Comparison of Structural Design Actions Part 4: Earthquake

School of Civil Engineering Sydney NSW 2006 AUSTRALIA http://www.civil.usyd.edu.au/ Comparison of Structural Design Actions Part 4: Earthquake Actions in Australia AS1170.4 – 1993 & 2007 Research Report No R897

Deborah G Hegarty B.Sc. (Eng), Dip. Eng, MIEAust CPEng May 2009 ISSN 1833-2781

School of Civil Engineering

http://www.civil.usyd.edu.au/

Comparison of Structural Design Actions Part 4: Earthquake Actions in Australia AS1170.4 – 1993 & 2007

Research Report No R897

Deborah G Hegarty B.Sc. (Eng), Dip. Eng, MIEAust CPEng

May 2009

ABSTRACT:

This report investigates the differences between the old AS1170.4:1993 code and the new

AS1170.4:2007 code and has examined the implications to building frame structural systems. The

principles of seismic design and the advances in the field that lead to development of the new

AS1170.4:2007 code [7] have been presented.

A detailed comparison of the differences between the Layout, Notation, Factors and Calculation of

the Design Base shear has been examined. The magnitude of the design base shear applied for all

structural system types and for all sub-soil classes has been carried out. Graphs showing the

percentage of seismic weight applied to structural systems for all the sub-soil classes have been

included in Appendix A. To highlight the revisions and implications of the new AS1170.4:2007

code, analysis of a typical concrete building frame structural system with reinforced concrete shear

walls has been carried out. A comparison of the calculation methods and the errors and

discrepancies of analysis procedures has been carried out and presented.

Keywords: Seismic Response Spectrum, Elastic, Dynamic, Natural Period, Earthquake Base Shear,

Structural Systems, Reinforced Concrete

Comparison of Structural Design Actions Part 4: Earthqauke Actions in Australia AS1170.4 – 1993 & 2007

May 2009

School of Civil Engineering Research Report No R897

ii

COPYRIGHT NOTICE

School of Civil Engineering, Research Report R897 COMPARISON OF STRUCTURAL DESIGN ACTIONS PART 4: EARTHQUAKE ACTIONS IN AUSTRALIA AS1170.4 – 1993 & 2007 © 2009 Deborah G Hegarty [email protected], [email protected]

ISSN 1833-2781

This publication April be redistributed freely in its entirety and in its original form without the consent of the copyright owner. Use of material contained in this publication in any other published works must be appropriately referenced, and, if necessary, permission sought from the author.

Published by: School of Civil Engineering The University of Sydney Sydney NSW 2006 AUSTRALIA

May 2009 This report and other Research Reports published by the School of Civil Engineering are available on the Internet: http://www.civil.usyd.edu.au


May 2009


iii

ACKNOWLEDGEMENTS

First I would like to thank Kourosh Kayvani and Kim Rasmussen for introducing me to the

field of seismology. They gave me the freedom to choose the direction of this research while

providing valuable support and feedback. I would also like to thank Joseph Hegarty who

offered many helpful suggestions on how to improve the structure of this report. I would like

to thank everyone who works at the Connell Wagner, whose company and support make an

invaluable working environment.

Connell Wagner also provided me with an education grant without which it would not have

been feasible to further my professional development in such a beneficial course.

Finally, I would like to thank my family and friends.


May 2009


iv

TABLE OF CONTENTS

ABSTRACT: ............................................................................................................... I

COPYRIGHT NOTICE .......................................................................................... II

ACKNOWLEDGEMENTS ................................................................................... III

TABLE OF CONTENTS ....................................................................................... IV

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

LIST OF TABLES ................................................................................................ XII

1 INTRODUCTION ............................................................................................ 1 1.1 AIMS AND OBJECTIVES ................................................................................ 2 1.2 REPORT OUTLINE ........................................................................................ 3

2 EARTHQUAKE ENGINEERING BACKGROUND ................................. 5 2.1 STRUCTURAL SYSTEMS ............................................................................... 5

2.1.1 Bearing Wall Systems ............................................................................. 6 2.1.2 Building Frame Systems ......................................................................... 6 2.1.3 Moment Resisting Frame Systems ......................................................... 6 2.1.4 Dual Systems ........................................................................................... 6

2.2 DUCTILITY – ELASTIC AND DYNAMIC RESPONSE ....................................... 8 2.2.1 Elastic Response ..................................................................................... 8 2.2.2 Ductile Response .................................................................................. 10 2.2.3 Structural Ductility Factor (µ) ............................................................. 10 2.2.4 The Structural Response Factor (Rf) & The Structural Performance Factor (Sp) ......................................................................................................... 11 2.2.5 Ductility Detailing ................................................................................ 14 2.2.6 Capacity Design ................................................................................... 20 2.2.7 Hysteretic Loops ................................................................................... 22

2.3 SEISMIC RESPONSE AND STRUCTURAL CONFIGURATION .......................... 24 2.3.1 Response in Elevation .......................................................................... 26 2.3.2 Estimates of Deflection and Drift ........................................................ 27 2.3.3 P-Delta Effects in framed structures.................................................... 28 2.3.4 Response in Plan .................................................................................. 29

2.4 THE INFLUENCE OF SOIL STIFFNESS ON EARTHQUAKE MAGNITUDE AND INTENSITY .............................................................................................................. 32

2.4.1 Site Classification using Shear Wave Velocity and Bedrock Properties 33 2.4.2 Site Classification using Site Natural Period ...................................... 36

2.5 SEISMIC RISK ............................................................................................. 37 2.5.1 Design Limit States ............................................................................... 38 2.5.2 Serviceability Limit State ..................................................................... 39 2.5.3 Economic Considerations .................................................................... 39


May 2009


v

2.6 DESIGN METHODS ..................................................................................... 40 2.6.1 Dynamic inelastic time-history analysis .............................................. 40 2.6.2 Modal superposition techniques .......................................................... 40 2.6.3 Equivalent lateral force procedures .................................................... 42 2.6.4 Capacity Spectrum Method .................................................................. 42

2.7 DISCUSSION ............................................................................................... 44

3 AS1170.4: 1993 & 2007 CODE COMPARISON ........................................ 47 3.1 CODE LAYOUT COMPARISON .................................................................... 47

3.1.1 AS1170.4:1993 Layout ......................................................................... 47 3.1.2 AS1170.4:2007 Layout and Revisions ................................................. 48

3.2 SITE HAZARD ............................................................................................. 51 3.2.1 Return Period Calculation ................................................................... 52

3.3 SITE FACTOR / SUB SOIL CLASS AND SPECTRAL SHAPE FACTOR ............. 55 3.3.1 Response Spectra and Spectral Shape Factor (Ch(T)) ........................ 56 3.3.2 Site Classification ................................................................................. 60

3.4 SELECTION OF EARTHQUAKE DESIGN CATEGORY .................................... 62 3.4.1 AS1170.4:1993 ..................................................................................... 62 3.4.2 AS1170.4:1993 (with AS1170.4:2002 Appendix D considerations) ... 64 3.4.3 AS1170.4:2007 ..................................................................................... 64 3.4.4 Earthquake Design Category Comparison ......................................... 66

3.5 PERIOD CALCULATION .............................................................................. 67 3.5.1 AS1170.4:1993 Approximated Formulae ............................................ 67 3.5.2 AS1170.4:2007 ..................................................................................... 67 3.5.3 Period Calculation Comparison .......................................................... 68 3.5.4 The Rayleigh Method ........................................................................... 70

3.6 RESPONSE FACTOR (RF), STRUCTURAL DUCTILITY FACTOR, µ, AND THE STRUCTURAL PERFORMANCE FACTOR, SP ............................................................. 70 3.7 EARTHQUAKE BASE SHEAR ....................................................................... 72

3.7.1 AS1170.4: 1993 Earthquake Base shear ............................................. 72 3.7.2 AS1170.4: 2007 Earthquake Base shear ............................................. 72

3.8 STRUCTURAL SYSTEMS AND RESTRICTIONS ............................................. 73 3.8.1 Bearing wall systems ............................................................................ 74 3.8.2 Building Frame systems ....................................................................... 79 3.8.3 Moment Resisting Frame System ......................................................... 87 3.8.4 Dual System .......................................................................................... 94

3.9 TORSION .................................................................................................... 94 3.9.1 AS1170.4:1993 Approximated Formulae ............................................ 95 3.9.2 AS1170.4:2007 Approximated Formulae ............................................ 96 3.9.3 Torsion Comparison ............................................................................. 97

3.10 DRIFT AND P-DELTA EFFECTS................................................................... 98 3.10.1 AS1170.4: 1993 Storey Drift Determination and P-delta Effects ... 98 3.10.2 AS1170.4: 2007 Storey Drift Determination and P-delta Effects ... 99 3.10.3 Storey Drift Determination and P-delta Effects Comparison ....... 100


May 2009


vi

3.11 DYNAMIC ANALYSIS ............................................................................... 101 3.12 DISCUSSION ............................................................................................. 104

4 ANALYSIS COMPARISON OF A TYPICAL CONCRETE STRUCTURAL SYSTEM ................................................................................... 106

4.1 BUILDING, SITE AND DESIGN METHOD SELECTION ................................ 106 4.1.1 Structural System ................................................................................ 106 4.1.2 Elevation ............................................................................................. 108 4.1.3 Plan ..................................................................................................... 110 4.1.4 Core Properties .................................................................................. 113 4.1.5 Shear Centre and Centre of Mass ...................................................... 114 4.1.6 AS1170.4:1993 Design Eccentricity Calculation .............................. 116 4.1.7 AS1170.4:2007 Design Eccentricity Calculation .............................. 117 4.1.8 Analysis Method ................................................................................. 118 4.1.9 Site & Structural Factors ................................................................... 119 4.1.10 Structural Classification / Importance level for the structure ...... 119 4.1.11 Acceleration Coefficient/Hazard Factor ....................................... 119 4.1.12 Probability Factor kp ...................................................................... 119 4.1.13 Site Factor / Sub Soil Class ............................................................ 119 4.1.14 Period Calculation for the buildings ............................................. 119 4.1.15 Response Factor and Ductility Ratio ............................................. 120 4.1.16 Earthquake Base shear Multiplier ................................................. 120 4.1.17 Loads .............................................................................................. 121

4.2 HAND CALCULATION ANALYSIS ............................................................. 124 4.2.1 First-Mode of Natural Period ............................................................ 124 4.2.2 Seismic Design Base Shear ................................................................ 124 4.2.3 Overturning Moment .......................................................................... 125 4.2.4 Torsion ................................................................................................ 127 4.2.5 Tension & Compression Core Stresses due to Overturning Moment129 4.2.6 Shear Force on Core due to Base Shear & Torsion ......................... 133 4.2.7 Structural Displacements ................................................................... 137 4.2.8 Deflection at Roof Level ..................................................................... 138 4.2.9 Storey Drift and P-Delta Effects ........................................................ 143

4.3 ETABS ANALYSIS ..................................................................................... 148 4.3.1 First-Mode of Natural Period ............................................................ 149 4.3.2 Seismic Design Base Shear ................................................................ 149 4.3.3 Overturning Moment .......................................................................... 150 4.3.4 Stresses ................................................................................................ 151 4.3.5 Deflections .......................................................................................... 152

4.4 CONCLUSIONS .......................................................................................... 155

5 CONCLUSIONS AND FUTURE WORK ................................................. 157 5.1 LIMITATIONS ........................................................................................... 160 5.2 CONTRIBUTIONS ...................................................................................... 161 5.3 SUCCESS CRITERIA .................................................................................. 161


May 2009


vii

5.4 FUTURE WORK ........................................................................................ 162 5.4.1 Fragility Curves .................................................................................. 162 5.4.2 Design and Detailing of the Lateral Supporting System ................... 162

5.5 FINAL NOTE ............................................................................................. 163

BIBLIOGRAPHY ................................................................................................. 164

APPENDIX A: CODE COMPARISON GRAPHS FOR STRUCTURAL SYSTEMS AND SITE SUB SOIL CLASSES ................................................... 166

BEARING WALL SYSTEM ..................................................................................... 167 BUILDING FRAME SYSTEM WITH REINFORCED CONCRETE WALLS .................... 172 BUILDING FRAME SYSTEM WITH CONCENTRICALLY BRACED FRAMES .............. 177 ORDINARY MOMENT RESISTING FRAME SYSTEM ............................................... 182 INTERMEDIATE MOMENT RESISTING FRAME SYSTEM ........................................ 187

APPENDIX B: STATIC ANALYSIS BUILDING COMPARISON CALCULATIONS ................................................................................................ 192

BLD 1 (14.4M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 193 BLD 1 (14.4M) SOIL CLASS AE TENSION & COMP CORE STRESS ........................ 196 BLD 1 (14.4M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 198 BLD 1 (14.4M) SOIL CLASS DE TENSION & COMP CORE STRESS ........................ 201 BLD 2 (29.7M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 203 BLD 2 (29.7M) SOIL CLASS AE TENSION & COMP CORE STRESS ........................ 206 BLD 2 (29.7M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 208 BLD 2 (29.7M) SOIL CLASS DE TENSION & COMP CORE STRESS ........................ 211 BLD 3 (56.1M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 213 BLD 3 (56.1MM) SOIL CLASS AE TENSION & COMP CORE STRESS ..................... 216 BLD 3 (56.1M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 218 BLD 3 (56.1MM) SOIL CLASS DE TENSION & COMP CORE STRESS ..................... 221 BLD 4 (97.9M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 223 BLD 4 (97.9M) SOIL CLASS AE TENSION & COMP CORE STRESS ........................ 226 BLD 4 (97.9M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 228 BLD 4 (97.9M) SOIL CLASS DE TENSION & COMP CORE STRESS ........................ 231

APPENDIX C: GLOSSARY .............................................................................. 233


May 2009


viii

LIST OF FIGURES

Figure 2-1 Shows three of the structural systems used for supporting lateral loads. A dual system uses a combination of these systems [13]. ................................................................................................................ 7

Figure 2-2 shows a cantilever subjected to a horizontal load [12]. ...................................................................................... 8

Figure 2-3 Shows the stress strain relationship of an element responding elastically. ........................................................ 9

Figure 2-4 Shows the relationship between strength and ductility [30]. ............................................................................ 10

Figure 2-5 Shows the typical load-displacement relationship for a reinforced concrete element [30]. ...................................................................................................................................................................................... 11

Figure 2-6 shows the relationship between ductility and force reduction factor [30]. ....................................................... 12

Figure 2-7 shows the influence of period on ductile force reduction [30]. ........................................................................ 13

Figure 2-8 Shows the preferred location of plastic hinges within the beams of a multistorey structure compared to the formation of a soft-storey due to plastic hinges forming in the columns [30]. ....................................................................................................................................................................... 15

Figure 2-9 shows the failure of a structure due to the development of a soft-storey ......................................................... 15

Figure 2-10 Shows plastic hinge rotations and deformations in beams [28] [29] .............................................................. 16

Figure 2-11 shows a standard detail for a typical beam. ..................................................................................................... 17

Figure 2-12 shows the typical beam reinforcement for a beam in an intermediate moment resisting frame for AS 1170.4:1993. ................................................................................................................................... 17

Figure 2-13 shows a standard detail for a typical beam with bottom layer continuity steel provided at the support. ....................................................................................................................................................... 18

Figure 2-14 shows a standard detail for a typical column to prevent the forming of a plastic hinge at the base of the column. .......................................................................................................................................... 19

Figure 2-15 shows failure at the base of a structural column due to the formation of a plastic hinge ..................................................................................................................................................................................... 20

Figure 2-16 shows the failure of a column in a soft storey due to lack of ties to constrain the vertical reinforcement during large deflection demands. .................................................................................................... 21

Figure 2-17 shows Wilson and Lams capacity response spectrum method [34][35][36] .................................................. 22

Figure 2-18 shows typical force-displacement hysteresis loop shapes for elastic and inelastic systems during a loading and unloading cycle [34] ............................................................................................................ 23

Figure 2-19 shows the comparison of hysteretic loops for an ideal case and where plastic hinges occur in a beam [30]. ................................................................................................................................................ 24


May 2009


ix

Figure 2-20 Shows plastic mechanisms in frame and wall systems; (a) soft-storey mechanism in a weak column/strong beam frame; (b, c) beam-sway mechanisms in a strong column/weak beam frame; (d, e) beam-sway mechanisms in a wall system [16]. ................................................................................... 25

Figure 2-21 shows the response of a structure to lateral loads. .......................................................................................... 26

Figure 2-22 shows diagrammatically the acceleration displacement response spectrum for a range of natural period [32]. ................................................................................................................................................ 27

Figure 2-23 Shows typical sway of multistorey frames [28][29] ....................................................................................... 28

Figure 2-24 Amplification of column bending moments in ductile frames due to P-delta hinges [28] [29] .................................................................................................................................................................... 29

Figure 2-25 shows a force equal to the total resultant horizontal earthquake force and a moment acting through the shear centre [30] ...................................................................................................................... 30

Figure 2-26 Shows various floor plans for symmetrical and unsymmetrical buildings. The shear centre and centre of mass relationship is shown [30]. ............................................................................................... 31

Figure 2-27 Sketch of some engineering topics [12]. ......................................................................................................... 32

Figure 2-28 Shows the first generation national site classification map of Australia based on modified NEHRP site classes [25][26]. .............................................................................................................................. 33

Figure 2-29 shows the earthquake shear waves propagating from the focus of the event. ................................................ 34

Figure 2-30 shows a schematic diagram illustrating local geology and soil features [14] ................................................ 35

Figure 2-31 Shows an example of modal superpositioning [12]. ....................................................................................... 41

Figure 2-32 shows Wilson and Lams capacity spectrum approach [34] [35] [36] ............................................................ 43

Figure 2-33 shows the push-over analysis of a building with a soft-storey [34] ............................................................... 43

Figure 2-34 Bi-linear approximation of the push-over curve [34] ..................................................................................... 44

Figure 3-1 shows the Earthquake Hazard Map of Australia from the AS1170.4:2007 code. ........................................... 51

Figure 3-2 Comparison of proposed R-Factors for New Zealand with Hazard curves for 0.5s Spectral Accelerations [29] .................................................................................................................................................. 54

Figure 3-3 shows the RSA acceleration and RSV velocity response spectra [34] ............................................................. 56

Figure 3-4 shows the RSD, displacement response spectra [34] ........................................................................................ 57

Figure 3-5 shows the displacement, velocity and acceleration response spectrum format [22] ........................................ 58

Figure 3-6 Recommended response spectrum model in tripartite presentation [21] ......................................................... 59

Figure 3-7 shows the demand curve consistent with the AS1170.4 model [34] [35] ........................................................ 60


May 2009


x

Figure 3-8 this figure shows the variation in the Periods with heights for the AS1170.4:1993 and 2007 Codes [4][7] ......................................................................................................................................................... 69

Figure 3-9 shows the comparison of the Rf and µ/Sp relationship [34] .............................................................................. 70

Figure 3-10 this figure shows the comparison of the base shear multiplier for BWS, for AS1170.4: 1993 & 2007 [4][7], for Soil Class Ae. ............................................................................................................. 74

Figure 3-11 this figure shows the comparison of the base shear multiplier for BWS, for AS1170.4: 1993 & 2007 [4][7], for Soil Class De. ............................................................................................................. 77

Figure 3-12 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4][7], for Soil Class Ae. ............................................................................................................. 79

Figure 3-13 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4] [7], for Soil Class De. ............................................................................................................ 81

Figure 3-14 this figure shows the comparison of the base shear multiplier for CBF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class Ae. .............................................................................................................. 83

Figure 3-15 this figure shows the comparison of the base shear multiplier for CBF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class De. .............................................................................................................. 85

Figure 3-16 this figure shows the comparison of the base shear multiplier for OMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class Ae. .............................................................................................................. 87

Figure 3-17 this figure shows the comparison of the base shear multiplier for OMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class De. .............................................................................................................. 89

Figure 3-18 this figure shows the comparison of the base shear multiplier for IMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class Ae. .............................................................................................................. 91

Figure 3-19 this figure shows the comparison of the base shear multiplier for IMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class De. .............................................................................................................. 93

Figure 3-20 shows the geometric eccentricities from the AS1170.4:1993 code. ............................................................... 96

Figure 3-21 Shows the translation and torsion effects on a floor plate [30]. .................................................................... 103

Figure 4-1 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4][7], for Soil Class Ae. ........................................................................................................... 107

Figure 4-2 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4] [7], for Soil Class De. .......................................................................................................... 108

Figure 4-3 shows a typical architectural section through Building Type 3. ..................................................................... 110

Figure 4-4 shows the typical architectural floor plate for all 4 buildings used in the comparison. ........................................................................................................................................................................ 111

Figure 4-5 shows the typical structural plan for the buildings, highlighting the two lateral resisting cores. .................................................................................................................................................................... 112


May 2009


xi

Figure 4-6 shows the core properties for core number 1 for building 3. .......................................................................... 113

Figure 4-7 shows the core properties for core number 2 for building 3. .......................................................................... 114

Figure 4-8 shows the calculation of the centre of mass and shear centre in the x-x direction. ........................................ 115

Figure 4-9 shows the calculation of the centre of mass and shear centre for the y-y direction ....................................... 116

Figure 4-10 shows the vertical distribution of the earthquake base shear for both AS1170.4:1993 & 2007 [4] [7] and [34] ........................................................................................................................... 126

Figure 4-11 shows the model used for building 3 within the Etabs model ...................................................................... 148

Figure 4-12 shows the meshing of the supporting cores by the Etabs model the colour of the segments represents the stress in the element. Etabs uses a colour range to express the stresses. .................................. 152

Figure 4-13 shows the deflective shape in the Y-direction for the most onerous 185mm deflection from Etabs ......................................................................................................................................................... 154


May 2009


xii

LIST OF TABLES

Table 3-1 lists six of the listed cities that have a revised value for the Hazard Factor. ..................................................... 52

Table 3-2 this table shows the differences between the Probability factor (kp) for AS1170.0:2002 Appendix D [6] and the AS1170.4:2007 Values [7] ................................................................................ 53

Table 3-3 this table shows the differences between the current and previous annual probability of exceedance values from the BCA [11]. .......................................................................................................................... 55

Table 3-4 shows the difference in the Sub-Soil Class values [21] [22] .............................................................................. 61

Table 3-5 shows the design category selections for Sydney using the AS1170.4:1993 code. .......................................... 63

Table 3-6 show the change in terminology for earthquake design categories in Sydney using the AS1170.4:2002 Appendix D ......................................................................................................................................... 64

Table 3-7 shows the earthquake design categories for Importance level 2 structures in Sydney using AS1170.4:2007........................................................................................................................................................... 65

Table 3-8 shows the earthquake design categories for importance level 3 structures in Sydney using AS1170.4:2007........................................................................................................................................................... 65

Table 3-9 shows the earthquake design categories for importance level 4 structures in Sydney using AS1170.4:2007........................................................................................................................................................... 66

Table 3-10 shows the revised ductility and over-strength factors used in the code but not in used notation [36]................................................................................................................................................................. 71

Table 3-11 show the comparison of the base shear multiplier for BWS, for AS1170.4:1993 and 2007, for soil class Ae ................................................................................................................................................... 75

Table 3-12 show the comparison of the base shear multiplier for BWS, for AS1170.4:1993 and 2007, for soil class De ................................................................................................................................................... 78

Table 3-13 show the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4:1993 and 2007, for soil class Ae. ....................................................................................................................... 80

Table 3-14 show the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4:1993 and 2007, for soil class De. ....................................................................................................................... 82

Table 3-15 show the comparison of the base shear multiplier for CBF, for AS1170.4:1993 and 2007, for soil class Ae. ......................................................................................................................................................... 84

Table 3-16 show the comparison of the base shear multiplier for CBF, for AS1170.4:1993 and 2007, for soil class De. ......................................................................................................................................................... 86

Table 3-17 show the comparison of the base shear multiplier for OMRF, for AS1170.4:1993 and 2007, for soil class Ae ................................................................................................................................................... 88


May 2009


xiii

Table 3-18 show the comparison of the base shear multiplier for OMRF, for AS1170.4:1993 and 2007, for soil class De ................................................................................................................................................... 90

Table 3-19 show the comparison of the base shear multiplier for IMRF, for AS1170.4:1993 and 2007, for soil class Ae ................................................................................................................................................... 92

Table 3-20 show the comparison of the base shear multiplier for IMRF, for AS1170.4:1993 and 2007, for soil class De ................................................................................................................................................... 94

Table 3-21 Shows the percentage difference in the design deflections multiplier for storey drift calculation for AS1170.4: 1993 & 2007 ........................................................................................................................... 100

Table 3-22 Shows the percentage difference in the inter-storey stability coefficient for P-delta effects multiplier for AS1170.4: 1993 & 2007 ................................................................................................................. 101

Table 3-23 shows the comparison (percentage differences) of the seismic weight loading multiplier for AS1170.4:1993 and 2007 code [4] [7]. ...................................................................................................... 104

Table 4-1 shows the typical loading to be taken for the basement areas .......................................................................... 121

Table 4-2 shows the typical loading to be taken for the floor areas ................................................................................. 121

Table 4-3 shows the typical loading to be taken for the plant floor ................................................................................. 122

Table 4-4 shows the typical loading to be taken for the steel roof ................................................................................... 122

Table 4-5 shows the typical loading to be taken for each core ......................................................................................... 122

Table 4-6 shows the minimum and maximum axial load acting on the cores. ................................................................ 123

Table 4-7 shows the first mode of natural period and base shear multiplier for the four buildings ............................................................................................................................................................................. 124

Table 4-8 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes for minimum loading ......................................................................................................................................................... 125

Table 4-9 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes for maximum loading......................................................................................................................................................... 125

Table 4-10 shows the differences in the overturning moment values for the AS110.4:1993 & 2007 codes for minimum loading ...................................................................................................................................... 126

Table 4-11 shows the differences in the overturning moment values for the AS110.4:1993 & 2007 codes for maximum loading ..................................................................................................................................... 127

Table 4-12 shows the differences in the torsion values for the AS110.4:1993 & 2007 codes for minimum loading ............................................................................................................................................................... 128

Table 4-13 shows the differences in the torsion values for the AS110.4:1993 & 2007 codes for maximum loading .............................................................................................................................................................. 129

Table 4-14 shows the comparison of tension stress induced in core 1 for minimum loading ......................................... 129


May 2009


xiv

Table 4-15 shows the comparison of tension stress induced in core 1 for maximum loading ........................................ 130

Table 4-16 shows the comparison of compression stress induced in core 1 for minimum loading ................................................................................................................................................................................ 130

Table 4-17 shows the comparison of compression stress induced in core 1 for maximum loading ................................................................................................................................................................................ 131

Table 4-18 shows the comparison of tension stress induced in core 2 for minimum loading ......................................... 131

Table 4-19 shows the comparison of tension stress induced in core 2 for maximum loading ........................................ 132

Table 4-20 shows the comparison of compression stress induced in core 2 for minimum loading ................................................................................................................................................................................ 132

Table 4-21 shows the comparison of compression stress induced in core 2 for maximum loading ................................................................................................................................................................................ 133

Table 4-22 shows the comparison of additional shear due to torsion for minimum loading ........................................... 133

Table 4-23 shows the comparison of additional shear due to torsion maximum loading ................................................ 134

Table 4-24 shows the comparison of equivalent horizontal base shear for minimum loading ....................................... 135

Table 4-25 shows the comparison of equivalent horizontal base shear for maximum loading ...................................... 135

Table 4-26 shows the comparison of total torsional and equivalent horizontal base shear for minimum loading ............................................................................................................................................................... 136

Table 4-27 shows the comparison of percentage difference in the total torsional and equivalent horizontal base shear for minimum loading ...................................................................................................................... 136

Table 4-28 shows the comparison of total torsional and equivalent horizontal base shear for maximum loading .............................................................................................................................................................. 137

Table 4-29 shows the comparison of percentage difference in the total torsional and equivalent horizontal base shear for minimum loading ...................................................................................................................... 137

Table 4-30 shows the comparison of percentage difference in the deflection at roof level of Core 1 for minimum loading (X-direction) ....................................................................................................................... 139

Table 4-31 shows the comparison of percentage difference in the deflection at roof level of Core 1 for maximum loading (X-direction) ...................................................................................................................... 139

Table 4-32 shows the comparison of percentage difference in the deflection at roof level of Core 2 for minimum loading (X-direction) ....................................................................................................................... 140

Table 4-33 shows the comparison of percentage difference in the deflection at roof level of Core 2 for maximum loading (X-direction) ...................................................................................................................... 140

Table 4-34 shows the comparison of percentage difference in the deflection at roof level of Core 1 for minimum loading (Y-direction) ....................................................................................................................... 141


May 2009


xv

Table 4-35 shows the comparison of percentage difference in the deflection at roof level of Core 1 for maximum loading (Y-direction) ...................................................................................................................... 141

Table 4-36 shows the comparison of percentage difference in the deflection at roof level of Core 2 for minimum loading (Y-direction) ....................................................................................................................... 142

Table 4-37 shows the comparison of percentage difference in the deflection at roof level of Core 2 for maximum loading (Y-direction) ...................................................................................................................... 142

Table 4-38 shows the comparison of storey drift and P-delta consideration of Core 1 for minimum and maximum roof loading (X-direction) ........................................................................................................ 144

Table 4-39 shows the comparison of storey drift and P-delta consideration of Core 2 for minimum and maximum roof loading (X-direction) ........................................................................................................ 145

Table 4-40 shows the comparison of storey drift and P-delta consideration of Core 1 for minimum and maximum loading (Y-direction) ................................................................................................................ 146

Table 4-41 shows the comparison of storey drift and P-delta consideration of Core 2 for minimum and maximum loading (Y-direction) ................................................................................................................ 147

Table 4-42 shows the first mode of natural period and base shear multiplier for building 3 .......................................... 149

Table 4-43 shows the first five modes of natural period for building 3 calculated using Etabs ...................................... 149

Table 4-44 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes by hand calculations ........................................................................................................................................................... 150

Table 4-45 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes using Etabs ......................................................................................................................................................................... 150

Table 4-46 shows the differences in the overturning moment values for the AS110.4:1993 & 2007 codes for minimum loading ...................................................................................................................................... 150

Table 4-47 shows the differences in the over turning moment for the AS110.4:1993 & 2007 codes using Etabs ............................................................................................................................................................... 151

Table 4-48 shows the differences in the stresses (Core 2) for the AS110.4:1993 & 2007 codes .................................... 151

Table 4-49 shows the differences in the stresses in the cores for the AS110.4:1993 & 2007 codes ................................................................................................................................................................................... 151

Table 4-50 shows the comparison of percentage difference in the deflection at roof level in the X-direction ......................................................................................................................................................................... 153





May 2009


xvi


May 2009


1

1 INTRODUCTION

An increased global awareness of natural disasters due to environmental changes has influenced our

assessment of risk. Historically seismic risk in Australia was considered to have low seismicity and

that events have mainly effect unpopulated areas. Due to Australia’s low seismicity, buildings have

not been designed for the ductility required for higher return period events which increases

vulnerability to a catastrophic disaster. Risk is the combination of the event and the vulnerability of

structures.

The prevention of structural failure due to natural disaster events such as earthquakes in Australia

has been of utmost concern since the development of the first code in the 1970’s. Since then there

has been tremendous development in understanding the physical geological element, structural

behaviour and risk assessment. Further to the unexpected disaster in Newcastle, NSW, in 1989 there

was an updating of the AS2121 1979 code which was the AS1170.4:1993 code, which has been

developed further into the AS1170:2007 code of today.

Why, How and So What? Are all questions that must be answered to understand the reasons there

have been revisions and the implications of them for the safe design of structures to withstand a

seismic event.

The building code for Australia has increased the return periods for events, which are to be

considered for buildings of varying importance levels. The revisions of the probability factor in the

BCA, amplifies the horizontal lateral loadings that the buildings are required to resist.

It was originally considered to develop a new code to replace the AS1170.4: 1993 code in

combination with the earthquake codes of New Zealand and Australia but due to extreme difficulties

in the drafting stages due to differences in the seismicity of the two countries it was decided to draft

two individual documents. In the new AS1170.4:2007 the design methods have been simplified and

were possible similar notation has been used to the New Zealand code [28] [29].


May 2009


2

There are two main factors that are required to be understood when considering seismic design and

preventing of failure:

Soil Behaviour – soils behaviour during a seismic event and its amplification potential are of utmost

concern for predicting structural behaviour. One particular development has been in the

understanding of the resonance of shear waves through bedrock and how it amplifies structural

response during an earthquake.

Structural Systems Behaviour – A structures predicted behaviour during event is critical.

Along with the two behaviours of the global systems above there are three structural properties that

must be considered when designing a building for survival in an earthquake event:

Period of the Structure – Does the accurate calculation of the period of a structure effect the design

of the system?

Torsion – Do larger accidental torsions have large implications on core and lateral resisting element

design? Symmetry of the torsion resistance elements is crucial to stop deflections occurring within a

floor plate.

Deflection demand and P-delta Effects – Does the structural system allow for the large deflection

demands? Do we design the structure to be ductile or elastic?

1.1 Aims and Objectives

The objectives of this report are as follows:

To investigate the reasons behind revising the code.

Establish what has been revised.

Study the implications of the changes on a typical concrete structural system.


May 2009


3

This report demonstrates the differences and the implications of the new AS1170.4:2007 code [7].

The techniques used in this report towards achieving the objectives are based on ascertaining an

understanding within the following areas:

Building Selection for Comparison – By using a variety of structural heights it is possible to obtain

a wide spectrum of design implications for buildings.

Soil Factor – By identifying the implications of the soil factor used for the site being considered.

Period Calculation – By using new formulae to obtain a less conservative natural period for a

structure, a more accurate behaviour of the structural system being designed can be achieved.

Structural Response Performance and Ductility – By considering the structural performance and

choosing a ductility factor that is to be designed, the onus is on the detailing to be achieved to

achieve compliance.

Base shear Magnitude – By determining the magnitude differences in the base shear it allows

immediate implication recognition.

Torsion – By examining the increase to accidental torsion being applied to symmetrical and

unsymmetrical systems a table of buildings affected has been developed.

Drift – Variations to drift have been examined.

Hand and Computer Aided Design – By comparing the difference in design methods and the

errors and discrepancies the requirement for accurate building models is highlighted.

1.2 Report Outline

The remainder of this report is divided into the following sections:

Chapter 2 investigates the principles of seismic design and the advances in the field that lead to

development of the new 1170.4:2007 code [7].


May 2009


4

Chapter 3 describes the differences in the new and the old code and the implications of the new

code to current building design.

Chapter 4 describes the design philosophy behind the choice of structural system, geometry and

design methods to best illustrate design implications.

Describes the hand calculation procedure for equivalent static analysis and examines the

implications on building detailing.

Describes the computer aided design procedure using ETABS for the static and dynamic analysis

required.

Investigates the comparison on the calculation methods and examines the errors and discrepancies of

the procedures.

Chapter 5 presents conclusions and indicates future work to expand the scope of implications.


May 2009


5

2 EARTHQUAKE ENGINEERING BACKGROUND

The following six topics are relevant to the revisions implemented in the new AS1170.4:2007 code

[7]:

Structural Systems

Ductility - Elastic and Dynamic Response

Seismic Response and Structure Configuration

The influence of Soil Stiffness on Earthquake Magnitude and Intensity

Seismic Risk

Design Methods

The background to and related research in these topics are examined in this chapter.

2.1 Structural Systems

“All buildings are not created equal when response to earthquake-induced forces is of concern” [30]

The challenge in seismic design of building structures is primarily to conceive and detail a structural

system that is capable of surviving a given level of lateral ground shaking with an acceptable level

of damage and a low probability of collapse. The choice of structural system and its ability to

perform under earthquake induced forces is of paramount importance in the early design stages of a

project. The geometry and occupation requirements, set-out by the architectural intensions can have

large influence on the selection of system and construction type.

A structure can be classified into one of four earthquake resisting systems:

Bearing Wall System,

Building Frame System,

Moment Resisting Frame System


May 2009


6

Dual System

Brief descriptions of these are as follows:

2.1.1 Bearing Wall Systems

These are a structural system with load bearing walls providing support for all or most of the vertical

loads, where shear walls or braced frames provide the horizontal earthquake resistance. The

presence of minor load bearing walls in a structure that would normally be classified as a building

frame system does not necessarily mean that the structure should be categorized as a bearing wall

system, as their contribution to lateral force resistance, if any, is often negligible.

2.1.2 Building Frame Systems

Are structural systems in which an essentially complete space frame supports the vertical loads and

the shear walls or braced frames provided the horizontal earthquake resistance. While there is no

requirement to provide horizontal resistance in the vertical-load framing, it is strongly recommended

that nominal moment resistance be incorporated in the vertical-load frame design. The vertical-load

frame provides a nominal secondary line of defence, although all required horizontal forces are

resisted by other earthquake resisting structural systems. However, consideration should be given to

the deformation compatibility between individual members. The presence of a frame can provide

vertical stability to the structure and prevent collapse after damage to shear walls or braced frames.

The frame also acts to tie the structure together and redistribute the horizontal force to undamaged

elements of the horizontal force resisting system.

2.1.3 Moment Resisting Frame Systems

A structural system in which an essentially complete space frame supports the vertical loads and the

total prescribed horizontal earthquake forces by the flexural action of the members. The beams,

supporting floors, and columns are continuous and meet at nodes, often called “rigid” joints. The

entire horizontal force stipulated should be capable of being resisted by moment resisting frames.

2.1.4 Dual Systems

A dual system is a structural system in which an essentially complete space frame supports the

vertical loads and at least a quarter of the prescribed horizontal earthquake forces. The total


May 2009


7

horizontal earthquake resistance is provided by the combination of the moment frame, shear walls or

braced frames in proportion to their relative rigidities.

Figure 2-1 Shows three of the structural systems used for supporting lateral loads. A dual system uses a combination of these systems [13].

Figure 2-1 above shows diagrammatically the three structural systems and their deflection response

under lateral loading.

Apart from lateral response of structures, vertical response and ground dislocation are other aspects

to be considered. For buildings the response to vertical accelerations are almost always a lesser

problem than the response to horizontal accelerations due to the characteristically high reserve

strength provided as a result of design for gravity loads. Although ground dislocation by faulting

directly under a building could have potentially disastrous consequences, the probability of

occurrence is extremely low. Where fault locations are identified it is common to legislate against

the building over the fault. Strong foundations generally tend to deflect the path of faulting around

the building perimeter, however large civil and infrastructural construction could be effected by this

but are outside the scope of this report.


May 2009


8

2.2 Ductility – Elastic and Dynamic Response

Another way of classifying the structural system is in terms of its design ductility level. The

structural systems above can all be classified to have varying ductility depending on construction

material used and detailing of elements and connections. There are four principles that must be

considered when assuming structural ductility: elastic response, ductile response, ductility detailing

and capacity design.

2.2.1 Elastic Response

A building can be described as having the response of a cantilever system (shown in Figure 2-2

below). The response to lateral ground accelerations can be idealised by a single degree of freedom

system using the D’Alembert’s principle, where the total inertial response of the system is dependant

on the displacement of the structure relative to the ground and the displacement of the ground itself.

Figure 2-2 shows a cantilever subjected to a horizontal load [12].

An elastic responding system has an idealized response. It has a linear strength-displacement

relationship. The structure is designed so that the maximum displacement (strain) is very close to the

displacement of the ideal elastic structure and when the lateral loading is removed the structure

recovers elastically (full recovery). Figure 2-3 shows the idealized elastic response of an element.

The importance of a building and to what extent damage is acceptable, are the two parameters that

define whether an elastic response is desirable.


May 2009


9

Figure 2-3 Shows the stress strain relationship of an element responding elastically.

It might be considered necessary for existing building of importance, such as historic buildings or

buildings required after an emergency e.g. Hospitals, to have adequate strength to ensure elastic or

near elastic response. Existing old buildings may possess a level of inherent strength such that elastic

response is assured however due to the lack of ductility in materials they need to withstand much

larger loads.

It is generally uneconomic, often unnecessary, and arguably undesirable to design structures to

respond to the design-level earthquakes in the elastic range.

Figure 2-4 shows the relationship between strength and ductility required to resist seismic forces,

where a ductility of µ=1.0 represents the ideal elastic response. It can be seen that for a similar

seismic event, the strength of an elastic element is required to be much larger than an element that

yields at a lesser strength but has the ability to achieve the deformations due to ductility.


May 2009


10

Figure 2-4 Shows the relationship between strength and ductility [30].

2.2.2 Ductile Response

Most ordinary buildings are designed to resist lateral seismic forces which are much smaller than

those that would be developed in an elastically responding structure, implying that inelastic

deformations and hence ductility will be required of the structure to absorb seismic energy,

involving yielding of reinforcement and possibly crushing of concrete. Provided that the strength

does not degrade as a result of inelastic action, acceptable response can be obtained. Displacement

and damage, however, must be controlled at acceptable levels.

2.2.3 Structural Ductility Factor (µ)

(µ) [29], is a numerical assessment of the ability of a structure to sustain cyclic displacements in the

inelastic range. Its value depends upon the structural form, the ductility of the materials and the

structural damping characteristics.

Once the value of µ is selected the structure must be detailed to achieve that selected ductility. For

moderately ductile structures such as ordinary moment resistant frames (OMRF), braced frames, and

similar, there is no explicit design of plastic hinges. The ductility is achieved by applying the

detailing provided in the material design standards currently in use.


May 2009


11

Figure 2-5, shows the typical load-displacement relationship of a reinforced concrete element. The

ductility demand for the element is required once the yield strength has been reached. It is seen that

if the element is not ductile brittle failure will occur, which is a failure without warning and can lead

to catastrophic events.

Figure 2-5 Shows the typical load-displacement relationship for a reinforced concrete element [30].

The level of ductility a structure requires may vary from low, requiring no special detailing, to high,

requiring careful consideration of detailing.

2.2.4 The Structural Response Factor (Rf) & The Structural Performance Factor (Sp)

The structural response factor is a reduction factor, Rf, used in the old code and is applied to account

for both damping and the ductility inherent in the structural system.

Where the structural performance factor, Sp, used in the new code is a numerical assessment of the

additional ability of the total building (structure and other components) to survive earthquake

motion. The performance factor represents a number of effects that are not explicitly represented in

an analysis. Those effects can be defined as follows [29]:


May 2009


12

Calculated loads correspond to the peak acceleration which happens only once and therefore is unlikely to lead to significant damage.

Individual structural elements are typically stronger than predicted by our analysis (higher material strength, strain hardening, strain rate effects)

The total structural capacity is typically higher than predicted (redundancy, non-structural elements)

The energy dissipation of structure is typically higher than assumed (damping from non structural elements and foundations)

The performance factors intend to account for these effects by a simple scaling of the design loads. It

is therefore necessarily limited but represents a practical attempt to capture those effects which can

not easily be modelled. Overall, these factors allow the design loads to be set to a level which

intends to represent a balance between risk and economical considerations.

As can be seen in Figure 2-6 below, for a lightly damped building structure of brittle material that

would be unable to tolerate any appreciable deformation beyond the elastic range the response factor

would be close to unity.

Figure 2-6 shows the relationship between ductility and force reduction factor [30].


May 2009


13

At the other extreme, a heavily damped building structure with a very ductile structural system

would be able to withstand deformations considerably in excess of initial yield and would, therefore,

justify the assignment of a larger response factor.

The response is dependant on the structural period of the building also and Figure 2-7 shows the

relationship of natural period and acceleration. For buildings with a natural period greater than that

corresponding to the peak elastic spectral response, the maximum displacements are very similar for

the elastic and inelastic shown in Figure 2-6 a), thus implying that the ductility achieved by the

inelastic system is approximately equal to the force reduction factor, R. This observation is

sometimes referred to as the equal-displacement principle.

Figure 2-7 shows the influence of period on ductile force reduction [30].

For shorter period structures, equal to or less than a natural period greater than that corresponding to

the peak elastic spectral response, this is not conservative. That is to say that the displacement

ductility demand is greater than the force reduction factor, shown in Figure 2-6 b). Using the equal-

energy principle, the peak displacement ductility factor can be found by equating the area under the

elastic force–displacement curve with the inelastic curve.


May 2009


14

For very short period structures the force reduction factor is still not conservative. This is due to the

period lengthening, due to stiffness degradation towards the period range of high response. Where as

medium to long period structures lengthen away from this critical period range. When the period

approaches zero (infinitely rigid structures) the maximum peak response is equal to the peak ground

acceleration, and the structural deformations become insignificant compared with the ground motion

deformations. Consequently if the structure cannot sustain the peak ground acceleration, failure will

occur. Therefore very short-period structures should not be designed for force levels less than the

peak ground acceleration. This behaviour may be termed equal-acceleration principle.

2.2.5 Ductility Detailing

“The quality of ductile detailing is more important to performance in actual earthquakes than the

value of earthquake design load” [18]

In elastic design the “limited” ductility case is considered, which is that no special detailing is

required, and “intermediate” and “special” are used for increased deflection demands in earthquake

design.

The purpose of ductility detailing is to allow the formation of plastic hinges in precise locations to

achieve the preferred structural response. For example, the ideal location of plastic hinges is within

the beam elements of a frame as development of plastic hinges in columns can lead to a soft storey

and hence reduced overall ductility.

Figure 2-8 shows the preferred locations of plastic hinges in the beams of a multi-storey frame rather

than in the columns.


May 2009


15

Figure 2-8 Shows the preferred location of plastic hinges within the beams of a multistorey structure compared to the formation of a soft-storey due to plastic hinges forming in the columns [30].

Figure 2-9 shows the failure of a structure due to the development of a soft-storey


May 2009


16

Figure 2-9 above shows the failure of a building due to the development of a soft storey. The

preliminary aim of capacity design is to prohibit the formation of a soft storey by forming plastic

hinges in the beams of a multi-storey frame instead.

Figure 2-10 Shows plastic hinge rotations and deformations in beams [28] [29]

Figure 2-10 shows the formation and rotations experienced within a beam during the development of

plastic hinges under lateral loads on a frame. Figure 2-11 and Figure 2-12 show the typical detailing

for a beam within an “ordinary” and “intermediate” moment resisting frame.

It can be seen that additional ductility in the “intermediate” moment resisting frame is provided by a

large increase in the level of shear reinforcement (stirrups, etc).


May 2009


17

Figure 2-11 shows a standard detail for a typical beam.

Figure 2-12 shows the typical beam reinforcement for a beam in an intermediate moment resisting frame for AS 1170.4:1993.

Gurley [18] discusses the consideration of robust design in earthquake engineering. He highlights

the removal of structural elements in a seismic event, such as corner columns, could lead to

“progressive” or “disproportional” collapse, as is considered in terrorist attacks.


May 2009


18

The minimum standard of ductility detailing now relates to the importance level of the structure as

defined in the BCA [11].

Figure 2-13 shows a standard detail for a typical beam with bottom layer continuity steel provided at the support.

Earthquake performance and design for redundant elements are related and the most crucial

parameters for achieving robust requirements is continuity of bottom reinforcement bars through

columns and other intermediate supports and the provision of secondary reinforcement for shear

strength and confinement of compression bars for buckling. These additional requirements can be

seen in Figure 2-13 above and are defined in the material standards.


May 2009


19

Figure 2-14 shows a standard detail for a typical column to prevent the forming of a plastic hinge at the base of the column.

Figure 2-14 above shows a typical reinforcement detail with additional tie restraints provided at the

base of the column to allow the formation of a ductile plastic hinge. This detail is for intermediate

moment resisting frames and it should be noted that there is no requirement for additional ties in

ordinary moment resisting frame columns. Figure 2-15 show failure of a column due to low ductility

in a plastic hinge at the base. Lack of sufficient tie reinforcement is evident in the photograph.


May 2009


20

Figure 2-15 shows failure at the base of a structural column due to the formation of a plastic hinge

2.2.6 Capacity Design

In the traditional capacity (force based) design of structures for earthquake resistance, the main

elements of the primary lateral force resisting system are chosen and suitably designed and detailed

for energy dissipation under severe imposed deformations. The critical regions of these members,

often termed plastic hinges, are detailed for inelastic flexural action, with shear failure inhibited by a

suitable strength differential. By this method all other structural elements are then protected against

actions that could cause failure, by providing them with strength greater than that corresponding to

the development of maximum feasible strength in the potential plastic hinge regions.

The capacity design procedure is characterised by the following features:

Plastic hinge location and detailing – They are clearly defined and carefully detailed to ensure ductility demands are readily accommodated.

Inelastic deformation – Undesirable deformation due to shear, anchorage failures or instability, within members containing plastic hinges are inhibited by ensuring that the strength of these modes exceeds the capacity of the plastic hinges.


May 2009


21

Brittle element protection – Potentially brittle areas are protected by designing them to remain elastic irrespective of the intensity of the ground shaking or the magnitudes of inelastic deformations that may occur. This approach enables the traditional detailing of these elements, such as used for structures designed to resist only gravity loads or wind loads.

The area of greatest uncertainty of response of capacity-designed structures is the level of inelastic

deformations that might occur under strong ground motion. These designed ductile structures rely on

being very tolerant with respect to imposed seismic deformations due to the high level of detailing

of the potential plastic regions.

Figure 2-16 shows the failure of a column in a soft storey due to lack of ties to constrain the vertical reinforcement during large deflection demands.

In Figure 2-16 above it can be seen that the column failed under the large seismic deflection

demands required during the event.


May 2009


22

There are new methods being proposed at present such as a displacement based method by Wilson

et al. [34]. It is proposed that using a capacity spectrum a structures displacement demand and

capacity can be conveniently demonstrated to assess if “survival” of the element can be ensured.

Figure 2-17 shows a typical acceleration-displacement response spectrum (ADRS) which is plotted

for the full natural period range (0<T<infinity) of structures.

Figure 2-17 shows Wilson and Lams capacity response spectrum method [34][35][36]

The performance of the structure in terms of force and displacement can be readily observed for

both new and existing structures by creating a capacity line, constructed from a non-linear static

push-over analysis.

Neither the old nor the new code specify Capacity Design but research is continuing to produce

fragility curves for various structures to facilitate this method of design [31].

2.2.7 Hysteretic Loops

A further observation to be aware of is that for short period structures (say T=0.5s) reduction in the

energy dissipation in columns and walls compared with plastic hinge mechanisms in beams, will

imply larger ductility demand. Therefore, the plastic mechanisms chosen should be carefully


May 2009


23

considered. The method used to calculate the energy that can be dissipated by a plastic hinge is the

area inside a hysteretic loop.

Figure 2-18 shows typical force-displacement hysteresis loop shapes for elastic and inelastic systems during a loading and unloading cycle [34]

Figure 2-18 shows a typical hysteresis loop. Perfect ductility is defined by the ideal elastic/plastic

(elastoplastic) models of hysteresis loops. They show the response of elements in terms of inertia

force (mass x acceleration) versus displacement at the centre of mass.


May 2009


24

Figure 2-19 shows the comparison of hysteretic loops for an ideal case and where plastic hinges occur in a beam [30].

As can be seen in Figure 2-19 above when plastic hinges are located in beam elements the energy

dissipation is large and the behaviour is equivalent to elastoplastic behaviour. Kayvani and Barzegar

[19] discuss the benefits of finite element (FE) models to obtain hysteretic models for tubular

members. In their paper they show that hysteretic results for FE methods correlate satisfactorily with

experimental data. Hysteretic loops for elements give great understanding into the cyclic behaviour

of elements and are a very useful design tool.

2.3 Seismic Response and Structural Configuration

A buildings response, from an engineers view point, focuses on sway collapse mechanisms in which

the building as a whole moves sideways and may collapse under its own weight.

Figure 2-20 above shows typical structural systems and configurations that are used within multi-

storey construction. Diagram a) shows the development of a soft-storey. It is easily imagined that

excessive deflections at the top of the building would cause tilting of the lower columns and collapse

would occur.


May 2009


25

Figure 2-20 Shows plastic mechanisms in frame and wall systems; (a) soft-storey mechanism in a weak column/strong beam frame; (b, c) beam-sway mechanisms in a strong column/weak beam frame; (d, e) beam-sway mechanisms in a wall system

[16].


May 2009


26

2.3.1 Response in Elevation

When subjected to lateral forces only, a building will act as a vertical cantilever. The total resulting

horizontal force and the overturning moment will be transmitted at the level of the foundations as

can be seen in Figure 2-21 . Once the lateral forces, such as may act at each level of the building, are

known, the storey shear forces, as well as the magnitude of overturning moments at any level can

readily be derived from usual equilibrium relationships.

Figure 2-21 shows the response of a structure to lateral loads.

Tall and slender buildings and those with concentration of masses at the top may require large

foundations to enable large over turning moments to be transmitted in a stable manner. Irregularities

such as set backs and staggered floors should be avoided. In the new code [7] all buildings are

considered irregular, which is simpler and not unrealistic. Also pounding (knocking into the adjacent

structures) is not considered an issue if the building is set back 1% of the structural height from the

boundary.

Variations with height of both stiffness and strength are likely to invite poor and often dangerous

structural response. Because of the abrupt changes of story stiffness’s the dynamic response may be

dominated by the flexible storey or soft-storey. Reduced stiffness is generally accompanied with

reduced strength and this may result in large inelastic deformations in such a storey. Collapse of the


May 2009


27

building is imminent when the energy absorption capacity or displacement capacity of the soft-

storey columns is exceeded by the energy demand or displacement demand. This is the general

cause of the majority of collapses in recent earthquakes.

Figure 2-22 shows diagrammatically the acceleration displacement response spectrum for a range of natural period [32].

In Figure 2-22 the structural response of a building in relation to its height has been shown in the

acceleration displacement response spectrum format. The diagonal lines extending to the curved

portion of the diagram represent the acceleration-displacement behaviour of a linear elastic system.

2.3.2 Estimates of Deflection and Drift

The analysis carried out under the design-level forces will produce estimates of lateral deflections,

∆. However if these forces were calculated assuming a structural ductility of say µ it must be

realised that the actual deflections achieved will be greater than the elastic values predicted by

analysis. A reasonable approximation is;

∆total = µ ∆


May 2009


28

The above equation can only be applied to the displacement at the centre of seismic force which is

typically located at about two-thirds of the building height. It should be noted that storey drifts,

particularly in the lower floors of framed buildings, may be substantially higher than estimated by

multiplying elastic drifts by the structural ductility factor.

It must also be noted that displacements are associated with a mode shape.

2.3.3 P-Delta Effects in framed structures

When flexible structures, such as reinforced concrete frames are subjected to lateral forces, the

resulting horizontal displacement leads to additional overturning moments because the gravity load

is also displaced. Therefore in addition to the overturning moment produced by lateral forces, the

secondary moment (the product of gravity x ∆) must also be resisted. This additional moment will in

turn increase the lateral displacement. In very flexible structures, instability, resulting in collapse,

may occur.

Figure 2-23 Shows typical sway of multistorey frames [28][29]

Figure 2-23 shows the three types of multi-storey building frame arrangements with the same typical

the global displacement demands. This global displacement ductility parameter is defined on the

basis of the horizontal displacement of the building either at the roof or preferably at the height of

application of the resultant lateral force. This should be spread as uniformly as possible over the

entire height of the building and preventing a soft-storey developing.


May 2009


29

Figure 2-24 Amplification of column bending moments in ductile frames due to P-delta hinges [28] [29]

Figure 2-24 shows diagrammatically the P-delta effects on a structure. In Figure 2-24a) the

deflection of the building is shown due to lateral forces being applied. The gravity loads to be

supported by the column structures are highlighted and it can be seen that they are now not vertical

over the base of the column. Additional moment caused by the product of the deflection and the

gravity load is induced at the base of the column. In Figure 2-24c) it can be assumed that once a

plastic hinge forms in the beams that the additional moment from P-delta effects will not be

distributed into the beams increasing the resultant moment in the column. If moments increase to

levels where a plastic hinge forms in the column instability and failure will occur.

2.3.4 Response in Plan

A structures response in plan depends on two important concepts;

1) Centre of Mass – During an earthquake, acceleration-induced inertia forces will be generated at each floor level, where the mass of an entire story may be assumed to be concentrated. Hence the location of a force at a particular level will be determined by the centre of the accelerated mass at that level.

Note in AS1170.4:2007 [7] general requirements for all buildings have been provided for both regular and irregular. It should also be noted that irregular mass distributions


May 2009


30

over the height of the building may lead to variations in the centre of mass at different floors.

Figure 2-25 shows a force equal to the total resultant horizontal earthquake force and a moment acting through the shear centre [30]

2) Centre of Rigidity (Shear Centre) – The shear centre is observed to be the point through which if a load is acting there will be bending without twisting.

As the shear force induced by earthquake acts through the centre of mass and not the shear centre, there will be floor rotation as well as floor translation. For convenience in design we replace the shear force though the centre of mass with an equal force through the shear centre and a moment due to eccentricity. This is seen in Figure 2-25.

To reduce Torsion (storey twist) the distance between the centre of mass and the centre of rigidity

should be reduced.


May 2009


31

Figure 2-26 Shows various floor plans for symmetrical and unsymmetrical buildings. The shear centre and centre of mass relationship is shown [30].

Analysis may show that in some buildings torsional effects may be negligible. However, as a result

of normal variations in material properties and section geometry and effects of torsional components

of ground motion, torsion may arise also in theoretically perfectly symmetrical buildings. These

maybe due to the presence and participation in the structural response of elements such as stairs,

partitions and infill walls. Hence codes require that allowance be made in all buildings for so-called

“accidental” torsional effects. AS1170.4:2007 applies a much larger “accidental” torsion to

symmetrical buildings and will be discussed further in Section 3.9.

Amplification also occurs in the dynamic response of a structure. This amplification is dependent on

the relationship between the natural vibration frequencies of the structure, the magnitude of the static

eccentricities and the degree of inelastic response. The amplification effect is larger for structures

responding elastically, compared with those that are excited beyond the elastic limit.


May 2009


32

2.4 The influence of soil stiffness on earthquake magnitude and intensity

It is generally accepted that soft soils modify the characteristics of strong ground motion transmitted

to the surface from the underlying bedrock [35]. Physical properties of soil influence the degree of

amplification or attenuation experienced by the travelling waves, of which the most important

characteristics being impedance, absorption and basin geometry.

Figure 2-27 Sketch of some engineering topics [12].

As can be seen in Figure 2-27 the shear wave properties vary as they pass though different layers of

material. Venkatesan et al. [33] discuss the conservation of energy as the mechanism behind the soil

amplification phenomenon, observed where seismic waves pass from a material of high impedance

(rock or stiff soils) to that of a lower impedance (soft soils).

The impedance of a medium is represented by the product of density (r) and shear wave velocity

(V).


May 2009


33

2.4.1 Site Classification using Shear Wave Velocity and Bedrock Properties

McPherson [25] [26] established the requirement for site classification in Australia on a national-

scale, as an essential requirement for determining the potential response of structures, after the 1989

Newcastle earthquake.

Figure 2-28 Shows the first generation national site classification map of Australia based on modified NEHRP site classes [25][26].

The site classification map is based on the shear wave velocity of the top 30m below ground surface

(Vs30), but as data is often not available for sites, especially in Australia, the use of geology as a

surrogate for shear wave velocity has been used to group bedrock materials likely to exhibit a

similar response to earthquake ground shaking.

The National Earthquake Hazard Reduction Program (NEHRP) published a map of Australia for

country wide coverage for site classification. Local variability, however, must be considered for site

specific assessments.


May 2009


34

For example, Sydney’s Botany Bay contains some of the largest accumulations of Quaternary sands

and silts. Where this sediment exceeds 30m Vs30 values from 200-250 m/s are reported, equivalent to

a site class of D or De and the underlying Hawkesbury Sandstone has Vs30 values in the range of

1200-2500 m/s, class B. Where the basement is less than 30m Vs30 values increase and site classes

change accordingly. These findings are very important for establishing implications due to these soil

types within the Sydney area.

Venkatesan et al. [33] emphasise the high impedance contrast between bedrock and upper soils as

potential hazardous conditions for resonance, due to the containment of energy within the soil, i.e.

seismic waves reflected from the soil surface back down to the bedrock interface would be reflected

back up through the bedrock again. Wilson et al. [35] relates the periodicity of the soft soil surface

motion directly to the time taken for the incident, or reflected, wave front to propagate through the

soil medium. Consequently the site natural period Tg correlates very well with soil depth H.

Figure 2-29 shows the earthquake shear waves propagating from the focus of the event.


May 2009


35

Also directional effects and geographical features have significant influence on local intensity of

ground motion, as does the peak ground attenuation relationship of the soil with distance from the

epicentre. Figure 2-29 shows waves propagating from the focus of a seismic event.

Figure 2-30 shows a schematic diagram illustrating local geology and soil features [14]

In Figure 2-30, soil conditions and local geological features are illustrated and are described below:

The greater the horizontal extent (L1 or L2) of the softer soils, the less the boundary effects of the bedrock on the site response.

As discussed previously, the depth of the soil (H1 or H2) affects the dynamic response of the site. With the natural period of the site increasing with depth.

The slope of the bedding planes (valley 2 and 3) affect the dynamic response but it is less easy to deal with non-horizontal strata.

Changes of soil types horizontally across a site (F and G) affect the site locally especially if a building straddles the two soil types.

Ridges and valleys can amplify resonance behaviour of a site.

Slopes of sedimentary may, of course, completely fail (H)

The above conditions are site specific and are not considered individually in the old or new codes.

The new code considers soil depth only and special studies should be considered under any of the

above mentioned site conditions.


May 2009


36

2.4.2 Site Classification using Site Natural Period

Venkatesan et al. [33] proposes the use of the sites natural period as a parameter for site

classification as it has the attribute of providing an objective, and direct, representation of the risk of

a soil site developing resonance behaviour.

Soil resonance can lead to catastrophic failures, when buildings and soils have the same natural

period. The old code [4] code does not specifically parameterise the natural period of the site which

characterises potential resonance behaviour. In theory, resonance can be avoided by ensuring that

the natural period of the building is not close to the natural period of the site, however this can be

difficult to define. If a structure is designed with an initial natural period significantly lower than the

natural period of the site, the stiffness degradation to the building that occurs during an event will

increase the natural period and could subsequently shift it closer to the natural period of the site. In

the new AS1170.4:2007 [7] code the natural period for the site has been incorporated as a site

classification criterion.

Venkatesan et al [33] presents an Extended Component Attenuation Model (ECAM) that

incorporates site period, shear wave velocity, soil damping properties, impedance contrasts with the

bed rock and frequency content of the earthquake excitation.

S = Sξ.Sλ.Sψ.Sτ

Where,

Sξ represents the effects of hysteretic and viscous damping.

Sλ represents the effects of impedance contrasts between soil and bedrock.

Sψ and Sτ both represent the effects of the form of the shear wave velocity profile.

The ECAM model predicts the soil amplification factor (S) which is defined as the ordinate of the

soil response spectrum at the fundamental natural period of the site divided by the respective

response spectrum ordinate of the rock outcrop. Since the model is intended to fully account for the


May 2009


37

effects of resonance, the predicted factor is expected to be generally higher than that stipulated in the

AS1170.4:1993 [4] code.

Wilson et al. [35] soil amplification factors have been adopted in the new code and are much larger

than previously used in the short-period range and for only the most onerous soil class in the longer-

period range.

In this report the implications to design, of site natural period accounting of resonance, applied in the

new code [7] will be discussed further in Section 3.3.

2.5 Seismic Risk

Severe natural hazard events are an inherent risk within the Australian environment and Seismic

disasters in Australia have resulted in significant loss of life and property. Therefore it is vital that

our understanding be improved. Gibson [17] acknowledges that the understanding of earth structure

and processes is necessary for earthquake risk mitigation. Risk mitigation actions can be associated

with past, present and future earthquakes.

The term earthquake hazard refers to the occurrence probability of damaging ground motions,

exclusively related to natural phenomena and processes, while risk and loss results from the

combing the earthquake hazard with the vulnerability of the building stock. Bungum [12] points out

that it is the combination of earthquake magnitude, poor building quality, and high population

density of the area of highest shaking that causes the disasters.

To assess the seismic risk associated with a given site, it is necessary to know not only the

characteristics of strong ground shaking that are feasible for a given site, but also the frequency with

which such events are expected. It is common to express this by the return period of an earthquake

of given magnitude, which is the average recurrence interval for earthquakes of equal or larger

magnitude.

From past experiences, average recurrence intervals between occurrences of earthquakes of given

magnitude can be obtained and they can be used to estimate the probability that the design ground

motion will be exceeded during the life of the structure (Hazard maps in AS1170.4: 2007). Wilson


May 2009


38

and Lam [36] present the history of the creation of these maps and the “bulls-eye” type contours

which coincide in location with recorded earthquake epicentres, and also current multidiscipline

research into the understanding of spatial distributions of seismic activity. By delineating the earth-

quake areas and to understand in detail the factors that turn an earthquake into a disaster is the

prevention method best employed to prevent the same.

Small earthquakes occur more frequently than large earthquakes and can generate peak ground

accelerations of similar magnitude to those of much larger earthquakes, but over a much smaller

area. The quantification of seismic risk at a site thus involves assessing the probability of occurrence

of ground shaking of a given intensity as a result of the combined effects of frequent moderate

earthquakes occurring close to the site, and infrequently larger earthquakes occurring at greater

distances. This also implies that the earthquake damage increases strongly with decreasing

occurrence probability (increasing return period), which in turn means that the largest ones are rare

but very destructive.

2.5.1 Design Limit States

The acceptable risk for a level of structural response will depend on the social and economic

importance of the building. These risk values are set out in the BCA [11]. The three levels or limit

states are serviceability, damage control and survival limit state. A hospital, for example, should be

designed for the survival limit state due to its requirement in post disaster service. Earthquake

disasters are of course caused by the combination of strong ground shaking and buildings having

low structural capacity, thus showing a poor performance during earthquake action and being unable

to withstand the shaking without damages.

It should be noted that seismic and wind loadings are not the only lateral considerations that need to

be designed for. In AS1170.0 [1] a minimum lateral resistance equivalent to 2.5% of (G+ψcQ) is to

be applied for robustness considerations, however, this is currently under review and intends to be

reduced to 1% for buildings over 15m tall and 1.5% for all other structures. It will be shown in

Section 3.8 that the minimum seismic loadings to be applied to structures 15m in height or less, will

be a lot more onerous than this lower 1% limit. In AS1170.4:2007, the minimum 1% of seismic

weight limit has been eliminated, therefore this new 1.5% limit for robustness takes precedence for


May 2009


39

taller structures. With the application of lateral design revisions to this new AS1170.4:2007 code, it

will have to be considered that all detailing requirements for both ductility and robustness need to be

reviewed in parallel.

2.5.2 Serviceability Limit State

Guidelines on limits for the design of members for serviceability are given in Appendix C of

AS1170.0:2002. It identifies deflection limits related to actions with an annual probability of

exceedance of 1:20 beyond which serviceability problems have been observed. Such boundaries for

acceptance are imprecise and should be treated as a guide only. They set out possible control

phenomena, such as acceptable response which is observer dependant. For the earthquake load

actions applied to walls and facades noticeable cracking and façade and glass damage is acceptable,

however the environment of the observer influences the tolerance of people to sensory deflection

and acceptable damage limits.

2.5.3 Economic Considerations

The main factor that can be economically quantified is the cost of providing a given level of seismic

protection, since this is objective. The key unquantifiable factor is the value of human life, which is

subjective and controversial. Some cost considerations are listed below:

Initial cost of providing increased seismic resistance

Reduced cost of repair and replacement, both structural and non structural as a result of damage or collapse

Reduce loss of revenue resulting from loss of serviceability

Reduced costs caused by third-party consequences of collapse

Possible reduced insurance costs

Reduced costs arising from injury or loss of life

The extent to which the initial cost is balanced by the latter factors depends on circumstances, i.e.

specific client requirement, site location, economic state of the country the structure.


May 2009


40

However the cost to provide increased seismic resistance is generally significantly less than

believed, as costs of doubling strength are only a fraction of a percent within the total building cost

and the costs associated with providing increased ductility are even less as it’s a detailing process.

2.6 Design Methods

There are three traditional analysis methods available to the designer to estimate the seismic forces

applied to a multi-storey building; Dynamic inelastic time-history analysis, Modal superposition

techniques and Equivalent lateral force procedures. Wilson and Lam [34][35][36] have also

developed a Capacity spectrum method.

2.6.1 Dynamic inelastic time-history analysis

This method is a very complex analysis and involves the solving the equations of motion of the

system using a time interval approach. It is generally not used in the preliminary stages as strengths

and stiffness’s of individual members are required. It requires the input of earthquake characteristics

and therefore interpretation of the results is dependant on assumptions made and can imply

considerable uncertainty in the predicted response. In this report, the computer aided design

package, ETABS, is used to carry out a dynamic analysis comparison between the two AS1170.4

[4][7] codes.

2.6.2 Modal superposition techniques

This method is an elastic dynamic approach and relies on the assumption that the dynamic response

of a structure may be found by considering the independent response of each natural mode of

vibration and then combining the responses in some way. Its advantage is that only a few of the

lowest modes of vibration have significance when calculating moments, shears and deflections at

different levels of the building.


May 2009


41

Figure 2-31 Shows an example of modal superpositioning [12].

The response to a given accelerogram in each significant mode of vibration is calculated as a time

history of forces and displacements and these responses are combined to provide a complete time

history of the structural response.

Combining the modes must be considered carefully as direct numerical addition is not consistent.

An example of this is if we consider the first two modes of a cantilever system and the modes are in

phase at the top of the system then they will be out of phase at mid height.

Careful consideration of combining modal contributions must be given to structures that have modal

frequencies very close together, which can occur in buildings with symmetrical floor plans,

subjected to torsional response as a result of eccentric mass.


May 2009


42

Figure 2-31 shows an example of the combining of modal moments to get the total base seismic

moment i.e. the square root sum of squares scheme (SRSS).

The drawback to this approach is that it is based on elastic response. The applicability of modal

superposition decreases with the reliance of ductility. It also relies on structural and earthquake

characteristic inputs at an early stage therefore it must be remembered that there are large

uncertainties in the results due to assumptions made.

2.6.3 Equivalent lateral force procedures

This method when combined with a capacity design philosophy, ensuring that ductility can occur

only in carefully selected and detailed plastic regions, it is still the most used method for designing

seismic resistance. This force-based method trades-off strength with ductility to ensure that the

structure has sufficient energy absorption capacity although as seen in the reference papers

[31][33][34][35] a move towards a displacement demand method is being recommended and

described below.

2.6.4 Capacity Spectrum Method

Wilson and Lam [34][35][36] have recommended a method to obtain the seismic demand by using

an acceleration-displacement response spectrum and the structural capacity curves (obtained by non-

linear push-over analysis) as shown in Figure 2-32, Figure 2-33 and Figure 2-34. The structure is

considered to survive the design earthquake if the capacity curve intersects the demand curve and

collapses if the curves do not intersect.


May 2009


43

Figure 2-32 shows Wilson and Lams capacity spectrum approach [34] [35] [36]

This method has been used to establish fragility curves for use in risk modelling. Rodsin et al. [31],

uses the CSM (capacity spectrum method) to present fragility curves for soft-storey buildings.

Figure 2-33 shows the push-over analysis of a building with a soft-storey [34]

This is beneficial as although the aim of capacity design is to prevent the creation of a soft-storey as

they are notoriously vulnerable to collapse under strong earthquake motion, depending on the drift


May 2009


44

demand on the soft-storey; a building subjected to a small or moderate magnitude earthquake has a

fair chance of survival.

Wilson and Lam [36] identify that research is currently being undertaken into retrofitting techniques

for improving the drift capacity of soft storey structures, and this highlights the question, what are

implications of revisions in the new code to existing structures and possible retrofitting

requirements?

Figure 2-34 Bi-linear approximation of the push-over curve [34]

2.7 Discussion

This chapter gave a brief synopsis of the theory for seismic design, as well as looking at the

advancements in six of the more significant topics that contribute to the revision of the Earthquake

code for Australia [4][7].

Section 2.1 – Structural Systems, highlighted the choice of structural systems. It is vital that the

performance of the structure mirrors the design assumptions made.

Section 2.2 – Ductility - Elastic and Inelastic Response, discusses the background in elastic and

inelastic theory required for the understanding of ductility design and detailing. Large background


May 2009


45

knowledge is still required by the engineer prior to using the coefficients provided in the code for

ductility and structural performance coefficients.

Section 2.3 – Seismic Response and Structure Configuration, looked at approaches to building

response due to geometry and physical attributes. The problems that effect structures in elevation

such as deflection demand were discussed. Consideration of deflection, ductility and loads are all to

be defined within a design philosophy prior to analysis being carried out. Response of the structure

in plan is based on torsional effects. The building responds due to torsion under the laterally applied

loads having an eccentricity at each floor level and also an accidental torsion load is considered to

allow for the torsional effects of the ground motion. As a result of geometrical influences for the

comparison of the codes on structures the decision was made to consider three different heights of

structures, but maintain a symmetrically responding system in plan in one direction while

unsymmetrical in the other.

Section 2.4–The influence of soil stiffness on earthquake magnitude and intensity, discusses the

problem of adequate earthquake input characteristics into analysis that needs to be addressed when

considering site soil impact on design. The work of most interest from this section is that of

Valkatensan et al. [33], which demonstrated the requirement to consider the natural period of a site

in the structures response to a seismic event, due to resonance and Also McPherson and Hall’s [26]

suggested soil classifications of D or De for Sydney’s Quaternary sand. It was detracted from this,

that a soil class of Ae and De should be considered to demonstrate the influence on structures in this

report that would be of most benefit to local design.

Section 2.5 – Seismic Risk, describes how the prediction of the frequency and size of events is

crucial to assessing the Risk of structures to seismic events. The work involved in forecasting

seismic events is outside the scope of this report, however, the recognition of designing to a

structures importance level is of consequence.

Section 2.6– Design Methods, looks at the traditional design methods and how they can be applied,

as well as highlighting some of the strengths and weaknesses associated with the different

approaches. Although Dynamic inelastic time-history, modal superposition and equivalent lateral


May 2009


46

force procedures are capable of modelling relatively complex building structures a new capacity

spectrum method is being developed and is currently being implemented into design analysis.

However, the established techniques are still intended for use within the new AS1170.4 code [7].

Therefore it was decided to compare the traditional methods for this report, and establish a basis for

consequences. For this reason the newly developed Capacity Spectrum Method was avoided.

The next chapter discusses the direct code differences, by comparing and contrasting factors and

calculation methods.


May 2009


47

3 AS1170.4: 1993 & 2007 CODE COMPARISON

With the introduction of the new code AS1170.4 – 2007 Earthquake Actions in Australia (AS-

2007), it was vital to establish the consequences to the design of structures. This chapter describes

how the old and new Codes [4] [7] are layed out and how they differ in Notation, Factors and

Calculation of the Design Base shear. The chapter concludes by looking at the main differences

between the codes and determines if these differences would have a major effect on the design of

current and future design projects.

3.1 Code Layout Comparison

The old and the new codes [4] [7] vary in presentation and therefore firstly a brief summary of the

revisions should be described.

3.1.1 AS1170.4:1993 Layout

The old code is layed out in 8 Sections and has 5 Appendices A through to E. A brief description of

each section is presented below.

Foreword – Flow charts are provided to establish if a structure needs to be designed for earthquake

loads.

1 Scope and General – The scope of the code is set out in this section, stating phenomena and

structures not covered by the code. Reference documents, definitions, notation and load

determination and combinations are also described.

2 General requirements – Requirements for structural and system classification, coefficients,

factors and limits are all described in this section.

3 Domestic structures – Covers the requirements for domestic structures.

4 Structural detailing requirements for general structures – The detailing of the structural force-

resisting system, including minimum forces to be resisted are set out in this section.


May 2009


48

5 Requirements for non-structural components – This section describes the requirements for

non-structural components, categorised as mechanical, electrical or architectural.

6 Static analysis – The structural properties, horizontal forces, distribution and effects are defined in

this section for the static analysis of structures.

7 Dynamic analysis – The earthquake actions and analysis procedures are established in this

section.

8 Structural Alterations – This section does not apply for domestic structures and states that

alterations are permitted provided that the resistance to horizontal earthquake forces is not less than

before alterations were made.

A Structure classification (informative) – Expands on the identification of structural

configurations.

B Structural systems (informative) – Defines the classification of the structural systems and states

the appropriate material codes to be used for design and detailing.

C Domestic structures (informative) – Gives guidance on improving horizontal earthquake

resistance for domestic structures.

D Types of dynamic analysis (informative) – Gives guidance and technical references for types of

dynamic analysis.

E Structural Alterations (informative) – Gives guidance on the strengthening required to existing

buildings due to structural alterations.

3.1.2 AS1170.4:2007 Layout and Revisions

The new code is also layed out in 8 Sections but has only 1 Appendix A. The layout and brief

description of each section is presented below.


May 2009


49

Preface – A comprehensive list is presented of all the revisions that have been made in the new

code. There have been 20 revisions noted.

1 Scope and General – The scope of the code is set out in this section, stating phenomena and

structures not covered by the code. The most significant revision is that structures with first mode

periods greater than 5 s have been excluded by this new code.

Normative reference documents, definitions and notation are described as in the old code. Loading

and combinations have been removed and are now given in the AS/NZS 1170.0 and the BCA

[1][11].

It should be noted that the term “normative” means that it is integral part of the code.

The structural components to be included in the calculation of the seismic weight and the position of

application have been described in this section with provision of illustrations for easy reference.

2 Design procedures – Requirements for structural and system classification and sets out the design

procedure to be used and a flow chart is provided for design reference requirements.

All clauses for domestic structures have been simplified and moved to the Appendix A.

Structural type classification has been replaced with importance level reference, as per AS/NZS

1170.0:2002 Appendix D. Importance level 1 structures do not require any analysis or detailing to

this code.

3 Site hazard – Coefficients and factors relating to the site hazards are described in this section. In

AS/NZS 1170.0:2002 the importance factors were replaced with the annual probability of

exceedance, to enable design to be set by the use of a single performance parameter. Values of

hazard are determined using the return period factor determined from the annual probability of

exceedance and the hazard factor for the site.


May 2009


50

4 Site sub-soil class – Soil profile descriptors have been replaced with 5 new site sub-soil classes.

Site factors and the effect of sub-soil conditions have been replaced with spectral shape factors in the

form of response spectra that vary depending on the fundamental natural period of the structure.

5 Earthquake design – This section describes all the requirements and limitations for the

earthquake design categories.

6 Equivalent static analysis – The structural properties, horizontal forces, distribution and effects

are defined in this section for the static analysis of structures.

The equation for base shear has been aligned with international standards and therefore notation and

factors have been revised.

Due to the new site sub-soil spectra, adjustments were needed to simple design rules throughout the

standard.

A new method has been introduced for the calculation of the fundamental natural period of the

structure.

The clause on torsion effects has been simplified, while the clause on stability effects has been

removed.

7 Dynamic analysis – The earthquake actions and analysis procedures are established in this

section.

The basic dynamic methods have not changed, however, scaling of results have been removed.

8 Design of parts and components – The section on structural alterations has been removed. In this

section all clauses on parts and components have been simplified.

A Domestic structures (normative) – All the “informative” Appendices have been removed. This

appendix gives guidance on improving horizontal earthquake resistance for domestic structures.


May 2009


51

3.2 Site Hazard

The acceleration coefficient, a, “an index related to the severity of earthquake ground motion” in the

old code [4], is now notated as the Hazard Factor, Z in the new code [7].

Figure 3-1 shows the Earthquake Hazard Map of Australia from the AS1170.4:2007 code.

Wilson and Lam [36] tell of the uncertainty in the rate of seismic activity of individual faults and

how earthquake sources have been modelled as “polygonal area source zones” on the Map of

Australia. The size and geometry of these source zones have been delineated in accordance with

information of localizing geological structures of “groups of faults” that have the potential of

generating future earthquakes. Figure 3-1 shows the hazard factor Z map of Australia from the new

code [7]. This map shows the “bulls-eye” effect of the contours that have been the locations of

historical events.

The maps used represent the 10% chance of exceedance in 50 years of the derived force,

corresponding to the average recurrence level of approximately 500 years. Therefore there is a 10%


May 2009


52

chance that a 500-year earthquake will occur during a 50 year period, and a 90% chance that it

won’t.

1993 2007 City A Z

Brisbane 0.06 0.05 Decreased

Darwin 0.08 0.09 Increased

Gold Coast 0.06 0.05 Decreased

Hobart (Tasmania) 0.05 0.03 Decreased

Launceston (Tasmania) 0.06 0.04 Decreased

Wollongong 0.08 0.09 Increased Table 3-1 lists six of the listed cities that have a revised value for the Hazard Factor.

Table 3-1 lists six of the cities around Australia that have revised site hazard factors however the

contour maps have remained the same. Darwin for example in both the old and new maps was

located on the 0.09 contour therefore this revision is more a correction of the old table than an

increase in the hazard factor.

3.2.1 Return Period Calculation

General probability P is the probability that an event will occur or be exceeded during an interval of

n years. The annual probability of occurrence of an event is approximately equal to 1/R (for large R

in years) and the annual probability of non-occurrence is approximately 1 - 1/R.

The probability that it won’t be exceeded during the design life L-year period is: (1.0 – 1/R) L.

The probability/risk of exceedance (r) that it will be exceeded during the L-year period is 1.0 minus

the probability of non-exceedance.

r = 1 – (1 – (1/R)) L

To find the return period therefore for the 10% chance of exceedance in 50 years, we use the

equation above and use a design life of 50 years.

0.10 = 1 – (1 – (1/R)) 50


May 2009


53

(1- 0.10) 1/50= 1 - (1/R)

(0.90) 1/50= 1 - (1/R)

(-0.0022) = - (1/R)

R = 475 years

Therefore there is approximately a 10% chance that a 500-year earthquake will occur during a 50

year period, and a 90% chance that it won’t.

With the introduction of AS 1170.0 – 2002 [1] an Appendix D [6] had to be introduced in order to

be able to use the old code [4] with the new requirements in Part B1 of the BCA [11]. The policy

criteria are in the form of importance levels and the associated annual probabilities of exceedance.

In Appendix D, the importance factor (I) was replaced by the variation of annual probability of

exceedance. This is expressed by the probability factor kp, applied to the acceleration coefficient (a).

The new adjusted value for (a = kp x a) was then to be used wherever (a) occurs.

The probability factor allows for the use of annual probability of exceedance as a means of setting

the level of performance. The structure type reference was replaced by importance level.

The probability factor is required to scale spectra to return periods other than 500 years, as required

for the serviceability limit state and for various combinations of structural importance level and

reference periods. A portion of the probability factors are shown below and compared

Probability Factor (kp) Annual Probability of Exceedance 2002 2007

1:2500 1.80 1:1500 1.50 1:1000 1.40 1.30 1:800 1.25 1.25 1:500 1.00 1.00 1:250 0.75

Table 3-2 this table shows the differences between the Probability factor (kp) for AS1170.0:2002 Appendix D [6] and the AS1170.4:2007 Values [7]


May 2009


54

Table 3-2 shows the difference in the values of the probability factor kp for the AS1170.0:2002 code

[6] and the new code [7]. The values shown in Table 3-2 are similar to the New Zealand Return

Period Factor Rs and Ru, Refer to Figure 3-2 (extract from NZS 1170.5 Supp1:2004 [29]). This is

contrary to beliefs that for a low seismic country such as Australia, a ratio of the hazard factor

between the 2500yr and the 500yr event would be greater than the same ratio for a high seismic

country such as New Zealand.

Figure 3-2 Comparison of proposed R-Factors for New Zealand with Hazard curves for 0.5s Spectral Accelerations [29]

In Figure 3-2 the values for kp have been derived by drawing a representative line through the hazard

curves (response spectrum acceleration as a function of return period) normalized by the 500 year

values for various structural periods for a range of locations. An equivalent representative line for

Australia would be expected to have been chosen. This response factor does not consider the

magnitude of the event and therefore is a scaling factor only.


May 2009


55

BCA Table B1.2b

Annual probability of exceedance

Importance Level 2007 Previous

1 1:250 1:500 2 1:500 1:500 3 1:1000 1:500 4 1:1500 1:800

Table 3-3 this table shows the differences between the current and previous annual probability of exceedance values from the BCA [11].

There has also been significant revision to the BCA [11] decreasing the annual probability of

exceedance values for importance level 3 and 4 structures, see Table 3-3.

With the decrease of the 3 and 4 Importance level to an annual probability of exceedance of 1:1000

and 1:1500 respectively means a multiplication factor of 1.3 and 1.5 on the base shear value rather

than a 1.0 and 1.25 previously used. In the new code [7] a special study is required for importance

level 4 classification to ensure that they remain serviceable for immediate use following the design

event for importance level 2 structures. That is to say that the building deflections are to be

calculated using a kp factor of 1.0 for the seismic load.

Most structures will now have to be designed for some earthquake actions to ensure minimum levels

of robustness.

3.3 Site Factor / Sub Soil Class and Spectral Shape Factor

Both the new and old codes use response spectra to define the magnitude of peak response of a

single degree of freedom system to a given seismic event. In earthquake engineering, response

spectra for a defined level of strong ground shaking are commonly used to define peak structural

response in terms of peak acceleration, velocity and displacement. a site and structural response.

However the response spectrum used to define the site classification in the new code has been

revised.


May 2009


56

3.3.1 Response Spectra and Spectral Shape Factor (Ch(T))

In the new code the spectral shape factor is introduced and is the combined multiplication factor

taking into account the sites condition and the structural period of the building. The resonance of the

site has large implications on the amplifying of the ground motion.

Response spectra are derived by the dynamic analysis of a large number of single degree of freedom

oscillators to the specified earthquake motion. Variables are the natural period of the oscillator and

the equivalent viscous damping. Although duration effects are not taken into consideration in

response spectra they identify easily the key design parameter of peak response that is used in

equivalent static response design.

Figure 3-3 shows the RSA acceleration and RSV velocity response spectra [34]

Wilson et al. [34] discuss the recommended earthquake response spectrum, for rock sites, in

Australia and the revision requirements to the old code [4] due to significant developments in the

area of response spectrum analysis. The results of the findings are shown in Figure 3-3 and Figure

3-4.


May 2009


57

Figure 3-4 shows the RSD, displacement response spectra [34]

It is shown that the old code [4] is conservative as velocity and displacement responses increase

indefinitely with increasing natural period, which does not reflect the physical reality. Equations for

the calculation and producing of response spectra are shown in Figure 3-5. These response spectra

have been adopted into the code and the implications to the base shear multiplier has been presented

in Section 3.8.

It is shown, in Figure 3-6, using a tripartite response spectra, that for the corner period controlling

maximum response spectral displacement a conservative recommendation of T2 =1.5secs is used for

Australia. The corner period T1 controlling the maximum response spectral acceleration, is in the

range of T1 =0.3-0.4secs. Note the period at which peak elastic response occurs depends on the

earthquake characteristics and the ground conditions.


May 2009


58

Figure 3-5 shows the displacement, velocity and acceleration response spectrum format [22]

The first corner period (T1) separates the two parts of the response spectrum and varies with soil

type. The response spectrum of this form indicates that the structural systems with natural period

less than T1 are subjected to the most onerous seismic actions, and with systems with periods

exceeding T1 seismic actions would decrease rapidly with increasing natural period.

For elastic response, peak acceleration, velocity and displacement are approximately interrelated by

equations of sinusoidal steady state motion. The interrelations enable peak velocity and

displacement to be calculated from peak acceleration.


May 2009


59

Figure 3-6 Recommended response spectrum model in tripartite presentation [21]

As seen in Figure 3-6, the tripartite response spectra include acceleration, displacement and velocity

information on the one logarithmic graph. The following equations show the interrelationships for

creating an acceleration displacement demand curve, shown in Figure 3-7:

T1 = 2π (RSVmax/RSAmax)

T2 = 0.5 + 0.5 (M-5)

In AS1170.4 1993: RSAmax = 2.5(PGA) = 2.5(a)S

In AS1170.4 2007: RSAmax = 3.0(PGA) = 3.0(KpZ)Fa

Where M is the magnitude of the seismic event, PGA is the peak ground acceleration/coefficient,

a/Z

Lam and Wilson [21] specify 3(kpZ)Fa where “3” (instead of 2.5) reflects the well known

phenomenon of high spectral amplification in the short period range with interplate earthquakes. In

calculating the acceleration response spectrum model the formulae have been revised to obtain a

more realistic response level to take account the resonance.


May 2009


60

In AS1170.4 1993: RSVmax = 1.8(PGV)S

In AS1170.4 2007: RSVmax = 1.8(PGV)Fv

In both codes RSDmax = (T2 / 2π) RSVmax

Where PGV is the peak ground velocity was 750 a and is now 750KpZ.

Figure 3-7 shows the demand curve consistent with the AS1170.4 model [34] [35]

In Figure 3-7, Fa is the site coefficient for the acceleration controlled region of the response spectrum

and Fv is the site coefficient for the velocity and displacement controlled region of the response

spectrum. In the new code the value of T2 is implicitly taken as 1.5secs based on an upper moment

magnitude limit of M7.

This maximum acceleration can be obtained in the new code [7] using the constant values of Ch(T)

for each sub-soil class, from Table 6.4, multiplied by the Z factor.

3.3.2 Site Classification

The new code [7] determines the Site class according to both soil type and depth, which determines

the site’s dynamic stiffness and period. The soil type and depth are major factors in determining the


May 2009


61

site’s dynamic response characteristics, along with the impedance contrast with underlying rock, the

damping of the soil, and its degree of nonlinearity.

The Sub-Soil Class Factors S values in the new code [7] have been increased significantly compared

to the old code [4] Site Factor, S. However the new values of S have not been defined in the new

code [7]. The Sub-Soil class factor has now been combined in the spectral shape factor.

Values for S, in the Table 3-4 are from Wilson and Lam’s paper [22] and are the recommended

values for a return period of 500yr.

1997 2007

Site Factor S Sub-Soil Class

0.67 Strong Rock Ae (0.8 Fa) (0.8 Fv)

1 Rock Be (1.0 Fa) (1.0 Fv)

1.25 Shallow Soil Ce (1.25 Fa) (1.4 Fv)

1.5 Deep or Soft Soil De (1.25 Fa) (2.3 Fv)

2 Very Soft Soil Ee (1.25 Fa) (3.50 Fv) Table 3-4 shows the difference in the Sub-Soil Class values [21] [22]

For sites consisting of layers of several types of material, the low amplitude natural period of the site

may be estimated by summing the contributions to the natural period of each layer. Venkatesan et al.

[33] present a simple model to establish the soil amplification factor (S) for the soil resonance

phenomenon, and was described in Section 2.4. The model takes into account the generic

classification of soil with depth and also the lateral profile. The new code [7] states that the

contributions of each layer may be estimated by determining the soil type of each layer and

multiplying the ratio of each layer’s thickness to the maximum depth of soil for that soil type by

0.6s.

Both the old and new codes do not consider the effect on a structure of earthquake induced

settlement, slides, subsidence, liquefaction or faulting. However, in the new code for structures sited

on sub-soil Class Ee (except houses in accordance with appendix A) the design shall consider the

effects of subsidence or differential settlement of the foundation material under the earthquake

actions determined for the structure. This will require a special study to be carried out.


May 2009


62

Particular caution is required where different foundation types or supporting soils occur under the

same building. In such cases the differing foundation stiffness’s may have a direct influence on the

distribution of seismic forces in the structure. Ignoring foundation flexibility will generally lead to a

conservative assessment of the seismic forces, but it is likely to result in a low estimate of the

seismic deformations.

Neither the old or new code covers the difficulties in assigning an appropriate site class for

structures founded on piles that extend through soil to a stronger, less flexible layer. However it

should be noted that in general, the classification of a site will be dependent on the surface soils even

where vertical piles or piers extend down to a harder underlying stratum, in that it is these that drive

the structural response. However, with raking piles or with stubby vertical piles or piers, the possible

adverse effects of the upper layers of soil can be reduced by considering the stiffer foundations

below. Also there may be situations with sleeve piles and specifically-designed separation of the

structure above the basement from the surrounding soil, as occurs in some type of seismic isolation

for example, where the structure is clearly likely to be subjected to the motions in the underlying

stratum rather than those of the surface soils.

3.4 Selection of Earthquake Design Category

The design requirements are constructed to reflect the relationship between the use of a structure and

the level of earthquake motion it may be exposed. The main concern with earthquake design is the

protection of life and the degree of exposure of the public to earthquake risk.

3.4.1 AS1170.4:1993

In the old code [4], there were five earthquake categories, A through to E. The intensity of the event

predicted gets more sever as you move from A to E. Each design category relates the soil type,

ground acceleration and importance factor based on type of structure. Once a design category has

been defined for a structure certain limits are placed on construction material, detailing or geometry.


May 2009


63

AS1170.4:1993 Earthquake Design Category

Soil Factor Sydney (I = 1.0) Design (I = 1.25) Design

S a aSI Category aSI Category

0.67 0.08 0.0536 C 0.067 C

1 0.08 0.08 C 0.1 D

1.25 0.08 0.1 D 0.125 D

1.5 0.08 0.12 D 0.15 D

2 0.08 0.16 D 0.2 E Table 3-5 shows the design category selections for Sydney using the AS1170.4:1993 code.

Table 3-5 shows the earthquake categories for all types of structure (I, II and III) using an

importance factor (I) value of 1.25 for type III structures. Note type III structures are now classified

as Importance level 4 structures required for post disaster facilities.

For Earthquake design category D, it was stated that regular structures only required a static analysis

while irregular structures required a dynamic analysis. This is therefore very open to interpretation,

and can lead to structures being classified as regular but having inherent irregular qualities that have

not been considered.

Earthquake design category E has the same analysis requirements for regular and irregular structures

but has large limitations on structural systems being used. However, this is only defined for

importance level 4 structures on the worst soil classification in the Sydney area.

Bearing wall systems are limited to 50m

Building frame systems are limited to 70m

Moment resisting frame and dual system with height over 30m requires a special moment resisting frame to continue down to the footing.

However, the worst alluvial soil condition is not very common in the Sydney area. Therefore

earthquake design category D is the most prevalent.


May 2009


64

3.4.2 AS1170.4:1993 (with AS1170.4:2002 Appendix D considerations)

With the introduction of AS1170.0:2002, which required the application of importance levels rather

than building type, and the probability of exceedance terminology revisions to the earthquake design

category selection table occurred.

However, as can be seen in Table 3-6 there were no revisions to the category definitions it was

purely a notational change.

AS1170.4:1993 & AS1170.0:2002 Appendix D Earthquake Design Category

Soil Importance Level

1,2 & 3 Importance Level 4

Factor Sydney 1/500 Design 1/800 Design

S a Kp kpaS Category Kp kpaS Category

0.67 0.08 1.0 0.0536 C 1.25 0.067 C

1 0.08 1.0 0.08 C 1.25 0.1 D

1.25 0.08 1.0 0.1 D 1.25 0.125 D

1.5 0.08 1.0 0.12 D 1.25 0.15 D

2 0.08 1.0 0.16 D 1.25 0.2 E

Table 3-6 show the change in terminology for earthquake design categories in Sydney using the AS1170.4:2002 Appendix D

3.4.3 AS1170.4:2007

In the new code [7], there are three earthquake categories, I through to III. The intensity of the event

predicted gets more severe as you move from I to III. The simple description is as follows:

I – a minimum static check

II – static analysis

III – dynamic analysis

As in the 1993 code, each design category relates the soil type, ground acceleration and importance

factor based on type of structure but they have also considered the height of the structure.


May 2009


65


Importance Level 2

Height <=12 12 < Height < 50 Height > 50

Sydney 1/500 Design Design Design

Soil Factors Z kp kpZ Category Category Category

0.8 0.08 1 0.08 I II III

1 0.08 1 0.08 I II III

1.4 0.08 1 0.08 I II III

2.25 0.08 1 0.08 NA II III

3.5 0.08 1 0.08 NA II III

Table 3-7 shows the earthquake design categories for Importance level 2 structures in Sydney using AS1170.4:2007

As the new code gives general requirements for all structures (regular or irregular), the inclusion of

height in the definition of earthquake design category allows for the definition of analysis type to be

selected.

As can be seen in Table 3-7, for importance level 2 structures a minimum of static analysis is

required for all buildings between 12m and 50m irrespective of the soil classification. For buildings

over 50m in height a dynamic analysis must be carried out. This is a significant increase in the

number and types of structures requiring dynamic analysis.


Importance Level 3

Height <50

Height > 50

Height < 25

Height >=25

Sydney 1/1000 Design Design Design Design Soil

Factors Z Kp kpZ Category Category Category Category

0.8 0.08 1.3 0.104 II III NA NA

1 0.08 1.3 0.104 II III NA NA

1.4 0.08 1.3 0.104 II III NA NA

2.25 0.08 1.3 0.104 II III NA NA

3.5 0.08 1.3 0.104 NA NA II III

Table 3-8 shows the earthquake design categories for importance level 3 structures in Sydney using AS1170.4:2007

As can be seen in Table 3-8, for importance level 3 structures static analysis is required for all

buildings less than 50m irrespective of the soil classification. For the most onerous soil classification

static analysis is required for buildings less than 25m. For buildings over 50m in height a dynamic

analysis must be carried out and for structures greater than 25m on soil class Ee. As for important


May 2009


66

level 2 structures this is a significant increase in the number and types of structures requiring

dynamic analysis.

Although importance level 4 structures require a special study to ensure that they remain serviceable

for immediate use following a design earthquake event, it should be noted that all importance level 4

structures greater than 12m in height require dynamic analysis, shown in Table 3-9.

AS1170.4:2007 Earthquake Design Category Importance Level 4

Height <12 Height >12

Sydney 1/1500 Design Design

Soil Factors Z kp kpZ Category Category

0.8 0.08 1.5 0.12 II III

1 0.08 1.5 0.12 II III

1.4 0.08 1.5 0.12 II III

2.25 0.08 1.5 0.12 II III

3.5 0.08 1.5 0.12 II III

Table 3-9 shows the earthquake design categories for importance level 4 structures in Sydney using AS1170.4:2007

3.4.4 Earthquake Design Category Comparison

With height restrictions governing the analysis method, a more onerous but practical determination

of structural behaviour than previously considered is required. The engineer gains a greater

understanding of modal response and building irregularities when a computer generated model is

produced and analysed.

There are an increased number of structures requiring dynamic analysis with the new height

classifications used and this ensures that analysis is carried out for buildings, however regular they

were once considered. It will be seen in the comparison of building systems in Section 3.8 that the

loadings for buildings within the longer period ranges have been reduced. However, it is vital that

the dynamic behaviour is understood for sensitive deflection demands.

The height of a structure is therefore a justified method for design analysis classification and the

revision will guarantee all structural implications have been considered by the engineer.


May 2009


67

3.5 Period Calculation

In calculating the periods of vibration, the influence of the flexibility of the supporting soils has been

considered, however, the topic of calculating the natural period for the structure is still to be

expanded. The calculation of natural period of a structure, with reasonable accuracy, is very

difficult.

The old and the new codes [4] [7] differ in the calculation of the natural period for structures. The

empirical methods used in both codes, as seen below, do not use material and sectional properties

appropriate to the limit state under consideration.

As it is the period associated with elastic response at just below flexural yield which is of relevance,

the period should not be based on properties of un-cracked concrete. However, the estimates in both

the old and new codes are likely to be conservative for multi-storey frames in so far as they are

likely to predict a shorter natural period and as a consequence increase response.

These methods also do not take into account the actual shape or properties of each structure. Hence

it should be noted that these approximate methods can be used for initial estimates in preliminary

design, or in structural checking. It is strongly recommended that a refined estimate be made based

on Rayleigh’s method once member sizes have been selected.

3.5.1 AS1170.4:1993 Approximated Formulae

The equations listed below give a conservative estimation of the natural structural period.

Fundamental Period T = hn/46

Period for the orthogonal direction T = hn/58

3.5.2 AS1170.4:2007

The method detailed below in the new code [7] is an empirical method set out in the NZS

1170.5:2004 [29] commentary. The New Zealand [29] code specifies the Rayleigh method this is

due to the fact that the formulae below are derived from a high-seismicity region and are considered


May 2009


68

conservative. When used in a region of moderate - or low- seismicity they are even more

conservative, where structures have lower required earthquake resistance and hence are less stiff.

T1 = 1.25kthn0.75 for the ultimate limit state

Where Kt = 0.11 for Moment Resisting Steel Frames

=0.075 for Moment Resisting Concrete Frames

= 0.06 for Eccentrically Braced Steel Frames

= 0.05 for all other structures

Hn is the height from the base of the structure to the utmost seismic weight or mass in meters.

Again it should be noted that structures with first mode periods greater than 5seconds are outside the

scope of the new code [7].

The “k” exponent factor for vertical load distribution, seen in the equation below, is dependent on

the buildings period. It takes into account the influence of higher modes in the increase of moment

and shear in upper level members for high period structures.

Fi = (Wi hi)k V/ ∑( Wj hj)

k

3.5.3 Period Calculation Comparison

The values obtained from the empirical formulae of the two codes vary significantly as can be see in

the Figure 3-8.


May 2009


69

Figure 3-8 this figure shows the variation in the Periods with heights for the AS1170.4:1993 and 2007 Codes [4][7]

It can be seen in Figure 3-8 that the value for the natural period of most structures calculated using

the new code [7], are less conservative. It is shown that the periods are longer for moment resisting

frame structures and eccentrically braced frames have increased period values until an approximate

height of 140m. A less conservative view is taken for “all other structures”, including building frame

systems with reinforced concrete walls which will be the structural system concentrated on in

comparisons of structures later in this report, having a lower value of fundamental period when

greater than approximately 65m.

It is clear that a less conservative method for calculating the natural period has been provided. The

new code [7] tries to bring the results of the lateral force method closer to those of modal response

spectrum analysis. From the figure above, due to the variation in period calculation for different

structural systems, a direct comparison of the base shear multiplier can not be carried out on a period

basis. The base shear multiplier will be compared in Section 3.8 on a structural height basis.


May 2009


70

3.5.4 The Rayleigh Method

It should be noted that the Rayleigh Method is used in the New Zealand [27] code, for calculating

the fundamental natural period of vibration.

The Rayleigh method estimates the natural period from lateral displacements induced by a system of

lateral forces applied at floor levels. This method is based on structural dynamics and utilizes the

actual material and member properties to form a structural stiffness matrix.

The method also determines the modal shape and can be used to determine the second and third

natural periods and their modal shapes.

3.6 Response Factor (Rf), Structural Ductility Factor, µ, and the Structural

Performance Factor, Sp

The theory of the structural response was discussed in detail in Section 2.2. But a quick discussion

of the old and the new notation with relevance to code revisions will be looked at here.

Figure 3-9 shows the relationship of the reduction factor Rf with the ductility to structural

performance ratio.

Figure 3-9 shows the comparison of the Rf and µ/Sp relationship [34]


May 2009


71

Wilson and Lam [36] demonstrate that the structural response factor (Rf) was previously proposed to

be defined in the new AS1170.4:2007 code [7] by using a ductility factor µ and an over-strength

factor Ω but notation similar to the New Zealand [28] code has been used.

System Ductility ( µ) Over-strength ( Ω) Rf = µxΩ URM 1.25 1.3 1.6 Limited Ductility 2 1.3 2.6 Moderate Ductility 3 1.5 4.5 Ductile 4 1.5 6

Table 3-10 shows the revised ductility and over-strength factors used in the code but not in used notation [36].

In design using reduced or inelastic spectra it is vital that the design includes ductility capacity to at

least equal to that corresponding to the assumed force reduction factor.

As the Sp/µ value reduces (i.e. Rf and Sp/µ increase), the structure will absorb increasing energy and

therefore is designed for less direct load but for more plastic capacity.

“Limited ductility” classification in the new code [7], with µ=2, requires basic detailing as specified

in the material standard [8]. While “Moderate ductility”, with µ=3, requires special detailing set-out

in the appendices of the material code [8]. Fully ductile structures, with µ=4, are out of the scope of

the new Australian standards and reference to the NZ 1170.5 [28] standards is required, where

sophisticated methods are employed to establish the plastic capacity and ductility available at the

joints and designated hinges. Detailing rules to achieve these levels of ductility can be highly

complex. At the other extreme, for the value of µ = 1.0 the structure is designed to remain fully

elastic under the full loads.

Structures outside the scope of AS1170.4 2007 are listed below which have ductility factor µ greater

than 3 and should be designed in accordance with NZS 1170.5.:

Special Moment Resisting Frames in Steel and Concrete. (Frames designed and detailed to achieve

high structural ductility and where plastic deformation is planned under ultimate actions)

Fully ductile eccentrically braced frames (steel)


May 2009


72

Ductile Coupled Walls (concrete - fully ductile)

Ductile Shear Walls (concrete - fully ductile)

3.7 Earthquake Base shear

The horizontal equivalent static shear force (V) acting at the base of the structure (base shear) in the

direction being considered is calculated from the following equations;

3.7.1 AS1170.4: 1993 Earthquake Base shear

V = I (CS/Rf) Gg

C = 1.25a/T2/3 n=2/3; Factor=1.25

Base shear multiplier = V/Gg = 1.25aIS/RfT2/3

There is a lower limit of V >0.01Gg and an upper limit of V< I (2.5a/Rf) Gg.

Also it should be noted that kp replaced I in AS1170.4:2002 Appendix D.

3.7.2 AS1170.4: 2007 Earthquake Base shear

V = Cd(T1)Wt

Cd(T1) = C(T1) (Sp/µ)

C(T1) = Ch(T1)kpZ

Base shear multiplier = V/Wt = Ch(T1)kpZ(Sp/µ)

There is no lower limit in the 2007 code, however, as stated above structures with a period greater

than 5 is not covered in the new 2007 code [7].

For comparison of the earthquake base shear, the percentage of seismic weight of the structure to be

used for calculating the base shear was obtained and graphed for structural system types in Section

3.8. This percentage will be referred to as the base shear multiplier within this report.


May 2009


73

3.8 Structural Systems and Restrictions

The best way to understand implications to structural design is in terms of loading magnitude (either

increasing or decreasing), the comparison of the horizontal design action coefficient was calculated

using both codes and the differences compared for structural systems.

The base shear multiplier will be represented by a varying structural height rather than by structural

period.

It should be noted that the following comparisons use a probability factor kp of unity and therefore if

the annual probability of exceedance is greater than 1/500 years, refer to the BCA [11] then the base

shear multiplier will have to be increased to suit.

It should also be noted that the new proposed of 1.0% robustness limit for buildings less than 15m in

height and 1.5% robustness limit for buildings taller than 15m in height has not been considered in

this comparison and will now be the more onerous loading for longer period buildings.

As note previously, for simple analysis a load factor of 0.10Wt can be applied to structures less than

12m in height for Earthquake Category I, unless a more detailed analysis is carried out. This only

applies for sub-soil class Ae, Be and Ce for Importance Level 2 structures.


May 2009


74

3.8.1 Bearing wall systems

Bearing Wall System; Soil Class Ae Comparison

Figure 3-10 this figure shows the comparison of the base shear multiplier for BWS, for AS1170.4: 1993 & 2007 [4][7], for Soil Class Ae.

The old code [4] limited the height of a bearing wall system to a structural height limit of 50m in a

category E earthquake. With the introduction of AS1170.0:2002 Appendix D, this limitation only

effects Importance level 4 structures on the most onerous of soil classes.

In Figure 3-10 above it can be seen that there is significant difference in the base shear multiplier. In

the old code [4] the reinforced shear walls were considered to be ductile if they are designed,

detailed and constructed to AS 3600 [8] and no additional detailing was required for consideration in

its Appendix A.. In the new code [7] concrete elements are not categorised into structural systems,

therefore allowing both “limited” and “ductile” detailing for any choice of system. “Limited”

ductility has been defined as design and detailing to the standard without Appendix A requirements

and the definition of “Ductile” is to include detailing to Appendix A of the material standard. This

begs the question; is the requirement of no additional detailing, as stated in A10 of Appendix A,

satisfactory for the assumed ductility level?


May 2009


75

As “limited” elements in the new code are considered to have no specific detailing requirements and

act relatively elastically, this is reflected in the increased loading that is applied to these elements as

seen in the Figure 3-10. In Table 3-11 it can be seen that for limited ductility walls there is an

increase in the multiplier for buildings less than 87.3m in height. The total percentage of the seismic

weight of the building applied to “limited” ductile shear walls in base shear is 7.23%. That is an

increase of 2.88% of the seismic load being applied to these elements for buildings less than 8m in

height.

In Figure 3-10 it is shown that for ductile walls, there is a decrease in the base shear multiplier for

both short and long period structures. The total percentage of the seismic weight of the building

applied to “ductile” shear walls in base shear is 4.18%. The loading applied in the old code is 4.35%

therefore no significant decrease (0.18%).

The previous height limit for the application of 1% of the seismic weight to be applied has been

reduced from 78m and 98m, in the fundamental and orthogonal direction respectively, to 51m for

“ductile” structures; implying that taller buildings experience a reduced seismic loading applied.

Bearing Wall Frame System Comparison

AS1170.4:1993 AS1170.4:2007 Multiplier % Difference

Soil Class Ae Fundamental Orthogonal Limited Ductile Limited Ductile

Comparing Upper Limit Multiplier at T1

Period secs T <= 0.2 0.2 0.3 0.3 Height m <= 9.2 11.6 8.1 8.1 Cd(T1) Upper Limit % 4.35% 4.35% 7.23% 4.18% 2.88% -0.18%

Comparing Upper Limit Multiplier at 1993 Orthogonal Maximum % Loading Height

Period secs T 0.3 0.2 0.4 0.4 Height m 11.6 11.6 24.9 11.6 Cd(T1) % 2.49% 3.27% 3.00% 3.13% -0.27% -0.14%

Comparing Structural Heights for 1993 Lower Limit Multiplier

Period secs T 1.7 1.7 1.8 1.2 Height m 78.2 98.6 87.3 50.9 Cd(T1) % 1% 1% 1% 1% 0% 0%

Table 3-11 show the comparison of the base shear multiplier for BWS, for AS1170.4:1993 and 2007, for soil class Ae


May 2009


76

For the “limited” ductility case there is a slight reduction in height where the previous limit was

applied, being at 88m rather than 98m, for the orthogonal direction and increasing in height from

78m to 88m for the fundamental direction.

The above Figure 3-10 and Table 3-11 only considered loading for soil class Ae, and it is seen that

there is reductions in the applied loading for “ductile” wall construction. The increase of 2.88% to

the applied loading for “limited” ductility wall construction within the short period range has large

implications for 1 and 2 storey buildings using this structural building system. And as the definition

for “limited” and “ductile” are ambiguous at present; should the increased loading be applied to all

bearing wall systems? Or just “limited” detailed walls such as unreinforced masonry elements used

within this system?

It is stated in the code that for earthquake category I, a simple application of 10% of the seismic

weight can be applied to buildings under 12m to eliminate the requirement for seismic design and as

can be seen in Figure 3-10, this loading seems even more onerous especially when assessing

performance levels for modifications to existing buildings.

Bearing Wall System; Soil Class De Comparison

In Figure 3-11 it can be seen that there is significant difference in the base shear multiplier.

In Figure 3-11 for “limited” ductility elements it is shown that there is large increased loading

applied for buildings less than 100m in height. At 100m in height there is a 2.3% seismic load

applied to the building. The total percentage of the seismic weight of the building applied to

“limited” ductile shear walls in base shear is 11.32%. That is an increase of 6.88% of the seismic

load being applied to these elements for buildings less than 16m in height.


May 2009


77

Figure 3-11 this figure shows the comparison of the base shear multiplier for BWS, for AS1170.4: 1993 & 2007 [4][7], for Soil Class De.

“Ductile” walls in the new code [7], as in the old code [4], have to be detailed to the material

standard AS3600 Appendix A[8]. However as no additional requirement is stated, concern as to

ability of the standard detailing to achieve this assumed ductility is of utmost concern.

In Figure 3-11 it is shown that for ductile walls, there is an increase in the base shear multiplier for

short period structures however this increase is experienced up to a natural period of 0.7 and 0.5secs,

for the fundamental and orthogonal directions respectively (corresponding to a height of 30m),

where after a decrease can be seen in the longer period range.

The total percentage of the seismic weight of the building applied to “ductile” shear walls in base

shear is 6.54%. That is an increase of 2.1% which is shown in Table 3-12 for structures of height

less than 16m.


May 2009


78

Bearing Wall Frame System Comparison


Soil Class De Fundamental Orthogonal Limited Ductile Limited Ductile


Period secs T <= 0.6 0.6 0.5 0.5 Height m <= 27.6 34.8 15.9 15.9 Cd(T1) Upper Limit % 4.44% 4.44% 11.32% 6.54% 6.88% 2.10%


Period secs T 0.8 0.6 0.9 0.9 Height m 34.8 34.8 34.8 34.8 Cd(T1) % 4.44% 4.44% 6.77% 3.91% 2.32% -0.53%

Comparing Maximum Height for a Ductile Load Difference of Zero



Period secs T 5.0 5.0 3.0 2.3 Height m 230.0 290.0 172.2 120.9 Cd(T1) % 1.0% 1.0% 1.0% 1.0% 0% 0%

Table 3-12 show the comparison of the base shear multiplier for BWS, for AS1170.4:1993 and 2007, for soil class De

The above Figure 3-11 and Table 3-12 considered loading increases for soil class De, which is

considered the “worst” founding material for Sydney’s Quaternary sands. The increases within the

short period range have large implications for 10 and 11 storey buildings using this structural

building system for both “limited” ductility and “ductile” designed elements. As can be seen the

maximum applied load is larger than the 10% used in the simplified method confirming that the soil

class De requires design for Earthquake Category II. Charts showing loading multipliers for all soil

classes are provided in the Appendix A.


May 2009


79

3.8.2 Building Frame systems

The old 1993 code [4] limited the height of a building frame system to a structural height limit of

70m in design category E.

Building Frame System with Shear Walls; Soil Class Ae Comparison

Figure 3-12 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4][7], for Soil Class Ae.

In Figure 3-12 above it can be seen that there is an increase in the applied base shear multiplier. In

the old code [4] the reinforced shear walls were considered to be ductile and were to be detailed to

AS 3600 Appendix A [8]. In the new code [7] concrete elements are not categorised into structural

systems, therefore allowing both “limited” and “ductile” detailing choice for any system choice.

As “limited” elements are considered to require no specific detailing and act relatively elastically,

this is reflected in the increased loading that is applied to these elements as seen in the Figure 3-12.

In Table 3-13 it can be seen that for limited ductility walls there is an increase in the multiplier for

buildings less than 87m in height. The total percentage of the seismic weight of the building applied


May 2009


80

to “limited” ductile shear walls in base shear is 7.23%. That is an increase of 3.97% of the seismic

load being applied to these elements for buildings less than 8m in height.

Building Frame System with RC Walls Comparison




Period secs T <= 0.2 0.2 0.3 0.3 Height m <= 9.2 11.6 8.1 8.1 Cd(T1)Upper Limit % 3.27% 3.27% 7.23% 4.18% 3.97% 0.91%


Period secs T 0.3 0.2 0.4 0.4 Height m 11.6 11.6 11.6 11.6 Cd(T1)% 2.49% 3.27% 5.42% 3.13% 2.92% -0.14%


Period secs T 0.25 0.20 0.39 0.39

Height m 11.60 11.60 11.60 11.60

Cd(T1)% 2.49% 3.27% 5.42% 3.13% 2.92% -0.14%


Period secs T 1.1 1.1 1.8 1.2 Height m 50.6 63.8 87.3 50.9 Cd(T1) % 1.0% 1.0% 1.0% 1.0% 0.00% 0.00%

Table 3-13 show the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4:1993 and 2007, for soil class Ae.

“Ductile” shear walls in the new code [7], as in the old code [4], have to be detailed to the material

standard AS3600 Appendix A [8]. In Figure 3-12 above it is shown that for ductile walls, there is an

increase in the base shear multiplier for short period structures and a significant decrease in the

longer period range.


shear is 4.18%, an increase of 0.91%. It can be seen in Table 3-13 that for structures of height less

than 12m there is an increase in the base shear multiplier. The height at which the previous limit of

1% of the seismic weight was to be applied has reduced to 50m from 64m.

Figure 3-12 and Table 3-13 only considered loading increases for soil class Ae. Even on this soil

class, the increases within the short period range have implications for 3 and 4 storey buildings using


May 2009


81

this structural building system for both “limited” ductility and “ductile” designed elements. The

simplified method, applying 10% of the seismic weight to buildings under 12m, is even more

onerous especially when assessing performance levels for modifications to existing buildings.

Building Frame System with Shear Walls; Soil Class De Comparison

In Figure 3-13 it can be seen that there is significant difference in the base shear multiplier.

For “limited” ductility elements it is shown that there is large increased loading applied, as seen in

Table 3-14, for buildings less than 170m in height. The total percentage of the seismic weight of the

building applied to “limited” ductile shear walls in base shear is 11.32%. That is an increase of

7.99% of the seismic load being applied to these elements for buildings less than 16m in height.

Figure 3-13 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4] [7], for Soil Class De.

“Ductile” shear walls in the new code [7], as in the old code [4], have to be detailed to the material

standard AS3600 Appendix A [8]. In Figure 3-13 above it is shown that for ductile walls, there is an


May 2009


82

increase in the base shear multiplier for short period structures however this increase is experienced

up to a natural period of 1.6secs (corresponding to a height of 75m), where after a decrease can be

seen in the longer period range.


shear is 6.54%. That is an increase of 3.21% which is shown in Table 3-14.

Building Frame System with RC Walls Comparison











Table 3-14 show the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4:1993 and 2007, for soil class De.

The increases within the short period range have large implications for 5 and 6 storey buildings

using this structural building system for both “limited” ductility and “ductile” designed elements. As

can be seen the maximum applied load is larger than the 10% used in the simplified method

confirming that the soil class De requires design for Earthquake Category II.

Building Frame System with CB Frames; Soil Class Ae Comparison

In Figure 3-14 it can be seen that there is a difference in the base shear multiplier. In the old code [4]

the reinforced braced frames where considered to be ductile if detailed to AS 3600 Appendix A [8].


May 2009


83

In the new code [7] concrete elements are similarly not categorised into structural systems, therefore

allowing both “limited” and “moderately ductile” detailing choice for any system choice depending

on the detailing.

Figure 3-14 this figure shows the comparison of the base shear multiplier for CBF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class Ae.

In Table 3-15 it can be seen that for limited ductility frames there is an increase in the multiplier for

buildings less than 87m in height. The total percentage of the seismic weight of the building applied

to “limited” ductile braced frames in base shear is 7%. That is an increase of 3% of the seismic load

being applied to these elements for buildings less than 8m.

“Moderately ductile” braced frames in the new code [7], as shown in Figure 3-14, there is a decrease

in the base shear multiplier for long period structures. For the short period structures the total

percentage of the seismic weight of the building applied in base shear is 4.18%. The loading applied

in the old code is 4.0% therefore no significant increase (0.18%). The previous height limit for the

application of 1% of the seismic weight to be applied has been reduced from 69m and 87m, in the

fundamental and orthogonal direction respectively, to 51m for “moderately ductile” structures;


May 2009


84

implying that taller buildings experience a reduced seismic loading applied and is maintained at 87m

for the “limited” ductile concentrically braced frame.

Building Frame System with Concentrically Braced Frames Comparison









Table 3-15 show the comparison of the base shear multiplier for CBF, for AS1170.4:1993 and 2007, for soil class Ae.

Even on soil class Ae (Rock), the increases within the short period range for the “limited” ductility

case have large implications for 1 and 2 storey buildings using this structural building system. The

application of 10% of the seismic weight seems too conservative for design.

Building Frame System with CB Frames; Soil Class De Comparison

In Figure 3-15 it can be seen that there is sizeable difference in the base shear multiplier.


May 2009


85

Figure 3-15 this figure shows the comparison of the base shear multiplier for CBF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class De.

For “limited” ductility elements it is shown that there is large increased loading applied, as seen in

Table 3-16, for buildings less than 145m in height. The total percentage of the seismic weight of the

building applied to “limited” ductile concentrically braced frames in base shear is 11.32%. That is an

increase of 7.35% of the seismic load being applied to these elements for buildings less than 16m in

height.

The total percentage of the seismic weight of the building applied to “moderately ductile” frames in

base shear is 6.54%. That is an increase of 2.57% which is shown in Table 3-16.


May 2009


86

Building Frame System with Concentrically Braced Frames Comparison











Table 3-16 show the comparison of the base shear multiplier for CBF, for AS1170.4:1993 and 2007, for soil class De.

“Moderately ductile” braced frames in the new code [7], as in the old code [4], have to be detailed to

the material standard AS3600 Appendix A [8]. In Figure 3-15 above it is shown that for ductile

frames, there is an increase in the base shear multiplier for short period structures, however after a

height of 29m the difference is only 0.44% and after a height of 75m (corresponding to a natural

period of 1.6secs), decrease can be seen in the longer period range.

The above Figure 3-15 and Table 3-16 considered loading increases for soil class De. The increases

within the short period range have implications for 5 and 6 storey buildings. As can be seen the


class De requires design for Earthquake Category II.


May 2009


87

3.8.3 Moment Resisting Frame System

Ordinary Moment Resisting Frames (OMRF) – Moment resisting frame with no particular earthquake detailing, specified in the relevant material standard AS 3600 [8] and AS4100 [9]. In the old code [4], a height limitation of 50m above the structural base of the structure applied for ordinary moment resisting frames where the product of acceleration coefficient and site factor (aS) is greater than or equal to 0.1 (1993) i.e. kpaS (2002)

Intermediate Moment Resisting Frames (IMRF) – Moment resisting frame of concrete or steel which is designed and detailed to achieve moderate structural ductility.

Ordinary Moment Resisting Frame; Soil Class Ae Comparison

Figure 3-16 this figure shows the comparison of the base shear multiplier for OMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class Ae.

In Figure 3-16 it can be seen that there is a difference in the base shear multiplier for both steel and

concrete ordinary moment resisting frames.

In Table 3-17 it can be seen that for both RC and Steel OMRF there is a significant increase in the

multiplier for buildings less than 4.7m and 2.8m in height respectively. The total percentage of the

seismic weight of the building applied to both material ordinary moment resisting frames is 7.23%.


May 2009


88

That is an increase of 2.23% of the seismic load being applied to this construction type for single to

two storey buildings.

In Figure 3-16, it is shown that this increased loading has a very steep decrease rate within the short

period range, and at the 12m height (corresponding to a period of 0.6 sec and 0.9 sec for RC and

Steel material type) it can be seen that there is a reduction in applied load.

Table 3-17 show the comparison of the base shear multiplier for OMRF, for AS1170.4:1993 and 2007, for soil class Ae

Figure 3-16 and Table 3-17 only considered loading increases for soil class Ae. The increases within

the short period range are significant and have large implications for 1 and 2 storey buildings for

both concrete and steel construction. There is a lot of usage of this construction type for warehouses

and storage facilities and increased loading will effect both connection design and deflection and

sway considerations. If the simplified method is used a far more conservative loading will be applied

to these structures.

Ordinary Moment Resisting Frame; Soil Class De Comparison

In Figure 3-17 it can be seen that there is substantial difference in the base shear multiplier for both

steel and concrete ordinary moment resisting frames on soil class De.


May 2009


89

Figure 3-17 this figure shows the comparison of the base shear multiplier for OMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class De.

The total percentage of the seismic weight of the building applied to ordinary moment resisting

frames is 11.32% and 6.54% for RC and Steel respectively. That is an increase of 6.32% for

reinforced concrete and 1.54% for steel, of the seismic load being applied to this construction type

for 2 to 3 storey buildings. This increased loading however is only applied to structures less than

9.3m and 5.6m in height for RC and Steel OMRF respectively.

In Figure 3-17 it is shown that this increased loading has a very steep decrease rate within the short

period range with reductions in loading occurring at 30m and 22m (corresponding to a period of 1.3

sec and 1.4 sec) for RC and Steel. In the old code the minimum loading of 5% and 4.44% for both

RC and Steel was applied for all structures under 35m in height (corresponding to a period of 0.8 sec

and 0.6 sec for the fundamental and orthogonal direction for both material types).

In Table 3-18 it can be seen that the reduction in the height at which the minimum loading of 1% of

the loading is applied to ordinary moment frames is significant. There is a reduction of loading for

building structures of 170m and taller for RC structures and 120m and taller for steel structures.


May 2009


90

Table 3-18 show the comparison of the base shear multiplier for OMRF, for AS1170.4:1993 and 2007, for soil class De

Figure 3-17 and Table 3-18 considered loading increases for soil class De. As this construction type

is very common in low rise building in industrial areas on the outskirts of Sydney, such as Botany

Bay, this increased loading will have significant impact on the design of new buildings as well as the

assessment of existing buildings that either requires upgrading or modifications. As can be seen the


class De requires design for Earthquake Category II. Charts showing loading multipliers for all soil

classes are provided in the Appendix A.

Intermediate Moment Resisting Frame; Soil Class Ae Comparison

In Figure 3-18 it can be seen that there is considerable difference in the base shear multiplier for

both steel and concrete intermediate moment resisting frames.


May 2009


91

Figure 3-18 this figure shows the comparison of the base shear multiplier for IMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class Ae.

In the old code and new code [4] [7] IMRF of reinforced concrete or steel are considered to require

additional detailing for ductility specified in the material standards AS 3600 Appendix A [8] and

AS4100 Section 13 [9].

In Table 3-19 it can be seen that for both RC and Steel IMRF there is only a slight increase in the

multiplier for buildings less than 4.7m and 2.8m in height respectively. The total percentage of the

seismic weight of the building applied to both materials is 4.18%. That is an increase of 0.91% of

the seismic load being applied to this construction type for single to two storey buildings.


period range and by 12m in height (corresponding to a period of 0.6 sec and 0.9 sec for RC and Steel

material type) it can be seen that there is a 1% and 1.88% reduction in applied load for steel and RC

respectively. The percentage loads start to reduce at approximately 6.9m and 4.1m in height for RC

and Steel respectively.


May 2009


92

Table 3-19 show the comparison of the base shear multiplier for IMRF, for AS1170.4:1993 and 2007, for soil class Ae

Figure 3-18 and Table 3-19 considered loading increases for soil class Ae. There are large

reductions to the loads being applied to this construction type. The assumption of this structural

system inherently requires ductility detailing therefore ensuring consideration of performance level

achievable.

The 10% minimum loading for simplified design would be extremely onerous for this building

system and full analysis would ensure a much more economical design.

The height at which the previous minimum loading of 1% of the loading is applied to intermediate

moment frames has been significantly revised. The load has been reduced until a structure reaches a

height of 128m for RC and 77m for Steel. It must be noted that the minimum loading for robustness

will in these cases then governs design.

Intermediate Moment Resisting Frame; Soil Class De Comparison

In Figure 3-19 it can be seen that there is large difference in the base shear multiplier for both steel

and concrete intermediate moment resisting frames, for soil class De.


May 2009


93

Figure 3-19 this figure shows the comparison of the base shear multiplier for IMRF, for AS1170.4: 1993 & 2007 [4][7] for Soil Class De.

In Table 3-20 it can be seen that for both RC and Steel IMRF there is an increase in the multiplier

for buildings less than 9.3m and 5.6m in height respectively. The total percentage of the seismic

weight of the building applied to both material intermediate moment resisting frames is 11.32% and

6.54% for RC and Steel. That is an increase of 7.99% for reinforced concrete and 3.21% for steel of

the seismic load being applied to this construction type for 2 to 3 storey buildings.


period range with reductions in loading occurring at 26m and 16m (corresponding to a period of 1.1

sec and 1.1 sec) for RC and Steel. In the old code the minimum loading of 3.33% and 3.08% for

both RC and Steel was applied for all structures under 28m and 35m in height (corresponding to a

period of 0.8 sec and 0.6 sec for the fundamental and orthogonal direction for RC and steel

respectively).


May 2009


94

Table 3-20 show the comparison of the base shear multiplier for IMRF, for AS1170.4:1993 and 2007, for soil class De

Figure 3-19 and Table 3-20 considered loading increases for soil class De. As with ordinary moment

resisting frames this increased loading will have significant impact on the design of new buildings as

well as the assessment of existing buildings that either requires upgrading or modifications. As can

be seen the maximum applied load is larger than the 10% used in the simplified method confirming

that the soil class De requires design for Earthquake Category II. Charts showing loading multipliers

for all soil classes are provided in the Appendix A.

3.8.4 Dual System

In the old 1993 code a moment resisting frame in a dual system had a restriction that if over a

structural height of 30m then a special moment resisting frames had to be provided down to the

footing.

3.9 Torsion

As discussed in Section 2.3.4 a structures response in plan has large implications to the forces

induced in the lateral support system due to torsion effects.


May 2009


95


In the old code [4] the actual response of the structure is calculated by applying the earthquake

actions through the shear centre obtaining the static eccentricity and then factoring it to include

dynamic amplification of the response.

ed1 = A1es + 0.05b and ed2 = A2es - 0.05b

Where, A1 = 2.6– 3.6(es/b)] or 1.4, whichever is greater

A2 = dynamic eccentricity factor = 0.5

The accidental eccentricities are added and subtracted from the appropriate factored case to ensure

that the more unfavourable case for the resisting elements on each side of the structure is included.


May 2009


96

Figure 3-20 shows the geometric eccentricities from the AS1170.4:1993 code.


Unlike the old code [4], amplification or de-amplification of the static eccentricities between the

centre of mass and stiffness is not required. This is convenient because normally the storey stiffness

centre cannot be uniquely defined with accuracy. Therefore in reality the amplification factor

requires tremendous analyses.

For buildings with full symmetry of stiffness and nominal masses in plan, the analysis for the

horizontal components of the seismic action gives no torsional response at all. However, variations

in stiffness and uncertainty of possible torsional components of ground motion may produce a

torsional response even in the most fully symmetrical building.


May 2009


97

Therefore to ensure a minimum torsional resistance and stiffness and limit the consequence of

unforeseen torsional response, the new code [7] introduces accidental torsion effects by applying the

earthquake actions at a position ± 0.1b from the nominal centre of mass, where b is the plan

dimension of the structure at right angles to the direction of the action.

It is conservative to assume that all the masses of the structure are displaced along the same

horizontal direction and in the same sense (+/-) at a time, however orientated to produce the most

adverse torsion moment must be considered.

It is completely impractical to study the effects of displacing the masses through dynamic analysis:

the dynamic characteristics of the system will change with the location of the masses. Therefore

accidental eccentricity of the total horizontal seismic component is considered with respect to the

centre of all the masses.

3.9.3 Torsion Comparison

As discussed above the fraction of the storey plan dimension has increased from 5% to 10% for the

calculation on accidental torsional effects. There is not a direct comparison to be drawn between

these two figures though as in the 1993 code [4], the accidental eccentricity is added or subtracted,

to or from the static eccentricities and an equal force is applied directly through the shear centre of

the building with an equivalent moment.

In the new code [7], the earthquake force is to be applied at the 10% dimension factor around the

centre of mass. Therefore the distance between the lines of action of the earthquake force (at ±10%

dimension from the centre of mass) to the shear centre is the true eccentricity to be calculated for.

For theoretically complete symmetrical buildings (in plan), this is a significant doubling of the

accidental torsional moment. Conversely, for the most onerous unsymmetrical buildings the

torsional effects produced by the two codes correlate well.

It is clear that in the new code [7] a more conservative method for calculating the torsional effects

has been provided for symmetrical buildings while maintaining similar presentation of effects for

largely unsymmetrical buildings.


May 2009


98

3.10 Drift and P-Delta Effects

3.10.1 AS1170.4: 1993 Storey Drift Determination and P-delta Effects

The design storey drift (∆) shall be calculated as the difference of the deflections (δX) at the top and

the bottom of the storey under consideration.

(δX) = KdδXe

Where

Kd = deflection amplification factor

δXe = deflection determined by an elastic analysis using ultimate earthquake forces.

Note that the horizontal force specified is applied through the centre of mass for each floor but

accidental torsional effects may be neglected. The inter-storey drift at the ultimate limit state is

limited to 1.5% of the storey height for each level.

P-delta effects need not be considered when the stability coefficient (m), from the equation below is

less than 0.10.

m = Px∆/VxhsxKd

Where

Px = total vertical design load at storey x

∆ = design storey drift

Vx = total horizontal earthquake shear force at the storey x

hsx = height of storey x


May 2009


99

When m is greater than 0.10, the deflection amplification factor (Kd) related to P-delta effects shall

be determined by rational analysis. However, if m is greater than 0.25 then the P-delta effects must

be examined very carefully and structures are considered unstable and shall be re-designed.

The design storey drift shall be multiplied by the factor (0.9/ (1-m)), which is greater than or equal to

unity, to obtain the storey drift including P-delta effects.

Alternatively, a second order analysis may be used to obtain the storey drift including P-delta

effects.

The increase in horizontal earthquake shear forces and moments resulting from the increase in storey

drift shall be added to the corresponding shear forces and moments determined without

consideration of the P-delta effects.

3.10.2 AS1170.4: 2007 Storey Drift Determination and P-delta Effects

As in the 1993 code, the design storey drift (dst) shall be calculated as the difference of the

deflections (δi) at the top and the bottom of the storey under consideration.

di = dieµ/Sp Where,

die = deflection determined by an elastic analysis using ultimate earthquake forces.

The inter-storey stability coefficient (θ) replaces the notation (m), and also limits the inter-storey

drift at the ultimate limit state to 1.5% of the storey height for each level.

As per the old code [4] P-delta effects need not be when the stability coefficient (θ), from the

equation below is less than 0.10, however, in the new code [7] structures are considered unstable

and shall be re-designed if θ is greater than 0.25.

θ = dst∑ Wj / [hsiµ∑Fj]

The calculation of P-delta effects is similar to the old code [4] however the ductility µ is used rather

than the deflection amplification factor to reduce the inter-storey stability coefficient.


May 2009


100

3.10.3 Storey Drift Determination and P-delta Effects Comparison

As can be seen in Table 3-21 and Table 3-22 there are revisions to the storey drift and inter-storey

coefficient values. The percentage differences are calculated using the following equations;

Storey drift % difference = [(µ/Sp) - Kd] / Kd

Inter-storey Stability Coefficient % difference = [(1/µ) - (1/Kd)] / (1/Kd)

The percentage difference for the design deflection multiplier (amplification factor) is significantly

reduced for all “limited” reinforced concrete wall systems but with a 30% increase for ordinary

moment resisting frames in concrete.

1993 Kd

2007 µ/Sp

% Difference in Design Deflections Multiplier

System Steel RC Limited Ductile Limited Ductile

Bearing Wall 4 2.6 4.5 -35% 13%

Building Frame with RC Shear Walls 5 2.6 4.5 -48% -10%

Building Frame with CB Frames 4.5 4.5 2.6 4.5 -42% 0%

Ordinary Moment Resisting Frames

Steel 4 2.6 -35%

Concrete 2 2.6 30%

Intermediate Moment Resisting Frames

Steel 4.5 4.5 0%

Concrete 3.5 4.5 29% Table 3-21 Shows the percentage difference in the design deflections multiplier for storey drift calculation for AS1170.4: 1993

& 2007

For “ductile” bearing wall systems there is an increase of 13% in the design deflection multiplier

and an increase of 29% for intermediate moment resisting frames of concrete.


May 2009


101

1993 1/Kd

2007 1/ µ

% Difference in inter- storey Stability Coefficient

Multiplier

System Steel RC Limited Ductile Limited Ductile

Bearing Wall 0.25 0.5 0.33 100% 33%

Building Frame with RC Shear Walls 0.2 0.5 0.33 150% 67%

Building Frame with CB Frames 0.22 0.22 0.5 0.33 125% 50%


Steel 0.25 0.5 100%

Concrete 0.5 0.5 0% Intermediate Moment Resisting Frames

Steel 0.22 0.33 50%

Concrete 0.29 0.33 17% Table 3-22 Shows the percentage difference in the inter-storey stability coefficient for P-delta effects multiplier for AS1170.4:

1993 & 2007

It can be seen that the multiplier for the inter-storey stability coefficient has been significantly

increased for all systems by using the ductility factor rather than the deflection amplification factor

in the denominator. This more conservative approach has been used by proposing the use of ductility

alone, but note that this isn’t as conservative as the New Zealand code which does not use the

ductility factor at all.

It is clear that a less conservative method for overall stability of the building has been provided with

reductions in both the design deflection multiplier for the calculation of inter-storey drift and the

increasing of the inter-storey stability coefficient.

3.11 Dynamic Analysis

As the two codes only provide minimum coverage of requirements, limitations and guidelines it can

be seen that there is very little difference between the two codes for dynamic analysis procedures.

The analysis methods chosen can be either a response spectrum analysis or a time history analysis. A

time history analysis involves the calculating of the response of a structure at each increment of time

when the base is subjected to a specific ground motion time history. It should be noted that the

dynamic analysis is only as good as the assumptions and inputs initial made.


May 2009


102

Dynamic analysis however can give a much better insight into structures dynamic response

characteristics such as variances in the loading distribution, torsional effects and combined mode

contributions.

The procedure specified in the codes is an elastic dynamic analysis. This is considered to be

satisfactory because structures designed elastically for the appropriately reduced earthquake forces,

is deemed to have the inelastic capacity to withstand the earthquake forces and deformations. This

again highlights the role of the engineer in understanding the detailing and capacity requirements of

the structure to obtain the ductility required.

Earthquake Actions – As in the equivalent static method the horizontal design response spectrum

(Cd(T)), including the site hazard spectrum and the effects of the structural response is;

Cd(T) = C(T) (Sp/µ) = Ch(T)kpZ(Sp/µ)

In static analysis only the 1st mode of natural period is considered, however, in dynamic analysis T is

the period of vibration appropriate for the mode of vibration of the structure being considered. Site

specific design response spectra are required as recommended in Section 2.4, relating to resonance

and soil classification and profile.

For vertical considerations the old code [4], recommend taking 50% of the horizontal accelerations

but in the new code [4], vertical earthquake actions shall be calculated using the following equation;

Cvd(T) = Cv(Tv)Sp = 0.5C(Tv)Sp =0.5Ch(Tv)SpkpZ

Where,

Cv(Tv) = the elastic site hazard spectrum for vertical loading for the vertical period of vibration.

Vertical earthquake actions however according to the new code [4] need not be considered for any

of the three design categories. It has been published recently, on the national geographic website

[27], that large vertical wave components may occur in seismic activity, although as the frequency

of such waves are of such high frequency they are relatively weak.


May 2009


103

Modal Analysis – The modal response method has been recommended in the new code [7], which

uses the peak response of all modes having a significant contribution to the total structural response.

The peak modal response shall be calculated using the ordinates of the appropriate response

spectrum curve.

In two-dimensional analysis, sufficient modes shall be included in the analysis to ensure that at least

90% of the mass of the structure is participating for the direction under consideration.

In three-dimensional analysis, all modes not relating to the seismic-force-resisting system shall be

ignored. Further, all modes with periods less than 5% of the natural period T1 may be ignored.

All peak member forces, displacements, horizontal earthquake shear forces and base reactions are

then to be combined using recognised methods. The effects of closely spaced modal periods must be

careful considered.

When considering torsion it is vital that accidental torsion be considered. This can be done by

making appropriate adjustments in the model, such as mass location adjustments. When using a two-

dimensional model the action effects arising form torsion shall be combined with the translational

action effects by direct summation.

Figure 3-21 Shows the translation and torsion effects on a floor plate [30].

Mathematical Model – For both the new and the old code [4] [7] there is no revision to this clause

with a mathematical model of the physical structure representing the spatial distribution of the mass


May 2009


104

and stiffness of the structure to an extent that is adequate for the calculation of the significant

features of its dynamic response being the requirement in both.

Drift Determination and P-delta Effects – For both the new and the old code [4] [7] the storey

drifts, member forces and moments due to P-delta effects shall be calculated as per the static

analysis methods as discussed in Section 3.10.3, using the deflections, forces and moments

calculated by the dynamic response method discussed above.

3.12 Discussion

There are significant implications to structures due to the revisions noted in this Chapter.

Infrastructure designed and built over the past 15 years in Australia, have been based on the design

parameters set out in the old AS1170.4:1993 code.

Structural System Comparison of the Base shear Multiplier (Percentage of Seismic Weight)

AS1170.4:1993 AS1170.4:2007 Difference

kp=1.0 & Z=0.08 Ae De Ae De Ae De

Bearing Wall System

Limited 4.35% 4.44% 7.23% 11.32% 2.88% 6.88%

Ductile 4.35% 4.44% 4.18% 6.54% -0.17% 2.10%

Building Frame System with Shear Walls

Limited 3.27% 3.33% 7.23% 11.32% 3.96% 7.99%

Ductile 3.27% 3.33% 4.18% 6.54% 0.91% 3.21%

Building Frame System with Concentrically Braced Frames

Limited 4.00% 3.97% 7.23% 11.32% 3.23% 7.35%

Moderately ductile 4.00% 3.97% 4.18% 6.54% 0.18% 2.57%


Concrete 5.00% 5.00% 7.23% 11.32% 2.23% 6.32%

Steel 4.35% 4.44% 7.23% 6.54% 2.88% 2.10%

Intermediate Moment Resisting Frames

Concrete 3.27% 3.33% 4.18% 11.32% 0.91% 7.99%

Steel 3.08% 3.08% 4.18% 6.54% 1.10% 3.46% Table 3-23 shows the comparison (percentage differences) of the seismic weight loading multiplier for AS1170.4:1993 and

2007 code [4] [7].

Whilst seismic activity modelling has developed, and must continue to do so, research efforts are

now crucial for assessing and comparing, the potential seismic performance of existing structures

and their components.


May 2009


105

As can be seen in Table 3-23 the increase in the loading applied to the different structural systems is

significant. Charts for all systems and Soil types have been provided in Appendix A.

The assumptions made for site soil class, structural system and performance have large implication

to the percentage of seismic loading that the lateral resisting system are subjected to. Where as

revision in the calculation of the natural period of the structures also has a bearing in relation to the

height of structures that are most effected.

Please note that the kp factor has been taken as equal to unity for this comparison and the Z factor

has been taken as 0.08, representing implications for Sydney structures.


May 2009


106

4 ANALYSIS COMPARISON OF A TYPICAL CONCRETE

STRUCTURAL SYSTEM

In order to highlight the revisions and implications of the new AS1170.4:2007 code it was decided

to compare the analysis and design of a typical concrete structural system. As was shown in Chapter

3 there is large increases in the seismic weight percentages applied as lateral loads to structures in

calculating the earthquake base shear, as well as revisions in torsion and deflection criteria. To

ensure a comparison of value is obtained the selection of building and site is of utmost importance.

Since the aim of this report is to demonstrate the differences in the two codes and determine the

structural implications the decision to use one structural construction type was taken so that the

focus of the research would remain on the original aim and not be split across related but separate

problems with multiple construction systems. A reinforced concrete building frame system with

reinforced concrete shear walls was therefore chosen.

4.1 Building, Site and Design Method Selection

4.1.1 Structural System

One of the most common construction systems due to its versatility is a reinforced concrete frame

with reinforced concrete shear walls. It is basically a column and beam frame system (which

supports the vertical loads) and where a reinforced concrete lift shaft has a dual purpose providing

the main obvious function and the primary lateral load resisting system of the building.

As was discussed 3.8.2 there are significant implications to building frame systems. The graphs

showing the differences in the base shear multiplication factor have been repeated in Figure 4-1 and

Figure 4-2 below for ease. In the old code all reinforced shear walls were to be considered “ductile”

and were to be detailed as such. Therefore, for the purpose of comparison the “ductile” design

response of the building frame system is only considered in this comparison.


May 2009


107

Figure 4-1 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4][7], for Soil Class Ae.

Figure 4-1 above shows the comparison on a soil class of Ae and it has been discussed previously

that the maximum loading that is applied to a “ductile” building frame system within the short

period range is 4.18%, which is an increase of 1%. There is a reduction of loading occurring after a

height of 12m for longer period structures.

Figure 4-2 above shows the comparison on a soil class of De and it has been discussed previously

that the maximum loading that is applied to a “ductile” building frame system within the short

period range is 6.54%, which is an increase of 3.21%. There is a reduction of loading occurring after

a height of 75m for longer period structures.


May 2009


108

Figure 4-2 shows the comparison of the base shear multiplier for BFS with RC walls, for AS1170.4: 1993 & 2007 [4] [7], for Soil Class De.

4.1.2 Elevation

Having established the percentage differences in the values of the base shear multiplier, a range of

structural heights were chosen to compare the structural implications. The revisions in the

earthquake design category were also considered when making these choices, as discussed in

Section 3.4.

Building 1 – A structural height of 14.4m was chosen (corresponding to actual physical floor height

requirements). This would allow the comparison for the loading applied in the short period range.

This height is within the earthquake design category 2. This requires a static analysis however there

is also a simplified design of structures method in clause 5.4.2.3 of the 2007 code that can be used

for structures not exceeding 15m. This simplified method is extremely conservative. This simplified

method applies a 6% seismic weight multiplier for rock sites and 13% for class De in Sydney for

building 1 (14.4m) which is extremely conservative. Therefore static analysis is considered here

using the calculation methods set-out in the code.


May 2009


109

Building 2 – A structural height of 29.7m was chosen (corresponding to actual physical floor height

requirements). This would allow the comparison for the maximum loading applied in the old code

for the De soil class.

Building 3 – A structural height of 56.1m was chosen (corresponding to actual physical floor

height). This would allow the comparison for the loading applied in the middle period range.

Building 4 – A structural height of 97.9ms was chosen (corresponding to actual physical floor

height requirements). This would allow the comparison for the loading applied in the longer period

range.

It should be noted also, that structural walls with aspect ratios greater than 2 are generally classified

as tall walls and tend to be flexure controlled, whilst squat walls with aspect ratios less than 2 tend to

be shear controlled. Tall walls tend to be lightly loaded axially, posses reasonable lateral load

capacity and behave like flexible cantilevers with drifts in excess of 1.0% possible. In contrast, shear

controlled walls, typically are very stiff with considerable lateral strength but crack at around 0.1%

drift and rapidly lose strength at around 0.75% drift.

It was decided to choose a building that has been designed in the office and then modify the

elevation by adding or subtracting floors to obtain the height required. Figure 4-3 shows the typical

architectural cross section through building type 3 being compared. For practicality an underground

car park, of various depths, was also considered for the other buildings.


May 2009


110

Figure 4-3 shows a typical architectural section through Building Type 3.

4.1.3 Plan

A standard floor plan for the above building chosen used a post tensioned reinforced slab with band

beams. The system allowed the office floors to have minimum requirement for columns and

interfaced well with a planning grid of 8250mm. The structural elements of the building are to be

designed to provide adequate performance for a minimum design life of 50 years.


May 2009


111

Figure 4-4 shows the typical architectural floor plate for all 4 buildings used in the comparison.

The floor plate was to be used in all four buildings being compared. The typical floor plate size is

1750m2 (50m x 35m).

The building comprises of a central core with a side atrium. Two shared passenger/goods goods lift

& stair cores, fronting both the main lobby and a service corridor at the rear, will be provided and

will serve all levels including the basement and roof plant room.


May 2009


112

The building floor plan is almost symmetrical. There is a slight difference in the properties of the

two cores and therefore there will be slight eccentricities in the x-x direction. The lateral resisting

element properties will be discussed next.

Figure 4-5 shows the typical structural plan for the buildings, highlighting the two lateral resisting cores.


May 2009


113

4.1.4 Core Properties

The cores in each building are maintained in the same dimensions. However depending on structural

height, they are required to be different thicknesses in order to withstand the stresses induced. Figure

4-6 and Figure 4-7 below show the typical core properties for the two cores of building 3.

Figure 4-6 shows the core properties for core number 1 for building 3.

The tributary area for vertical load carried by core number 1 is 115 m2. This is required when

calculating the stability and stresses acting on the core. It is generally considered beneficial to create

the largest possible tributary area for vertical load to be applied to the cores. However in this floor

plate design this is restricted by the columns between the cores. There small tributary area on the

cores will have a significant impact on the stresses in the cores.


May 2009


114

Figure 4-7 shows the core properties for core number 2 for building 3.

The tributary area for vertical load carried by core number 1 is 115 m2. This is required when

calculating the stability and stresses acting on the core.

4.1.5 Shear Centre and Centre of Mass

As discussed in Section 2.3 during an earthquake the acceleration-induced inertia forces will be

generated at each floor level, where the mass of the entire storey are assumed to be concentrated

(centre of mass). Torsion will be induced in the floor plate unless these forces act through the shear


May 2009


115

centre of the floor plate. Therefore both the location of the centre of mass and the shear centre

location are required to determine the behaviour of the structure in plan.

Figure 4-8 shows the calculation of the centre of mass and shear centre in the x-x direction.


May 2009


116

Figure 4-9 shows the calculation of the centre of mass and shear centre for the y-y direction

4.1.6 AS1170.4:1993 Design Eccentricity Calculation

The design eccentricities have to be calculated for both directions:

X-X Direction (Load applied perpendicular) –

ed1 = A1es + 0.05b

ed2 = A2es - 0.05b


May 2009


117

Where,

A1 = dynamic eccentricity factor = [2.6– 3.6(es/b)] or 1.4, whichever is greater

es = 8.59 – 7.05 = 1.54 (Figure 4-8)

b = 49.5

A1 = [2.6– 3.6(1.54/49.5)] = 2.488


ed1 = 2.488(1.54) + 0.05(49.5) = 6.30m

ed2 = 0.5(1.54) - 0.05(49.5) = -1.705m

Y-Y Direction (Load applied perpendicular) –

es = 0 (Figure 4-9)

b = 35

A1 = [2.6– 3.6(0/49.5)] = 2.6


ed1 = 2.6(0) + 0.05(35) = +1.75m

ed2 = 0.5(0) - 0.05(35) = -1.75m

4.1.7 AS1170.4:2007 Design Eccentricity Calculation

The design eccentricities have to be calculated for each direction of earthquake action and orientated

to produce the most adverse torsion moment:

X-X Direction (Load applied perpendicular) –


May 2009


118

ed1 = es + 0.1b

ed2 = es - 0.1b

es = 8.59 – 7.05 = 1.54 (Figure 4-8)

b = 49.5

ed1 = (1.54) + 0.1(49.5) = 6.49m

ed2 = (1.54) - 0.1(49.5) = -3.41m

Y-Y Direction (Load applied perpendicular) –

es = 0 (Figure 4-9)

b = 35

ed1 = (0) + 0.1(35) = +3.5m

ed2 = (0) - 0.1(35) = -3.5m

4.1.8 Analysis Method

The equivalent lateral force method procedure is used for the static analysis and consists of the

following steps;

Estimate the first-mode natural period.

Choose the appropriate seismic base shear coefficient.

Calculate the seismic design base shear

Distribute the base shear as component forces acting at different levels of the structure.

Analyze the structure under the design lateral forces to obtain design actions, such as moments and shears


May 2009


119

Estimate structural displacements and particularly, story drifts.

4.1.9 Site & Structural Factors

4.1.10 Structural Classification / Importance level for the structure

When considering the old 1993 code the structure for building 3, is classified as Type II, which are

structures including buildings that are designed to contain a large number of people, or people of

restricted or impaired mobility.

For the new 2007 code this corresponding to Importance Level 3 (2002 & 2007) defined in Part B of

the Building Code of Australia [11].

4.1.11 Acceleration Coefficient/Hazard Factor

The site of the structure is to be considered in Sydney therefore:

a = 0.08 (1993 & 2002)

Z = 0.08 (2007)

4.1.12 Probability Factor k p

For Importance Level 3 Structure in the BCA 2002, for a 1:500 year annual probability of

exceedance: kp = 1.0

For Importance Level 3 Structure in the BCA 2007, for a 1:1000 year annual probability of

exceedance: kp = 1.3

4.1.13 Site Factor / Sub Soil Class

Comparison has been done for soil class Ae and De and both these soil types will be considered for

the four buildings being compared.

4.1.14 Period Calculation for the buildings

An example of the natural period of the structure will be shown below for building 3.

For 1993: The Fundamental Period: hn/46 = 56.1/46 = 1.22 sec


May 2009


120

For 1993: The Orthogonal Direction: hn/58 = 56.1/58 = 0.97 sec

For 2007: T1 = 1.25kthn0.75 for the ultimate limit state

Where Kt = 0.05 for all other structures

T1 = 1.25 x 0.05 x (56.1)0.75 = 1.28 sec

4.1.15 Response Factor and Ductility Ratio

For 1993: Response Factor Rf for a Building Frame System: Rf = 6.0

For 2007: Ductility Ratio: Sp/µ = 0.22

The inverse of the ductility ratio corresponds to Rf = µ/ Sp = 4.5

4.1.16 Earthquake Base shear Multiplier

An example of the calculation for the earthquake base shear multiplier is shown below for building

3.

1993: For Soil Class Ae

V = I (CS/Rf) Gg

C = 1.25a/T2/3 n=2/3; Factor=1.25

V/Gg = 1.25aIS/RfT2/3

V Fund = 1.0 x 0.08 x (1/6) x ((1.25 x 0.67)/1.222/3) = 0.009 Gg

V Orth = 1.0 x 0.08 x (1/6) x ((1.25 x 0.67)/0.962/3) = 0.011 % Gg

2007: For Soil Class Ae

V = Cd(T1)Wt


May 2009


121

Cd(T1) = C(T1) (Sp/µ)

C(T1) = Ch(T1)kpZ

Ch(T1) = 0.558 (Obtained from Table 6.4)

V ductile walls = (1.3 x 0.08) x (0.558) x (0.22) = 0.013 Wt

These values will be used to calculate the total horizontal equivalent static forces for the building

and the over turning moment on the lateral resisting system.

4.1.17 Loads

The following Table 4-1, Table 4-2, Table 4-3, Table 4-4, Table 4-5 and Table 4-6 show the typical

loading that has been used in the analysis and design of the buildings being compared.

Basement Levels: (1750m2) kPa Min Max

Live Load: (ψc= 0.4) Car Parking 2.5 0 1

Dead Load Slab (160mm Dp) 24 x 0.16 = 3.84 3.84 3.84

Beams (300mm Dp) Equivalent 1.2 1.2 1.2 Service Dead Loads Screeds 1.2 1.2

Services 0.3 0.3

Total UDL (kPa) 6.54 7.54

Load per Floor (kN) 11445 13195 Table 4-1 shows the typical loading to be taken for the basement areas

Typical Floor: (1750m2) kPa Min Max

Live Load: (ψc= 0.4) Office 3 0 1.2


Beams (450mm Dp) Equivalent 1.7 1.7 1.7 Service Dead Loads Walls 1 1

Screeds 1.2 1.2

Services 0.3 0.3


Load per Floor (kN) 14490 16590 Table 4-2 shows the typical loading to be taken for the floor areas


May 2009


122

Plant Room Floor: (1750m2) kPa Min Max

Live Load: (ψc= 0.4) Plant 5 0 2


Beams (500mm Dp) Equivalent 1.5 1.5 1.5

Service Dead Loads Walls 1 1

Screeds 1.2 1.2

Services 0.3 0.3


Load per Floor (kN) 15400 18900 Table 4-3 shows the typical loading to be taken for the plant floor

Steel Roof: (770m2) kPa Min Max

Live Load: (ψc= 0.0) Roof 0.25 0.1 0.1

Dead Load Steel Beams 0.5 0.5

Cladding 0.4 0.4 Service Dead Loads Services 0.3 0.3


Total Roof Load (kN) 1001 2002 Table 4-4 shows the typical loading to be taken for the steel roof

Core Walls: kPa Min/Max

Core Dead Load:

(300mm Wd)

24 x 0.3 = 7.2

Area Core 1: 6.66 m2 Area Core 2: 10.83 m2

Height of Core Walls Building 1: 14.4 m 104 kN

Total Load Core 1 691 kN











Table 4-5 shows the typical loading to be taken for each core


May 2009


123

Axial Load on Core Walls: Min Max

Trib Area for Each Core 115 m2 kPa kPa

Roof 1.3 2.6

Plant 8.8 10.8

Typical Floor 8.28 9.48

Basement 6.54 7.54

Height of Core Walls Building 1: (4 Floors & Roof) P

Min P

Max

Total Load Core 1 691 kN 4649 5351


Height of Core Walls Building 1: (2 Base, 4 Floors, Plant & Roof)

P Min

P Max




P Min

P Max




P Min

P Max



Table 4-6 shows the minimum and maximum axial load acting on the cores.

It should be noted at this point also that the kp factor used for the building type for the two codes will

be used, which will produce higher values. However the lateral loading to be considered for

robustness will not apply to these comparisons, only due to the fact that currently it is 2.5% but they

are proposing to reduce this to 1.5% for buildings taller than 15m and 1% for less than 15m but this

has not been issued for public discussion yet.

It should also be noted that for the sake of safety dead loads are customarily overestimated. This fact

should be considered in seismic design, whenever gravity load effects enhance strength when

combined with effects of seismic forces, such as may occur when estimating stresses within the

cores.


May 2009


124

4.2 Hand Calculation Analysis

After defining the building geometry above, spreadsheets were created using Microsoft Excel for all

four building and two soil class cases Ae and De. The equivalent lateral force method is used for the

static analysis.

All spreadsheets for all four buildings can be referenced in Appendix B

4.2.1 First-Mode of Natural Period

As discussed previously in Section 3.5, there was revision in the calculation of the period of a

structure. This revision can be seen below in Table 4-7. Also the base shear multipliers have been

shown for comparison.

First Mode Natural Period (sec) Base shear Multiplier

Ae Base shear Multiplier

De

AS1170.4:1993 AS1170.4:2007 AS1170.4:1993 AS1170.4:1993

Ref Height

(m) Fund Orth kt = 0.05 Fund Orth 2007 Fund Orth 2007

1 14.40 0.31 0.25 0.46 0.024 0.028 0.04 0.033 0.033 0.084

2 29.70 0.65 0.51 0.80 0.015 0.017 0.02 0.033 0.033 0.084

3 56.10 1.22 0.97 1.28 0.1 0.1 0.13 0.022 0.025 0.056

4 97.90 2.13 1.69 1.95 0.1 0.1 0.01 0.015 0.017 0.028 Table 4-7 shows the first mode of natural period and base shear multiplier for the four buildings

The 2007 values in Table 4-7 have been calculated using a kp factor of 1.3. As can be seen in the

Table 4-7 above the building 1 (14.4m) has the most significant increase in loading with a 42%

increase for buildings founded on rock and 150% for the more onerous soil class De. Building 2

(29.70m) and Building 3 (56.10m) have less of an increase in the loading multiplier for rock, which

is 17% and 30%, but have far larger increase for the soil class De, being 154% and 124%

respectively. These values have been expressed in the comparison tables for the base shear for the

four buildings in Table 4-8 and Table 4-9.

4.2.2 Seismic Design Base Shear

The base shear was calculated for all four buildings using the Multiplier set out in Table 4-7 and the

loadings defined in Section 4.1.17.


May 2009


125

As can be seen in Table 4-8 and Table 4-9 there is an approximate average increase of 18% in the

base shear for Class Ae soils within the short period range but there is a decrease of 35% for the

longer period building 4. There are significant increases of 150% in the base shear for all buildings

within the short and medium period range on soil class De, although it is not as large for the longer

period buildings, being shown as 60%.

Base shear Min Loading (G only)

Ref Height (m)

Soil Class

Fundamental kN

Orthogonal kN

2007 kN

Numerical Difference

% Difference

1 14.4 Ae 1478 1725 2165 439 25.5

De 2035 2035 5139 3105 152.6

2 29.7 Ae 1424 1662 1946 284 17.1

De 3176 3176 8021 4846 152.6

3 56.1 Ae 2023 2310 2583 273 11.8

De 4431 5172 11249 6077 117.5

4 97.9 Ae 3670 3670 2368 -1302 -35.5

De 5545 6472 10479 4007 61.9 Table 4-8 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes for minimum loading

Base shear Max Loading (G + ψcG)

Ref Height (m)

Soil Class

Fundamental kN

Orthogonal kN

2007 kN


% Difference

1 14.4 Ae 1687 1969 2471 501 25.5

De 2341 2341 5913 3572 152.6

2 29.7 Ae 1653 1929 2259 330 17.1

De 3686 3686 9310 5625 152.6

3 56.1 Ae 2333 2663 2978 315 11.8

De 5109 5962 12969 7006 117.5

4 97.9 Ae 4241 4241 2736 -1504 -35.5

De 6407 7478 12109 4630 61.9 Table 4-9 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes for maximum loading

4.2.3 Overturning Moment

The overturning moment was calculated for all four buildings using the vertical load distribution as

shown in Figure 4-10 below.


May 2009


126

Figure 4-10 shows the vertical distribution of the earthquake base shear for both AS1170.4:1993 & 2007 [4] [7] and [34]

As can be seen in Table 4-10 and Table 4-11 there is an approximate average increase of 18% in the

overturning moment for Class Ae soils within the short period range but there is a decrease of 35%

for the longer period building 4.

Overturning Moment (G only)

Ref Height (m) Soil Class Fundamental

kNm Orthogonal

kNm 2007 kNm


% Difference

1 14.4 Ae 21288 24846 31170 6324 25.5

De 29297 29297 74004 44706 152.6

2 29.7 Ae 42294 49362 57809 8447 17.1

De 94312 94312 238228 143916 152.6

3 56.1 Ae 113507 129596 144915 15319 11.8

De 248595 290139 631081 340942 117.5

4 97.9 Ae 359277 359277 231811 -127466 -35.5

De 542858 633578 1025889 392311 61.9 Table 4-10 shows the differences in the overturning moment values for the AS110.4:1993 & 2007 codes for minimum loading

There are significant increases of 150% in the overturning moment for all buildings within the short

and medium period range on soil class De, although it is not as large for the longer period buildings,

being shown as 60% in Table 4-10 and Table 4-11.


May 2009


127

Overturning Moment (G + ψcG)

Ref Height (m) Soil Class

Fundamental kNm

Orthogonal kNm

2007 kNm

Numerical Difference % Difference

1 14.4 Ae 24297 28357 35576 7218 25.5

De 33707 33707 85142 51435 152.6

2 29.7 Ae 49092 57296 67101 9805 17.1

De 109471 109471 276519 167048 152.6

3 56.1 Ae 130857 149404 167065 17661 11.8

De 286593 334487 727542 393055 117.5

4 97.9 Ae 415148 415148 267860 -147288 -35.5

De 627277 732104 1 185423 453319 61.9 Table 4-11 shows the differences in the overturning moment values for the AS110.4:1993 & 2007 codes for maximum loading

4.2.4 Torsion

The torsion was calculated using the equations described in Section 4.1.5 and the percentage

differences are shown in Table 4-12 and Table 4-13 below. There are similar percentage differences

tabulated for the torsion in the x-direction as was seen for the overturning moment in Section 4.2.3.

However in the y-direction there are much larger percentage increases.

Torsion (G only) Fundamental Orthogonal 2007

X-Direction Y-Direction X-Direction Y-Direction X-Dir Y-Dir

Ref Height (m)

Soil Class (ed1) (ed2) (ed1) (ed2) (ed1) (ed2) (ed1) (ed2) (e) (e)

1 14.4 Ae 9323 2521 2587 2587 10881 2942 3019 3019 14048 7576

De 12831 3469 3560 3560 12831 3469 3560 3560 33353 17987

2 29.7 Ae 8981 2428 2492 2492 10482 2834 2909 2909 12632 6813

De 20026 5414 5557 5557 20026 5414 5557 5557 52057 28074

3 56.1 Ae 12760 3450 3541 3541 14569 3939 4043 4043 16765 9041

De 27946 7555 7755 7755 32616 8818 9051 9051 73007 39372

4 97.9 Ae 23144 6257 6422 6422 23144 6257 6422 6422 15367 8287

De 34970 9454 9704 9704 40814 11034 11325 11325 68008 36676


May 2009


128

Torsion (G only)



Ref Height (m) Soil Class

X-Direction kN X-Direction

Y-Direction kN Y-Direction

1 14.4 Ae 3167 29.1 4557 150.9

De 20522 159.9 14427 405.2

2 29.7 Ae 2151 20.5 3904 134.2

De 32031 159.9 22517 405.2

3 56.1 Ae 2196 15.1 4998 123.6

De 40391 123.8 30322 335.0

4 97.9 Ae -7777 -33.6 1865 29.0

De 27195 66.6 25351 223.8 Table 4-12 shows the differences in the torsion values for the AS110.4:1993 & 2007 codes for minimum loading

The increases in the y-direction account for the increase in the allowance for accidental torsion, even

when the building is considered ideally symmetrical.

Torsion ( G + ψcG) Fundamental Orthogonal 2007

X-Direction Y-Direction X-Direction Y-Direction X-Dir Y-Dir

Ref Height (m)

Soil Class (ed1) (ed2) (ed1) (ed2) (ed1) (ed2) (ed1) (ed2) (e) (e)

1 14.4 Ae 10641 2877 2953 2953 12419 3358 3446 3446 16034 8647

De 14762 3991 4096 4096 14762 3991 4096 4096 38373 20694

2 29.7 Ae 10424 2818 2893 2893 12166 3289 3376 3376 14663 7908

De 23245 6284 6450 6450 23245 6284 6450 6450 60425 32586

3 56.1 Ae 14710 3977 4082 4082 16795 4541 4661 4661 19327 10423

De 32218 8710 8940 8940 37602 10166 10434 10434 84167 45390

4 97.9 Ae 26743 7230 7421 7421 26743 7230 7421 7421 17757 9576

De 40408 10924 11213 11213 47161 12750 13087 13087 78584 42380


May 2009


129

Torsion (G + ψcG)



Ref Height (m)

Soil Class X-Direction X-Direction Y-Direction Y-Direction

1 14.4 Ae 3615 29.1 5201 150.9

De 23611 159.9 16598 405.2

2 29.7 Ae 2496 20.5 4531 134.2

De 37179 159.9 26136 405.2

3 56.1 Ae 2532 15.1 5762 123.6

De 46565 123.8 34956 335.0

4 97.9 Ae -8986 -33.6 2155 29.0

De 31424 66.6 29293 223.8 Table 4-13 shows the differences in the torsion values for the AS110.4:1993 & 2007 codes for maximum loading

4.2.5 Tension & Compression Core Stresses due to Overturning Moment

The calculation of the tension and compression stresses in the core allows the easy identification of

the cores ability to satisfactorily support the lateral loads. The tension and compression stresses have

been calculated using the following equation: P/A ± M/Z. The distribution of the lateral loads to the

cores was based on the stiffness proportion of each core to the stiffness of the cores together.

Core 1 Tension Min Loading (G only) N/mm2

Ref Height (m)

Core Thickness (mm) Soil Class Fundamental Orthogonal 2007


% Difference

1 14.40 200 Ae 0.10 -0.06 -0.03 0.13 134

200 De -0.11 -0.11 -2.10 1.99 1809

2 29.70 200 Ae -0.11 -0.42 -0.80 0.69 627

400 De -1.39 -1.39 -4.88 3.49 251

3 56.10 300 Ae -1.11 -1.61 -2.08 0.97 87

400 De -4.21 -5.22 -13.49 9.28 220

4 97.90 400 Ae -5.48 -5.48 -2.39 -3.09 56

400 De -9.93 -12.13 -21.64 11.71 118 Table 4-14 shows the comparison of tension stress induced in core 1 for minimum loading

In Table 4-14 and Table 4-15 the tension stresses induced in core 1 due to the over turning moment

have been presented. It can be see that for the minimum loading that the tension stresses developed

within the cores are just on the limits of the tensile capacity of 60MPa concrete. For Building 3 on

soil class De and Building 4 on both rock and soil, core 1 is over stressed. Further revision to the


May 2009


130

configuration or thickness of the shear walls is required in order for building 4 to be stabilised. Also

the cores require a larger axial force to be applied in order to reduce the tensile induction.

Core 1 Tension Max Loading (G + ψcG) N/mm2

Ref Height (m)

Core Thickness (mm)

Soil Class Fundamental Orthogonal 2007


% Difference

1 14.40 200 Ae -0.04 -0.22 -0.54 0.50 1250

200 De -0.45 -0.45 -2.73 2.28 507

2 29.70 200 Ae -0.41 -0.77 -1.21 0.80 195

400 De -1.76 -1.76 -5.81 4.05 230

3 56.10 300 Ae -1.65 -2.22 -2.77 1.12 68

400 De -5.13 -6.30 -15.83 10.70 209

4 97.90 400 Ae -6.83 -6.83 -3.26 -3.57 52

400 De -11.98 -14.52 -25.51 13.53 113 Table 4-15 shows the comparison of tension stress induced in core 1 for maximum loading

In Table 4-15 the tensile capacity for the maximum loading is shown. Core 1 within the analysis of

Building 3 and 4 are shown as outside the tensile capacity range of 60MPa concrete.

Core 1 Compression Min Loading (G only) N/mm2

Ref Height (m)

Core Thickness (mm)



% Difference

1 14.40 200 Ae 2.14 2.30 2.58 0.44 21

200 De 2.50 2.50 4.48 1.98 79

2 29.70 200 Ae 3.90 4.22 4.59 0.69 18

400 De 3.18 3.18 6.67 3.49 110

3 56.10 300 Ae 6.24 6.73 7.21 0.97 16

400 De 8.09 9.10 17.37 9.28 115

4 97.90 400 Ae 12.37 12.37 9.28 -3.09 -25

400 De 16.82 19.02 28.53 11.71 70 Table 4-16 shows the comparison of compression stress induced in core 1 for minimum loading

Table 4-16 and Table 4-17 show the compressive stresses in core 1 for the four buildings. Although

there is a large increase in the compression stresses in core 1 for all four buildings they are all with

limits.


May 2009


131

Core 1 Compression Max Loading (G + ψcG) N/mm2

Ref Height (m)

Core Thickness (mm)



% Difference

1 14.40 200 Ae 2.27 2.45 2.78 0.51 22

200 De 2.69 2.69 4.98 2.29 85

2 29.70 200 Ae 4.20 4.57 5.00 0.80 19

400 De 3.55 3.55 7.60 4.05 114

3 56.10 300 Ae 6.77 7.35 7.89 1.12 17

400 De 9.01 10.17 19.71 10.70 119

4 97.90 400 Ae 13.72 13.72 10.15 -3.57 -26

400 De 18.87 21.41 32.40 13.53 72 Table 4-17 shows the comparison of compression stress induced in core 1 for maximum loading

The stresses have been distributed to the two cores in proportion to their stiffness. Therefore core 2

has a larger portion of the load applied to it. This is reflected in the larger stress values shown in

Table 4-18 and Table 4-19.

Core 2 Tension Min Loading (G only) N/mm2

Ref Height (m)

Core Thickness (mm)



1 14.40 200 Ae -1.29 -1.63 -2.22 0.93 -72

200 De -2.04 -2.04 -6.23 4.19 205

2 29.70 200 Ae -2.75 -3.41 -4.20 1.45 53

200 De -4.60 -4.60 -11.20 6.60 143

3 56.10 300 Ae -5.69 -6.73 -7.72 2.03 36

400 De -11.10 -13.16 -30.06 18.96 171

4 97.90 400 Ae -15.63 -15.63 -9.31 -6.32 40

400 De -24.73 -29.23 -48.69 23.96 97 Table 4-18 shows the comparison of tension stress induced in core 2 for minimum loading

As shown in Table 4-18 and Table 4-19 core 2 of buildings 3 and 4 are over stressed in tension if

60MPa concrete is specified. Additional axial load is required on the cores or modifications of the

properties of the cores or configuration.


May 2009


132

Core 2 Tension Max Loading (G + ψcG) N/mm2

Ref Height (m)

Core Thickness (mm)



% Difference

1 14.40 200 Ae -1.58 -1.96 -2.63 1.05 66

200 De -2.46 -2.46 -7.28 4.82 196

2 29.70 200 Ae -3.39 -4.16 -5.08 1.69 50

400 De -4.82 -4.82 -13.10 8.28 172

3 56.10 300 Ae -6.81 -8.01 -9.15 2.34 34

400 De -12.98 -15.36 -34.85 21.87 168

4 97.90 400 Ae -18.40 -18.40 -11.10 -7.30 40

400 De -28.92 -34.12 -56.60 27.68 96 Table 4-19 shows the comparison of tension stress induced in core 2 for maximum loading

Table 4-20 and Table 4-21 show the core 2 compression stresses in the cores. Although there are

large increases in the stresses especially on the more onerous soil case they are within limits of

60MPa concrete.

Core 2 Compression Min Loading (G only) N/mm2

Ref Height (m)

Core Thickness (mm)



1 14.40 200 Ae 2.79 3.12 3.72 0.93 33

200 De 3.54 3.54 7.73 4.19 118

2 29.70 200 Ae 5.33 6.00 6.79 1.46 27

400 De 5.37 5.37 12.51 7.14 133

3 56.10 300 Ae 9.16 10.20 11.19 2.03 22

400 De 13.71 15.77 32.68 18.97 138

4 97.90 400 Ae 20.26 20.26 13.94 -6.32 -31

400 De 29.36 33.86 53.32 23.96 82 Table 4-20 shows the comparison of compression stress induced in core 2 for minimum loading

Building 4 has been compared although the cores are not satisfactory for building stability. Further

work would be required to obtain a lateral support system that would provide adequate resistance for

the loads applied.


May 2009


133

Core 2 Compression Max Loading (G + ψcG) N/mm2

Ref Height (m)

Core Thickness (mm)



% Difference

1 14.40 200 Ae 3.07 3.45 4.13 1.06 35

200 De 3.95 3.95 8.77 4.82 122

2 29.70 200 Ae 5.97 6.74 7.66 1.69 28

400 De 6.12 6.12 14.41 8.29 135

3 56.10 300 Ae 10.28 11.48 12.62 2.34 23

400 De 15.59 17.97 37.46 21.87 140

4 97.90 400 Ae 23.03 23.03 15.73 -7.30 -32

400 De 33.55 38.75 61.23 27.68 83 Table 4-21 shows the comparison of compression stress induced in core 2 for maximum loading

4.2.6 Shear Force on Core due to Base Shear & Torsion

Base shear and shear load due to torsion are to be combined to give the worst effect on the core

walls. The torsion values have been presented above.

Base shear Due to Torsion Minimum Loading (G only) kN

Fundamental Orthogonal 2007

X-Direction Y-Direction X-Direction Y-Direction X-Direction Y-Direction

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Ref Height (m)

Soil Class 8.59 5.88 11.58 2.89 8.59 5.88 11.58 2.89 8.59 5.88 11.58 2.89

1 14.4 Ae 382 71 143 36 446 306 167 42 576 395 419 105

De 526 97 197 49 526 360 197 49 1368 937 995 248

2 29.7 Ae 368 68 138 34 430 294 161 40 518 355 377 94

De 822 152 307 77 822 562 307 77 2136 1462 1553 387

3 56.1 Ae 523 97 196 49 598 409 224 56 688 471 500 125

De 1147 212 429 107 1338 916 501 125 2995 2050 2178 543

4 97.9 Ae 949 176 355 89 949 650 355 89 630 432 458 114

De 1435 266 537 134 1674 1146 626 156 2790 1910 2028 506 Table 4-22 shows the comparison of additional shear due to torsion for minimum loading


May 2009


134

Base shear Due to Torsion Maximum Loading (G + ψcG) kN



Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Ref Height (m)

Soil Class 8.59 5.88 11.58 2.89 8.59 5.88 11.58 2.89 8.59 5.88 11.58 2.89

1 14.4 Ae 437 81 163 41 510 349 191 48 658 450 478 119

De 606 112 227 57 606 415 227 57 1574 1078 1145 286

2 29.7 Ae 428 79 160 40 499 342 187 47 602 412 437 109

De 954 176 357 89 954 653 357 89 2479 1697 1802 450

3 56.1 Ae 604 112 226 56 689 472 258 64 793 543 576 144

De 1322 245 494 123 1543 1056 577 144 3453 2364 2510 627

4 97.9 Ae 1097 203 410 102 1097 751 410 102 728 499 530 132

De 1658 307 620 155 1935 1324 724 181 3224 2207 2344 585 Table 4-23 shows the comparison of additional shear due to torsion maximum loading

As can be seen in Table 4-22 and Table 4-23 there is a large increase in the base shear in the

perpendicular direction due to the introduction of 10% building width eccentricity rather than the

previously defined 5%.

The torsion loading has been distributed to the two cores in proportion to the rotational stiffness of

the cores in each direction. This loading is then to be included in the total loading applied to the

walls. The base shear is also distributed to the cores in relation to the stiffness properties of the

cores.

When analysising for seismic events the effect of cracking must be considered but for the

distribution of stresses in the cores the proportion will be distributed similarly so it is not relevant for

this sections stress comparison.


May 2009


135

Base shear Minimum Loading (G only) kN



Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Ref Height (m)

Soil Class 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8

1 14.4 Ae 606 872 296 1183 707 1018 345 1380 887 1277 433 1732

De 834 1200 407 1628 834 1200 407 1628 2107 3032 1028 4111

2 29.7 Ae 584 840 285 1139 681 981 332 1330 798 1148 389 1557

De 1302 1874 635 2540 1302 1874 635 2540 3289 4732 1604 6417

3 56.1 Ae 830 1194 405 1619 947 1363 462 1848 1059 1524 517 2067

De 1817 2614 886 3545 2120 3051 1034 4137 4612 6637 2250 8999

4 97.9 Ae 1505 2165 734 2936 1505 2165 734 2936 971 1397 474 1894

De 2273 3272 1109 4436 2653 3818 1294 5177 4296 6183 2096 8383 Table 4-24 shows the comparison of equivalent horizontal base shear for minimum loading

Table 4-24 and Table 4-25 show the values of the total base shear applied to the two cores within the

building in proportion to the stiffness of the cores.

Base shear Maximum Loading (G + ψcG) kN



Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Ref Height (m)

Soil Class 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8

1 14.4 Ae 692 996 337 1350 807 1162 394 1575 1013 1458 494 1976

De 960 1381 468 1873 960 1381 468 1873 2424 3488 1183 4730

2 29.7 Ae 678 975 331 1322 791 1138 386 1543 926 1333 452 1807

De 1511 2175 737 2949 1511 2175 737 2949 3817 5493 1862 7448

3 56.1 Ae 956 1376 467 1866 1092 1571 533 2131 1221 1757 596 2382

De 2095 3014 1022 4087 2445 3518 1192 4770 5317 7652 2594 10375

4 97.9 Ae 1739 2502 848 3392 1739 2502 848 3392 1122 1614 547 2189

De 2627 3780 1281 5126 3066 4412 1496 5982 4964 7144 2422 9687 Table 4-25 shows the comparison of equivalent horizontal base shear for maximum loading


May 2009


136

Total Base shear Minimum Loading (G only) kN



Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Ref Height (m)

Soil Class 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8

1 14.4 Ae 989 943 439 1218 1154 1324 512 1422 1464 1672 852 1836

De 1361 1298 604 1677 1361 1561 604 1677 3475 3969 2023 4360

2 29.7 Ae 952 908 423 1174 1111 1275 493 1370 1316 1503 766 1651

De 2124 2026 942 2617 2124 2436 942 2617 5424 6194 3157 6804

3 56.1 Ae 1353 1291 600 1668 1545 1772 686 1904 1747 1995 1017 2191

De 2963 2827 1315 3652 3459 3967 1535 4262 7607 8687 4427 9543

4 97.9 Ae 2454 2341 1089 3025 2454 2815 1089 3025 1601 1829 932 2009

De 3708 3537 1646 4570 4328 4964 1921 5334 7086 8092 4124 8889 Table 4-26 shows the comparison of total torsional and equivalent horizontal base shear for minimum loading

Table 4-26 and Table 4-27 show the total shear loading distributed to the two cores for minimum

applied loading while Table 4-28 and Table 4-29 show the total base shear for the maximum applied

base shear loading.

Total Base shear Minimum Loading (G only) kN


X-Direction Y-Direction X-Direction Y-Direction

Core 1 Core 2 Core 1 Core 2 Core 1 Core 2 Core 1 Core 2

Ref Height (m) Soil Class 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8

1 14.4 Ae 310 348 340 414 27 26 66 29

De 2115 2408 1419 2683 155 154 235 160

2 29.7 Ae 205 228 273 281 18 18 55 21

De 3301 3758 2214 4187 155 154 235 160

3 56.1 Ae 202 223 331 287 13 13 48 15

De 4149 4720 2892 5280 120 119 188 124

4 97.9 Ae -853 -987 -157 -1016 -35 -35 -14 -34

De 2759 3128 2204 3556 64 63 115 67 Table 4-27 shows the comparison of percentage difference in the total torsional and equivalent horizontal base shear for

minimum loading


May 2009


137

It is shown that core 2 attracts much larger loadings in the y-direction due to its larger stiffness’s. It

would be beneficial to have cores of relatively similar stiffness in order to distribute the loading

evenly between the two cores.

Total Base shear Maximum Loading (G + ψcG) kN



Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Core 1

Core 2

Ref Height (m)

Soil Class 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8

1 14.4 Ae 1128 1076 501 1391 1317 1511 584 1623 1671 1908 972 2096

De 1565 1493 695 1929 1565 1796 695 1929 3998 4566 2327 5016

2 29.7 Ae 1105 1054 491 1362 1290 1480 573 1590 1528 1745 889 1917

De 2465 2351 1094 3038 2465 2827 1094 3038 6296 7190 3664 7898

3 56.1 Ae 1560 1488 692 1922 1781 2043 790 2195 2014 2300 1172 2526

De 3416 3259 1516 4210 3987 4574 1770 4914 8770 10015 5104 11001

4 97.9 Ae 2836 2705 1259 3495 2836 3253 1259 3495 1850 2113 1077 2321

De 4285 4087 1902 5281 5001 5736 2219 6163 8188 9351 4766 10272 Table 4-28 shows the comparison of total torsional and equivalent horizontal base shear for maximum loading

Total Base shear Maximum Loading (G + ψcG) kN

Difference % Difference

X-Direction Y-Direction X-Direction Y-Direction

Core 1 Core 2 Core 1 Core 2 Core 1 Core 2 Core 1 Core 2

Ref Height (m) Soil Class 0.41 0.59 0.2 0.8 0.41 0.59 0.2 0.8

1 14.4 Ae 354 397 388 473 27 26 66 29

De 2433 2770 1632 3087 155 154 235 160

2 29.7 Ae 238 265 317 327 18 18 55 21

De 3831 4363 2570 4860 155 154 235 160

3 56.1 Ae 233 257 382 331 13 13 48 15

De 4783 5441 3335 6088 120 119 188 124

4 97.9 Ae -985 -1140 -182 -1174 -35 -35 -14 -34

De 3188 3614 2546 4109 64 63 115 67 Table 4-29 shows the comparison of percentage difference in the total torsional and equivalent horizontal base shear for

minimum loading

4.2.7 Structural Displacements

A fundamental assumption underlying the provisions in AS1170.4:1993 and 2007 for the design of

energy dissipation and ductility is that the global inelastic response of a structure to monotonic


May 2009


138

lateral forces is bilinear, i.e. close to linear elastic-perfectly-plastic. The elastic stiffness used in

analysis should correspond to the stiffness of the elastic part of bilinear global force-displacement

response. This is to say, that the use of the full elastic stiffness of uncracked concrete or masonry in

the analysis is completely inappropriate. For this reason in calculating the deflection, the analysis

should take into account the effect of cracking. Moreover the stiffness of concrete members

corresponding to the initiation of yielding of the reinforcement should be used.

Without modelling it is considered that a stiffness of 50% of the uncracked stiffness will give a

every conservative result. The lower the stiffness value used in the analysis the higher the second-

order effects observed, which is on the safe-side of design.

Paulay [30] recommends an equivalent moment of inertia Ie for a cantilever wall to be calculated

using the following formula:

Ie = [(100/fy) + (Pu/fc’A g)] Ig

For the calculations in this section the equation above was used but it can be seen that the effective

stiffness are then approximately 25% which is extremely conservative.

4.2.8 Deflection at Roof Level

Using the equation above to calculate the equivalent moment of inertia for the cores the deflections

at each storey were calculated. Table 4-30 through to Table 4-37, present the deflections at roof level

of the two cores under minimum and maximum loading in both directions.

It can be seen in Table 4-30 and Table 4-31, for minimum and maximum loads respectively, that the

deflection at roof level for Core 1 for building 1 (14.4m) and 2 (29.70m) are within the serviceability

limits of span/500 for both the soil class Ae and De. For building 3 (56.10m) the deflections are

satisfactorily within the limits for the soil class Ae, however they are just outside the limits at

span/450 for soil class De.


May 2009


139

Core 1 Deflection Min Loading (G only) mm X-Direction

Ref Height (m) Limit

Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 0.7 0.8 1.0 0.21 27

200 De 0.9 0.9 2.3 1.38 147

2 29.70 59.40 200 Ae 4.6 5.2 6.5 1.25 27

400 De 5.7 5.5 14.7 9.21 163

3 56.10 112.20 300 Ae 28.0 30.1 36.2 6.05 22

400 De 48.6 53.6 125.2 71.58 147

4 97.90 195.80 400 Ae 278.0 261.8 175.1 -86.62 31

400 De 420.0 461.6 775.0 313.45 75 Table 4-30 shows the comparison of percentage difference in the deflection at roof level of Core 1 for minimum loading (X-

direction)

The core arrangement for Building 4 has been shown to be too highly stressed in tension in Section

4.2.5. It is however clear that the deflection for buildings on rock class Ae have been reduced by

approximately 30%. It is also shown that there is a large increase in deflection observed for

buildings on the more onerous soil class De. Both cores require their properties to be revised to

satisfy the stress and deflection demands.

Core 1 Deflection Max Loading (G + ψcG) mm X-Direction


Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 0.8 1.0 1.2 0.28 34

200 De 1.1 1.1 2.9 1.82 160

2 29.70 59.40 200 Ae 5.5 6.2 7.7 1.48 27

400 De 6.7 6.5 17.5 10.92 163

3 56.10 112.20 300 Ae 32.7 35.3 42.3 7.08 22

400 De 57.0 62.7 146.5 83.79 147

4 97.90 195.80 400 Ae 328.0 309.0 206.7 -102.27 31

400 De 495.7 544.8 914.7 369.87 75 Table 4-31 shows the comparison of percentage difference in the deflection at roof level of Core 1 for maximum loading (X-

direction)


May 2009


140

Core 2 Deflection Min Loading (G only) mm X-Direction


Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 0.7 0.8 1.0 0.22 31

200 De 1.0 1.0 2.4 1.41 145

2 29.70 59.40 200 Ae 6.7 7.6 9.4 1.80 27

200 De 8.4 8.1 21.8 13.62 163

3 56.10 112.20 300 Ae 40.8 44.0 52.8 8.84 22

400 De 71.9 79.2 185.0 105.83 147

4 97.90 195.80 400 Ae 411.0 387.0 258.9 -128.08 31

400 De 621.0 682.5 1146.0 463.47 75 Table 4-32 shows the comparison of percentage difference in the deflection at roof level of Core 2 for minimum loading (X-

direction)

It is observed that Core 2 has similar scale deflections as shown in Table 4-32 and Table 4-33, for

the minimum and maximum loading. Building 3 is shown to have satisfactory deflection for both

soil cases. Building 4 still remains unsatisfactory for deflection demands.

Core 2 Deflection Max Loading (G + ψcG) mm X-Direction


Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 0.8 1.0 1.2 0.25 30

200 De 1.2 1.2 2.8 1.66 142

2 29.70 59.40 200 Ae 7.9 9.0 11.1 2.14 27

400 De 9.9 9.7 25.8 16.14 163

3 56.10 112.20 300 Ae 47.8 51.5 61.9 10.35 22

400 De 84.2 92.8 216.7 123.90 147

4 97.90 195.80 400 Ae 474.5 447.6 299.1 -148.46 31

400 De 716.9 789.3 1323.9 534.54 75 Table 4-33 shows the comparison of percentage difference in the deflection at roof level of Core 2 for maximum loading (X-

direction)

The deflection in the Y-direction is shown to be larger, which is due to a smaller stiffness value in

this direction. As can be seen in Table 4-34 and Table 4-35, the values are similarly scaled as shown

above for the x-direction however the values are more significant.


May 2009


141

Core 1 Deflection Min Loading (G only) mm Y-Direction


Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 2.11 2.44 3.17 0.73 35

200 De 2.91 2.88 7.53 4.65 160

2 29.70 59.40 200 Ae 2.28 2.59 3.21 0.62 27

400 De 2.76 2.68 7.17 4.49 163

3 56.10 112.20 300 Ae 13.74 14.80 17.78 2.98 22

400 De 23.71 26.11 61.00 34.89 147

4 97.90 195.80 400 Ae 135.50 127.59 85.37 -42.22 -31

400 De 204.74 225.00 377.80 152.80 75

Table 4-34 shows the comparison of percentage difference in the deflection at roof level of Core 1 for minimum loading (Y-direction)

Building 1, 2 and 3 are all within the deflection limits. As discussed previously Building 4 fails the

deflection limits specified for soil class De however the deflection is shown to reduce for soil class

Ae, giving satisfactory deflections.

Core 1 Deflection Max Loading (G + ψcG) mm Y-Direction


Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 2.53 2.92 3.80 0.88 35

200 De 3.51 3.47 9.09 5.62 160

2 29.70 59.40 200 Ae 2.71 3.07 3.81 0.74 27

400 De 3.27 3.18 8.50 5.32 163

3 56.10 112.20 300 Ae 16.09 17.33 20.81 3.48 22

400 De 27.76 30.58 71.42 40.84 147

4 97.90 195.80 400 Ae 156.41 147.56 98.62 -48.94 -31

400 De 236.34 260.22 436.45 176.23 75 Table 4-35 shows the comparison of percentage difference in the deflection at roof level of Core 1 for maximum loading (Y-

direction)

As core 2 is stiffer and attracts a larger applied loading, the deflection in the Y-direction is far

greater than in the x-direction. As can be seen in Table 4-36 and Table 4-37, Building 1, 2 and 3 for

soil class Ae are all within the deflection limits. In the soil class De analysis Building 3 is shown to

have 150% larger deflections than in the old code.


May 2009


142

Core 2 Deflection Min Loading (G only) mm Y-Direction


Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 2.18 2.47 3.07 0.60 28

200 De 3.00 2.98 6.93 3.95 132

2 29.70 59.40 200 Ae 8.97 10.19 12.62 2.43 27

400 De 11.27 10.97 29.31 18.34 163

3 56.10 112.20 300 Ae 55.68 59.31 71.23 11.92 21

400 De 96.85 106.67 249.19 142.52 147

4 97.90 195.80 400 Ae 553.48 521.18 348.71 -172.47 -31

400 De 836.30 919.00 1543.21 624.21 75 Table 4-36 shows the comparison of percentage difference in the deflection at roof level of Core 2 for minimum loading (Y-

direction)

Even with the large reductions in the deflections shown in the tables below for Building 4 on soil

class Ae the core still fails the deflection limits specified. Further work on modifying the cores to

meet the deflection criteria should be carried out to get a fairer assessment of the values.

Core 2 Deflection Max Loading (G + ψcG) mm Y-Direction


Core Thickness (mm)



% Difference

1 14.40 28.80 200 Ae 2.42 2.75 3.43 0.68 28

200 De 3.36 3.34 7.85 4.51 134

2 29.70 59.40 200 Ae 10.64 12.08 14.96 2.88 27

400 De 13.36 13.00 34.74 21.74 163

3 56.10 112.20 300 Ae 64.45 69.45 83.39 13.94 22

400 De 113.40 124.91 291.75 166.84 147

4 97.90 195.80 400 Ae 638.91 602.75 402.84 -199.91 -31

400 De 965.37 1062.94 1782.78 719.84 75 Table 4-37 shows the comparison of percentage difference in the deflection at roof level of Core 2 for maximum loading (Y-

direction)

The percentage difference values will not change significantly with a modification of wall

thicknesses, however if the cores are modified without relation to each other loading distribution

between the cores could result in significant differences.

It should also be noted that the two cores analysed in the above tables are considered to be deflecting

independently of each other. If the cores were coupled using coupler beams the forces, over turning


May 2009


143

moments, stresses and deflections would have completely different patterns. An investigation into

the differences in the codes on a coupled wall system would be very beneficial.

4.2.9 Storey Drift and P-Delta Effects

The instability effects due to second-order P-Delta effects can be substantial. The deflections in the

previous section are used to calculate the storey drift in order to establish if second-order effects

need to be considered.

The two cores were individually analysed for p-delta effects were to be considered.

Table 4-38 to Table 4-41, show the inter-storey drift for the top floor of each building for both soil

classes for the two cores in both directions. They show the stability coefficient for the floors being

considered and then states whether P-delta effects should be considered. If the stability factor is less

than 0.1 then P-delta effects do not have to be considered. If the stability coefficient is greater than

0.2 then the structure is potentially unstable and shall be redesigned. The calculations for the

stability coefficient were made easier by using the same axial load for the roof level presented in

Table 4-3 for all four buildings. The earthquake horizontal shear values where taken form the tables

for each building and soil class provided in Appendix B for the roof level.


May 2009


144

Table 4-38 shows the comparison of storey drift and P-delta consideration of Core 1 for minimum and maximum roof loading (X-direction)


May 2009


145

Table 4-39 shows the comparison of storey drift and P-delta consideration of Core 2 for minimum and maximum roof loading (X-direction)


May 2009


146

Table 4-40 shows the comparison of storey drift and P-delta consideration of Core 1 for minimum and maximum loading (Y-direction)


May 2009


147

Table 4-41 shows the comparison of storey drift and P-delta consideration of Core 2 for minimum and maximum loading (Y-direction)


May 2009


148

As can be seen in Table 4-38 to Table 4-41 Building 4 is unstable with the cores as specified and

needs to be reanalysed using modified core properties. It can be seen that there is a large increase in

the values of the inter-storey drift values for all buildings for both soil cases.

P-delta effects are required to be considered for Core 2 in Building 3, this is due to the large

deflections imposed on the structural core due to its relatively larger stiffness. It should be

considered that a less onerous effective moment of inertia is used in the deflection analysis therefore

both reducing the deflections and the instability of second-order effects.

Please note that due to the number of spreadsheets for the deflection and P-delta effects it was not

feasible to provide them all in the appendix, therefore they have been included with this report on

the attached CD-ROM.

4.3 Etabs Analysis

In this section the Hand Calculations and Etabs computer model results are compared for accuracy

of the methods used for building 3.

Figure 4-11 shows the model used for building 3 within the Etabs model


May 2009


149

The model was created for the structure and then the equivalent earthquake loads for the two codes

for the two soil classes being analysed were inputted.

4.3.1 First-Mode of Natural Period

Table 4-42 shows the results for building 3 from the hand calculations carried out in Section 4.2.1.

Table 4-43 show the calculated first 5 modes for the building using Etabs. As can be seen the period

of the structure for the first mode of natural frequency is significantly larger in the computer model.

This large first mode of natural period would ensure that less load can be considered when analysing

and designing the building. It also highlights the crudeness of the empirical methods stated in the

codes.

First Mode Natural Period (sec) Base shear Multiplier

Ae Base shear Multiplier

De

AS1170.4:1993 AS1170.4:2007 AS1170.4:1993 AS1170.4:1993

Ref Height

(m) Fund Orth kt = 0.05 Fund Orth 2007 Fund Orth 2007

3 56.10 1.22 0.97 1.28 0.1 0.1 0.13 0.022 0.025 0.056 Table 4-42 shows the first mode of natural period and base shear multiplier for building 3

Etabs Model

Mode Natural Period (sec) Frequency

1 3.48 0.28

2 3.44 0.29

3 2.16 0.46

4 0.99 1.01

5 0.78 1.28 Table 4-43 shows the first five modes of natural period for building 3 calculated using Etabs

4.3.2 Seismic Design Base Shear

The base shear has been compared. However the equivalent earthquake shear forces were inputted

into the system from the hand calculations and therefore this allows a model comparison rather than

an output result comparison. Table 4-44 shows the hand calculated base shears and Table 4-45

shows the base shear out puts from the Etabs model. The results shown in the tables numerically

correlate well therefore giving the first indication that the model will satisfactorily represent

expected responses.


May 2009


150

Base shear Hand Calculation

Ref Load Case

Soil Class

Fundamental kN

Orthogonal kN

2007 kN


% Difference

3 Min Ae 2023 2310 2583 273 11.8

De 4431 5172 11249 6077 117.5

3 Max Ae 2333 2663 2978 315 11.8

De 5109 5962 12969 7006 117.5 Table 4-44 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes by hand calculations

Base shear for Building 3 By Etabs

Dir Load Case

Soil Class 1993 kN 2007 kN

Difference kN

Percentage Difference

X Min Ae 2060 2630 570 27%

De 4510 11500 6990 155%

X Max Ae 2370 3040 670 28%

De 5200 13200 8000 154%

Y Min Ae 2310 2590 280 12%

De 4440 11300 6860 155%

Y Max Ae 2660 2980 320 12%

De 5110 18000 12890 252% Table 4-45 shows the differences in the base shear values for the AS110.4:1993 & 2007 codes using Etabs

4.3.3 Overturning Moment

The overturning moment was calculated for building 3 using the Etabs model. Table 4-46 shows the

difference in the over turning moments calculated by hand. Table 4-47 shows the difference in the

overturning moment for the two soil classes Ae and De for building 3.

Overturning Moment Hand Calculations

Ref Load Case Soil Class Fundamental

kNm Orthogonal

kNm 2007 kNm


% Difference

3 Min Ae 113507 129596 144915 15319 11.8

De 248595 290139 631081 340942 117.5

3 Max Ae 130857 149404 167065 17661 11.8

De 286593 334487 727542 393055 117.5 Table 4-46 shows the differences in the overturning moment values for the AS110.4:1993 & 2007 codes for minimum loading

The over turning moment has increased in projection with the percentages seen in the hand

calculations however the numerical value of the over turning moment is significantly less.


May 2009


151

Overturning Moment for Building 3 By Etabs

Dir Load Case

Soil Class 1993 kNm 2007 kNm

Difference kNm


X Min Ae 53800 69200 15400 28%

De 118000 301000 183000 155%

X Max Ae 62500 80500 18000 28%

De 137000 350000 213000 154%

Y Min Ae 59100 69100 10000 17%

De 118000 301000 183000 155%

Y Max Ae 68600 80400 11800 17%

De 137000 596000 459000 335% Table 4-47 shows the differences in the over turning moment for the AS110.4:1993 & 2007 codes using Etabs

4.3.4 Stresses

The stresses obtained from the computer model are not similar in magnitude to the stresses

calculated by hand. Table 4-48 shows the stresses calculated by hand while Table 4-49 shows the

stresses calculated by Etabs.

Maximum Stresses For Core 2 Hand Calculations

Ref Load Case Soil Class Fundamental

N/mm2 Orthogonal N/mm2

2007 N/mm2


% Difference

3 Tension Ae -6.81 -8.01 -9.15 2.34 34%

De -12.98 -15.36 -34.85 21.87 168%

3 Compression Ae 10.28 11.48 12.62 2.34 23%

De 15.59 17.97 37.46 21.87 140% Table 4-48 shows the differences in the stresses (Core 2) for the AS110.4:1993 & 2007 codes

Maximum Stresses For Cores Etabs Calculations

Ref Load Case Soil Class 1993

N/mm2 2007

N/mm2 Difference N/mm2


3 Tension Ae -1.887 -4.912 3.025 160%

De -7.216 -21.394 14.178 196%

3 Compression Ae 1.814 4.053 2.239 123%

De 6.037 17.652 11.615 192% Table 4-49 shows the differences in the stresses in the cores for the AS110.4:1993 & 2007 codes

As can be seen in the tables above the compression stresses in the cores are of much smaller

magnitude than in the hand calculations; however the tension stresses are of slightly smaller

magnitude.


May 2009


152

Figure 4-12 shows the meshing of the supporting cores by the Etabs model the colour of the segments represents the stress in the element. Etabs uses a colour range to express the stresses.

4.3.5 Deflections

The deflections were calculated for the minimum and maximum loads applied through the centre of

mass. Table 4-50 and Table 4-51 show the hand calculated values predicted for the deflection, while

Table 4-52 and Table 4-53 show the values obtained from the Etabs model. As can be seen the

values correlate very well for the rock Ae soil class however there is larger deflections predicted for

the more onerous soils class De. A maximum predicted deflection is 183 mm which is outside the

limit of 112mm.


May 2009


153

Core Deflections mm X-Direction Hand Calculations


Core Thickness (mm)



% Difference

Core 1 Min 112.20 300 Ae 28.0 30.1 36.2 6.05 22

400 De 48.6 53.6 125.2 71.58 147

Core 2 Min 112.20 300 Ae 40.8 44.0 52.8 8.84 22

400 De 71.9 79.2 185.0 105.83 147

Core 1 Max 112.20 300 Ae 32.7 35.3 42.3 7.08 22

400 De 57.0 62.7 146.5 83.79 147

Core 2 Max 112.20 300 Ae 47.8 51.5 61.9 10.35 22

400 De 84.2 92.8 216.7 123.90 147 Table 4-50 shows the comparison of percentage difference in the deflection at roof level in the X-direction

Core Deflections mm Y-Direction Hand Calculations


Core Thickness (mm)



% Difference

Core 1 Min 112.20 300 Ae 13.74 14.80 17.78 2.98 22

400 De 23.71 26.11 61.00 34.89 147

Core 2 Min 112.20 300 Ae 55.68 59.31 71.23 11.92 21

400 De 96.85 106.67 249.19 142.52 147

Core 1 Max 112.20 300 Ae 16.09 17.33 20.81 3.48 22

400 De 27.76 30.58 71.42 40.84 147

Core 2 Max 112.20 300 Ae 64.45 69.45 83.39 13.94 22

400 De 113.40 124.91 291.75 166.84 147 Table 4-51 shows the comparison of percentage difference in the deflection at roof level in the X-direction


May 2009


154

Figure 4-13 shows the deflective shape in the Y-direction for the most onerous 185mm deflection from Etabs

The building analysed fails in deflection for the most onerous soil class and the cores and lateral

stability supports are to be reconsidered.

Core Deflections mm X-DirectionEtabs

Ref Load Class Limit

Core Thickness (mm)

Soil Class 1993 2007 Difference


Core 1 Min 112.20 300 Ae 28.0 36.0 8 28%

400 De 63.0 160 97 154%

Core 2 Min 112.20 300 Ae 29.0 36.0 27 94%

400 De 63.0 160 97 154%

Core 1 Max 112.20 300 Ae 34.0 42 8 23%

400 De 74.0 185 111 150%

Core 2 Max 112.20 300 Ae 34.0 42 4 12%

400 De 74.0 185 111 150% Table 4-52 shows the comparison of percentage difference in the deflection at roof level in the X-direction


May 2009


155

Core Deflections mm Y-Direction Etabs

Ref Load Class Limit

Core Thickness (mm)

Soil Class 1993 2007 Difference


Core 1 Min 112.20 300 Ae 13.0 13.0 0 0%

400 De 25.0 60.0 35.0 140%

Core 2 Min 112.20 300 Ae 13.0 18.0 5.0 38%

400 De 26.0 78.0 52.0 200%

Core 1 Max 112.20 300 Ae 14.0 18.0 3.0 21%

400 De 30.0 133.0 103.0 343%

Core 2 Max 112.20 300 Ae 15.0 20.0 5.0 33%

400 De 30.0 183.0 153.0 510% Table 4-53 shows the comparison of percentage difference in the deflection at roof level in the X-direction

4.4 Conclusions

At this point the differences in the code have become apparent and it is evident that there are

significant implications to varying structural heights.

By comparing the values in the tables in this section it is clear that the loading applied to all

buildings has increased within the short and medium period range. The tension induced in lateral

supporting cores has increased dramatically on the soil class De. As can be seen in Table 4-18 (Core

2 tension stresses for minimum loading) for a building approximately 30m in height the tension load

has increased by 150%. This building height is very common for residential developments and

commercial developments. Therefore using the values for the 2007 code, an average 200mm thick

reinforced core wall (60MPa) will fail in tension. Careful consideration must be now considered in

the assessing of existing buildings constructed on this soil class.

The total base shear due to the equivalent earthquake horizontal force and torsion has been

calculated and presented. As an example of the implications to a 15m high building, consider the

base shear increase from 1350kN to 3475kN applied to Core 1 of Building 1, on soil class De,

shown in Table 4-26 and Table 4-27. The walls would not be designed for this increased value.

AS3826 – 1998 [9] was introduced to set out limits for the assessment and analysis of the earthquake

resistance of existing buildings for the old AS1170.4:1993 code. There has been no revision to this

code to-date to reflect the changes introduced by the 2007 code.


May 2009


156

The deflections predicted have increased considerably. However the larger deflections obtained in

the Tables could relate to the excessively conservative cracked stiffness value used in the

calculations.

Errors and Discrepancies may have occurred due to the following two reasons:

1) Equivalent Moment of Inertia Ie, used to represent reinforcement yield. (Section 4.2.7).

2) Core Configuration and Stiffness Distribution

These are highlighted by the failure of the structural cores by force, moment or deflection in the

above calculations for the building comparisons. The same structural core system is satisfactory for

the short to medium height buildings (period) however for taller buildings a reconfiguration of cores

is required. Additional stiffness and a more even distribution of load would provide a satisfactory

result.

Comparing the analysis of the structures using hand calculations and Etabs, it is quiet clear that the

calculation methods produced similar expectations however there have been large differences in the

magnitude relating to some of the values.

Sources of discrepancies may relate to values of material properties and additional stiffness within

the model. The results of the comparison show that if calculations are carried out by hand the results

would be generally conservative however careful consideration should be given to deflection

calculations and material and element properties assumed.

The next chapter presents the final conclusions and suggestions for further work to expand the scope

of this topic.


May 2009


157

5 CONCLUSIONS AND FUTURE WORK

There are many differences in the two codes as shown in Section 3 and Section 4. Wilson and Lam’s

advancement in relation to the elastic response spectra for Australia has been a major influence in

revisions in the code in both soil and structural response behaviour which has been discussed in

Section 2.2 and Section 3.3. The soils resonance behaviour and the repercussions this has on a

structure with a coinciding natural frequency of vibration has been identified as a critical

consideration requirement. This has been included in the new code as a spectral shape factor which

combines the soil and structural period influences for calculating the earthquake lateral forces

applied to the structure. One of the main impacts of considering this phenomenon are increased

loadings being applied to the more onerous soil classes than have been previously considered.

The elastic and dynamic response of structures has been considered in greater detail in the new 2007

code. The performance of the structure and the detailing requirements to obtain the appropriate

response are highlighted by notational revisions. The influence of ductility and structural

performance has created large increases in the inter-storey drift and P-delta effects.

During the course of this research a number of questions were raised as to why this research was

necessary.

What are the differences between the AS1170.4:1993 and 2007 code?

This question has been answer in depth in Section 3 and was achieved by identifying the main

factors revised in the code and demonstrating how they influence the analysis of structural behaviour

during a seismic event.

These factors are:

Site factors

Sub-soil Classes


May 2009


158

Structural Natural Period

Torsion

Drift and P-delta effects

What are the differences between the calculation methods?

This is a practical question for general application within a design office environment. This question

is raised as a result of the code comparison for inelastic/dynamic analysis, and the ability to calculate

accurately structural properties manually.

The comparison of the hand calculations and computer aided analysis was carried out to establish

the conservative nature of the code provisions. The calculations required for the analysis of the four

structures for this comparison was the most time consuming task in the analysis process. The

decision was made to limit the dynamic analysis to one structure (Building 3, 56.1m). The

comparison of the hand and etabs model showed that the two methods correlate, however, the

magnitude of both stresses and deflection vary in magnitude. The reasons for errors and

discrepancies has been discussed in Section 4.4

What are the implications for new and existing building design due to revisions in Site Factor/ Sub

Soil Class Classification?

This question needed to be answered to ascertain the practical repercussions of the code revisions

and is one of the most onerous. The old code did not take into account the resonance effect of the top

founding soil layers. The new AS1170.4:2007 code has been updated to cater for this large structural

response amplification.

What are the implications for new and existing building design due to revisions in Period

Calculation?

This is the first of three questions that needed to be answered to establish the impacts to new and

past design and detailing methods. This question was raised as a result of the previous application of


May 2009


159

an empirical measurement of period for the structure not taking into account the construction system

used and the more accurate method provided in the new code AS1170.4: 2007 [7].

The consideration of structural system type and construction material within the calculation of the

natural period for the structure has allowed for a less conservative natural period to be obtained. The

steep increase of natural period observed for moment resisting frames shown in Figure 3-8

corresponds with reduction in loading applied to these structural systems shown in the graphs in

Section 3.8 and Appendix A. If differentiating between systems is not carried out an “all other

structures” category is provided which produces similar natural period’s values for structures as

produced in the old 1993 code.

What are the implications for new and existing building design due to revisions in Torsion?

This was the second of three questions. The increase in accidental torsion applied to ideally

symmetrical buildings has significant increased effects on the total base shear while it doesn’t

decrease the effects for unsymmetrical buildings. The comparisons are discussed in detail in Section

4.2.4.

What are the implications for new and existing building design due to revisions in Drift

determination and P-Delta Effects?

This was the third and last of three questions relating to the effects of vertical structural geometry

and elastic and inelastic response. The impact of the revision in the new code [7] for the calculation

of inter-storey drift and P-delta effects is due to revisions in the elastic and inelastic response

spectrum and the revisions to the ductility of a structure.

The influence of increased loading has large implications on the deflections observed in the analysis;

these subsequently have implications on the inter-storey drift and the instability effects of P-delta. It

is shown how the stiffness properties of the lateral load resisting elements are the most crucial when

assessing the deflections predicted for the cores. The influence of cracked concrete and

reinforcement at yield must be accurately assessed in order to achieve creditable predictions.


May 2009


160

What are the limitations to structural systems and combinations?

This question was raised as a result of code restrictions and system limitations. Limitations specified

in the new code have used the approach that favours site specific studies and the requirement for

consideration of special detailing requirements of structures and the economic benefit balance of

system selection or site location.

The new code considers all buildings as irregular and does not limit height of any of the structural

systems. It does however, restrict the used of the ductility factor µ to less than 3, for design within

the limits of AS1170.4:2007 and the Appendix A of AS3600. This means any special moment

resisting frames have to be designed and detailed to NZ 1170.5. There is also a restriction on the

hazard factor Z before the use of the New Zealand code is used. The hazard factor must be under 0.3

to be within the AS1170.4 code. Importance level 4 structures that are required for post disaster

service require a special study to be carried out to ensure serviceability for a design event for an

importance level 2 building is maintained.

5.1 Limitations

The main limitation of the analysis comparison is the input values for the core values inputted in the

hand calculations. For example as discussed in Section 4.2.7 the influence of cracked concrete and

reinforcement yield must be considered in the calculation of deflections and P-delta effects. This

value of equivalent moment of inertia has not been specified and therefore general rules should be

used. 50% is considered conservative although Paulay has suggested a value that has an approximate

value of 25% for the core and axial load distributions within the 4 buildings considered in this

comparison. The use o this approximate 25% values in the calculation of deflection have lead to

large predicted values for both deflections and inter-storey drift. By considering a larger percentage

of the equivalent moment of inertia (Ie) of the cores, the values for deflection predicted would be

less however resulting also in less ductility. This illustrates the difference in design for elastic and

ductile response. The difference in the deflections that are predicted if the building was to be

considered to be more elastic would be in the order of 50%, for a consideration of 50% of the

moment of inertia of the cores.


May 2009


161

The other observation with the building plan chosen for comparison within this report is that the

cores are highly stressed due to lack of tributary area for the axial load to be supported. This induces

large tensions in the cores due to the over-turning moment. Possible improvements that could

alleviate this would be rearrangement of the columns around the core to induce larger axial loads in

the cores would reduce the tensile stresses in the cores.

5.2 Contributions

The goal of this report was to identify the reasons and revisions to the AS1170.4 earthquake design

code. Also implications were to be demonstrated by analysing four buildings on two soil types to

establish the range of influence. The techniques proposed in this report towards achieving this

overall goal were based on the following ideas:

Establish the Magnitude of Applied Load for the 4 Structures – By using a standard floor plate

and only altering the structural height of the building for determining the applied structural load, the

calculations for the equivalent horizontal shears were minimised and it is possible to establish tables

for easy comparison of loading implications.

Establish the Stresses and Deflection Implications – By identifying the loadings the stresses in the

cores and deflection predictions were then obtained and equated for comparison.

Etabs Model – To obtain a comparison of the values for the hand calculations and also carry out a

dynamic response comparison, Building 3 was chosen to be modelled in ETABS. By creating the

model and in putting the loads specified in Section 4.1.17 a comparison of Building 3 was presented.

5.3 Success Criteria

T two criteria questions can be raise to assess to what extent this report has succeeded in achieving

its goals:

1) In what ways can the implications be readily used?

2) To what extent information contained here in can be applied to a wide variety of building types?


May 2009


162

This report has provided easy to use base shear multiplier comparison graphs for all building

structural systems within Appendix A. Add to this the comparison tables produced in Section

4showing the difference in the natural period, base shear, over-turning moment , torsion, core

stresses, deflection, inter-storey drift and P-delta effects and the result is a significant comparison of

the codes with a reduction in the length of time required to complete a building systems base shear

calculation and assess what implications it may have on the structure.

5.4 Future Work

The next step in the comparison of the AS1170.4 1993 and 2007 code is in establishing the

implications to existing buildings and to develop an assessment method for renovations if required.

The following subsections will discuss possible areas that require further development to show the

differences between the codes and the introduction of the new capacity response spectrum analysis

methods introduced by Wilson et al.

5.4.1 Fragility Curves

The capacity response spectrum represents seismic demand in the form of acceleration-displacement

response spectrum and the structural capacity is estimated from a non-linear push-over analysis.

Wilson and Lam [36] consider the development of fragility curves for different structural systems

the next challenge for Australian earthquake engineering to assist in risk modelling.

Therefore the development of fragility curves for cores and shear walls for the four buildings

analysed in Section 4 would be establish the structural behaviour of these elements in a seismic

events.

5.4.2 Design and Detailing of the Lateral Supporting System

No implementation details of the experimental results were presented, which implies that these

analyses are purely theoretical.

Using the comparison and loads presented in this report it would be beneficial to design the above

core shear walls and specify detailing requirements to ascertain satisfactory lateral capacity.


May 2009


163

5.5 Final Note

The research presented in this report has demonstrated that the new code AS1170.4:2007 has

implemented technical advances in the area of soil and structural behaviour during seismic events

from the past 15 years. Although loadings have been increased dramatically, especially for the more

onerous soil classes the actual cost of designing a structure to survive a seismic event is a fraction of

the overall cost of a building.

The basis of a buildings survival during a seismic event is in the ductility detailing of the lateral

supporting system and the behaviour of framing elements whether structural or non structural to

deformations and stresses induced by the event.

By using the comparison tables of the earthquake base shear multiplier in Appendix A for the

calculation of applied lateral loads to a structural system and using the comparison tables set out in

Section 4 implications of the new AS1170.4: 2007code have been demonstrated.


May 2009


164

BIBLIOGRAPHY

[1] AS/NZS 1170.0:2002 Structural design actions Part 0: General Principles [2] AS/NZS 1170.1:2002 Structural design actions Part 1: Permanent, imposed and other

actions [3] AS/NZS 1170.2:2002 Structural design actions Part 3: Wind Actions [4] AS/NZS 1170.4:1993 Minimum design loads on structures Part 4: Earthquake loads [5] AS/NZS 1170.4 Supp 1:1993 Minimum design loads on structures Part 4: Earthquake

loads - Commentary [6] AS/NZS 1170:2002 Structural design actions Part 0: General Principles General Principles,

Appendix D, Factors for use with AS1170.4-1993 [7] AS 1170.4:2007 Structural design actions Part 4: Earthquake actions in Australia [8] AS 3600:2001 Concrete Structures [9] AS 3826:1998 Strengthening existing buildings for earthquake [10] AS 4100:1998 Steel Structures [11] Building Code of Australia [12] Bungum, H., Project 3: Hazard, Risk and Loss. ICG Assessment,

http://www.geohazards.no/projects/project3_08/project_3_earthq.htm, Accessed November 2008

[13] Corus in Construction, Teaching resources, Structural Principles, http://www.corusconstruction.com/en/reference/teaching_resources/architectural_studio_reference/design/choice_of_structural_systems_for_multi/structural_principles/, Accessed November 2008.

[14] Dowrick, J.D. Earthquake Resistant Design, SJohn Wiley & Sons, Ltd., 1977. [15] European Steel Designers Education Program. ESDEP. http://www.esdep.org Accessed

November 2008. [16] Fardis et al. Designers’ guide to EN1998-1 and EN1998-5 Eurocode 8: Design of

structures for earthquake resistance. Thomas Telford, 2005. [17] Gibson, Gary. Seismological contributions to earthquake risk mitigation. In Proceedings of AEEC

Conference, November 2006. [18] Gurley, Colin. Protecting life and reducing damage in earthquakes and terrorist attacks. In

Proceedings of AEEC Conference, November 2006. [19] Kayvani, K. and Barzegar, F. Influence of local inertia on seismic response of offshore jackets.

Engineering Structures, Vol 18, No 2, pp. 93-101, 1996. [20] Kayvani, K. and Barzegar, F. Hysteretic modelling of tubular members and offshore platforms. Journal

of Structural Engineering, Vol 123, No 1, January 1997. [21] Kayvani, K., Schmidt, B., Steele, J. and Sidwell, G. HSeismic Engineering for Replacement

Research Reactor in Australia. EIn Proceedings of Earthquake Engineering, Pacific Conference, 2003.

[22] Lam, N. and Wilson, J. The new response spectrum model for Australia. eJSE International Special Issue, Earthquake Engineering in the low and moderate seismic regions of Southeast Asia and Australia, http://www.ejse.org/Archives/Fulltext/2008/Special1/200801.pdf, Accessed November 2008.

[23] Li, B., Duffield, C.F. and Hutchinson G.L. A parametric study of the lateral performance of a high-rise structure. In Proceedings of ASEC Conference, June 2008.


May 2009


165

[24] Lumantarna, E.,Vaculik, J., Griffith, M., Lam, N. and Wilson, J. Seismic fragility curves for un-reinforced masonry walls. In Proceedings of AEES Conference, November 2006.

[25] McPherson, A. and Allen, T. An improved understanding of earthquake ground shaking in Australia. In Proceedings of AEES Conference, November 2006.

[26] McPherson, A. and Hall, L. Site Classification for earthquake hazard and risk assessment in Australia. In Proceedings of AEES Conference, November 2006.

[27] National Geographic. Large earthquake “Bounces” are stronger than gravity. http://news.nationalgeographic.com/news/2008/10/081030-earthquake-bounce.html?source=rss Accessed November 2008.

[28] NZS 1170.5:2004 Structural Design Actions Part 5: Earthquake actions – New Zealand [29] NZS 1170.5 Supp 1:2004 Structural Design Actions Part 5: Earthquake actions – New

Zealand - Commentary [30] Paulay, T. and Priestley, M.J.N. Seismic Design of Reinforced Concrete and Masonry Buildings. John

Wiley & Sons, Inc., 1992. [31] Rodsin, K., Lam, N., Wilson, J. and Goldsworthy, H. Seismic fragility curves for soft-storey

buildings. In Proceedings of AEES Conference, November 2006. [32] SAI Global, AS1170.4 Earthquake Actions in Australia. In Proceedings of SAI Seminar, May

2007. [33] Venkatesan, S., Lam, N. and Wilson, J. Simple model accounting for the soil resonance phenomenon.

In Proceedings of AEES Conference, November 2006. [34] Wilson, J. and Lam, N. Earthquake design of buildings in Australia using Velocity and Displacement

Principles. In Proceedings of SAI Global Training, May 2007, Australian Journal of Structural Engineers, Vol 6, No 2, 2006

[35] Wilson, J. and Lam, N. A recommended Earthquake Response Spectrum Model for Australian. In Proceedings of SAI Global Training, May 2007, Australian Journal of Structural Engineers, Vol 5, No 1, 2003

[36] Wilson, J. and Lam, N. Recent developments in the research and practice of earthquake engineering in Australia. In Proceedings of AEES Conference, November 2006.


May 2009


166

APPENDIX A: CODE COMPARISON GRAPHS FOR STRUCTURAL SYSTEMS

AND SITE SUB SOIL CLASSES

This appendix includes the graphs for the comparison of the Seismic Weight Multiplier for the

Calculation of the Earthquake Base shear, V.

1) Hazard Factor (Z)/ Acceleration Coefficient (a) Used – Sydney was chosen as the main city for comparison, emphasizing implications to the immediate locality and projects where my design office is based. 0.08 is the value used.

2) Probability Factor (k p) – This factor was chosen as unity, representing the 1 in 500 year probability of exceedance. All results from this comparison can then be factored according to the serviceability requirements of the structure.

Comparisons were carried out using Excel 2003.


May 2009


167

Bearing Wall System

Bea

ring

Wal

l Sys

tem

Com

paris

on fo

r S

oil C

lass

Ae

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ae

1993

(F

und

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ae

1993

(O

rth

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ae

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Ae

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


168

Bea

ring

Wal

l Sys

tem

Com

paris

on fo

r S

oil C

lass

Be

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Be

1993

(F

und

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Be

1993

(O

rth

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Be

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Be

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


169

Bea

ring

Wal

l Sys

tem

Com

paris

on fo

r S

oil C

lass

Ce

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ce

1993

(F

und

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ce

1993

(O

rth

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ce

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Ce

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


170

Bea

ring

Wal

l Sys

tem

Com

paris

on fo

r S

oil C

lass

De

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

De

1993

(F

und

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

De

1993

(O

rth

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

De

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

De

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


171

Bea

ring

Wal

l Sys

tem

Com

paris

on fo

r S

oil C

lass

Ee

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ee

1993

(F

und

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ee

1993

(O

rth

P)

Rf =

4.5

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ee

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Ee

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


172

Building Frame System with Reinforced Concrete Walls

Bui

ldin

g Fr

ame

Sys

tem

Com

paris

on fo

r Soi

l Cla

ss A

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ae

1993

(Fun

d P

) R

f = 6

.0 R

einf

orce

d C

oncr

ete

She

ar W

alls

Ae

1993

(Ort

h P

) R

f = 6

.0 R

einf

orce

d C

oncr

ete

She

ar W

alls

Ae

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Ae

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


173

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss B

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Be

1993

(F

und

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Be

1993

(O

rth

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Be

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Be

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


174

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss C

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ce

1993

(F

und

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ce

1993

(O

rth

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ce

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Ce

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


175

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss D

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

De

1993

(F

und

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

De

1993

(O

rth

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

De

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

De

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


176

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss E

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ee

1993

(F

und

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ee

1993

(O

rth

P)

Rf =

6.0

Rei

nfor

ced

Con

cret

e S

hear

Wal

ls

Ee

2007

u/S

p =

4.5

Duc

tile

She

ar W

alls

Ee

2007

u/S

p =

2.6

Lim

ited

Duc

tile

She

ar W

alls


May 2009


177

Building Frame System with Concentrically Braced Frames

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss A

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ae

1993

(F

und

P)

Rf =

5.0

Con

cent

rical

ly-B

race

d F

ram

esA

e 19

93 (

Ort

h P

) R

f = 5

.0 C

once

ntric

ally

-Bra

ced

Fra

mes

Ae

2007

u/S

p =

4.5

Mod

erat

ely

Duc

tile

Con

cent

rical

ly-B

race

d F

ram

esA

e 20

07 u

/Sp

= 2.

6 Li

mite

d D

uctil

e C

once

ntric

ally

-Bra

ced

Fra

mes


May 2009


178

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss B

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Be

1993

(F

und

P)

Rf =

5.0

Con

cent

rical

ly-B

race

d F

ram

esB

e 19

93 (

Ort

h P

) R

f = 5

.0 C

once

ntric

ally

-Bra

ced

Fra

mes

Be

2007

u/S

p =

4.5

Mod

erat

ely

Duc

tile

Con

cent

rical

ly-B

race

d F

ram

esB

e 20

07 u

/Sp

= 2.

6 Li

mite

d D

uctil

e C

once

ntric

ally

-Bra

ced

Fra

mes


May 2009


179

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss C

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ce

1993

(F

und

P)

Rf =

5.0

Con

cent

rical

ly-B

race

d F

ram

esC

e 19

93 (

Ort

h P

) R

f = 5

.0 C

once

ntric

ally

-Bra

ced

Fra

mes

Ce

2007

u/S

p =

4.5

Mod

erat

ely

Duc

tile

Con

cent

rical

ly-B

race

d F

ram

esC

e 20

07 u

/Sp

= 2.

6 Li

mite

d D

uctil

e C

once

ntric

ally

-Bra

ced

Fra

mes


May 2009


180

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss D

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

De

1993

(F

und

P)

Rf =

5.0

Con

cent

rical

ly-B

race

d F

ram

es

De

1993

(O

rth

P)

Rf =

5.0

Con

cent

rical

ly-B

race

d F

ram

es

De

2007

u/S

p =

4.5

Mod

erat

ely

Duc

tile

Con

cent

rical

ly-B

race

d F

ram

es

De

2007

u/S

p =

2.6

Lim

ited

Duc

tile

Con

cent

rical

ly-B

race

d F

ram

es


May 2009


181

Bui

ldin

g F

ram

e S

yste

m C

ompa

rison

for

Soi

l Cla

ss E

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ee

1993

(F

und

P)

Rf =

5.0

Con

cent

rical

ly-B

race

d F

ram

esE

e 19

93 (

Ort

h P

) R

f = 5

.0 C

once

ntric

ally

-Bra

ced

Fra

mes

Ee

2007

u/S

p =

4.5

Mod

erat

ely

Duc

tile

Con

cent

rical

ly-B

race

d F

ram

esE

e 20

07 u

/Sp

= 2.

6 Li

mite

d D

uctil

e C

once

ntric

ally

-Bra

ced

Fra

mes


May 2009


182

Ordinary Moment Resisting Frame System

Ord

inar

y M

omen

t Res

ista

nt F

ram

e C

ompa

rison

for

Soi

l Cla

ss A

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ae

1993

(F

und

P)

Rf =

4.0

RC

Ae

1993

(O

rth

P)

Rf =

4.0

RC

Ae

2007

u/S

p =

2.6

RC

Ae

1993

(F

und

P)

Rf =

4.5

Ste

el

Ae

1993

(O

rth

P)

Rf =

4.5

Ste

el

Ae

2007

u/S

p =

2.6

Ste

el


May 2009


183

Ord

inar

y M

omen

t Res

ista

nt F

ram

e C

ompa

rison

for S

oil C

lass

Be

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Be

1993

(F

und

P)

Rf =

4.0

RC

Be

1993

(O

rth

P)

Rf =

4.0

RC

Be

2007

u/S

p =

2.6

RC

Be

1993

(F

und

P)

Rf =

4.5

Ste

el

Be

1993

(O

rth

P)

Rf =

4.5

Ste

el

Be

2007

u/S

p =

2.6

Ste

el


May 2009


184

Ord

inar

y M

omen

t Res

ista

nt F

ram

e C

ompa

rison

for

Soi

l Cla

ss C

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ce

2007

u/S

p =

2.6

RC

Ce

1993

(F

und

P)

Rf =

4.0

RC

Ce

2007

u/S

p =

2.6

Ste

el

Ce

1993

(F

und

P)

Rf =

4.5

Ste

el

Ce

1993

(O

rth

P)

Rf =

4.0

RC

Ce

1993

(O

rth

P)

Rf =

4.5

Ste

el


May 2009


185

Ord

inar

y M

omen

t Res

ista

nt F

ram

e C

ompa

rison

for

Soi

l Cla

ss D

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

De

2007

u/S

p =

2.6

RC

De

1993

(F

und

P)

Rf =

4.0

RC

De

2007

u/S

p =

2.6

Ste

el

De

1993

(F

und

P)

Rf =

4.5

Ste

el

De

1993

(O

rth

P)

Rf =

4.0

RC

De

1993

(O

rth

P)

Rf =

4.5

Ste

el


May 2009


186

Ord

inar

y M

omen

t Res

ista

nt F

ram

e C

ompa

rison

for

Soi

l Cla

ss E

e

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)E

e 20

07 u

/Sp

= 2.

6 R

C

Ee

1993

(F

und

P)

Rf =

4.0

RC

Ee

2007

u/S

p =

2.6

Ste

el

Ee

1993

(F

und

P)

Rf =

4.5

Ste

el

Ee

1993

(O

rth

P)

Rf =

4.0

RC

Ee

1993

(O

rth

P)

Rf =

4.5

Ste

el


May 2009


187

Intermediate Moment Resisting Frame System

Inte

rmed

iate

Mom

ent R

esis

tant

Fra

me

Com

paris

on fo

r S

oil C

lass

Ae

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ae

1993

(F

und

P)

Rf =

6.0

RC

Ae

1993

(O

rth

P)

Rf

= 6

.0 R

C

Ae

2007

u/S

p =

4.5

RC

Ae

1993

(F

und

P)

Rf =

6.5

Ste

el

Ae

1993

(O

rth

P)

Rf

= 6

.5 S

teel

Ae

2007

u/S

p =

4.5

Ste

el


May 2009


188

Inte

rmed

iate

Mom

ent R

esis

tant

Fra

me

Com

paris

on fo

r S

oil C

lass

Be

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Be

1993

(F

und

P)

Rf =

6.0

RC

Be

1993

(O

rth

P)

Rf =

6.0

RC

Be

2007

u/S

p =

4.5

RC

Be

1993

(F

und

P)

Rf =

6.5

Ste

el

Be

1993

(O

rth

P)

Rf =

6.5

Ste

el

Be

2007

u/S

p =

4.5

Ste

el


May 2009


189

Inte

rmed

iate

Mom

ent R

esis

tant

Fra

me

Com

paris

on fo

r S

oil C

lass

Ce

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ce

1993

(F

und

P)

Rf =

6.0

RC

Ce

1993

(O

rth

P)

Rf =

6.0

RC

Ce

2007

u/S

p =

4.5

RC

Ce

1993

(Fun

d P

) R

f = 6

.5 S

teel

Ce

1993

(Ort

h P

) R

f = 6

.5 S

teel

Ce

2007

u/S

p =

4.5

Ste

el


May 2009


190

Inte

rmed

iate

Mom

ent R

esis

tant

Fra

me

Com

paris

on fo

r S

oil C

lass

De

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

De

1993

(F

und

P)

Rf =

6.0

RC

De

1993

(O

rth

P)

Rf =

6.0

RC

De

2007

u/S

p =

4.5

RC

De

1993

(F

und

P)

Rf =

6.5

Ste

el

De

1993

(O

rth

P)

Rf =

6.5

Ste

el

De

2007

u/S

p =

4.5

Ste

el


May 2009


191

Inte

rmed

iate

Mom

ent R

esis

tant

Fra

me

Com

paris

on fo

r S

oil C

lass

Ee

0.00

0

0.02

0

0.04

0

0.06

0

0.08

0

0.10

0

0.12

0

050

100

150

200

250

300

350

Hei

ght m

Multiplier Cd(T1)

Ee

1993

(F

und

P)

Rf =

6.0

RC

Ee

1993

(O

rth

P)

Rf =

6.0

RC

Ee

2007

u/S

p =

4.5

RC

Ee

1993

(F

und

P)

Rf =

6.5

Ste

el

Ee

1993

(O

rth

P)

Rf =

6.5

Ste

el

Ee

2007

u/S

p =

4.5

Ste

el


May 2009


192

APPENDIX B: STATIC ANALYSIS BUILDING COMPARISON

CALCULATIONS

This appendix includes the spreadsheets used to calculate the horizontal earthquake base shear,

overturning moment and torsion for the four buildings being compared.

1) Hazard Factor (Z)/ Acceleration Coefficient (a) Used – Sydney was chosen as the main city for comparison, emphasizing implications to the immediate locality and projects where my design office is based. 0.08 is the value used.

2) Probability Factor (k p) – This factor value was chosen as 1.0, representing the 1 in 500 year probability of exceedance for the 1993 calculations but 1.3 representing the 1 in 1000 year probability of exceedance for the 2007 calculation.

Comparisons were carried out using Excel 2003.


May 2009


193

Bld 1 (14.4m) Soil Class Ae Base shear & Moment


May 2009


194


May 2009


195


May 2009


196

Bld 1 (14.4m) Soil Class Ae Tension & Comp Core Stress


May 2009


197

v


May 2009


198

Bld 1 (14.4m) Soil Class De Base shear & Moment


May 2009


199


May 2009


200


May 2009


201

Bld 1 (14.4m) Soil Class De Tension & Comp Core Stress


May 2009


202


May 2009


203



May 2009


204


May 2009


205


May 2009


206



May 2009


207


May 2009


208



May 2009


209


May 2009


210


May 2009


211



May 2009


212


May 2009


213



May 2009


214


May 2009


215


May 2009


216

Bld 3 (56.1mm) Soil Class Ae Tension & Comp Core Stress


May 2009


217


May 2009


218



May 2009


219


May 2009


220


May 2009


221

Bld 3 (56.1mm) Soil Class De Tension & Comp Core Stress


May 2009


222


May 2009


223



May 2009


224


May 2009


225


May 2009


226



May 2009


227


May 2009


228



May 2009


229


May 2009


230


May 2009


231



May 2009


232


May 2009


233

APPENDIX C: GLOSSARY

Base shear The total horizontal earthquake shear force at the base of the structure.

Bearing wall system Structural system in which load bearing walls provide support for all or most of the vertical loads while shear walls or braced frames provide the horizontal earthquake resistance.

Braced frame Two-dimensional structural system composed of an essentially vertical truss (or its equivalent) where the members are subject primarily to axial forces when resisting earthquake actions.

Concentric braced frame A braced frame in which the members are subjected primarily to axial forces

Connection Mechanical means that provide a load path for actions between structural elements, non-structural elements and structural and non-structural elements.

Drift See definition of Storey Drift

Dual System A structural system in which an essentially complete space frame provides support for the vertical loads and at least a quarter of the prescribed horizontal earthquake forces. The total horizontal earthquake resistance is provided by the combination of the moment frame, shear walls or braced frames, in proportion to their relative rigidities.

Ductility (of a structure) The Ability of a structure to sustain its load-carrying capacity and dissipate energy when responding to cyclic displacements in the inelastic range during an earthquake.

Earthquake actions Inertia-induced actions arising from the response to earthquake of the structure.

Global System Whole of system including soil and the structural system

Impedance The impedance of a medium is represented by the product of density (r) and shear wave velocity (V).

Intermediate moment resisting frame (IMRF) A concrete or steel space frame designed in accordance with AS 3600 or AS 4100 , respectively, in which members and joints are capable of resisting forces by flexure as well as axial forces along the axis of the members, including specific ductility requirements


May 2009


234

Moment-resisting frame essentially complete space frame that supports the vertical and horizontal actions by both flexural and axial resistance of its members and connections.

Moment-resisting frame, intermediate Concrete or steel moment-resisting frame designed and detailed to achieve moderate structural ductility.

Moment-resisting frame, ordinary Moment-resisting frame with no particular earthquake detailing, specified in the relevant material standard

Moment-resisting frame, special Concrete or steel moment-resisting frame designed and detailed to achieve high structural ductility and where plastic deformation is planned under ultimate actions

Partition Permanent or relocatable internal dividing wall between floor spaces.

P -delta effect Additional induced structural forces that develop as a consequence of the vertical loads acting on the horizontally-displaced building frame

Plastic Hinge Localized zone of yielding where the moment capacity is reached

Bedrock (Greek: "blanket rock") is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other planets. The term was first defined by George P. Merrill in 1897 who stated, "In places this covering is made up of material originating through rock-weathering or plant growth in situ. In other instances it is of fragmental and more or less decomposed matter drifted by wind, water or ice from other sources. This entire mantle of unconsolidated material, whatever its nature or origin, it is proposed to call the Bedrock."

Resonance The destructive effect of resonance mainly stems form the natural period of the waveform causing the amplitude of response of the structure to significantly exceed the amplitude of response of the ground.

Seismic-force-resisting system Part of the structural system that provides resistance to the earthquake forces and effects.

Shear wall A wall designed to resist horizontal earthquake forces acting in the plane of the wall. A shear wall can be either load bearing or non-load bearing

Soft Storey One in which the horizontal stiffness of the storey is less than 70% of that in the storey above or less than 80% of the average stiffness of the three storeys above.

Space frame A three-dimensional structural system composed of interconnected members (other than load bearing walls) that are capable of supporting vertical loads, which may also provide horizontal resistance to earthquake forces.


May 2009


235

Static eccentricity The distance from the shear centre to the centre of mass at the level considered, measured perpendicular to the direction of loading

Storey Space between levels including the space between the structural base and the level above.

Storey Drift The displacement of one level relative to the level above or below

Storey Height The distance from floor level to floor level.

Storey Shear The summation of all the design horizontal forces acting on the levels above the storey under consideration

Structure An assemblage of members designed to support gravity loads and resist horizontal forces and may be either a building structure or a non-building structure.

Structural base The level at which the earthquake ground motions are considered to be imparted to the structure or the level at which the structure as a dynamic vibrator is supported.

Structural performance factor ( Sp ) The Numerical assessment of the additional ability of the total building (structure and other parts) to survive earthquake motion.

Structural ductility factor ( µ ) The Numerical assessment of the ability of a structure to sustain cyclic displacements in the inelastic range. Its value depends upon the structural form, the ductility of the materials and structural damping characteristics.


May 2009

236

Documents

Comparison of Structural Design Actions Part 4: Earthquake