dissertation christian schembri final

SHEAR ENHANCEMENT OF

TIMBER BEAMS

By

CHRISTIAN SCHEMBRI

Dissertation presented to the

Department of Building and Civil Engineering

Faculty for the Built Environment

University of Malta

In partial fulfilment of the requirements for the degree of

Bachelor of Engineering and Architecture

JUNE 2010

i

To my parents, Laurence and Maria Dolores

ii

Declaration

I, the undersigned, hereby declare that this dissertation is my original work and that

all references made to other sources have been appropriately acknowledged.

_________________

Christian Schembri

June 2010

iii

Acknowledgments

I would like to express my sincere gratitude to my tutor Professor A. Torpiano,

B.E.&A.(Hons), M.Sc.(Lond), Ph.D.(Bath), D.I.C., M.I.Struct.E., C.Eng., Eur.Ing., for his guidance, technical support, patience and encouragement.

I would also like to thank Dr. M. A. Bonello, B.E.&A.(Hons.), M.Sc.(Lond.), Ph.D.(Lond.), D.I.C., Eur.Ing., and Professor S. Buhagiar, B.E.&A.(Hons.), M.Sc.(Lond.), D.I.C, Ph.D.(Lond.), M.I.Struct.E., C.Eng., for inspiring me with the idea of this dissertation; the Lab Technicians of the Civil Engineering Laboratory,

Mr N. Azzopardi A.M.I.C.T.(UK), A.I.A.T. and Mr A. Falzon A.I.A.T. for their assistance during the preparation and testing in the same laboratory; Ing. M.

Fenech B.Eng.(Hons.) from the Department of Metallurgy and Materials Engineering, Faculty of Engineering for his valuable assistance in the preparation

and testing of the pull-out tests and the staff from the latter department and at the

engineering workshop.

Special thanks go to JMV Ltd. for sponsoring the GFRP reinforcement and some

materials required in the preparation of the tests. My sincere appreciation is due to

Mr. R. Vassallo and Mr. J. Bonello of JMV Ltd.

A thanks goes to all friends for their support and encouragement.

iv

Last but not least I would like to thank my father Laurence, my mother Maria

Dolores, my sister Roberta, her husband James and his father Vince, my brother

Jurgen and his fiance Yvette, and my fiance Rosanne and her family for their

invaluable help and moral support.

v

Abstract

The use of timber in construction is characterised by several difficulties. Not least is

its low strength perpendicular to the grain which is likely to lead to shear failure

parallel to the grain. The occurrence of several forms of decay and weathering

further reduce timber strength. The use of 6mm diameter GFRP rebars for the

shear enhancement of timber beams, inserted at angles of 900 and 600 to the main

bending axis, was therefore studied. An epoxy-acrylate adhesive was used. The

same configurations were carried out on both new timber beams and damaged

timber beams to investigate the potential of the shear enhancement method

studied in strengthening and repair respectively. Intentional damage was induced

to simulate weathering. Pull-out tests were also carried out to investigate bond

between the GFRP rebars and the timber for the adhesive used.

The results show that the effectiveness of this method depends on the beam

condition. The average ultimate loads obtained for the reinforced new beams did

not show any increase when compared with that obtained by the control new

beams while those for the reinforced damaged beams showed increases in the

order of 22% when compared to the control damaged beams. These results

should not be taken as the general case and further investigation is required.

Keywords: Shear enhancement, Timber beams, Glass Fibre Reinforced Polymer

(GFRP) rebars, strengthening, repair, bond.

vi

Contents

Declaration ........................................................................................................... iiAcknowledgments ............................................................................................... iiiAbstract ................................................................................................................ vContents .............................................................................................................. viList of Figures ...................................................................................................... xiList of Tables .................................................................................................... xviiiAbbreviations and Notation ................................................................................ xix

Chapter 1 - Introduction ..................................................................................... 11.1 Introduction .............................................................................................. 11.2 Main Objectives and Structure of this Dissertation .................................. 3

Chapter 2 - Literature Review ............................................................................ 52.1 Overview.................................................................................................. 52.2 Shear strength of timber beams .............................................................. 62.3 Shear Enhancement of Timber Beams .................................................... 8

2.3.1 The requirement for shear enhancement .......................................... 82.3.2 Research in shear enhancement techniques .................................... 82.3.3 The case of using Fibre Reinforced Polymers (FRPs) .................... 102.3.4 Research using GFRP rebars as shear enhancement of timber beams ..............................................................................................122.3.5 Research using other FRP types for shear strengthening of timber beams .........................................................................................................17

Contents

vii

2.3.6 Research using shear spikes to increase bending stiffness ............ 20 2.4 More considerations ................................................................................. 22

2.4.1 Shear Connections ......................................................................... 222.4.2 Bond Strength ................................................................................. 23

Chapter 3 Experimental Methodology ......................................................... 263.1 Overview................................................................................................ 263.2 Full Scale Beam Loading Test ............................................................... 27

3.2.1 Shear stresses ................................................................................ 273.2.2 Test Setup ....................................................................................... 293.2.3 Materials ......................................................................................... 30

3.2.3.1 Timber ........................................................................................ 303.2.3.2 Reinforcement ............................................................................ 313.2.3.3 Adhesive ..................................................................................... 31

3.2.4 Testing Configurations .................................................................... 313.2.5 Preparation Procedures .................................................................. 35

3.2.5.1 Timber Beams ............................................................................ 353.2.5.2 Strain Gauges ............................................................................. 373.2.5.3 Insertion of GFRP rebars ............................................................ 38

3.2.6 Testing Procedures ......................................................................... 393.3 Direct Pull-out Testing ........................................................................... 41

3.3.1 Materials ......................................................................................... 413.3.2 Test Setup and Testing Configurations ........................................... 413.3.3 Preparation Procedures .................................................................. 43

3.3.3.1 Attachment to Tensile Machine .................................................. 433.3.3.2 Timber Blocks ............................................................................. 433.3.3.3 GFRP Rebars ............................................................................. 443.3.3.4 Insertion of GFRP bars ............................................................... 45

3.3.4 Testing Procedures ......................................................................... 45

Chapter 4 Results and Analysis of Results ................................................. 474.1 Overview................................................................................................ 47

Contents

viii

4.2 Ultimate Loads ....................................................................................... 474.3 Failure Modes ........................................................................................ 50

4.3.1 C1N ................................................................................................. 524.3.2 C2N ................................................................................................. 544.3.3 C3N ................................................................................................. 554.3.4 I1N .................................................................................................. 584.3.5 I2N .................................................................................................. 604.3.6 I3N .................................................................................................. 624.3.7 V1N ................................................................................................. 634.3.8 V2N ................................................................................................. 664.3.9 V3N ................................................................................................. 684.3.10 C1D ............................................................................................. 704.3.11 C2D ............................................................................................. 714.3.12 C3D ............................................................................................. 724.3.13 I1D ............................................................................................... 734.3.14 I2D ............................................................................................... 754.3.15 I3D ............................................................................................... 774.3.16 V1D .............................................................................................. 774.3.17 V2D .............................................................................................. 794.3.18 V3D .............................................................................................. 80

4.4 General Observations of Failure Modes ................................................ 824.5 Direct Pull-out Test Results ................................................................... 84

4.5.1 Failure modes ................................................................................. 884.6 Rebar Forces in Full Scale Beam Loading Test .................................... 92

4.6.1 Failure Modes ................................................................................. 95

Chapter 5 Conclusions and Recommendations for Future Work............ 1015.1 Overview.............................................................................................. 1015.2 Conclusions ......................................................................................... 101

5.2.1 Effectiveness of Shear Enhancement Method Applied ................. 1015.2.2 Rebar Forces ................................................................................ 1025.2.3 Failure Modes ............................................................................... 103

Contents

ix

5.2.4 Insertion Angle of Rebar ............................................................... 1035.2.5 Adhesive ....................................................................................... 104

5.3 Recommendations for Future Work ..................................................... 104

References ...................................................................................................... 107

Appendix A Results ..................................................................................... 112A.1 Graphs .................................................................................................... 112

A.1.1 Beam C1N ....................................................................................... 113A.1.2 Beam C2N ....................................................................................... 114A.1.3 Beam C3N ....................................................................................... 114A.1.4 Beam I1N ......................................................................................... 115A.1.5 Beam I2N ......................................................................................... 116A.1.6 Beam I3N ......................................................................................... 117A.1.7 Beam V1N ....................................................................................... 118A.1.8 Beam V2N ....................................................................................... 119A.1.9 Beam V3N ....................................................................................... 120A.1.10 Beam C1D ..................................................................................... 121A.1.11 Beam C2D ..................................................................................... 121A.1.12 Beam C3D ..................................................................................... 122A.1.13 Beam I1D ....................................................................................... 123A.1.14 Beam I2D ....................................................................................... 124A.1.15 Beam I3D ....................................................................................... 125A.1.16 Beam V1D ..................................................................................... 126A.1.17 Beam V2D ..................................................................................... 127A.1.18 Beam V3D ..................................................................................... 128

A.2 Pull-out Test Photos ................................................................................ 129A.2.1 Sample 45-2 .................................................................................... 129A.2.2 Sample 45-3 .................................................................................... 130A.2.3 Sample 60-2 .................................................................................... 130A.2.4 Sample 60-5 .................................................................................... 131A.2.5 Sample 90-2 .................................................................................... 132

Contents

x

A.2.6 Sample 90-4 .................................................................................... 133

Appendix B - Testing and Materials Data ..................................................... 134B.1 Computation of principal stresses and their direction .............................. 134B.2 Calculation of Loading Rate .................................................................... 138B.3 Tensile testing report of Aslan 100 6mm GFRP Rebar ........................... 139B.4 Aslan 100, Product Data Sheets ............................................................. 141B.5 Sika Anchor-Fix 2, Product Data Sheet................................................... 156B.6 Test Rig Setup ........................................................................................ 166B.7 Attachment to Tensile Testing Machine .................................................. 166

xi

List of Figures

Fig. 2.1 Schematic of reinforced timber beam test configurations carried out by Svecova and Eden (2004)

13

Fig. 2.2 Schematic of reinforced timber beam test configurations carried out by Amy and Svecova (2004)

16

Fig. 2.3 Schematic of reinforced timber beam test configuration carried out by Triantafillou (1997)

18

Fig. 2.4 Schematic of reinforced timber beam test configurations carried out by Buell and Saadatmanesh (2005)

19

Fig. 2.5 Single shear connection modelling the use of hex bolts and lag screws

23

Fig. 3.1 Principal stresses and principal directions of test setup used (the magnitude of the arrows are indicative of the stress magnitude)

28

Fig. 3.2 (a) The test setup as recommended by ASTM D 198-99, (b) The test setup as used in this experimental programme

28

Fig. 3.3 Schematic of the test setup (dimensions are in millimeters)

30

Fig. 3.4 Diagrams of configurations tested, the dimensions of the damaged series are the same as those for the new series (all dimensions are in millimetres)

33

Fig. 3.5 Making of the simulated damage

35

Fig. 3.6 Creation of drill jig

36

List of Figures

xii

Fig. 3.7 Drilling of holes 36

Fig. 3.8 Checking the electrical resistance by means of an ohm metre

38

Fig. 3.9 Inserting the GFRP rebars

39

Fig. 3.10 Pull-out Test Setup (dimensions are in millimeters)

42

Fig. 3.11 The pull-out tested configurations

42

Fig. 3.12 Preparation of timber block samples

44

Fig. 3.13 Preparation of the GFRP rebars

45

Fig. 3.14 Preparation of the GFRP rebars

46

Fig. 4.1 Ultimate loads of new timber beam series

48

Fig. 4.2 Ultimate loads of damaged timber beam series

49

Fig. 4.3 Convention used for the presentation of crack patterns

51

Fig. 4.4 Horizontal shear failure of beam C1N

52

Fig. 4.5 Bending failure of beam C1N

53

Fig. 4.6 Crack pattern for beam C1N

53

Fig. 4.7 Bending failure of beam C2N

54


55

Fig. 4.9 Beam C3N at ultimate failure

56


57

Fig. 4.11 Right side of beam I1N after test

58

Fig. 4.12 Crack pattern for beam I1N

59

Fig. 4.13 Bending cracks of beam I2N

60

List of Figures

xiii


61

Fig. 4.15 Beam I3N after the test

62


63

Fig. 4.17 First bending crack on the right side of beam V1N

64

Fig. 4.18 Shear displacement at the end of beam V1N

64

Fig. 4.19 Crack pattern for beam V1N

65

Fig. 4.20 Beam V2N after failure

66

Fig. 4.21 Crack pattern for beam V2N

67

Fig. 4.22 First bending crack of beam V3N

68

Fig. 4.23 Shear failure of beam V3N

68

Fig. 4.24 First bending crack on the left side of beam V3N

69

Fig 4.25 Crack pattern for beam V3N

69

Fig. 4.26 Crack pattern for beam C1D

70


71

Fig. 4.28 Beam C3D after failure

72


73

Fig. 4.30 Beam I1D after failure

74

Fig. 4.31 Crack pattern for beam I1D

75

Fig. 4.32 Beam I2D at ultimate failure

76

Fig 4.33 Crack pattern for beam I2D

76

Fig. 4.34 Crack pattern for beam I3D 77

List of Figures

xiv

Fig. 4.35 Beam V1D at ultimate failure

78

Fig. 4.36 Crack pattern for beam V1D

78

Fig. 4.37 Beam V2D after failure

79


80

Fig. 4.39 First bending crack on the right side of beam V3D

81

Fig. 4.40 Beam V3D at ultimate failure

81


82

Fig. 4.42 Typical timber block position at initiation of test

84

Fig. 4.43 Force against Displacement for 450 series

85


85


86

Fig. 4.46 Pull-out samples prior to testing

87

Fig. 4.47 Typical pull-out failure

88

Fig. 4.48 Sample 45-2

89


89


89


90


90


90

Fig. 4.54 (a) Bond stresses in pull-out testing, (b) Bond stresses in structural elements

92

List of Figures

xv

Fig. 4.55 I1N Rebar 1

96

Fig. 4.56 V1N Rebar 1

97

Fig. 4.57 (a) & (b) V2N Rebar 2, (c) V3N Rebar 1

98

Fig. 4.58 I1D Rebar 1

99

Fig. 4.59 I3D Rebar 1

99

Fig. 4.60 V1D Rebar 1

100

Fig. 4.61 V3D Rebar 1

100

Fig. A.1 Rebar marking

113

Fig. A.1.1 C1N Load against Time

113


114


114

Fig. A.1.4.a I1N Load against Time

115

Fig. A.1.4.b I1N Tensile forces in Rebars against Time

115


116


116


117


117

Fig. A.1.7.a V1N Load against Time

118

Fig. A.1.7.b V1N Tensile forces in Rebars against Time

118


119

Fig. A.1.8.b V2N Tensile forces in Rebars against Time 119

List of Figures

xvi


120

Fig. A.1.9.b V3N Tensile forces in Rebars against Time

120

Fig. A.1.10 C1D Load against Time

121


121


122

Fig. A.1.13.a I1D Load against Time

123

Fig. A.1.13.b I1D Tensile forces in Rebars against Time

123


124


124


125


125

Fig. A.1.16.a V1D Load against Time

126

Fig. A.1.16.b V1D Tensile forces in Rebars against Time

126


127


127


128


128

Fig. A.2.1 Stereoscope images of sample 45-2

129


130


131

Fig. A.2.4 Stereoscope images of sample 60-5 131

List of Figures

xvii


132


133

Fig. B.1.a Bending and shear stresses of a rectangular beam

134

Fig. B.1.b Points considered in the calculation of principal stresses (dimensions are in millimetres)

135

Fig. B.1.c The conversion of stresses to principal stresses for a point (in bending compression) above the Neutral Axis

136

Fig. B.6 Test Rig Setup 166

xviii

List of Tables

Table 2.1 Characteristic strength values of designation C timbers (extracted from EN 338:2003)

7

Table 3.1 Properties of GFRP rebars (refer also to Appendices B.3 and B.4)

31

Table 3.2 Details of the tested configurations

32

Table 3.3 Dimensions and surface moisture of the nine timber beams used with their respective marking at each end (all dimensions are in millimetres)

34

Table 4.1 Average ultimate loads and variance of tested timber beams

49

Table 4.2 Failure modes of the tested timber beams

83

Table 4.3 Pull-out test results at ultimate

86

Table 4.4 Pull-out average test results and variance for each tested series at ultimate

87

Table 4.5 Ultimate bond forces for GFRP rebars as used in the full-scale beam configurations

93

Table 4.6 Forces in rebars at ultimate failure of beam

93

Table 4.7 Rebar bond failures in full scale beam loading test specimen

94

Table 4.8 Opened up rebars from the full scale beam specimens

95

Table B.1.a Computations of principal stresses together with principal directions from the quoted equations

137

xix

Abbreviations and Notation

SFD Shear Force Diagram

BMD Bending Moment Diagram NA Neutral Axis

A = area of shaded cross-section A

= area of cross-section

b = width of rectangular cross-section d = depth of rectangular cross-section I

= moment of inertia

L = rebar embedment length

M

= bending moment applied at a cross-section

V

= shear force applied at a cross-section

x = distance of a point along the span from the support y = distance of a point at a cross-section from the neutral axis

= principal plane direction

xy = horizontal and vertical shear stress in a rectangular beam at a point x = bending stress at a point

1 = tensile principal stress

2 = compressive principal stress

b = bond stress

= diameter

1

Chapter 1 - Introduction

1.1 Introduction

Timber is one of mans oldest used materials. Its long history is due to it being a

natural material, and often easily sourced from nearby locations. Timber was

employed by man to serve several purposes, such as to build boats, in

construction of houses, furniture and paper making. Several wood products have

been developed in recent history which made its use more widespread.

The use of timber in construction throughout history has been an extensive one.

Timber offered possibilities of building forms which were difficult to construct by

other building materials, namely stone. Large span timber beams were employed

in large span roof structures, such as at the Parthenon in Greece and Roman

basilicas. Larger spans were later achieved by using two rafters connected by a

cross beam. Timber beams were also used locally to support stone slabs at

storeys where a stone arch could not be constructed because of the side thrusts

produced. An interesting composite timber beam design was carried out by

Leonardo. This design involved the use of four pieces of timber connected together

by the use of dowels (Tampone, 1996).

Chapter 1 Introduction

2

Developments are a characteristic of mankind. In his nature man tries to improve

on what he already has. Further developments could supersede other practices

previously employed by man. In fact during the last two centuries, the use of

traditional materials such as timber saw a decline, with the advent of new materials

such as steel and concrete. Enhanced properties and reliability over those offered

by traditional materials made them more attractive. In this context, therefore, one

could argue that methods of strengthening timber elements could place timber

materials on a platform to compete with more advanced construction materials.

In addition since timber was a widely used material throughout history, timber is

found in buildings subjected to various forms of degradation, giving rise to the need for repair. Several repair methods exist, and these can be mainly classified into

traditional methods, including scarf joints, tenons and dovetails, mechanically- fastened methods, including bolted metal side plates, flitch beams and bolted

joints, and adhesive methods, including various epoxy resin formulations with the use or not of metallic or non-metallic reinforcement (TRADA, 1992). Rehabilitation of timber structures is not only required when damage is inflicted to the timber

element but also to extend service life of a structure or to cope with increasing

loads.

It was against this background that it was decided to study the effectiveness of

specific reinforcement configurations, as applied to new timber beams and to

damaged timber beams. Small diameter Glass Fibre Reinforced Polymer (GFRP) rebars were inserted vertically (or quasi-vertically) to the bending axis of the timber beams to resist horizontal shear displacement. GFRP rebars thus acted as dowels

between the top section and bottom section of the timber beams. Insertion of


3

dowels tends to result in greater beam stiffness and strength, reduced weight-to-

strength ratio, reduced end-grain splitting, are aesthetically discrete which is of an

advantage especially in the case of conservation projects, provide greater ease and speed to prepare and install, and are capable of transferring high local

stresses.

1.2 Main Objectives and Structure of this Dissertation

In this dissertation, the use of 6mm GFRP rebars for the strength enhancement of

timber beams in the shear zone will be investigated. Full scale beam loading tests

will be carried out. The spacing of the GFRP rebars was taken as equal to the

beams depth, following a recommendation made by Svecova and Eden (2004).

The variables include the inclination angle of the GFRP rebars with respect to the

horizontal, and the beam condition, being either new or damaged.

The two angles considered are the 900 and the 600 angles with the horizontal. The

reason for choosing these two angles, rather than lower angles, is due to the

expected stress trajectories being more vertical, because of higher shear forces resulting from the choice of a short span, and the fact that the load point is close to

the support. Part of the action of the GFRP rebars action is expected to be tension,

but they will also resist shear in the horizontal direction.

The same configurations of GFRP rebars will be used on both new timber beams

and damaged timber beams. The selected damage was induced by a horizontal

cut at mid-depth of the beam cross-section along the shear span. This damage

simulates horizontal splits which are quite common to timber members especially


4

when they are subjected to wetting and drying. This type of damage was reported by Arda Akbiyik (2005) to be the most commonly encountered type of damage in timber stringers taken from timber bridges in the United States.

Compared with the diameters used by others, a 6mm diameter GFRP rebar is quite

small, however it was felt that it would still be useful for shear enhancement of

timber beams.

A pull-out test of the GFRP rebars from timber blocks, with varying grain angle with

respect to the insertion direction of the GFRP rebars, will also be carried out, to

investigate the bond between the GFRP rebars and the timber blocks for the

adhesive used.

This dissertation is organised in the following manner. Chapter 1 provides a brief

introduction to this study. Chapter 2 consists of a literature review, in which, among

other topics, research on the use of dowels for shear enhancement of timber

beams was reviewed in detail. Chapter 3 presents the experimental methodology

adopted in this study. In chapter 4, the results and their analysis are presented.

Finally in Chapter 5 the conclusions are presented together with recommendations

for future work.

5

Chapter 2 - Literature Review

2.1 Overview

This literature review is divided into three main parts. The first part is a brief section

in which an overview is given of papers wherein the shear strength of timber

beams, and the difficulty of its accurate determination, were addressed. The

second part, which is the main part of the literature review, reviews briefly the need

for shear enhancement of timber beams, including a description of several shear

enhancement methods for timber beams researched, and some properties of Fibre

Reinforced Polymers (FRP) studied. The last section of the second part reviews, in more detail, research on shear enhancement methods using Glass Fibre

Reinforced Polymer (GFRP) rebars and Carbon Fibre Reinforced Polymer (CFRP) fabric or laminates. Research on GFRP rebars, used to increase the flexural

stiffness of deteriorated timber beams by improving the interlayer horizontal shear

transfer, was also reviewed. The third and last part of this chapter reviews some

other considerations, such as the assimilation of shear enhancement methods by

the use of dowels to dowel-like shear connections, and the issue of bond.

Chapter 2 Literature Review

6

2.2 Shear strength of timber beams

One of the most important properties of structural timber is that of shear strength.

However the determination of the shear strength of timber is not simple as many

variables are involved, to some of which variables is attributed a lot of uncertainty.

Timber mechanical properties vary with grain direction, wood species, locality from

which the wood was obtained, density, moisture content, temperature, the rate and

duration of loading, size, the presence of natural defects and their location

(including slope of grain, knots, checks, splits and shakes), rot or decay, and other anatomical features such as cell length, and the occurrence of tension or

compression wood.

Some of these variables may cause the shear strength to become critical. A good

example of such criticality is that created with the presence of checks and splits

from uneven drying, especially if their location is in close proximity to the position of

the neutral plane of the structure (Akbiyik, 2005). They act as planes of weakness where shear strength is mostly required (Bodig and Jayne, 1982). Shear strength is not something separate from bending strength. The presence of discontinuities

also affects the moment capacity of timber sections, since the moment of inertia of

a cross-section is reduced noticeably. The forming of checks and splits and up to

which degree they form are difficult to predict especially if the environment is

uncontrolled.

With all these variables affecting the shear strength of timber beams, and the

difficulty to quantify them, a lot of uncertainty results. In 2006, Denzler and Glos

argued that no test method was available that covered all factors influencing shear


7

strength. It was concluded that the test method proposed in EN 408 does not cover

all the factors influencing shear strength. It was pointed out that a disadvantage

with this test method is that it involves small specimens.

Studies were also carried out to determine the shear strength of timber beams as

opposed to small-scale shear testing on timber samples. It was found that the

longitudinal shear strength of beams was lower than the shear strength obtained

from small clear block tests and that beams with a larger cross-sectional area have

lower shear strength (Rammer et. al., 1996). In 2005, Akbiyik commented that the size effect apparent in experimental studies has not yet been reproduced in finite

element analysis (Foschi and Barrett, 1976; Longworth, 1977; Rammer and Lebow, 1997; Cofer et al., 1997; Lam et al., 1997; as referred to by Akbiyik, 2005). In the determination of shear strength of timber beams uncertainty remains.

Table 2.1 is an extract from EN 338 of characteristic strength values of designation

C timbers.

Strength properties (N/mm2)Bending fm,k 14 16 18 22 24 27 30 35 40Tension parallel ft,0,k 8 10 11 13 14 16 18 21 24Tension perpendicular ft,90,k 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.4Compression parallel fc,0,k 16 17 18 20 21 22 23 25 26Compression perpendicular fc,90,k 4.3 4.6 4.8 5.1 5.3 5.6 5.7 6.0 6.3Shear fv ,k 1.7 1.8 2.0 2.4 2.5 2.8 3.0 3.4 3.8

C30 C35 C40

Species type

Strength class

Poplar and conifer species

C14 C16 C18 C22 C24 C27

Table 2.1 Characteristic strength values of designation C timbers (extracted from EN 338:2003)


8

2.3 Shear Enhancement of Timber Beams

2.3.1 The requirement for shear enhancement

Timber can be very attractive as a constructional material. In addition, from a

sustainability point of view, timber is attractive, not least because it can be grown

close to the location of use. However, some difficulties which might limit its use do

exist. One limitation is its poor strength perpendicular to the grain, which may result

in low shear resistance parallel to the grain (Triantafillou, 1997). The presence of checks and splits further reduce timbers shear resistance. For these reasons

shear strength enhancement could be useful. Other reasons where shear strength

enhancement is relevant include situations where it is desired to extend the service

life of a structure or to cope with increasing loading levels, for important

conservation projects or to make timber structures more competitive, when compared with other constructional forms, whilst reducing the variability, and thus

the uncertainty, involved in the behaviour of timber elements. Shear enhancement

may be required in beams loaded close to supports, at the occurrence of drilled

holes and cut-outs and also when the bending capacity of timber beams is

enhanced (Triantafillou, 1997; Buell and Saadatmanesh, 2005).

2.3.2 Research in shear enhancement techniques

Research has been carried out on several techniques to increase the capacity of

timber beams both in bending and in shear. In this section some research that has

been carried out on some techniques for the shear enhancement of timber

members will be mentioned. Some of these techniques are applicable both to new,

as well to existing, timber structures.


9

Some early studies in shear reinforcement of timber involved the use of steel

plates, aluminum plates or light gauge steel inserted vertically, either between

selected vertical laminations, on the sides, or between lumber bonded by resins

(Sliker, 1962; Stern and Kumar, 1973; Stern and Kumar, 1973; as referred to by Triantafillou, 1997).

Studies on timber reinforced with FRP materials are limited (Triantafillou, 1997; Alann Andre, 2006). This may be due to the fact that shear failure mode is a less common failure mode than bending failure (Alann Andre, 2006). Some of the studies making use of FRPs include the reinforcement of glulam beams in

proximity to circular holes, and the enhancement of the shear strength of curved

and cambered glulam beams (Blom and Backlund, 1980; Larsen et al., 1992; Hallstrom, 1995; as referred to by Triantafillou, 1997 and by Svecova and Eden,

2004). In 1997, Triantafillou conducted experimental research using FRP sheets externally bonded to the shear critical zones of timber beams. In 2000, Johns and

Lacroix used GFRP sheets which were applied in a U-shaped manner up the sides

of the beam in two layers (as referred to by Amy and Svecova, 2004). In 2004, Svecova and Eden studied the behaviour of GFRP bars for the shear and flexural

enhancement of timber beams. A continuation of this study was published in the

same year by Amy and Svecova, with the application of GFRP bars to dapped

timber beams. In 2005, Buell and Saadatmanesh studied the behaviour of fabric

wraps or laminate strips on long and short spans. Some of these techniques will be

viewed in detail.


10

2.3.3 The case of using Fibre Reinforced Polymers (FRPs) FRPs offer an attractive option to be considered in construction. They are

lightweight, requiring no heavy-duty equipment during installation thus helping to

keep labour costs down, and site constraints minimal. They have a high strength-

to-weight ratio, and are especially strong in tension. However when compared to

steel, FRPs have a lower elastic modulus, leading to greater deflections in

elements reinforced for flexure. Brittle failure is exhibited by FRPs, as they behave

linearly elastic up to the breaking point. Ongoing work seeks to achieve a more

ductile failure of FRP bars, by combining fibres of different ultimate strain, and

orientation, in the reinforcement (Somboonsong et al., 1998 as referred to by Bakis et al., 2002).

FRPs may offer an effective solution to steel durability problems, where an

improved corrosion resistance is required, and where the electrical and magnetic

properties of steel are undesirable (Balendran et al., 2002; Bakis et al., 2002). However FRPs may deteriorate by the diffusion of moisture and other chemical

solutions. Glass fibres may experience serious durability problems, when subjected to alkaline environments, such as in concrete or in high temperature environments

(Tannous and Saadatmanesh, 1999; Katz and Berman, 2000; Pisani, 1998; Kumahara et al., 1993; Sen et al., 1993; Katsuki and Uomoyo, 1995; as referred to

by Balendran et al., 2002). An alternative glass fibre, used to improve performance in alkaline environments, is alkali-resistant glass. Thermosetting resins, widely

used in FRP matrices, have durability disadvantages. If heated, thermosetting

resins will not regain their original strength when cooled. In addition, the re-shaping

of FRPs, made of thermosetting resins, is not possible after production, since then


11

they will not regain their original strength after re-shaping. The use of thermoplastic

resins is currently under consideration as an alternative.

FRPs can be produced by several processes, of which the most common process

used for commercially available FRP rebars is pultrusion. They can be produced in

various forms, and can be used in the interior, near surface or surface of the main

structure. Several surface deformations are applied to FRP rebars to enhance their

bond characteristics, by providing a better mechanical interlock. FRPs are

orthtotropic materials, and are fabricated in one-dimensional or multi-dimensional

shapes. Of the latter, two-dimensional orthogonal grids are the most common.

One major disadvantage with the use of FRPs is their high cost when compared to other materials.

There are several possibilities of using FRPs in timber structures. They can be

used with various timber elements or types, including trusses, solid-sawn timber,

glulam, engineered timber products or even in connections of timber elements.

FRP reinforcement can be used to strengthen or re-strengthen and repair, either

globally or locally to a structure. Applications of prestressed FRP to timber have

also been studied (Steiger).

Significant increases in strength and stiffness can be achieved by the use of

metallic reinforcement; however other problems are encountered due to the

incompatibilities between the wood and the metal (Dagher and Lindyberg, 2000; as referred to by Amy and Svecova, 2004). These differences include the different hygro-expansion, and the large stiffness difference of wood and the metallic

reinforcement, and can lead to separation or tension failure at or near the glue line


12

(Amy and Svecova, 2004). An inferior bond performance between steel dowels and the timber when compared to GFRP dowels bonded in timber, was commented

upon by Svecova and Eden (2004) when conducting research of using rebars as shear enhancement of timber beams.

2.3.4 Research using GFRP rebars as shear enhancement of

timber beams

GFRP rebars are used for structural strengthening. Their use in providing flexural

and shear reinforcement has been researched, with the former being more

common. Their use is not only being explored in connection with concrete

structures as a possible substitute to steel reinforcement but also as a possible

strength enhancement method for timber beams. This section looks at research

work including shear enhancement alone, and a concurrent use of shear and

flexural enhancement by the use of GFRP rebars.


13

(a)

(b)

(c)

(d) Fig 2.1 Schematic of reinforced timber beam test configurations carried out by Svecova and Eden

(2004), (a) dowels in the shear span only, (b) dowels throughout the beam span, (c) dowels in the shear span and flexural reinforcement in the constant moment region, (d) dowels and flexural reinforcement both throughout the beam span.

Svecova and Eden (2004) carried out studies wherein the load carrying capacity of timber beams, in both shear and flexure, was increased by the use of GFRP rebar

dowels (16mm in diameter, 255mm in length), and near-surface-mounted GFRP


14

rebars (5mm in diameter) respectively (fig. 2.1 a, b, c, d). The timber beams used had some weathering damage as they were cut from Douglas Fir bridge stringers

which had been in construction for around 40 years. Four point bending tests were

carried out according to ASTM D198-99.

The following variables were studied: dowel spacing (spacing equal to half beams depth and to beams depth), the effect of the flexural reinforcement used together with the dowel reinforcement, the span along which the reinforcement was installed

(shear span, constant moment span and beam span) and the reinforcement material. Only one test was carried out using steel dowels (12mm in diameter, 255mm in length); for all the other tests GFRP rebars were used.

Beams, reinforced with dowels only, experienced an increase in the Modulus of

Rupture (MOR) in the range between 17% to 25% for configurations as in fig. 2.1a, and 33% to 35% for configurations as in fig. 2.1b. The introduction of dowels

changed the failure mode from cross-grain tension, or horizontal shear failure, to

simple tension at mid-span for beams as in configuration fig. 2.1a. For beams

configured as in fig. 2.1b, the mode of failure remained simple tension at mid-span,

but was arrested between two shear dowels. It was apparent that the avoidance of

tension failure enhances the performance of timber beams. This was also expected

from a previous research carried out by Gentile et al. in 2002.

Beams reinforced in both shear and flexure experienced an increase in the MOR in

the range varying between 47% and 52%. The predominant failure mode for

beams configured as in fig. 2.1c, remained tensile at mid-span, as the flexural

reinforcement was not long enough to bridge the defects in the tension zone. For


15

beams configured as in fig. 2.1d, the predominant failure mode shifted to a

compression failure.

The failure mode for reinforced beams changed from a sudden brittle one to a type

of failure mimicking a more ductile failure. Load was transferred to the GFRP

reinforcement after the timber had initial cracking.

The GFRP reinforcement increased the ultimate load capacity of timber beams.

The highest ultimate load reached by the control beams became an average for

beams reinforced with dowels only and minimum for beams reinforced with both

dowels and rebars along the span length. In addition, with increasing

reinforcement, less variability was apparent in the ultimate load capacity of a

group. The ductility increased, larger load levels were accompanied with larger

deflections, allowing for ample time of warning. With reduced variability, a less

conservative approach to timber design can be reached.

The GFRP reinforcement used changed the behaviour of the beam to that similar

to a truss. The tension chord members and vertical members were made of GFRP,

and the diagonal members and the compression chords were made of timber. This

system exploits the best characteristics of both materials used, timber having a

high compressive strength parallel to the grain, while the GFRP has a high tensile

strength. The success of this system then depends on the bond between the two.


16

(a)

(b) Fig 2.2 Schematic of reinforced timber beam test configurations carried out by Amy and Svecova

(2004) (a) GFRP rebars as flexural reinforcement between dapped ends, (b) GFRP rebars as flexural reinforcement between dapped ends and as dowels inclined at 300 to the vertical

Amy and Svecova (2004) continued on previous research carried out by Gentile et al. (2002) and Svecova and Eden (2004). Douglas-fir timber beams that had been in construction were used, with the main difference that they had a dapped end.

The tests were carried out under monotonic loading, in three-point bending, with

the point load applied at mid-span point.

Testing configurations are shown in fig. 2.2 a, b, together with control beams which

were visually graded to be of superior quality. In order to take advantage of the

high tensile strength in the longitudinal direction of the pultruded GFRP rebars,

(12mm in diameter), the bonded length of the GFRP dowels was increased by inclining them at an angle of 600 to the horizontal. This angle was aimed to

increase dowel resistance, while limiting the drilled length for ease of installation.


17

For the control beams, dap and horizontal shear failure modes, starting from the

dapped portion and continuing to the mid-span, dominated. For beams configured

as in fig. 2.2a the behaviour was of the same order as the control beams. Two

reasons could account for this. Flexural reinforcement did not affect dap failure,

and that the control group consisted of timber of a higher grade. Some of the

beams reinforced for flexure were able to attain larger deflections, and to sustain

some loading after first cracking.

The beams reinforced as in fig. 2.2b experienced a 22% increase in the ultimate

load compared with the control beams. This estimate is conservative, since the

beams used for this configuration were of a much lower grade when compared to

the control. These specimen sustained larger deflections, resulting in higher

ductility. Dap and shear failure modes did not dominate, even though horizontal

splits were evident during testing. The splits and dap failure were arrested by the

dowel bars. Failure modes, such as compression perpendicular to the grain in the

compression zone, and bearing under the loading point or at the support, occurred,

all being stronger modes of failure.

2.3.5 Research using other FRP types for shear strengthening of

timber beams

Triantafillou (1997) conducted research study in the use of CFRP fabric or laminates bonded to the sides of timber beams in the shear-critical zones (fig. 2.3). An effort was made to sample small uniform and clear specimens without defects

to reduce the uncertainties involved with timber mechanical properties. The beams


18

were designed to fail in shear by reducing the width of the beams in the shear-

critical zones.

Fig 2.3 Schematic of reinforced timber beam test configuration carried out by Triantafillou (1997)

An analytical method that transforms the FRP fabric or laminate to an equivalent

timber section was proposed. It resulted in very close agreement to the

experimental results, with a slight overestimation. Analytically it was found that

shear capacity increases with increasing FRP cross-sectional area and FRP

Youngs Modulus in relation to those of the timber section, and with decreasing

ratio of the vertical height of the FRP to that of the timber member. A lower bound

is needed to this last condition to avoid timber shear failure in the unreinforced

section from occurring before timber shear failure in the reinforced section.

The variables looked at include the fibre direction (either horizontal, vertical or a combination of both), the number of CFRP layers (either one or two), and the ratio of the vertical height of the FRP to that of timber (either 1 or 0.6).

From the experimental results, it was observed that FRP reinforcement increased

the shear capacity. The FRP material use could be optimised for a given shear

capacity enhancement by placing the fibre direction horizontally, and by using an

FRP vertical height slightly larger than the minimum limiting value for which FRP

failure occurs before timber failure. Higher differences between the experimental


19

and analytical approaches were mostly observed when using two layers of CFRP

fabric.

(a)

(b) Fig 2.4 Schematic of reinforced timber beam test configurations carried out by Buell and

Saadatmanesh (2005) (a) CFRP fabric with its longitudinal direction parallel to the longitudinal direction of the beam, (b) CFRP fabric with its longitudinal direction perpendicular to the longitudinal direction of the beam overlapped on the sides and on the top of the beam

Buell and Saadatmanesh (2005) researched the use of CFRPs in the form of bi-directional fabric wrap, and laminate strips, to investigate whether they would

increase the bending strength, shear strength and stiffness of timber beams. Both

flexural tests and shear tests on structural beam sizes were carried out. Shorter

beams were used for the shear tests, and the shear span-to-depth ratio was within

the limits suggested by ASTM D 198.

For the shear tests, two control specimens were tested, as one of them had fewer

defects than the rest; it gave very strong results in horizontal shear. In fact the

beam reinforced as in fig. 2.4b did not exhibit horizontal shear strength


20

enhancement when compared to the stronger control beam. The beam reinforced

as in fig. 2.4a recorded a horizontal shear strength increase of 68% when

compared to the weaker control beam; in this case both cut from the same original

timber beam. Increases in the deflection ductility were also recorded.

The increase in horizontal shear strength was an important result, since many

timber bridges are structurally inefficient, because of insufficient strength in

horizontal shear. It was concluded that the carbon fabric reduces the effects of

defects present in timber, and thus it allows the strength of timber beams to

approach the strength of timber beams without defects.

2.3.6 Research using shear spikes to increase bending stiffness

Research has been conducted on the enhancement of bending stiffness of

deteriorated timber beam elements, by the use of pultruded glass fibre rebars

known as shear spikes (or Z-spikes) (Radford et al., 2000; Schilling et al., 2004; Burgers et al., 2005; Gutkowski and Forsling, 2007; and Gutkowski et al., 2008). The ultimate goal of this research programme, (which includes other studies not mentioned here), was to find a repair method that is easy to apply to full-scale bridges, without interrupting railroad operation. Radford et al. (2000) initiated this research on small-scale timber beams. Research then proceeded on full-scale

timber beams and on full-scale bridge chord members.

The main idea behind this technique is to enhance the interlayer shear

performance of deteriorated timber beams by bridging deteriorated regions with

sound material, and thus improve their flexural stiffness. For this purpose, shear

spikes were inserted in a direction perpendicular to the primary bending axis.


21

When timber beams to be tested were deemed to be of good quality, intentional

horizontal cuts were induced at mid-depth of the beams to simulate damaged

beams. These cuts were generally located between points of load application and

supports.

Experimental investigation involved mainly flexural load testing. Other tests such

as cyclic loading and resin shear strength testing were carried out.

The process of shear spiking involved the cutting of glass fibre rebars to small

lengths. Their leading edge was then shaped to a sharp point by using an angle

grinder. Holes were drilled in timber beams at the chosen points of application with

a diameter slightly larger than that of the spikes. Shear spikes were then driven

into these holes by a dead blow hammer, to minimise the risk of splitting the ends

of the spikes. This process was facilitated by the pointed edge. This pointed edge

was also deemed to avoid that epoxy resin, on the side of the hole, being scraped

off during the installation of the shear spike.

The initial flexural stiffness of a timber beam was measured by non-destructive

load testing, and by collecting load-deflection data. In many of these studies, shear

spikes were installed incrementally in pairs, and the flexural stiffness was recorded

at each stage. It was observed that the main increment in flexural stiffness

occurred after the insertion of the first pair at each respective beam end.

It was generally observed that the effectiveness of the method depended on the

deterioration degree of the timber beam, with the highly deteriorated beams

showing the most potential for repair. The flexural stiffness in the undamaged state

seems to be an upper limit of the stiffness that can be regained by this method


22

(Gutkowski and Forsling, 2007). An insertion of a shear spike where it was not needed, left a decayed void without repair. It was concluded therefore that repair is

related to the location and number of shear spikes. The use of epoxy combined

with shear spikes was highly effective (Radford et al., 2000). Similarity between small scale beams and full scale beams testing was observed.

When load testing was carried out to ultimate failure, it was observed that the

predominant failure mode was flexure, signifying a failure in the timber rather than

in the shear spike system (Gutkowski et al., 2007). Other observations include the following. Epoxy resin formed a better bond with

wood, resulting in better strength than with polyester resin. The bond was also

improved by lightly sanding the spikes, and by using a slightly oversized hole than

previously used. Fibreglass grindings used with the epoxy mixture resulted with

better fill-up of timber voids, while the strength of the epoxy was not compromised.

(Miller et al., 2008)

2.4 More considerations

2.4.1 Shear Connections

In 2005, in a study on shear repair of timber beams, Akbiyik tried several repair

methods using long hex bolts and lag screws. Beams with splits were tested to

determine the residual strength, and checked beams were tested to shear failure.

Both types of beams were then repaired. All beams were then tested to failure to

determine the effectiveness of a repair method. The effectiveness was determined

by comparing the unstrengthened post-failure capacity of original beam to ultimate

failure capacity of repaired beam.


23

Fig 2.5 Single shear connection modelling the use of hex bolts and lag screws

Considering these beams as having a complete discontinuity, Akbiyik compared

the repair methods to dowel-like shear connections as shown in fig. 2.5. The aim

was to reach the ultimate failure load that would be reached by an undamaged

timber beam. The researcher used The American Forest and Paper Association

(AFPA) mechanical connection concepts, to obtain the number of hex bolts or lag screws required. These guidelines provide different yield failure modes from which

the dominating yield failure mode was chosen in design. From the results obtained

it can be seen that better mathematical models are needed to predict the capacity

of such repair methods as the ultimate failure load predictions were not reliable in

most of the cases.

2.4.2 Bond Strength

The research work reviewed in this section is generally concerned with direct pull-

out testing of bonded-in rods. The design issues and performance requirements

are not the same as for rods used in shear enhancement of timber beams,

however these studies give us a good idea of the main issues involved with bond

strength. Many joint characteristics are common to both steel rods and FRP rods (Broughton and Hutchinson, 2001).


24

Bond increased with increasing embedment length, and in many cases with

increasing bond line thickness, but this depended also on the adhesive type

(Connolly and Mettem, 2003; Broughton and Hutchinson, 2001; Felligioni et al., 2003; Harvey and Ansell). A larger bondline thickness resulted in a reduced peak shear stress in the adhesive, corresponding to an increase in the experimental

failure load (Broughton and Hutchinson, 2001). The joint thickness did not only affect pull-out strength, but also affected failure mode. For example, it was

observed that a lower glue thickness resulted in wood failure with a shift towards a

glue-steel failure with higher glue thicknesses (Felligioni et al., 2003).

Bond improved with larger adhesive shear strength and tensile modulus. Adhesive

types also affected the failure modes. It was observed that epoxy adhesives

generally led to timber failures, close to, and along, the adhesive/timber interface,

while other types of adhesive (acrylics, polyurethane and phenol-resorcinol-formaldehyde) led to adhesive failure or adhesion failure at the adhesive/timber interface. The latter corresponded with lower pull-out strengths. Epoxy has better

gap-filling qualities. (Broughton and Hutchinson, 2001)

In pull-out testing, the peak shear stress is also a function of end-constraint, which

is the hole diameter in the base plate, against which the pull-out is made

(Broughton and Hutchinson, 2001).

Joint design can be arranged in such a way to increase stress transfer always

keeping in consideration the failure mode. One can try to deal with a dominating

failure mode for a particular joint design to further increase strength. In improving


25

bond, one should keep in mind that bond can be of two main types, mechanical

and chemical. Several methods to enhance both types of bond exist.

26

Chapter 3 Experimental Methodology

3.1 Overview

As pointed out in Chapter 1 the experimental programme of this study consists in

the testing of full scale beams loading and pull-outs. This chapter is organised in

the following manner.

Firstly, a brief discussion on principal stresses is made. Then the experimental

method adopted for the full scale beam loading test is explained. This consists in

the test setup adopted, a description of the materials used, a description of the

testing configurations, the preparation procedures and the testing procedures.

Lastly the experimental method adopted for the pull-out tests is presented in a

similar way to that of the full-scale beam loading tests.

For the pull-out test a bond length of 100mm was tested which is approximately

half the length of GFRP rebars used for the full scale beam loading test. The same

bondline thickness, and materials were used as well.

Some additional information is given in Appendix B.


27

3.2 Full Scale Beam Loading Test

3.2.1 Shear stresses

A beam element is inevitably subjected to both flexure and shear. It can be loaded in such a manner so that it will be more likely to fail in shear than in flexure. If the

shear span length (marked a in fig. 3.2 a, b) is low, the ratio of applied shear load to moment increases. ASTM D 198-99 states that timber beams with a shear span-

depth ratio less than 5 are most likely to fail in shear. When the shear span-depth

is low, the applied load is close to the support and the principal stresses within the

region are rotated to close to 450 to the horizontal. Recalling Mohrs Circle of

stresses reminds that in these circumstances, vertical and horizontal shear

stresses are close to maximum. These principal stress lines can be considered as

the load paths through which the forces in a structure flow (fig. 3.1). The computation of principal stresses and their direction shown diagrammatically in fig.

3.1 is shown in Appendix B.1.

Although the test setup used in this experimental programme is slightly different

from that recommended by ASTM D198-99, the effect of the applied force is still

the same as can be observed in fig. 3.2 a, b, in the sense that a large shear force

results in the region between support and point load.


28

Fig 3.1 Principal stresses and principal directions of test setup used (the magnitude of the arrows are indicative of the stress magnitude)

(a) (b)

Fig 3.2 (a) The test setup as recommended by ASTM D 198-99, (b) The test setup as used in this experimental programme


29

3.2.2 Test Setup

Testing of full scale timber beams was carried out under a three-point loading test

setup, as shown in fig. 3.3 (refer also to Appendix B.6). All beams spanned 1500mm between simple supports, with a point load applied at a distance 500mm

away from the support. The shear span-depth ratio adopted was equal to 2.5,

which followed the recommendation made in ASTM D198-99, of limiting the shear

span-depth ratio to 5 for timber beams, so that they would be likely to fail in shear.

When one end of a timber beam was tested, the beam was inverted by 1800 on

plan, and then the other end was tested in a different test. This procedure made it

possible that, with limited resources, more results could be obtained, since from

every one timber beam, two results were obtained instead of one.

EN 408 recommends that testing of full-scale beams should take 300 seconds +/-

120 seconds to reach ultimate failure. An assumption that the timber grade was

C16 was made. Following Eurocode 5 design equations for shear and bending,

without safety factors, it was predicted that the ultimate failure load was equal to

about 64kN (Appendix B.2). On this basis the loading rate used was of 5kN every 30 seconds. During testing, it was found that the ultimate failure load was much

higher than 64kN, but nevertheless the loading rate was kept as 5kN every 30

seconds.


30

Fig. 3.3 Schematic of the test setup (dimensions are in millimeters)

3.2.3 Materials

3.2.3.1 Timber

The timber beams used in this study were sourced from local supplier Joseph

Caruana Co. Ltd. and imported from Austria. The beams were made of larch wood

(Larix deciduas, known locally as red deal or ta l-ahmar), a softwood. No certification of the timber beams quality was available. However BS EN 1912: 2004

indicates that this species can have a grade of C30, C24 or C16. The grading of

the timber beams quality was not considered to be important as comparison of the

performance of the reinforced timber beam configurations was made with that of

the control beams. Larch wood is a moderately heavy timber, with density being in

the range between 480 and 640 kg/m3 when dry (Patterson 1988).

The beams were stored in a private garage for about three months after being

bought and then transported to the Civil Engineering Laboratory at the Faculty for

the Built Environment at the University of Malta, about a month before testing

commenced.

The beams nominal cross-section was 200mm by 200mm.


31

3.2.3.2 Reinforcement

Aslan 100 GFRP rebars of 6mm rebar diameter (6.35mm nominal diameter) were used. These are manufactured by Hughes Brothers, Inc., USA and were supplied

by J.M.V. Ltd.

Aslan 100 GFRP rebars are made up of E-glass fibres in a vinyl ester matrix. The

surface of Aslan 100 GFRP is finished by helically over-wound fibres, and a sand

coating to enhance bond. Some properties are given in table 3.1 and are those

quoted from the manufacturer.

Bar Size (mm)

Cross Sectional Area

(mm2) Shear

Strength (MPa)

Tensile Strength

(MPa) Tensile Modulus of

Elasticity (GPa)

6 31.67 152 825 40.8 Table 3.1 Properties of GFRP rebars (refer also to Appendices B.3 and B.4)

3.2.3.3 Adhesive

Sika AnchorFix-2, a two-part epoxy-acrylate adhesive was used to fix rebars. Its

compressive strength is quoted by its manufacturer as being 60N/mm2 tested

according to ASTM D695 (refer also to Appendix B.5). A pull-out test was carried out to investigate the bond strength developed with the wood and with the rebars.

This adhesive was applied by a gun which facilitates the filling up of holes made to

receive the rebars.

3.2.4 Testing Configurations

In this research nine timber beams were tested. Each beam was tested twice in

two separate three-point loading tests. The variables for this research were the

angle of the GFRP rebars with the horizontal and the timber beam condition.


32

Details and diagrams of the configurations that were tested are shown in table 3.2

and in fig. 3.4. One 6mm diameter GFRP rebar was installed in the centre of the

beams width at the positions shown in the elevations of fig. 3.4. Each configuration

was tested three times to obtain certain statistical reliability from the test results.

Strain gauges were fixed to each GFRP rebar at the centre of their length at which

position the tensile stresses were expected to be maximum due to the highest

bonded length. Tensile stresses were not expected to be large, because of the

very short bonded length, which is a problem characteristic of shear reinforcement.

The GFRP rebars were expected to act as dowels, resisting horizontal shear

displacement, as is likely in timber beams, because of their orthotropic nature.

Beam End

Series

Beam End Mark

Reinforcement Spacing of Reinforcement

Angle of Reinforcement with Horizontal

Timber Beam End

Condition

C1NC2NC3NC1DC2DC3DI1NI2NI3NI1DI2DI3DV1NV2NV3NV1DV2DV3D

VD "damaged"

IN

3* 6mm GFRP rebars 200mm

60 degrees

"new"

ID "damaged"

VN

90 degrees

"new"

CN

none none none

"new"

CD "damaged"

Table 3.2 Details of the tested configurations

Notes: C Control specimen


33

V Vertically inserted GFRP rebars into specimen I Inclined inserted GFRP rebars into specimen N New timber beams D Simulated Damaged timber beams A number 1,2,3 was added for the three identical test configurations so that each tested beam can be easily distinguishable.

Fig. 3.4 Diagrams of configurations tested, the dimensions of the damaged series are the same as those for the new series (all dimensions are in millimetres)


34

End A End B Width (b) Depth (d) Length (l) Surface Moisture (%)V1D V1N 195 197 2564 9.3V2D V2N 197 196 2586 11.3V3N V3D 198 198 2548 11.5I1D I1N 198 199 2564 12.3I2D I2N 198 199 2553 12.6I3D I3N 198 198 2539 10.4C2D C1D 194 198 2567 9.8C1N C2N 197 197 2560 11.3C3N C3D 196 196 2586 10.2

Table 3.3 Dimensions and surface moisture of the nine timber beams used with their respective marking at each end (all dimensions are in millimetres)

Table 3.3 presents the dimensions and surface moisture contents of all the tested

timber beams measured some few days before testing commenced. The width

dimension refers to the horizontal dimension while the depth dimension refers to

the vertical dimension of the timber beam cross-section. Widths, depths and

surface moisture contents were measured at three different locations along the

beam length on all four sides of the beam; 300mm from each end and at the centre

of the length. The values shown in the table are averages of values obtained at

these three locations. The quoted length of the timber beam is the minimum length

when measuring the length along the four corners of the cross-section. The length

varied at these locations due to the fact that the timber beams were bought double

the size needed and were cut manually by a chain saw.

Each beams longitudinal side was marked as being the left, right, top or bottom

side. The applied load was applied to the side chosen to be the top. The right side

of the beam is that side on the right hand side when looking from end A towards

end B with the top side of the beam facing up.


35

3.2.5 Preparation Procedures

3.2.5.1 Timber Beams

The horizontal cuts were made at mid-depth, from the beam end up to the position

of the load application point for those timber beams to be tested as damaged. This

simulated a weathered timber beam with a horizontal split. The cuts were initiated

by means of a drunking saw (cross cut), and finished by a hand saw (fig. 3.5). This method made the best use of the tools available.

Fig. 3.5 Making of the simulated damage

A procedure to drill holes to receive the GFRP rebars was then initiated. The

positions of the holes were marked on the bottom side of the beams. Drill jigs were then created for the 600 and the 900 holes. The 600 drill jig was created from a rectangular piece of timber following the described procedure. A 600 angle was

marked accurately on it. This mark was then placed parallel with a punch drill bit

and the rotating table was set parallel to the bottom side of the rectangular piece of

timber. This was achieved by the aid of a rotating L-square (fig. 3.6). A 900 drill jig was created following the same procedure.


36

Fig. 3.6 Creation of drill jig

The drill jigs were positioned and clamped on the timber beams with the marked guides (fig. 3.7b). The holes were drilled by a hand drill, firstly to a diameter of 10mm and then re-drilled to a diameter of 12mm. The hole diameter of 12mm was

chosen so that, to have around 2.5mm bondline with a GFRP nominal diameter of

6.35mm. A wood drill bit of 11mm would have been preferred but was not found on

the market. The last 12mm of the beams depth were left undrilled to facilitate the

application of the adhesive. This was achieved by marking the drill bit with a piece

of tape to act as guide (fig. 3.7a).

(a) (b) Fig. 3.7 Drilling of holes


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The checks and knots of all sides of the beams were plotted prior to testing and are

shown later on the same plots of the cracks.

3.2.5.2 Strain Gauges

Type TML BFLA-2-5-5L strain gauges were fixed to the GFRP rebars. Prior to

fixing of the strain gauges, the GFRP rebars were smoothened by a hand file to an

area slightly larger than that of the strain gauge at the position where the strain

gauges were to be fixed, in order to ensure good adhesion. The smoothened area

was cleaned by means of cotton buds immersed in white spirit. The white spirit was

then dried by a tissue paper. The strain gauges were then placed with bonding

face down on a plastic sheet. A transparent tape was bonded to the other side of

the strain gauge. The tape was then lifted carefully, bending as little as possible the

strain gauge. The strain gauge was now fixed to the tape with the bonding face

exposed. CN adhesive was applied on the cleaned surface of the GFRP rebar and

the strain gauge was placed on this surface. The strain gauge was pressed by the

finger through the tape for a couple of minutes to allow for curing. The tape was

then removed. A layer of electrical insulating tape was applied around the strain

gauge and the exposed wire to protect the strain gauge. Finally each strain gauge

resistance was checked by means of an ohm metre and all strain gauges were

found to be in the range of the required resistance specified by the manufacturer of

121.0 +/- 0.5 ohms (fig. 3.8). Therefore it was ensured that none of the strain gauges was damaged in the process.


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Fig. 3.8 Checking the electrical resistance by means of an ohm metre

3.2.5.3 Insertion of GFRP rebars

The holes were cleaned by firstly placing the timber beams with holes down so as

to aid any timber debris to fall. An air gun connected to a compressor was used to

further clean the holes. The beam was then rotated so the holes would point

upwards. The adhesive cartridge was opened. The static mixer fixed with an

extension to reach the entire depth of the holes was screwed to the cartridge. The

cartridge was placed into a gun. The first few pumps of the adhesive were

discarded so as to ensure adequate mixing of the two-part adhesive. After this the

holes were filled up to about two-thirds of their volume with the adhesive (fig. 3.9a). The GFRP rebars were then inserted in a rotating manner so as to expel any

trapped air (fig. 3.9b). It was observed that in all insertions some extra adhesive flowed out of the hole. This ensured adequate filling of the holes. The installation of


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the GFRP rebars was carried out at an ambient temperature of 16.30C. A relative

humidity reader was unavailable.

(a) (b) Fig. 3.9 Inserting the GFRP rebars

3.2.6 Testing Procedures

The beams were transported and placed in the rig by hand. Two transverse

Universal Beam sections where clamped in position on top of the rig frame to

support the test specimens. Steel spacers were used both at the supports and at

the load point together with steel bearing plates. The dimensions of the bearing

plates at the supports were of 12mm thickness, 90mm width and 215mm length

which was enough to span the width of the beams. The bearing plate at the point

load was circular with a thickness of 30mm and a diameter of 220mm. This bearing

plate was used from the fourth test onwards after another two bearing plates were

used without spacers and were bent. The three tests that used different bearing

plates and their dimensions are indicated in Chapter 4.

The hydraulic jack used at the point load position was of 200kN capacity. In order to ensure calibration of the loading equipment used, the load cell together with the

Peckel Data Logger2500 system was tested by a compression testing machine.


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Calibration was ensured by observing that the results obtained by the data logger

were in agreement with those of the compression testing machine.

Load-displacement data was not recorded. This was not considered to be

dissertation christian schembri final

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