meng dissertation final tom lequeux

8/2/2019 Meng Dissertation Final Tom Lequeux

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The Reinforcement of Existing Bolted TimberConnections

by Thomas Lequeuxsupervised by Dr. Mark Evernden

Department of Architecture and Civil EngineeringThe University of Bath

2012

AR40223 MEng dissertation


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I

Abstract

Within timber structures, the connections are widely recognised as the determining factor of

timber member size and geometry. In order to keep up with the architectural and engineering

demands on timber construction, developments in the efficiency of these connections must

match the increases in capacity of newly developed timber materials. The aim of this studywas to develop and investigate an original method of reinforcing existing bolted timber

connections. The effectiveness of embedding Densified Veneer Wood (DVW) strips into

parallel and perpendicular to the grain timber connections was investigated experimentally

and analysed using various mathematical models.

For the parallel to the grain connections, tensile pull out tests were carried out for four

reduced EC5 end spacing connections. One of these was unreinforced; the other 3 had

reinforcement introduced at varied distances between the dowel and the end distance. These

tests were repeated for connections designed to EC5 end distance rules. The introduction of

reinforcement in the reduced end distance connections lead to a 95% increase in connectioncapacity, which represented a capacity close to that of the non-reinforced EC5 end spacing

connection. The addition of a net tension shear failure to the shear plug failure experienced in

the connections was used to analyse the capacity increase due the reinforcement using a shear

plug analysis model.

The reinforcement of the EC5 determined end distance connection led to no significant

increase in connection capacity, and a brittle tensile failure mode was experienced in all four

connections. The reinforcement within these connections failed in axial tension. A

mathematical model was developed to determine the axial tensile load subjected to the

reinforcements within the connections, in order to determine the effectiveness of

reinforcement as a function of the end distances of the connections. Although the model failedto quantify this, its limitations were analysed and its function in determining the influence of

the embedded reinforcement on the splitting capacity timber connections was discussed.

A reduction in connection capacity was witnessed in the reinforced perpendicular to the grain

timber connections. Shear plane analysis models were successfully used to explain this, and

the development of reinforcement introduction to increase the connection capacity was

discussed.

Acknowledgements

I would like to extend my thanks to Mark Evernden for his guidance throughout this project. I

am also very grateful for the support offered to me from Antony Darby, Tim Ibell, Will

Bazeley, Sophie Hayward and Andrew Thomson.


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III

4.3.1 Reinforcement end distance ............................................................................26

4.3.2 Connection capacity and failure mode predictions .........................................26

4.3.3 Results of parallel to grain connection testing ................................................26

4.3.4 Analysis of results of parallel to grain connection testing

for reduced EC5 end distance 3d .....................................................................27

4.4 Conclusions ..................................................................................................................28

5 Stiffness and Strength Analysis of Reinforced Timber Connections .................................29

5.1 Summary .....................................................................................................................29

5.2 Failure modes ...............................................................................................................29

5.3 Parallel to the grain stiffness analysis ..........................................................................29

5.4 Parallel to the grain strength analysis ...........................................................................30

5.4.1 Flexural capacity analysis ..............................................................................31

5.4.2 Full thickness shear plug strength analysis method ........................................32

5.5 Perpendicular to grain strength analysis .......................................................................34

5.5.1 Strength analysis of reinforcement contribution .............................................345.6 Concluding comments .................................................................................................35

6 Conclusion and development of findings...............................................................................36

6.1 Summary .....................................................................................................................36

6.2 Conclusions ..................................................................................................................36

6.2.1 Reinforcement characteristics ........................................................................36

6.2.2 Connection fabrication ...................................................................................36

6.2.3 Connection testing ...........................................................................................36

6.2.4 Strength modelling .........................................................................................36

6.3 Discussion and further work .....................................................................................................37

7 References ................................................................................................................................39


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IV

List of figures

1-1 Typical mortice and tenon joint (Thomson, 2010)

1-2 The Metropol Parasol, Seville

1-3 Timber beam connection: Increased efficiency through reinforcement

2-1 General dowel failure modes for built in metallic dowels (Larsen, 1973)2-2 Assumed behaviour of metallic dowels in bending (Larsen, 1973)

2-3 Assumed behaviour for dowel embedment (Larsen, 1973)

2-4 EYM failure modes for metallic dowel-plate timber connections (Thomson, 2010)

2-5 Brittle Failure Modes- a) net tensions, b) splitting, c) plug shear

2-6 Definition of fastener spacings

2-7 Perpendicular to grain splitting

2-8 Connection failure response perpendicular to grain (Van der Put and Leitjen, 2000)

2-9 Assumed model for beam on elastic foundation analysis (Thomson, 2010)

2-10 Strengthening Schemes (Yang and Smith, 2010)

2-11 Rupture failure mode observed in specimen D (Yang and Smith, 2010)

2-12 Spruce beams reinforced on both faces (Ansell, 2009)

3-1 Perpendicular to the grain embedment testing configuration3-2 Double shear connection testing. a) perpendicular b) parallel post failure

3-3 Load-slip plots for double shear connection testing

3-4 Parallel to the grain pull out testing setup

3-5 EYM failure modes for metallic dowel-plate timber connections (Thomson, 2010)

3-6 Parallel to the grain reinforcement designs. a) cylindrical b) strip

3-7 Perpendicular to the grain loading capacity test setup

3-8 Perpendicular to the grain reinforcement designs. A) non-reinforced B) reinforced

3-9 Reinforcement embedment into timber connection

3-10 Three point bending test experimental setup

3-11 Three point bending test characteristics

3-12 Load-slip plot of three point testing experiment

4-1 Parallel to the grain experimental setup

4-2 Load-slip response for connections loaded parallel to the grain

4-3 Timber element end view post failure: 5

4-4 Load-slip response for connections loaded parallel to the grain

4-5 Timber connection G post failure

4-6 Perpendicular to the grain experimental setup

4-7 Load-slip perpendicular to the grain connection response. 4-8 Failure modes of perpendicular to the grain connections

5-1 Connection failure modes of parallel and perpendicular to the grain timber connections

5-2 Comparison of theoretical and experimental values for connection stiffness

5-3 Two-phase timber failure. a) plug shear b) net tension shear

5-4 Tensile split through reinforcement, connection G5-5 Dowel loading onto timber members and reinforcement

5-6 End view of shear plug perimeter. (a) definition (b) connection A shear plug

5-7 End view of shear plug perimeter. (a) definition (b) connection B shear plug

5-8 Plot comparing theoretical and experimental values of the relationship between the ultimate

load and effective perimeter of the connections

5-9 Plot comparing theoretical and experimental values of connection stiffness and splitting

capacity

5-10 Splitting shear surface of unreinforced connection

5-11 a) Splitting shear surface of unreinforced connection b) Net tension failure bellow dowel

6-1 Perpendicular to the grain connection reinforcement- inside face near surface mounting


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List of tables

2.1 Minimum spacing and end and edge distances for dowels

3.1 Embedment strength and foundation modulus for C16 Whitewood PSE

3.2 Results summary for double shear connection testing3.3 Experimentally obtained material properties of DVW

4.1 Summary of connection dimensions (parallel to the grain)

4.2 Summary of load-slip response of parallel to the grain pull-out tests

4.3 Summary of connection dimensions (perpendicular to the grain)

4.4 Summary of load-slip response of perpendicular to the grain pull-out tests

5.1 Connection tensile fracture loads

List of equations

3.1 EYM failure mode I

3.2 EYM failure mode II3.3 Characteristic fastener yield moment

3.4 Characteristic splitting capacity of the connection3.5 Flexural strength calculation from 3 point bending test

3.6 Youngs Modulus calculation from 3 point bending5.1 Theoretical stiffness of timber connections

5.2 Point load calculation

5.3 Theoretical induced tensile load List of symbols

Embedment strength

Embedment strength parallel to the grain Foundation Modulus of timber Characteristic load carrying capacity of connection Fastener diameter Timber thickness Characteristic fastener yield moment Characteristic splitting capacity of the connection Timber member thickness Timber member depth Distance between loaded edge and centre of most distant fastener mm

Flexural stress

Youngs modulus Connection stiffness Dry density Effective point load subjected from dowel to DVW strip Theoretical induced tensile load of DVW strip Effective length of DVW strip Ultimate connection capacity Total plug shear length (connection end distance) Effective perimeter length of shear plug

Characteristic shear strength of timber


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Chapter 1

Introduction

We are witnessing a revival of the use of timber in structural engineering. Growing concerns

on the exhaustion of fossil fuels and the devastating impact of commonly used construction

materials on the environment has led many to revert back to the use of this sustainable

building materials. Responsibly managed forests are supplying the requirement for timber

with very little carbon impact on the planet. This thesis aims to contribute to existing

developments in timber construction to match the increase in scale at which it is being used.

In the context of construction, timber provides an impressively high strength to weight ratio.

The natural characteristics of timber such as the variations in species, size and shape render it

extremely versatile. The size limitations of timber members due to natural growth geometry

have been overcome by the development of laminated type materials such as glulam. These

materials have revolutionised the scale at which timber structures are being built. The increasein flexural, compressive and tensile properties of laminated timber also permit the design of

reduced sections of timber beams, to satisfy the architectural and engineering demands of the

structure as well reducing the general cost attributed to the construction. The dominant

limiting factor of timber construction is the design of connections. Often the connection

requirements define the form and proportion of a structure. This thesis aims to develop

mechanical reinforcements to improve the efficiency of timber connections.

Modern timber construction has been derived following centuries of knowledge, from which

safe methods of construction, connection details and design limitations have been developed.

Mortice and tenon configurations (Figure 1-1) represent the most common form of connection

used in traditional carpentry (Harris, 1978). The design of entirely timber connections hasexperienced a recent surge in popularity, due to their increased sustainability and low energy

properties. On a structural basis, the use of entirely timber connection holds several key

advantages. Timber connection elements have corrosion resistance properties, as well as

relatively low heat conductivity properties relative to metallic components, increasing their

fire resistance (Thomson, 2010). However, the design of these connections remains widely

the same as those used in traditional carpentry, due to a lack of engineering development.

Research carried out at the University of Bath has led to the successful development of

several non-metallic connection materials.

Figure 1-1: Typical mortice and tenon joint (Thomson, 2010)


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Metallic connections are now common practice in the field of timber construction. The

development of these connections is attributed to timber and steel shortages experienced

during the two world wars which pushed for more economical and efficient designs

(Thomson, 2010). The original timber design code used in the UK BS EN 5268 was derived

from war time construction practice in Canada (Thomson, 2010). This code has been

developed into Eurocode 5 (EC5), which represents the design limits of both the serviceabilityand the ultimate load of structures. The advances of metal connection design have progress

alongside developments of timber members. As the scale of timber structures continue to

increase, connectors have been designed to match the increases in load transfer, as

demonstrated in Figure 1-2.

T

he development of increasingly sophisticated metal connectors have reverted the limitations

in capacity of timber connections back to the timber elements themselves. The research

carried out in this thesis concentrates on an original method of reinforcing timber elementswithin connections, in order to increase connection capacity and efficiency (Figure 2-3).

Fi ure 1-2: The Metro ol Parasol, Seville

Fi ure 1-3: Timber beam connection: Increased efficienc throu h reinforcement

connector

original beam geometry

efficient geometry of reinforced beam


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Chapter 2

Literature Review

2.1 Summary

This chapter includes the summation of the research that has previously been undertaken in

timber connections that use dowel type fasteners. Since the purpose of this thesis is to

discover how timber connections can be reinforced, it is important to establish which

connection type is to be used as a constant upon which the different reinforcement methods

can be tested. The extensive research previously carried out on metallic type fasteners will

assist in deciding which connection to use. Research into non-metallic connections will

provide a number of materials potentially suitable for in-situ composite connection

reinforcements.

An initial review of the current analysis methods and design of timber connections including

metallic dowel fasteners and slot in plate is required. This provides context for thereinforcement types created and subsequent analysis to be carried out. The primary subjects

analysed include connection design in accordance to Eurocode 5 (EC5), connection yield

capacity, brittle splitting, timber shear failure and the deformation of timber connections

under loading.

2.2 Design of Metallic Dowel and Plate Connections

Metallic dowel type connections are designed for two distinct modes of failure; ductile

bearing failures and brittle fracture or shear failures. The design is therefore ruled by the

lower bound calculated capacity.

The European Yield Model (EYM, established by Johansen in 1949) is used to analyse ductile

failure. It is a force equilibrium model whereby the applied load is balanced with the

embedment resistance of the connected structural member and the bending resistance of the

fastener. Brittle facture or shear failure has since been observed at lower loads than those

suggested by the EYM through research carried out by Quenneville and Mohammad (2000)

and Leijten and Van der Put (2004). This research has since been incorporated into EC5 and

will be discussed in Section 2.4.

2.3 European Yield Model

The EYM provides a simple and effective method for calculating the bearing capacity of

metallic dowel connections. It was derived from observations of different failure modes

during experiments and developed into theory through material properties and connection

geometry. It also includes variations in timber member capacity due to different characteristic

embedment strengths (Larsen, 1973).

The dowel effect of the fastener is dependent on its bending resistance and the crushing

resistance of the timber. A second tensile effect of the fastener is dependent on the end

restraint of the fastener and friction between the fastener and timber, and is accounted for by

an appropriate addition of strength to the yield model calculations.

Figure 2-1 overleaf presents the two general cases of built-in dowel failure as derived by

Larsen (1973). Case A depicts the bearing failure of the timber under a relatively stiff dowel,


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Figure 2-2: Assumed behaviour of metallic dowels in bending (Larsen, 1973)

whereas Case B shows the combined bending failure of the dowel and bearing failure of the

timber. Analytical expressions for the two cases have been derived from assumptions made

about the material properties of the metallic dowel and timber connection. Larsen (1973)

modelled the steel material behaviour to be stiff-plastic for simplicity.

The embedment strength of the timber under contact loading of a metallic dowel is

demonstrated in Figure 2-3. Under this arrangement the load-displacement response of dowel

bearing can also be obtained, where P represents the force per unit depth of timber. The forceper unit strength of timber is expressed as the product of the diameter of the fastener d and the

mean stress under the dowel . The assumption is made that pressure beneath the dowel dueto the embedment strength is uniformly distributed (Figure 2-3).In the case of a dowel and thin plate timber connection, the EYM considers three potential

failure modes, represented in Figure 2-4. Mode 1 illustrates the plastic embedment of the

dowel into the timber connection. It represents a connection consisting of a very low

embedment resistance timber coupled with a stiff dowel. Modes 2 and 3 represent

simultaneous failure of the timber in bearing and the formation of plastic dowel yield points.

Connection which fail this way are considered to be more efficient, as they make more use of

the composite strength of the connection and generally use more slender fasteners (Thomson,2010). Research carried out by Thomson (2010) at the University of Bath investigated the

performance of non-metallic dowels within dowel-plate connections, in which modifications

Figure 2-1: General dowel failure modes for built in metallic dowels-

represents embedment

strength (Larsen, 1973)

Case A Case B


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Figure 2-3: Assumed behaviour for dowel embedment (Larsen, 1973)

were applied to the EYM model to carry out strength analysis of GFRP, compressed wood

and oak dowels. From these findings a greater understanding of the fundamental components

of the yield model was acquired for the purpose of this thesis.

2.4 Brittle Failure of Timber Connections

Through the introduction of reinforcement components into timber connections in this thesis,

there is a potential for not only increasing the strength capacity but also changing the failure

modes of the connections. It is therefore important to review brittle failure models as well as

the EYM. Experimental work carried out by Quennville (2009) suggests that dowel-plate

timber connections often fail in brittle modes rather than modes suggested by the EYM under

generally lower loads than predicted. Eurocode 5 dictates the required minimum dimensions

of the connection as well as the required end and edge distances and the spacings between

fasteners to avoid brittle failure.

Metallic dowels required much larger spacings than timber dowels used in traditional

carpentry, as they are much stiffer and stronger. This must be considered when establishing

which failure mode is to be induced, from which the effectiveness of the reinforcement can be

investigated. Figure 2-5 illustrates the various brittle failure modes obtained for single dowel

and plate timber connections.

EC5 contains the relevant minimum spacing requirements to prevent brittle failure in timber

connections, which will be used for the design of the laboratory connections in this thesis.

Table 2-1 summarises the relevant end minimum spacing requirements for dowel and plate

connections. Another consideration highlighted by the EC5 codes is that the load carrying

Figure 2-4: EYM failure modes for metallic dowel-plate timber connections (Thomson, 2010)


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capacity of a multiple fastener connection may be lower than the sum of the individual load

carrying capacities of each fastener. Although this thesis considers single dowel connections

alone, this phenomenon should be considered were the effects of reinforcement on multiple

dowel connections would be investigated.

Research into the failure modes of timber connections loaded parallel to the grain has been

carried out by Quennville and Mohammad (2000). This included a series of tests of the

failure modes and strength of steel bolted timber connections, specifically in the row shear out

and group tear out failure modes. From these results the theory that the longitudinal shear

stress at failure is a function of the smaller of the end spacing or the dowel spacing and the

member thickness. This means that when brittle shear failure governs, there is no advantage

of using different end/dowel spacings since the smaller of the two would cause failure.

Connections loaded perpendicular to the grain have their capacity limited through lowperpendicular strength compared to axial strength of timber. These loads are difficult to

negate since the timber connections at truss node in practice are perpendicular to the grain.

Spacing and end Angle to grain Minimum spacing or

distances edge/end distances ||

Table 2.1 Minimum spacing and end and edge distances for dowels

FFF

Figure 2-5: Brittle Failure Modes- a) net tensions, b) splitting, c) plug shearF F F

(a) (b) (c)

Figure 2-6: Definition of fastener spacings


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The linear elastic fracture mechanics model developed by Van der Put and Leitjen (2000) is

now part of the basis of design of EC5.

A fracture load F for the timber connection shown in Figure 2-7 can be calculated using the

section and connection properties and the deflection of the section at the point where the force

is applied. This mechanism can be used to determine whether splitting will be the dominant

mode of failure. Van der Put and Leitjen (2000) characterised the four modes of failure shownin Figure 2-8.

Mode A- The connection is much stronger than the splitting strength. Embedmentstresses under the fastener will be low. The connection is therefore

overdesigned.

Mode B- The connection strength equals the splitting strength. The embedment stresses

are high. This is an optimally designed connection.

Mode C- The connection causes splitting only after significant slip due to high

embedment stresses and hardening of the timber after yield. This is an under-

designed connection.

Mode D- No Splitting will occur. This connection is under designed.

In his thesis on non-metallic timber connections, Thomson (2010) relates the characterisation

of timber connection failure to the design of reinforced concrete. He states that the capacity ofmechanical fasteners in a timber connection should be under-designed to ensure connection

Figure 2-8: Connection failure response perpendicular to grain (Van der Put and Leitjen, 2000)

Figure 2-7: perpendicular to grain splitting


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ductility prior to ultimate splitting failure, since the over-design of steel reinforcement causes

brittle failure by concrete crushing in RC beams.

2.5 Connection Stiffness

Thomson (2010) derived a method of analysing the stiffness of non-metallic dowel-plateconnections from the beam on elastic foundation theory. By adapting the widely used

Aydoans model (1993) he calculated the effect of using different material dowels on the

overall stiffness of the connection, by deriving the stiffness matrix from the model illustrated

in Figure 2-9:

By simply inputting the connection geometry, foundation moduli and dowel stiffness of a

dowel-plate connection, the stiffness matrix could be used to determine its characteristic

stiffness. The calculated theoretical values for the connection stiffness were on average 55%

of the experimental values obtained, a considerable difference that was attributed to the

significance of the variations in foundation moduli (Thomson, 2010).

There is a possibility of deriving a similar model to establish the influence of reinforcement

on the overall stiffness of dowel-plate connections. The predominant focus for this thesis is

the influence of reinforcement on overall connection strength and not stiffness, as is the case

in the practice of timber design. The application of this model was therefore considered

unnecessary for this thesis. EC5 proposes a simple calculation for an unreinforced connection

stiffness based on the timber density and dowel diameter, a method widely used in timberconnection analysis. This calculation does not consider the effect of the timber grain

orientation when calculating the stiffness of the connection, and can therefore be assumed to

be conservative.

2.6 Carbon Fibre Reinforced Polymer Composite Reinforcement

The use of CFRP composite reinforcements in timber connections has been researched by

Yang and Smith (2010) at the University of Hong Kong. Their research comprised of the

strengthening of single-bolted timber joints with different configurations of CFR which were

then subsequently tested under monotonic loading. The 0.166mm thick CFRP sheets where

arranged to strengthen the timber specimens as illustrated in Figure 2-10.

Figure 2-9: Assumed model for beam on elastic foundation analysis (Thomson, 2010)


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Analysis of the calculated shear stress induced between the CFRP and timber substrate in

scheme D (wrapped), let to the conclusion that the strength of this bolted timber joint was

enhanced by an average of 58%.

The actual effectiveness of the reinforcement is questionable. From the rupture failure mode

observed in specimen D (Figure 2-11), it becomes apparent that the CFRP wrapping around

the bolt provided direct resistance to the connector rather than reinforcing the timber itself.

This led to a failure mode dictating the capacity of the connection shifting from timber

bearing and splitting to an instantaneous failure from CFRP deboning. This failure mode is

undesirable as it limits the capacity to the capability of the CFRP.

2.7 Mechanical repair of timber beams fractured in flexure using bonded-in

reinforcements

The use of bonded-in reinforcement to repair timber beams was investigated the University of

Bath (Ansell, 2009). The reinforcement was introduced in the form of steel or composite

CFRP and GFRP pultruded rods. The rectangular sections were introduced into grooves

following the straightening direction of the fractured beams. The effectiveness of the

reinforcement in restoring the flexural strength of the beam to its un-fractured value was

investigated through a series of 4-point bending tests. The reinforced sections are illustrated in

Figure 2-12.

Figure 2-10: Strengthening Schemes (Yang and Smith, 2010)

- v

Figure 2-12: Spruce beams reinforced on both faces (Ansell, 2009)


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Of the reinforcement materials, the Steel and CFRP rods proved to be the most effective, as in

most cases the flexural strength of the timber was restored to its pre-fracture value, and in

some cases this value was exceeded through reinforcement. In one case the case of the CFRP

reinforcement introduced on both the compressive and tensile face of the beam, a repaired

flexural strength of 251% of the original un-fractured value was achieved. Introducing the

reinforcement into both the top and bottom faces of the beams proved more effective thansimply reinforcing only one of the sides. In most cases the failure mode of the beams were

transformed due to the positioning of the reinforcement introduction, and the quality of the

adhesive to reinforcement bond (Ansel, 2009).

The results of the research carried out by Ansell are very impressive, and imply that boned in

reinforcement would have an even greater influence on the flexural capacity if introduced into

the un-fractured beams. The use of in situ steel to reinforce timber is questionable, due to its

high thermal conductivity and therefore negative impact on the fireproofing of timber

structures.

2.7 Reinforcement Materials

By reviewing findings into timber reinforcement materials and their properties, one can

predict which could potentially increase the bearing capacity of a dowel and plate timber

connection, and how they would influence the failure mode. Suitable materials can be

obtained from research into non-metallic timber connections, where the materials ability to

integrate with timber and increase the bearing capacity of the connection has been

investigated.

2.7.1 Compressed Wood

The use of compressed wood as a replacement for steel connectors in timber structures has

been researched in Japan. It is fabricated by compressing low density () wood suchas Japanese cedar in a hot press for 30 minutes at 130C , where the product is a high density

() wood (Jung, 2008). In the process the clear wood specimens are compressedperpendicular to the grain. The heat and pressure soften the lignin within the cells of the

wood, causing the cells to drift and collapse. A subsequent rapid drop in temperature freezes

the compression.

Where Jung (2008) tested full scale joints made from compressed wood dowel and plate

connections, there were favourable results in their loading capacities. Other advantages of

using compressed wood were cited in the research document such as adding value andstructural integrity to timber structures. The sustainability and fireproof advantages of using

non-metallic reinforcements in conjunction with non-metallic timber connections are also key

driving forces for this research, as discussed Chapter 1.

2.7.2 Densified Veneer Wood

Densified veneer wood (DVW) is a multi-layered material that comprises of beech wood

veneers and phenol formaldehyde resin. Its usage as a component of timber connections has

been widely reported by Leijten (1993). In the experiment carried out by Leitjen (1998),

DVW was glued at the interface between steel fasteners and timber structural members to

provide ductility within the connection. The research also showed that the reinforcementprovided high embedment resistance and prevented splitting of the timber.


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Leitjen (1998) proposed several key advantages of using DVW in timber connections. DVW

is commercially available in a range of forms. Its density is similar to that of some hardwoods

negating the need for specialist equipment to size the reinforcement components. Cross-wise

layered veneers are less affected by load direction compared to unmodified timber. Tests

showed that DVW had embedment strength of up to 160MPA (Leitjen, 1998).

The resin content of DVWs affects directly its structural capacities and material properties.

Fully resin-impregnated DVW is impermeable to moisture, although it has reduced tensions

and bending strength. Leijten (1998) stated that partially resin-impregnated DVW is moresuitable for structural purposes. Because it isnt fully impermeable to moisture, it is only

applicable to certain service classes relating to moisture exposure discussed in EC5.

A series of short term loading tests of DVW was carried out by Leijten (1998) to determine its

embedment strength. The tests were carried out under tension and compression loading of the

material as well as at 45 orientation. The density of the material ranged between 1144-

1253

. This subsequently proved that the embedment strength of DVWs is independent

of the load orientation and type (Leijten, 1998).

2.8 Conclusions

The analysis of the publications featured in this chapter has provided the necessary basis for

the development of this thesis.

- An analysis of the reinforcement of both parallel and perpendicular to the grain

connections will provide the body of this thesis.

- The European Yield Model provides the required timber failure modes which will be

used to assess the influence of reinforcement on the capacity of connections.

- Eurocode 5 provides the guidance to the design of timber connections to which the

dowel, plate, and timber connection parameters will be set. The connections

investigated in this thesis will consist of single dowels, so that the EC5 minimum end

spacing rules will dictate the connection failure modes.

- Reinforcement methods of wrapping and near surface mounting of timber connections

have been proved to significantly increase connection capacity. An understanding of

how the failure mode of a connection may be altered through reinforcement is

important to determine its effectiveness.

- Modified woods provide a high strength alternative reinforcement material to

previously used composite FRPs. They are more sustainable, fire resistant and

provide structural integrity. Studies of various modified wood materials have

provided suitable choices for the reinforcement material.


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Chapter 3

Characterisation of connection components and

selection of reinforcement material

3.1 Summary

This chapter focuses on selecting appropriate timber connections upon which the effect of the

chosen reinforcement would be investigated experimentally, including that of different

reinforcement designs. The development and strength analysis of the chosen connections will

be further assessed in Chapters 4 and 5. For certain connection components, a series of

characteristic tests were carried out to obtain the relevant properties of the materials.

3.2 Timber material and dimensions

The specimens used for the timber connections in experimental work carried out in this thesis

were 48

98 mm planed Whitewood PSE beams, machine graded to C16 strength. This

selection was made on the basis that this timber is particularly prone to splitting (BS EN

1995, 2004), and therefore matches the desired lower bound resistance connection from which

the effect of reinforcement could be analysed. The timber was delivered at 15% moisture

content, and had a mean density of.3.2.1 Timber embedment strength

The embedment strength and stiffness response of the softwood were system properties to

obtain for use in the strength calculations in Chapter 5. The method of acquiring the

embedment strength of the timber specimens a simplified version of BS EN 383 (2003) was

used as proposed by Wilkinson (1991). This method consists of a dowel being loaded incompression in a half hole. The test was carried out for both perpendicular and parallel to the

grain orientations, and was set up as shown below in Figure 3-1. The timber elements were

tested in a Dartec loading machine at a rate of 3mm/minute with the intention of reaching

failure within 300s (Thompson, 2010). Failure in the elements was defined as either the point

where maximum load resistance was reached, or a 5mm displacement (BS EN 383, 2003).

The cyclic loading method for obtaining the embedment strength was carried out as required

by BS EN 383 (2003). The elements used for parallel and perpendicular to the grain testing

measured 10010050 (lengthwidththickness). The dowel used for the test measured16mm in diameter.

3.2.2 Results of embedment strength tests and 5th

percentile analysis method

The characteristic values for the embedment strength and foundation modulus are presented intable 3.1, calculated in accordance with BS EN 14358 (2006). The standard BS EN 26891

Figure 3-1 Perpendicular to the grain embedment testing configuration

F


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(1991) was used to obtain the embedment strength and foundation modulus from theexperimental data, in accordance with the 5

thpercentile data analysis method taken from ASTM

5652-95. Theembedment strengthis a system specific characteristic equal to the maximumload resistance divided by the loaded dowel area. The foundation modulus is taken as theload resistance per unit displacement per unit dowel length (stiffness/unit dowel length) in the

elastic loading range (Thomson, 2010).

3.3 Dowel material and dimensions

The material and dimensions of the dowel and plate components are to remain constantthroughout the experiments. In order to induce connection failure through the timber elements

of the connections, the dowel and plate components must be selected to be sufficiently stiff

and of a relatively high capacity.

In accordance with EC5, the dowel diameter must measure between 8-30mm. The double

shear test proposed in BS EN 26891 (1991) can be used to determine the stiffness and

strength of a dowel relative to the timber capacity of a connection. In his thesis, Thomson

(2010) found that a 12mm steel dowel provided a relatively strong and stiff connection, where

it experienced plastic yielding before eventual timber failure. This experiment was replicated

using a 16mm diameter dowel. It could therefore be established whether the dowel was

sufficiently stiff and strong to induce failure in the timber element before it experiencedplastic deformation.

The testing method is illustrated in Figure 3-2, where the single dowel double shear

connections are subjected to compressive monotonic loading. The connection slip was

measured from the plate displacement. Two connection configurations were tested; in the first

the side members were orientated parallel to the central member, whereas in the second the

orientation was perpendicular. A Dartec universal loading machine rig was used for both tests

to apply loading rate of 1.5mm/minute. This rate was chosen as it provided connection failure

within 300 seconds as required in BS EN 26891 (1991).

Grain orientation Embedment Strength Foundation Modulus () ()

Parallel 24.1 989

Perpendicular 19.8 271

Table 3.1: Embedment strength and foundation modulus for C16 Whitewood PSE

Figure 3-2: Double shear connection testing. Grain orientation a) perpendicular b) parallel post failurea)

b)


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3.3.1 Results and discussion of dowel characterisation tests

The connection yield load, ultimate load and stiffness were extrapolated from the results in

Figure 3-3 using the methods described in Section 3.2.3. In accordance with BS EN 26891

(1991), failure in the connection was defined as a complete loss in loading capacity or the

load at which the slip displacement reached 15mm.

The 16mm steel dowel provided a relatively stiff connection and a similar ultimate load of

approximately 21kN for both grain orientations. The connection hardening experienced

beyond the yield load was due to the bearing of the timber under the dowel loading. The

element cross section illustrated in Figure 3-2 (b) confirms this, as the dowel has experiencedlittle if any plastic deformation.

For the connections in this thesis to qualify as thin plate (BS EN 1995 (2004)) the

connection plates used must have a thickness of less that the diameter of the dowel passing

through. It is common practice for connections of type to have plate thicknesses of between

8mm and 12mm (BS EN 1995 (2004)). A steel plate thickness of 10mm was used throughout

the connection tests in this thesis.

3.4 Parallel to the grain timber connection design

The dimensions for the timber side members for connection testing were selected in

accordance to a series of requirements. For parallel to the grain testing, the overall length was

decided from the EC5 end and dowel spacing requirements as described in Chapter 2. Figure

3-4 illustrates the testing setup, which consists of a high strength connection at the top,

achieved by combining a two dowel in line and plate connection with dimensions determined

by the EC5 minimum spacing rules. The bottom single dowel connection was therefore

designed to be of lower capacity and therefore to fail first. This is where the reinforcement

will be introduced for comparative testing. The side member thickness and width was set at

48mm and 98mm respectively (Figure 3-4). The connection failure achieved in the double

shear testing proved this width to be sufficiently large to mitigate against net shear failure

along the side elements cross section.

Connection orientation Yield Load (kN) Ultimate Load (kN) Stiffness (kN/mm)

Parallel 17.0 21.1 11.8

Perpendicular 11.4 21.7 10.7

Table 3.2: Results summary for double shear connection testing

parallel perpendicular

0

5

10

15

20

25

0 1 2 3 4 5

LoadkN

Connection Slip (mm)

Figure 3-3: Load-slip plots for double shear connection testing


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15

3.4.1 Theoretical capacity of selected connection

The characteristic load carrying capacity of the connection selected above can be calculated

using the EYM equations in EC5. The purpose of the model is to determine whether the

capacity of the connection is determined by the timber bearing capacity (Figure 3-6 (I)) or the

dowel yield capacity (Figure 3-5 (II), BS EN 1995 (2004)).

{ ( )

characteristic load carrying capacity of connection (N) embedment strength parallel to the grain ()

fastener diameter (mm)

characteristic fastener yield moment (Nmm) timber thickness (mm)

Figure 3-5: EYM failure modes

I II

I (3.1)

II (3.2)

Figure 3-4: Parallel to the grain pull out testing setup


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EYM failure mode I- Timber load carrying capacity

EYM failure mode II- Dowel load carrying capacity

49.1kN

Timber member capacity < Dowel yield capacity < plate capacity

These solutions demonstrate that the timber element load bearing capacities the tested

connections are far lower than the characteristic dowel load bearing capacity as desired.

3.4.2 Reinforcement characterisation

This section focuses on the introduction of a reinforcement mechanism into the connection

selected Section 3.4.1. The primary function of the reinforcement is to increase the overall

capacity of the connection. The focus of the selection of the reinforcement in this thesis lies

with its material properties and its method of integration into the existing connection.

3.4.3 Parallel to the grain reinforcement introduction

The basis of reinforcement design in this thesis is to introduce in-situ elements into theconnection to increase the overall capacity and potentially induce a more ductile mode of

failure. By incorporating elements in-situ, there would be no overall change in connection

geometry. The two initial designs are illustrated in Figure 3-6.

Both designs function by redistributing the force applied to the connection through loading of

the central dowel. A modified wood element of a higher strength and stiffness to the timber

was to be near surface mounted to the connection. The following predictions have been made

on the reinforcement performance of each design.

Design A (Figure 3-6, A) illustrates a cylindrical element of the modified wood surrounding

the dowel. By bonding this material into the timber, it would be loaded directly by the dowel,from which the load would be redistributed radially to a larger surface area. Design B (Figure

3-6, B) comprises of a near surface mounted strip of modified wood between the dowel and

the end of the timber connection. Through the introduction a reinforcement element of higher

axial tensile strength than the timber, the connection split should form at a higher load.

In this thesis reinforcement design B will be investigated, as it has been predicted to provide a

significant increase in connection capacity while using considerably less material that design

A. In practice it would also be possible to introduce reinforcement B into an in service frame,

whereas introducing reinforcement around the bolt in service (connection A) would not be. As

well as influence on loading capacity, the influence of the reinforcement on the failure mode

of the connection will be investigated.

(3.3)


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3.5 Perpendicular to the grain timber connection design

The scope of this thesis includes the potential to reinforce perpendicular to grain timber

connections. The primary weakness of these connections is described in Chapter 2 as the

relatively low perpendicular strength compared to the axial strength of timber. When loaded,these connections tend to fail in a fracture mode, where the unzipping of the timber

propagates along the grain from the dowel connection (Haller, 1998).

The test setup used to investigate the reinforcement of perpendicular to the grain connections

is illustrated in Figure 3-7. This is a simplified version of the fracture toughness testing

described in ISO 22157. The timber members were loaded from a central dowel to induce

fracture failure. The side member thickness and depth measured 48mm and 98mm

respectively. The end distance between the dowel and the bottom edge of the timber element

was set to 3d in accordance with EC5 minimum spacing rules. Each side member spanned a

total of 240mm, providing an effective span of 200mm and 20mm of contact between the

timber and steel plates each support (Figure 3-7). A preliminary test showed that this

span/depth ratio was sufficient to induced complete splitting failure horizontally from the

dowel before significant shear failure took place at the supports. The dowel and plate

materials and dimensions used are the same as those described in Section 3.5.

3.5.1 Theoretical capacity of selected connection

As described in Chapter 2, the theoretical fracture load of a connection loaded perpendicularlyto the grain can be calculated using the model derived by Van der Put and Leitjen (2000)

which uses the following equation:

Figure 3-7: Perpendicular to the grain loading capacity test setup

Figure 3-6: Parallel to the grain reinforcement designs. a) cylindrical b) strip

F F

(a) (b)

dowel

DVW strip

timber

Tensile splitting

force


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characteristic splitting capacity of the connection (N)

loaded member thickness (mm)

timber member depth (mm) distance between loaded edge and centre of most distant fastener (mm)Using this calculation the characteristic splitting capacity of the connection is found to be

14kN. As described in Chapter 2, this calculation tends to provide lower bound values for the

splitting capacity of perpendicular connections, and comparisons between the theoretical and

experimental values obtained can be found in Chapter 5.

3.5.2 Perpendicular to the grain reinforcement introduction

A similar reinforcement mechanism to that designed for parallel to the grain connections is

proposed for perpendicular to grain testing. By near surface mounting reinforcement strips of

the modified wood into the timber members as illustrated in Figure 3-8, the connections can

be tested and the influence of the reinforcement on the loading capacity was analysed. Figure

3-8, B illustrates how the reinforcement is designed to function; by introducing the strips in

proximity to the dowel, they will obstruct the unzipping of the timber member and increase

the tensile splitting capacity of the connection. This was predicted induce a loading higher

capacity.

3.7.1 Reinforcement dimensions

The reinforcement strips dimensions to be inserted into the timber connections measure mm (widthheightlength). With the connection side members measuring48mm in width, the strips would cover approximately 25% of this. This proposed width was

chosen to provide an increase in connection capacity through reinforcement than was larger

than the reduction in capacity through removal of timber material. This reasoning has been

illustrated in Figure 3-9. All reinforcement strips are to be introduced to the outside face ofthe side member of the timber connections, as this is considerably more practical to

manufacture than to embed them into the inside face of a full scale connection.

(3.4)

F

Figure 3-8: Perpendicular to the grain reinforcement designs. A) non-reinforced B) reinforced

(A)

(B)

F

F

tensile splitting force


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3.6 Reinforcement material properties

Densified Veneer Wood (DVW) was chosen as the reinforcement material. The advantages of

using this product have been discussed in Chapter 2. For the purpose of strength analysis in

Chapter 5, the flexural capacity and youngs modulus of DVW investigated through a set of

three point flexural strength tests. The material was tested parallel and perpendicular to the

laminate direction. The experimental method adopted was proposed by BS EN 373 589.

The flexural strength of the timber can be calculated from the ultimate load under three point

bending and the dimensions of the elements from equation 3.5.

The flexural strength represents the highest stress experienced in the material at the point of

rupture. Due to the bending nature of the test only the surface fibres experience the highest

stress and therefore determine the strength of the material. If the material was to be testing

solely in tension, the entire element would be subjected to the maximum stress meaning the

weakest fibre would determine its strength. It is therefore evident flexural strength can be

larger than the tensile strength for a given material. For the purpose of the strength analysis in

this thesis they will be considered equal. The Elastic Modulus of the reinforcement elements

are determined from the following equation, where both the ultimate load and the strain

are derived from Figure 3-12.

Figure 3-9: Reinforcement embedment into timber connection

10mm

10mm

F

F/2F/2L/2=70mm L/2=70mm

Figure 3-11: Three point bending test characteristics

(3.5)

(3.6)

Figure 3-10: Three point bending test experimental setup


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The flexural strengths, elastic modulus and compressive strength of the DVW strips have

been summarised in Table 3.3.

Due to the higher value for flexural strength obtained under perpendicular to the laminate

orientation loading, the reinforcement was introduced into the connection so that it was

loaded in this orientation.

3.7 Conclusions

The connections to be tested have been designed in accordance to the relevant British

Standards and EC5. DVW strips were chosen as the reinforcement type to be embedded into

both the parallel and perpendicular timber connections to increase their tensile splitting

capacity. EYM models were used to determine the theoretical capacities and failure modes of

each connection. Material test where carried out to characterise the properties of theconnection elements:

- A compression test provided the embedment strength of the C16 strength class timber

elements

- Double shear compression tests determined that the 16mm diameter steel dowels were

sufficiently stiff and strong to induce timber splitting failure before yielding.

- The tensile strength of the DVW reinforcement was derived from a 3 point bending

test to provide the material characteristic for subsequent analysis.

DVW grain Ultimate loadFlexural

Strength Elastic Modulus Compressive Strengthorientation (kN) () () ()

Perpendicular 0.47 98.7 7173 590

Parallel 0.37 72.1 5525 590

0.0

0.1

0.2

0.3

0.4

0.5

0 1 2 3 4 5

Load(kN)

Deflection (mm)

Figure 3-12: Load-slip plot of three point testing experiment

perpendicularparallel

Table 3.3: Experimentally obtained material properties of DVW


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Chapter 4

Experimental study of DVW reinforced timber connections

4.1 Summary

A series of parallel and perpendicular DVW reinforced timber connections where tested

experimentally. The objective was to assess the stiffness and strength of each connection and

analyse the resulting failure modes. Characterising the failure modes was essential to assess

the impact of the reinforcement on the stiffness and strength of the connections. They

subsequently determined the analysis methods used in Chapter 5.

The material properties of the softwood used throughout can be found in Chapter 4. Care was

taken into ensuring that all the tested connections within a category were of the same

dimensions, where the length was kept constant using a guide attachment to the band saw, and

any fluctuations in member thicknesses where planed to achieve continuity in element

dimension. It was ensured that the timber elements used contained few knots, although in thesamples where some were found they were kept on stronger side of the connection and so

away from the connections to be reinforced.

A vertical axis pillar drill was used to drill the dowel hole in both the timber elements and the

steel plates. The dowel holes were manufactured accurately to ensure a correct alignment in

each connection. This prevented prestresses between timber elements developing from a poor

fit, which would lead to undesired variations in the results. All reinforcement material was

bonded in using Polyten, a high grade assembly adhesive.

4.2 Parallel to grain connection testing

A total of eight parallel to the grain connection pull out tests were carried out. In connections

A, B, C, and D a reduced end spacing of 3d was used for the single dowel connection. For

connections E, F, G and H the EC5 determined 80mm end distance was applied (Chapter 2).

For each set of end distances, three variations in reinforcement distance from the dowel were

tested, to be compared to a non-reinforced connection. These connection dimensions are

summarised in Table 4.1. The connections with the reduced 3d end spacing were tested to

establish whether the reinforcement would in fact increase the capacity of the connection to

that equal to or higher than an equivalent 5d end spacing connection. In practice a reduced

end distance of 3d represents an inefficient connection since the timber failure through

splitting is induced much before dowel failure (EC5). For the purposed of this thesis, an

induced brittle timber failure provided a clear failure mode upon which any influence from

the reinforcement was observed. The 5d end distance tests were carried out to establish

whether there would be an additional increase in capacity of the connection that had already

been designed in accordance to EC5 to mitigate against timber splitting failure (Chapter 2).

The connections were loaded monotonically in tension at a rate of 1mm/minute. This load rate

was in accordance with BS EN 26891 (1991), and was designed induce ultimate failure within

180 to 600 seconds. Ultimate load was taken as complete loss of load resistance or connection

slip of 15mm or over according to BS EN 26891 (1991). The high stiffness of the steel plate

and dowel component meant that load slip was attributed solely to the timber and could

therefore be measured directly from the platen displacement of the plates (Chapter 3). ADartec universal loading machine was used to apply the tensile load throughout. The load and

slip was recorded using strain smart data acquisition system 5000.


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4.2.1 Reinforcement End Distance

The dimensions (width/thickness), end distances and reinforcement distance from dowelx of each timber connection components are tabulated below:

4.2.2 Connection capacity and failure mode predictions

Queenville and Mohammad (2000) concluded in their study of metallic bolted connections

that the typical failure mode for dowel type connections was initialised by a single full

thickness shear plug, followed by complete unzipping of remaining timber. This failure modewas predicted for unreinforced connections A and E. It was expected that where

reinforcements were near surface mounted, there would be an overall increase in strength

capacity of the connections. In the case of the reduced end distance tests B, C, D, it was

expected that the failure mode would be more ductile, due to load distribution along the

reinforcement (Chapter 3.) As for the specimens F, G, H where timber splitting had beenmitigated against through the use of EC5 minimal end spacing rules, it was expected that the

reinforcement would increase the bearing resistance under the loading of the dowel.

Test Connection Side member cross End distance Reinforcement distance

section (b/t) (mm) (x) (mm)A 9648 3d /B 9648 3d 0C 9648 3d 10D 9648 3d 20E 9648 5d /F 9648 5d 0G 96

48 5d 25

H 9648 5d 50

Table 4.1: Summary of connection dimensions

Figure 4-1: Parallel to the grain experimental setup


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4.2.3 Results of parallel to grain connection testing

Table 4.2 summarises the parallel to grain test results. The ultimate load was defined in

accordance with BS EN 26891 (1991) using the ASTM D 5652-95 5th

percentile method to

determine yield load. Figure 4-2 combines the four reduced end spacing connection testresults into a typical load-slip plot.

Test connection Yield Load (kN) Ultimate Load (kN) Stiffness (kN/mm)

A 17.3 17.3 15.2

B 29.8 29.8 13.8

C 32.1 33.8 12.8

D 31.6 32.1 10.35

E 31.1 40.2 17.0

F 28.2 41.2 14.7

G 32.8 37.0 16.2H 31.741 34.9 17.7

4.2.4 Analysis of results of parallel to grain connection testing for reduced EC5 end

distance 3d

All connections with a reduced end spacing 3di.e. A, B, C and D displayed a linear load slip

response up to yielding. Loading was continued until brittle failure was achieved for all

connections. Brittle failure of the timber side elements of all connections occurred post yield,except for connection C where there is evidence of connection ductility beyond elastic

yielding (Figure 4-2).

Table 4.2: Summary of load-slip response of parallel to the grain pull-out tests

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10 12

Force(kN)

Connection slip (mm)

A

B

C

D

E

Figure 4-2: Load-slip response for connections loaded parallel to the grain


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There is clear evidence of a significant increase in both connection yield and ultimate strength

from the unreinforced connection A to the reinforced connections B, C and D. The highest

ultimate load obtained was that of connection D, measuring 33.8kN. This represented a 95%

increase in connection ultimate strength when compared to the non-reinforced connection A.

This increase in connection capacity was predicted in Section 4.2.3.

The load-slip plot of the unreinforced 5d end distance connection E has been included in the

Figure 4-2 for the purpose of comparison. It is clear that the reinforced, reduced end distance

connections achieved a relatively competitive ultimate strength (33.8kN) to that of the

unreinforced 5d end distance connection (40.2kN).

The variations in yield and ultimate strength between reinforced connections B, C and D are

low and are attributed to material inconsistencies and the variable nature of the composition

of timber. Although a reduction in connection stiffness has been recorded from connection A

to connections B, C, D, it is relatively small and is also attributed to material inconsistencies.

A partial thickness shear plug developed in all the 3d end spacing connections. This failuremode represents an increase in the ductility of the connection as opposed to brittle splitting

failure of the timber members (Thompson (2010). The influence of the partial thickness shear

plug on the overall capacity of the connections is unclear from the data in Figure 4-2, and so

has been analysed in the strength analysis section of Chapter 5.

In connections C and D, a crack initially propagated from the dowel down to the

reinforcement at the point of failure, which subsequently formed a partial thickness shear

plug. Connections A and B experienced an instant partial thickness shear plug at the point of

failure. In all the connections the formation of this plug led to a redistribution of loading onto

the second timber element, where a splitting crack formed. In connection A, a partial shear

plug formed in the second element as shown in Figure 4-3.

A change in failure mode was observed for the reinforced connections B,C, and D from that

of connection A. Figure 4-3 illustrates clearly that connection B experienced initial failure

through a partial shear plug. Interestingly, net tension failure subsequently developed from thereinforcement strip downwards. This additional shear plane will be discussed in further detail

in Chapter 5.

Figure 4-3: Timber element end view post failure:

Connection A Connection B


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4.2.5 Results of 5d end distance parallel to grain connection testing

The results of the 5d end distance connection testing has been summarised in a typical load-

slip plot in Figure 4-4. It illustrates that the reinforcement in the connection provided little

influences on the connection capacities, as the ultimate load capacity achieved by the

reinforced connections is similar to that achieved by the unreinforced connection.

The highest ultimate load achieved was through testing of connection F, at 41.2kN. The

stiffness of the 5d end spacing connections was similar to that of the 3d end spacing

connections. The variations in stiffness between the connections were relatively small and

also attributed to material inconsistencies.

Connections E, F, G and H demonstrated a relatively instantaneous connection failure post

yield, caused from a tensile split forming along the entire length of the timber members at the

ultimate load capacity (Figure 4-5). In contrast to the reduced end spacing connections, the

reinforcement strips split along the connection failure.

4.3 Perpendicular to grain connection testing

A total of four perpendicular to the grain connection test were carried out. The dimensions of

the side timber elements of the connections were kept constant and determined in Chapter 3.Connection I was tested without reinforcement, whereas connections J, K, L all contained

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5 6

Force(kN)

Slip displacement (mm)

E

F

G

H

Figure 4-4: Load-slip response for connections loaded parallel to the grain

Figure 4-5: Timber connection G post failure


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reinforcements introduced at different distances x from the central dowel (Figure 4-6). The

connection characteristics are summarised in table 4.3.

Figure 4-6 illustrates the test setup used, where the dowel is loaded into the timber in

compression from the central plate. This method effectively replicated a perpendicular to the

grain pull out test. The timber end distances above and below the dowel are identical, and so

the direction of the load is not relevant and the desired splitting failure mode was achieved.

The connections were loaded monotonically in compression at a rate of 1mm/minute. This

load rate is in accordance with BS EN 26891 (1991). The justification for the use of this load

rate along with definitions of load failure are identical to the parallel to the grain loading test,

and can be found in Section 4.2.1.

4.3.1 Reinforcement end distance

Test Side member cross End distance Reinforcement distance

Connection section (b/t) (mm) (x) (mm)A 9648 3d /B 9648 3d 0C 9648 3d 20D 9648 3d 50

4.3.2 Connection capacity and failure mode predictions

The failure mode predicted for the non-reinforced connection A consists of an initial high

plastic deformation, during which the dowel embeds into the timber. This was predicted to

cause simultaneous hardening of the timber and horizontal splitting of the timber to propagate

from the central dowel. The reinforcement introduced into the connections B, C and D was

expected to obstruct the induced splitting, leading to a failure mode shift from timber

unzipping to a more ductile bearing failure, which would in theory increase the connections

loading capacity.

4.3.3 Results of parallel to grain connection testing

Table 4.4 summarises the parallel to grain test results. The ultimate load was defined from themethod in BS EN 26891 (1991) described in Section 3.2.3. The stiffness plot was calculated

using the ASTM D 5652-95 method described also found Section 3.2.3, from which the Yield

Figure 4-6: Perpendicular to the grain experimental setup

Table 4.3: Summary of connection dimensions


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load of each connection was derived. Figure 4-7 combines the four perpendicular to the grain

connection test results into a typical load-slip plot.

4.3.4 Analysis of results of parallel to grain connection testing for reduced EC5 end

distance 3d

All four perpendicular to the grain connections displayed a linear load slip response up to

yielding. Elastic yielding was followed by significant plastic yielding of the timber (Figure 4-

7), demonstrating connection hardening. Loading was continued until brittle failure was

achieved for all connections. This was represented by timber splitting which propagated fromthe dowel along the grain as predicted in Section 4.3.2. The highest ultimate loading capacity

was achieved by the non-reinforced connection at a load of 22.8kN. The reinforced

connections B, C and D all failed under a lower load, ranging from 21.kN to 22 kN. This

reduction in capacity through the introduction of reinforcement is relatively low. It was

concluded that the reinforcement provided no positive contribution to the connection loading

capacity. However the results do show that all reinforced connection failed at a lower capacity

than the non-reinforced connection. Connection strength analysis in Chapter 5 evaluates how

this reduction in capacity was induced.

The influence of reinforcement on the connection stiffness was negligible and, as explained in

section 4.2.5, was attributed to variations in the timber consistency. The influence of the

Test connection Yield Load (kN) Ultimate Load (kN) Stiffness (kN/mm)A 16.3 22.8 10.1

B 11.9 21.6 8.7

C 14.6 21.2 6.1

D 14.8 22.0 8.1

Table 4.4: Summary of load-slip response of perpendicular to the grain pull-out tests

0

5

10

15

20

25

30

0 2 4 6 8 10

Load(kN)


A

B

C

D

Figure 4-7: Load-slip perpendicular to the grain connection response.


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reinforcement to the connection stiffness could be analysed in further detail, but is

unnecessary as the emphasis of this thesis is on the ultimate capacity of the connections.

Analysis of the non-reinforced Connection A displays the failure mode predicted in Section

4.3.2. Timber splitting along the grain from the dowel occurred at the ultimate load (Figure 4-

8 A). The influence of the reinforcement onto the failure mode of Connection D can be seen

in Figure 4-8 B). The induced splitting was obstructed by the reinforcement strips to a certaindepth, although a full length split along the inside face of the connections proved that the

reinforcement did not stop splitting uniformly across the elements depth. Further analysis of

the failure modes achieved can be found in Chapter 5.

4.4 Conclusions

Parallel and Perpendicular to the grain connection tests have been carried out to determine the

influence of reinforcement on the overall connection capacity and failure modes. From theseresults the following conclusions were drawn:

- The addition of reinforcement to the parallel to the grain 3d end distance connections

led to an increase in connection capacity of up to 95%, with the reinforcement proving

similarly effective in all reduced end distance connections. This increase lead to a

connection capacity close to that of a nonreinforced 5d end distance connection.

The non-reinforced connection failure was initiated by a partial shear plug; whilst

the reinforced connections displayed an additional net tension failure bellow the

DVW strips.

- The addition of reinforcement to the parallel to the grain 5d end distance connectionsproved ineffective to the connection capacity or failure mode. The failure mode

experienced in all 5d end distance connections consisted of instantaneous brittle

failure post yield through tensile splitting.

- The addition of reinforcement to the perpendicular to the grain caused a reduction in

connection capacity. An obstruction to the longitudinal tensile splitting was observed

at the reinforcements.

- Recorded variations to connection stiffness were relatively small and attributed to

material inconsistencies in all tested connections.

Connection A Connection D

Figure 4-8: Failure modes of perpendicular to the grain connections


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Chapter 5

Stiffness and Strength Analysis of Reinforced Timber Connections

5.1 Summary

This section outlines techniques used to predict the failure modes of the reinforced timber

connections and their corresponding capacities. The non-reinforced parallel and perpendicular

to the grain timber connection connections are analysed using the European Yield Model

featured in EC5. An adaptation of this model is subsequently used to predict the influence of

the embedded reinforcement on the strength of the connections.

5.2 Failure modes

For the purpose of strength analysis the connection failure modes obtained experimentally in

Chapter 4 have been categorised using the EYM and illustrated in Figure 5-1 plot. Mode Arepresents an overdesigned connection where brittle timber failure is experienced prior to

steel dowel yielding. This was the dominant failure mode experienced in the parallel to the

grain connection experiments. Mode B represents the failure mode experienced in the

perpendicular to the grain connection tests, illustrating connection hardening followed by

splitting failure of the timber members.

5.3 Parallel to the grain stiffness analysis

The theoretical stiffness of a timber connection can be obtained through a simple calculation

provided in EC5, which considers the connection dowel diameter and the density of thetimber elements used (Equation 5.1).

dry density d dowel diameter (mm)

Equation 5.1 provides a stiffness value per dowel per shear face. The timber density in the

experiments was constant at 350

and the dowel diameter used measured 16mm. The

theoretical parallel to the grain connection stiffness is therefore 9.1kN . A graphicalcomparison is shown in Figure 5-2.

Figure 5-1: Connection failure modes of parallel and perpendicular to the grain timber

connections

(5.1)


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The theoretical calculation provided by EC5 does not take into account the effect of grainorientation on connection stiffness, which suggests it is a conservative method of analysis.

5.4 Parallel to the grain strength analysis

It is clear from the results in Chapter 4 that the reinforcement of the timber connections has

led to a significant increase in connection capacity. This section aims to quantify this increase

in capacity in terms of the change in failure mode induced by the reinforcement.

As described in Chapter 4, the reduced end distance connections failed through the formation

of a full thickness shear plug. The reinforced connections displayed an additional net tension

shear failure which formed from the reinforcement strips (Figure 5-3). In order for the net

shear failure to occur bellow the reinforcement, the reinforcement axial tensile capacity under

dowel loading must have be higher than the force required to induce the failure mode

illustrated in Figure 5-3. Since the reinforcement within the EC5 minimum end spacingconnection failed in axial tension the ultimate connection failure (Figure 5-4), it is assumed

that the axial tensile capacity of the reinforcement was reached.

theoretical stiffness

a)

0

10

20

30

40

50

0 1 2 3 4 5 6 7 8

Forc

e(kN)

Slip displacement (mm)

Figure 5-2: Comparison of theoretical and experimental values for connection stiffness

E) Unreinforced,

Figure 5-3: Two-phase timber failure. a) plug shear b) net tension shear

a) b)

A) Unreinforced,


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The model in Section 5.4.1 attempts to determine at which end distance between 3d and 5d

does the reinforcement fail in tensions before shear plug failure, and therefore at which end

distance does the reinforcement become ineffective.

5.4.1 Reinforcement influence on fracture strength model

This section attempts to quantify the effectiveness of the reinforcement in increasing the

fracture strength of the connections through the characteristic tensile stress capacity of theDVW strips and the ultimate loads of the connections obtained experimentally.

The lateral tensile stress capacity of the DVW strips was derived from the calculations carried

out in Section 3.7.2 as 100N/. For simplification of analysis, the force subjected to thereinforcement through the centrally loaded dowel is taken as a point load, derived from the

UDL subjected to the connection from the dowel (Figure 5-5).This point load P is therefore a

function of the ultimate load, reinforcement width to the timber side member width. The

flexural stress induced to each DVW strip by the point load P is derived from equation 5.2.

The ultimate load subjected onto the reinforcement is taken as the highest recorded ultimate

load achieved experimentally for each parallel to the grain end distance group

. This is to

establish the worst case loading subjected onto the reinforcement. The tensile stress induced

from the point load P to the reinforcement is defined from Equation 5.3.

CV

Figure 5-5: Dowel loading onto timber members and reinforcement

(5.2)

(5.3)

platedowel reinforcement

timber

P

P/2P/2L/2 L/2

Figure 5-4: Tensile split through reinforcement, connection G


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It is clear from the results in table 5.1 that the model has incorrectly derived the theoretical

induced tensile load of the reinforcement, since these values are up to nine times the tensile

strength value obtained experimentally. There are several limitations to this model which may

explain why this is the case. The assumption that the dowel loading can be translated to a

point load onto the DVW is not accurate, since the load has been partially redistributed

through the timber to a more uniform loading arrangement (Figure 3-6,(b)) along the strip.

The load from the dowel itself cannot be considered as a UDL, since a slight rotation of the

dowel during plate loading induces a greater force on the inside face of the timber (Section

5.5). The DVW strip has also been modelled as a simply supported beam whereas in fact it is

supported along its whole length by the timber section bellow it. This means that the 3 point

bending tensile strength value established is not completely representative of thereinforcement effectiveness in the connection, and a beam on elastic foundation of the

reinforcement embedded into the timber would potentially provide a more realistic value of

capacity.

The model does however represent part of the fracture behaviour of the reinforced

connections. From the failure modes obtained, it is apparent that the 3d end distance

connections failed in a series of shear modes before the reinforcement failed in tension

(Chapter 4). The 5d end distance connections failed in instantaneous fracture post yield,

including the splitting of the reinforcement strips. The tensile strength of the reinforcement

was therefore reached between the loads experienced in the 3d and 5d connections. A model

which included the fracture behaviour of timber when loaded by a single dowel could providea more realistic theoretical effectiveness of the embedded reinforcement, and will be

discussed further in chapter 6.

5.4.2 Full thickness shear plug strength analysis method

The theoretical strength of a connection can be determined from the analysis of its post failure

full thickness shear plug (Quenneville and Mohammad (2000); Thomson (2010); BS EN

1995, 2004). To calculate these values the shear strength value of the timber and the area of

timber loaded in shear must be obtained. The characteristic shear strength value per shear

plane,

, for the C16 softwood is taken as

(BS EN 338,2009). The area is taken

as the length of the plug (Figure 5-6 (a)) multiplied by the timber thickness, whichcorresponds to the end distance of the connection.

Connection DVW Tensile strength Theoretical induced tensile load orientation (N) () ()

D 3600 100 756

F 4300 100 903

Figure 5-6: end view of shear plug perimeter. (a) definition (b) connection A shear plug

(a) (b)

Table 5.1: Connection tensile fracture loads


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The theoretical connection strength is calculated using Equation 5.2:

ultimate connection capacity (kN)

total plug shear length (connection end distance ) (mm) effective perimeter length of shear plug (mm) characteristic shear strength of timber (N/m)In order to compare the theoretical connection strength of the reinforced connections, the

shear plug of the connections was analysed using this method. The assumption was made that

the additional shear plug perimeter obtained net tension failure bellow the reinforcement

contributed to the connection strength, and so was added to the effective perimeter of the

shear plug (Figure 5-7). Figure 5-7 illustrates the predicted theoretical relationship between

ultimate capacity and effective shear plug perimeter of each connection, compared to the

actual connection capacity and measured shear plug perimeter of each connection. These

values are summarised in Figure A- of Appendix A.

Figure 5-8 illustrates a clear correlation between the theoretically and experimentally results,

proving that the use of the shear plug strength analysis method is suitable for not only reducedend distance connections but also the reinforced connections. The assumption that the

(5.2)

Figure 5-7: End view of shear plug perimeter. (a) definition (b) connection B shear plug

(a) (b)

theoretical, A

BC

D

E F

GH

0

10

20

30

40

50

60

0 50 100 150 200 250

Ultimate

Load(kN)

Effective Perimeter (mm)

theoretical,

Figure 5-8: Plot comparing theoretical and experimental values of the relationship between the ultimate

load and effective perimeter of the connections

theoretical,


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34

addition of the net tension shear perimeter to the effective perimeter of the connections is

reasonable in the sense that the failure to which is contributed forms in the same plane as the

shear plug. However this method has not been certified and is therefore only appropriate for

the strength analysis of this thesis.

5.5 Perpendicular to grain strength analysis

The theoretical stiffness defined calculated in Section 5.3 also applies to the perpendicular to

the grain connection, since the method by which it was defined doesnt take into account

grain orientation.

Additionally, the experimental results were compared to the characteristic splitting capacity of

the connections, calculated in Section 3.6.2 to be 14kN for the non-reinforced connection

perpendicular to the grain connection (EC 5). Figure 5-9 graphically compares the theoretical

and experimental values obtained for connection stiffness and splitting capacity.

The theoretical stiffness is very close to the experimental values obtained from connection

testing (Figure 5-9). However the predicted splitting capacity is lower than the experimentally

obtained values in all connection results. This splitting capacity is in fact similar than the

yield strength of the connections. This may be because the splitting capacity is a lower bound

value, and the connection actually experienced considerable plastic deformation beforeeventual splitting.

5.5.1 Strength analysis of reinforcement contribution

The experimental findings in Chapter 4 stated that the reinforcement provided no increase in

connection capacity when embedded into the perpendicular to the gain connections. In fact,

the reinforced connections should a reduction in connection capacity.

Through the analysis of the timber splitting at connection failure, this phenomenon may be

explained. Figure 5-10 illustrates the shear surface responsible for connection failure during

splitting.

theoretical

stiffness

theoretical

splitting

capacity

0

5

10

15

20

25

0 2 4 6 8 10

Load(kN)


A

B

C

D

Figure 5-9: Plot comparing theoretical and experimental values of connection stiffness and

splitt

meng dissertation final tom lequeux

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