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
Fig. 4.8 Crack pattern for beam C2N
55
Fig. 4.9 Beam C3N at ultimate failure
56
Fig. 4.10 Crack pattern for beam C3N
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
Fig. 4.14 Crack pattern for beam I2N
61
Fig. 4.15 Beam I3N after the test
62
Fig. 4.16 Crack pattern for beam I3N
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
Fig. 4.27 Crack pattern for beam C2D
71
Fig. 4.28 Beam C3D after failure
72
Fig. 4.29 Crack pattern for beam C3D
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
Fig. 4.38 Crack pattern for beam V2D
80
Fig. 4.39 First bending crack on the right side of beam V3D
81
Fig. 4.40 Beam V3D at ultimate failure
81
Fig. 4.41 Crack pattern for beam V3D
82
Fig. 4.42 Typical timber block position at initiation of test
84
Fig. 4.43 Force against Displacement for 450 series
85
Fig. 4.44 Force against Displacement for 600 series
85
Fig. 4.45 Force against Displacement for 900 series
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
Fig. 4.49 Sample 45-3
89
Fig. 4.50 Sample 60-5
89
Fig. 4.51 Sample 60-2
90
Fig. 4.52 Sample 90-2
90
Fig. 4.53 Sample 90-4
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
Fig. A.1.2 C2N Load against Time
114
Fig. A.1.3 C3N Load against Time
114
Fig. A.1.4.a I1N Load against Time
115
Fig. A.1.4.b I1N Tensile forces in Rebars against Time
115
Fig. A.1.5.a I2N Load against Time
116
Fig. A.1.5.b I2N Tensile forces in Rebars against Time
116
Fig. A.1.6.a I3N Load against Time
117
Fig. A.1.6.b I3N Tensile forces in Rebars against Time
117
Fig. A.1.7.a V1N Load against Time
118
Fig. A.1.7.b V1N Tensile forces in Rebars against Time
118
Fig. A.1.8.a V2N Load against Time
119
Fig. A.1.8.b V2N Tensile forces in Rebars against Time 119
List of Figures
xvi
Fig. A.1.9.a V3N Load against Time
120
Fig. A.1.9.b V3N Tensile forces in Rebars against Time
120
Fig. A.1.10 C1D Load against Time
121
Fig. A.1.11 C2D Load against Time
121
Fig. A.1.12 C3D Load against Time
122
Fig. A.1.13.a I1D Load against Time
123
Fig. A.1.13.b I1D Tensile forces in Rebars against Time
123
Fig. A.1.14.a I2D Load against Time
124
Fig. A.1.14.b I2D Tensile forces in Rebars against Time
124
Fig. A.1.15.a I3D Load against Time
125
Fig. A.1.15.b I3D Tensile forces in Rebars against Time
125
Fig. A.1.16.a V1D Load against Time
126
Fig. A.1.16.b V1D Tensile forces in Rebars against Time
126
Fig. A.1.17.a V2D Load against Time
127
Fig. A.1.17.b V2D Tensile forces in Rebars against Time
127
Fig. A.1.18.a V3D Load against Time
128
Fig. A.1.18.b V3D Tensile forces in Rebars against Time
128
Fig. A.2.1 Stereoscope images of sample 45-2
129
Fig. A.2.2 Stereoscope images of sample 45-3
130
Fig. A.2.3 Stereoscope images of sample 60-2
131
Fig. A.2.4 Stereoscope images of sample 60-5 131
List of Figures
xvii
Fig. A.2.5 Stereoscope images of sample 90-2
132
Fig. A.2.6 Stereoscope images of sample 90-4
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
Chapter 1 Introduction
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
Chapter 1 Introduction
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
Chapter 2 Literature Review
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)
Chapter 2 Literature Review
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.
Chapter 2 Literature Review
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.
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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.
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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.
Chapter 2 Literature Review
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.
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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.
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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.
Chapter 2 Literature Review
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).
Chapter 2 Literature Review
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
Chapter 2 Literature Review
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.
Chapter 3 Experimental Methodology
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.
Chapter 3 Experimental Methodology
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
Chapter 3 Experimental Methodology
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.
Chapter 3 Experimental Methodology
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.
Chapter 3 Experimental Methodology
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.
Chapter 3 Experimental Methodology
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
Chapter 3 Experimental Methodology
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)
Chapter 3 Experimental Methodology
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.
Chapter 3 Experimental Methodology
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.
Chapter 3 Experimental Methodology
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
Chapter 3 Experimental Methodology
37
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.
Chapter 3 Experimental Methodology
38
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
Chapter 3 Experimental Methodology
39
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.
Chapter 3 Experimental Methodology
40
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
Top Related