BENDING TEST FOR SMALL-DIAMETER TIMBER

Bachelor’s thesis

Visamäki Campus

Degree Programme in Construction Engineering

Autumn, 2019

Viet Cuong Dao

 ABSTRACT

Degree Programme in Construction Engineering

Hämeenlinna University of Applied Sciences Centre

Author Viet Cuong Dao Year 2019

Subject Bending Test for Small-Diameter Timbers

Supervisor(s) Professor Andrew K. Petersen

ABSTRACT

In the last two decades, the utilisation of small diameter timbers in

constructions has gained increasing attention from both researchers and
sustainable building enthusiasts. Timbers, when used as structural
elements in their natural circular form, showed not only a notably higher
strength per unit weight than other timber materials but also lower
environmental impacts than any other structural materials. In recent
years, innovations in forest management policies and connection design
have significantly reduced the barriers to adoption of structural small
diameter timbers.
The purpose of this Bachelor´s Thesis was to examine the development of
standardised strength testing methods for small-diameter timber to
stimulate the structural use of natural round timber.
The existing data was collected from the bending test for round timbers
from the Delft University of Technology laboratory. The extant information
from the previous test was then analysed and recommendations for future
development in bending test arrangement were provided.

Keywords small-diameter timber, bending test.

Pages 41 pages + 4 appendices

ACKNOWLEDGEMENTS

I place on record, my sincere gratitude to:

Herr Andrew K. Petersen, Professor of International Construction

Management Department of Civil Engineering, Hochschule Mainz
University of Applied Sciences, for providing me with all the necessary
guidance for the research.

Herr Tarick Chahade, Assistant, Department of Civil Engineering

Hochschule Mainz University of Applied Sciences for the detailed and
constructive list of related literature and research.

Herr Erick Weiller, International Civil Engineering Co-ordinator,

Department of Civil Engineering, Hochschule Mainz University of Applied
Sciences, for his enthusiastic help with the fabrication of the sample test
piece.

Herr Dieter Hahner, Assistant, Institute of Innovative Structures,

Hochschule Mainz University of Applied Sciences, for his generous help in
the laboratory.

Niina Kovanen, International Co-ordinator, Department of Construction

Engineering, Häme University of Applied Sciences, for her support to
finalise my thesis in Finland.
CONTENTS

1 INTRODUCTION ........................................................................................................... 1
1.1 Background.......................................................................................................... 1
1.2 Motivation ........................................................................................................... 1
1.3 Research Aim and Objectives .............................................................................. 2
1.4 Scope of the Research ......................................................................................... 2

2 LITERATURE REVIEW.................................................................................................... 4
2.1 Overview ............................................................................................................. 4
2.2 Round Timber as a Construction Material .......................................................... 5
2.2.1 Historical Background.............................................................................. 5
2.2.2 Contemporary Application ...................................................................... 7
2.2.3 Benefits of Round Timbers ...................................................................... 9
2.2.4 Challenges.............................................................................................. 12
2.2.5 Available Connections for Round Timbers ............................................ 13
2.3 Testing Methods................................................................................................ 15
2.3.1 Overview ................................................................................................ 15
2.3.2 Bending Test EN 408 .............................................................................. 16
2.3.3 Bending Test for Round Timber............................................................. 17
2.3.4 Test Pieces ............................................................................................. 19
2.3.5 Test Results ............................................................................................ 21
2.4 Summary Table.................................................................................................. 23

3 METHODOLOGY ......................................................................................................... 26
3.1 Research Strategy.............................................................................................. 26
3.2 Research Method .............................................................................................. 26
3.3 Research Approach ........................................................................................... 26
3.4 Data Collection Method .................................................................................... 27
3.5 Research Process ............................................................................................... 27
3.6 Methods of Data Analysis ................................................................................. 27

4 DATA ANALYSIS AND FINDINGS ................................................................................. 29

4.1 Notable Findings................................................................................................ 29
4.1.1 Bending Test .......................................................................................... 29
4.1.2 Bundled Column Method ...................................................................... 29
4.2 Bundled Test Piece ............................................................................................ 29

5 DISCUSSION AND RECOMMENDATIONS ................................................................... 32

5.1 Discussion of Findings ....................................................................................... 32
5.1.1 Test Arrangement .................................................................................. 32
5.1.2 Bundled Column Method ...................................................................... 32
5.2 Limitations of the Study .................................................................................... 34
5.3 Recommendations for Future Research ........................................................... 34
5.3.1 Application of High Technologies .......................................................... 34
5.3.2 Design of Supports................................................................................. 35
 5.3.3 Hemp ..................................................................................................... 36

6 CONCLUSION ............................................................................................................. 37

REFERENCES.................................................................................................................... 39

BIBLIOGRAPHY ................................................................................................................ 41

Appendices
Appendix 1 Selected whole timber structural connections
Appendix 2 Selected structural system in whole timber
Appendix 3 The roof truss of the Muroto Indoor Stadium in Japan
Appendix 4 Classification of whole timber by degree and type of processing
LIST OF FIGURES

Figure Page

Figure 1. Different cross-sections of round timbers (Hochschule Mainz, 2018) .......... 2

Figure 2. Norwegian Stave Church (Wikimedia Commons, 2018) ................................ 5
Figure 3. The Xianju Bridge in China (Wikimedia Commons, 2018) ............................. 6
Figure 4. Roman camp with fortifications mad of small-diameter timbers (Wikimedia
Commons, 2018)............................................................................................................... 6
Figure 5. Simple timber shelter with an interlocking corner (ShutterStock Image, 2019)
…………………………………………………………………………………………………………………..7
Figure 6. Timber Rounding machine (Woodlandia, 2018) ............................................ 7
Figure 7. Machine rounded timbers (Wooden Supplies, 2019) .................................... 8
Figure 8. Modern summer log house (Pinterest, 2019) ................................................ 8
Figure 9. Overstocked small-diameter timber stands (Northwest Natural Resource
Group, 2018) ..................................................................................................................... 9
Figure 10. Effect of knots and other imperfections in dimensioned lumber vs. whole-
timber as shown by grain patterns (Bukauskas, 2015) .................................................. 10
Figure 11. Cross-section of a whole timber with its largest inscribed square
(Bukauskas, 2015) ........................................................................................................... 11
Figure 12. Naturally tapered whole-timber with the largest prismatic member it could
yield (Bukauskas, 2015) .................................................................................................. 11
Figure 13. Bolted joints connection (Pinterest, 2019)............................................... 14
Figure 14. Bolted joints with slotted-in metal plates variations (Lokaj & Klajmonova,
2014)……………….. ............................................................................................................ 14
Figure 15. Test arrangement for measuring modulus of elasticity in bending (BSI,
1995)………….. .................................................................................................................. 16
Figure 16. 𝐸𝑚 𝑡𝑜𝑡𝑎𝑙 deflection measurement (de Vries, 1998) ............................... 17
Figure 17. 𝐸𝑚 𝑙𝑜𝑐𝑎𝑙 deflection measurement (de Vries, 1998) ............................... 17
Figure 18. Test Arrangement (de Vries, 1998) .......................................................... 18
Figure 19. Linear Variable Displacement Transducers (Direct Industry, 2019)......... 18
Figure 20. Measurement of EN 408 deflection (de Vries, 1998) ............................... 19
Figure 21. Loading head cross-section with total LVDT placements (de Vries,
1998)………….. .................................................................................................................. 19
Figure 22. Bundled column using 4 tapered timbers (Bukauskas, 2015) .................. 21
Figure 23. Relative Different between the local and total value of Douglas and Larch
specimen (de Vries, 1998) .............................................................................................. 23
Figure 24. Debarked small-diameter timbers ........................................................... 30
Figure 25. Temporary connection using plastic cable tie .......................................... 30
Figure 26. Stabilising the piece .................................................................................. 31
Figure 27. Power drill and screw for connecting the members ................................ 31
Figure 28. Connections in elements compresses (Brito & Junior, 2012) .................. 33
Figure 29. A system with metal rings, steel bars, washers and nuts (Brito & Junior,
2012)………….. .................................................................................................................. 33
Figure 30. “Skeleton” representation of a whole timber generated from 3D scanning
(GIM International, 2019) ............................................................................................... 35
Figure 31. Hardened support on a standard bend fixture (ADMET, 2019) ............... 36
LIST OF TABLES

Table Page

Table 1. Embodied Carbon Coefficients of Various Structural materials. Unit: kg CO2

e/ kg…………………… .......................................................................................................... 12
Table 2. Results of the Modulus of Elasticity measurements.................................... 22
Table 3. Literature Reviews Summary Table ............................................................. 23
GLOSSARY OF TERM

In natural language and in technical reference words, like learning outcomes, have a
variety of meanings that can lead to confusion. To avoid this problem, a consistent set
of definitions is provided below.

Terminology Definition
BSI British Standard Institution
CAD Computer-Aided Design
CEN European Committee of Standardisation
DUT Delft University of Technology
LVDT Linear Variable Displacement Transducer
3D Scanning Three-Dimension Scanning

LIST OF SYMBOLS

𝐸 Local Modulus of Elasticity

𝐸 Total Modulus of Elasticity
𝑓 Bending Strength
G Shear Modulus
𝑅𝐷 Relative Difference between Total and Local Values
 1

1 INTRODUCTION

1.1 Background

In the past two decades, the diversity and sophistication of natural round
timber construction practices have increased significantly. However, in the
current European standards for the design of timber structures, the issues
of strength-testing methods, strength classes, and grading are solved only
for sawn timber. Previous research has been conducted to determine the
characteristic strength of round timber and to establish a reliable grading
system (de Vries, 1998). The research task is to identify the characteristic
strength of round timber before developing suitable joints and connectors
for the material.

1.2 Motivation

Natural round timber carries all of the beneficial characteristics of timber

in general, with some additional advantages. Timber has been reported to
have a lower effective embodied energy than the most commonly used
structural systems, namely reinforced concrete and steel (Bukauskas,
2015, p. 11). Compared to conventional dimensioned timber products such
as glue-laminated or sawn timber, natural round timber has lower
embodied carbon, higher strength per unit weight and great
environmental credentials.

The major obstacle preventing wider acceptance of small-diameter round

timbers as a structural material is the complex and costly connections. The
imperfections in the roundness and straightness of natural timber make it
difficult to design structural connections that can transfer load between
members in a structural system. Moreover, the reverse design process, in
which the available lengths and sections of timber dictate the geometry of
the structure, is not well-received by conventional designers.

In order to stimulate the wider adoption of structural small diameter round

timber, non-destructive static transverse bending tests, typically
"mechanical grading" or "machine stress grading" (de Vries, 1998), have
been developed based on existing evaluation technique for sawn timber.
The main objective of bending tests is to estimate the modulus of elasticity
of the timber element, which is strongly correlated with the strength
properties of the material. However, the highly irregular geometries of
small-diameter timber make it challenging to obtain accurate
measurements for the modulus of elasticity. Therefore, improvement in
test arrangement and supports is necessary.
 2

1.3 Research Aim and Objectives

Against the background earlier outlined, this research project will be

undertaken with the aim of analysing the existing data generated from the
previous Bending test conducted by the Delft University of Technology (de
Vries, 1998).

To achieve this aim, the following objectives will be pursued:

Objective #1: Conduct research of existing literature for a

comparable study.

Objective #2: Adapt and adopt the methodology for collecting data.

Objective #3: Analyse the collected data from the bending test for
round timbers.

Objective #4: Discuss the findings.

The expected outcomes of the study include:

 The methodology will be adapted from the comparable study.
 The Bending Test for Round Timber will be used for the analysis of
the existing data.
 The results will be similar to the findings of the other studies.
 The result of this research will determine the characteristic strength
of round timbers in different cross-sections (Figure 1).

Figure 1. Different cross-sections of round timbers (Hochschule

Mainz, 2018)

1.4 Scope of the Research

The scope of the study is limited to consider only one machine grading
system for round timber, particularly bending test.

Chapter 1 introduces the background to the research problem with

particular respect to bending test for small diameter round timber. The
aims and objectives of the research along with the scope of the research
are stated. The structure of the paper is also explained.
 3

Chapter 2 presents and critiques the existing literature related to the

utilization of round timber as a construction material and strength testing
methods for small-diameter timber, in particular, bending test. The
chapter introduces the historical background, recent policies, beneficial
aspects, challenges, and existing types of connection for round timbers in
the construction industry. After illustrating the test EN 408:1995 as a
reference, the bending test for round timbers (de Vries, 1998) is described
in detail in terms of test arrangement, design of test specimen, and test
result.

Chapter 3 demonstrates the research methodology implemented for this

research. The chapter gives an outline of the research strategy, the
research method, the research approach, the methods of data collection,
the research process, and the type of data analysis.

Chapter 4 contains an analysis of notable findings from the literature

reviews. Key findings concerning the result of the bending test described
by de Vries (1998), and the bundled column method (Bukauskas, 2015, p.
22) are discussed. The optimum bundled test piece is presented and
analysed in this chapter.

Chapter 5 discusses the findings of the thesis and suggest directions for
future research. The main focus of the findings interprets the arrangement
of the bending test for round timbers, and the bundled column method.
Future research topics including the determination of physical
characteristics of round timbers, the design of bending test supports, and
the application of hemp are given.

Chapter 6 concludes the research with a summary of findings, a summary

of contributions, future work, and concluding remarks.
 4

2 LITERATURE REVIEW

2.1 Overview

Timber, when used in their natural circular form, is the earliest

construction material discovered by mankind. In the primeval time, round
timber was utilised in simple pole structures for high axial, bending
strength and flexibility to withstand lateral loads (Batchelar, 2012, p.558).
Round timber elements appeared in classical churches, bridges, marine
constructions, animal barns, fortifications, etc. for thousands of years.

With the introduction of advanced mechanical methods in fabrication,

timbers of constant cross-section, including mechanically shaped
cylindrical round timbers, became popular (Thépaut & Hislop, 2004, p. 3).
Structural components processed by mechanical technique benefited from
the simplicity in detailing, jointing construction, and more synchronised
appearance. However, the mechanical debarking and manufacturing
process significantly reduce the strength of the components and increase
material wastage, raising both economic and environmental concerns
(Bukauskas, 2015, p. 13).

The benefits of applying natural round wood in construction included low

construction cost, higher strength per unit and strong environmental
credentials (Thépaut & Hislop, 2004, p. 3). Round timber was reported to
have a loading capacity of five times greater than the largest piece of
dimensioned timber that be produced from the same cross-section (Wolfe,
2000, p. 21). The much lower effective embodied energy of timber
compared to steel and reinforced concrete also drew high attention from
the future low-carbon and renewable construction (Bukauskas, 2015, p.5).

Challenges faced by researchers in expanding the application of small-

diameter timber were categorised into economics, forest management,
engineered design, and construction (Wolfe, 2000, p. 21). The most well-
known issue that limited the structural use of round timber was the
artisanal and troublesome connections (Brito & Junior, 2012, p. 244).

While the demand for timber in the natural circular form rose significantly
in recent times, the need for standardised grading methods for round
timber remained unsolved. Many testing methods, including bending test,
compression test, buckling test, tension test, dynamic modulus of elasticity
test and X-ray density measurements, were developed based on the
existing test for sawn plank in the European standards (Ranta-Maunus,
1999, p. 59).

In 1998, de Vries, and Gard described the development of a strength

grading method for small diameter timber, reporting the outcome of the
bending test to determine the Modulus of Elasticity (𝐸 ) and bending
strength (Modulus of Rupture, 𝑓 ) of round timber specimens. The result
 5

of the test demonstrated potential accurate measurements with specific

modifications from the test built for sawn timbers.

In order to satisfy the design challenges for natural round timber,

Bukauskas (2015, p. 14) described the bundled column method, which was
expected to produce a practical, easily fabricated, and flexible structural
system. The promising result in the buckling test for bundled column set a
new task to develop other structural elements using an equivalent
mechanism.

The goal of this literature review is to give an overview of small-diameter

round timber as a construction material and to report existing testing
methods related to round timber, in particular, bending test.

2.2 Round Timber as a Construction Material

2.2.1 Historical Background

Natural timber was the oldest construction material used by mankind.

Evidence of the early application of timber members appeared in many
societies around the world where an abundant natural resource of timber
existed. Both classical Western and Oriental architecture were strongly
influenced by the use of round wood for supports (Thépaut & Hislop, 2004,
p. 2). This included the early historic bridges (see Figure 3) and agricultural
buildings of China and Japan through to the Norwegian Stave Church
constructed in the 12th century (see Figure 2). Another utilisation of round
timber from the ancient time could be found in marine constructions such
as jetties, piers, docks.

Figure 2. Norwegian Stave Church (Wikimedia Commons, 2018)

 6

Figure 3. The Xianju Bridge in China (Wikimedia Commons, 2018)

In the old days, the total volume of construction was considerably smaller
and the construction quality was considered secondary importance. This
led to the preference of small-diameter timber as a construction material
due to its convenient size, related to handling and transportation (Ranta-
Maunus, 1991, p.15). Temporary construction and low-value buildings
benefited from a tremendous volume of minimally processed small-
diameter round timber. Notable examples illustrated the early application
of small-diameter round timber include the Roman camp (see Figure 4)
and fortification, barns, animal shelters, and artisan houses.

Figure 4. Roman camp with fortifications mad of small-diameter

timbers (Wikimedia Commons, 2018)

At the beginning of the construction industry, round timber members were

utilised as simple pole structures after minimal processing using hand
tools. Prior to the introduction of mechanical jointing, lashings were the
main type of connection. Timber members in its natural circular form
optimised high axial, bending strength and flexibility to accommodate
lateral loads (Batchelar, 2012, p. 558). In those countries where plenty
source of natural timber is available above the ground, the log
construction was born (Thépaut & Hislop, 2004, p. 4). Figure 5 illustrates a
simple shelter made of the tree trunks from coniferous forests stacked on
top of each other with the interlocking corner in the most elementary
manner.
 7

Figure 5. Simple timber shelter with an interlocking corner

(ShutterStock Image, 2019)

2.2.2 Contemporary Application

With the involvement of mechanical methods (Figure 6) in the

manufacture of round timbers, machine debarked elements with a circular
cross-section (Figure 7) became more popular. The major goal in producing
machine rounded timber is to obtain mechanically shaped cylindrical
components of constant cross-section without affecting the natural taper
of the original tree (Thépaut & Hislop, 2004, p. 5). Structural elements
processed by mechanical methods can simplify detailing, jointing, and
construction, as well as enhance the appearance of the structure.

Figure 6. Timber Rounding machine (Woodlandia, 2018)

However, mechanical debarking significantly reduced the strength of the

components and increased material wastage. This was due to the fact that
mechanically removal of bark damage the natural arrangements of the
fibre that give the timber its strength (Gorman, 2012). An alternative to
mechanical debarking was manual methods which can limit the harmful
effects on the properties of natural timber (Thépaut & Hislop, 2004, p. 5).
 8

Figure 7. Machine rounded timbers (Wooden Supplies, 2019)

Nowadays, the timber constructions built from the round timber members
have become increasingly popular. In recent years, the enormous growth
of the leisure industry and access to the countryside had directly
associated with the need for round wood (Lokaj & Klajmonova, 2014). The
utilisation of modern log construction (Figure 8) for residential buildings
such as summerhouses or cabin and leisure facilities can be found all over
Europe and North America (Thépaut & Hislop, 2004, p. 3). The machine-
debarked round timbers were also widely used as conventional structural
elements such as columns, beams, rafters, etc and as assemblies such as
trusses and portal frames.

Figure 8. Modern summer log house (Pinterest, 2019)

In 2000, Wolfe revealed research from the US Forest Products Laboratory

exposing an interesting fact that forest stands around the United States
are overstocked with small-diameter trees due to their insignificant
market value. Forest managers considered the overstocked small-
diameter tree stands critical forest health issues. The forests which were
crowded with suppressed and unhealthy trees were subject to attack from
insects and disease and the risk of total destruction by fire (Gorman, 2012).
In order to improve the health of wood and mitigate the risk from
overstocked stands (Figure 9), excess woody biomass, including small-
diameter trees, dead trees, excessive slash on the ground must be thinned
out. Small-diameter timbers could be achieved in a large amount from the
forest thinnings (Ranta-Maunus, 1999, p. 14).
 9

Prior to more intensive investigations on the utilisation of small-diameter

timbers, the most common approach towards this resource of wood was
the manufacturing of low-value byproducts such as firewood or wood
chips (Burton, Dickson, & Harris, 1998, p. 77). Another less
environmentally friendly but economic method was burning.

Figure 9. Overstocked small-diameter timber stands (Northwest

Natural Resource Group, 2018)

In the current forestry industry, forest thinnings has no longer been

considered a secondary byproduct (Bukauskas, 2015, p.7). Changes have
been undertaken with the aim of maximizing the value of thinnings as a
profitable forestry resource. Upcoming developments concentrated on
buildings that are both environmentally acceptable and incorporated with
high technology and high quality (Wolfe, 2000, p.21). Small diameter
timber was considered perfectly matched with this demand due to its
environmentally friendly image and the application of high-tech
connection systems.

2.2.3 Benefits of Round Timbers

The favourable context of round timbers in construction includes low

construction costs, higher strength per unit and strong environmental
credentials.

Research by Wolfe at the US Forest Product Laboratory (2000, p.21)

demonstrated the low construction cost of small-diameter round timber
as a structural material. In the manufacturing process of round timbers,
felling, debarking, and any necessary cuts made were involved. All the
steps included in the processing of natural timber into structural elements
other than long-distance transport could be achieved by hand without the
use of heavy machines. In terms of environmental impacts, this means
potential energy savings and diminished reliance on fossil fuels. This
interesting feature makes natural round timber notably appropriate for
the construction market of low-cost materials in developing regions
(Bukauskas, 2015, p.5).
 10

When being compared to timber structural elements in the original circular

form, the largest piece of dimensioned timber could only yield 20% of the
load capacities that have been assigned to round timber (Gorman, 2012,
p.155). This higher strength per unit weight of round timber was explained
by three reasons, in particular, fibre continuity, sectional properties, and
taper.

In natural conditions, the grain of the tree curves around knots and other
imperfections as a way to cover these local weaknesses. The process of
sawing timbers into prismatic elements (Figure 10) cuts through the
valuable tree grain, which introduces the ends of the fibres that give a tree
strength and uncovers local weaknesses. As a result, affecting fibre
continuity reduced the strength of prismatic members by up to 2-3 times
(Bukauskas, 2015, p.13).

Figure 10. Effect of knots and other imperfections in dimensioned

lumber vs. whole-timber as shown by grain patterns (Bukauskas,
2015, p. 13)

The sectional properties of round wood elements, in particular, cross-

section and moment of inertia, witness great changes under the
manufacturing process to sawn plank (Ranta-Maunus, 1999, p.30). A
recent study (Bukaukas, 2015) reported that the largest prismatic
component that can be derived from round wood (Figure 11) has its cross-
section and moment of inertia reduced by 36% and 58% respectively.
These directly related to a decrease in crushing and buckling capacity of
sawn plank.
 11

Figure 11. Cross-section of a whole timber with its largest inscribed

square (Bukauskas, 2015, p.13)

Figure 12 demonstrates how the smaller end of a tapered timber

determines the size of any prismatic or uniform-section circular member
that can be manufactured from that timber. A calculation method for
computing the buckling capacity of natural tapered wood showed a
significant 43% greater in the buckling capacity of these elements in
comparison with prismatic or uniform-section derived from them
(Bukauskas, 2015, p. 14).

Figure 12. Naturally tapered whole-timber with the largest prismatic

member it could yield (Bukauskas, 2015, p.14)

The most beneficial aspect of round timber as a structural material is its

remarkable environmental credentials due to low embodied energy
compared to other construction materials and renewability (Wolfe, 2000,
p. 22). While analysis had proved that the structure of a building accounts
for 71% of the energy needed in the construction process (Bukauskas,
2015, p. 11), it was crucial to diminish the embodied energy of the
structural systems. In order to achieve this goal, the use of materials with
low embodied energy in effective structural configurations that maximize
structural capacity while minimizing material use should be promoted. The
timber structural system was proved to have a lower effective embodied
energy than its commonly used counterparts (see Table 1) such as steel
and reinforced concrete (Hammond & Jones, 2011). In addition,
atmospheric carbon was temporarily sequestered in a timber structure. At the
end of life of a building, timber members can be recycled, left to biograde, or
be burned in the form of biofuel. Hence, timber has all the potential to
become the most applicable low-carbon and renewable material in the future.
 12

Table 1. Embodied Carbon Coefficients of Various Structural

materials. Unit: kg CO2 e/ kg

ECfos ECbio ECtotal

Steel (General) n/a n/a 1.46
Glue-Laminated Timber 0.42 0.45 0.87
Sawn Softwood 0.2 0.39 0.59
Whole-timber 0.2 n/a 0.2
Note: For all embodied carbon coefficients, CO2e, meaning CO2 equivalent
is used (Bukauskas, 2015, p. 12)

2.2.4 Challenges

The development of structural use of small-diameter round timber faces a

number of challenges in economics, forest management, engineered
design and construction (Wolfe, 2000, p. 23).

A major economic barrier to expanding the use of small-diameter round

timber encompassed the perception that the market value of the material
does not surpass the cost of harvesting, processing and transporting.
Promoters have emphasized the better characteristics of round timber
compared to sawn plank when introducing the material (Wolfe, 2000).
However, the majority of design engineers, building contractors and
architects hold a sceptical and conventional opinion against the use of
small-diameter timber due to various technical issues (Brito & Junior,
2012). Hence, the feasibility of utilising round wood requires extensive
changes in the marketing philosophy of the forest products industry as well
as explorations of possible new markets.

In terms of forest management, the challenges include the cost of forest

thinnings and the limited means to identify the quality of structural small-
diameter timber. Forest managers did not consider thinning small-
diameter timber cost-effective due to the absence of subcontracted
harvest. Moreover, the lack of existing grading methods for natural round
timber also complicates the categorising process after thinnings (Ranta-
Maunus, 1999, p. 29).

The most well-known obstacle against the acceptance of small-diameter

timber in the construction industry is the complexity of its connections.
Although connections play the key role in the safety of round timber
structures (Brito & Junior, 2012, p. 244), detailed guidance on the design
of those connections in accordance with official building codes is not
readily available. Hence, contractors held resistance against the use of
small-diameter timber unless a standardized way of manufacturing the
connections is provided.
 13

2.2.5 Available Connections for Round Timbers

Connections of round timber members plays a key role in the safety of

timber structures. However, the joints of the structural components made
of round timbers require a more intensive approach than the sawn plank
counterparts (Lokaj & Klajmonova, 2014, p. 103). In most cases, the
connections must be sawn by hand under the supervision of experienced
carpenters to achieve the expected quality (Brito & Junior, 2012, p. 244).
This posed the major problem of utilising round wood in construction that
people held strong resistance against the artisanal and troublesome
methods of manufacturing the connections.

In order to stimulate a more efficient use of the connection between round

wood structural components, various types of joints were introduced in
the construction industry. Brito and Junior (2012, p. 244) classified the
main varieties of connections with round timber for structural components
in accordance with its function, including:
 notches in wood,
 wooden dowel,
 threaded rods with washers and nuts,
 dowel nut,
 steel plate external fixed with screws,
 steel plate internal fixed with screws,
 galvanised perforated steel plate and nailed,
 steel straps woven into,
 steel rings with threaded rods fixed with washers and nuts,
 steel connectors for structures mixed round wood and concrete,
 details on the interface of the timber structure with masonry,
 connection system for log home walls,
 connections in parts compressed, and
 connections to bases of columns.

In most of the listed joints, the connections include metal components

such as threaded steel bar, metal pin, and metal ring.

According to Lokaj and Klajmonova (2014, p. 103), bolted joints with or

without internal metal plates bolted are the most popular connections
used in the recent construction of round timbers.

In the bolted joints (Figure 13), threaded steel rods are placed through
transverse pre-drilled holes passing through the longitudinal axis of the
elements and tightened by washers and nuts on the extremities. Any
further tightening process can be carried out when the timber has reached
the equilibrium moisture content (Brito & Junior, 2012, p.246).
 14

Figure 13. Bolted joints connection (Pinterest, 2019)

The bolted connections with slotted-in metal plates described in Figure 14

share the same operating principle with the bolted joints. These
connections contain metal plates being imputed into the cleft in the
longitudinal axis of the timber component. The joint is performed via the
bolts traverse across the plates and pieces of round timber (Brito & Junior,
2012, p. 246). When the washers and nuts on the end of the steel rods are
being tightened, the inner faces of the wood and the faces of the metal
sheet are compressed by the locking system.

Figure 14. Bolted joints with slotted-in metal plates variations (Lokaj
& Klajmonova, 2014, p. 104)

The design of steel connection for round timbers is based on the principle
of avoiding the splitting failure mechanisms. The reinforcement is
expected to possess two fundamental effects: transferring the tensile
stresses perpendicular to the grain and increasing the embedding capacity
of the reinforced timber area (Lokaj & Klajmonova, 2014, p. 104).

In 2014, the tension test carried out by researchers from the Technical
University of Ostrava showed that round timber samples with bolted joints
or bolted joints with slotted-in steel plates had the ability to absorb more
pressure before collapse than the unreinforced ones. Both connections
showed profitable due to the usage of affordable parts.
 15

2.3 Testing Methods

2.3.1 Overview

As the use of timber in the natural circular form has increased recently, a
complete method of sorting round timber according to strength classes is
crucial. In the current European standards for the design of timber
structures, the issues of strength-testing methods, strength classes, and
grading are solved only for sawn timber (de Vries, 1998, p. 184). For round
timber, there was no existing grading system that associates the
characteristics of the material to its strength values.

Researchers had concentrated on the applicability of the European

Committee of Standardisation (CEN) standards written for sawn timber to
develop more completed procedures for natural round timber. Various
issues have been discussed including the strength testing methods,
strength classes, strength grading and the effect of size on strength. The
main objective of researchers was to resolve the characteristic values
needed in the design of load-bearing round timber structures, examine the
practicability of non-destructive methods in strength-grading, and
establish a technique for developing international standards for visual
strength-grading of structural round timber (Ranta-Maunus, 1999, p. 59).
According to the Technical Research Center of Finland (1999), about 1400
bending tests and a minor number of compression and tension tests had
been carried out to fulfil the proposed objective.

The strength test for round timber was closely related to EN 408:1995
procedures (BSI, 1995, p. 2), which are designed for testing sawn timber
(de Vries, 1998). Due to the variation of the diameter of round wood, the
deviations from the standard were adopted if necessary. In 1999, Ranta -
Maunus drafted a proposal for the testing standard of circular timber with
modified testing methods. According to researchers from the Delft
University of Technology (DUT), the real test showed minor tolerances
with no significant influence on the result.

In the EN 408:1995 Standard issued by the CEN, specific laboratory

methods for the determination of mechanical and physical properties of
timber structural sizes are described. The following properties of structural
timber can be determined by test methods: modulus of elasticity in
bending; shear modulus; bending strength; modulus of elasticity in tension
parallel to the grain; tension strength parallel to the grain; modulus of
elasticity in compression parallel to the grain; compression strength
parallel to the grain.

Additionally, the determination of dimensions, moisture content, and

density are specified. The methods illustrated in the standard are
applicable to rectangular or circular shapes of a considerably constant
cross-section of solid unjointed, finger-jointed, and glue-laminated wood.
 16

It is also worth noting that the standard is not intended for quality-control
test purposes (BSI, 1995, p. 3).

Ranta-Maunus (1999, p. 59) introduced six different available strength

testing methods for small-diameter round timber, namely bending test,
compression test, buckling test, tension test, dynamic modulus of elasticity
test and X-ray density measurements.

2.3.2 Bending Test EN 408

A four-point bending test was conducted to determine the modulus of

elasticity of round timber elements. The modulus of elasticity would be
obtained by measuring the local deflection between the load. A
comparison between the global and local deflection was carried out to
verify the value of the modulus of elasticity. The average diameter and the
minimum diameter close to the collapsing position were used for
calculating local Modulus of Elasticity (𝐸 ) and bending stress (𝑓 )
respectively.

The procedure of conducting the test and test arrangement were

demonstrated in the EN 408: 1995 Standard. In principle, the test piece
shall be symmetrically loaded in bending at two points over a span of 18
times the diameter (Figure 15). Two symmetrical loads are placed at a
distance of six times the diameter between them. In case these conditions
were not precisely satisfied, the distance between the load points and the
supports may be adjusted by an amount not greater than 1.5 times the
piece depth, and the span and test piece length may be changed by an
amount not greater than three times the element diameter,
while maintaining the symmetry of the test.

Figure 15. Test arrangement for measuring modulus of elasticity in

bending (BSI, 1995, p. 6)

The supports for the test piece shall be simple. In order to minimize the
indentation, small steel plates of length not greater than one-half of the
depth of the test piece may be inserted between the piece and the loading
heads or supports. Buckling would be avoided using any necessary lateral
 17

restraint that allows the test piece to deflect without significant frictional
resistance.

The load applied to the piece shall be maintained at a constant rate

movement not greater than 0,003 h mm/s. The maximum load applied
shall not exceed the proportional limit load or cause damage to the piece.

An accuracy of 1% of the load applied to the test piece or, for loads less
than 10 % of the applied maximum load, an accuracy of 0,1 % of the
maximum applied load must be satisfied by the loading equipment.

Deformations shall be measured at the center of a central gauge length of

five times the depth of the section. An accuracy of 1 % or, for deformations
less than 2 mm, an accuracy of 0,02 mm is required in the measurement
of the deformation (BSI, 1995, p. 7).

2.3.3 Bending Test for Round Timber

The test arrangement built for small-diameter timber based on the EN 408
description for testing sawn timber (de Vries, 1998, p. 188). With respect
to the difference between sawn timbers and round timbers, the deflection
measurements had been modified as shown in Figure 16 and Figure 17.

Figure 16. 𝐸 deflection measurement (de Vries, 1998, p. 189)

Figure 17. 𝐸 deflection measurement (de Vries, 1998, p. 189)

In the test conducted by the DUT laboratories, the test arrangement

included an Instron universal testing machine model 119S and a 3.5meter
 18

long I-beam. The test piece was reinforced by two end supports attached
to the I-beam that allowed different specimen lengths to be tested (Figure
18).

Figure 18. Test Arrangement (de Vries, 1998, p. 190)

The deflection was measured by a Linear Variable Displacement

Transducers (LVDTs). The LVDTs (Figure 19) were attached to the test piece
at three different reference points. A frame with the LVDT was mounted
on two outer reference points. On a flat horizontal plane rotating around
the center reference points axis, the LVDT core was installed. The
deflection measurement accuracy was controlled using special LVDT
configurations. Any potential rotation of the specimen during the
measurement of local and global deflection was regulated by two LVDTs.
The location of the LVDTs was described in Figure 20 and Figure 21.

Figure 19. Linear Variable Displacement Transducers (Direct Industry,

2019)
 19

Figure 20. Measurement of EN 408 deflection (de Vries, 1998, p. 191)

The loading heads (Figure 21) were designed in the fundamental that
enabled the loaded sections to rotate and alter the test specimen surface
during the experiment. The axis of rotation of the loading heads located
within the neutral plane of the test piece.

Figure 21. Loading head cross-section with total LVDT placements (de
Vries, 1998, p. 191)

A small Modulus of Rupture test series for each diameter group was
required to determine the expected failure load of the test piece. During
the Modulus of Elasticity test, only 40% of the failure load was applied to
the specimen. The test piece was loaded to failure again after relief to
resolve the bending strength.

2.3.4 Test Pieces

Researchers harvested different materials from different countries for

investigation, including Norway spruce (Picea abies) and Scots pine (Pinus
sylvestris) from Finland, Sitka spruce (Picea sitchensis) from the UK, Larch
(Larix kaempferi) from the Netherlands and Douglas fir (Pseudotsuga
menziesii F.) from France (Ranta-Maunus, 1999, p. 71).
 20

According to the EN 408 standard (BSI, 1995, p. 6), the determination of

test pieces must satisfy requirements in dimension, moisture content,
density and modulus of elasticity in bending.

The dimensions of the test pieces are measured to an accuracy of 1% and

the measurements are taken not closer than 1500 mm to the ends. In the
case of varying widths and length, the average of three separate
measurements at different positions on the length of each piece will be
the outcome dimensions. The test piece shall have a minimum length of
19 times the depth of the section.

All measurements were expected to be made at the standard environment

of (20±2) °C and (65 ± 5) % relative humidity. The moisture content and
density of the test piece shall be determined on a full cross-section cut as
close as possible to the fracture.

A constant mass, which is attained from two successive weighings at an

interval of six hours with no more than 0.1% difference, is also required.
In addition to the test arrangement description in the EN 408:1995, various
practical issues caused by the difference in the nature between sawn
timber and natural round wood should be considered.

The most significant difference between round wood and sawn wood is
the initial curvature possessed by natural round timber. This characteristic
leads to the appearance of an elliptical-shaped cross-section that caused
difficulty in measuring and calculating the moment of inertia. Diameter
ranges could be used as an alternative to determining the span length, the
moment of inertia, and the limit of deformation speed.

Exceptional attention should be paid to the rotation of the test piece when
measuring the displacements by external devices during loading:
 The installing process of displacement measurement reference points
at prescribed locations would be affected by the asymmetrical and
cracked surface of round timber,
 A neutral plane of the pole should be identified in order for the load
to be applied, and
 The determination of this neutral plane requires special load
headings that can adapt to various diameter of the tapered poles.

Figure 22 described a potential method in fabricating the test pieces for

bending tests using the "bundled column" method introduced by
Bukauskas (2015, p. 14). In the bundled column system, round timber
elements were packed together using shear bolt connections. The weight
of all steel connectors in the column was estimated at 10% of the total
timber weight. This method of connecting timber elements provided an
applicable, easily fabricated and adaptable system that possessed high and
equal crushing capacity at the end. The biggest challenge in the design and
fabrication of bundled components is to establish a method to fasten the
 21

timbers together while enhancing adequate shear resistance against

buckling.

Figure 22. Bundled column using 4 tapered timbers (Bukauskas, 2015,

p. 14)

2.3.5 Test Results

The Modulus of Elasticity in bending was determined in two different ways

of measurement.

In the EN 408 experiment, the force- deflection graph was influenced by

only local specimen characteristics and modulus of elasticity. The EN 408
bending test resulted in the local modulus of elasticity in bending 𝐸
and bending stress 𝑓 , which could be expressed as:

( )
𝐸 = ( )
(1)

𝑓 = (2)

where:
 𝑎 is the distance between a loading position and the nearest support
in a bending test, in millimetres;
 F is the maximum load, in Newtons;
 F − F is an increment of load on the straight-line portion of the
load
 deformation curve, in Newtons;
 𝐼 is the second moment of area, in millimetres to the fourth power;
 𝑙 is the gauge length for the determination of modulus of elasticity,
in millimetres;
 𝑤 − 𝑤 is the increment of deformation corresponding to F2 – F1, in
 millimetres; and
 𝑊 is the section modulus, in millimetres to the third power.
(BSI, 1995, p. 7).

In the bending test using the measurement reference points in the middle
section and on both ends of the specimens, the full specimen
characteristics, modulus of elasticity and the shear modulus (G) had an
 22

influence on the force-deflection graph. The total value 𝐸 was

achieved by recording the displacement of the test piece middle section
with respect to the end supports.

In theory, the relative difference between 𝐸 and 𝐸 would be

used as the characteristics of the test arrangement. The results from
different test arrangements were expected to yield comparable relative
difference values. The relative difference between 𝐸 and 𝐸 ,
𝑅𝐷 , would be calculated using the formula: 𝑅𝐷 = .

The results presented by researchers from DUT laboratory in Table 2.

showed correspondence to the theory. The mean value of 𝐸 was
greater than 𝐸 in both test specimens. The standard deviation of the
Douglas test piece was notably higher than that of the Larch.

Table 2. Results of the Modulus of Elasticity measurements

Douglas Larch

N Min Max Mea Std. N Min Max Mean Std.

n Dev. Dev.
𝐸 145 5.5 12.1 8.9 1.4 137 7.3 17.4 12.6 1.9 [GPa]

𝐸 138 4.2 21.6 9.6 3.1 137 7.3 20.5 13.3 2.4 [GPa]

Note: From " The development of a strength grading system for small
diameter round wood " by P. de Vries, 1998, Center for Timber Research,
p.193

The significant differences between the local and total values could be
explained by identifying the error sources in the measurements, including:
 The effects of shear deformation resulted in lower values for 𝐸 ,
 The variation caused by the difference in the characteristics such as
density and knot along the test piece, and
 The variations of the central part properties used in the 𝐸
measurement and the average properties used in 𝐸
measurement resulted in the correlation coefficient (de Vries, 1998,
p. 194).

The distribution of 𝑅𝐷 values between the total and local moduli of

elasticity was illustrated in Figure 23. Overall, the average values for 𝑅𝐷
from both Douglas and Larch specimens showed a comparable rate of 5%.
However, the standard deviations, which were 20% and 7% of Douglas and
Larch respectively witnessed a significant difference between the test
arrangements.
 23

Figure 23. Relative Different between the local and total value of
Douglas and Larch specimen (de Vries, 1998, p. 195)

The conclusion drew by the researchers from DUT laboratory (1998)

showed the possibility of accurate measurements using both local and
total bending test arrangements. However, it was recommended to use
the modified EN 408 test to determine 𝐸 as a reference due to error-
prone measurements from the original standard.

2.4 Summary Table

The following is a summary table of literature used throughout the

research, including the topic of the reference, method of data analysis
used, and significant outcomes provided.

Table 3. Summary Table of Literature Reviews

Citation Topic Method Significant outcomes

Batchelar Innovative use of timber Quantitative The use of round timber was
(2012) rounds in high-performance descriptive not limited to simple
structures. structures.
"To introduce and highlight
the potential applications of
cored rounds for high-
performance structures" (p.
1)
Brito & Types of connections for Quantitative Most connections used for
Junior structural elements descriptive round timber structural
(2012) roundwood used in Brazil. elements used steel connectors.
"To present the main usual
types of connections used in
structural systems and
construction" (p. 1)
 24

Bukauskas New structural systems in Quantitative  Natural round timber had

(2015) small diameter timber. explanatory
additional advantages
"Focuses on the design of a
structurally independent compared to other types of
column in whole-timber" (p.
11) timber construction
materials, especially in the
environmental aspect.
 The bundled column system
provided a potential method
to manufacture the test
piece.
de Vries The development of a Quantitative  The bending test built for
(1998) strength grading system for explanatory
round timber closely based
small diameter round wood.
" Concentrates on the test on the EN 408 test for sawn
methods to determine the
timber.
Modulus of Elasticity (𝐸 )
and on the potentiality of 𝐸  The result of bending tests
as a grading parameter with
respect to strength class showed corresponding
classification" (p. 184) results to the theory.
Gorman Assessing the capacity of Quantitative  Connection using bolts and
(2012) three types of round-wood explanatory plates showed acceptable
connections. results in terms of loading
"To improve the utilization of capacity.
small-diameter round wood
for use as structural members
by evaluating structural
connections" (p. 155)
Lokaj & Selected problems in using Quantitative The most popular types of
Klajmonov round timber in building explanatory connections used for structural
a (2014) structures. round timbers were bolted
"To present specific joints and bolted joints with
problems of designing round slotted-in plates.
timber structures, mainly
joints" (p. 1)
Ranta- Round small-diameter timber Quantitative  Various testing methods had
Maunus for construction. descriptive
(1991) "To present research on been developed concerning
material production and the strength of round timber
developments of small-
structural components.
diameter round timber
structures " (p. 12)  Small diameter timbers
could be harvested in large
amounts from forest
thinnings.
 25

Thépaut & Round timber in Quantitative  Round timbers had been

Hislop construction: An descriptive
used in construction from
(2004) introduction.
"To give an introduction of the prehistoric era
round timber in the
construction industry" (p. 1)  The machine rounded timber
was described.
Wolfe Research challenges for Quantitative The utilization of small-
(2000) structural use of small- descriptive diameter timbers in
diameter round timbers. construction faced both
"To give an overview of the economic and technical
options for round timber challenges.
structural applications and
contains recommendations
for research needed to
promote
acceptance of engineered
applications" (p. 1)
 26

3 METHODOLOGY

This chapter includes the research methodology implemented in this

study. In more detail, in this part, the author outlines the research strategy,
the research method, the research approach, the methods of data
collection, the research process, and the type of data analysis.

3.1 Research Strategy

The research held regarding this study was an applied one, but not new.
Rather, various pieces of previous academic research exist with respect to
the utilisation and testing method for small-diameter timbers, not only for
Europe in specific, but also for other places of the world. Hence, the
proposed research took the form of new research but on an existing
research subject.

3.2 Research Method

In order to meet the objectives of the study, qualitative research was

conducted. The typical attribute of qualitative research is that it is
recommended during preliminary phases of research projects, while the
design is expected to emerge as the study unfolds. The basic advantage of
qualitative research, which also creates its basic difference with
quantitative counterpart, is that it offers a complete description and
analysis of a research subject, without limiting the scope of the research.

However, the effectiveness of qualitative research is heavily dependent on

the skills and abilities of researchers, while the valuable findings from the
research may be difficult to present. The qualitative research also requires
a longer time to collect data and industry-related expertise from
researchers.

3.3 Research Approach

The research approach implemented in this study was the inductive

approach. According to this approach, researchers begin with specific
observations and propose theories towards the end of the research
process as a result of observations. The reason for occupying the inductive
approach was that it aims to generate meanings from the data set
collected in order to identify patterns and relationships to build a theory;
however, the inductive approach does not prevent the researcher from
using existing theory to formulate the research question to be explored.
Inductive reasoning is based on learning from premises, in which patterns,
resemblances, and regularities are observed in order to reach conclusions.
 27

3.4 Data Collection Method

For the purposes of this research, secondary data collection method and
experimental data collection method were used.

Secondary data is a type of data that has already been published in books,
newspapers, magazines, journals, online portals, etc. The main advantage
of secondary data is the availability of data used in previous work, which
makes it time-saving and convenient for future research. In addition,
secondary data also provides a baseline for primary research, to which the
results of the primarily collected data could be compared. An appropriate
set of criteria to select secondary data to be used in the study to minimise
the risks in the research validity and reliability, including but not limited to
date of publication, credential of the author, reliability of the source,
quality of discussions, depth of analyses, the extent of contribution of the
text to the development of the research area etc., must be applied.

Experimental data collection method was utilised to generate data from

the optimum test piece fabrication. The key features of the experimental
data collection method included accurate measurements which enabled
the possibility to replicate the experiment.

3.5 Research Process

Data from secondary sources with respect to small-diameter round timber

utilisation and testing methods, especially bending test, were collected
during January and May 2019. More specially, the researcher held regular
meetings with the supervisor to explain what have been found and discuss
the findings, which could lead to new theories and areas relating to the
research subject. The data is then analysed to identify resemblances
between sources. In June 2019, an optimum test piece was fabricated
using the bundled column method. Finally, new theories and research
questions were formulated in the form of recommendations for future
research.

3.6 Methods of Data Analysis

Secondary data analysis was used to analyse the data gathered from
existing literature. Appropriate findings were extracted from validated
sources and critique on existing data was given by the researcher. The
main advantage of secondary data analysis is that it saves time by
providing access to high-quality data sources while replicating findings
using similar analyses. However, secondary data may not answer the
specific research questions of the researcher or contain specific
information that the researcher would like to have.
 28

Qualitative data analysis was used to analyse the data generated from the
optimum test piece fabrication. After the hypothesis arising during the
secondary data analysed had been scientifically tested, the data from the
experiment was analysed in order to compare the primary and secondary
findings. The most important purpose of the qualitative data analysis in
this study was to link the research findings to the research aim and
objectives.
 29

4 DATA ANALYSIS AND FINDINGS

This chapter contains an analysis of the findings. More specially, in this

chapter, the author presents notable findings with respect to small-
diameter round timber utilization, summarises the results of the bending
test conducted by the DUT laboratory (1998), and demonstrates the
bundled column method.

4.1 Notable Findings

4.1.1 Bending Test

The bending test carried out by the DUT laboratory (1998) showed a good
result with respect to the theory. The mean value of 𝐸 was greater
than 𝐸 in both laboratories. The standard deviation of the Douglas
test piece was notably higher than that of the Larch. The correlation
coefficients between local and total moduli of elasticity of Douglas and
Larch were 0.74 and 0.91 respectively. It was reported that although the
average values for 𝑅𝐷 (5%) can be compared; the standard deviations
(20%, 7%) show significant differences between test arrangements.

Possible errors resulting in the differences between local and total values
of the test were identified. In order to mitigate the risk of errors in
measurements, researchers from the DUT laboratory suggested using the
modified EN 408 test to determine 𝐸 as a reference.

4.1.2 Bundled Column Method

Bukauskas (2015, p. 14) introduced bundled column methods as an

applicable, easily fabricated and adaptable system with high crushing
capacity. The bundled system made use of shear bolts connections, which
accounted for 10% of the total timber weight, to connect timber elements
with a different cross-section. Although the bundling method was
undeniable an attractive way to fabricate structural components made of
small-diameter round timbers, the method of fastening timber elements
within the systems remained a great challenge.

4.2 Bundled Test Piece

A prototype of the bundled column was built using four 1.4 meters long
and 45-55 millimetre diameter small-diameter round timbers (Figure 24).

Each timber was first debarked using a crosscut saw. The debarked timbers
were then measured to identify the original length, the diameters at both
 30

ends, and the length of the straightest part along the longitudinal axis of
the member. A rip cut saw was used to cut the timbers into the desired
length that can optimise the straightest parts of the elements. The system
was joined temporarily using plastic cable ties (Figure 25). After being
stabilised as shown in Figure 26, the members within the bundled system
were connected by metal screws using a power drill. The 100 millimetres
long and 5-millimeter diameter Torx compatible wood screw was used in
the bundled system (Figure 27). Finally, an angle grinder was used to
remove any exceed parts of the screws.

The connection method showed a good result with respect to the bundled
column method with negligible damages in the timbers.

Figure 24. Debarked small-diameter timbers

Figure 25. Temporary connection using plastic cable tie

 31

Figure 26. Stabilising the piece

Figure 27. Power drill and screw for connecting the members
 32

5 DISCUSSION AND RECOMMENDATIONS

5.1 Discussion of Findings

As stated in Chapter 1, the aim of this research was to analyse the existing
data generated from the previous Bending test undertaken by the Delft
University of Technology laboratory (de Vries, 1998). At this stage of the
research, a literature review on the utilization of small-diameter timber as
a construction material and the bending test for round timber was
conducted. Key findings from the research include the arrangement of the
bending test for round timber and the bundled column method. In
addition, a bundled test was fabricated using metal screws to join four
small-diameter timbers.

5.1.1 Test Arrangement

The bending test for round timber (de Vries, 1998, p. 189) was developed
based on the EN 408 test with modifications in supports and loading heads.

According to the CEN description for the bending test of timber, the test
pieces shall be simply supported. The application of small steel plates
inserted between the supports and the test pieces or any necessary lateral
restraint would be considered in order to minimize the indentation and
allow the test piece to deflect without significant frictional resistance. The
same concept was applied in the supports of the bending test for round
timbers. However, the variable cross-section of small-diameter round
timber may require universal supports that could adapt to the various
cross-sections. In this case, small steel plates would potentially play the
main role in the supports.

In the bending test carried out by the DUT laboratory, the loading head
was designed with the aim to enable the loaded sections to rotate and
alter the test piece surface during the experiment. The most important
aspect that may affect the design of loading heads made for the round
timber bending test may be the determination of the neutral plane. In
theory, the neutral plane of the test piece must be identified in order to
locate the axis of rotation for the loading heads. However, the neutral
plane of small-diameter timber test piece with curvature would require
more investigation.

5.1.2 Bundled Column Method

The bundled column method introduced by Bukauskas (2015, p. 14)

showed a potential way to fabricate the test piece for the bending test.
The concept of bundled column system was to pack tapered timbers in
even multiples using shear bolts as connectors between elements, which
accounted for 10% of the total timber weight in the system. Theoretically,
 33

the bundled column system was expected to carry positive attributes of

round timber in general and provide high and equal crushing capacity. The
system showed good performance under the compression test.

However, certain modifications would be concerned in the design of the

bundled specimen used in the bending test. Unlike the tapered timbers
used in the bundled column system, small-diameter round timbers were
characterised by the unruly initial curvature along the longitudinal axis of
the pole. The significant curvature was developed as a reaction to the
natural growing conditions of trees. The joining between small diameter
timber members within a bundled system would be significantly affected
by the straightness of the material. This unfavourable but typical
characteristic also resulted in the appearance of an elliptical-shaped cross-
section varying across the length of the pole. Consequently, the design of
the test piece using the bundling principle would face the same challenges
in determining the diameter and neutral plane of the specimen.

The connections between members within the bundled system would play
an important role in the designing process. The design of a small-diameter
round timber bundled test piece may consider adapting the connections in
parts compressed (Brito & Junior, 2012, p. 242) to obtain a more stable
cross-section. In these connections (Figure 28), cylindrical logs were
connected using steel threaded bushings, washers and nuts, while certain
parts of the member logs within the system may be sawn off. In addition
to the bolted connections previously used by Bukauskas (2015), the
application of slotted-in metal plates may be considered. Figure 26
described another possible type of connection for a bundled system using
connections with metal rings, steel bars, washers, and nuts.

Figure 28. Connections in elements compresses (Brito & Junior, 2012,

p.245)

Figure 29. A system with metal rings, steel bars, washers and nuts
(Brito & Junior, 2012, p. 245)
 34

5.2 Limitations of the Study

This study used secondary data analysis method to evaluate the data
gathered from the extant literature. The data collected from secondary
sources were first validated in terms of date of publication, credential of
the author, reliability of the source, quality of discussions, depth of
analyses, the extent of contribution of the text to the development of the
research area. However, the huge gap in the dates of publication of
reviewed literature would put questions on the validity and relevancy of
previous data.

The recently increasing diversity and sophistication of round timber

practices resulted in a wide range of secondary data concerning various
issues related to small diameter timbers. However, information about the
specific research focus on bending test arrangement was neither readily
available nor up to date.

Another consideration was the limitation of the qualitative research

method implemented for this thesis. Although the chosen research
method offered a complete description and analysis of the research
subject, it had limited the measurement from the actual bending test.

5.3 Recommendations for Future Research

5.3.1 Application of High Technologies

In Section 5.1, the author discussed the challenge in determining the

neutral plane of the small diameter round timber test piece. Due to the
diversity in the curvature of small diameter timbers, the neutral plane
would require an advanced measuring method. Traditionally, geometric
information about round timber has been gathered using conventional
manual measurement tools such as tape measure or tree calipers. In these
cases, only basic measures could be recorded, while more sophisticated
measurements such as taper or curve could be assumed. Nowadays, the
measurements of highly irregular geometries in small-diameter round
timber specimens would benefit from the development of three-
dimension (3D) scanning technologies and computer-aided design (CAD) in
the construction industry. With the involvement of 3D scanning
technologies, a digital prototype of the specimen would be created with
more precise physical properties regardless of irregular geometries. Future
researchers would then import the digital prototype into the CAD program
to analyse and determine necessary characteristic values required by the
bending test description.

A key challenge with utilizing 3D scanning technologies for investigating

physical characteristics of small-diameter round timbers was a
representation of the scanning result into convenient forms for structural
 35

design, analysis, and fabrication. Future researchers are recommended to

investigate the skeleton representations (Figure 30), in which the area
centroid of circles fitted to a timber surface representation was used for
the alignment of structural members during design.

Figure 30. “Skeleton” representation of a whole timber generated

from 3D scanning (GIM International, 2019)

At this point in the research, the author suggested using Autodesk Revit to
create the digital prototype of the test piece. Future researchers would
model curved beams using the Spline function when sketching the piece.
The properties of the material could then be chosen from the Revit
material library, while changes in properties of round timbers would be
modified. The tapering characteristic of round timber could also be
represented by altering the dimensions of the cross-sections in both ends
of the beam.

However, in order to model a curved beam, only one reference plane is

needed, which means that the beam can only be bent within one plane.
Therefore, prototypes with highly irregular geometries that bend in more
than one plane may not be effectively represented in Revit. Another
challenge in utilising Revit was to select the suitable connections for the
specimen. The author suggested investigating further on the connections
for round timber prototype in Revit, possibly self-modified templates.

5.3.2 Design of Supports

As described in the EN 408 standard, the test piece should be simply

supported. In the existing test for sawn timber specimen, the most popular
types of support for the four-point bending test included cylindrical rollers
and hardened knife-edge supports. While cylindrical rollers showed little
potential for the circular cross-section of small diameter timbers, the knife-
edge support (Figure 31) may be utilized with further investigation. The
manufacturer of bending test equipment provided options of customized
non-standard equipment unique to the specimen due to the awareness of
the diversity and sophistication of testing methods and test pieces. The
author recommended further research on the design of customized
supports with unique attributes satisfying the test requirements.
 36

Figure 31. Hardened support on a standard bend fixture (ADMET,

2019)

The design of universal supports had been mentioned in Section 5.1.

Theoretically, the universal support would be designed in the way that
various different cross-sections could be accommodated. In the bending
test described by de Vries (1998), about 300 specimens with at least five
different diameters had been tested. The supports used in de Vries
experiment showed potential for the design of a universal support.
However, there has been very little information about the development of
universal supports for bending tests from other researchers. In addition,
the supports used in the de Vries test only considered specimens with
single circular cross-sections, while the adaptability of these supports to
more complex cross-sections remained unknown. Therefore, future
researchers should investigate the feasibility of universal support
thoroughly before any further design would be discussed.

5.3.3 Hemp

In order to enhance the physical appearance of the structural members

made of small-diameter timbers, the application of hemp should be
considered. The use of hemp as a building material includes hempcrete,
hemp fibre, and hemp oil sealant. In this case, hempcrete or hemp fibre
could be wrapped around the small-diameter timber elements with highly
irregular geometries to achieve elements with common cross-section and
straightness. This process could potentially increase the overall strength of
the structural members. In addition, the external hemp cover could act as
a perfect insulating material for the timber core.
 37

6 CONCLUSION

The aim of this thesis was to analyse the existing data generated from the
previous Bending test conducted by the Delft University of Technology (de
Vries, 1998). To achieve this aim, the following objectives were proposed
to be pursued:

Objective #1: Conduct research of existing literature for a

comparable study.

Objective #2: Adapt and adopt the methodology for collecting data.

Objective #3: Analyse the collected data from the bending test for
round timbers.

Objective #4: Discuss the findings.

The expected outcomes of the study included adapting the methodology

from the comparable study, using the bending test for round timber to
analyse the existing data, achieving similar results to that of the other
studies, and determining the characteristic strength of round timbers in
different cross-sections by applying the result from this study.

To approach the aim of the study, a literature review was conducted. The
literature review first focused on various aspects concerning the utilisation
of round timber as a construction material including the historical
background, the contemporary applications, benefits of the material, the
challenges for wider acceptance in the construction industry, and available
structural connections for round timbers. The information from the extant
literature which related to the bending test for round timbers in terms of
test reference, test arrangement, test piece, and test result were then
presented.

The research method in this thesis was to conduct a qualitative research

using the inductive approach. The notable findings obtained from the
secondary sources in the literature review, which were the utilisation of
round timbers, the bending test arrangement, and the bundled column
method, were analysed and discussed. Recommendations for future
research on the application of high technologies and the design of test
supports were given after the discussion.

At this stage of the research, the expected objectives were completed as

follows:
 38

Objective #1: Conduct research of existing literature for a

comparable study.

A literature review was conducted concerning the

utilisation of round timbers in construction and
bending test for round timbers

Objective #2: Adapt and adopt the methodology for collecting data.

The bundled column method was adapted and adopted

to fabricate an optimum bundled test piece

Objective #3: Analyse the collected data from the bending test for
round timbers.

The bundled test piece had been analysed.

Objective #4: Discuss the findings.

The notable findings from literature review had been

discussed.

The ultimate goal of this research was to stimulate the adoption of small-
diameter round timber as a structural material. The purpose of this
research was to analyse the existing data generated from the previous
Bending test conducted by the Delft University of Technology (de Vries,
1998) in order to develop suitable test arrangement and test specimen
design, which ultimately increase the accuracy of the experiment.

To address the challenge of effectively determining physical characteristics

of irregular small diameter timbers and designing suitable supports for the
bending test, some important research questions to be explored has been
recommended in this thesis. These include:
 Accurately measuring the physical characteristics of small-diameter
timbers with the application of 3D scanning and CAD programme.
 Developing customised supports with unique attributes satisfying the
bending test requirements
 Adapting the application of hemp in the fabrication of test pieces.

It can be concluded that the most significant finding from this study seems
to be the bundled system as a method to fabricate test pieces for the
bending test. By adapting the bundled column method, an optimum test
piece was produced. The design challenges of a bundled system are
expected to be largely addressed through innovations in connections and
the involvement of advanced measuring techniques. On the other hand,
this research has shown that the design of appropriate supports is the key
objective in the development of bending test for natural round timbers.
 39

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 42

Appendix 1
SELECTED WHOLE TIMBER STRUCTURAL CONNECTIONS (Bukauskas, 2019, p. 759)
 43

Appendix 2
SELECTED STRUCTURAL SYSTEM IN WHOLE TIMBER (Bukauskas, 2019, p. 761)
 44

Appendix 3
THE ROOF TRUSS OF MUTORO INDOOR STADIUM IN JAPAN (Bukauskas, 2019, p. 777)
 45

Appendix 4
CLASSIFICATION OF WHOLE TIMBER BY DEGREE AND TYPE OF PROCESSING (Bukauskas,
2019, p. 759)