Comparison AS11704 1993&2007

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: ............................................................................................................... ICOPYRIGHT NOTICE .......................................................................................... IIACKNOWLEDGEMENTS ................................................................................... IIITABLE OF CONTENTS ....................................................................................... IVLIST OF FIGURES ............................................................................................. VIIILIST OF TABLES ................................................................................................ XII1 INTRODUCTION ............................................................................................ 1

1.1 AIMS AND OBJECTIVES ................................................................................ 21.2 REPORT OUTLINE ........................................................................................ 3

2 EARTHQUAKE ENGINEERING BACKGROUND ................................. 52.1 STRUCTURAL SYSTEMS ............................................................................... 5

2.1.1 Bearing Wall Systems ............................................................................. 62.1.2 Building Frame Systems ......................................................................... 62.1.3 Moment Resisting Frame Systems ......................................................... 62.1.4 Dual Systems ........................................................................................... 6

2.2 DUCTILITY ELASTIC AND DYNAMIC RESPONSE ....................................... 82.2.1 Elastic Response ..................................................................................... 82.2.2 Ductile Response .................................................................................. 102.2.3 Structural Ductility Factor () ............................................................. 102.2.4 The Structural Response Factor (Rf) & The Structural Performance Factor (Sp) ......................................................................................................... 112.2.5 Ductility Detailing ................................................................................ 142.2.6 Capacity Design ................................................................................... 202.2.7 Hysteretic Loops ................................................................................... 22

2.3 SEISMIC RESPONSE AND STRUCTURAL CONFIGURATION .......................... 242.3.1 Response in Elevation .......................................................................... 262.3.2 Estimates of Deflection and Drift ........................................................ 272.3.3 P-Delta Effects in framed structures.................................................... 282.3.4 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 332.4.2 Site Classification using Site Natural Period ...................................... 36

2.5 SEISMIC RISK ............................................................................................. 372.5.1 Design Limit States ............................................................................... 382.5.2 Serviceability Limit State ..................................................................... 392.5.3 Economic Considerations .................................................................... 39


May 2009


v

2.6 DESIGN METHODS ..................................................................................... 402.6.1 Dynamic inelastic time-history analysis .............................................. 402.6.2 Modal superposition techniques .......................................................... 402.6.3 Equivalent lateral force procedures .................................................... 422.6.4 Capacity Spectrum Method .................................................................. 42

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

3 AS1170.4: 1993 & 2007 CODE COMPARISON ........................................ 473.1 CODE LAYOUT COMPARISON .................................................................... 47

3.1.1 AS1170.4:1993 Layout ......................................................................... 473.1.2 AS1170.4:2007 Layout and Revisions ................................................. 48

3.2 SITE HAZARD ............................................................................................. 513.2.1 Return Period Calculation ................................................................... 52

3.3 SITE FACTOR / SUB SOIL CLASS AND SPECTRAL SHAPE FACTOR ............. 553.3.1 Response Spectra and Spectral Shape Factor (Ch(T)) ........................ 563.3.2 Site Classification ................................................................................. 60

3.4 SELECTION OF EARTHQUAKE DESIGN CATEGORY .................................... 623.4.1 AS1170.4:1993 ..................................................................................... 623.4.2 AS1170.4:1993 (with AS1170.4:2002 Appendix D considerations) ... 643.4.3 AS1170.4:2007 ..................................................................................... 643.4.4 Earthquake Design Category Comparison ......................................... 66

3.5 PERIOD CALCULATION .............................................................................. 673.5.1 AS1170.4:1993 Approximated Formulae ............................................ 673.5.2 AS1170.4:2007 ..................................................................................... 673.5.3 Period Calculation Comparison .......................................................... 683.5.4 The Rayleigh Method ........................................................................... 70

3.6 RESPONSE FACTOR (RF), STRUCTURAL DUCTILITY FACTOR, , AND THE STRUCTURAL PERFORMANCE FACTOR, SP ............................................................. 703.7 EARTHQUAKE BASE SHEAR ....................................................................... 72

3.7.1 AS1170.4: 1993 Earthquake Base shear ............................................. 723.7.2 AS1170.4: 2007 Earthquake Base shear ............................................. 72

3.8 STRUCTURAL SYSTEMS AND RESTRICTIONS ............................................. 733.8.1 Bearing wall systems ............................................................................ 743.8.2 Building Frame systems ....................................................................... 793.8.3 Moment Resisting Frame System ......................................................... 873.8.4 Dual System .......................................................................................... 94

3.9 TORSION .................................................................................................... 943.9.1 AS1170.4:1993 Approximated Formulae ............................................ 953.9.2 AS1170.4:2007 Approximated Formulae ............................................ 963.9.3 Torsion Comparison ............................................................................. 97

3.10 DRIFT AND P-DELTA EFFECTS................................................................... 983.10.1 AS1170.4: 1993 Storey Drift Determination and P-delta Effects ... 983.10.2 AS1170.4: 2007 Storey Drift Determination and P-delta Effects ... 993.10.3 Storey Drift Determination and P-delta Effects Comparison ....... 100


May 2009


vi

3.11 DYNAMIC ANALYSIS ............................................................................... 1013.12 DISCUSSION ............................................................................................. 104

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

4.1 BUILDING, SITE AND DESIGN METHOD SELECTION ................................ 1064.1.1 Structural System ................................................................................ 1064.1.2 Elevation ............................................................................................. 1084.1.3 Plan ..................................................................................................... 1104.1.4 Core Properties .................................................................................. 1134.1.5 Shear Centre and Centre of Mass ...................................................... 1144.1.6 AS1170.4:1993 Design Eccentricity Calculation .............................. 1164.1.7 AS1170.4:2007 Design Eccentricity Calculation .............................. 1174.1.8 Analysis Method ................................................................................. 1184.1.9 Site & Structural Factors ................................................................... 1194.1.10 Structural Classification / Importance level for the structure ...... 1194.1.11 Acceleration Coefficient/Hazard Factor ....................................... 1194.1.12 Probability Factor kp ...................................................................... 1194.1.13 Site Factor / Sub Soil Class ............................................................ 1194.1.14 Period Calculation for the buildings ............................................. 1194.1.15 Response Factor and Ductility Ratio ............................................. 1204.1.16 Earthquake Base shear Multiplier ................................................. 1204.1.17 Loads .............................................................................................. 121

4.2 HAND CALCULATION ANALYSIS ............................................................. 1244.2.1 First-Mode of Natural Period ............................................................ 1244.2.2 Seismic Design Base Shear ................................................................ 1244.2.3 Overturning Moment .......................................................................... 1254.2.4 Torsion ................................................................................................ 1274.2.5 Tension & Compression Core Stresses due to Overturning Moment1294.2.6 Shear Force on Core due to Base Shear & Torsion ......................... 1334.2.7 Structural Displacements ................................................................... 1374.2.8 Deflection at Roof Level ..................................................................... 1384.2.9 Storey Drift and P-Delta Effects ........................................................ 143

4.3 ETABS ANALYSIS ..................................................................................... 1484.3.1 First-Mode of Natural Period ............................................................ 1494.3.2 Seismic Design Base Shear ................................................................ 1494.3.3 Overturning Moment .......................................................................... 1504.3.4 Stresses ................................................................................................ 1514.3.5 Deflections .......................................................................................... 152

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

5 CONCLUSIONS AND FUTURE WORK ................................................. 1575.1 LIMITATIONS ........................................................................................... 1605.2 CONTRIBUTIONS ...................................................................................... 1615.3 SUCCESS CRITERIA .................................................................................. 161


May 2009


vii

5.4 FUTURE WORK ........................................................................................ 1625.4.1 Fragility Curves .................................................................................. 1625.4.2 Design and Detailing of the Lateral Supporting System ................... 162

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

BIBLIOGRAPHY ................................................................................................. 164APPENDIX A: CODE COMPARISON GRAPHS FOR STRUCTURAL SYSTEMS AND SITE SUB SOIL CLASSES ................................................... 166

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

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

BLD 1 (14.4M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 193BLD 1 (14.4M) SOIL CLASS AE TENSION & COMP CORE STRESS ........................ 196BLD 1 (14.4M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 198BLD 1 (14.4M) SOIL CLASS DE TENSION & COMP CORE STRESS ........................ 201BLD 2 (29.7M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 203BLD 2 (29.7M) SOIL CLASS AE TENSION & COMP CORE STRESS ........................ 206BLD 2 (29.7M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 208BLD 2 (29.7M) SOIL CLASS DE TENSION & COMP CORE STRESS ........................ 211BLD 3 (56.1M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 213BLD 3 (56.1MM) SOIL CLASS AE TENSION & COMP CORE STRESS ..................... 216BLD 3 (56.1M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 218BLD 3 (56.1MM) SOIL CLASS DE TENSION & COMP CORE STRESS ..................... 221BLD 4 (97.9M) SOIL CLASS AE BASE SHEAR & MOMENT ................................... 223BLD 4 (97.9M) SOIL CLASS AE TENSION & COMP CORE STRESS ........................ 226BLD 4 (97.9M) SOIL CLASS DE BASE SHEAR & MOMENT ................................... 228BLD 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 /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 Australias 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 1970s. 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 DAlemberts 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 =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 () () [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 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 forcedisplacement 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


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 stiffnesss 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

School of Civil Engineering Research Repor

Date post:	06-Jan-2016
Category:	Documents
Upload:	don-deol
View:	28 times
Download:	4 times

Comparison AS11704 1993&2007

Documents