characterisation and seismc assessment of unreinforced masorny buildings

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Characterisation and Seismic

Assessment of Unreinforced Masonry

Buildings

Alistair Peter Russell

A thesis submitted in partial fulfilment of the requirements for the degreeof Doctor of Philosophy

Supervised by Associate Professor Jason M. Ingham

The University of AucklandDepartment of Civil and Environmental Engineering

New Zealand

October 2010


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Preface

The research reported in this thesis was undertaken at the Department of Civil and En-vironmental Engineering, The University of Auckland, between March 2006 and October2010. The contents of this thesis is the original work of the author, except where specif-ically acknowledged in the text, and includes nothing that is the outcome of work donein collaboration. No part of the thesis has been submitted for a degree to any otherUniversity.

The opinions, conclusions and recommendations presented herein are those of the authorand do not necessarily reflect those of the University of Auckland or any of the sponsoringparties to this project.

The thesis is approximately 85,000 words in length, including appendices, tables, refer-ences and equations, and contains exactly 140 figures.

Alistair Russell

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Abstract

This thesis describes the characterisation and seismic assessment of unreinforced masonry(URM) buildings.

The research in this thesis was conducted with the primary aim of developing a deeper

understanding of the response of URM buildings, with an emphasis on the response ofURM walls responding in-plane, and in particular, a comprehensive inspection of thein-plane response of flanged URM walls. Most previous expressions for determining thelateral strength capacity of URM walls responding in-plane have not taken into accountthe effect of flanges, and consequently have underestimated the expected capacity. Thisthesis identified that in order to accurately account for the strength inherently availablein URM buildings, the effect of flanges should be incorporated into the seismic assessmentprocedure.

The New Zealand URM building stock was analysed to determine typical building charac-

teristics, and URM buildings were classified into seven typologies on the basis of buildingheight and building footprint. Further analysis of the building stock determined that mostURM buildings in New Zealand are one and two storeys in height. The characterisationof the New Zealand URM building stock was used to form the basis of the experimentalprogramme reported in this thesis, such that specimens accurately reflected constructioncharacteristic of existing New Zealand URM buildings.

Testing of URM walls showed that the presence of flanges has a significant effect on thebehaviour of walls responding in-plane. Flanges increase the force and displacement ca-pacity of in-plane loaded walls, when compared with in-plane loaded walls without flanges.The results from the experimentation that was conducted in order to determine the limit-ing strength of the shear walls were compared with analytical results from other research,with a high level of correlation. Consequently, equations were recommended for deter-mining the in-plane lateral strength limits of URM walls both with and without flanges.Drift limits and energy dissipation characteristics were also proposed.

Finally, a procedure was developed for assessing the performance of URM buildings, basedon displacement based design principles. The procedure was demonstrated using an ex-ample.

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Acknowledgements

Though writing is a solitary activity, no research is a solo venture.

Firstly, I would like to thank my supervisor and friend Jason Ingham, without whom Iwould not have embarked on this study. I am grateful for the encouragement, support

and many of the initial ideas which he provided, and not least of all for the reliable andrapid critiquing of each part of this thesis.

I would like to thank my wife, Myra, without whom I would not have completed thisstudy. The constant encouragement, support, love and patience, particularly during thelong hours of writing, were sine qua nonin allowing me to complete my thesis.

This project would not have been possible without the financial assistance provided bythe Foundation for Research, Science and Technology (grant number UOAX0411) and by

the Department of Civil and Environmental Engineering at The University of Auckland.

The valuable input from Ken Elwood, John Butterworth, Mike Griffith and Guido Ma-genes, particularly in the initial stages of formulating ideas and the later stages of analysingdata, is gratefully acknowledged.

The assistance of Hank Mooy, Tony Daligan, Jeffrey Ang, Mark Byrami and Noel Perin-panayagam in many aspects of lab work and experimentation is also sincerely appreciated.

David Hopkins provided valuable feedback on the background and history of URM build-ings in New Zealand and associated legislation. Patrick Cummuskey, Win Clark, RussellGreen, John Buchan, Neil McLeod, Bruce Mutton, Claire Stevens, Katherine Wheeler andRichard Deakin provided invaluable data on the background, development and number ofURM buildings in various jurisdictions around New Zealand. This assistance is gratefullyacknowledged.

Finally, many thanks to all the graduate students and staff in the retrofit group atThe University of Auckland, for helping in the lab and for bouncing ideas off, but mostimportantly for the continuous support and friendship throughout this study.

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Nomenclature

Roman

ai Distance between inertia centre and compression edge of wall mmai Effective floor acceleration at level i m/s2

af Distance between centre of flange and compression edge of wall mmAn Area of net mortared section mm

2

Af Cross-sectional area of flange mm2

b Factor to account for wall aspect ratio -bf Width of flange mmbw Width of wall mmc Cohesion MPaC(T) Elastic site hazard spectrum -Ch(T) Spectral shape factor -COV Coefficient of variation %d Displacement mmdc Spectral corner point diaplacement mmde Elastic displacement mmdo Maximum displacement for one hysteretic cycle mmdu Ultimate wall displacement mmdT Design displacement mmdVmax Wall displacement at Vmax mmE Youngs modulus (elastic modulus) MPaED Energy dissipated by damping kNmm

ES Strain energy kNmmf b Compressive strength of bricks MPafbt Direct tensile strength of bricks MPafD Damping force kNfdt Diagonal tension strength of masonry MPaFi Equivalent static horizontal force at leveli kNf j Compressive strength of mortar MPafm Axial compressive stress MPaf m Compressive strength of masonry MPaf j Compressive strength of mortar MPa

G Shear modulus (modulus of rigidity) MPaGeff Effective post-cracked shear modulus (modulus of rigidity) MPaGi Permanent action at leveli kN

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g Acceleration due to gravity m/s2

g Diagonal gauge length mmglong Diagonal gauge length perpendicular to applied force mmgshort Diagonal gauge length parallel to applied force mmh Height of wall mmheff Height to resultant of lateral force MPaI Moment of inertia mm4

Ig Gross moment of inertia mm4

Ieff Effective moment of inertia mm4

Keff Effective stiffness of equivalent SDOF system kN/mKi Initial stiffness kN/mlw Length of wall mmM Base moment kNmmeff Effective mass of equivalent SDOF system kgn Number of storeys -

N Normal force on cross section kNND Superimposed dead load at top of wall kNN(T,D) Near fault factor -%NBS Percentage New Building Standard -%NBSb Baseline percentage New Building Standard -%NBSnom Nominal percentage New Building Standard -n Proportion of gross solid area of unit %P Applied force kNPCE Expected gravity compressive force applied to a wall or pier kNQi Imposed action at leveli kN

R Earthquake return period factor -r Post-yield stiffness ratio -R Reduction factor applied to displacement spectrum for damping eq -s Sample standard deviation -Sa Spectral acceleration m/s

2

Sd Spectral displacement mmT Translational period sTc Corner period sTeff Effective period of equivalent SDOF system sV Applied lateral force kNVb Shear strength corresponding to diagonal tension failure involving

cracking through bricks kNVbase Base shear kNVcrack Base shear at first crack kNVdt Shear strength corresponding to diagonal tension failure kNVj Shear strength corresponding to diagonal tension failure involving

damage in mortar joints kNVmax Maximum base shear kNvme Cohesive strength of masonry bed joint MPaVn Nominal shear strength kNVr Shear strength corresponding to onset of rocking kN

Vs Shear strength corresponding to sliding kNVtc Shear strength corresponding to toe crushing kN

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Vu Equivalent ultimate base shear kNVy Shear strength corresponding to effective yield kNWf Weight of flange kNWi Seismic weight at leveli kNWt Total seismic weight kNWw Weight of in-plane wall kNx Sample mean -z Distance from extreme compression fibre to line of

action of normal force (N) mmZ Earthquake hazard factor -

Greek

Factor equal to 0.5 for wall fixed at base-free at top -

c Effective aspect ratio - Factor to account for non-linear vertical stress distribution - Shear strain mm/mmm Unit weight of masonry kN/m

3

H Horizontal extension mmlong Diagonal extension mmshort Vertical shortening mmV Diagonal shortening mmc Direct strain in compression mm/mmt Direct strain in tension mm/mm

Drift %crack Wall drift at cracking %u Ultimate wall drift %Vmax Wall drift at Vmax mm Coefficient of friction - Population mean - Poissons ratio -eq Equivalent viscous damping ratio - Population standard deviation -n Axial compressive stress perpendicular to the bed joint MPa1

Maximum principal stress MPa2 Minimum principal stress MPas Shear stress MPaE Earthquake imposed action combination factor - Frequency rad/sn Natural frequency rad/s

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Contents

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvi

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiii

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 New Zealand Seismicity . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 URM Building Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 Typical Structural Deficiencies in URM Buildings . . . . . . . . 4

1.2.2 In-Plane Wall Response . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Research Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Background on Potentially Earthquake Prone Buildings . . . . . . . . 13

2.1 Earthquake Prone Building Legislation . . . . . . . . . . . . . . . . . . 14

2.2 URM Heritage Considerations . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 New Zealand Building Codes . . . . . . . . . . . . . . . . . . . . . . . . 18

2.4 Provisions for the Seismic Upgrade of Existing Buildings . . . . . . . . 20

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Contents

2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3 Architectural Characterisation . . . . . . . . . . . . . . . . . . . . . . . . 27

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2 Background of Building Typologies . . . . . . . . . . . . . . . . . . . . 29

3.2.1 European URM Typology Studies . . . . . . . . . . . . . . . . 31

3.2.2 North American URM Typology Studies . . . . . . . . . . . . . 37

3.2.3 Iranian URM Typology Studies . . . . . . . . . . . . . . . . . . 39

3.2.4 Previous Research into URM Typologies in New Zealand . . . . 40

3.3 New Zealand URM Building Typologies . . . . . . . . . . . . . . . . . . 40

3.3.1 Parameters for Differentiating Typologies . . . . . . . . . . . . 44

3.3.2 Details of New Zealand URM Typologies . . . . . . . . . . . . . 46

3.4 International Comparisons with New Zealand Typologies . . . . . . . . 81

3.5 Supplementary Characteristics of New Zealand URM . . . . . . . . . . 82

3.5.1 Bond Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

3.5.2 Wall Height and Thickness . . . . . . . . . . . . . . . . . . . . 86

3.5.3 Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3.5.4 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

4 New Zealand URM Building Stock . . . . . . . . . . . . . . . . . . . . . . 93

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.2 Estimation of URM Population and Distribution . . . . . . . . . . . . . 94

4.3 Estimation of URM Population and Value . . . . . . . . . . . . . . . . 100

4.4 Estimated Vulnerability of URM Buildings . . . . . . . . . . . . . . . . 105

4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5 Diagonal Shear Testing of Wall Panels . . . . . . . . . . . . . . . . . . . 113

5.1 Diagonal Shear Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.1.1 Test Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

5.1.2 Interpretation of Diagonal Shear Test . . . . . . . . . . . . . . 116

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Contents

5.1.3 Derivation of Drift . . . . . . . . . . . . . . . . . . . . . . . . . 121

5.2 Construction Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

5.4.1 Prediction of Shear Strength . . . . . . . . . . . . . . . . . . . 130

5.4.2 Comparison of Shear Strength with ASCE 41-06 . . . . . . . . 131

5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

6 In-Plane Cyclic Testing of Rectangular URM Walls . . . . . . . . . . . 135

6.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136


6.2.1 Wall Specifications . . . . . . . . . . . . . . . . . . . . . . . . . 138

6.2.2 Wall Construction . . . . . . . . . . . . . . . . . . . . . . . . . 139

6.2.3 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . 142

6.3 Testing Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

6.3.1 Test Setup and Instrumentation . . . . . . . . . . . . . . . . . . 143

6.3.2 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

6.3.3 Predicted Flexural and Shear Strength . . . . . . . . . . . . . . 147

6.4 Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

6.4.1 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

6.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

6.5.1 Force-Displacement Response . . . . . . . . . . . . . . . . . . . 154

6.5.2 Energy Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . 155

6.5.3 Bilinear Approximation . . . . . . . . . . . . . . . . . . . . . . 157

6.5.4 Multi-Linear Approximation . . . . . . . . . . . . . . . . . . . . 159

6.5.5 Ultimate Drift . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

6.5.6 Predicted Behaviour and Measured Behaviour . . . . . . . . . . 163

6.5.7 Consolidation of Predictive Equations . . . . . . . . . . . . . . 165

6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

6.7 Photos of Wall Response . . . . . . . . . . . . . . . . . . . . . . . . . . 169

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7 In-Plane Cyclic Testing of Flanged URM Walls . . . . . . . . . . . . . . 173

7.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174


7.2.1 Wall Specifications . . . . . . . . . . . . . . . . . . . . . . . . . 179

7.2.2 Wall Construction . . . . . . . . . . . . . . . . . . . . . . . . . 181

7.2.3 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . 186

7.3 Testing Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

7.3.1 Test Setup and Instrumentation . . . . . . . . . . . . . . . . . . 186

7.3.2 Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

7.4 Predicted Flexural and Shear Strength . . . . . . . . . . . . . . . . . . 190

7.5 Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

7.5.1 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192

7.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

7.6.1 Force-Displacement Response . . . . . . . . . . . . . . . . . . . 195

7.6.2 Energy Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . 199

7.6.3 Bilinear Approximation . . . . . . . . . . . . . . . . . . . . . . 200

7.6.4 Multi-Linear Approximation . . . . . . . . . . . . . . . . . . . . 201

7.6.5 Ultimate Drift . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

7.6.6 Initial Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

7.6.7 Crack Pattern Analysis . . . . . . . . . . . . . . . . . . . . . . 211

7.6.8 Predicted Behaviour and Measured Behaviour . . . . . . . . . . 214

7.6.9 Consolidation of Predictive Equations . . . . . . . . . . . . . . 216

7.7 General Force-Displacement Response of URM Walls With Flanges . . 217

7.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219

7.9 Photos of Wall Response . . . . . . . . . . . . . . . . . . . . . . . . . . 221

8 URM Building Assessment Procedure . . . . . . . . . . . . . . . . . . . . 227

8.1 New Zealand Existing Building Seismic Performance Criteria . . . . . . 229

8.2 Direct Displacement Based Design . . . . . . . . . . . . . . . . . . . . . 231

8.2.1 Determining Base Shear Demand . . . . . . . . . . . . . . . . . 234

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Contents

8.2.2 Distribution of Base Shear . . . . . . . . . . . . . . . . . . . . . 235

8.3 Assessment Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

8.3.1 Walls Responding Out-of-Plane . . . . . . . . . . . . . . . . . . 243

8.3.2 Walls Responding In-Plane . . . . . . . . . . . . . . . . . . . . 243

8.3.3 Response of Diaphragms and Connections . . . . . . . . . . . . 247

8.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

9 Summary of Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

9.1 Architectural Characterisation Chapter 3 . . . . . . . . . . . . . . . . 253

9.2 Characterisation of New Zealands URM Building Stock Chapter 4 . . 253

9.3 Diagonal Shear Testing of Wall Panels Chapter 5 . . . . . . . . . . . 254

9.4 In-Plane Cyclic Testing of URM Walls Chapters 6 and 7 . . . . . . . 255

9.4.1 Rectangular Walls . . . . . . . . . . . . . . . . . . . . . . . . . 255

9.4.2 Flanged Walls . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

9.5 URM Building Assessment Procedure Chapter 8 . . . . . . . . . . . . 258

9.6 Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259

10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

A Photographic Examples of New Zealand URM Building Typologies . 283

A.1 Typology A Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

A.2 Typology B Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

A.3 Typology C Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

A.4 Typology D Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290A.5 Typology E Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

A.6 Typology F Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

A.7 Typology G Photos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

B Example of Assessment Procedure Typology C . . . . . . . . . . . . . 303

B.1 Seismic Assessment of Typology C Structure . . . . . . . . . . . . . . . 304

B.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316

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List of Figures

List of Figures

1.1 The plate boundary in New Zealand (reproduced with permission from

GNS Science (2007)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 In-plane behaviour modes of a laterally loaded URM wall (after Marzahn,

1998; Voon, 2007) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 Prior cracking in the wall of a two storey URM isolated building as a

potential sliding plane. . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Map of seismic zones [from NZSS 1900 Chapter 8:1965 (New Zealand

Standards Institute, 1965)] . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.1 Typologies for the Italian URM building stock (reproduced from Binda

(2006a)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.2 Typical masonry buildings in Ljubljana, Slovenia (reproduced from (Tomazevic

and Lutman, 2007)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.3 Typology in Iranian rural residential building stock; comparison of model

and prototype (reproduced from (Ghannad et al., 2006)) . . . . . . . . . 39

3.4 Locations of buildings surveyed throughout New Zealand . . . . . . . . 423.5 Building importance levels from AS/NZS 1170.0 Table 3.1 . . . . . . . 44

3.6 Overall dimensions of a Typology A1 building . . . . . . . . . . . . . . . 47

3.7 Overall dimensions of a Typology A2 building . . . . . . . . . . . . . . . 48

3.8 Typology A buildings single storey isolated . . . . . . . . . . . . . . . 50

3.9 Overall dimensions of a Typology B building . . . . . . . . . . . . . . . 52

3.10 Typology B buildings single storey row . . . . . . . . . . . . . . . . . 53

3.11 Timber joist details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.12 Spikes on exterior wall for connecting internal floor diaphragm . . . . . 56

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List of Figures

4.3 Construction date of URM buildings in NZ . . . . . . . . . . . . . . . . 99

4.4 Number of URM buildings from QV according to construction date . . . 102

4.5 Number of URM buildings according to storey height . . . . . . . . . . 104

4.6 Valuation of URM building stock according to height . . . . . . . . . . . 104

4.7 Number of low rise (1 and 2 storey) buildings as a proportion of all

New Zealand URM buildings . . . . . . . . . . . . . . . . . . . . . . . . 105

4.8 Estimated %NBS of URM buildings in provinces throughout New Zealand110

4.9 National estimate of potentially earthquake prone and earthquake risk

U RM bui l di ngs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

4.10 Estimated %NBS of URM buildings in New Zealand . . . . . . . . . . . 112

5.1 Orientation of wall panels for ASTM procedure and modified procedure 115

5.2 Diagonal shear test setup . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5.3 Gauge lengths for measuring wall panel distortion . . . . . . . . . . . . 117

5.4 Mohrs Circle interpretation of the diagonal shear test . . . . . . . . . . 120

5.5 Derivation of drift angle . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

5.6 Shear strength of a wall and diagonal tension strength of masonry . . . 124

5.7 Crack patterns at failure of wall panels . . . . . . . . . . . . . . . . . . 125

5.8 Photos of cracking through bricks in wall panels . . . . . . . . . . . . . 125

5.9 Photos of cracking through mortar in wall panels . . . . . . . . . . . . . 126

5.10 Shear failure mode of Wall AP1 . . . . . . . . . . . . . . . . . . . . . . 127

5.11 Force-displacement response of wall panels . . . . . . . . . . . . . . . . 128

5.12 Maximum principal stress-drift response of wall panels . . . . . . . . . . 129

6.1 Side wall of a Typology A structure . . . . . . . . . . . . . . . . . . . . 1396.2 Wall dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

6.3 Wall construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

6.4 Mortar sand gradation curve . . . . . . . . . . . . . . . . . . . . . . . . 142

6.5 Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

6.6 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

6.7 Imposed cyclic displacement history . . . . . . . . . . . . . . . . . . . . 145

6.8 Cracking patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

6.9 Force-displacement response . . . . . . . . . . . . . . . . . . . . . . . . 154

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List of Figures

6.10 Energy dissipated ED in one force-displacement loop (from (Chopra,

2007)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

6.11 Equivalent viscous damping ratio of Wall A2 and A4 . . . . . . . . . . . 157

6.12 Equivalent bilinear approximation (from Magenes and Calvi (1997)) . . 158

6.13 Equivalent bi-linear response of Wall A1, A2, A4 . . . . . . . . . . . . . 158

6.14 Component forces versus displacement curves (reproduced from ASCE

(2007)) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

6.15 Multi-linear approximation of Wall A1 . . . . . . . . . . . . . . . . . . . 160



6.18 Approximated response of Wall A4 . . . . . . . . . . . . . . . . . . . . . 162

6.19 Photos of Wall A1 response . . . . . . . . . . . . . . . . . . . . . . . . . 169



7.1 Shear stress distribution in the in-plane URM wall with and without

flange (from Yi et al. (2008)) . . . . . . . . . . . . . . . . . . . . . . . . 177

7.2 Wall dimensions (continued) . . . . . . . . . . . . . . . . . . . . . . . . 184

7.3 Wall construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

7.4 Timber diaphragm used in Wall A3 . . . . . . . . . . . . . . . . . . . . 187

7.5 Application of axial load on flanged walls . . . . . . . . . . . . . . . . . 188

7.6 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

7.7 Cracking patterns (continued) . . . . . . . . . . . . . . . . . . . . . . . 197

7.8 Force-displacement response . . . . . . . . . . . . . . . . . . . . . . . . 1987.9 Equivalent viscous damping ratio of Wall A6, A7 and A8 . . . . . . . . 200

7.10 Equivalent bi-linear response of Wall A5, A6, A7, A8 . . . . . . . . . . . 201

7.11 Multi-linear approximation of Wall A3a . . . . . . . . . . . . . . . . . . 202





7.16 Average multi-linear approximations of flanged walls . . . . . . . . . . . 206

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List of Figures

7.17 Generalised force-deformation relation for masonry elements or compo-

nents (reproduced from Figure 7-1(a), ASCE (2007)) . . . . . . . . . . . 207

7.18 Flange effects on the orientation of cracking of in-plane wall . . . . . . . 212

7.19 Shear stress distribution for a shear force applied parallel to the web . . 212

7.20 Direction of principal stresses at ends of in-plane wall . . . . . . . . . . 213

7.21 Generalised force-deformation relation for masonry elements or compo-

nents (reproduced from Figure 7-1, ASCE (2007)) . . . . . . . . . . . . 217


7.23 Photos of Wall A3a response . . . . . . . . . . . . . . . . . . . . . . . . 222





8.1 Strength versus risk and ultimate limit state as reference point (repro-

duced from NZSEE (2006)) . . . . . . . . . . . . . . . . . . . . . . . . . 230

8.2 Displacement response spectra . . . . . . . . . . . . . . . . . . . . . . . 232

8.3 Effective stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

8.4 URM building assessment procedure flowchart . . . . . . . . . . . . . . 237

8.5 Tributary areas for axial load on each wall . . . . . . . . . . . . . . . . . 240

8.6 Two storey URM analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 242

A.1 Typology A buildings single storey isolated . . . . . . . . . . . . . . . 284

A.2 Typology B buildings single storey row . . . . . . . . . . . . . . . . . 285

A.3 Typology B buildings single storey row . . . . . . . . . . . . . . . . . 286A.4 Typology C buildings two storey isolated . . . . . . . . . . . . . . . . 287

A.5 Typology C buildings two storey isolated . . . . . . . . . . . . . . . . 288

A.6 Typology C buildings two storey isolated . . . . . . . . . . . . . . . . 289

A.7 Typology D buildings two storey isolated . . . . . . . . . . . . . . . . 290

A.8 Typology D buildings two storey row . . . . . . . . . . . . . . . . . . 291




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List of Figures


A.13 Typology E buildings three + storey isolated . . . . . . . . . . . . . . 296

A.14 Typology F buildings three + storey row . . . . . . . . . . . . . . . . 297

A.15 Typology F buildings three + storey row . . . . . . . . . . . . . . . . 298A.16 Typology F buildings three + storey row . . . . . . . . . . . . . . . . 299

A.17 Typology G buildings monumental, religious and institutional . . . . . 300

A.18 Typology G buildings monumental, religious and institutional . . . . . 301

B.1 Typology C building for seismic assessment . . . . . . . . . . . . . . . . 304

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List of Tables

List of Tables

3.1 URM typologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.2 Typical dimensions of a Typology A1 building . . . . . . . . . . . . . . 483.3 Typical dimensions of a Typology A2 building . . . . . . . . . . . . . . 49

3.4 Typical dimensions of a Typology B building . . . . . . . . . . . . . . . 52

3.5 Typical dimensions of a Typology C1 building . . . . . . . . . . . . . . 59



3.8 Typical dimensions of a Typology D building . . . . . . . . . . . . . . . 68

3.9 Typical dimensions of a Typology E building . . . . . . . . . . . . . . . 72

3.10 Typical dimensions of a Typology F building . . . . . . . . . . . . . . . 77

4.1 Auckland City pre-1940 potentially earthquake prone buildings . . . . . 95

4.2 Population data and URM buildings for Auckland City and Auckland

Province . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

4.3 Provincial populations and estimated number of existing URM buildings 99

4.4 Number of URM buildings from QV according to construction decade . 101

4.5 URM building stock according to storey height . . . . . . . . . . . . . . 103

4.6 Baseline %NBSb for provinces . . . . . . . . . . . . . . . . . . . . . . . 108

4.7 Estimated number of potentially earthquake prone and earthquake risk

U RM bui l di ngs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

5.1 Construction details of wall panels . . . . . . . . . . . . . . . . . . . . . 122

5.2 Summary of results of wall panels . . . . . . . . . . . . . . . . . . . . . 124

5.3 Default lower bound masonry properties (from ASCE (2007)) . . . . . . 132

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CHAPTER 1

Introduction

All new structural masonry in New Zealand consists of concrete blocks laid in a bed of

mortar and reinforced with steel bars, similar to reinforced concrete. This is termed re-

inforced masonry. In many older masonry buildings, the masonry consists of clay bricks

laid on a bed of mortar, but with no reinforcement. This is known as unreinforced ma-

sonry, or URM. In many parts of the world URM is still constructed today, but due to

its poor performance in earthquakes and New Zealands relatively high seismicity, URM

is no longer permitted in this country as a structural material for new buildings.

Unreinforced masonry is known to perform poorly when subjected to lateral forces pro-

duced by earthquakes of large magnitude (Drysdale et al., 1999; Megget, 2006; Paulay and

Priestley, 1992). This has been shown numerous times in earthquakes such as the 1989

Newcastle earthquake in Australia (Page, 1996), and the 1994 Northridge earthquake in

California (Klingner, 2006). In New Zealand, URM was a common building material in

the later part of the 19th and early part of the 20th centuries, but since the 1931 Napier

earthquake in which nearly 260 people were killed and many URM buildings in the city

were destroyed (Dowrick, 1998; Scott, 1999), the use of this construction material declined

(see Chapter 4 for details). The use of unreinforced masonry was also restricted by gov-

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Chapter 1. Introduction

ernment legislation after the introduction of the building bylaw NZS 1900 in 1965 (New

Zealand Standards Institute, 1965). The current masonry design standard NZS 4230:2004

refers only to reinforced masonry structural elements (Standards New Zealand, 2004b).

Nevertheless many historic URM buildings still exist in New Zealand.

1.1 New Zealand Seismicity

New Zealand is seismically active and lies on the boundary of the Australian and Pacific

tectonic plates. To the east of the North Island the Pacific plate is forced under the Aus-

tralian plate. Under the South Island the two plates push past each other sideways, and to

the south of New Zealand the Australian plate is forced under the Pacific plate, as shown

in Figure 1.1. The seismicity within New Zealand varies depending on the proximity to a

fault line associated with this plate boundary. For example, Wellington is considered to

have a high level of seismicity, compared with Auckland, which is considered to have low

seismicity.

It is estimated that New Zealand has approximately 14,000 earthquakes each year, and

most are small, but that between 100 and 150 have a magnitude sufficient to be felt (GNS

Science, 2007). In the past 150 years, New Zealand has had approximately 15 earthquakes

registering over M7.0 magnitude on the Richter scale, with centres less than 30 km deep.

By comparison, the Kocaeli earthquake in Turkey in 1999 registered M7.4 and had an

epicentre at a depth of 16 km. Casualties numbered nearly 16,000 and it was estimated

that over 66,000 buildings collapsed or were heavily damaged (Sezen et al., 2000). Al-

though New Zealands current methods of construction are considered to employ more

advanced seismic design than those used in Turkey, New Zealands URM building stock

was constructed before the current understanding of seismic design principles. Conse-

quently, those existing buildings which were not designed to withstand the lateral forces

generated in an earthquake, of which the URM building stock forms a significant pro-

portion, are still at risk of significant damage and/or catastrophic structural failure in a

major earthquake.

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1.2. URM Building Behaviour

Figure 1.1: The plate boundary in New Zealand (reproduced with permission from GNSScience (2007))

1.2 URM Building Behaviour

URM buildings typically consist of foundations, URM walls and piers oriented in or-

thogonal directions and timber floors, acting as diaphragms, connected to walls by wall-

diaphragm ties. Other building components may include parapets and appendages such

as awnings. URM walls are typically stiff structural elements and can be categorised into

in-plane and out-of-plane walls depending on the direction of earthquake motion relative

to the plane of the walls. Walls oriented parallel to the motion of earthquakes are called

in-plane walls, and walls perpendicular to in-plane walls are defined as out-of-plane walls.

URM buildings are characterised by a limited number of storeys (typically up to three,

and no greater than six in New Zealand). As a generalisation they have regular plan

shapes and the external walls form part of the horizontal force resisting system.



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1.2.1 Typical Structural Deficiencies in URM Buildings

As stated above, URM has been shown to perform poorly in earthquakes. There are a

number of common details and aspects of URM construction which have been identifiedas deficient. Bruneau (1994b) summarises common failure modes of URM buildings as

follows:

Lack of anchorage

Anchor failure

In-plane failures

Out-of-plane failures

Combined in-plane and out-of-plane effects

Diaphragm related failures.

Lack of anchorage refers to the absence of positive anchorage between floors or roof el-

ements and walls, in which exterior walls behave as cantilevers over the total building

height. This cantilever effect increases the risk of out-of-plane failure as building height

increases, and additionally increases the risk of global structural failure due to floor and

roof collapse from de-seating. Anchor failure is similar in consequence to lack of an-

chorage and occurs when any connections between diaphragms and walls were poor or

inadequate in the original construction, or have deteriorated over time. Out-of-plane fail-

ures occur when inertia forces are of sufficient magnitude to cause displacements in an

out-of-plane direction such that the wall loses integrity and collapses. This can be sudden

and explosive. When adequate connections are present between walls and diaphragms,the out-of-plane excitation occurs at each floor level, and when there is inadequate con-

nection, the walls become tall unrestrained cantilevers. Parapets are vulnerable to this

type of failure, and are subject to the greatest amplification of ground motions. Failure

of the diaphragm itself is not common and is mostly related to connection and anchor

failures.

When a building is to be retrofitted, or its seismic performance is to be improved, the first

and most obvious course of action is to restrain any unrestrained parapets and gables.



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The consequences of parapet failure may not be significant for the buildings structural in-

tegrity, but may have life safety consequences from falling to the ground below. Secondly,

the connections between the walls and diaphragms must be made robust to prevent floor

or roof collapse from de-seating. Magenes and Calvi (1997) note that once out-of-planefailure is prevented by proper measures (e.g. reinforced concrete ring beams or steel ties

at the floor levels) the in-plane walls provide the stability necessary to avoid collapse.

Bothara et al. (2010) note that URM buildings may be satisfactory in a medium earth-

quake risk zone if anchorage and out-of-plane failure of the walls can be prevented. For

countries with high seismicity such as New Zealand (Wellington, in particular), in-plane

walls still have the potential for significant damage. Consequently, the work reported here

focuses on the in-plane response on URM walls.

1.2.2 In-Plane Wall Response

Numerous researchers have reported the different failure modes for in-plane URM walls

(see Calderini et al., 2009a,b; Cattari and Lagomarsino, 2009; Lee et al., 2008; Magenes

and Calvi, 1997; Mann and Muller, 1973, 1982, 1985; Marzahn, 1998; Priestley et al.,

2007). Failure modes of masonry piers subjected to seismic actions and gravity actions

can be generally classified as either flexure dominated or shear dominated, and are related

primarily to the aspect ratio of the wall and the axial load ratio. Figure 1.2 shows in-plane

behaviour modes of a laterally loaded URM wall.

Flexural response (Figure 1.2(a)) is sometimes differentiated from rocking response (Fig-

ure 1.2(b)), but in general there is little difference in URM and this response is charac-

terised by the wall behaving as a vertical in-plane cantilever when subjected to lateral

forces and displacements. Cracks occur in the masonry tension zone through the opening

of bedjoints and shear is carried in the compression zone. Failure occurs by crushing at

the wall corner (compression toe) and possibly overturning of the wall. Wall overturning

is unlikely in URM buildings as drifts of up to 10% are required, and other failures will

occur before this drift limit is reached. The extent of toe crushing depends largely on

the ratio of applied axial stress to axial strength (axial load ratio), and when the axial



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(a) Flexure (b) Rocking/Toe crushing

(c) Sliding (d) Diagonal shear

Figure 1.2: In-plane behaviour modes of a laterally loaded URM wall (after Marzahn,1998; Voon, 2007)

load is low the behaviour appears more like rocking of a rigid block. On the tension side

of the wall cracking may be distributed up the height of the wall, but in the absence of

reinforcement to limit crack widths and spread out crack locations, cracking more often

occurs at the base of the wall, either between the wall and foundation, or at the opening

of a bedjoint several courses above the base. Consequently, although flexural response is

sometimes differentiated from rocking and toe crushing, there is significant overlap, and in



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general defining this mode of behaviour as flexural response is satisfactory. Strength and

displacement limits are difficult to define for flexural response as toe crushing is the only

mode which provides any practical limiting constraints. For wall integrity and gravity

load carrying capacity, as well as non-structural damage limitation, a drift limit of 0.8%for flexural response of non-retrofitted URM walls has been suggested in the literature

(Cattari and Lagomarsino, 2009; Magenes and Calvi, 1997; Priestley et al., 2007).

Shear failure is often classified as either sliding shear failure (Figure 1.2(c)) or diagonal

tension failure (Figure 1.2(d)). Sliding shear occurs when entire parts of the wall displace

horizontally on a sliding plane. A sliding plane can form along cracked bedjoints due to

the formation of horizontal cracks subjected to reversed seismic action, or on damp proof

courses. This mode of failure is more likely to occur when axial load levels and/or friction

coefficients are low, or the bond (cohesion) between bricks and mortar is weak. Cracks

induced by deflection such as foundation settlement (see Figure 1.3) can create a failure

plane in much the same way as a damp-proof course. Magenes and Calvi (1997) note that

sliding on horizontal bedjoints is a stable mechanism as high displacements are possible

without the loss of integrity of the wall. Damage is concentrated in a bedjoint and as

long as some axial load is present high energy dissipation is possible. Similar to a flexural

(rocking) response, a drift or displacement limit for sliding has no real meaning. Diagonal

tension failure is often simply termed shear failure and is characterised by a critical

Figure 1.3: Prior cracking in the wall of a two storey URM isolated building as a potentialsliding plane.



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combination of principal tension and compression stresses as a result of combining shear

and compression and leads to the formation and development of inclined diagonal cracks.

Depending on the relative strength of mortar joints, the brick-mortar interface, and brick

units, the cracks may follow the path of the bed- and headjoints, or may go through thebricks. In some instances, strength limits are given simply in terms of diagonal tension

failure (ASCE, 2007), whilst in other instances equations are given which differentiate

between diagonal cracking through joints and diagonal cracking through bricks (Calderini

et al., 2009b; Cattari and Lagomarsino, 2009). For walls where a shear failure dominates,

a drift limit of 0.4% for non-retrofitted URM walls has been suggested in the literature

(Cattari and Lagomarsino, 2009; Priestley et al., 2007).

ASCE (2007) classifies rocking and bedjoint sliding behaviour modes as deformation

controlled actions because of the large displacements available without significant loss

of strength1, while diagonal tension and toe crushing behaviour modes are classified as

force controlled actions because the ultimate failure can be abrupt with little or no

subsequent deformation.

Calderini et al. (2009a) note that it is not always easy to distinguish the occurrence of a

specific type of mechanism, as many interactions may occur between them.

1.3 Research Motivation

Much research has been previously presented on in-plane wall response, but it has been

identified in the literature (see Lee et al., 2008; Moon et al., 2006; Yi et al., 2006a,b, 2008)

that codified equations for assessing the strength and displacement capacity of walls are

overly conservative, particularly when assessing URM walls with flanges (return walls)

1Deformation tends to imply a change of shape, which may or may not be considered a brittle failure(see Chapter 5), and consequently where large lateral displacements are available without significant lossof strength, it may be more correct to classify such behaviour modes as displacement controlled actions.

Because this could be confusing when considering displacement based design and assessment of structures(see Chapter 8), and also because it is already defined in ASCE (2007), the definition of deformationcontrolled actions from ASCE (2007) is used in this thesis.



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1.3. Research Motivation

at either or both ends of the wall. Consequently, one objective of this research was to

investigate the response of flanged URM walls, in the context of previous research into

failure modes, and to determine strength and displacement limits.

Furthermore, in order to quantify the risk which New Zealands URM building stock

poses, another objective of this research was to develop a comprehensive characterisation

of the national URM building stock, in terms of the building properties (architectural

characteristics and typical deficient details) and the prevalence, distribution and finan-

cial value of URM buildings. This characterisation was undertaken in order to provide

a framework to support both other aspects of the research reported herein, and to assist

the parallel research activities of others considering the seismic response and retrofit of

URM buildings in New Zealand.

Finally, NZSEE (2006) has provided guidelines for assessing the structural performance of

buildings in earthquakes in the context of New Zealands legislative environment. Simple

assessment procedures to determine if a building is potentially earthquake-prone (IEP, Ini-

tial Evaluation Procedure) as well as detailed assessment procedures are provided. It hasbeen communicated by practitioners that whilst the IEP is useful for quickly determining

if a building is potentially-earthquake prone, the detailed assessment procedures are com-

plicated and difficult to follow. It has been identified that an assessment procedure needs

to fill the gap between the existing IEP and the NZSEE detailed assessment procedures.

Consequently, a significant emphasis in this thesis is placed on the seismic response of

URM walls subjected to in-plane loading, particularly accounting for the effects of flanges,

and on the development of a detailed seismic assessment procedure which is straightfor-

ward to follow and implement, and is appropriate for the financial and heritage value

attached to each unreinforced masonry building.



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1.4 Thesis Outline

As stated above, the primary aim of this doctoral investigation was to develop a deeper

understanding of the seismic response of URM buildings in New Zealand. To achieve this

aim, it was necessary to first characterise the New Zealand URM building stock on both

an individual building and nationwide basis. Once the characteristics of URM buildings

were known, it was then possible to assess their seismic performance, focussing on the

behaviour of walls responding in-plane. As such, this thesis has two main parts. The first

part constitutes the background to URM history and development in New Zealand, inves-

tigations into characterising buildings on an individual basis and characterisation of the

New Zealand URM building stock on a nationwide basis. This characterisation exercise

is covered in Chapters 2 4. The second part of the thesis constitutes Chapters 5 7,

where the focus changes from a broad level context to the detailed experimentation and

analysis of URM walls responding in-plane. The experimental programme was formed

on the basis of results from the first part of the thesis, such that wall specimens were

constructed in a manner that replicated the structural characteristics of existing New

Zealand URM buildings. Finally, the two main parts of the thesis are drawn together in

Chapter 8, where a framework is presented for assessing the seismic performance of the

in-plane loaded walls of a typical New Zealand URM building.

thesis haec divisa in partes novem est. Chapter 1 describes the aim and scope of the

present work. Chapter 2 presents a brief background to the legislative environment in New

Zealand for assessing and improving the seismic performance of potentially earthquake

prone buildings, as well as a brief consideration of principles for assessing and retrofitting

buildings of an historic nature. Furthermore, this chapter traces the development of New

Zealand building codes and the associated provisions for assessing existing earthquake

risk buildings, and provides some background to the history of the URM building stock

in New Zealand. Chapter 3 outlines general structural configurations (typologies) which

apply specifically to New Zealand URM buildings. Other characteristics are outlined, in

terms of materials and wall geometries, and how these characteristics relate to the overall

architectural typologies. Chapter 4 gives an estimate of the number of URM structures in

New Zealand, and the financial value of these buildings, both collectively and individually,



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1.4. Thesis Outline

as well as an estimate of their seismic vulnerability. The results of diagonal shear tests on

eight two-leaf unreinforced masonry wall panels are presented in Chapter 5. The aim of

the experimentation in Chapter 5 was to investigate the diagonal tension (shear) strength

of unreinforced masonry wallettes having different mortar properties and bond patterns,and also to provide a baseline against which to compare other retrofitted masonry samples

and also samples tested in-situ in existing buildings. Chapter 6 presents the in-plane cyclic

response of three unreinforced masonry walls designed to replicate typical New Zealand

construction in the early 20th Century, with different aspect ratios and different levels of

axial load. Chapter 7 describes the results of experimentation investigating the in-plane

response of walls with perpendicular flanges or return walls at the wall ends. Flanges of

different lengths and at different locations were investigated. Chapter 8 presents a proce-

dure for assessing the seismic performance of URM buildings, with particular emphasis on

the response of in-plane walls. Finally, Chapter 9 contains a summary of the conclusions

drawn from this doctoral investigation.

This thesis does not feature a chapter containing a comprehensive literature review be-

cause the investigation reported here draws on several different fields, and consequently

a review of previous research on each topic is presented separately in each chapter.



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CHAPTER 2

Background on Potentially Earthquake

Prone Buildings

This chapter presents a brief background to the legislative environment in New Zealand for

assessing and improving the seismic performance of potentially earthquake prone buildings

(EPBs), as well as a brief consideration of principles for assessing and retrofitting buildings

of historic nature. Furthermore, this chapter traces the evolution of New Zealand building

codes and the associated provisions for assessing existing earthquake risk buildings, and

provides some background to the history of the development of the URM building stock

in New Zealand.

Much of the information included in this chapter is a summary from different working

group reports and regulatory documents in New Zealand, particularly by the New Zealand

Society for Earthquake Engineeering (NZSEE) and the Department of Building and Hous-

ing (DBH). This information is intended to provide a context for the research conducted

in subsequent chapters.



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2.2. URM Heritage Considerations

ad hoc, where evaluations of buildings are conducted on a case-by-case basis, and usually

triggered by an application to the TA under the Building Act for building alteration,

change of use, extension of life or subdivision. For larger TAs, an active approach would

be more appropriate as it enables the adoption of the best possible risk-reduction pro-gramme and sets and controls the level of any work required to mitigate risk. Buildings

that the preliminary investigation suggests may be earthquake-prone should be subject

to an initial evaluation procedure (IEP). The objective of the IEP is to identify as closely

as possible all earthquake-prone buildings within a TAs jurisdiction. At the same time,

this initial evaluation should limit the number of buildings that would, on a further de-

tailed evaluation, be found to be not earthquake-prone (DBH, 2005). Where an initial

evaluation indicates that a building is likely to be earthquake-prone but the precise EPB

status of the building may be in doubt, it is desirable that a detailed assessment of the

building is undertaken to determine more precisely whether the building falls within the

Building Acts definition of earthquake-prone.

2.2 URM Heritage Considerations

An understanding of the principles involved in maintaining the character and integrity

of heritage buildings is of primary importance when changing a URM building during an

upgrade or strengthening. Indeed, a consideration of such principles is worth including

from the beginning of any building improvement project, and as such, a brief outline of

heritage conservation principles is summarised here. A more comprehensive coverage can

be found in Goodwin (2008) and McClean (2007, 2009).

Robinson and Bowman (2000) state that good conservation practice balances two com-

plimentary principles: the safe, private and public enjoyment of an historically significant

building; and the continuing practical use of a building as a property asset. If these

principles are observed, the result will be the maintenance or enhancement of a buildings

heritage significance, its continuing useful life and its value as an asset for its owner and

the community. Goodwin (2009) states that it is important to understand that there



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Chapter 2. Background on Potentially Earthquake Prone Buildings

is more to a building than simply its physical material. This intangible component is

comprised of concepts such as its history and social use, and spiritual significance of the

place, which are essentially what the building means to an observer or group at a point

in time. When combined with the physical elements, these are collectively known as itsheritage value.

There are some parts of a building which are less important than others. Some parts can

be altered or removed, but others are more important to maintain. Buildings have an

intrinsic logic, and retrofit interventions need to follow the intrinsic logic of the building.

As an example, parts of a structure which were originally designed to be visible should

still be visible after the intervention. Invasiveness is the key characteristic leading to

the success or otherwise of the intervention from a heritage conservation perspective.

Robinson and Bowman (2000) state that for any required strengthening or stabilisation,

the objective should be to minimise the adverse effects on the building fabric and the

spaces around the building. There are four principles which should be considered for this

purpose:

Knowledge of the important characteristics of a building;

The golden rule for changes to heritage buildings is as much as necessary, as little

as possible. All work should involve minimum intrusion;

Strengthening work should be reversible and aim to achieve structural effectiveness

at reasonable cost. This approach will allow for more improved strengthening sys-

tems to be incorporated at a later date and for further adaptive reuse or restoration

of original forms in the future;

Strengthening systems should respect the character and integrity of a heritage build-

ing.

The Department of Building and Housing also recognises the unique need for heritage

buildings to be treated with special consideration. The following is quoted from the

Department of Building and Housings guidance for territorial authorities for developing

earthquake prone building policies (DBH, 2005),

The Building Act requires TAs to state in their EPB policies how they



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2.2. URM Heritage Considerations

intend to manage heritage buildings that are earthquake-prone. The age, lay-

out, structure, type of construction and the cultural and aesthetic sensitivity

of heritage buildings are such that the cost of their structural improvement

is likely to be very high. These special considerations and constraints meanthat TAs will need to engage fully with the owners of heritage buildings and

the Historic Places Trust. TA policies should also indicate how the TA would

manage the different needs of private and public owners of heritage build-

ings. In determining a suitable standard of performance improvement, TAs

will need to take into account the high priority that owners and the Historic

Places Trust will place on the protection of a buildings fabric, in addition to

meeting its EPB policy requirements concerning the life safety of occupants.

Given the importance of heritage buildings to the historical and cultural life of

the nation and the local community, TAs may wish to consider special imple-

mentation measures in relation to these buildings. These could include setting

an extended period in which structural improvements are to be completed or

providing incentives to owners to upgrade buildings.

Finally, NZSEE (2006) notes that historical buildings of special cultural significance

should be assigned Importance level 3 unless this classification would result in signifi-

cant disruption to historical fabric. In such cases Importance Level 2 may be assigned

but with the expectation of greater damage in a large (low probability) earthquake. This

approach takes into account the need to balance the preservation of a heritage building

for life safety and collapse prevention purposes with the consideration of the architec-

tural impacts of the improvement measures. A minimalist intervention approach will be

considered a failure if, in the event of an earthquake, the building collapses and is lost

forever. Conversely, a more invasive retrofit intervention which changes the buildings

historic fabric permanently, while allowing the structure to withstand earthquakes which

it may be subjected to, could also be considered a failure as the buildings original and

historic nature is lost forever.



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2.3 New Zealand Building Codes

The construction of URM buildings in New Zealand peaked in the decade between 1920

and 1930 and subsequently declined (see Figures 4.3 and 4.4), with one of the most im-

portant factors in this decline being the economic conditions of the time. The Great

Depression in the 1930s and the outbreak of World War II slowed progress in the con-

struction sector significantly, and few large buildings of any material were constructed in

the period between 1935 and 1955 (Megget, 2006; Stacpoole and Beaven, 1972). Equally

important in the history of URM buildings in New Zealand was the 1931 M7.8 Hawkes

Bay Earthquake, and the changes in building provisions which it precipitated. The de-struction of many URM buildings in Napier graphically illustrated that as a construction

material, URM provided insufficient strength to resist lateral forces induced in an earth-

quake due to its brittle nature and inability to dissipate energy. Later in 1931, in response

to that earthquake, the Building Regulations Committee presented a report to the Par-

liament of New Zealand entitled Draft General Building By-Law (Cull, 1931). This

was the first step towards requiring seismic provisions in the design and construction of

new buildings. In 1935, this report evolved into NZSS No. 95, published by the newly

formed New Zealand Standards Institute, and required a horizontal design acceleration of

0.1g, and this requirement applied to the whole of New Zealand (New Zealand Standards

Institute, 1935). NZSS No. 95 also suggested that buildings for public gatherings should

have frames constructed of reinforced concrete or steel. The By-Law was not enforceable,

but it is understood that it was widely used especially in the larger centres of Auck-

land, Napier, Wellington, Christchurch and Dunedin (Megget, 2006). The provisions of

NZSS No. 95 were confined to new buildings only, but the draft report acknowledged that

strengthening of existing buildings should also be considered, and alterations to existing

buildings were required to comply with the provisions (Davenport, 2004). In 1939 and

1955 new editions of this By-Law were published, and apart from suggesting in 1955 that

the seismic coefficient vary linearly from zero at the base to 0.12 at the top of the building

(formerly the seismic coefficient was uniform up the height of the building), there were

few significant changes (Beattie et al., 2008). It was not until 1965 that much of the re-

cent research at the time into seismic design was incorporated into legislation. The New

Zealand Standard Model Building By-Law NZSS 1900 Chapter 8:1965 explicitly prohib-



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2.3. New Zealand Building Codes

(a) North Island (b) South Island

Figure 2.1: Map of seismic zones [from NZSS 1900 Chapter 8:1965 (New Zealand Stan-dards Institute, 1965)]

ited the use of URM: (a) in Zone A; (b) of more than one storey or 15 ft (4.6 m) eaves

height in Zone B; (c) of more than two storeys or 25 ft (7.6 m) eaves height in Zone C.

These zones refer to the seismic zonation at the time, which have subsequently changed

and evolved. Zone A consisted of regions of the highest seismic risk and Zone C consisted

of regions of the lowest seismic risk (New Zealand Standards Institute, 1965). Details of

the seismic zonation in NZSS 1900 are shown in Figure 2.1. Again, the provisions of this

By-Law did not apply automatically and had to be adopted by local authorities.

The 1965 code required that buildings be designed and built with adequate ductility,

although further details were not given. The next version of the loadings code was pub-

lished in 1976 as NZS 4203 (Standards Association of New Zealand, 1976), and was a

major advance on the 1965 code. Most importantly, the 1976 loadings code was used in

conjunction with revised material codes: steel, reinforced concrete, timber and reinforced

masonry, which all required specific detailing for ductility. Thus after the publication of

this code in 1976, unreinforced masonry was explicitly prohibited as a building material



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throughout the whole of New Zealand.

The use of URM was implicitly discouraged through legislation from as early as 1935, and

although it was still allowed in some forms after 1965, observations of existing building

stock show its minimal use from 1935 onwards, especially for larger buildings. This is

thought to be significantly attributable to the exceptionally rigorous quality of design

and construction by the Ministry of Works at the time (Johnson, 1963; Megget, 2006).

2.4 Provisions for the Seismic Upgrade of Existing

Buildings

As building codes were being developed for the design of new buildings, attention was

also given to the performance of existing buildings in earthquakes. The first time this was

addressed in legislation was Amendment 301A to the 1968 Municipal Corporations Act

(New Zealand Parliament, 1968). This Act allowed territorial authorities, usually being

boroughs, cities or district councils, to categorise themselves as earthquake risk areas and

thus to apply to the government to take up powers to classify earthquake prone build-

ings and require owners to reduce or remove the danger. Buildings (or parts thereof) of

high earthquake risk were defined as being those of unreinforced concrete or unreinforced

masonry with insufficient capacity to resist earthquake forces that were 50% of the mag-

nitude of those forces defined by NZS 1900 Chapter 8:1965. If the building was assessed

as being potentially dangerous in an earthquake, the council could then require the

owner of the building within the time specified in the notice to remove the danger, either

by securing the building to the satisfaction of the council, or if the council so required,

by demolishing the building. Most major cities and towns took up the legislation, and as

an indication of the effect of this Act, between 1968 and 2003 Wellington City Council

achieved strengthening or demolition of 500 out of 700 buildings identified as earthquake

prone (Hopkins et al., 2008). Auckland, in spite of having a low seismicity, took a strong

interest in the legislation and this led to considerable activity in strengthening buildings

(see Boardman (1983)). In Christchurch, a moderately high seismic zone, the City Council



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2.4. Provisions for the Seismic Upgrade of Existing Buildings

implemented the legislation, but adopted a more passive approach, generally waiting for

significant developments to trigger the requirements. In Dunedin, now seen to be of low

seismic risk, little was done in response to the 1968 legislation. Strengthening of schools,

public buildings and some commercial premises was achieved. As a result, Dunedin hasa high percentage of URM buildings compared with many other cities in New Zealand

(Hopkins, 2009). Megget (2006) states that much of the strengthening in Wellington

was accomplished with extra shear walls, diagonal bracing or buttressing and the tying

of structural floors and walls together, and that many brittle hazards such as parapets

and clock towers had been removed after the two damaging 1942 South Wairarapa earth-

quakes (M7 & M7.1) which were felt strongly in Wellington. Hopkins et al. (2008) noted

that there was criticism at the loss of many older heritage buildings and at the use of

intrusive retrofitting measures which were not harmonious with the architectural fabric

of the building. At the same time, this did provide an opportunity in many cases for the

land on which the old building was situated to be better utilised with new, larger and

more efficiently designed structures.

A major drawback of the 1968 legislation, which endured until 2004, surviving intact with

the passage of the Building Act 1991, was that the definition of an earthquake prone build-

ing and the required level to which such buildings should be improved remained tied to the

1965 code. Most territorial authorities called for strengthening to one-half or two-thirds of

the 1965 code, and many buildings which were strengthened to these requirements, were

subsequently found to fall well short of the requirements of later design standards for new

buildings.2 This situation was recognised by the New Zealand Society for Earthquake

Engineering (NZSEE), which was also concerned about the performance of more modernbuildings, particularly after the observed poor performance of similarly aged buildings in

earthquakes in Northridge, California (1994) and Kobe, Japan (1995). NZSEE pushed for

new, more up-to-date and wide-ranging legislation. This was supported by the Building

Industry Authority, later to become part of the Department of Building and Housing,

and a new Building Act came into effect in August 2004 (New Zealand Parliament, 2004).

This brought in new changes as to what constituted an Earthquake Prone Building. In

2Wellington City Council found that in January 2008, of 97 buildings which had been previouslystrengthened, 61 were subsequently identified as potentially earthquake prone (Bothara et al., 2008;Stevens and Wheeler, 2008).



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particular, the definition of an earthquake prone building was tied to the current design

standard of the time, and no longer to the design standard of any particular year. The

legislation allowed any territorial authority that is satisfied that a building is earthquake

prone to require the owner to take action to reduce or remove the danger. Each territorialauthority was required to have a policy on earthquake prone buildings, and to consult

publicly on this policy before its adoption. Policies were required to address the approach

and priorities and to state what special provisions would be made for heritage buildings.

The 2004 legislation applied to all types of building except small residential ones, (resi-

dential buildings were excluded unless they comprised 2 or more storeys andcontained 3

or more household units).

As soon as the 1968 legislation came into effect to attempt to mitigate the effects of

earthquake prone buildings, the New Zealand National Society for Earthquake Engineer-

ing set up a steering committee to provide a code of practice in an effort to assist local

authorities to implement the legislation. Since the first draft code of practice published by

the NZNSEE (1972), several successive publications have been produced, each extending

on the previous version. These documents have been instrumental in helping engineers

and territorial authorities to assess the expected seismic performance of existing buildings

consistent with the requirements of the legislation. Guidelines for assessing and upgrading

earthquake risk buildings were published as a bulletin article in 1972 (NZNSEE) and then

separately published the following year, which became colloquially known as the Brown

Book (NZNSEE, 1973). The Brown Book provided guidelines for surveying earthquake

risk buildin

characterisation and seismc assessment of unreinforced masorny buildings

Documents