Bamboo as Earthquake Reinforcement Honours 2018

School of Civil, Environmental and Mining Engineering

Final Year Research Project Report

INVESTIGATION OF BAMBOO REINFORCED SEISMIC BANDS WITHIN EARTHQUAKE-RESISTANT CONSTRUCTION

By F18S31 Hannah Jury, HJ Daniel Paukner, DP Ned Magarey, NM Alex Savory, AS

Supervisor:

October 2018

Dr. Michael Griffith

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We give permission for this project to be reproduced and submitted for the purposes of assessment, and archived in the School repository as reference material.

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ABSTRACT The objective of this research is to investigate the use of bamboo reinforced seismic bands within Nepalese vernacular masonry, for the purposes of providing a low-cost, accessible seismic strengthening technique. Much of the devastation of the 2015 Gorkha Earthquake in Nepal was was due to the inability of the current building stock in Nepal to resist earthquake forces that are inevitably caused by the high seismicity of the region. Laboratory testing was undertaken to determine bamboo tensile and compressive strength as well as the interaction properties between bamboo and mortar. A finite element analysis was used to model the bamboo-mortar interaction and calibrated with laboratory results. Interaction properties from this model, as well as material testing results were then inputted into a model of a simple masonry structure where different bamboo reinforcement situations were generated and optimised through analysis of global performance of the structure under cyclic loading.

Analysis of results found that inclusion of bamboo reinforced seismic bands were most effective at the lintel level, where they reduced maximum stress experienced in the masonry between openings by approximately 14%.

i

ACKNOWLEDGEMENTS We would like to acknowledge and give thanks to a number of people who have provided significant guidance and advice to us whilst undertaking this research project; To our supervisor, Professor Michael Griffith, for sharing his extensive knowledge and expertise, and for his ongoing assistance with all aspects of the project; To laboratory technicians Scott Letton, Jon Ayoub, Ian Ogier and Dale Hodson for generously giving their time and providing direction with all laboratory testing; To Young Kol Park for his generosity in sharing his time and extensive knowledge in finite element analysis to help achieve the computer modelling components of this research; To Jeff Lugg and Adelaide Zoo for kindly donating bamboo for all laboratory testing; To Martijn Schildkamp of Smart Shelter Research for imparting his extensive knowledge in the field of seismic building design in Nepal; And to Engineers Without Borders for providing general support and insight on the humanitarian engineering context of the project.

ii

CONTENTS

CONTENTS

ABSTRACT

i

ACKNOWLEDGEMENTS

ii

LIST OF FIGURES

viii

LIST OF TABLES

xii

LIST OF SYMBOLS & ABBREVIATIONS

xiii

1

INTRODUCTION

1

2

LITERATURE REVIEW

3

2.1

Earthquake Activity in Nepal . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

2.2

Social Aspects to Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

2.3

Vernacular Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4

2.3.1

Vernacular building in Nepal . . . . . . . . . . . . . . . . . . . . . . .

5

2.3.2

Unreinforced masonry (URM) . . . . . . . . . . . . . . . . . . . . . .

5

Non-Vernacular Construction . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

2.4.1

Reinforced concrete frames . . . . . . . . . . . . . . . . . . . . . . . .

8

2.4.2

Other common building typologies . . . . . . . . . . . . . . . . . . .

8

Seismic Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

2.5.1

New construction techniques . . . . . . . . . . . . . . . . . . . . . . .

9

2.5.2

Retrofitting techniques . . . . . . . . . . . . . . . . . . . . . . . . . . .

12

Critical Assessment of Existing Techniques . . . . . . . . . . . . . . . . . . .

13

2.4

2.5

2.6

3

AIMS

14 iii

CONTENTS

3.1

Summary of Literature Review Findings . . . . . . . . . . . . . . . . . . . .

14

3.2

Research Gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15

3.3

Aims & Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15

4

METHODOLOGY

18

5

LABORATORY TESTING PHASE (AIMS 1, 2)

19

5.1

Preliminary Preparation of Bamboo . . . . . . . . . . . . . . . . . . . . . . .

19

5.1.1

Choice of bamboo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19

5.1.2

Bamboo macro dimensions . . . . . . . . . . . . . . . . . . . . . . . .

20

Bamboo Density Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20

5.2.1

Bamboo density testing procedure . . . . . . . . . . . . . . . . . . . .

20

5.2.2

Density tests results . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

Bamboo Tensile Strength Testing . . . . . . . . . . . . . . . . . . . . . . . . .

22

5.3.1

Bamboo coupon preparation . . . . . . . . . . . . . . . . . . . . . . .

22

5.3.2

Bamboo coupon testing procedure

. . . . . . . . . . . . . . . . . . .

26

5.3.3

Coupon results data processing . . . . . . . . . . . . . . . . . . . . .

27

5.3.4

Tensile tests results . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27

Bamboo Compressive Strength Testing . . . . . . . . . . . . . . . . . . . . .

29

5.4.1

Compression tests sample preparation . . . . . . . . . . . . . . . . .

29

5.4.2

Bamboo compression testing procedure

. . . . . . . . . . . . . . . .

30

5.4.3

Compression testing data processing . . . . . . . . . . . . . . . . . .

30

5.4.4

Compression tests results . . . . . . . . . . . . . . . . . . . . . . . . .

30

Bamboo-Mortar Pull-out Testing . . . . . . . . . . . . . . . . . . . . . . . . .

32

5.5.1

32

5.2

5.3

5.4

5.5

Pull-out test bamboo preparation . . . . . . . . . . . . . . . . . . . .

iv

CONTENTS

5.6

6

5.5.2

Pull-out test mortar preparation . . . . . . . . . . . . . . . . . . . . .

32

5.5.3

Pull-out test mould preparation . . . . . . . . . . . . . . . . . . . . .

34

5.5.4

Pull-out testing procedure

. . . . . . . . . . . . . . . . . . . . . . . .

34

5.5.5

Bamboo pull-out test data processing . . . . . . . . . . . . . . . . . .

35

5.5.6

Pull-out tests results . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40

Summary of Laboratory Testing Results . . . . . . . . . . . . . . . . . . . .

43

FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

44

6.1

Finite Element Analysis Background . . . . . . . . . . . . . . . . . . . . . . .

44

6.1.1

Modelling Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44

6.1.2

Concrete damage plasticity model background . . . . . . . . . . . .

45

Model of Bamboo Pull-Out Tests . . . . . . . . . . . . . . . . . . . . . . . . .

46

6.2.1

Geometry of bamboo pull-out test models . . . . . . . . . . . . . . .

47

6.2.2

Meshing of bamboo pull-out test models . . . . . . . . . . . . . . . .

47

6.2.3

Material properties of bamboo and mortar . . . . . . . . . . . . . .

47

6.2.4

Boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . .

50

6.2.5

Bamboo-mortar interaction classification . . . . . . . . . . . . . . . .

51

6.2.6

Bamboo-mortar interaction calibration . . . . . . . . . . . . . . . . .

52

6.2.7

Summary of pull-out test FEA results . . . . . . . . . . . . . . . . .

58

FEA of Bamboo Reinforced Masonry structure . . . . . . . . . . . . . . . .

59

6.3.1

Masonry modelling techniques . . . . . . . . . . . . . . . . . . . . . .

59

6.3.2

Masonry properties and use of CDP . . . . . . . . . . . . . . . . . .

60

6.3.3

Modelling the seismic band . . . . . . . . . . . . . . . . . . . . . . . .

63

6.3.4

Sample building geometry . . . . . . . . . . . . . . . . . . . . . . . . .

63

6.3.5

Loading and boundary conditions . . . . . . . . . . . . . . . . . . . .

64

6.2

6.3

v

CONTENTS

7

Mesh generation and element type . . . . . . . . . . . . . . . . . . .

65

6.3.7

Sensitivity analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

66

6.3.8

Data processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

66

6.3.9

Optimisation of quantity of reinforcement . . . . . . . . . . . . . . .

68

6.3.10 Building case comparison results . . . . . . . . . . . . . . . . . . . . .

69

DISCUSSION OF LABORATORY AND MODELLING TESTING

71

7.1

Laboratory Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

71

7.1.1

Density of bamboo . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

71

7.1.2

Tensile strength of bamboo . . . . . . . . . . . . . . . . . . . . . . . .

73

7.1.3

Compressive strength of bamboo

. . . . . . . . . . . . . . . . . . . .

76

7.1.4

Bamboo/mortar pull-out test interaction . . . . . . . . . . . . . . . .

78

Finite Element Analysis Discussion . . . . . . . . . . . . . . . . . . . . . . . .

84

7.2.1

Pull-out test mortar modelling inaccuracies . . . . . . . . . . . . . .

84

7.2.2

Pull-out test bamboo geometric variability

. . . . . . . . . . . . . .

86

7.2.3

Inconsistencies in FEA and experimental pull-out results . . . . . .

87

7.2.4

Masonry model inputs . . . . . . . . . . . . . . . . . . . . . . . . . . .

89

7.2.5

Masonry model behaviour . . . . . . . . . . . . . . . . . . . . . . . . .

91

7.2.6

Recommended improvements to model . . . . . . . . . . . . . . . . .

93

7.2.7

Analysis of model results and outputs . . . . . . . . . . . . . . . . .

93

7.2

8

6.3.6

CONCLUSION AND RECOMMENDATIONS

95

8.1

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95

8.2

Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

97

8.2.1

97

Implementation of research . . . . . . . . . . . . . . . . . . . . . . . .

vi

CONTENTS

9

8.2.2

Improvements in methodology . . . . . . . . . . . . . . . . . . . . . .

98

8.2.3

Future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

99

REFERENCES

101

Appendices

107

A LITERATURE REVIEW - FURTHER INFORMATION

108

B FURTHER METHODOLOGY, RESULTS AND DATA

110

C CONTACTS

113

vii

CONTENTS

LIST OF FIGURES 1.1

Effect of seismic bands under lateral loading (adapted from Caon 2009) . . . . . . . . . .

2.1

Illustration of plate mechanisms in Nepal. MFT, MBT and MCT refer to the main tectonic zones/faults in the region (Sapkota et al. 2013 in Chiaro et al. 2015, p. 1031) . . . . . . .

2.2

4

Masonry wall failures (Shakya et al. 2013, p.723; Guatam et al. 2016, p.14; Chen et al.

2017, p. 566) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4

3

Location of Gorkha earthquake in Nepal. MFT, MBT and MCT refer to the main tectonic zones/faults in the region (Gautam, de Magistris & Fabbrocino 2017, p. 38) . . . . . . . .

2.3

2

7

Diagonal/vertical shear failure of URM wall, and horizontal shear failure (Bhattacharya S 2013, p.57) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

2.5

Cement-bonded masonry (Chen et al. 2017, p. 568-569) . . . . . . . . . . . . . . . . . . .

9

2.6

Use of bamboo reinforcement in new masonry structures (Blondet et al. 2011, p.11) . . .

10

2.7

Implementation of horizontal bands during construction (Blondet et al.

2011, p.9;

Langenback 2015, p.5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

2.8

Gabion bands in use during construction (Langenbach 2015, p.7) . . . . . . . . . . . . . .

12

2.9

An example of retrofitted bamboo reinforcement arrangement (Blondet et al. 2011) . . . .

13

3.1

Summary of aims of research showing relationship with each other in the project . . . . .

17

4.1

Overview of Project Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

18

5.1

Bamboo with key sections labelled . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21

5.2

The dimensions of the two different bamboo ’dog-bone’ coupons from which results were derived

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

5.3

Bamboo cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

23

5.4

Different coupon orientations (Richard & Harries 2014, p. 101) . . . . . . . . . . . . . . .

24

5.5

Apparatus setup for coupon preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

5.6

Comparison between design and actual coupon side profile . . . . . . . . . . . . . . . . . .

26

5.7

Extensometer attached to a coupon during tensile testing . . . . . . . . . . . . . . . . . .

26

5.8

Sample tensile coupons post-testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27

5.9

Stress-strain graph of 2nd coupon iteration . . . . . . . . . . . . . . . . . . . . . . . . . .

28

5.10 Stress-strain graph of 3rd coupon iteration . . . . . . . . . . . . . . . . . . . . . . . . . . .

29

5.11 Bamboo samples used in compression tests . . . . . . . . . . . . . . . . . . . . . . . . . .

30

viii

CONTENTS

5.12 Detail of reinforcement bars being added

. . . . . . . . . . . . . . . . . . . . . . . . . . .

33

5.13 Effects of creep and shrinkage due to water absorption in bamboo (Ghavami 2004, p. 642)

33

5.14 Apparatus setup for pull-out test curing. The three concrete cylinders on the front are used for determining compressive strength of the concrete . . . . . . . . . . . . . . . . . .

35

5.15 Apparatus used to perform pull-out tests on bamboo . . . . . . . . . . . . . . . . . . . . .

36

5.16 Compressive test stress-strain graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

37

5.17 Typical pull-out test arrangement describing analysis concept (Haskett, Oehlers & Ali 2008) 38 5.18 Linear and bi-linear local τ -δ relationships (Haskett, Oehlers & Ali 2008) . . . . . . . . .

38

5.19 Local τ -δ rule including residual friction, representing bond behaviour along bond interface (Vaculik et al. 2017) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

38

5.20 Example of global load-slip response showing graphical location of PIC and δmax (Haskett, Oehlers & Mohamed Ali 2006) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39

5.21 Method of obtaining parameters from test data, using data from pull out sample 3 . . . .

40

5.22 Non-noded load vs. displacement behaviour of sample 1 . . . . . . . . . . . . . . . . . . .

41

5.23 Graph showing average local stress-slip relationship for bamboo section containing a node, overlaid on individual results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42

5.24 Graph showing average local stress-slip relationship for bamboo section containing no node, overlaid on individual results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42

6.1

Response to uniaxial loading (Abaqus User Manual) . . . . . . . . . . . . . . . . . . . . .

46

6.2

Diagram showing normal and frictional forces acting on bamboo during pull-out tests . .

48

6.3

Idealised tensile and compressive CDP graphs . . . . . . . . . . . . . . . . . . . . . . . . .

49

6.4

Failure modes of bonds/joints (a) tensile cracking, (b)&(c) shear sliding (K.F. Abdulla et al. 2017) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51

6.5

Traction Separation Stiffness Relationships (K.F. Abdulla et al. 2017) . . . . . . . . . . .

51

6.6

Finite element modelling methodology for determining interaction behaviour between bamboo and mortar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

53

6.7

FEM results of un-noded test with kss = ktt = 9.82 N/mm3 . . . . . . . . . . . . . . . . .

54

6.8

FEM results of un-noded test with kss = ktt = 6.00 N/mm3 . . . . . . . . . . . . . . . . .

55

6.9

Graph showing FEA achieved stress-slip relationship for bamboo section containing no node, overlaid on individual and average experimental results . . . . . . . . . . . . . . . .

ix

55

CONTENTS 6.10 FEM results of noded test geometry with adjusted un-noded kss = ktt = 6.00 N/mm3 . .

56

6.11 Graph showing FEA achieved stress-slip relationship for bamboo section containing one node using noded geometry and un-noded stiffness value Kss = Ktt = 6.00 N/mm3 , overlaid on individual and average experimental results . . . . . . . . . . . . . . . . . . .

56

6.12 FEM results of un-noded test geometry with noded experimental mean kss = ktt = 16.2 N/mm3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57

6.13 FEM results of un-noded test geometry with adjusted noded kss = ktt = 7.9 N/mm3 . . .

58

6.14 Graph showing FEA achieved stress-slip relationship for bamboo section containing one node using un-noded geometry and calibrating a noded stiffness value Kss = Ktt = 7.90 N/mm3 , overlaid on individual and average experimental results . . . . . . . . . . . .

58

6.15 Masonry modelling techniques (a) typical masonry sample (b) micro modelling (c) simplified micro modelling (d) macro modelling (Abdelmoneim, 2016) . . . . . . . . . . .

60

6.16 Tensile and compressive CDP inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61

6.17 Sample building case C1, C2 and C3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

64

6.18 Loading definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

64

6.19 Geometry, mesh and boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

6.20 Sensitivity analysis to show change in global maximum displacement and maximum principal stress with changing wall thickness . . . . . . . . . . . . . . . . . . . . . . . . . .

66

6.21 Maximum principal tensile stress at different points in lintel band model over loading cycles 67 6.22 Contour plot results of building case C2, lintel band only, under maximum loading . . . .

67

6.23 Points of model where detailed stress/displacement analysis was undertaken . . . . . . . .

67

6.24 Relationship between maximum displacement at point A and number of culms of bamboo reinforcement at lintel level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

68

6.25 Maximum principal stress under maximum load . . . . . . . . . . . . . . . . . . . . . . . .

69

6.26 Reduction in maximum principal stress between windows due to addition of seismic bands (Point F) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69

6.27 Reduction in maximum principal stress at top of wall due to addition of seismic bands (Point A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70

7.1

Potential areas of improvement and discussion for density tests . . . . . . . . . . . . . . .

73

7.2

Preliminary coupon design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

73

7.3

First iteration of dog bone coupons with wide gauge region . . . . . . . . . . . . . . . . .

74

x

CONTENTS

7.4

Second iteration of dog bone coupons with narrower gauge region . . . . . . . . . . . . . .

75

7.5

Photos of compression tests both during and after loading showing failure mechanisms . .

77

7.6

Barrelling under loading when there is friction between surfaces. Points A, B and C show how as deformation increases, the sides of the sample effectively ’wrap’ around the contact surface (adapted from ASM 2008) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

77

7.7

Graph showing increase in compressive strength with cross-sectional area of bamboo . . .

78

7.8

Pull-out test split in half showing internal movement of bamboo and nodal interlocking .

79

7.9

Effect of localised bend in bamboo on resistive forces (bend exaggerated for illustrative purposes) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

81

7.10 Crushing and shearing of mortar as a result of pull-out testing . . . . . . . . . . . . . . .

85

7.11 Graph of idealised vs experimental CDP data . . . . . . . . . . . . . . . . . . . . . . . . .

86

7.12 Lintel band separation under max loading . . . . . . . . . . . . . . . . . . . . . . . . . . .

92

7.13 Failure of masonry walls between openings (Nanjunda 2004) . . . . . . . . . . . . . . . . .

94

B.1 Dogbone with no aspect ratio used, thickness set at whatever the bamboo thickness is (Shao et al. 2010, p. 135) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 B.2 Dogbone for glue-laminated bamboo (He et al. 2015, p. 495) . . . . . . . . . . . . . . . . 110 B.3 Dogbone for scrimber bamboo. Note the long grip length (Xu et al. 2017, p. 734) . . . . 110 B.4 Pull-out test geometry and boundary condition application . . . . . . . . . . . . . . . . . 111 B.5 Allowed geomtery of openings as specified in NBC . . . . . . . . . . . . . . . . . . . . . . 111

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CONTENTS

LIST OF TABLES 5.1

Summary of density of bamboo samples, found through water displacement . . . . . . . .

22

5.2

Summary of tensile strength test results of bamboo samples . . . . . . . . . . . . . . . . .

28

5.3

Summary of compressive strength of bamboo samples . . . . . . . . . . . . . . . . . . . .

31

5.4

Average cross-sectional areas of different culms of bamboo . . . . . . . . . . . . . . . . . .

31

5.5

Summary of cement mortar strength properties used for pull-out tests . . . . . . . . . . .

35

5.6

Summary of local bond stress-slip properties for no node and node sections . . . . . . . .

41

5.7

Summary of properties of bamboo and mortar as found during this research . . . . . . . .

43

6.1

CDP data used in Pull Test from Bolhassani et al. (2015) . . . . . . . . . . . . . . . . . .

48

6.2

Idealised tensile and compressive CDP inputs . . . . . . . . . . . . . . . . . . . . . . . . .

50

6.3

Experimental stiffness values found through pull-out tests . . . . . . . . . . . . . . . . . .

52

6.4

FEM calibrated stiffness results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59

6.5

Masonry smear elastic and plastic mechanical properties . . . . . . . . . . . . . . . . . . .

60

6.6

CDP compressive and tensile behaviour definition

. . . . . . . . . . . . . . . . . . . . . .

61

6.7

CDP damage parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

62

7.1

Summary of density of bamboo samples, found using measurement technique . . . . . . .

72

7.2

Masonry and mortar modulus of elasticity comparison . . . . . . . . . . . . . . . . . . . .

90

A.1 Detail of research into bamboo reinforcement in construction . . . . . . . . . . . . . . . . 108 A.2 Summary of merits of seismic improvement techniques . . . . . . . . . . . . . . . . . . . . 109 B.1 Required steel in seismic bands according to NBC . . . . . . . . . . . . . . . . . . . . . . 112

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CONTENTS

LIST OF SYMBOLS & ABBREVIATIONS Aband

cross sectional area of seismic band, mm2

Ainner

interior cross sectional area of bamboo mm2

Ap

cross-sectional area of bamboo, mm2

dc

compressive CDP damage parameter

dint

diameter of internode bamboo sections, mm

dn

diameter of node bamboo sections, mm

dt

tensile CDP damage parameter

E

Young’s Modulus of Elasticity, MPa

F

force, N

Gf

interfacial fracture energy, N/mm

g

gravitational acceleration = 9.81kg/m2

H

wall height, mm

hm

height of mortar bed joints, mm

hu

height of masonry units, mm

Knn

bond stiffness in normal direction to the culm, N/mm3

Kss

bond stiffness in tangential direction along the culm, N/mm3

Ktt

bond stiffness in tangential direction around the culm, N/mm3

Lcrit

critical embedment length of bamboo in mortar, mm

Lint

length of internode bamboo sections, mm

Ln

length of node bamboo sections, mm

Lper

circumference of bamboo section, mm

m

mass, kg

n

number of coarses of masonry

P

load, N

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CONTENTS

PIC

load to cause inter-crack debonding, N

s

speed, m/s

t

thickness of bamboo wall, mm

V

volume, m3

δmax

slip at loaded end for PIC , mm

δ1

slip at free end for PIC , mm

strain, dimensionless

ρ

density, kg/m3

σ

stress, MPa

τave

average bond stress, MPa

τmax

bond stress at failure, MPa

τr

residual bond stress, MPa

µ

coefficient of friction, dimensionless

Abbreviations CDP

concrete damage plasticity

FEA

finite element analysis

FEM

finite element model

LVDT

linear variable differential transformer

NBC

Nepalese building code

RC

reinforced concrete

URM

unreinforced masonry

xiv

1

1

INTRODUCTION

INTRODUCTION

The objective of this research is to investigate the use of bamboo reinforced seismic bands within Nepalese vernacular masonry, for the purposes of providing a low-cost, accessible solution. In April of 2015, a 7.8 magnitude earthquake struck Nepal causing upwards of 10 billion USD of damage equating to approximately one third of Nepal’s GDP - but more importantly claiming 9,000 lives.

This single event highlights the necessity for drastic structural improvement with much of the building failure in the country the result of poor construction practices or a lack of seismic features in building design (Baitao & Peitei 2015).

Likely due to a lack of engineering education in developing countries and economic incentive for foreign investment into low-cost seismic improvement, innovative research is disproportionately weighted towards cutting edge, expensive, and inherently ‘first world’ seismic improvement (Arya et al. 2013).

This comes as little help to developing communities with a predominantly unskilled labour force and general lack of construction education. To fill this void, ‘international organisations should provide efficient equipment or methods to help retrofit or reconstruct damaged structures as soon as possible’ (Chen et al. 2017, pp. 576-577).

As such, an appreciation of the vernacular architecture of the region is key to understanding design motivations and allows for greater insights into design proposals. Vernacular architecture refers to the way in which architecture in a particular culture or place organically evolves to respond to environmental or societal constraints produced by the ‘spontaneous and continuing activity of a whole people with a common heritage, acting under a community of experience’ (Belluschi referenced in Rudofsky 1964, np). The term vernacular is generally used in reference to traditional construction techniques based on appropriate material use and effective climatic management.

Due partially to Nepal’s economic disadvantage, along with minimal institutional construction education, vernacular masonry is of great significance to this study. Strict steel and reinforced-concrete centered post-earthquake regulations have been made by the Nepalese government, but it has been predicted by Adhikary et al.

(2016) that these regulations will go unfollowed as families and

communities ‘will continue to build as they used to build ... The government’s policy is – sadly – self-defeating’ (Adhikary et.al. 2016). Unreinforced masonry construction (URM) makes up a large percentage of building type in Nepal, and historically, it performs poorly under seismic loading. Under standard vertical loading in compression, masonry performs well. However, this construction is highly inductile and offers little to no flexural rigidity or tensile strength. Due to this, ‘rubble [masonry] structures performed the worst during the Nepal earthquake [of 2015]’ and with their high prevalence in rural regions, were the main cause of human casualties (Baitao & Peilei 2015, p. 573).

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INTRODUCTION

Figure 1.1: Effect of seismic bands under lateral loading (adapted from Caon 2009)

Within the literature there is research detailing potential material and design solutions that explore methods of earthquake resistance, however these solutions are often expensive and unsustainable.

There is currently minimal research exploring innovative use of bamboo; a cheap, renewable and locally-accessible resource. In particular, the application of computer modelling of bamboo in the context of earthquake reinforcement has had little to no exploration. Bamboo is present in the Nepalese building codes (NBC 203: Guidelines for Earthquake Resistant Building Construction: Low Strength Masonry), however there is no research available to conclusively justify its use. It is important to substantiate why and how bamboo may be used and what potential impact it realistically has to reduce building damage in a seismic event.

As such, the aim of this research is to explore the possibility of improving the earthquake resistance of masonry through integration of bamboo reinforced seismic bands. Seismic bands help to strengthen masonry under lateral loading as seen in Figure 1.1 and are usually reinforced with steel. The investigation into the appropriateness of bamboo as reinforcement will be achieved through material testing of bamboo samples to determine key properties and parameters. These findings are then used in finite element analysis to model the bamboo and masonry, and the interaction between materials used during construction of a typical Nepalese house. This computer model is iterated and optimised to investigate the application of bamboo reinforced masonry such that the performance of the reinforced structure can be analysed and the potential improvements in performance can be quantified, identifying the opportunities for implementation in URM buildings in rural Nepal.

This research has the potential to highlight significant opportunities within this innovative yet appropriate design solution, to encourage its implementation and save lives in earthquake events.

2

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2

LITERATURE REVIEW

LITERATURE REVIEW

To understand the background of the problem domain and identify knowledge gaps, a review of relevant literature was undertaken. This encompassed a review of literature regarding damage due to seismic activity in Nepal, construction and building stock, and techniques to improve building performance under seismic loading. This was particularly relevant and necessary given the broad and complex scope of the problem domain.

2.1

Earthquake Activity in Nepal

Nepal has a long history of earthquakes, and consequent widespread damages and loss of life associated with these events. In the 20th century, two major earthquakes - an 8.0 magnitude in 1934 and a 6.6 magnitude in 1988 - claimed the lives of 8,500 and 1,000 people respectively, with large earthquakes also recorded in the centuries preceding this (NGDC/WDS 2017). As aforementioned, the more recent 2015 Gorkha Earthquake (named after the region at its epicentre, 80 km north of Kathmandu (Goda et al. 2015, p. 1)) caused 9,000 deaths and 10 billion USD of damage. The complex cyclic loading caused by earthquakes in multiple directions results in both in-plane and out-of-plane stress and potential failure of various building elements, resulting in significant damage and human casualties. The high seismic activity is due to the country’s location on the overlap of the ‘subducting Indian plate and the overriding Eurasian plate’ (Goda et al. 2015 p. 2), as depicted in Figure 2.1 and Figure 2.2. Fault lines are a result of this movement, something also responsible for ‘driving the uplift of the Himalayan mountain range’ (Chiaro et al. 2015, p. 1031). It was this thrust faulting which caused the Gorkha earthquake.

Figure 2.1: Illustration of plate mechanisms in Nepal. MFT, MBT and MCT refer to the main tectonic zones/faults in the region (Sapkota et al. 2013 in Chiaro et al. 2015, p. 1031)

2.2

Social Aspects to Design

The issue of earthquake resistant construction in Nepal is more complex than simply implementing design solutions, due to key social conditions in the region. Arya et al. (2013) extensively details these limitations, citing lack of financial resources, training and construction material availability. ‘The main 3

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LITERATURE REVIEW

Figure 2.2: Location of Gorkha earthquake in Nepal. MFT, MBT and MCT refer to the main tectonic zones/faults in the region (Gautam, de Magistris & Fabbrocino 2017, p. 38)

factor that leads to the development of a seismic culture is earthquake hazard awareness’ (Ortega et al.

2017, p.2) however the relatively infrequent occurrence of earthquakes creates an absence of concern (Arya et al. 2013). There is also insufficient awareness as to economically-viable solutions, and so motivation to address seismic activity is missing (Arya et. al. 2013, p. 19).

Strict steel and reinforced-concrete centered post-earthquake regulations have been made by the Nepalese government, but it has been predicted by Adhikary et al. (2016) that these regulations will go unfollowed as families and communities ‘will continue to build as they used to build ...

The government’s policy is – sadly – self-defeating’ (Adhikary et.al. 2016). Furthermore, strict deforestation policy diminished the Nepalese hardwood industry and ‘adversely affected the traditional practices of making earthquake resistant wooden bands’ (Adhikary 2016, p.533) and timber-framed structures. Essentially, any attempts to improve Nepal’s ability to proactively reduce earthquake damage need to be low-cost, use readily-available resources, and effectively engage the prospective end users through assimilation of design with the current building vernacular. This is one of the key reasons driving the focus of the project on usage of bamboo for reinforcement, as it meets all these criteria.

2.3

Vernacular Construction

Appreciation of building typologies in Nepal is necessary to help drive choices regarding project direction. With the saturation of vernacular construction in the country, understanding the processes behind their design and construction enables project goals and aims to be better focused on proper problem solutions.

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2.3.1

LITERATURE REVIEW

Vernacular building in Nepal

Vernacular architecture is the organic evolution of building forms and construction techniques as a ‘result of hundreds of years of optimisation’ (Badoch et al. 2014, p.227) influenced by naturally available materials, local construction knowledge and technologies, culture, and climate (Asquith and Vellinga 2006; Badoch et al. 2014, p.227). Vernacular culture is not sculpted by engineers, architects or specialists, but is rather the pool of empirical knowledge passed over generations of the owner’s and communities who build homes, schools, and businesses (Ortega et al. 2017, p.2). Under threat of a globalising world with inherently homogenised building theology, it is important to safeguard vernacular culture and heritage ‘not only as a witness of the past, but also as a key factor for local development, boosting local economies’ (Ortega et al. 2017, p.1).

In areas of significant seismicity vernacular architecture is inherently vulnerable due to the ‘generalised use of poor materials, the potential scarcity of high-strength resources in developing communities, and poor maintenance, associated at times with lack of a good construction’ (Ortega et al. 2017, p.1). In 1994 the provisional first edition of the Nepalese seismic design code (NBC) was published, though engagement and use by the broader construction community has been minimal (Adhikary 2016, p.533; Baitao & Peilei 2015, p.571; Sharma, Deng & Noguez 2016, p.72). Furthermore, construction and damage patterns witnessed after the 1934 Nepal-Bihar earthquake correlate to that witnessed after the 2015 Gorkha earthquake suggesting insignificant post-event changes to construction habits and seismic culture (Guatam et al. 2016, p.2).

As of 2016, almost three quarters of buildings are made of locally sourced, non-engineered materials such as rubble/stone, timber, bamboo, and clays in the forms of brick, adobe and rammed earth (Adhikary 2016, pp.533-534; Sharma, Deng & Noguez 2016, p.63). Less than 25% of buildings are made with contemporary, potentially earthquake-proof (when properly engineered) materials such as concrete and steel, and due to their high cost and/or high technicality are intrinsic to higher socio-economic communities (Adhikary 2016, p.534; Sharma, Deng & Noguez 2016, p.63). It was, therefore, ‘the disadvantaged social groups in the poorer districts who suffered the largest damage and loss’ (Adhikary 2016, p.534) during the 2015 event.

2.3.2

Unreinforced masonry (URM)

Non-engineered or poorly engineered stone/rubble and brick masonry is globally recognised as seismically vulnerable due to its high mass, low strength, brittle nature and lack of ductility (Goda et al. 2015, p.8; Sathiparan et al. 2013, p.2177; Sharma, Deng and Noguez 2016, p.63). This results in ‘sudden catastrophic failures under seismic loading’ (Sathiparan et al. 2013, p.2177). Unreinforced masonry structures make up approximately 50%-60% construction in Nepal and are typical to rural 5

2

LITERATURE REVIEW

areas, causing the most significant economic and human losses in the 2015 earthquake (Baitao & Peilei 2015, p.573).

2.3.2.1

URM construction

Rarely constructed in compliance with the Nepal National Building Code (Guatam et al. 2016, pp.14-15), Nepalese URM buildings are typically constructed from undressed stones of irregular shape and/or sun-dried or fired clay brick. Furthermore, there is ‘no standardised brick dimension used in Nepal, which often results in uneven wall thicknesses’ (Bonapace & Sestini 2003, p.33). Although there is some use of cement mortar, low strength mud mortar is common. There are also examples of masonry un-bonded by mortar with gaps between elements filled with small stone aggregate and/or soil (Guatam et al. 2016,pp.14-15; Sharma, Deng & Noguez 2016, p.72; Shakya et al. 2013, p.722). URM walls are typically constructed with two wythes, the void in between filled with clay soil and/or rubble aggregate resulting in a heavy wall thickness of 360mm-600mm (Adhikary 2016, p.537; Bonapace & Sestini 2003, 33; Sathiparan et al. 2013, p.2177; Shakya et al. 2013, p.724). Flexural, shear and detachment resisting elements such as through stones, horizontal and vertical bands, ties, corner stones, and other adequate wall connections are seldom used (Guatam et al. 2016, p.11; Sharma, Deng & Noguez 2016, p.73). The use of wooden frames and seismic bands has also reduced due to the aforementioned diminishing hardwood industry and resultant ‘skyrocketing’ of hardwood prices (Adhikary 2016, p.538). Additionally, timber components that are available often have insufficient sectional dimensions (Shakya et al. 2013, p.722).

2.3.2.2

Unreinforced masonry failure mechanisms

Lack of monolithic structural integrity of unreinforced masonry accounts for a significant fraction of seismic damage witnessed after the 2015 Gorkha earthquake. Seismic loading lead to local flexural cracking, out-of-plane failure, separation of masonry wall wythes (see Figure 2.3a, 2.3b) and complete or partial wall collapse (see Figure 2.3c). These failures are due to thick wall elements composed of small sized rubble with an absence of bracing (both diagonal and vertical) and lateral support from high strength mortar, through stones, and lateral ties binding masonry wall elements (Adhikary 2016, p.537; Guatam et al. 2016, p.11; Sathiparan et al. 2013, p.2177; Shakya et al. 2013, p.723; Sharma, Deng & Noguez 2016, p.72-73). Chen et al. (2017) concludes that the severe damage or collapse of mud bonded masonry was far more prevalent than that of higher strength cement bonded masonry. The low tensile strength of mud mortar as well as a lack of horizontal bracing and adequate wall connections also caused in-plane shear cracking (see Figure 2.4) (Baitao & Peilei 2015, p.576; Shakya et al. 2013, p.725; Sharma, Deng & Noguez 2016, p.72). Globally, insufficient anchorage and significant mass was responsible for roof collapse (Guatam et al. 2016, p.14). Inadequate connections

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LITERATURE REVIEW

(a) Delamination of wall wyths

(b) segregation of mud mortar and associated delamination of wall wyths

(c) Gable failure and partial out of plane collapse Figure 2.3: Masonry wall failures (Shakya et al. 2013, p.723; Guatam et al. 2016, p.14; Chen et al. 2017, p. 566)

between orthogonal walls and floors resulted in ‘the formation of severe cracks near [wall] corners and at the location of openings’ (Sharma, Deng & Noguez 2016, p.72), out-of-plane failure, and collapse via the separation of building elements (Adhikary 2016, p.537; Guatam et al. 2016, p.13; Sathiparan et al. 2013, p.2177; Shakya et al. 2013, p.722). Gable walls were particularly vulnerable to this due to significant mass and detachment from the roof structure (Guatam et al. 2016, p.14; Sharma, Deng & Noguez 2016, p.72). Out-of-plane failure of load bearing elements could in turn trigger the global collapse of others. As witnessed by multiple engineering reconnaissance teams, ‘out-of-plane failures were more intense than any other type of failure’ (Guatam et al. 2016, p.11), therefore the most pressing problem in Nepalese vernacular seismic performance (Adhikary 2016, p.534).

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LITERATURE REVIEW

Figure 2.4: Diagonal/vertical shear failure of URM wall, and horizontal shear failure (Bhattacharya S 2013, p.57)

2.4 2.4.1

Non-Vernacular Construction Reinforced concrete frames

An understanding of the performance and failure mechanisms of building types in Nepal based on nonvernacular sources is necessary in deciding where further research will lie. The most prevalent type of this construction in Nepal is with reinforced concrete (RC) buildings. RC frames make up 10% of the country’s total building number (CBS 2012, p. 1), but this number rises up to a third of buildings in urban centres (Chen et al. 2017, p. 569). With construction of these RC-framed buildings beginning in the late 1970s (Goda et al. 2015, p. 8; Ohsumi, Mukai & Fujitani 2016, p. 1232), a national building code was only formally introduced in 2006 (Gautam et al. 2016, p. 3), and it is estimated that around 90% of RC-framed buildings in Nepal are non-engineered (JICA 2002 in Chaulagain et al. 2013, p. 284; Spence 2007, p. 218).

Failure mechanisms that occur in RC frames are mostly due to improper construction and design issues rather than material properties (Baitao & Peilei 2015; Shakya et al. 2013, p. 731). RC-frame’s vulnerabilities predominantly stem from social issues such as lack of education and inadequate access to good quality materials and funds and as such, there is limited scope for improvement from an engineering point of view. Furthermore, considering the relative smaller scale of numbers of RC framed buildings, as well as the success of the typology in comparison to brick or mortar counterparts, it was decided that the focus of the remainder of the literature review in regards to seismic improvements, would be based on the unreinforced masonry vernacular architecture.

2.4.2

Other common building typologies

Beyond RC frames, there are several other building typologies that draw upon aspects of both vernacular and non-vernacular in their design. The most common of these hybrid building types was cement-bonded masonry, (Chen et al. 2017, p. 564; Sharma, Deng & Noguez 2016, p. 63). These

8

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LITERATURE REVIEW

(a) well-constructed

(b) failed

Figure 2.5: Cement-bonded masonry (Chen et al. 2017, p. 568-569)

buildings performed better than their corresponding vernacular archetypes during seismic events, but fail through similar mechanisms as discussed in section 3.3.2.2. Chen et al. argues that the issue with this building type is not with the cement mortar, and that failures occur due to inadequate brick strength (Chen et al. 2017, p. 565-566). Typical construction and failure is highlighted in Figure 2.5.

2.5

Seismic Improvement

When considering the seismic improvement of masonry buildings, there are a variety of different methods available for both new construction and retrofitting of existing structures. It is important to assess the applicability and suitability of these techniques in the rural Nepalese context. As suggested

by Sathiparan et al. (2013, p. 2179), retrofitting techniques to strengthen unreinforced masonry buildings in developing countries should be: efficient in imparting strength and ductility to a structure and effective in dissipating energy, low cost/economically conscious, produced with readily available materials, and require low skill in implementation.

The following seismic improvement techniques will be assessed in respect to these criterion.

2.5.1 2.5.1.1

New construction techniques Vertical steel reinforcement

As previously mentioned, common failure mechanisms of masonry construction is through collapse of critical sections such as jambs of openings and corners of walls. The failure of these elements can be delayed with the addition of vertical reinforcements to provide tensile strength far greater than that of the mortar. Typically this is achieved through the addition of steel reinforcement. The reinforcement is placed in regular intervals through the wall with additional reinforcement in critical sections. During masonry construction, cavities are made which the steel is then added to and covered with cement concrete to bind to the bricks. (Arya et. al 2013; Blondet et al. 2011, pp.9-10). However as pointed 9

2

LITERATURE REVIEW

(a)

(b)

Figure 2.6: Use of bamboo reinforcement in new masonry structures (Blondet et al. 2011, p.11)

out by Moroz (2014, p.126), ‘in low cost housing applications, the high cost of steel reinforcement results in it being left out of construction’ and therefore it is potentially not the most suitable solution for dwellings in rural Nepal.

2.5.1.2

Bamboo reinforcement

The lack of accessibility of steel to developing communities has inspired research into the potential use of bamboo as reinforcement. Although there is relatively little research in this area, experimental results on the tensile and bonding strength of bamboo indicate there is high potential in this application (Lima 2008, p.985). The quick growth rate of bamboo, which can reach full size in just a few months, and its high prevalence in Nepal make the natural material accessible to even the lowest income and most isolated communities (Ghavami 2004; Moroz et al. 2014, pp.126-127; Akeju 2001, p.1464).

Bamboo can be used both as vertical tensile reinforcement or as horizontal reinforcement in the form of seismic bands (Blondet et al. 2011, p.10). As vertical reinforcement, bamboo is bonded in the mortar between the two wythes as can be seen in Figure 2.6.

2.5.1.3

Seismic bands

Seismic bands (also known as ring beams, collar beams or crown beams) improve seismic strength by increasing horizontal bending strength against plate action for out of plane inertia loads and by tying together perpendicular walls (Arya et. al 2013, Blondet et al. 2011, p.9, Bothara 2011). They help hold walls together causing the building to act like a box under seismic shaking, improving the strength of the construction. Seismic bands are continuous strips placed through load bearing longitudinal and transverse walls at different horizontal planes, such as at plinth, lintel and roof eave level and on top of gabels. (Arya et. al 2013) (see Figure 2.7b). They must match the width of the wall and can be made from a variety of materials, most commonly reinforced concrete. Wooden and

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LITERATURE REVIEW

(a) bamboo

(b) timber

Figure 2.7: Implementation of horizontal bands during construction (Blondet et al. 2011, p.9; Langenback 2015, p.5)

bamboo seismic bands are made with long strips placed on either edge of the wall with transverse members connecting the two strips (Blondet et al. 2011, p.9). This can be seen in Figure 2.7 showing an example of ring beams constructed from bamboo (2.7a) and timber (2.7b). As mentioned by Adhikary (2016, p.538), the use of hard wood seismic bands was common in masonry and rammed earth constructions, significantly improving their strength under seismic conditions. However from the early 1970s, conservation initiatives caused the price of hardwood to skyrocket, deterring people from buying it for seismic band construction. Due to financial constraints and the difficulty of access of industrially-produced construction materials such as steel and concrete in rural and mountainside sites, the use of steel reinforced concrete seismic bands is unviable for most households in Nepal and therefore the seismic bands are usually disregarded (Langenback 2015, p.2). For this reason, it was found that in the Gorkha earthquake, new masonry buildings performed worse than those built before the 1970s which had wooden seismic bands (Adhikary 2016, p.538).

2.5.1.4

Gabion bands

Gabion bands act similarly to seismic bands, but are formed through the use of a wire or polypropylene mesh (as highlighted in Figure 2.8) to constrain stones in a horizontal plane, making use of widely-available material. This mesh is lightweight and easy to transport to isolated communities and the overall cost of construction is very low (Langenback 2015, p.3). Research into this technique has been conducted by Langenback with several test constructions but there is little proof of concept.

2.5.1.5

Improved mortar

The proposed use of solid waste (foundry slag) mortar in masonry buildings as a replacement for the poorly-performing mud mortar was researched by Ahmad et. al. (2014). Promising results showed that a 20% replacement of cement with foundry slag can be safely adopted without endangering

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LITERATURE REVIEW

Figure 2.8: Gabion bands in use during construction (Langenbach 2015, p.7)

the safety of the structure. There is also suggestion of the use of mortar made from a mixture of plaster and mud or use of fine aggregates combined with cement and lime (Ohsumi, Mukai & Fujitani 2016, p.1242). However it is important to note that although improved mortar does increase the tensile strength of the masonry bonds, they are still very weak and additional tensile reinforcement is recommended.

2.5.2 2.5.2.1

Retrofitting techniques Mesh reinforcement

Mesh reinforcement is a simple and, depending on the variety, relatively low cost retrofitting technique that can be applied to URM structures to effectively hold the structural elements together, even if damaged or disintegrated (Blondet et al. 2011; Macabuag et al. 2012; Sathiparan et al. 2013). It is also relatively easy to construct. Common mesh types include PP, polymer and welded wire.

2.5.2.2

Stitching and grout/epoxy injection

Stitching and grout/epoxy injection is a relatively simply surface treatment technique than can be applied to older masonry that is showing signs of age such as cracking. Grout or epoxy can be injected into gaps that may have formed, with larger gaps stitched together with steel ties and mortar. The minimal cost and materials required along with its low technicality make this an accessible technique.

This may be a less effective technique in comparison to others as it will only restore the original stiffness of the wall (Bhattacharya et al. 2014, p. 58-65).

2.5.2.3

External reinforcement

Bamboo reinforcement As well as in new construction as described in section 4.1.2, bamboo reinforcement can be used as a retrofitting technique. A potential method described by Blondet et al. (2011, p. 12) and Bhattacharya et al. (2014, p. 59) involves fixing chicken wire mesh to the internal wall surface and vertical bamboo 12

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LITERATURE REVIEW

Figure 2.9: An example of retrofitted bamboo reinforcement arrangement (Blondet et al. 2011)

members to the exterior. These elements are then tied together via polypropylene ties inserted through the wall. An example of this arrangement can be seen in Figure 2.9. This system has been shown to significantly improve the behaviour of walls under significant seismic load, however can be inefficient in preventing cracking under low seismic loading (Bhattacharya et al. 2014, p. 59; Blondet et al.

2011, p. 12).

Tensioned car tire straps Used car tires with an inlaid steel mesh may also be recycled and utilised as a form of post tensioned reinforcement for URM. According to Bhattacharya et al. (2014, p. 60-65), this is a relatively cheap option, and from a sustainability standpoint, this is considered a sustainable technique as it is re-purposing what would otherwise be a waste material. The authors go on to state that in testing of the tire reinforcement, ‘combining horizontal and vertical reinforcements improved the lateral acceleration resistance of the masonry models by 100% and increased their ductility’.

2.6

Critical Assessment of Existing Techniques

The seismic improvement techniques mentioned in Section 4 have been summarised in Table A.2 (see Appendix A) as a tool to compare their effectiveness and applicability in the setting of low income rural Nepal. The variables identified by Sathiparan et. al (2013, p.2179) have been assessed for each technique and a score out of 3 allocated. The cumulative ‘score’ for each technique is then used to rank the techniques. Based on the Table A.2 ranking system, the most favourable retrofitting techniques were identified as bamboo reinforcing techniques, and gabion bands. From the literature review, it was observed that the level of research in the area of these techniques is either low or medium. This scarcity of existing research highlights the need for further investigation into these methods so as to benefit those at greatest risk in seismic events. This comparison was generated through analysis of a multitude of different research papers and sources meaning that it may be at risk of different author biases. It would be beneficial to have an overarching research project to objectively tests and assess the various techniques in a low-cost, development context.

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3 3.1

AIMS

AIMS Summary of Literature Review Findings

Nepal has a long history of significant earthquake damage and destruction with three major earthquakes in the last 100 years (including the 2015 earthquake), resulting in 18,500 deaths.

Strict steel and reinforced-concrete centred post-earthquake regulations have been put in place by the Nepalese government, but it is predicted that these regulations will go unfollowed. Furthermore, a governmental anti-deforestation policy has adversely affected traditional seismic reinforcement practices for masonry involving hardwood seismic band construction.

Failure mechanisms that occur in reinforced concrete frames are mostly due to improper construction and design issues rather than material properties, and as such there is limited scope for improvements to this building typology in Nepal from a purely engineering point of view – in other words, attempting to make technical improvements in this area would have minimal usefulness.

Attempts to improve Nepal’s ability to proactively reduce earthquake damage need to be low-cost, use readily-available resources and effectively engage with the prospective end users through assimilation of design with the current building vernacular.

As such, given the saturation of vernacular

unreinforced masonry (URM) construction, acute consideration of this construction type and its seismic performance is imperative to developing useable design solutions.

URM is recognised globally as being seismically vulnerable, with these structures making up a significant portion of buildings damaged during the 2015 Gorkha earthquake. Failures typically witnessed after the Gorkha earthquake included flexural cracking, out-of-plane failure, separation of masonry wall wythes and complete or partial wall collapse.

Seismic bands can improve seismic strength of URM buildings by increasing horizontal bending strength through resistance of out-of-plane loading and tying together perpendicular walls.

As

mentioned the use of hardwood seismic bands was common prior to the implementation of strict deforestation laws, with old URM buildings using these traditional bands performing better than new masonry buildings without bands in the 2015 earthquake. The use of reinforced concrete seismic bands is unviable for most households in Nepal given the limited availability of steel and concrete, resulting in seismic bands currently being largely disregarded.

This scenario has inspired research into the potential use of bamboo as reinforcement. Given its strength properties, fast rate of growth (i.e. not vulnerable to deforestation) and high prevalence in Nepal, bamboo presents a significant development opportunity. Through a ranking comparison

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system, some of the most favourable reinforcing techniques identified were those involving bamboo.

Bamboo also has the potential to assimilate with current building vernacular, which as mentioned is a key concern. There is a scarcity of research in the area of bamboo reinforcement however and this has led to the research aims identified in section 3 below.

3.2

Research Gaps

Nepal is at a high risk of seismic actions, and whilst it would be sensible to avoid the use of seismically vulnerable URM construction altogether this is not a realistic option, due to the saturation of this typology into the country’s building stock. In alignment with principles of a strength based approach and appropriate technology, two important concepts in humanitarian engineering, it was logical to focus research efforts on improving current vernacular building stock, rather than acting in opposition to current building culture. This is supported by de Maat (2014), who states that the focus should be on ‘user-building interaction’ that draws upon vernacular that is ‘effective, appropriate and humanly sustainable’. The author also reflects that ‘Western systematisation is not the only route to improvement’. The vast majority of research in seismic strengthening techniques involves high-tech and expensive solutions, unviable in a Nepalese context. Of the seismic strengthening techniques analysed in the literature review there are several which show promise, however further investigation of these methods is required.

The key knowledge gap found through the investigation of the literature was the lack of comprehensive investigation of the use of bamboo reinforcement in low cost masonry structures, including a: • lack of comprehensive testing and comparison between different low-cost seismic strengthening techniques • lack of comprehensive investigation of the use of bamboo as a seismic strengthening material • minimal implementation of finite element analysis (FEA) to model bamboo as a seismic strengthening material in masonry • minimal comparison within improvement methods as to optimum parameters, sizing and design

3.3

Aims & Objectives

Based on the literature review and an estimation of what could be achieved within the project time frame, the overall research objective was to investigate the appropriateness of bamboo members for seismic reinforcement of masonry construction typical to rural Nepal. In order to accomplish this, commercial finite element analysis software Abaqus FEA was used to model global structural 15

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AIMS

response to seismic action in terms of stress, strain and displacement, considering the change in performance as a result of the implementation of bamboo reinforcement. Bamboo material properties and bamboo-mortar interaction behaviour was determined through laboratory testing, followed by modelling of unreinforced and bamboo reinforced masonry structures. As research was undertaken, exploration of related construction and testing techniques was carried out as appropriate. These research objectives relate back to the original topic proposition; ‘Investigation of Bamboo Reinforced Seismic Bands Within Earthquake-Resistant Construction’ and are broken up into the following overarching aims, summarised in Figure 3.1; 1. Investigate bamboo material properties 1.1. Develop and refine laboratory testing methodology for tensile strength, compressive strength and density testing of bamboo 1.2. Determine bamboo material properties 2. Investigate material interactions and model with FEA 2.1. Develop laboratory testing methodology and perform pull-out tests to investigate bond behaviour between bamboo and cement mortar, deriving a local bond stress-slip relationship 2.2. Model bond behaviour with computer finite element analysis and validate a computer model of experimental pull-out tests 2.3. Calibrate bond behaviour for use in finite element model of bamboo reinforced masonry structure under seismic loading 3. Develop and analyse potential reinforcement solutions based on earthquake loading 3.1. Research into potential reinforcement options and opportunities for the use of bamboo reinforcement.

3.2. Model a simple masonry structure with different bamboo reinforcement situations 3.3. Refine and optimise quantity and arrangement of bamboo in the masonry structure 3.4. Optimise quantity and placement of bamboo reinforcement in the masonry structure based on analysis of results under cyclic loading.

By developing and optimising a computer model that uses laboratory-derived parameters for bamboo we can provide Engineers Without Borders, our research partner, with tangible, results-based, and potentially usable recommendations into best practice for earthquake-proofing houses in Nepal using bamboo, an area of research greatly under-represented in today’s academia. The combination of laboratory testing and computer modelling of the use of bamboo as a seismic band is a new approach to the development of design solutions in this context. This research is essentially a high-tech approach providing low-tech, simple recommendations to solve a wide-spread and immediate problem in Nepal.

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Figure 3.1: Summary of aims of research showing relationship with each other in the project

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4

METHODOLOGY

METHODOLOGY

To achieve the research aims outlined in Section 3, the overarching methodology in Figure 4.1 was developed. This flowchart presents a breakdown of the key tasks and their interrelation to the project aims.

The following two sections of the report, Section 5 and Section 6, contain the detailed methodology and relevant results for the whole project and encompass each Aim in its entirety. Section 7 then involves the key discussion of the report, with a final summary of findings in the conclusion.

Figure 4.1: Overview of Project Methodology

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LABORATORY TESTING PHASE (AIMS 1, 2)

LABORATORY TESTING PHASE (AIMS 1, 2)

This section of the report encompasses the detailed methodology and results of the laboratory testing phase, which makes up part of Aim 1 and Aim 2. This phase encompasses all aspects of the physical testing procedures performed to determine bamboo material properties and interactions, parameters which are necessary for the different FEA models used in Aims 2 and 3. Laboratory testing of the bamboo was necessary due to a lack of dependable data and information in existing literature relating to the mechanical properties of bamboo.

Key parameters necessary for FEA modelling were: • tensile and compressive strength of bamboo and mortar • Young’s Modulus of bamboo • density of bamboo and mortar • bond strength and bond-slip relationship between bamboo and mortar Procedures used during testing have been drawn from a review of relevant literature (as referenced throughout in relevant sections), existing testing precedents and consultation with industry specialists and adjusted based on the materials, apparatus and skills available.

Further discussion and

implications of the material property results is explored in detail in Section 7.

5.1 5.1.1

Preliminary Preparation of Bamboo Choice of bamboo

A bamboo species was chosen that is both common in Nepal and easy to obtain in Adelaide (therefore a feasible option in the context of laboratory testing, and appropriate for analysis of Nepalese construction). Bambusa vulgaris, or Golden Bamboo, was selected as an appropriate species to fit both of these criterion. Branches and leaves were removed and the bamboo left to dry for at least 12 weeks prior to testing. Whilst chemical treating for the purpose of waterproofing or protection against insect attack is commonplace in the use of bamboo for construction, this was not carried out (with exception of the density tests), as it is seen to have minimal impact on the mechanical properties of the bamboo. Each piece of bamboo was labelled for further comparison between pieces.

Bamboo is a highly versatile and dynamic material, ranging across many different species. However, the issue with this is that the literature shows there to be highly varied strength properties even within samples taken from the same culm, let alone across other species. Another issue with bamboo used is that the availability and viability of the species in Nepal may mean it is not applicable in different 19

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areas. Whilst this means there are some limitations with any results generated, it will not necessarily impact the usefulness of end deliverables. Having a key aim of the project be around development of a computer model means that ideally, different strength properties can be inputted as necessary if further testing is done, with minimal or even negligible changes to the model itself. Development of robust and generic laboratory testing techniques will also allow for simple replication of tests with other bamboo species to determine material properties which can easily be updated into the original computer model generated, with minimal changes made to the model itself. Once the computer model has been developed and validated using testing results, there is also scope for further research to carry out further testing and development of the model to better optimise and refine the system.

It should be noted that bamboo collected and utilised for testing has been limited to less than 50mm in diameter as larger specimens would be less desirable as embedded reinforcement within URM construction typical to Nepal.

5.1.2

Bamboo macro dimensions

Referential records of bamboo dimensions were made for all specimens. This was done so that appropriate computer models of the bamboo could be created. The following measurements were recorded, with key regions labelled in Figure 5.1: • Diameter of internode sections, dint (taken at the middle of three internodes using vernier calipers, and repeated at three different orientations to get a representative average diameter) • Diameter of node sections, dn (repeated for all nodes on the bamboo piece using vernier calipers, and repeated at three different orientations) • Thickness of bamboo, t (taken from top section of bamboo and repeated at three different orientations) • Distance between nodes, Lint • Length of nodal section, Ln (measured from where internodal bamboo diameter starts to increase)

5.2 5.2.1

Bamboo Density Testing Bamboo density testing procedure

The bamboo density for each respective culm of bamboo used for other tests was measured on dry samples using a simple water volume displacement procedure, as described in the Annual Book of ASTM Standards,volume 4.10 (ASTM International 2011). Firstly, elongated longitudinal sections 20

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(a) culms

(b) cross-section

Figure 5.1: Bamboo with key sections labelled

of bamboo were cut such that a proper representation of the bamboo material was taken (i.e. the effect of localised differences in mass are reduced). Taking longitudinal sections reduces the relative total area of exposed cut bamboo, which limits potential water intake during the measuring process.

The lengths of the bamboo were chosen such that there was a large enough mass so that it could be determined to a precision of ± 0.2% or less. Having a larger bamboo sample also increased the relative precision of volume measurements, as this resulted in a significant change in the water level of the measuring container. For the purposes of this investigation, lengths that are 2.5 times the diameter of the bamboo cross-section were used, as this creates an appropriate bamboo sample for the above criteria. Once samples were cut, all loose fibres were removed and the cut surfaces lightly sanded.

A key issue with measuring the volume of a timber sample is the possibility of water absorption, which would result in erroneous volume changes. To counter this, Huntsman Cearawax (a combination of Kerosene 30 and triethanolamine) was applied to the samples prior to testing, providing a water resistant coating.

The mass of the sample was measured post-wax application using an electronic scale. The samples were then immersed in a long cylindrical graduated container of water of similar cross-sectional dimension to the bamboo samples, and the volume change recorded. This change represented the volume of each sample. The density was then calculated as follows:

density (

mass (kg) kg )= m3 volume change (m3 )

21

(5.1)

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LABORATORY TESTING PHASE (AIMS 1, 2)

Density tests results

Table 5.1: Summary of density of bamboo samples, found through water displacement

Tests were conducted on nine samples to determine density of the bamboo being used. The results are summarised in Table 5.1, with an average density of 859 kg/m3 . The sample lettering (e.g. A1, A2, A3) indicates samples taken from the same bamboo culm.

Analysing the data from the density tests suggests that the results obtained are reasonably accurate.

The average density has a low coefficient of variation and correlates to values found in the literature.

There were however limitations with the methodology that inhibit the preciseness of results. Further discussion and analysis of these limitations and the data can be found in Section 7.1.1.

5.3 5.3.1

Bamboo Tensile Strength Testing Bamboo coupon preparation

Coupons were prepared to determine the tensile strength of the bamboo, one of the key properties in subsequent computer analysis. Using a water jet cutter, ‘dogbones’ were cut out of the bamboo, with dimensions shown in Figure 5.2. The sections are all cut from internodal sections, in a tangential direction. Derivation of these two different shapes is discussed below and in Section 5.3.

Deriving strength properties from bamboo coupons was not straightforward.

Bamboo is a

non-homogeneous, graded material, consisting mainly of lengthwise fibres which provide the tensile strength properties, and an organic polymer called ligdin which contributes to its rigidity. Its fibres increase in number the closer to the outside culm wall (see Figure 5.3a), which affects the uniformity of the tensile strength and results in non-linear stress and strain profiles across the cross section (Richard & Harries 2014, p. 118). Richard and Harries established that coupons should be taken in a tangential direction to the bamboo wall as it better reflects overall strength properties (Richard & Harries 2014,

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(a) first dog-bone

(b) second dog-bone Figure 5.2: The dimensions of the two different bamboo ’dog-bone’ coupons from which results were derived

(a) cross-section of bamboo showing increase in fibres

(b) tangential and radial directions

towards the outside culm wall

of bamboo

Figure 5.3: Bamboo cross-sections

p. 118). Coupons were also cut in the tangential direction due to the radial thickness being small for the collected bamboo. Figure 5.3b) and Figure 5.4 highlight the difference in radial and tangential directions when preparing coupons.

It is also well-documented that bamboo tensile strength increases the higher up the culm from its base, (Richard & Harries 2014; Oka et al. 2014) due to the increased ratio of area of bamboo fibres to total cross-sectional area - the fibres are thought to be continuous throughout the culm, and as there is a reduced cross-sectional area towards the top of the culm, there is a higher proportion of fibres. This is factored into data processing by taking coupons from different heights along a bamboo culm and averaging this data for the computer modelling. Further discussion about bamboo as a

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(a) difference in radial and tangential

(b) tangential coupon

(c) radial coupon

directions Figure 5.4: Different coupon orientations (Richard & Harries 2014, p. 101)

functionally-graded material can be found in the second section of the methodology (see Section 7.2.2.1) There are also varied testing procedures and sizes for bamboo coupons. Whilst there is an international standard on bamboo testing, there are issues with the dictated methodology (Richard & Harries 2014), as well as practical constraints regarding apparatus and material on hand during laboratory work. For example, grips for tangential testing are meant to be orientated such that they attach to the sides of the bamboo coupon (see Figure 5.4), however this is not possible given the small thickness of the bamboo being used. Other requirements include gauge length being 50-100 mm long, and the ends shaped to ensure failure occurs in this section (i.e. the dogbone) (ISO 22157-2004b, in Richard & Harries 2014).

Various coupon dimensions from different journals are shown in Appendix B (Figures B.1, B.2 and B.3) to demonstrate the range of sizes that have been used, and the final design of the coupons (Figure 5.2) based on a synthesis of this information, as well as material and apparatus constraints.

The small thickness and diameter of the bamboo used for this research was problematic in that it was too difficult to cut flat cross-sections. It also limited the width of the dogbones, and therefore the width of the gauge section. Richard and Harries proposed a gauge width to depth ratio of 4:1 (Richard & Harries 2014, p. 106), however this was not possible for the coupons prepared for this project due to the curvature of the bamboo and hence a compromised ratio of 2:1 was used instead.

This was a similar ratio to other coupons in the literature, such as those in Figures B.1, B.2, B.3 and hence was deemed acceptable. The width of the coupon ends was set at 16 mm to minimise the

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Figure 5.5: Apparatus setup for coupon preparation

tangential curvature of the coupon whilst still allowing for significant purchase of the aluminium tabs to the bamboo by use of the epoxy.

The dogbone sections were cut only out of internodal regions. In terms of tensile strength, bamboo is weaker at the node (Richard & Harries 2014, p. 112; Shao et al. 2010), and as such exploration of nodal properties would be required to create an accurate bamboo model. However due to time and project scope constraints, it was not feasible to undertake in-depth exploration of nodal material properties. For these reasons, these properties have been excluded from the FEA model and are an area of possible future research.

The cross-sectional area of each coupon’s gauge section was formulated for later strength calculations.

This was done by taking measurements of the internal coupon section using vernier calipers, and then constructing the cross-section in Autocad based on these measurements to find the area.

Aluminium tabs were then attached to each side of the ends of the dogbones using MasterBrace 4000 epoxy to create a level surface for the jaws of the testing apparatus to attach to. This was done as the bamboo coupons were curved and it helped to reduce potential crushing of the bamboo by the jaws. The tabs were cut such that they were larger than the ends of the dogbone to ensure maximum surface contact between the dogbone, epoxy and the tabs. The end sections of the bamboo and the surfaces of the tabs were then roughened up using a file and sandpaper to provide maximum surface contact and grip between the tabs, epoxy, bamboo and apparatus jaws.

In order to bond the tabs to the bamboo using the epoxy, a clamping arrangement with two metal plates was utilised to ensure the tabs were parallel. This ensured that there was a uniform flat surface for the apparatus jaws to grab during testing. Each coupon end was effectively sandwiched between two tabs that had epoxy applied to them and clamped until the epoxy set overnight. Figures 5.5 and 5.6 demonstrates the apparatus set up. A further seven day drying period was allowed for the epoxy to reach its maximum strength.

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(a) diagram of cross-section of apparatus for coupon

(b) side profile of coupons once clamps

preparation

have been removed

Figure 5.6: Comparison between design and actual coupon side profile

Figure 5.7: Extensometer attached to a coupon during tensile testing

5.3.2

Bamboo coupon testing procedure

After the epoxy was adequately set, the coupons were tested in a 50 kN Mohr & Federhaff tensile/compressive testing machine, to determine the tensile strength of the bamboo. Each coupon was placed such that there was maximum contact between the machine jaws and the aluminium coupon tabs at each end. An MTS 50mm Axial Extensometer was affixed to the coupon to record axial displacement data through the gauge region of the dogbone and thus strain data. Tensile force was then applied to the coupon at a displacement rate of approximately 1 millimetre/minute until failure.

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(a) wider coupon failure

(b) narrow coupon failure

Figure 5.8: Sample tensile coupons post-testing

5.3.3

Coupon results data processing

Once the failure forces were known, the ultimate tensile strength of each specimen was determined by dividing the force at which failure occurred by the cross-sectional area of the coupon (determined in 5.3.1):

ultimate tensile strength (M P a) =

F orce (N ) Area (mm2 )

(5.2)

Average strength was then determined to give a general tensile strength representative of the bamboo.

Young’s Modulus of elasticity, E, was identified as a key parameter required for modelling, and can be determined using equation 5.3;

Young’s Modulus, E (MPa) =

Stress, σ P/A PL = = Strain, ε ∆/L ∆A

(5.3)

where P represents load, L represents the initial length of the coupon, ∆ represents the change in length of the coupon (measured with the extensometer) and A represents the cross-sectional area.

5.3.4

Tensile tests results

Tensile tests were conducted on 12 coupons, based on the two different coupon designs highlighted in Figure 5.2. Example failures of the two different types of coupons are displayed in Figure 5.8a and Figure 5.8b. The tensile strength of the bamboo was found to be 133.9 MPa and Young’s Modulus of Elasticity was 14701 MPa. Final results for the bamboo tensile tests are presented in Table 5.2 and example stress-strain profiles provided in Figures 5.9 and 5.10.

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Table 5.2: Summary of tensile strength test results of bamboo samples

Figure 5.9: Stress-strain graph of 2nd coupon iteration

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Figure 5.10: Stress-strain graph of 3rd coupon iteration

Tensile testing produced appropriate results for computer modelling that were in agreement with expected strength parameters for the bamboo being tested, however the lower number of samples tested and variability in failure mechanisms raises questions about the validity of these results. Testing was conducted on two different coupon sizes due to difficulties forcing failure in the larger coupon sizes.

Further commentary and analysis of results including discussion of preliminary testing is summarised in Section 7.1.2.

5.4 5.4.1

Bamboo Compressive Strength Testing Compression tests sample preparation

Unlike the tensile samples, compression tests were undertaken on whole bamboo cross sections. This was done as there were less restrictions due to testing apparatus compared to the tensile tests, and whole sections were more appropriate for testing of compressive strength due to issues such as buckling.

Similar to the density tests, the methodology is largely founded on techniques described in the Annual Book of ASTM Standards, volume 4.10 (ASTM International 2011), where it is suggested that elongated longitudinal sections are prepared with a length a minimum 2.5 times the diameter of the sample. This length is a suggestion for solid timber specimens, and as bamboo is effectively a hollow tube, there are concerns with buckling at greater sample lengths. As such, longitudinal lengths were set at the minimum 2.5 times the diameter. This resulted in variable compression samples dependent on the culm from which they were taken, as demonstrated in Figure 5.11a.

To ensure uniform compression was applied to the sample, and to reduce the chance of buckling, efforts were made to have parallel contact surfaces between compression plates. One obstacle to this is the non-linear localised shape of bamboo specimens. The culms in Figure 5.1a highlight that whilst the

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(a) samples showing variable

(b) use of L-beam and steel

(c) sample under

longitudinal lengths

square for cutting localised right

compression loading

angles Figure 5.11: Bamboo samples used in compression tests

global shape of bamboo appears linear, on a local level there are often differences between different internodal sections in terms of curvature and direction of growth. As such, each cut along the bamboo section needed to be taken tangential to the local bamboo piece, rather than tangential to the overall bamboo shape. To this end, the bamboo was measured up for cutting inside a brace using a set square (see Figure 5.11b for details). Once the specimens were cut in this manner, further minor adjustments were made using a belt sander as necessary.

Similarly to the tensile coupons, cross-sectional areas of all samples were then calculated by taking measurements using vernier calipers, and then calculating area digitally using Autocad.

5.4.2

Bamboo compression testing procedure

Compression tests were carried out using the 50 kN Mohr & Federhaff tensile/compressive testing machine. An example is shown in Figure 5.11. Each sample was then loaded at a displacement control rate of 1 mm/minute until compressive failure occurred, and the load at failure recorded.

5.4.3

Compression testing data processing

Compressive strength was then calculated using 5.4 as with tensile strength:

ultimate compressive strength (M P a) =

5.4.4

F orce (N ) Area (mm2 )

(5.4)

Compression tests results

Compression tests were conducted on nine samples from four different bamboo culms, and strength properties calculated. The results are summarised in Table 5.3, demonstrating an average compressive

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strength of 83.9 MPa, with a coefficient of variation of 9.6%. Internodal samples were taken such that similar results were produced to tensile testing conducted (see 5.3.1 for further discussion and reasoning of why this happened). As with the density tests, the lettering designates samples taken from the same culm.

Table 5.3: Summary of compressive strength of bamboo samples

The compressive tests conducted were relatively successful in context with other tests performed during this research, given the relative uniformity of both failure mechanisms and compressive strength generated. More in-depth discussion of the results and analysis of any limitation in testing procedures can be found in Section 7.1.3.

The cross-sectional areas of the bamboo calculated for compression tests were also useful for the purposes of constructing FEM bamboo elements in Abaqus. To this end, both the actual area of the bamboo (i.e. the hollow cylinder section) and the area of the hollow section itself were needed (this was necessary in defining material properties for an idealised geometry of bamboo, as explained in Section 6.3.3). The average readings for five different culms, as well as the overall average across these culms is summarised in Table 5.4.

Table 5.4: Average cross-sectional areas of different culms of bamboo

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Bamboo-Mortar Pull-Out Testing

To analyse bamboo-mortar interaction behaviour and determine key bond parameters for inputting to FEA, pull-out tests were carried out. This was done to develop a local bond stress-slip relationship such that elastic bond stiffness could be determined, and also to determine an average overall bond strength. These tests were conducted on bamboo culms embedded in cylinders of cement mortar.

Upon curing of the mortar, the samples were axially loaded in tension so as to disembed the bamboo from the mortar.

5.5.1

Pull-out test bamboo preparation

To prepare the bamboo, specimens were cut into 450mm segments and all dimensions measured including cross sectional area and number of nodes. A 450mm segment size was chosen such that 200mm was embedded into the mortar, and 250mm sat above the top. This embedment length was selected based on laboratory constraints and existing embedment lengths in literature from Mahzuz et al. (2013), Jung (2006), Iyer (2002) and Agarwal, Nanda and Maity (2014). This length was deemed sufficient to fully develop bond strength between the bamboo and mortar. The 250mm of bamboo protruding above the mortar allowed for an adequate length of reinforcing bar to be inserted into the bamboo for testing purposes, as is explained below.

Six segments were cut such that two sets of testing were undertaken, as follows; • Three pull-out tests with no nodes embedded in the mortar • Three pull-out tests with one node embedded in the mortar These two test variations were undertaken so the effect on bond strength from nodal interlock could be explored and implemented into the FEA model.

In order to grip the bamboo during the pull-out tests, a steel reinforcing bar was inserted into the top opening of the bamboo and a sufficient length epoxied in place with MasterBrace. The length of bar protruding from the bamboo is set at approximately 150mm, such that there was sufficient space for the jaws of the testing apparatus to grip onto, and 150mm was epoxied inside the bamboo. Detail of this setup is displayed in Figure 5.12.

5.5.2

Pull-out test mortar preparation

The mortar used for the pull-out test was a cement mortar-mix of Adelaide Brighton Cement High Early Strength Cement, coarse sand and water, and was prepared immediately before pouring into

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(a) adding epoxy to bamboo

(b) detail of reinforcement bars post-setting

Figure 5.12: Detail of reinforcement bars being added

Figure 5.13: Effects of creep and shrinkage due to water absorption in bamboo (Ghavami 2004, p. 642)

moulds to avoid premature curing. A High Early Strength cement was used to ensure that testing could be performed relatively early with confidence that the mix had sufficient strength.

A drier mortar mix was used for this experiment - 11kg water : 14kg cement : 55kg sand - based on a 1:4 cement:sand ratio. This was such that the mortar was of high strength, but would not excessively wet the bamboo. It was important to minimise water absorption in the bamboo and hence reduce potential expansion and shrinkage as the mortar cured. If expansion and shrinkage of the bamboo occurs, full bond strength between the bamboo and mortar may not develop or cracking of the mortar could occur, both scenarios potentially affecting test results. Ghavami (2004) describes these potential effects of water absorption graphically as below (Figure 5.13; The permeability of bamboo in respect to water absorption when used in combination with mortar 33

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is a matter of some significance for the reasons highlighted above. In construction practice, there are a number of techniques available to minimise water absorption. This involves coating the bamboo with a water repellent or other impermeable coating such as epoxy or tar based paints (Ghavami, 2004). These coatings however may have an effect on the bond between bamboo and mortar and exploration of such effects is outside the scope of this project. As such, waterproofing and reduction of water absorption was not considered here further than reducing water content of the mortar. In some contexts it is also reasonable to assume that waterproofing would not be undertaken in practice.

5.5.3

Pull-out test mould preparation

200 mm lengths of 100 mm diameter PVC pipe were cut to serve as a mould/formwork for the mortar cylinders for the pull-out test. To house the moulds, a purpose-built wooden casket was constructed to constrain the moulds during initial curing, and act as formwork on the bottom side/base of the moulds. The casket was an open box, sized to fit the moulds snugly. At either short end of the box, taller pieces of timber acted as supports for a large piece of plywood to sit over the top. Holes were cut into this plywood such that the bamboo segments embedded into the mortar would fit through and be held vertically whilst mortar cured, ensuring a firm connection between mortar and bamboo.

This top plywood component was installed to the casket prior to pouring and also acted as a frame to ensure that the bamboo was placed centrally in the moulds. Figure 5.14 contains details of this arrangement.

The PVC pipe formwork/moulds were then placed into the wooden casket. To ensure that the formwork fit snugly in the casket, additional packers were used to minimise the risk of small movements.

Bamboo was placed in the centre of the moulds and the top plywood sheet placed over to ensure that the bamboo is central in the moulds. Mortar was then mixed and evenly poured into the moulds to 200mm height around the bamboo. A spirit level and a right angle was used to set the bamboo vertically such that the bamboo could be pulled straight out in pure shear without introducing forces in a horizontal plane, giving an accurate measure of shear bond strength. The moulds were then left to cure for 28 days to reach characteristic strength. The PVC formwork was not removed post-curing to mimic real-world confinement of mortar, and reduce potential splitting of the mortar during testing.

The curing setup is demonstrated in Figure 5.14.

5.5.4

Pull-out testing procedure

Once the characteristic strength of the mortar had been reached, each bamboo-mortar sample was then monotonically loaded axially in tension to dis-embed the bamboo from the mortar. This loading was undertaken using the Instron machine, where loading and displacement was measured and graphed.

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Figure 5.14: Apparatus setup for pull-out test curing. The three concrete cylinders on the front are used for determining compressive strength of the concrete

In order to properly grip the bamboo and apply an upwards force without crushing the sample, a steel bar was epoxied into the tops of the bamboo to an appropriate depth such that there was adequate bond strength between the bamboo and bar. A cage was fitted around the bamboo in order to apply a downwards restraint during loading to the top of the sample, and displacement at both ends of the embedded bamboo was measured using LVDTs. For the top displacement, a plate was fixed to the bamboo and average displacement measured using LVDTs at each end. Details of the testing method can be seen in Figure 5.15.

Lastly, compressive and tensile strength of the mortar was determined for input in the FEA model by performing standard compression tests and Brazilian tensile tests on samples of the same mortar.

Strain readings were taken for the compressive tests and stress-strain curves plotted as shown in Figure 5.16.

Table 5.5: Summary of cement mortar strength properties used for pull-out tests

5.5.5

Bamboo pull-out test data processing

In a standard pull-out test arrangement, the distribution of shear stress along the bamboo length is non-uniform. As such, to determine the critical bond stress and hence shear bond strength, an analysis technique outlined by Vaculik et al. (2018) and Haskett, Oehlers and Ali (2008) was utilised (Figure 5.17).

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Figure 5.15: Apparatus used to perform pull-out tests on bamboo

In testing, when a tensile load, P, is applied to the embedded material, a slip, ∆, occurs at the crack face and at the free end in what is known as the global P-∆ response. Data on these parameters was collected in laboratory testing (loads and deflections) and then utilised to determine material characteristics. This was done through analysis of the local bond stress versus slip response, or, local τ -δ response, that is drawn from the global P-∆ response. τ represents the shear stress at the interface of the two materials in a pull out situation that resists the load, P, and δ is the slip at the interface. Figure 5.17 highlights these parameters. When performing FEA modelling, it is this local bond behaviour that is used in interaction modelling. As such performing accurate analysis of this data in order to determine realistic stress-slip behaviour is particularly critical to the modelling components of the research objectives.

To create a simplified a model of the local bond behaviour, Haskett, Oehlers and Ali (2008) describe two model variations; a linear descending model, or a bi-linear model, as shown in Figure 5.18.

For analysis in this case the bi-linear model was deemed to be a more accurate representation of the expected failure mechanism during testing, incorporating elastic, softening and final debonding regions.

Vaculik et al. (2018) discuss adjusting the bi-linear rule to include a residual frictional component through the use of τr , a residual stress value, as highlighted in Figure 5.19. The method for extracting τr and other corresponding parameters requires the use of iterative inverse analysis techniques to produce accurate values representative of true local bond behaviour. Whilst this model suggested

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(a) Cylinder 1 stress-strain plot

(b) Cylinder 2 stress-strain plot Figure 5.16: Compressive test stress-strain graphs

by Vaculik et al. is a more precise representation of bonding behaviour in general, its development requires significant computational and manual effort and as such was deemed out of scope for this project. This presents an opportunity for future research to refine the bi-linear model developed for this project.

Haskett, Oehlers and Ali detail the method for analysing global data as follows. Firstly, the load to cause inter-crack debonding, PIC and corresponding displacement, δmax or δ2 were found graphically from the global load-slip response graph and adjusted for the stretch of the bamboo between the true loaded end, and the point at which loaded end displacement was measured (the LVDT was 100mm above the loaded end), as highlighted in Figure 5.20. Lper (the perimeter of the reinforcing bar, in this case the circumference of the bamboo), E (found through the bamboo tensile tests) and Ap (cross-sectional area of the specimen) are all known, and as such it was simply a matter of rearranging Equation 5.5 (from Haskett, Oehlers & Ali 2008) to find the corresponding shear bond strength, τmax .

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Figure 5.17: Typical pull-out test arrangement describing analysis concept (Haskett, Oehlers & Ali 2008)

Figure 5.18: Linear and bi-linear local τ -δ relationships (Haskett, Oehlers & Ali 2008)

Figure 5.19: Local τ -δ rule including residual friction, representing bond behaviour along bond interface (Vaculik et al. 2017)

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Figure 5.20: Example of global load-slip response showing graphical location of PIC and δmax (Haskett, Oehlers & Mohamed Ali 2006)

PIC =

p p p p 2Gf Lper EAp = τmax δmax Lper EAp

(5.5)

In Equation 5.5, Gf represents the interfacial fracture energy and is essentially the area under the local stress-slip graph, as shown in Figure 5.18.

Haskett, Oehlers and Ali (2008) state this method can only be utilised with sufficient embedment length that is greater than Lcrit , the equation for which is given below in Equation 5.6. This is necessary so that sufficient bond strength develops.

π Lcrit = q τmax Lper 2 δmax EAp

(5.6)

During data analysis, it was found that the bamboo embedment lengths were below the Lcrit values calculated using Equation 5.6 and as such the method detailed above was not applicable. Haskett, Oehlers and Ali suggest that taking an average bond stress, τav , over the embedment length is an appropriate approximation of τmax for bond lengths less than Lcrit , especially for very short bond lengths. As such, τmax is simply as follows:

τave = τmax =

PIC (N ) embedment length (mm) × Lper (mm)

(5.7)

Finally, the value for δ1 in the bi-linear model was found through the experimental global slip value at the free end for when maximum load P occurs (as detailed by Haskett, Oehlers and Ali).

Results of the testing and follow up calculations were then analysed to identify any trends or relationships of significance, such as a relationship between number of embedded nodes and critical 39

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LABORATORY TESTING PHASE (AIMS 1, 2)

bond stress. Once an average critical bond stress/critical debonding load has been identified, it can be used as a part of finite element analysis to assess the applicability of bamboo to reinforce masonry.

5.5.6

Pull-out tests results

As described in the methodology, during testing, measurements of load and displacement at either end of the bamboo-mortar block were taken and load-displacement graphs plotted for each case. Behaviour of these graphs was analysed and a max load, P, identified at which debonding occurs, as can be seen in Figure 5.21. Corresponding displacements δ1 and δ2 were also identified from the graphs for use in determining the local bond stress-slip relationship between the bamboo and mortar.

(a) δ1 obtained from Load vs. Free end displacement graph

(b) δ2 obtained from Load vs. Loaded end displacement graph Figure 5.21: Method of obtaining parameters from test data, using data from pull out sample 3

Figure 5.22 shows the load-displacement graph for a test using bamboo with no node embedded.

Whilst this graph exhibits similar behaviour to the noded tests in the elastic region of the graph prior

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to peak load, the zig-zag behaviour beyond this point is noticeably different. It is understood that this difference in behaviour is due to the mode by which the laboratory apparatus operates. The pull rate was set based upon a distance per time setting rather than increasing load per time setting, meaning that without the effects of nodal interlock, the apparatus was forced to rapidly decrease and increase the pull rate as the bamboo intermittently locked and slipped, resulting in the jerking behaviour visible in Figure 5.22. Further discussion of these results is located in Section 7.1.4.

Figure 5.22: Non-noded load vs. displacement behaviour of sample 1

Based on the techniques and equations outlined in the methodology, the following parameters in Table 5.6 defining the local bond stress-slip relationship were determined for all pull out samples. These parameters were then to be utilised in FEA modelling.

Table 5.6: Summary of local bond stress-slip properties for no node and node sections

These bond stress-slip parameters were plotted as shown in Figures 5.23 and 5.24 where average curves were calculated for node and no node cases. Discussion of these results is provided in Section 7.1.4.

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Figure 5.23: Graph showing average local stress-slip relationship for bamboo section containing a node, overlaid on individual results

Figure 5.24: Graph showing average local stress-slip relationship for bamboo section containing no node, overlaid on individual results

The nodal and non-nodal local bond properties were then used as inputs and verification for pull-out test FEA modelling, as described in Section 6.2.

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5.6

LABORATORY TESTING PHASE (AIMS 1, 2)

Summary of Laboratory Testing Results

The bamboo properties found from the laboratory research and detailed in the above text are reproduced in Table 5.7 for ease of reading and use. It is these values which were then used in computer modelling, a process detailed in the following pages (Section 6).

Table 5.7: Summary of properties of bamboo and mortar as found during this research

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FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

In order to develop design strategies to improve the seismic stability of URM buildings in rural Nepal, it is necessary to demonstrate in some way the performance of these suggestions and how they are going to be of advantage. This would be most convincingly achieved by building a full scale prototype in rural Nepal and study the influence of reinforcement under earthquake loading. However this is both a costly and difficult exercise and earthquake loading cannot be induced. A realistic alternative is to simulate earthquake loading through shake table testing of a scale model. However this is a significant time and cost burden and realistically only allows for the testing of one or two different arrangements.

As this project aims to investigate the potential for seismic strengthening using bamboo, which is still a new area in the masonry structures field, a more flexible mode of testing multiple configurations and arrangements is a better option. Finite element analysis (FEA) has many advantages such as low cost, relatively quick processing time and ability to easily iterate and refine the models to generate optimal results.

The first part of the computer modelling phase involved the generation of a finite element model of the pull out tests which were tested in the laboratory testing phase of the project. Geometry, loading and boundary conditions were implemented to best replicate the conditions in the laboratory and the results analysed to compare with experimental results and validate the finite element model. Special attention was paid to modelling the bamboo nodes and the effect they have in locking the bamboo culms in the mortar. Following from this, the mechanics of the interaction between bamboo and mortar along with the experimentally determined material properties of bamboo were input in a series of models of simple URM structure to investigate the potential benefits of bamboo reinforcement on the global behaviour of the reinforced masonry building under cyclic loading.

6.1

Finite Element Analysis Background

6.1.1

Modelling Steps

The following is an overarching method for the finite element analysis applied to each of the modelling phases.

Pre-processing: • Part: Geometry of the test specimens was modelled as closely as possible in the Abaqus software package.

• Property: Appropriate material properties and cross-section geometry was assigned. masonry, bamboo and mortar are each assigned a set of material properties.

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• Assembly: Separate geometries are assembled to create the global model.

• Step: Analysis steps and required output are defined, as well as changing in loading or boundary conditions over the step. Static general step is selected for both models, for the pull out model a displacement control is applied and for the masonry model, cyclic loading is applied.

• Interaction: the mechanical interactions between surfaces is defined. Unless an interaction is specified, different parts will act independently from each other, even if their geometry intersects.

In the pull out test model, the interface defined between the bamboo and mortar block is essential in calibrating the model with experimental results, which then feeds into the masonry model.

• Load: Loading and boundary conditions as well as the steps in which they should be active are defined. In the interaction model displacement control is applied and in the masonry model load control is applied.

• Mesh: An appropriate mesh size for each part of the model is applied to achieve convergence in results while maintaining computational efficiency.

• Analysis type is set, for both models linear analysis is used.

Simulation: • Job: The job is submitted for processing. Abaqus run solves the numerical problem defined by geometry, loading and constraints solving for every node in the mesh at the specified increments of the time step.

Post-processing: • Visualisation: The results are visualised and interpreted through analysis of major/minor principal stress and deflection of the model through the time step.

6.1.2

Concrete damage plasticity model background

The concrete damage plasticity model is employed to predict the smeared crack mechanism of quasi-brittle materials and is used in both the pull-out and masonry structure finite element models to model behaviour of mortar and masonry/mortar composite respectively. It uses concepts of isotropic damage elasticity in combination with isotropic tensile and compressive plasticity to represent the inelastic behaviour of concrete and is used to describe the non-reversible damage that occurs in the fracturing process. (Singh et al. 2017) The model assumes that the two main failure mechanisms are tensile cracking and compressive crushing of the material. The model was first proposed by Lubliner et al. (1989) for use in modelling of concrete, but has since been used for modelling of masonry, which also exhibits quasi-brittle behaviour.

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(a) tension

(b) compression

Figure 6.1: Response to uniaxial loading (Abaqus User Manual)

As can be seen in Figure 6.1a, under uniaxial tension, the stress versus strain relationship is linear elastic until the yield stress is reached. Once this stress has been exceeded, micro cracking in the masonry will occur. With increasing load beyond the yield stress σ 0 , the micro cracks form, which is represented with a strain-softening response. Whereas under uniaxial compression, as shown in Figure 6.1b, the response is linear until the value of initial yield. In the plastic region, the response is typically characterised by strain hardening followed by strain softening beyond the ultimate stress.

Under cyclic loading, degradation mechanisms must be included to account for micro cracks which form under initial damage. This is discussed further in Section 6.3.2.1.

6.2

Model of Bamboo Pull-Out Tests

The geometry, boundary conditions, loading conditions, and material properties of the bamboo culms and mortar blocks (as found through testing in Section 5.5.4) can be reconstructed in Abaqus and tested to assess the software’s render of such scenarios.

Firstly, an idealised model for a bamboo pull-out test with no node was adapted with the averaged bond stress and slip parameters found during the experimental testing phase. The deflections at the free and loaded ends of the bamboo culm were then compared with the experimental data to validate these interaction parameters. Next, an idealised model of the bamboo pull-out tests with a single node was created and a these verified interaction parameters were inputted. Deflection and pull force of this model were compared with the un-noded model in order to quantify the effect of the node.

Again, the deflections at the free and loaded ends of the bamboo culm were then compared with the corresponding experimental data to validate this noded model and, by extension, the quantified effect of a node.

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6.2.1

FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

Geometry of bamboo pull-out test models

The geometry of each anisotropic culm is complex and irregular. Each culm exhibits slight curvature both globally and irregularly between nodes. Cross sections are not perfectly circular or elliptical and both nodal and inter-nodal sections can be highly variable. Culm geometry will, therefore, be interpolated from measurements taken in phase 1 (see Section 5.1.2). An idealised hollow circle section with 24mm diameter and 2.6mm wall thickness is assumed for the cylindrical regions of the culm. For the noded bamboo culm, the idealised node is at 110mm from the free end of the culm and has a diameter of 29mm. These values are averages of the respective physical culms tested. Geometry diagrams can be seen in Figure B.4 in Appendix B.

6.2.2

Meshing of bamboo pull-out test models

The finite element mesh was generated as approximately 6mm3 prismatic elements for the mortar block and approximately 6mm3 prismatic elements for the bamboo culm. All elements were solid type C3D8R, un-noded pull-out model having 10686 elements noded pull-out model having 12750 elements.

6.2.3

Material properties of bamboo and mortar

Material properties determined from laboratory testing are given in Table 5.7. The concrete damage plasticity model is used to define mortar. This is to simulate the micro cracking and crushing of the mortar block in response to the normal force imposed by bamboo nodes. The compressive and tensile failure stresses are those found in during the tensile and compressive testing of the mortar used in the lab pull tests. These inputs are given below.

6.2.3.1

Bamboo material properties

Density is 859 kg/m3 as found by experimental bamboo density tests described in Section 5.2. Young’s modulus of elasticity is 14700 M P a as found by experimental bamboo tensile tests described in 5.3.

6.2.3.2

Mortar material properties

It was necessary to input mortar material properties due to the reactionary forces on the culm applied by the mortar in the noded pull-out tests.

The elastic portion of the mortar’s compression and tension curves is defined by a Young’s modulus of elasticity of approximately 16000 Mpa as found by experimental compression testing of mortar in Section 5.5.4.

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Figure 6.2: Diagram showing normal and frictional forces acting on bamboo during pull-out tests Table 6.1: CDP data used in Pull Test from Bolhassani et al. (2015)

Concrete damage plasticity, as introduced in Section 6.1.2, was used for modelling the plastic regions of the mortar’s compression and tension curves. The forces resisting movement of the bamboo under load are not only parallel to the bamboo surface, in the form of bond and shear friction forces, but also forces normal to the specimen at nodes where the bamboo’s cross-sectional area increases (see Figure 6.2). For this reason CDP was necessary to model the mortar’s behaviour, and by extension reaction forces against the bamboo node.

To obtain the compressive and tensile behavior data necessary for CDP input, extensive compression and tension testing, and by extension verification of data, of mortar needs to be carried out. As this is out of the scope of this project, sample CDP data values have been adopted from Bolhassani et al. (2015), as depicted in Table 6.1. The CDP model can only be used if compressive and tensile behaviour of the quasi-brittle material fits the uniaxial loading behaviour curves seen in Figure 6.1.

It has been assumed that the mortar fits this criteria because of its similar make up to standard concrete. Therefore, compressive and tensile behaviour curves have been idealised based on the results of preliminary mortar compression and tensile testing values of failure stress, strain and Young’s modulus, as found in Section 5.5.4. Such idealised behaviour data inputs are depicted in Table 6.2.

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(a) tensile behaviour

(b) compressive behaviour Figure 6.3: Idealised tensile and compressive CDP graphs

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Table 6.2: Idealised tensile and compressive CDP inputs (a) tensile behaviour

(b) compressive behaviour

6.2.4

Boundary conditions

Two boundary conditions are active, one on the exterior nodes of the top surface of the mortar block and one at the loaded end of the bamboo culm. Firstly, the exterior mortar surface was allowed full displacement restraint in x, y, z directions and full rotational restraint in ux, uy, uz directions.

Secondly, the loaded end of the bamboo culm was allowed zero degrees of freedom with displacement restraint in x, y directions and rotational restraint in ux, uy, uz directions. This condition applies a translation in the z direction over five minutes creating a slip between the confined bamboo surface and the interior mortar surface. This second boundary condition acts as the applied loading and mimics LVDT position and the deflection based loading of the Instron used in the physical experiments.

The magnitude of this displacement is the measured slip at the loaded end of the bamboo culm of the corresponding experimental pull tests. These boundary conditions can be seen in Figure B.4 in Appendix B and were defined as: • 0.483mm displacement 100mm above the mortar was applied to the un-noded model • 0.780mm displacement 100mm above the mortar was applied to the noded model

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6.2.5

FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

Bamboo-mortar interaction classification

The surface-to-surface interaction between the bamboo and mortar encompassing it was defined to represent the bond behaviour. Cohesive interaction behaviour was input as described below.

Cohesive behaviour was used to model the initial elastic response of the bond between mortar and bamboo surface in both the normal and shear directions. Such linear behaviour of the bond is expressed as the elastic stiffness matrix K where Knn , Kss , and Ktt represent the stiffness of the bond in the normal n, tangential s, and tangential t directions respectively. Such stiffnesses and reactions to shear and normal tension forces on the bond are depicted in Figure 6.4 as described by Abdulla et al. (2017).

Figure 6.4: Failure modes of bonds/joints (a) tensile cracking, (b)&(c) shear sliding (K.F. Abdulla et al. 2017)

Traction and separation values of each direction can be used to find said stiffness coefficients as depicted

by Figure 6.5. The stiffness of the bond in a single direction is the slope of the elastic region of the traction-separation/stress-slip graph prior to reaching the maximum shear or normal stress.

(a) traction separation equation

(b) traction separation relationship

Figure 6.5: Traction Separation Stiffness Relationships (K.F. Abdulla et al. 2017)

Of the the two stiffness values in tangential directions to the bamboo surface only that in the direction along the culm Kss is important as no torsion force is being applied to the culm. It is therefore assumed, for the sake of simplicity, that there are no micro torsion reactionary forces due to irregularities in experimental culm shape. This assumption holds in the finite element model as the culm shape is

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idealised as perfectly cylindrical for the no node model and the internodal culm shape is perfectly cylindrical in the node model, as stated in Section 6.2.1. Therefore, the effects of Ktt are negligible and it can be assumed that Kss = Ktt . Furthermore, Knn was ignored as tensile forces between the bamboo culm and mortar were not applied.

Kss = Ktt was first determined for each experimental interaction. The bond stiffness is defined as the slope of the line from the origin to the point of maximum bond stress as given by Equation 6.2.

Kss = Ktt values from each bond slip relationship provided by the experimental data were determined and are depicted in Table 6.3.

[h]Kss = Ktt = τmax /δ1

(6.1)

As highlighted in Section 5.5.5 τmax was approximated as τave . Therefore:

[h]Kss = Ktt = τave /δ1

(6.2)

Table 6.3: Experimental stiffness values found through pull-out tests

Stiffness coefficients Kss = Ktt were input and optimised to maximise analytic and experimental result similarity.

6.2.6

Bamboo-mortar interaction calibration

Figure 6.6 highlights the iterative methodology for determining bamboo-mortar interaction behaviour value Kss compatible with FEA of a masonry structure under seismic loading. This methodology is discussed in detail in this section.

Stiffness coefficients Kss = Ktt based on experimental data were entered to determine: • applied load needed to displace the loaded end of the culm by the amount specified in boundary conditions • displacement of free end of culm 52

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FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

Figure 6.6: Finite element modelling methodology for determining interaction behaviour between bamboo and mortar

These outputs were then checked against corresponding experimental results and the stiffness coefficients were adjusted as necessary.

Increasing stiffness in turn increased applied force and

decreased free end displacement.

6.2.6.1

Bamboo-mortar interaction calibration for un-noded tests

Firstly, the un-noded model interaction stiffness was calibrated. The mean experimental stiffness of Kss = Ktt = 9.82 N/mm3 was entered and the aforementioned output values seen in Figure 6.7 were compared with experimental as below; • free end displacement or δ1 = 0.0221 mm which lay outside the experimental bounds of 0.165 mm ≤ δ1 ≤ 0.037 mm given by Table 5.6.

• applied force or Pic = 9.28 kN which lay within the experimental bounds of 7.9 kN ≤ Pic ≤ 9.5 kN given by Table 5.6

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(a) Free end displacement of un-noded test

(b) Applied load of un-noded test with

with kss = ktt = 9.82 N/mm3

kss = ktt = 9.82 N/mm3

Figure 6.7: FEM results of un-noded test with kss = ktt = 9.82 N/mm3

This disparity between experimental and FEM free end displacement values indicates that a reduction in bond stiffness would give results more indicative of experimental culm-mortar interaction. After iteration and adjustment, a value of Kss = Ktt = 6.00 N/mm3 was reached. Kss and output values, seen in Figure 6.8, lay within experimental bounds of Table 5.6 also depicted in Figure 5.24. These output values are; • free end displacement or δ1 = 0.0405 mm which lay within the experimental bounds of 0.165 mm ≤ δ1 ≤ .037 mm • applied force or Pic = 8.44 kN which lays within the experimental bounds of 7.9kN ≤ Pic ≤ 9.5kN • bond stiffness Kss = Ktt = 6N/mm3 lays within the experimental bounds of 3.22 N/mm3 ≤ Kss ≤ 14.9 N/mm3 Therefore the model and stiffness value were validated. The resultant bond-slip relationship is plotted against corresponding experimental relationships in Figure 6.9.

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(a) Free end displacement of un-noded test

(b) Applied load of un-noded test with

3

kss = ktt = 6.00 N/mm3

with kss = ktt = 6.00 N/mm

Figure 6.8: FEM results of un-noded test with kss = ktt = 6.00 N/mm3

Figure 6.9: Graph showing FEA achieved stress-slip relationship for bamboo section containing no node, overlaid on individual and average experimental results

The un-noded bond stiffness achieved previously can be further validated against the noded experimental test results. For this the noded test culm geometry was used. A bond stiffness of Kss = Ktt = 6.00Ń /mm3 was declared and the resultant output values were compared against experimental results. Output values seen in Figure 6.10 lay within experimental bounds of Table 5.6 also depicted in Figure 5.24. These output values are: • Free end displacement or δ1 = 0.0443 mm which lay within the experimental bounds of 0.022 mm ≤ δ1 ≤ 0.074 mm.

• Applied force or Pic = 11.9 kN which lay within the experimental bounds of 8.7 kN ≤ Pic ≤ 12.1 kN .

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Therefore the un-noded bond stiffness and the noded finite element model were validated. The resultant bond-slip relationship is plotted against corresponding experimental relationships in Figure 6.11.

(a) Free end displacement of noded test

(b) Applied load of noded test geometry with

3

kss = ktt = 6.00 N/mm3

geometry with kss = ktt = 6.00 N/mm

Figure 6.10: FEM results of noded test geometry with adjusted un-noded kss = ktt = 6.00 N/mm3

Figure 6.11: Graph showing FEA achieved stress-slip relationship for bamboo section containing one node using noded geometry and un-noded stiffness value Kss = Ktt = 6.00 N/mm3 , overlaid on individual and average experimental results

Kss = Ktt = 6.00 N/mm3 is therefore a reasonable estimation for the cohesion between the smooth bamboo surface and tested mortar. It is assumed that any increase in applied force and reduction in free end displacement (when compared to the un-noded model) is entirely due to normal forces acting on the bamboo node and applied by the mortar. This exercise demonstrates the appropriateness of this iterative process of achieving an analytically bond stiffness value and can now be applied to the noded experimental results.

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6.2.6.2

FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

Bamboo-mortar interaction calibration for noded tests

The mean experimental noded stiffness of Kss = Ktt = 16.2 N/mm3 was entered and the output values were compared with experimental results. Output values seen in Figure 6.12 lay within experimental bounds of Table 5.6.

• Free end displacement or δ1 = 0.0122 mm which lay outside the experimental bounds of 0.022 mm ≤ δ1 ≤ 0.074 mm.

• Applied force or Pic = 13.8 kN which lay outside the experimental bounds of 8.7 kN ≤ Pic ≤ 12.1 kN .

(a) Free end displacement of ideal cylindrical 3

(b) Applied load ideal cylindrical test geometry with kss = ktt = 16.2 N/mm3

test geometry with kss = ktt = 16.2 N/mm

Figure 6.12: FEM results of un-noded test geometry with noded experimental mean kss = ktt = 16.2 N/mm3

This disparity between FEM and experiment free end displacement and applied load indicates that a reduction in bond stiffness would give results more indicative of experimental culm-mortar interaction and so, after iteration and adjustment, a value of Kss = Ktt = 7.9 N/mm3 was reached. This value, along with free and loaded end displacements, and applied force, lay within the experimental bounds depicted in Table 5.6 and Figure 5.24. Resultant free end displacements and applied force values are depicted in Figure 6.13 and the bond slip relationship can be seen in Figure 6.14. Output values are: • Free end displacement or δ1 = 0.0370 mm which lay within the experimental bounds of 0.022 mm ≤ δ1 ≤ 0.074 mm.

• Applied force or Pic = 12.1 kN which lay within the experimental bounds of 8.7 kN ≤ Pic ≤ 12.1 kN .

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Therefore the noded bond stiffness was calibrated for use in further finite element modelling and validated. The resultant bond-slip relationship is plotted against corresponding experimental relationships in Figure 6.14.

(a) Free end displacement of ideal cylindrical

(b) Applied load ideal cylindrical test

3

geometry with kss = ktt = 7.9 N/mm3

test geometry with kss = ktt = 7.9 N/mm

Figure 6.13: FEM results of un-noded test geometry with adjusted noded kss = ktt = 7.9 N/mm3

Figure 6.14: Graph showing FEA achieved stress-slip relationship for bamboo section containing one node using un-noded geometry and calibrating a noded stiffness value Kss = Ktt = 7.90 N/mm3 , overlaid on individual and average experimental results

6.2.7

Summary of pull-out test FEA results

In summary, FEM-calibrated bond stiffness for both noded and un-noded pull-out tests are depicted in Table 6.4. Though less than mean experimental stiffness values, these lay within experimental bounds

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Table 6.4: FEM calibrated stiffness results

and can therefore be considered validated conservative estimates for FEA compatible bamboo-mortar bond stiffness.

6.3

FEA of Bamboo Reinforced Masonry Structure

In order to gauge the potential effectiveness of bamboo reinforced seismic bands, a finite element model of a simple masonry structure with and without bamboo reinforced seismic bands at the lintel and roof level is developed and analysed under cyclic loading.

6.3.1

Masonry modelling techniques

Although masonry has been used as a construction material for thousands of years, details of modelling masonry structures under different loading conditions still has potential for significant future development. This complexity is largely concerned with difficulty in accurately representing the highly non-linear and variable behaviour of masonry and its degradation over cyclic seismic loading (Caddemi et al. 2017). Different strategies to model masonry include macro and micro modelling as shown in Figure 6.15. Micro modelling involves the generation of each individual masonry unit and the mortar between them with material properties for each defined. Macro modelling assumes the masonry to be a homogeneous material with averaged material properties assigned. Simplified micro modelling offers a hybrid alternative where the size of the masonry units is modified in the lateral and horizontal direction to occupy the mortar joints and a surface interaction between units is applied to represent the behaviour of the mortar joint (Senthivel & Lourenco 2009). Macro modelling offers a less accurate prediction of local behaviour due not being able to map macro failure due to the absence of interface joints (Abdelmoneim et al. 2016, Aguera 2016) however it does offer high computational efficiency and ability to effectively model the global behaviour of models with larger geometries (Lourenco, 1996). In this research, the global response of masonry under cyclic loading with and without bamboo reinforced seismic bands is investigated and macro modelling techniques are deemed suitable for this purpose. The relative simplicity of macro modelling also means that the model can be easily adapted and iterated to test different reinforcement situations.

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Table 6.5: Masonry smear elastic and plastic mechanical properties

Figure 6.15: Masonry modelling techniques (a) typical masonry sample (b) micro modelling (c) simplified micro modelling (d) macro modelling (Abdelmoneim, 2016)

6.3.2

Masonry properties and use of CDP

As discussed in Section 6.1.2, the concrete damage plasticity model is used to replicate the quasi-brittle behaviour of the masonry-mortar composite in this model. To obtain the compressive and tensile behaviour necessary to implement the CDP model, it is necessary to perform as series of tests including compressive strength of masonry prisms and diagonal tension tests as detailed in ASTM C1314 and ASTM E519. As this was outside of the scope of this project, results from tests conducted

by Bolhassani et al. (2015) have been adopted for the finite element model. As can be seen in Table 6.5 and Figure 6.16, both the compressive and tensile data collected fit well to the idealised CDP compressive and tensile curves (Figure 6.1), making the data a good fit and appropriate to input as CDP parameters in Abaqus.

6.3.2.1

CDP damage

In order to represent the irreversible damage which occurs to the masonry under cyclic loading, isotropic compression and tension damage parameters are introduced. These damage parameters evolve through increased loading cycles and act in conjunction with the elasto-plastic CDP parameters specified to represent the masonry’s inelastic behaviour. Equations 6.3 and 6.4 suggested by Birtel et al. provide a mechanism to calculate the compressive and tensile damage parameters. It is essential in the definition of these values that the resulting plastic strain is never negative or decreasing with

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Table 6.6: CDP compressive and tensile behaviour definition

(a) Compressive behaviour

(b) Tensile behaviour

Figure 6.16: Tensile and compressive CDP inputs

increasing inelastic strain. The plastic strain at any point can be calculated using Equation 6.5, which has been implemented as seen in Table 6.7. In addition to the damage parameters, a value of compressive recovery of 0.7 and a factor of tensile recovery of 0.1 are specified to model masonry’s ability to recover reasonably from compressive loading but poorly from tensile loading. These values are commonly used in the literature.

dc = 1 −

σc σc max

(6.3)

dt = 1 −

σt σt max

(6.4)

pl = in −

dc σc × 1 − dc E

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(6.5)

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Table 6.7: CDP damage parameters

6.3.2.2

Masonry elasticity

The elastic modulus of the combined masonry/mortar smear must be calculated to be representative of the two materials properties and geometry. Equation 6.6 proposed by Abdulla et al. is adopted where hu is height of masonry units and hm is the thickness of mortar. H represents the height of the masonry assemblage and n is the number of courses of masonry. Standard dimensions as specified in NBC 202: Load bearing masonry, are hu = 57mm and hm = 10mm. For the geometry specified, H=3000mm and n=45. Different values of E for brick and mortar were tested. As mentioned in section 6.3.2.1, the requirement that the plastic strain must always be positive and increasing with increasing inelastic strain meant that there were some values of E which would be inappropriate. Calculations showed that to satisfy this criterion, the modulus of elasticity for the masonry smear must be defined as at least 2134 MPa. This is dependent on the tensile behaviour, where change in plastic strain with a modulus of elasticity less than this becomes negative. The strength values decided on were Eu = 8500M P a and Em = 900M P a, which resulted in a modulus of elasticity for the masonry smear of E = 3800M P a.

E=

HEu Em nhu Em + (n − 1)hm Eu

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(6.6)

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FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

Modelling the seismic band

Seismic bands were integrated into the finite element model through generation of a thin band which wraps around the thickness of the wall at the lintel and roof level. The geometry of the masonry was split into two sections above and below the lintel band. Although in reality these bands would be 150mm thick composite mortar and reinforcement bands, for the purpose of this model only the equivalent area of bamboo was modelled. This is so that the surface to surface interaction of the band to the masonry is representative of the bond of the mortar to the bamboo, which was tested in the laboratory and generated in the first finite element model. This was defined with the same parameters explained in Section 6.2.5. The bond of the mortar band to the first course of bricks above is identical to the bond between courses of masonry which is encompassed in the smear model and therefore is unnecessary to separate. The interaction properties are replicated from the pull test, as explained in Section 6.2.7.

Using average cross-sectional area data as show in Table 5.4, the modulus of elasticity of bamboo used in the model was calculated using Equation 6.7 to represent the modulus of elasticity of gross cross sectional area. The geometry of the band was input at 20mm high by 350mm wide (thickness of the wall), giving a total cross sectional area of Aband = 7000mm2 . Based on average cross-sectional area of the bamboo tested, this equates to approximately 11 culms of bamboo.

Ebamboo(gross) =

6.3.4

AP AP + Ainner

(6.7)

Sample building geometry

As discussed in Section 2.5.1.3, seismic bands are a relatively well known application to increase resistance of masonry buildings to seismic action, however the possibility of using bamboo rather than steel to reinforce the bands has not been investigated in the literature. To study the influence of bamboo reinforced seismic bands on the performance of low cost masonry, a finite element model of a simple masonry building was developed to be tested with different reinforcement situations. The geometry of the structure including wall height, thickness and openings was chosen in accordance with the Nepalese Building Code 202 : 1994 - Mandatory Rules of Thumb Load Bearing Masonry (Appendix B). The structure consists of two 6m long walls with openings and two 3m long shear walls with no openings, all with a wall height of 3m and thickness of 350mm as can be seen in Figure 6.19a It is assumed that the structure has a ’light’ roof as the majority of masonry buildings in rural Nepal, which adds no structural stability and is therefore not modelled.

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Figure 6.17: Sample building case C1, C2 and C3

In order to test the effectiveness of bamboo reinforced seismic bands in different locations, the three reinforcement configurations pictured in Figure 6.17, were generated and compared. These configurations were: • Case C1: No seismic bands • Case C2: Lintel band only • Case C3: Lintel and roof band

(a) Peak ground acceleration of 2015

(b) Cyclic loading amplitude over 100 second

earthquake in Kathmandu vicinity, (USGS,

load step

2015) Figure 6.18: Loading definition

6.3.5

Loading and boundary conditions

Loading was applied to the structure in the form of a body force, which applies an acceleration on the structure and the resulting force is dependent on the user defined density (see Section 6.3.2) and geometry of the model. An initial acceleration of 0.2kg/m3 was applied to the building and then a cyclic amplitude was applied with a time changing factor multiplying this initial acceleration.

The maximum resultant acceleration was 20 × 0.2 = 0.4kg/m3 , or just over 0.4 time gravitational acceleration, where 20 is the maximum magnitude of the cyclic amplitude applied. As can been seen in Figure 6.18a, this is approximately equal to the peak ground acceleration experienced as close as 64

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(a) Sample geometry

(b) Mesh and boundary conditions

Figure 6.19: Geometry, mesh and boundary conditions

20km from but also as far as 120km from the epicentre as a part of the 2015 Ghorka earthquake. For the purpose of calculating the masonry weight, a density of 2600 kg/m3 for the composite masonry and mortar wall has been assumed. To best represent dynamic earthquake loading, the body force was applied in cycles of increasing amplitude as illustrated in Figure 6.18b. This loading history data is adopted from research undertaken by Snethivel et al. (2009) in their experimental testing of historical stone masonry. Originally this was used as cyclic displacement control, however for the purpose of this model, the same magnitude of time-amplitude variation has been applied to the body force. The structure is modelled to be restrained from displacement in all directions at the base of all walls, assuming they are tied into foundations adequate with stay in tact through a seismic event as seem in 6.19b.

6.3.6

Mesh generation and element type

The finite element mesh was generated as prismatic elements with edges approximate 150mm long for the masonry and prismatic elements with edges approximately 50mm long for the bamboo reinforcement. The geometry was modelled as solid deformable C3D8R elements, the standard Abaqus solid element with eight nodes and reduced integration at one central point for each element (Aguera 2016). Building case 1 was made up of 5156 elements, case 2 7092 elements and case 3 9372 elements.

A boundary condition resisting displacement in all directions was applied to the base of the structure as seen in 6.19b. Final results were generated using a step increment of 0.2 seconds, meaning the entire system is solved 500 times over the 100 second time step.

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Figure 6.20: Sensitivity analysis to show change in global maximum displacement and maximum principal stress with changing wall thickness

6.3.7

Sensitivity analysis

It is recognised that the results of this finite element analysis are only as accurate as the data and parameters fed in to the model. A sensitivity analysis was undertaken to identify the relative impact of variation in the value input for different material properties on the results generated and calibrate accordingly. As an example, Figure 6.20 shows the relationship between wall thickness modelled and global maximum principal stress and displacement.

6.3.8

Data processing

The bamboo reinforced masonry structure model was iterated and refined, with results generated for comparison using a relatively coarse precision of 0.5 seconds per step. To achieve final results for processing and analysis, a more precise step time of 0.2 seconds was chosen, meaning the model was solved 500 times over the 100 second loading cycle. The results analysed were displacement, maximum principal strain and maximum principal tensile stress. Figure 6.22 shows displacement, stress and strain at the maximum load as contour plots generated in the Abaqus environment, allowing for easy visual determination of regions where results are most extreme. Based upon this visual assessment, more detailed analysis of local ‘hot spots’ was undertaken. Points of interest further investigated included the top of the shear walls in both the interior and exterior face, the region between windows, the base of the wall near the door opening and the top corner of the window wall, as pictured in Figure 6.23, allocated names for reference. At each of these locations’ maximum principal stress, maximum principal strain and displacement data was collected over the 100 second cyclic loading step.

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Figure 6.21: Maximum principal tensile stress at different points in lintel band model over loading cycles

(a) Stress

(b) Strain

(c) Displacement

Figure 6.22: Contour plot results of building case C2, lintel band only, under maximum loading

Figure 6.23: Points of model where detailed stress/displacement analysis was undertaken

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For example, Figure 6.21 shows the variation in maximum principal stress at points A, B, D, F, G and H in building case C2 over the time step. As can be seen, the maximum stress of these locations is found at point D, on the top corner of the structure, on the inner face. Maximum stress in the entire model was recorded at point C, however this has been omitted from analysis as it is not impacted by the inclusion of seismic bands at the roof or lintel level. It can also be seen that the maximum stress at points B and H are at peak when stress at points A and G are at a minimum and vice versa. This is because these stresses are measured at the interior and exterior faces of the walls. When the interior face is experiencing maximum principal tensile strength, the exterior face is experiencing maximum principal compressive stress.

6.3.9

Optimisation of quantity of reinforcement

In order to test the impact of including different quantities of bamboo reinforcement in the seismic band, iteration and optimisation was undertaken by adjusting the cross sectional area of the bamboo band (as explained in Section 6.3.3). Figure 6.24 shows the relationship between maximum displacement at point A and number of culms of bamboo reinforcement at the lintel level as an example.

Figure 6.24: Relationship between maximum displacement at point A and number of culms of bamboo reinforcement at lintel level

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FINITE ELEMENT MODELLING PHASE (AIMS 2, 3)

Building case comparison results

Data collected over the time step for building cases C1, C2 and C3 allowed for comparison of the seismic bands effect on different parts of the building. Figure 6.25 shows contour plots of the maximum principal stress in each of the three cases at maximum loading. Small differences in stress can be seen, but this is more clearly demonstrated through Figures 6.26 and 6.27 which show the reduction of stress achieved at point F and point A (see Figure 6.23) due to addition of a lintel and roof band as compared to the base case C1.

(a) No Band

(b) Lintel band

(c) Lintel and roof band

Figure 6.25: Maximum principal stress under maximum load

Figure 6.26: Reduction in maximum principal stress between windows due to addition of seismic bands (Point F)

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Figure 6.27: Reduction in maximum principal stress at top of wall due to addition of seismic bands (Point A)

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DISCUSSION OF LABORATORY AND MODELLING TESTING

DISCUSSION OF LABORATORY AND MODELLING TESTING

This section encompasses discussion of all laboratory testing and finite element analysis components of this report as appropriate, including discussion of materials, methodology, results and the feasibility and useability of outcomes.

This incorporates discussion that is critical to understanding the

implications of research outcomes and to future research in the field.

7.1

Laboratory Testing

Following method formulation and data collection, the feasibility and application of data collected during laboratory work was then analysed in relation to its usability in the FEA model. As highlighted throughout the methodology, large parts of this testing required much iteration and refinement to arrive at appropriate results and so there is naturally a large degree of discussion relating to this process and the overall usefulness of the data collected.

7.1.1

Density of bamboo

The results of the density test appeared to be suitable for use as a parameter in the FEA model. The data corresponded to existing values found in the literature (experimental values for this research were approximately 860 kg/m3 compared to a value of 812 kg/m3 found by Mahzuz (2010), for example).

Furthermore, the coefficient of variation was found to be low (5.34%), at least in context with other test results collated during the research. This suggests that results for the dry bamboo samples tested were accurate and useful for inputting into the FEA model. In terms of trends amongst the data, when comparing data from the same culm there was minimal variation in density, within reasonable bounds given the lack of precision of instrumentation (an issue detailed below).

There were however areas that might influence test results. Several samples were weighed before and after the application of wax and it was found to have marginal impact on mass. When samples were left in the water-filled measuring tube for an extended period of time (for the purpose of checking the success of the wax application), no change in volume was observed. Beading of water on the inside surfaces of samples post-testing also reflected the success of the wax in creating a hydrophobic surface such that volume changes were an accurate representation of volume of the sample (see Figure 7.1a).

A key issue was the equipment available for performing density tests. There were no appropriate cylindrical flask that had both a high degree of precision and wide enough opening to allow bamboo samples inside. As such, volume changes could only be measured to the nearest ml, representing potential precision of ± 5 − 10%.

As mentioned in the methodology, this method was taken from Annual Book of ASTM Standards, 71

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volume 4.10, which also suggest other testing methods of density. One such method involves physically measuring dimensions of an object using vernier calipers and using these recordings to calculate a volume. Typically, this is only appropriate for regularly-shaped objects, and could not be used for something with irregular geometry such as bamboo. As the bamboo pieces being tested for density were taken from the same culms being used for compression tests, a close approximation of the surface area had already been calculated. Multiplying this surface area by the known length of the density samples gave another possible approximation for volume which was useful for comparison and analysis with the original method. The results of this are summarised in Table 7.1. When compared to the original density tests, the average density was higher (930 kg/m3 compared to 860 kg/m3 ) but still within an appropriate range. Whilst by no means exact, this comparison of data helps corroborate the relative accuracy of the density calculated.

Table 7.1: Summary of density of bamboo samples, found using measurement technique

Another method suggested by ASTM involves filling a container with water up to the brim, and then submerging an object of known mass into the container. The water that overflows is the volume displaced by the object, and as such collecting this water and weighing it can be used to generate a value for the object’s volume (given the simple conversion of water’s mass to volume). Whilst this has an increased level of difficulty in equipment set up and application, it represents a much more precise method for calculating density.

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(a) water beading on interior

(b) use of a push stick

of a sample Figure 7.1: Potential areas of improvement and discussion for density tests

7.1.2

Tensile strength of bamboo

As outlined in the methodology, to determine the tensile strength of the bamboo, ‘dog bone’ coupons were produced and tested.

This coupon design was however not the only design tested during

development of the methodology. An iterative design-and-test process was utilised to develop the final coupon design that exhibited the desired tensile failure mechanism.

Preliminary straight coupons were initially drafted up without a dog bone section and were successful in terms of delivering reasonable strength results, however there was evidence of crushing failure at the tab interface, rather than tensile failure occurring in the gauge length. This preliminary design is displayed in Figure 7.2.

Figure 7.2: Preliminary coupon design

Following this result, dog bone coupons were developed based on precedents in literature (as mentioned in the methodology), in an attempt to force tensile failure in the gauge region by reducing cross sectional area. Improved results were seen with this design, however a combined crushing and shearing 73

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failure was still observed in the end tab region, with no true tensile failure through the gauge region.

This was hypothesised to be due to the curvature of the bamboo in the tabs creating a region of weakness that was easily crushed in the apparatus jaws prior to any tensile failure occurring. This failure can be seen in Figure 7.3c below, and for comparison, the non-tabbed and tabbed coupons prior to testing in Figures 7.3a and 7.3b.

(a) Non-tabbed coupon

(b) Tabbed coupon

(c) Failed coupon showing crushing/shear failure

Figure 7.3: First iteration of dog bone coupons with wide gauge region

A third and final revision of the coupon design was successful in forcing tensile failure in the gauge region. This coupon was identical to the prior design but the gauge region trimmed down from the previous 16mm width to 7mm. This reduced cross section promoted tensile failure through the gauge region before crushing or shearing failure could occur in the tab region, resulting in a more accurate measure of the tensile strength of the bamboo. This new coupon design and observed failure mechanism is illustrated in 7.4.

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(a) Coupon prior to testing

(b) Tested coupon exhibiting tensile failure

Figure 7.4: Second iteration of dog bone coupons with narrower gauge region

From results in Table 5.2, the second coupons achieved a mean tensile strength of 137.5 MPa with a coefficient of variation of 19.2%, whereas for the refined third coupons, a lower mean tensile strength of 126.7 MPa was achieved, also with a lower coefficient of variation of 14.2%. The 5% difference in coefficient of variation is not a largely significant results, however the lower value for the third coupons is possibly due to the comparatively lower number of tests done.

An unexpected component of the results is the third coupons having a mean tensile strength 10.9 MPa, lower than the second coupons. As true tensile failure was not achieved with the second coupons, their average calculated tensile strength can only be considered as a lower bound estimate of the bamboo’s tensile strength. This is due to the previously mentioned crushing/shearing failure mechanism occurring in the tab region prior to desired tensile failure occurring in the gauge region.

As such, the third coupons were expected to yield a higher tensile strength by failing through the correct tensile failure mechanism. From the calculated results however, this was not the case. There is potential for this to be due to a relationship between increased cross sectional area and strength, however there is insufficient data from the testing in this report to draw any such conclusion and this is slightly out of scope for this project, hence it may be an area for future research. The difference in average tensile strength recorded was also less than 10% and so not of great significance given the small sample size.

Also from Table 5.2, an average Young’s modulus of elasticity, E, of 14.7 GPa was calculated based on the stress-strain data measured during testing, with an average coefficient of variation of 13%. In comparison to other reinforcing materials such as steel which may have a modulus of elasticity in the realm of 200 GPa (Chen et al. 2015), bamboo has a much lower capacity to behave elastically, with 75

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DISCUSSION OF LABORATORY AND MODELLING TESTING

lower loads causing permanent deformation. This behaviour was observed during testing, with sudden plastic failure occurring in every coupon. This is potentially a limitation in the use of bamboo as an alternative reinforcement material, with further investigation into this behaviour another possible area for future research.

There is also scope for future research into the implications of performing tensile tests on whole bamboo culms rather than coupons. It is possible that lesser or greater interlocking of the bamboo fibres may occur in a full culm, decreasing or increasing tensile strength beyond the capacity indicated by testing of coupons. As such, coupon testing may not be the most appropriate method to determine the tensile strength of bamboo. Greater challenges exist however in testing full culms due to the size and shape, and as such it was not possible to undertake such testing for this project due to limitations of available laboratory apparatus.

7.1.3

Compressive strength of bamboo

As highlighted in the methodology section, compression tests produced successful results for using in FEA modelling. Values attained also were comparable to compression tests done during other research, such as by Gupta et al. (2015). There was a fairly low coefficient of variation between all samples, and little variation between compressive strength for samples taken from the same culm. All tests appeared to fail in a relatively uniform way, in what appeared to be pure compression. This demonstrated the relative success of the shorter sample sizes in reducing the chance of buckling failure occurring.

A point of consideration was the end-friction effects between the compression plates and sample, something partially caused by having shorter specimen sizes. Essentially, the contact between the sample and plate restrains movement in the lateral direction and effectively helps to confine the sample, increasing recorded compressive strength beyond an accurate value. The combination of short sample length and frictional effects resulted in ’barrelling’ of the majority of samples tested, something demonstrated in Figure 7.5a. This barrelling response is well documented, highlighted in ASM Handbook, volume 8 (ASM, 2008). Unfortunately, there is little to no discussion in the literature about addressing or adjusting compression values for bamboo samples impacted by end-friction effects, or about parameters to use for compression testing. ASM suggests creating a frictionless surface between the compression plate and sample, but there is also no research evident that has attempted this with bamboo. Exploring the variation in measured compression strength and failure mechanism based on aspect ratio of sample tested is a potential area of future research. High contact friction is further demonstrated in the ends of samples post-loading, as shown in Figure 7.5 and explained in Figure 7.6.

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(a) barreling failure

(b) end-friction effect has caused the sample to squash/wrap around the contact surface

Figure 7.5: Photos of compression tests both during and after loading showing failure mechanisms

Figure 7.6: Barrelling under loading when there is friction between surfaces. Points A, B and C show how as deformation increases, the sides of the sample effectively ’wrap’ around the contact surface (adapted from ASM 2008)

Whilst slightly outside the scope of this research, it is interesting to consider the effect of increased area on compressive strength. Although data is limited, the graph in Figure 7.7 highlights the apparent slight upward trend in strength with increasing area (which may just be due to the natural variability in bamboo samples and lack of in-depth data). Prior research, such as that done by Richard and Harries (2014), has demonstrated that tensile strength of bamboo increases with decreased cross-sectional area, however there appears to be little to no research on the effect area has on compressive strength of bamboo. Of course, there are multiple factors which may be the cause of the trend in Figure 7.7, such as increased surface area leading to increased end-friction effects, or the bamboo tested being from different samples. This may be an area of future exploration for later research.

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Figure 7.7: Graph showing increase in compressive strength with cross-sectional area of bamboo

7.1.4

Bamboo/mortar pull-out test interaction

To the knowledge of the authors, there is little to no existing research involving the development of a local bond stress-slip relationship between bamboo and mortar based on pull-out test data, utilising the techniques outlined in this report, including a comparison between the behaviour of noded and non-noded embedment. There has also been no prior laboratory testing at the University of Adelaide on pull-out tests for bamboo samples, and as such there has been significant effort put into the development and post-testing analysis of the methodology utilised here. Challenges have presented in the form of comparing and corroborating results and data with existing literature, and verifying the accuracy and usefulness of data collected. As such, there are a number of points of commentary and recommendations for areas of future research.

7.1.4.1

Data analysis

Analysing the pull-out test results and data, a number of conclusions and conjectures can be drawn.

In terms of the original load-displacement graphs (examples of which are presented in Figures 5.21 and 5.22) it can be noted that the drop in load following bond breakage at maximum load, P, is approximately 1 kN in magnitude for the noded samples, and approximately 2 kN for the non-noded sample. This indicates that a drop in capacity in the realm of 12-25% may be expected immediately after bond breakage. However, reading further along the graph it can seen that this load begins to increase with displacement of the bamboo, which is likely attributed to interlocking of the bamboo nodes and interlocking due to the natural curvature of the bamboo, as is illustrated in Figure 7.8.

This suggests that breakage of the bond does not necessarily correspond to complete debonding failure, as the residual frictional and mechanical interlocking component of the bond may in fact match or surpass the load capacity at initial bond breakage. It could possibly also be further extrapolated that 78

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the embedment of multiple nodes over a greater bamboo length would further increase this effect and result in significantly higher loading being required to fully de-bond bamboo from mortar.

(a) Displacement at bottom and at a node

(b) Detail of node

Figure 7.8: Pull-out test split in half showing internal movement of bamboo and nodal interlocking

The effect of a single node is somewhat quantified in comparison between Figures 5.23 and 5.24. The variation in peak bond strength τf , of 0.129 MPa in magnitude suggests an approximate increase in capacity of 20% for the noded case. The greater difference in behaviour between the two cases however can be seen in the displacement required to reach the point of debonding at δmax ; the required displacement is almost double for the noded case at 0.843mm compared to the non-noded case at only 0.489mm.

These results clearly indicate that nodal embedment has a significant effect on the local bond strength between bamboo and mortar. This result is promising in terms of illustrating potential for the use of bamboo in the reinforcement of URM. Further exploration of the implications of these nodal effects are explored in the FEA modelling component of the project.

7.1.4.2

Implications of bi-linear model without friction component

As alluded to in the methodology, it was deemed to be not feasible to fit the tri-linear bond stress-slip model presented by Vaculik et al. (depicted in Figure 5.19) given the time constraints on the project.

Significant time would need to be allocated to developing an inverse analysis code to utilise the model and unfortunately this time was not available. Instead, the bi-linear model without a residual friction

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component was utilised. It should be noted that the tri-linear model has been developed relatively recently and whilst it is potentially a superior and more accurate model, it does not necessarily discredit the use of the bi-linear model as incorrect.

The issue with not using the tri-linear method is that ultimately, the data appears to better suit this type of model. From the graphs presented in Figure 5.21, it can be seen that there is fairly minimal drop off from peak loading once bond breakage has occurred. The resulting ’saw blade’ behaviour occurring in Figure 5.22 or smooth load increase in Figure 5.21 is indicative of a strong post-debonding frictional component of the local stress-slip behaviour, as evidently there is some resistive force preventing a clean pull out of the bamboo from the mortar. As mentioned previously, it is highly recommended that future research explores the implementation of the tri-linear model for these reasons.

Of course, it is worthwhile considering whether one can apply local stress-slip modelling (either bi-linear or tri-linear) to an organic material such as bamboo at all. These types of models are typically applied to ’bond-slip behaviour between reinforcement and brittle substrates’ (Vaculik et al.

2018, p. 245), such as FRP or steel reinforcing bars. There appears to be little to no evidence or usage of the model for bamboo pull-out strength, let alone for timber materials in general. Use of the model often assumes a highly regular shape to its constituents, for example when performing intermediate calculations, which is at odds with the highly irregular profile of bamboo (issues with the shape of bamboo are explored further in Section 7.1.4.3 below). The nodes themselves are responsible for a large degree of pull-out strength and are physically quite large, however this physical mass cannot be easily or readily incorporated into the bi-linear calculations. Ultimately, the lack of corroboration of this technique when used on bamboo does impact its feasibility in generating accurate local bond models and it would be appropriate to compare these results to any future studies in this area.

7.1.4.3

Limitations of testing

The irregularities naturally present in bamboo lead to certain complications with testing and data analysis. It is a highly non-homogeneous material with localised bends and non-uniform cross-sections.

The surface of the bamboo often contains a divot on one side which impacts the ease with which the cement can properly bond to the bamboo. The model essentially assumes that the bamboo is perfectly round and uniform in dimension, such as when utilising Lper . Recording and calculating these relevant values was conducted at various places along the bamboo to arrive at an appropriate average, however ideally it would be better to incorporate variable parameters when performing calculations, such as those in Equation 5.7 (e.g. splitting the bamboo up into sections with pre-recorded dimensions).

Localised bends to the bamboo also mean that post-debonding, the cement mortar would provide a higher resistive effect in some areas (due to physical mechanical resistance), and a lower resistive

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effect in others (due to a lack of surface contact between bamboo and mortar - see Figure 7.9 below).

This effect is loosely comparable to pulling a steel reinforcing hook through mortar - resistance is provided due to both its longitudinal frictional resistance and the mechanical interlock of the hook.

A similar effect being present in bamboo would of course impact any results and implications taken from post-debonding displacement of the bamboo.

Figure 7.9: Effect of localised bend in bamboo on resistive forces (bend exaggerated for illustrative purposes)

In terms of the testing apparatus and procedures, there were several areas of improvement that became evident during the course of the experiments. Firstly, the area of mortar cover for the pull-out test samples realistically should have been increased to minimise the chance of radial cracking whilst the bamboo was pulled from the mortar, as cracking may have influenced and reduced the dependability of results. Secondly, it is worthwhile considering the implications of not having full development of the bond - i.e. having an embedment length less than Lcrit . Whilst this ensures that failure does not occur in the bamboo, it also means that it is likely that full bond strength was not reached. The method for calculating τmax when embedment length is greater than Lcrit (see Equation 5.5) would appear to represent a more accurate method than the simplistic τave method that was utilised for lengths greater than Lcrit . It may be more suitable to test samples with lengths greater than estimated Lcrit values for this reason. Lcrit can only be found post-testing, and given that tests of these kind appear to have not been performed before, it was difficult to ascertain what this value would be. Perhaps more crucially, having a greater embedment length was not feasible with the current testing equipment available.

Calculated Lcrit values were in the realm of 500 - 600mm, which would struggle to fit into the Instron

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testing machine. Having greater lengths would also mean that more than one node would likely be embedded in the mortar, resulting in difficulties analysing the individual effects of nodes compared to sections without nodes. Thirdly in terms of methodology improvements, it would be ideal to increase the number of LVDTs used during measurements, in particular to measure potential slip/displacement at more areas along the testing apparatus. Given that the measured displacements are relatively small, it is possible that the total measured movement this was made up of a culmination of machine/jaw slippage, or settling. Having an idea of displacement along different areas of the samples, including at the gripped end, would allow actual movement of the bamboo to be properly calibrated.

Lastly, it would have been more suitable to increase the number of testing samples. Having only three samples for each arrangement (nodal and non-nodal) means there is uncertainty about conclusiveness of results, especially in context with bamboos inherent variability. However, increasing the number of samples used during this project was not very feasible given the intensive effort needed to test each sample in terms of set-up time and this is something that needs to be taken into consideration for any future work.

7.1.4.4

Future recommendations for pull-out tests

Whilst setting up and carrying out the pull-out testing it became apparent that there is significant scope for off-shoot research that would add value to this body of knowledge, but was unfortunately out of scope for this project. Future recommendations are described explicitly in detail here due to the breadth of potential improvements identified during this research (as opposed to in the conclusion, Section 8, which contains future recommendations derived through the rest of the project).

For example, as previously mentioned, it would be ideal to fit a tri-linear model to the bamboo-mortar bond stress-slip interface, incorporating a residual friction component to account for interlocking of the bamboo in a pull-out situation. This would require the development of a code to fit a local bond stress-slip relationship based on global load-slip behaviour. Ideally this model could also be compared to more basic models/methods to identify any potential benefits in terms of accurately modelling real-world behaviour.

In conjunction with this, a code for developing tri-linear bond stress-slip relationships would be generalised such that irregular shapes such as bamboo could be more accurately modelled without having to oversimplify geometry, such as in this case where it has essentially been modelled as a perfect cylinder.

Testing and analysis should also be undertaken to understand the effects on pull-out strength of multiple nodes over a greater embedment length.

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insufficient to draw any significant conclusions as to the relationship between multiple nodes and pull-out strength, and has more so been undertaken to establish some base properties for use in FEA modelling.

During testing it was noted that displacements measured at debonding were quite small, and whilst it was assumed that this movement was at the bamboo-mortar interface it was not possible to verify with full confidence that this was the case. Final displacements could be checked approximately by splitting open the mortar, however it was not possible to determine whether the displacement at initial debonding was correct. Slip may have occurred at the apparatus jaws or at the reinforcement bar/epoxy to bamboo interface, or settling of the mortar may have occurred, with a culmination of these errors possibly impacting upon the results of the test. To minimise this in future tests, it would be recommended to measure displacement at every interface to ensure an accurate measure of slip.

A limitation of the pull-out testing was the range of the LVDT’s utilised, which had a maximum capacity of approximately 20mm. Due to this, it was not possible to analyse the behaviour of a full pull out of bamboo from mortar. Test results suggested that residual friction may have made up a significant component of the bond strength between bamboo and mortar and as such it would be ideal to fully explore this behaviour in future research by utilising long range LVDT’s and associated testing apparatus.

In this research only one species of untreated bamboo was utilised for all testing, and as such these results can not necessarily be considered as general or true for other varieties. Hence another area for future research would be to analyse bond strength/pull-out strength for various species, as well as various bamboo dimensions. Different treatments of the bamboo could also be explored, including treatments to improve bond, and the effects of treatments to prevent breakdown of the bamboo.

Similarly, mortar variations could also be tested to determine an optimum mix, or simply to determine case specific bond strengths with the use of a pre-determined mortar.

Future research into bamboo-mortar interaction should ideally undertake pull-out testing not only under monotonic, continuous rate loading, but also under cyclic loading, so as to analyse the effects on bond deterioration after a number of loading cycles. This is especially important given the application in this instance of a bond required to resist cyclic loading that may be applied during earthquake events. Analysis of bond under cyclic loading is crucial to understanding the long term applicability of bamboo as a reinforcement and whether it has the ability to withstand one or more loading cycles.

The implications of this understanding may be critical to future construction recommendations and the implementation of a bamboo reinforcement scheme.

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Finite Element Analysis Discussion

Development of both finite element analysis models was the product of an iterative and constantly evolving process. While both models are the product of some assumptions and idealisations, they provide useful insights into the behaviour of bamboo as reinforcement helpful in trying to understand this unfamiliar research field.

7.2.1

Pull-out test mortar modelling inaccuracies

Various errors arise from the use of the concrete damage plasticity model in the simulation of the mortar block deformation, as discussed below.

7.2.1.1

Shear failure not considered

The CDP model does not allow for shearing failure within the mortar itself and assumes tensile cracking and compressive crushing are the two main failure mechanisms. This potentially causes inaccuracies as the vertical downward force on the top of the mortar block and the upward component of the normal force applied by the bamboo node may create a plane of shearing. This shear failure of the mortar above the bamboo node was not witnessed during the experimental pull-out tests. As can be seen in Figure 7.10 possible shear failure was, however, witnessed just below the node, towards the free end of the culm. Due to it’s position on the free end of the culm, the implications of this failure are minimal as mortar in this region would not experience as considerable trauma or shear stresses as mortar further up the culm. Possible explanations for this include but are not limited to: • There is the potential for localised mortar weakness due to inconsistencies in cast or mix • This region may have failed due to irregularities in culm shape causing local interlock between mortar and bamboo. Forces may accumulate along a local shear plane failing with bamboo slip • Disturbance of weakened mortar after testing is most likely. Mortar blocks were split in order for sections and failure mechanisms to be analysed. In this process mortar may have failed, cracked, and been pulled away from the specimen creating mortar trauma misleadingly indicative of failure during testing. This potential for error during analysis in unavoidable There were no other indications of shear failure within the mortar block itself and so the assumption that crushing (and tensile cracking) are the main failure mechanisms within the mortar is reasonable.

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Figure 7.10: Crushing and shearing of mortar as a result of pull-out testing

7.2.1.2

Separation of particles not considered

Tension ‘cracking is simulated with a softening stress-strain response, which induces strain localisation in the concrete [or mortar] structure’ (Abaqus Analysis User’s Manual 1-5, 2009). This means that adjacent mortar particles do not separate from each other in the finite element simulation. Instead, the entire part (in this case the entire mortar block) deforms through locally and globally stretching and compressing. Because of this, the assumption is made that a complete separation between mortar and bamboo occurs as the surface-surface bond fails and mortar particles are allowed to move around the bamboo nodes.

7.2.1.3

Appropriateness of CDP input data

As discussed in Section 6.1.2 the compressive and tensile behaviour CDP inputs (compressive yield stress, inelastic strain, tensile yield stress, and cracking strain) must be indicative of concrete’s characteristic elastic-plastic stress-strain relationships. In Section 6.2.3 it was stated that appropriate experimental CDP input values could not be found and so values were idealised based on typical concrete stress-strain relationship depicted in Figure 6.2 and extrapolated from experimental mortar compressive and tensile failure stresses, as well as Young’s modulus of elasticity seen in Figure 5.5.

This further introduces the potential for inaccuracies. Simply, a low-strength concrete with linear elastic behaviour and corresponding failure stresses has been modelled in place of the specific cement mortar mix used during pull out tests.

As can be seen in Figure 7.11 the modelled elastic behaviour and experimental elastic behaviour of the cement mortar in compression are sufficiently consistent. The plastic region of the experimental stress-strain relationship was not attained due to laboratory restriction and so was extrapolated.

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Figure 7.11: Graph of idealised vs experimental CDP data

7.2.2

Pull-out test bamboo geometric variability

As highlighted in Section 6.2 the irregular nature of bamboo culm geometry makes it virtually impossible to accurately recreate in Abaqus. For this reason culm geometry has been idealised as perfectly cylindrical, with the exception of the node. The diameter and thickness of modelled culms were approximated as an average of corresponding dimensions of relevant experimental culms.

Cross-sectional variation were approximated at points of interest along the culm (at the base of the mortar block, at the top of the mortar block, at the node) from measurements taken in the preparation of the laboratory pull tests but these are merely estimations. This means that various forces may not be adequately accounted for within the finite element model and potential geometric inaccuracies, and their resultant forces, are highlighted below; • Widening of culm from base to top of mortar block: Applied force will not be parallel to bamboo surface and there will be a resultant tension component acting on the bamboo-mortar bond. This has the potential to cause premature de-bonding. Furthermore, once de-bonding and slip has occurred bamboo with a smaller diameter corresponds to a location along the mortar block with a larger interior diameter, and the surfaces will no longer fit perfectly against one another. A reduced residual friction between surfaces is a result.

• Slimming of culm from base to top of mortar block: Applied force will not be parallel to bamboo surface and there will be a resultant compression component acting on the bamboo-mortar bond.

This preserves the bond as a smaller component of applied force will, therefore, be acting as a shearing force. Furthermore, there is a resultant component of normal force acting against the pull direction as a larger diameter section of bamboo is pulled towards a narrower interior diameter section of the mortar block.

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• Curvature of culm: Slight curvature in the length of the culm results in the applied force acting in a direction not always parallel to the surface of the bamboo. This again results in a reduced shear force against the bamboo-mortar surface and an resultant normal force with components acting as a reaction to the applied force. As the compressive strength of both bamboo and mortar is far greater than the bond strength between the two materials this curvature was not inputted into the finite element model. Curvature could not be accurately measured or recreated in an efficient manner geometrically but culms that exhibited visible curvature were identified and noted.

7.2.2.1

Composite nature of bamboo

The strong longitudinal fibres that make up a bamboo culm are not evenly spread over its cross-section.

This means that different tensile strength can be measured of a coupon from the outer portion of bamboo than of the inner portion. In the past researchers have attempted to model this distribution of strength over a bamboo cross-section by representing the bamboo as a functionally-graded material.

This involves the use of matrices to distribute the strength properties over the cross-section and increases computational time required to solve the model. The improvement in accuracy is relatively small considering the significant added complexity. Furthermore, the circular cross-section of bamboo used during pull-out testing means that the distribution of fibres is approximately symmetrical, minimising the effect of irregular fibre/strength distribution on this model. For the purpose of this research the assumption was made that the bamboo is entirely homogeneous over its cross-section and a uniform tensile strength was assigned.

7.2.3

Inconsistencies in FEA and experimental pull-out results

The ratio of FEA determined noded bond stiffness and un-noded bond stiffness was 79.4% the corresponding experimental ratio. Therefore there is a lesser improvement in bond cohesion with the addition of the node analytically, Compared to the additional stiffness suggested by noded experimental tests. For both the noded and un-noded pull-out test models a bond stiffness kss significantly lower than the corresponding experimental test was achieved. The un-noded pull-out tests analytical bond stiffness was 61.1% compared to the corresponding experimental mean and the noded pull-out tests analytical bond stiffness was 48.5% compared to the corresponding experimental mean. Though severe, both calibrated analytical bond stiffness values were within experimental bounds and are therefore of use and for further bamboo reinforcement modelling. This bond stiffness can be considered a conservative estimate. A reduction in bond stiffness implies that, according to the finite element model, both noded and un-noded bamboo slips further with loading in the elastic region of the bamboo-mortar bond than was witnessed during experimentation. Furthermore, during the iterative process of finding

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the most appropriate noded and un-noded bond stiffness, it was the applied force necessary to achieve a reasonable loaded end displacement that caused kss to reduce. In turn this kss reduction caused free end displacement to increase. There are multiple potential reasons for a reduction in bond stiffness and a reduction in nodal effect, most of which distil down to immense material variability.

7.2.3.1

Bamboo geometry simplification

As highlighted previously, should the culm widen from the base to the top of the mortar block, an applied force will not be parallel to bamboo surface and a resultant tension component acting on the bamboo-mortar bond would be present. Therefore, the bamboo-mortar bond tension stiffness, or knn , may have dictated bond failure. This stiffness was not found experimentally and could not be identified in the literature. The analysis of experimental pull-out test data could, therefore, have yielded a higher kss as the failure bond stress τf would have been reached at a lesser strain. Simply, a wider cross-sectional area of bamboo at the loaded end of the culm would have failed earlier, due to the simplification of bamboo geometry in the finite element model other bamboo geometric variations could have caused similar disparities between stress τf and strain, or free end displacement δ1 .

7.2.3.2

Bamboo material inconsistencies

As can be seen in Table 5.2 and as is discussed in Section 7.1.2 there is a high variability in bamboo material properties. Bamboo’s Young’s modulus of elasticity used in finite element modelling was that achieve from experimental bamboo tensile tests and inaccuracies introduced here propagate through causing inaccuracies in FEA pull-out test simulation. for example, an input Young’s modulus lower than the true value of the pull-out bamboo culms would result in greater stretching of the bamboo, a smaller free end deflection, and reduced applied force. Inconsistencies in failure mechanisms between bamboo tension tests, as described in Section 7.1.2 are indicative of the presence of such inaccuracies.

7.2.3.3

Loaded end displacement

Any inaccuracies introduced due to bamboo geometry simplification and/or bamboo material inconsistencies are made more severe due to the displacement controlled loading boundary condition being place 100 mm from the true loaded end in the unconfined region of bamboo. This was, however, necessary to replicate laboratory testing procedure and boundary condition displacement values, with the loaded end displacement LVDT being placed in this location due to laboratory limitations. Due to a lack of local bond and normal forces action on it, the 100 mm of unconfined bamboo would have stretched dis-proportionally more than the confined bamboo, after considering local material and geometric inconsistencies. Greater accuracy would be achieved by measuring displacement at the true

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loaded end, that is the top of the mortar block, during experimental testing and this is recommended as an improvement to the methodology.

7.2.3.4

Elastic bond analysis limitations

Limitations of bi-linear bond analysis of laboratory results and taking and approximation τave as τde−bonding in place of τmax as highlighted in 7.1.4.3 also introduces inaccuracies. With inaccuracies in peak bond stress come inaccuracies in bond stiffness due to their interdependence, as highlighted in Equation 6.2. Outputs that are potentially measured more accurately during laboratory testing, will therefore be inconsistent with modelling results.

7.2.4

Masonry model inputs

The following sections outline the key discussion points in regards to the masonry model inputs.

7.2.4.1

Loading definition

As discussed in Section 6.3.5, load was input as a body force, meaning that the magnitude of the seismic force was dependent on the masonry weight and a nominated acceleration. This allows for a realistic simulation of the increased force up the height of the structure. Typically in seismic design, forces are applied as point loads based on seismic weight with a design action coefficient to adjust for the intensity of the earthquake being designed for (for example Australian Standard 1170.4). In finite element analysis of the performance of masonry under seismic forces, often a displacement control is input instead. This is due to the fact that finite element analysis is usually trying to replicate a laboratory test where displacement control was used. However as the FEA modelling of the masonry structure was not endeavouring to replicate laboratory conditions, more free reign over definition was possible. After experimentation with different load definition regimes, a body force load definition was selected as a realistic representation of the true conditions a building is subjected to under seismic loading.

7.2.4.2

Masonry model material properties

Along with geometry, boundary conditions and loading, material properties are a critical part of FEA inputs. As discussed, bamboo material properties were able to be tested experimentally, with measurements of tensile and compressive strength and density collected for use in the model. However as experimental determination of masonry material properties was out of scope for this research project, appropriate values were adopted from the literature. Through research and preliminary testing of bamboo, it is clear that material properties of the natural fibre are highly inconsistent and

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Table 7.2: Masonry and mortar modulus of elasticity comparison

the same is true for mortar and masonry units. This is especially the case in developing countries where masonry production is less consistent. Testing by Engineers Without Borders in East Timor showed this to be the case, particularly when considering masonry made from clays sourced from different locations (Hamish Banks, personal communication, 7 May 2018). In Nepal, fired and un-fired masonry made from a variety of clays or rubble stone masonry of inconsistent geometry and stone type are all prevalent. Inaccurate material properties is often cited as the most significant cause of reduced accuracy in finite element modelling, and in the case of attempting to model and understand un-engineered, low-cost masonry in a development context, it is clear that this is an important factor in achieving reliable results. It was decided that there was to be a preference in choosing results from a reliable paper above choosing results which are specifically looking at low strength materials.

Plastic (CDP) properties Research by Parajuli looked into mechanical properties of masonry from heritage listed buildings in Nepal bonded with mud masonry. Findings were unfortunately insufficient to form the basis of a CDP model to be implemented in Abaqus. Instead parameters from Bolhassani et.al were selected, as seen in Table 6.6. These properties are for fully grouted masonry blocks, which although prevalent in Nepal, are not the primary form of masonry used in construction. As can be seen in in Figure 6.16, the stress vs. inelastic and cracking strain curves from this data is a good match to the idealised curves in Figure 6.1.

Elastic properties As mentioned in Section 6.3.2.2, the elastic properties of the masonry were calculated using Equation 6.6. The values of modulus of elasticity for masonry and mortar respectively were assumed through calibration with the finite element model, informed by values from literature (Bolhassani et.al, Laftah et.al). The research by Parajuli gave values of modulus of elasticity for both bricks and mortar in Nepal. However these computed to a modulus of elasticity for the smear of just 214MPa, which is significantly less than the required minimum E to comply with the CDP data of 2134MPa, as discussed in Section 6.3.2.2. Through iteration and calibration with the model, final values of E were selected as in Table 7.2. Included in this table are the strength parameters collected

by Parajuli from Nepalese masonry as well as properties of high strength materials (Bolhassani et.al, Laftah et.al). As can be seen, a compromise between high strength and low strength material properties has been made.

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7.2.4.3

DISCUSSION OF LABORATORY AND MODELLING TESTING

Masonry FEA model assumptions

Although best efforts were made to create an accurate model which successfully imitates real performance of masonry, assumptions made in the model generation have an undeniable impact on the validity of results. These assumptions and limitations are described below; • Corner connections: In this model, it has been assumed that corner joints between walls have a strong connection which does not separate under earthquake loading. As noted in the literature review Section 2.5.1.3, separation of walls is a common cause of failure of URM buildings in Nepal. Because the finite element analysis does not capture this type of failure, the extra benefit it provides is not able to be quantified.

• Boundary conditions: It as assumed that the base of the building is clamped and that no displacement in any direction can occur. In reality it is possible that there would be failure at the foundation level also. However this is not related to the performance of seismic band and therefore does not impact the comparative nature of the analysis undertaken.

• Use of CDP: It is assumed that the concrete damage plasticity model is able to realistically represent the behaviour of the masonry composite under cyclic loading. An alternative to CDP is the use of the Drucker Prager model, which represents the behaviour of reinforced concrete as homogenised continuum with yielding defined by the Drucker Prager criterion (Lin et.al 2004).

This model was used in Abdulla et.al’s simulation of masonry behaviour. The CDP model was selected as its use in masonry modelling is more common and it is easily integrated into the Abaqus environment.

7.2.5

Masonry model behaviour

The following sections outline the key discussion points in regards to the masonry model outputs and behaviour.

7.2.5.1

Issues with applying small amounts of bamboo reinforcement

Recommendations for the quantity of reinforcement in seismic bands from the NBC (Appendix B) are for walls spanning over 5 metres, 2 steel reinforcement bars of diameter 10mm and fy = 550N/mm2 are necessary. This equates to 43,200 MPa of tensile capacity, which based on the experimental results of bamboo tensile strength of 134 MPa, corresponds to just 330 mm2 of bamboo. As specified in section 6.3.3, the area of bamboo used in the model is far in excess of this. At first, the area of bamboo was entered as the equivalent of 2 culms of bamboo, equating to a cross-sectional area of 2 × (277 + 358) = 1170mm2 . Spreading this area of the 350mm width of the wall meant that the prismatic mass of 91

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Figure 7.12: Lintel band separation under max loading

bamboo was only 3.3mm thick. This created issues in the model prohibiting convergence of results.

Observation of preliminary outputs before divergence showed that the bamboo band had distorted and completely separated from the masonry. One reason for this is the dramatic difference in thickness of the band as compared to the masonry geometry causing unrealistic concentration of stress. Another cause is the fact that the mesh of the bamboo was very unfavourable. Ideally, edges mesh elements should be approximately equal in length. Because the height of the entire band was only 3mm, mesh size would have to defined as 3mm to achieve this. This however would results in the bamboo being divided into approximately 70000 elements, which is an unrealistic amount to solve for. Consequently a coarser mesh size of 50mm was chosen, which resulted in very oblong elements, which is not unideal and caused problems in solving they system.

Through testing of different arrangements, a thickness of 20mm was decided on as it provided stable results while representing a realistic amount of bamboo to be used as reinforcement.

7.2.5.2

Masonry-seismic band interface

As mentioned in Section 6.3.3, the interface between the bamboo and mortar was modelled using interaction properties determined in the laboratory testing phase calibrated in the pull out FEA. As can be seen in Figure 7.12, under maximum loading there was some separation between the lintel band and the masonry at points of maximum stress. This indicates that at high stress points such as the corner of windows, it is likely that the bamboo reinforcement will separate from the mortar. However as determined through laboratory testing of the pull-out behaviour of bamboo, this is unlikely to be a problem as the nodes of the bamboo act as an anchor. This anchorage resists the bamboo being pulled out of mortar, even after the initial cohesive bond has been broken.

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7.2.6

DISCUSSION OF LABORATORY AND MODELLING TESTING

Recommended improvements to model

As mentioned, the masonry model was the product of a continual and iterative process and there would always be extra improvements which could be made to improve the accuracy of the model, albeit at the expense of computational efficiency. The main recommended improvements are listed below: • Investigation into post-peak behaviour - this could be achieved through definition of a displacement control rather than an load control and would allow for detailed investigation into the plastic behaviour of the masonry, as defined using CDP damage parameters.

• Micro modelling of masonry - this allows for analysis of the effect of the bamboo reinforcement in mitigating the development of macro cracking

7.2.7

Analysis of model results and outputs

The following discussion outlines the results of the masonry FEM.

7.2.7.1

Sensitivity analysis

Sensitivity analysis showed that some parameters associated with the model had a significantly greater impact than others on results, as was expected. For example, as can be seen in Figure 6.20, the wall height had a significant impact on the stress and displacement of the model. Modulus of elasticity of the masonry was also highly significant with a 30% decrease in the value of E resulting in a 45% increase in maximum global displacement. This highlights the importance that the masonry material properties are defined accurately, but also shows that the model would like behave differently if material properties of low strength masonry were used, a recommendation for further investigation.

7.2.7.2

Bamboo quantity optimisation

As can be seen in Figure 6.24, the relationship between the amount of bamboo reinforcement modelled in the lintel band and the maximum global displacement is roughly linear. This make it difficult to optimise the quantity used, and the amount of bamboo used in the building case comparison models was chosen as a approximately 10 culms, or 7000mm2 as this is the upper limit of practical amounts to include in the seismic band which is usually just 150mm thick.

7.2.7.3

Impact of bamboo reinforced seismic bands

Despite the difficulties in accurately modelling the behaviour of masonry with finite element analysis without experimental data to validate results, the model generated is successful in acting as a 93

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By creating identical models with different reinforcement arrangements and

quantities, the potential improvements possible through integration of bamboo reinforced seismic bands is possible to gauge. In this new research area, this is helpful in laying down groundwork to encourage continued and detailed analysis of bamboo reinforced seismic bands to get the body of knowledge to a point where they can be confidently be implemented in developing countries.

7.2.7.4

Lintel band

Figure 6.26 illustrates the integration of a bamboo reinforced lintel band was able to reduce the maximum principal stress between windows by approximately 14%. Interestingly, this reduction in stress as a percentage is consistent over the loading cycle although magnitude of applied load increases to be 10 times that of the first cycle. The failure of masonry between window openings between As can be seen in Figure 7.13 showing the failure of the masonry between window openings in a building in rural India, this is a significant weak spot in masonry buildings. Hence even a small reduction of stress in this region is important.

Figure 7.13: Failure of masonry walls between openings (Nanjunda 2004)

7.2.7.5

Roof band

Illustrating the impact of the roof band, Figure 6.27 shows the reduction in maximum principal stress at point A achieved through addition of a roof band was approximately 2.5%. The reduction of stress at the same point due to only the addition of a lintel band was just 0.25%, providing good justification for the addition of a roof band as well as the lintel band. However the improvements achieved through the roof band are less significant than the improvements possible through integration of a lintel band, and if given the choice of just one, the lintel band should be implemented.

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8 8.1

CONCLUSION AND RECOMMENDATIONS

CONCLUSION AND RECOMMENDATIONS Findings

The objective of this research has been to investigate the optimisation of bamboo reinforcement within earthquake-resistant construction in Nepalese vernacular architecture. Nepal has a long history of major earthquake events, with three earthquakes in the last 100 years (as recent as 2015) claiming 18,500 lives. The 2015 Gorkha earthquake not only claimed significant life, but was responsible for upwards of 10 billion USD of damage, equating to a third of Nepal’s GDP. These statistics alone highlight the need for drastic and immediate structural improvement, with much of the building failure the result of poor seismic construction practices.

Unreinforced masonry is globally recognised as being seismically vulnerable and makes up a significant portion of the building typologies in Nepal.

Innovative research is currently disproportionately

weighted towards ’first world’, cutting edge solutions that are unfortunately largely inaccessible to developing communities. As such, it is imperative that research is undertaken into seismic improvement techniques that are accessible, low-cost, sustainable, will engage with the end user and impart significant strength to resist seismic forces.

Bamboo presents significant opportunity as a low-cost, innovative reinforcing material for masonry in developing contexts that has the potential to have a significant impact in delaying the failure of masonry structures, which can be the difference between life and death in serious seismic events. Lack of research into bamboo for reinforcement is the overarching knowledge gap that in-turn formed the research aims.

A range of laboratory testing was used to derive material properties of bamboo, and bond interaction properties of bamboo with mortar. An Abaqus finite element model of pull-out testing was validated and used to calibrate bamboo-mortar interaction properties derived from laboratory testing for use in further modelling of reinforcement. Finite Element Modelling was then used to assess the impact of different placements and volumes of bamboo reinforcement in seismic bands in a representative simple Nepalese URM structure. This model can be easily adapted to test the use of different material properties if further research into the strength of different bamboo species or low cost masonry is undertaken.

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CONCLUSION AND RECOMMENDATIONS

In relation to the aims and sub-aims of the project set out in Section 3, outcomes of the research are: Aim 1 – investigate bamboo material properties 1.1 Robust and generic laboratory testing methodologies have been developed for tensile strength, compressive strength and density testing of bamboo, with recommendations made for future testing and adjustment of methodologies.

1.2 A breadth of material properties were derived for Bambusa Vulgaris, a common species of bamboo.

Whilst material properties have been well-explored in the past for various types of bamboo, this data and research helps add to the body of knowledge, something particularly relevant given bamboo’s high variability across species. There is little pre-existing research on this species of bamboo in terms of these material properties.

Aim 2 - investigate material interactions and model with FEA 2.1 A local bond stress-slip curve for bamboo embedded in a standard cement mortar was derived.

This appears to be one of the first applications of the local bond-stress slip curve to bamboo in general.

Whilst the model itself is at this stage basic due to its preliminary stature, there is great scope for further research in this area and application of this knowledge.

2.2 Elastic bond behaviour was modelled with Abaqus FEA and verified within experimental bounds.

Experimental mean values of bond stiffness could not be verified however and an adjusted bond stiffness value was required to accurately recreate experimental bond behaviour. Examples of bamboo-mortar bond modelling were not found in a review of literature and and there is scope for further research into model refinement.

2.3 Bamboo-mortar elastic bond stiffness was calibrated with Abaqus FEA for use in finite element model of masonry strengthened with bamboo reinforced seismic bands. This calibrated bond stiffness was found to be within experimental bounds though limitations were present and further refinement could be achieved with improvements in both laboratory testing and finite element modelling methodologies.

Aim 3 - Develop and analyse potential reinforcement solutions tested under loading 3.1 Research into potential reinforcement options resulted in the choice of bamboo reinforced seismic bands (rather than conventional steel) as the chosen opportunity for the integration of bamboo to seismically strengthen masonry.

3.2 A finite element model of a simple unreinforced masonry structure was developed and analysed under cyclic loading with and without the addition of bamboo reinforced seismic bands at the lintel

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and roof level. Although failure of the structure could not be captured, the alleviation of stress and displacement in the masonry due to action of the bamboo reinforcement could be captured at different points of the building. Significant improvements were seen both at the top of walls and the regions between openings. The reinforcement was found to have a consistent percentage reduction in stress across different magnitude load cycles, indicating that it will engage even under small seismic loading, helping to reduce damage to the building as well as acting to maintain structural integrity in a major earthquake event.

8.2

Recommendations

Recommendations that have been drawn from the research conducted have been highlighted below and are based on three themes; • Implementation of research represents key findings based on original aims - in essence, tangible recommendations for design improvements using bamboo • Improvements in methodology covers areas in which further refinement could be made to testing and modelling procedures such that future experimentation is improved • Future research details areas of potential future research including future opportunities for further investigation into the problem domain

8.2.1

Implementation of research

Analysis of the three building cases undertaken suggests that the most important feature is inclusion of a bamboo reinforced lintel band, which can significantly alleviate the stress between openings and delay failure in this region. The addition of a roof band is especially helpful in reducing stresses at the top of walls and should also be included where a ’light roof’ is used. The quantity of reinforcement included in these band is proportional to the improvement in performance, but an initial recommendation for an appropriate amount of bamboo is approximately 10 culms, equating to a cross sectional area of 7000mm2 .

The development of generalised bamboo testing methodologies that were suitable for finding material parameters was also a key outcome of the research, something relevant given the current variability in the existing literature’s testing procedures. The methodology detailed in Section 5 can be utilised for future testing and development of testing procedures.

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8.2.2

CONCLUSION AND RECOMMENDATIONS

Improvements in methodology

As highlighted and alluded to throughout the discussion, there were several areas in which change in current practices and procedures would lead to tangibly better results. Whilst many of these changes were implemented as they became clear during the course of the research, there were areas in which it was impractical to do so, largely because of a lack of time, or resources at hand. What follows is a non-exhaustive re-summation of these potential improvements. Further detail and discussion can be found in the relevant discussion sections (see Section 7): • tensile testing – use of coupons with a more gradual curve leading up to the gauge section to have a better failure mechanism – mark on the gauge section where the extensometer was originally placed to allow for more critical analysis of validity of strain data collected • density testing – use of equipment with more precise measurements to allow for better data collection – use of overflowing water displacement technique to gather more precise data • compression tests – use of a frictionless surface between sample and compression plate to reduce end-friction effects, or optimise the length of samples tested such that end effects are minimised but buckling does not yet occur • pull-out tests (recommendations also highlighted in more detail in Section 7.1.4 due to breadth and depth of recommendations found) – development and use of a tri-linear local stress-slip model – increase embedment depth beyond Lcrit • quantity of testing – the need to iterate many of the designs for material testing, as well as the scope of testing done and time limitations meant that some of the testing done did not have enough data to produce conclusive results. It is suggested that future testing should look to use a higher volume of test samples for the majority of tests completed • modelling of bamboo masonry interaction – Plastic bond behaviour investigation and integration into model – Damage modelling of bamboo-mortar bond with cyclic loading applied, indicative of seismic events 98

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CONCLUSION AND RECOMMENDATIONS

– Shear failure and damage incorporated into mortar properties – Greater precision and accuracy in geometric representation of bamboo cum elements • modelling of bamboo reinforced masonry structure – Micro modelling of masonry to map macro cracking behaviour – Post-peak behaviour investigation of model behaviour – Integration of vertical reinforcement around wall openings – Model of failure by separation of walls at corners It should be noted that aspects of these improvements are only applicable and relevant to the laboratory equipment available at the University of Adelaide.

8.2.3

Future research

During the literature review, as well as the formulation of methodology and analysis of results, multiple areas of potential future research became apparent both in terms of bamboo-specific research and the field of seismic improvements. These are detailed below.

8.2.3.1

Seismic improvements outside of bamboo (literature review)

Other potential means of seismic reinforcement that still have large research gaps to be filled: • gabion bands as seismic bands, both wire and polypropylene mesh varieties • mesh reinforcement, including polypropylene, polymer and welded wire varieties • post-tensioned car tire strapping as an external reinforcement

8.2.3.2

Improvements to research of bamboo as earthquake-resistant material

Areas of bamboo research that have scope to further improve or build upon the research conducted in this paper: • a substantial model of bamboo drawing upon in-depth material properties as a functionally graded material • development of tri-linear bond stress-slip model for the bamboo-mortar bond, incorporating a residual friction region and involving iterative inverse analysis as described by Vaculik et al.

(2018) • undertake study of bamboo nodal properties to create a more accurate bamboo model

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• tensile tests - performing tensile tests on whole culms as opposed to coupons to gain insight into accuracy of coupon tests • further research into variability of bamboo strength properties within the species tested here • investigation of the variation in bamboo strength properties amongst different species of bamboo • research into the impact of seismic bands against specific failure mechanisms such as separation of wall elements and local failures • further research into effect of seismic bands on structure with different geometries • application of seismic band model on a mud masonry structure, as is highly prevalent in rural Nepal. Would involve the determination of mud masonry properties • research into bamboo as seismic reinforcement using other construction techniques, i.e. as vertical reinforcement • investigation and development of suitable detailing procedures of bamboo reinforcement to achieve strong embedment otherwise achieved through hooks or cogs in steel reinforcement • investigation into the potential of using PP bands as a low-cost alternative to steel as confinement reinforcement for bamboo rods • thorough investigation of techniques to preserve bamboo and make it resistant to rotting or being damaged by insects. There are already a variety of both technical and traditional techniques to achieve this but comparative testing would be highly beneficial

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Appendices

107

A

A

LITERATURE REVIEW - FURTHER INFORMATION

LITERATURE REVIEW - FURTHER INFORMATION

Table A.1: Detail of research into bamboo reinforcement in construction

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LITERATURE REVIEW - FURTHER INFORMATION

Table A.2: Summary of merits of seismic improvement techniques

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B

B

FURTHER METHODOLOGY, RESULTS AND DATA

FURTHER METHODOLOGY, RESULTS AND DATA

Figure B.1: Dogbone with no aspect ratio used, thickness set at whatever the bamboo thickness is (Shao et al. 2010, p. 135)

Figure B.2: Dogbone for glue-laminated bamboo (He et al. 2015, p. 495)

Figure B.3: Dogbone for scrimber bamboo. Note the long grip length (Xu et al. 2017, p. 734)

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FURTHER METHODOLOGY, RESULTS AND DATA

Figure B.4: Pull-out test geometry and boundary condition application

Figure B.5: Allowed geomtery of openings as specified in NBC

111

B

FURTHER METHODOLOGY, RESULTS AND DATA

Table B.1: Required steel in seismic bands according to NBC

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C

C

CONTACTS

CONTACTS

Dr Michael Griffith Professor School of Civil, Environmental and Mining Engineering The University of Adelaide michael.griffith@adelaide.edu.au

Scott Letton Senior Technician - Machines & Structures School of Civil, Environmental and Mining Engineering The University of Adelaide scott.letton@adelaide.edu.au

Martijn Schildkamp Director Smart Shelter Research smartshelterresearch@gmail.com

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