum 27.95 26.65 27.71 27.13 27.88 28.78

4 D EQX-Spectrum 64.76 61.68 64.24 62.85 64.67 66.64

5 E EQX-Spectrum 56.09 53.43 55.54 54.45 56.05 57.88

Top floor displacement (de )at possible impact point (cm)

Joints

Zone Soil type Output case a b c d

1 A EQX-Spectrum 4.87 5.76 5.76 4.87

2 B EQX-Spectrum 12.69 14.94 14.94 12.69

3 C EQX-Spectrum 20.79 24.42 24.42 20.79

4 D EQX-Spectrum 48.62 56.78 56.78 48.62

5 E EQX-Spectrum 42.19 49.54 49.54 42.19

Max. top floor displacement

in cm

Min- Seismic Separation (d) in (cm) according to

Zone Soil condition ds2 ds3 ES-EN:2015/EC-8

FEMA-1997 UBC-1997

1 A 7.09 5.76 9.13 120 9.13

2 B 17.74 14.94 23.20 120 23.20

3 C 28.78 24.42 37.74 120 37.74

4 D 66.64 56.78 87.55 120 87.55

5 E 57.88 49.54 76.19 120 76.19

`74

These adjacent buildings are assumed to be not aligned floor-to-floor; in this case also taller

building pound to the shorter one. But the result that calculated by ES-EN: 2015/EC-8 is not

factored with reduction factor of 0.7. As a result the minimum required separation distance

between the adjacent buildings (building 2 and building 3) based on ES-EN: 2015/EC-8 and

UBC-1997 are equal.

Table 4.11: Recommended minimum seismic separation (d min)

Location of adjacent building construction

Recommended min. seismic separation (d min) in cm Seismic zone Site condition

1 A 9.13

2 B 23.20

3 C 37.74

4 D 87.55

5 E 76.19

4.2.3 Case study -III (B1 vs. B3):-Adjacent buildings have different

arrangement

It has been discussed the modeling and analysis of the two adjacent buildings which one of

the sample building have regular configuration and the other sample building have irregular

configurations; in the previous topic; but here it will be interpret and discussed the result of

this analysis. See modal information versus period of the adjacent buildings to show the

buildings are vibrating in-phase or out-of –phase which causes pounding.

Table 4.12: Response Spectrum Modal Information of building 1 & 3

Step type

Mode B-1 B-3

1 2.68 2.74

2 2.47 2.29

3 2.26 1.94

4 2.08 1.47

5 1.85 1.36

6 1.6 0.94

7 1.32 0.69

8 1.16 0.59

9 0.85 0.52

10 0.82 0.47

11 0.68 0.45

12 0.65 0.4

Period(se)

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20

Per

iod

(se

c..)

Mode

RS modal information of buildings

B-1

B-3

`75

Based on the results in table 4.12 and chart above the two buildings (building 1 and 3 see

figure 4.5 and 4.6 below) have different period as it is observed from the first twelve mode of

the SAP output; as a result of this, it is possible to say that, the vibrations of these adjacent

buildings are out-of-phase. This indicates that the calculation of the separation distance

between the two buildings is necessary and it should be modified for the respective seismic

zones and site conditions.

Figure 4.5: The deformed shape of building 1(after analysis of 8th

mode)

Figure 4.6: The deformed shape of building 3(after analysis of 8th

mode)

The joint displacement at the top floor of building 1 and building 3 in all seismic zones and

soil conditions (in the possible facing side) are given below in the table (table 4.13 and table

4.14) respectively.

`76

Table 4.13: Joint Displacements of building 1

Table 4.14: Joint Displacements of building 3

Using these results, the minimum required separation distance (d) between the two buildings

calculated and summarized in the table 4.15 below (according to ES-EN-1998-1-1, 2015/EC-

8, FEMA-1997 and UBC-1997).

Table 4.15: Minimum required separation distance

Top floor displacement (de )at possible impact point (cm)

Joints

Zone Soil type Output case a b c d e

1 A EQX-Spectrum 3.36 3.19 3.14 3.19 3.36

2 B EQX-Spectrum 8.61 8.20 8.08 8.20 8.61

3 C EQX-Spectrum 14.06 13.42 13.23 13.42 14.06

4 D EQX-Spectrum 32.88 31.39 30.97 31.39 32.88

5 E EQX-Spectrum 28.48 27.17 26.80 27.17 28.48

Top floor displacement (de )at possible impact point (cm)

Joints

Zone Soil type Output case a b c d

1 A EQX-Spectrum 4.87 5.76 5.76 4.87

2 B EQX-Spectrum 12.69 14.94 14.94 12.69

3 C EQX-Spectrum 20.79 24.42 24.42 20.79

4 D EQX-Spectrum 48.62 56.78 56.78 48.62

5 E EQX-Spectrum 42.19 49.54 49.54 42.19

Max. top floor displacement

in cm

Min- Seismic Separation (d) in (cm) according to

Zone Soil condition ds1 ds3 ES-EN:2015/EC-8

FEMA-1997 UBC-1997

1 A 3.36 5.76 4.67 100 6.67

2 B 8.61 14.94 12.07 100 17.24

3 C 14.06 24.42 19.73 100 28.18

4 D 32.88 56.78 45.93 100 65.61

5 E 28.48 49.54 40.00 100 57.14

`77

These adjacent buildings are assumed to be aligned floor-to-floor; therefore the result that

calculated by ES-EN: 2015/EC-8 is factored with reduction factor of 0.7. As a result the

minimum required separation distance between the adjacent buildings (building 1 and

building 3) based on ES-EN: 2015/EC-8 and UBC-1997 which are widely used codes in

Ethiopia, take the greater of the two.

Table 4.16: Recommended minimum seismic separation (d min)

Location of adjacent building construction

Recommended min. seismic separation (d min) in cm Seismic zone Site condition

1 A 6.67

2 B 17.24

3 C 28.18

4 D 65.61

5 E 57.14

4.2.4 Time History Analysis

The top floor displacement of sample buildings at the possible impact points are given

in the table below

Table 4.17: Peak top floor displacements of sample buildings (d e)

Peak top floor displacement (d e ) at the possible impact point

(cm)

Joints

S. buildings Output case a b c d e f g

B-1 Elcentro EQ 28.92 27.24 26.98 27.24 28.92 - -

B-2 Elcentro EQ 62.80 61.94 65.02 - - - -

B-3 Elcentro EQ 45.23 53.76 53.76 45.23 - - -

B-4 Elcentro EQ 20.04 22.95 25.11 26.39 25.11 22.95 20.04

Using these results, the minimum required separation distance (d) between the two buildings

calculated and summarized in the table 4.18 below (according to ES-EN-1998-1-1, 2015/EC-

8, and UBC-1997).

`78

Table 4.18: Minimum required separation distance of all three cases

Min- Seismic Separation (d) in (cm)

according to

Cases

Max. top floor displacement in

cm

ES-EN:2015/EC-8

UBC-1997

Case-I(B1 vs. B4)

ds1 ds4

27.41

39.15 28.92 26.39

Case-II(B2 vs. B3)

ds2 ds3

84.37

84.37 65.02 53.76

Case-III(B1 vs. B3)

ds1 ds3

42.73

61.05 28.92 53.76

As a result the minimum required separation distance between the adjacent buildings based

on ES-EN: 2015/EC-8 and UBC-1997 which are widely used codes in Ethiopia, take the

greater of the two.

4.2.5 Summary on the discussion of all three cases

It is nearly impossible to construct a building which has similar seismic behavior to another

building. Structural poundings happen because of swaying of adjacent buildings with

different mode shapes and periods under seismic loads which are not separated from each

other properly.

The above three cases shows that if similar adjacent buildings are constructed in different

seismic zones and soil conditions of the country the required seismic separation for each

seismic zones and soil conditions is completely different. Even if the gap between adjacent

buildings is sufficient in one seismic zone and site condition, it may not be sufficient for the

other to withstand seismic pounding.

Some researcher observation shows that pounding cases of regular adjacent buildings are

better than that of irregular buildings, if pounding building is regular, an impact surface can

be formed between two adjacent buildings. During earthquakes, structures mass and rigidity

affect seismic behavior due to these researchers conclude that Poundings may occur because

of structural irregularities.

Different modal behavior of two adjacent buildings causes different pounding types. This

makes pounding more complex. Various types of impact (pounding) seen in the recent

earthquakes phenomenon this can be categorized in to five that describe in different

literatures as: Pounding of the structure on the column of an adjacent building, Pounding of a

`79

heavier building on a lighter one, Pounding of a shorter building on a taller one, pounding of

two adjacent buildings with non-coaxial mass centers, Adjacent buildings with equal height

and with aligned floor levels. In this thesis work additional impact type is describes and

discussed due to adjacent building configurations behavior. Table 4.19 and Figure 4.7 below

shows that the effect of adjacent building configurations on the requirement of minimum

seismic gap for all three cases.

Table 4.19: Recommended minimum seismic gap provide for the three cases

seismic gap (cm)

seismic zone vs. soil type case-1 case-2 case-3

1-A 4.47 9.13 6.67

2-B 11.52 23.2 17.24

3-C 18.84 37.74 28.18

4-D 43.96 87.55 65.61

5-E 38.17 76.19 57.14

Figure 4.7: The effect of adjacent building configurations on seismic gap value

From the above discussions, the seismic gap requirement of adjacent buildings are increased

with in the consecutive increments of seismic zones, except that the seismic gap requirement

of the three cases in seismic zone 4 is greater than that of the gap required by seismic zone 5,

This is due to the soil/site condition characteristics.

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

Sei

smic

gap

(cm

)

Zone & site condition

case-1

case-2

case-3

`80

4.3 Method of Mitigation using passive structural controls

4.3.1 Reduce the Dynamic Response of Adjacent Buildings for all Cases

There are different techniques to reduce the dynamic response of new and existing adjacent

buildings as discussed previously. But in this case, only water tank as tuned mass damper has

been providing to reduce the dynamic response of the adjacent buildings in order to reduce

the lateral displacement or deformation of the adjacent buildings consequently, to reduce the

minimum required separation distance between them and use the land effectively. A

significant change in lateral displacement has been observed due to introducing of the water

tank at the top floor of buildings in all cases. The detail discussion will present here after.

4.3.2 Water Tank dimension and Capacity Calculation

Water tanks are integral part of all buildings and they impart large dead load on the structure.

Here a suitable TMD system (water tank as a lumped mass without sloshing effect of water)

is modeled on top story of buildings. The tank plan dimension is calculated based on mass

ratio and the amount of water required on that particular building and it is placed above a top

floor about two meter height columns. The beam-column supports for the tank were concrete

sections and the walls and roof of tank also modeled as concrete sections.

Calculations for Single TMD - One Water Tank: - This is done by considering the main

structure top story slab, beam and column cross sections and dimensions in addition to this

water requirement of sample buildings are basic criteria for calculation of water tank size.

Stiffness Calculation: - for Building one (B1)

Moment of inertia of top story columns = (1/12) x 0.2 x 0.33 = 4.5 x 10

-4 m

4

Stiffness of each column = K = 12EI/L3 = 12 x 29000 x 10

3 x 4.5 x 10

-4 / 3

3 = 5800 kN/m

Total stiffness = Number of columns x stiffness of each column = 570075.6 kN/m

Use mass ratio of 3% for each water tank

Stiffness of columns of water tank = (3/100) x 570075.6 = 17102.268 kN/m

Stiffness of each column of water tank = (1/4) x 17102.268 = 4275.567 kN/m

`81

Calculation of Depth of Column for Water Tank

Let D1, B1 be the depth and width of water tank column respectively.

Stiffness of each column of water tank = 12EI1/L3 = 4275.567 kN/m, use E=29Gpa

I1 = 3.32 x 10-4

m4

Assuming width of column of water tank (B1) = 0.2 m

I1 = B1 D1 3/12 = 3.32 x 10

-4 m

4 D1= 0.27m, use D1= 0.3m

Size of each column of water tank = 0.2 m x 0.3 m

Calculation of Total Weight of top story

Total weight of top story = weight of (slab + beams + columns + outer walls + inner walls

+imposed loads) = 21038.8847 KN

Weight of both water tank with columns = (6/100) x 21038.8847 = 1262.333KN

Weight of columns of both water tank = 8 x 0.2 x 0.3 x 2 x 24 = 23.04 KN

Weight of water tank for both tanks = 1262.3 – 23.04= 1239.3 KN

Consumption of water in liters per day for a single person as per IS 1172 – 1993 = 150litres

On other water supply design manuals 80 - 100 liter per day is recommended. Therefore, in

this particular study assume 80 liters of water per day is enough for a single person.

Estimated no. of persons occupying in the building = 450nos.

Total amount of water consumed per day = 450X 80

= 36000 liters

Use 36 m3

Maximum volume of water tank required for consumption purpose = 36m3

The water tank is assumed to be located with aligned to the building corner frame structure

plan dimensions. Therefore,

L*B*H=36m3

Building Corner frame plan dimension is L=6.5m and B=5m Substitute this value

We get the single tank depth of H=1.1m for each tank use H=0.55m

The assumed water tank is located with symmetrical corner place of the building with

dimension of (5.0m*6.5m *0.55m).

`82

Load calculation of the TMD (for each water tank)

Normal weight concrete is used for slab; wall and frame elements on the other way lime

mortar is use as floor finish.

Top Slab of the tank

t slab=10cm & t mortar=2cm

DL = 0.1 x 5 x 6.5 x 24 + 0.02 x 5 x 6.5* 19 = 91kN

LL = 0.5 x 5 x 6.5 = 16.25kN (assumed as flat roof LL=0.5kN/m2)

Bottom Slab of the tank

t slab=12cm & t mortar=3cm

DL = 0.12 x 5 x 6.5 x 24 + 0.03 x 5 x 6.5 x 19 =113.1kN

LL = Weight of water in the tank = 5x 6.5 x 0.55 x 10 = 178.75 kN

Side Walls of the tank

t wall=15cm

DL = 0.15 x 5 x 0.55 x 2 x 24 + 0.15 x 6.5 x 0.55 x 2 x 24 = 45.54 kN

Total DL = 91 + 113.1 + 45.54 = 249.64 kN

Total LL = 16.25+ 178.75 = 195kN

Total load = 444.64kN

The assumed water tank beam sections used are 0.2m*0.3m

The concentrated DL and LL are converted in to distributed load and the transfer of load on

each beam is based on EBCS-2, 1995. Beam and column loads are automatically considered

by the software program use SAP2000. Other sample buildings water tank dimension and

load calculations are follow the same procedure.

Table 4.20: Determination of Water Tank Size for all sample buildings

S. buildings

No. of Tank

Tank size(m)

W*L*H

Tank-Column

Cross-section(m)

Tank-beam

Cross-section(m)

Total load (kN/m2)

DL LL

B1 2 5.0*6.5*0.55 0.2*0.3 0.2*0.3 7.68 6.0

B2 2 4.7*4.8*0.5 Φ0.25(circular) 0.2*0.25 7.74 5.5

B3 2 3.4*6.3*0.7 0.2*0.3 0.2*0.3 8.51 7.5

B4 2 4.5*5.0*0.8 0.2*0.3 0.2*0.3 8.66 8.5

`83

4.3.3 Reduce the Dynamic Response for case-I

As we have been discussed under the section 3.1.1 (Building description of case I) and 4.2.1

(Result and Discussion of case-I), that the adjacent buildings has been designated as building

1 and 4 (see figure 3.5). Under the result and discussion section, we have determined the

required minimum seismic separation distance between adjacent buildings in different

seismic zone and site conditions without providing any energy-dissipating devices on the

sample buildings by using different building code standards.

As we have been discussed previously, water tank has been introduced to reduce the

earthquake-generated forces acting up on structures this also led to reduce the lateral

displacement of structures. The TMDs (water tank) layout has been provided in symmetrical

manner (as much as possible) to avoid undesirable twisting or torsional effects due to

unsymmetrical location of water tank which cases non uniformly distribution of lateral load

on the structure. The provided water tank dimension and other important parameters are

discussed above in Table 4.20. Moreover the water tank has been placed symmetrically on

the top floor corner of the buildings. See figure 4.8 and 4.9.

Figure: 4.8 3D view deformed shape of building 1 (after analysis of 8th

mode) with TMDs

`84

Figure: 4.9 3D view deformed shape of building 4 (after analysis of 10th

mode) with TMDs

Hence, after providing of TMDs (water tank) on the building the lateral displacements and

the time periods of the two adjacent buildings are reduces from the previous result. For

building 1 the lateral displacement is reduced by average of 79.20% and the time period is

also reduced by 13%. On the other hand the lateral displacements and time periods of

building 4 is reduced by average of 70.12% and 16% respectively. Due to the reduction of

lateral displacement in both buildings, the minimum required separation distance between the

adjacent buildings are reduces from the previous seismic gap value. The joint displacement at

the possible impact point and the calculated minimum required separation distance are listed

in the tables 4.21.

Table 4.21: Minimum required separation distance of buildings with TMDs

Max. top floor displacement

in cm

Min- Seismic Separation (d) in (cm) according

to

Zone Soil condition ds1 ds4 ES-EN:2015/EC-8

FEMA-1997 UBC-1997

1 A 0.75 0.89 0.81 96 1.16

2 B 1.75 2.30 2.02 96 2.89

3 C 2.81 3.78 3.30 96 4.71

4 D 6.45 8.75 7.61 96 10.87

5 E 5.69 7.63 6.66 96 9.52

`85

The result that calculated by FEMA-1997 is depend on taller buildings height due to this it

does not have any difference from the previous result. The minimum required separation

distance between the adjacent buildings (building 1 and building 4) based on ES-EN:

2015/EC-8 and UBC-1997 which is widely used codes in Ethiopia; take the greater of the

two. Table 4.22 below shows that the minimum recommended seismic gap between adjacent

buildings with and without TMDs and its percent of reduction.

Table 4.22: Recommended minimum seismic gap of buildings with and without TMDs

Location of adjacent building construction Min. seismic gap (d min) in cm

% of reduction Seismic zone Site condition Without TMDs With TMDs

1 A 4.47 1.16 74.05

2 B 11.52 2.89 74.91

3 C 18.84 4.71 75.00

4 D 43.96 10.87 75.27

5 E 38.17 9.52 75.06

4.3.4 Reduce the Dynamic Response for case-II

As we have been discussed under the section 3.1.2 (Building description of case II) and 4.2.2

(Result and Discussion of case-II), that the adjacent buildings has been designated as building

2 and 3 (see figure 3.7). Under the result and discussion section, we have determined the

required minimum seismic separation distance between adjacent buildings in different

seismic zone and site conditions without providing any energy-dissipating devices on the

sample buildings by using different building code standards.

As we have been discussed previously, water tank has been introduced to reduce the

earthquake-generated forces acting up on structures this also led to reduce the lateral

displacement of structures. The TMDs (water tank) layout has been provided in symmetrical

manner (as much as possible) to avoid undesirable twisting or torsional effects due to

unsymmetrical location of water tank which cases non uniformly distribution of lateral load

on the structure. The provided water tank dimension and other important parameters are

discussed above in Table 4.20. Moreover the water tank has been placed symmetrically on

the top floor corner of the buildings. See figure 4.10 and 4.11.

`86

Figure: 4.10 3D view deformed shape of building 2 (after analysis of 9th

mode) with TMDs

Figure: 4.11 3D view deformed shape of building 3 (after analysis of 7th

mode) with TMDs

Hence, after providing of TMDs (water tank) on the building the lateral displacements and

the time periods of the two adjacent buildings are reduces from the previous result. For

building 2 the lateral displacement is reduced by average of 75.30% and the time period is

also reduced by 22%. On the other hand the lateral displacements and time periods of

building 3 is reduced by average of 94.2% and 13% respectively. Due to the reduction of

lateral displacement in both buildings, the minimum required separation distance between the

`87

adjacent buildings are reduces from the previous seismic gap value. The joint displacement at

the possible impact point and the calculated minimum required separation distance are listed

in the tables 4.23.

Table 4.23: Minimum required separation distance of buildings with TMDs

The result that calculated by FEMA-1997 is depend on taller buildings height due to this it

does not have any difference from the previous result. In this case the minimum required

separation distance between the adjacent buildings (building 2 and building 3) based on ES-

EN: 2015/EC-8 and UBC-1997 are equal. Table 4.24 below shows that the minimum

recommended seismic gap between adjacent buildings with and without TMDs and its

percent of reduction.

Table 4.24: Recommended minimum seismic gap of buildings with and without TMDs

Location of adjacent building construction Min. seismic gap (d min) in cm

% of reduction Seismic zone Site condition Without TMDs With TMDs

1 A 9.13 2.00 78.10

2 B 23.20 4.64 80.00

3 C 37.74 7.36 80.50

4 D 87.55 16.77 80.84

5 E 76.19 14.57 80.88

Max. top floor displacement

in cm

Min- Seismic Separation (d) in (cm) according

to

Zone Soil condition ds2 ds3 ES-EN:2015/EC-8

FEMA-1997 UBC-1997

1 A 1.97 0.36 2.00 120 2.00

2 B 4.55 0.89 4.64 120 4.64

3 C 7.22 1.42 7.36 120 7.36

4 D 16.45 3.28 16.77 120 16.77

5 E 14.28 2.89 14.57 120 14.57

`88

4.3.5 Reduce the Dynamic Response for case-III

As we have been discussed under the section 3.1.3 (Building description of case III) and 4.2.3

(Result and Discussion of case-III), that the adjacent buildings has been designated as

building 1 and 3 (see figure 3.9). Under the result and discussion section, we have determined

the required minimum seismic separation distance between adjacent buildings in different

seismic zone and site conditions without providing any energy-dissipating devices on the

sample buildings by using different building code standards.

As we have been discussed previously, water tank has been introduced to reduce the

earthquake-generated forces acting up on structures this also led to reduce the lateral

displacement of structures. The TMDs (water tank) layout has been provided in symmetrical

manner (as much as possible) to avoid undesirable twisting or torsional effects due to

unsymmetrical location of water tank which cases non uniformly distribution of lateral load

on the structure. The provided water tank dimension and other important parameters are

discussed above in Table 4.20. Moreover the water tank has been placed symmetrically on

the top floor corner of the buildings. See previous figure 4.8 and 4.11.

Hence, after providing of TMDs (water tank) on the building the lateral displacements and

the time periods of the two adjacent buildings are reduces from the previous result. For

building 1 the lateral displacement is reduced by average of 79.20% and the time period is

also reduced by 13%. On the other hand the lateral displacements and time periods of

building 3 is reduced by average of 94.2% and 13% respectively. Due to the reduction of

lateral displacement in both buildings, the minimum required separation distance between the

adjacent buildings are reduces from the previous seismic gap value. The joint displacement at

the possible impact point and the calculated minimum required separation distance are listed

in the tables 4.25.

`89

Table 4.25: Minimum required separation distance of buildings with TMDs

The result that calculated by FEMA-1997 is depend on taller buildings height due to this it

does not have any difference from the previous result. The minimum required separation

distance between the adjacent buildings (building 1 and building 3) based on ES-EN:

2015/EC-8 and UBC-1997 which are widely used codes in Ethiopia; take the greater of the

two. Table 4.26 below shows that the minimum recommended seismic gap between adjacent

buildings with and without TMDs and its percent of reduction.

Table 4.26: Recommended minimum seismic gap of buildings with and without TMDs

Location of adjacent building construction Min. seismic gap (d min) in cm

% of reduction Seismic zone Site condition Without TMDs With TMDs

1 A 6.67 0.83 87.56

2 B 17.24 1.96 88.63

3 C 28.18 3.15 88.82

4 D 65.61 7.24 88.97

5 E 57.14 6.38 88.83

Max. top floor displacement

in cm

Min- Seismic Separation (d) in (cm) according

to

Zone Soil condition ds1 ds3 ES-EN:2015/EC-8

FEMA-1997 UBC-1997

1 A 0.75 0.36 0.58 100 0.83

2 B 1.75 0.89 1.37 100 1.96

3 C 2.81 1.42 2.20 100 3.15

4 D 6.45 3.28 5.07 100 7.24

5 E 5.69 2.89 4.47 100 6.38

`90

Generally the required seismic gaps of all sample adjacent buildings with and without TMDs

are described in (Figure 4.12) below.

Figure: 4.12 Sample adjacent buildings seismic gap requirement with and without TMDs in

different seismic zones and site conditions

From the previous discussions, the seismic gap requirement of adjacent buildings with and

without TMDs are increased with in the consecutive increments of seismic zones, but in both

cases the seismic gap requirement of seismic zone 4 is greater than that of the gap required in

seismic zone 5, This is due to the soil/site condition characteristics.

0

10

20

30

40

50

60

70

80

90

100

case-1/wo

TMDs

case-1/w

TMDs

case-2/wo

TMDs

case-2/w

TMDs

case-3/wo

TMDs

case-3/w

TMDs

Sei

smic

gap

(C

m)

cases

1-A

2-B

3-C

4-D

5-E

`91

CHAPTER FIVE

5. CONCLUSION AND RECOMMENDATIONS

5.1 Conclusions

Seismic gap requirement of selected sample adjacent buildings that was assumed to be

constructed in different area of Ethiopia with and without TMDs was assessed. Linear

dynamic analysis was performed on four different reinforced concrete structures using

SAP2000 software.

Based on the analysis results of the three cases the following conclusion can be drawn:

The seismic gap requirement of all selected sample adjacent buildings in different seismic

zones and soil conditions of the country is absolutely different to withstand seismic pounding

effect. In other word, Even if the gap between adjacent buildings is sufficient in one seismic

zone and site condition, it may not be sufficient for the other to withstand pounding damage.

Based on the analysis results, case-1 (Both adjacent buildings have regular configurations)

require less seismic gap between them in all seismic zones and site conditions as compared to

case-2 and case-3.From this we conclude that the pounding damage effect between regular

adjacent building configurations are much better than that of irregular adjacent building

configurations. Generally the configuration of adjacent building is the main criteria to

determine the ponding damage effect.

From the result the seismic gap requirement of case-1 with TMDs (water tank) is reduced by

an average of 74.86% from that of buildings without TMDs. Similarly the seismic gap

requirement of case-2 and case-3 with TMDs (water tank) is reduced by an average of

80.06% and 88.56% from that of buildings without TMDs in all seismic zones and site

conditions respectively. From this we conclude that introducing of water tank as TMDs is

used to minimize the required seismic gap between adjacent buildings and use the land

effectively by reducing the dynamic response of buildings under earthquake excitation. This

innovation method of building mitigation shows the significant change in lateral

displacement and the period of the buildings.

`92

5.2 Recommendations

The primary thing for preventing of pounding is provide the minimum required seismic gap

between adjacent buildings for the respective seismic zones and site conditions. Providing a

sufficient seismic gap has been the commonly accepted strategy adopted by building codes

throughout the world.

Designers can play a great role in minimizing pounding effect through eliminating designing

of highly irregular adjacent buildings arrangement. City municipality will have a great role in

solving such kind of problem, by permitting an appropriate design for construction and

supervising the construction process whether constructed or not according to the approved

design.

During design and construction of new buildings which are neighboring to existing high rise

buildings the behavior of existing adjacent buildings under earth quake excitation should be

known and take account of to determine the seismic gap requirement between them.

Providing of water tank on appropriate position of the buildings can be used to reduce the

seismic gap requirement of adjacent buildings in addition to supplying water for the peoples

live on that particular building. It is also recommended using of passive structural controls

instead of shear wall and steel bracing is much better to withstand the lateral force or

earthquake load for minimizing the pounding risk.

For Further Study

Finally the author suggests that such a study shall further be conducted by considering the

limitations made in this study. Therefore, one can extend this study to include the following

points:-

i. This study consider only four buildings in three configurations, thus one can extend

this by considering several number of building configurations, in order to get a

detailed and applicable result.

ii. This study conducted only the application of Water tank as TMDs (without sloshing

effect), thus one can extend this to the application of Water tank as TLDs (with

sloshing effect).

iii. This study may also extend to the future by using experimental simulation (setup)

including the methodology and feasibility of TMDs connection to the primary

structure, in order to get a realistic conclusion.

`93

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`94

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`97

APPENDIX

Spectral acceleration coefficients (Sa/g) vs. Period

THE RSA CURVE FOR DIFFERENT SITE CONDITIONS

period (T) in sec,

Soil type

A B C D E

zone-1 zone-2 zone-3 zone-4 zone-5

Spectral Acceleration Coefficient

T in sec Sa/g Sa/g Sa/g Sa/g Sa/g

0 1.000 1.200 1.150 1.35 1.4

0.02 1.200 1.440 1.323 1.5525 1.68

0.04 1.400 1.680 1.495 1.755 1.96

0.06 1.600 1.920 1.668 1.9575 2.24

0.08 1.800 2.160 1.840 2.16 2.52

0.1 2.000 2.400 2.013 2.3625 2.8

0.12 2.200 2.640 2.185 2.565 3.08

0.14 2.400 2.880 2.358 2.7675 3.36

0.16 2.500 3.000 2.530 2.97 3.5

0.18 2.500 3.000 2.703 3.1725 3.5

0.2 2.500 3.000 2.875 3.375 3.5

0.22 2.500 3.000 2.875 3.375 3.5

0.24 2.500 3.000 2.875 3.375 3.5

0.26 2.500 3.000 2.875 3.375 3.5

0.28 2.500 3.000 2.875 3.375 3.5

0.3 2.500 3.000 2.875 3.375 3.5

0.32 2.500 3.000 2.875 3.375 3.5

0.34 2.500 3.000 2.875 3.375 3.5

0.36 2.500 3.000 2.875 3.375 3.5

0.38 2.500 3.000 2.875 3.375 3.5

0.4 2.500 3.000 2.875 3.375 3.5

0.42 2.381 3.000 2.875 3.375 3.5

0.44 2.273 3.000 2.875 3.375 3.5

0.46 2.174 3.000 2.875 3.375 3.5

0.48 2.083 3.000 2.875 3.375 3.5

0.5 2.000 3.000 2.875 3.375 3.5

0.52 1.923 2.885 2.875 3.375 3.365

0.54 1.852 2.778 2.875 3.375 3.241

0.56 1.786 2.679 2.875 3.375 3.125

`98

0.58 1.724 2.586 2.875 3.375 3.017

0.6 1.667 2.500 2.875 3.375 2.917

0.62 1.613 2.419 2.782 3.375 2.823

0.64 1.563 2.344 2.695 3.375 2.734

0.66 1.515 2.273 2.614 3.375 2.652

0.68 1.471 2.206 2.537 3.375 2.574

0.7 1.429 2.143 2.464 3.375 2.500

0.72 1.389 2.083 2.396 3.375 2.431

0.74 1.351 2.027 2.331 3.375 2.365

0.76 1.316 1.974 2.270 3.375 2.303

0.78 1.282 1.923 2.212 3.375 2.244

0.8 1.250 1.875 2.156 3.375 2.188

0.82 1.220 1.829 2.104 3.293 2.134

0.84 1.190 1.786 2.054 3.214 2.083

0.86 1.163 1.744 2.006 3.140 2.035

0.88 1.136 1.705 1.960 3.068 1.989

0.9 1.111 1.667 1.917 3.000 1.944

0.92 1.087 1.630 1.875 2.935 1.902

0.94 1.064 1.596 1.835 2.872 1.862

0.96 1.042 1.563 1.797 2.813 1.823

0.98 1.020 1.531 1.760 2.755 1.786

1 1.000 1.500 1.725 2.700 1.750

1.02 0.980 1.471 1.691 2.647 1.716

1.04 0.962 1.442 1.659 2.596 1.683

1.06 0.943 1.415 1.627 2.547 1.651

1.08 0.926 1.389 1.597 2.500 1.620

1.1 0.909 1.364 1.568 2.455 1.591

1.12 0.893 1.339 1.540 2.411 1.563

1.14 0.877 1.316 1.513 2.368 1.535

1.16 0.862 1.293 1.487 2.328 1.509

1.18 0.847 1.271 1.462 2.288 1.483

1.2 0.833 1.250 1.438 2.250 1.458

1.22 0.820 1.230 1.414 2.213 1.434

1.24 0.806 1.210 1.391 2.177 1.411

1.26 0.794 1.190 1.369 2.143 1.389

1.28 0.781 1.172 1.348 2.109 1.367

1.3 0.769 1.154 1.327 2.077 1.346

1.32 0.758 1.136 1.307 2.045 1.326

1.34 0.746 1.119 1.287 2.015 1.306

1.36 0.735 1.103 1.268 1.985 1.287

1.38 0.725 1.087 1.250 1.957 1.268

1.4 0.714 1.071 1.232 1.929 1.250

1.42 0.704 1.056 1.215 1.901 1.232

1.44 0.694 1.042 1.198 1.875 1.215

1.46 0.685 1.027 1.182 1.849 1.199

`99

1.48 0.676 1.014 1.166 1.824 1.182

1.5 0.667 1.000 1.150 1.800 1.167

1.52 0.658 0.987 1.135 1.776 1.151

1.54 0.649 0.974 1.120 1.753 1.136

1.56 0.641 0.962 1.106 1.731 1.122

1.58 0.633 0.949 1.092 1.709 1.108

1.6 0.625 0.937 1.078 1.688 1.094

1.62 0.617 0.926 1.065 1.667 1.080

1.64 0.610 0.915 1.052 1.646 1.067

1.66 0.602 0.904 1.039 1.627 1.054

1.68 0.595 0.893 1.027 1.607 1.042

1.7 0.588 0.882 1.015 1.588 1.029

1.72 0.581 0.872 1.003 1.570 1.017

1.74 0.575 0.862 0.991 1.552 1.006

1.76 0.568 0.852 0.980 1.534 0.994

1.78 0.562 0.843 0.969 1.517 0.983

1.8 0.556 0.833 0.958 1.500 0.972

1.82 0.549 0.824 0.948 1.484 0.962

1.84 0.543 0.815 0.937 1.467 0.951

1.86 0.538 0.806 0.927 1.452 0.941

1.88 0.532 0.798 0.918 1.436 0.931

1.9 0.526 0.789 0.908 1.421 0.921

1.92 0.521 0.781 0.898 1.406 0.911

1.94 0.515 0.773 0.889 1.392 0.902

1.96 0.510 0.765 0.880 1.378 0.893

1.98 0.505 0.758 0.871 1.364 0.884

2 0.500 0.750 0.862 1.350 0.875

2.02 0.490 0.735 0.846 1.323 0.858

2.04 0.481 0.721 0.829 1.298 0.841

2.06 0.471 0.707 0.813 1.273 0.825

2.08 0.462 0.693 0.797 1.248 0.809

2.1 0.454 0.680 0.782 1.224 0.794

2.12 0.445 0.667 0.768 1.201 0.779

2.14 0.437 0.655 0.753 1.179 0.764

2.16 0.429 0.643 0.739 1.157 0.750

2.18 0.421 0.631 0.726 1.136 0.736

2.2 0.413 0.620 0.713 1.116 0.723

2.22 0.406 0.609 0.700 1.096 0.710

2.24 0.399 0.598 0.688 1.076 0.698

2.26 0.392 0.587 0.675 1.057 0.685

2.28 0.385 0.577 0.664 1.039 0.673

2.3 0.378 0.567 0.652 1.021 0.662

2.32 0.372 0.557 0.641 1.003 0.650

2.34 0.365 0.548 0.630 0.986 0.639

2.36 0.359 0.539 0.619 0.970 0.628

`100

2.38 0.353 0.530 0.609 0.953 0.618

2.4 0.347 0.521 0.599 0.937 0.608

2.42 0.342 0.512 0.589 0.922 0.598

2.44 0.336 0.504 0.579 0.907 0.588

2.46 0.330 0.496 0.570 0.892 0.578

2.48 0.325 0.488 0.561 0.878 0.569

2.5 0.320 0.480 0.552 0.864 0.560

2.52 0.315 0.472 0.543 0.850 0.551

2.54 0.310 0.465 0.535 0.837 0.543

2.56 0.305 0.458 0.526 0.824 0.534

2.58 0.300 0.451 0.518 0.811 0.526

2.6 0.296 0.444 0.510 0.799 0.518

2.62 0.291 0.437 0.503 0.787 0.510

2.64 0.287 0.430 0.495 0.775 0.502

2.66 0.283 0.424 0.488 0.763 0.495

2.68 0.278 0.418 0.480 0.752 0.487

2.7 0.274 0.412 0.473 0.741 0.480

2.72 0.270 0.405 0.466 0.730 0.473

2.74 0.266 0.400 0.460 0.719 0.466

2.76 0.263 0.394 0.453 0.709 0.459

2.78 0.259 0.388 0.446 0.699 0.453

2.8 0.255 0.383 0.440 0.689 0.446

2.82 0.251 0.377 0.434 0.679 0.440

2.84 0.248 0.372 0.428 0.670 0.434

2.86 0.245 0.367 0.422 0.660 0.428

2.88 0.241 0.362 0.416 0.651 0.422

2.9 0.238 0.357 0.410 0.642 0.416

2.92 0.235 0.352 0.405 0.633 0.410

2.94 0.231 0.347 0.399 0.625 0.405

2.96 0.228 0.342 0.394 0.616 0.399

2.98 0.225 0.338 0.388 0.608 0.394

3 0.222 0.333 0.383 0.600 0.389

3.02 0.219 0.329 0.378 0.592 0.384

3.04 0.216 0.325 0.373 0.584 0.379

3.06 0.214 0.320 0.368 0.577 0.374

3.08 0.211 0.316 0.364 0.569 0.369

3.1 0.208 0.312 0.359 0.562 0.364

3.12 0.205 0.308 0.354 0.555 0.360

3.14 0.203 0.304 0.350 0.548 0.355

3.16 0.200 0.300 0.345 0.541 0.351

3.18 0.198 0.297 0.341 0.534 0.346

3.2 0.195 0.293 0.337 0.527 0.342

3.22 0.193 0.289 0.333 0.521 0.338

3.24 0.191 0.286 0.329 0.514 0.333

3.26 0.188 0.282 0.325 0.508 0.329

`101

3.28 0.186 0.279 0.321 0.502 0.325

3.3 0.184 0.275 0.317 0.496 0.321

3.32 0.181 0.272 0.313 0.490 0.318

3.34 0.179 0.269 0.309 0.484 0.314

3.36 0.177 0.266 0.306 0.478 0.310

3.38 0.175 0.263 0.302 0.473 0.306

3.4 0.173 0.260 0.298 0.467 0.303

3.42 0.171 0.256 0.295 0.462 0.299

3.44 0.169 0.254 0.292 0.456 0.296

3.46 0.167 0.251 0.288 0.451 0.292

3.48 0.165 0.248 0.285 0.446 0.289

3.5 0.163 0.245 0.282 0.441 0.286

3.52 0.161 0.242 0.278 0.436 0.282

3.54 0.160 0.239 0.275 0.431 0.279

3.56 0.158 0.237 0.272 0.426 0.276

3.58 0.156 0.234 0.269 0.421 0.273

3.6 0.154 0.231 0.266 0.417 0.270

3.62 0.153 0.229 0.263 0.412 0.267

3.64 0.151 0.226 0.260 0.408 0.264

3.66 0.149 0.224 0.258 0.403 0.261

3.68 0.148 0.222 0.255 0.399 0.258

3.7 0.146 0.219 0.252 0.394 0.256

3.72 0.145 0.217 0.249 0.390 0.253

3.74 0.143 0.214 0.247 0.386 0.250

3.76 0.141 0.212 0.244 0.382 0.248

3.78 0.140 0.210 0.241 0.378 0.245

3.8 0.139 0.208 0.239 0.374 0.242

3.82 0.137 0.206 0.236 0.370 0.240

3.84 0.136 0.203 0.234 0.366 0.237

3.86 0.134 0.201 0.232 0.362 0.235

3.88 0.133 0.199 0.229 0.359 0.232

3.9 0.131 0.197 0.227 0.355 0.230

3.92 0.130 0.195 0.225 0.351 0.228

3.94 0.129 0.193 0.222 0.348 0.225

3.96 0.128 0.191 0.220 0.344 0.223

3.98 0.126 0.189 0.218 0.341 0.221

4 0.125 0.188 0.216 0.338 0.219