Engineering solutions and connectionsMichael Flach1, Caroline Frenette2

The act of construction is primarily the addition and the connection of separate elements. The first timber structures weresimply made of piled wood pieces attached together to ensure stability. Later, the introduction of triangular systemsbrought the concept of members and nodes, where the nodes ensured a link by transmitting compression and tractionforces from one member to the others in order to achieve equilibrium.

Later on, the emergence of more complex and effective spatial structures asked for the development of more sophisticatedconnection technologies, as well as industrialised and adaptable assembly systems. The combination of wood with othermaterials, such as steel, concrete, and new synthetic glues and resins lead to a new generation of assembly technologies.

Today, the engineering designer is confronted with a wide spread of technologies that allow wood to be associated withother materials to create complex structural systems. It represents a very interesting challenge to find out the most suitablesolution integrating feasibility, reliability, economy and aesthetics.

The design of connections, being a key-point in the overall design of a structure, always reveals a great source ofcreativity. The examples presented in this paper show the great requirements and technological challenges faced bytoday’s structural wood engineers to elaborate satisfying connection systems.

1. INTRODUCTION

Since man has the tools to cut trees and work out wood, he has used it to build. As long as the length and the size of thewood pieces were sufficient for his needs, he would pile and attach them together.

Later on, building techniques were improved and handcrafted connections were developed. With time, these techniqueswere refined and sophisticated connections entered as a reference in the culture of carpentry as we find it in traditionalJapanese Joinery [1], as well as in basic techniques of carpenter companionship [2] in Europe. Many historical buildingsdemonstrate the ingenuity of traditional assembly techniques and the ability of the carpenters who had the time and thepatience to adjust each piece of wood by hand. This precious inheritance still inspires today’s most spectacular timberstructures.

Nonetheless, the development of the steel industry did not wait to revolutionise the connection techniques for woodstructures. Nails, bolts, and steel pins rapidly replaced Wood dowels. The increasing need for efficient structures pushedthe development of larger steel connectors. The replacement of traditional wood to wood assembly techniques by visiblesteel connections lead to disgraceful decorations found on wood structures built in the modern time of industrialisation.

To follow architects’ and clients’ wishes, and also to improve the constructive protection, visible steel plates have slowlybeen abandoned in the past few years. Inserted inside the wood, and associated to composite glues, today’s steelconnections are more sophisticated and discreet. A whole range of modern assembling techniques allow today’s engineersto design reliable connections adapted to a particular project while maintaining aesthetics and being easy to realise. Butthis design choice requires a good knowledge of the available technologies, as well as competence regarding all theassociated materials, such as metals, concrete, glue, carbon fibres, and many others.

The following examples show the rapid evolution of assembling techniques, which contributed to the development ofinnovative wood structures, always pushing back the limits wood use in modern construction.

1 Consulting Eng., Ass. Prof.2 MASc.

2. STRUCTURAL DESIGN

Connections, joints and junctions are accidents in the structure: they weaken the sections, reduce the fire resistance, andcost money. However, they are necessary to recreate the continuity of structures, to allow triangulation, and to fastendifferent structures together.

Therefore, their numbers have to be minimised in the design of the structure: not only for a better resistance of thestructure but also for economy, since the cost of a truss structure can be estimated by multiplying the cost of one assemblynode by the number of nodes. The following two structures answer to the same structural requirements, are composed ofapproximately the same amount of wood, but are very different regarding the number of nodes (fig.1). The cost and thedesign of each connection are consequently very different.

Figure 1 – Examples of structural systems

3. WOOD TO WOOD CONNECTIONS

Knowing that a connection in compression is easier to realise since the force can transfer from one element to the other bydirect contact, it is convenient to organise the elements of a truss structure in order to have the diagonals work incompression instead of in tension. For the elements that have to work in tension, such as the bottom cord, it is convenientto consider the combination with another material such as steel to reduce the cost of the structure.

Diagonals working in tension Diagonals working in compressionFigure 2 – Types of truss systems

To transmit compression forces, the traditional techniques using direct contact between the two wood elements, such asmortise and tenon and in-cut recess, are suitable, and no visible fasteners are therefore required. Traditional techniquesalways have their place when dealing with small forces, and it is important to remember to use these old tricks that enablewood connections to be made without steel.

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Figure 3 – Chapel in Oyonnax -Diagonals are blocked on hardwood bracketsthat transmit the force through contact slit

is is an example of a modern wood structure built with a technique used for old wood bridges (fig.3). The fabricationhniques were adapted to today’s technologies by replacing the handcrafting by high precision numerical cutting.

ordd to

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is te cweg.6

Figure 4 – La Sanne bridges - The blocking of the diagonals is made trough a steel bracketusing the area of the entire cross-section

er to improve the resistance of this type of connection, it is possible to increase the contact area using steel bracket reinforce to wood surface by resin injection to ensure a perfect fit with no gap between the two surfaces (fig.4).

igure 5 – Chaumont High School - The central nodeompression forces among the diagonals. Since no stee

ype of connection is particularly effective since it usesonnection’s capacity goes up to several hundred tons wiver, it is necessary to introduce a perfect hinge using a).

is filled with a no shrinkage concrete to transfer thel parts are visible, it has a 60-min. fire resistance [3].

the wood capacity through its entire cross-section (fig.5).thout using any sophisticated connectors. For the high loads, metal axle to eliminate secondary effects due to moments

AnothConstnodesonly mannconnetechn

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Figure 6 – Fayette Bridge - Steel brackets fixed with a rotational steel axle to transfera compression force of 2000 kN.

er technique also allows one to eliminate visible connectors; it is the sandwich crossing of continuous elements.ruction using superposed elements such as grid and ribbed shells eliminates the use of heavy steel connectors at the. The connectors are no longer concentrated at few nodes but spread out along the elements. Sandwich crossing ispossible if the ribs are composed of several sections, so that they can be recomposed in a linear and repetitiveer. The connections are made with steel dowels, screws or nails, depending on the thickness the layers. The steelctors are hammered inside the wood, and therefore invisible. There are many possible applications for thisique:

re 7 – Cantercel - The beam grid, covering 1000 mgirders. The connecting steel dow

e connection systems such as steel dowels hammeetry using standardised wood elements put in place ted inside the wood and can take the shear forces, wh

ood elements (fig.8).

Figure 8 – The retaining wall, 135 m long and 7 m h

spectacular structures made of superposed elements ahe continuity kept in each direction in every other lansure the continuity of the broken layer. Screws or nai

2, is composed of two times 2 crossing layers of 10 x 10 cmels are hammered every 12 to 20 cm.

red at each intersection allow the construction of complexon the site. For this retaining wall, the steel connectors areile the compression forces are taken by direct contact between

igh, is made of piled logs

re ribbed shells (fig.9). Thyer. When needed, the nodls are used to connect the d

connected with steel dowels

e ribs are made of crossing plankses are reinforced using steel platesifferent layers together.

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igure 9 – St-Quentin swimming pool [4] -The ribs are mscr

e interest for wood on wood connections is definitely to knsequently, the connection looks nicer, has better protectinomical. However, applications with these connections aar forces. When the engineer has the opportunity to prom more sophisticated and thus costly connections are requige for these connections to resist.

4. COMPOSITE

e nail has undoubtedly been the most popular connectoard to traditional handcrafted wood to wood connections.nails have been produced going from the profiled nail toe been optimised to improve its performance. Hence due

the best force/area ratios among the various known connn hammered. The use of a large number of ribbed naiods allows the transmission of large forces.

Figure 10 – Groissiat pay station – A perfect pin joThe nailed steel plates are hidden o

s therefore possible to create a perfect hinge connection el plates through the nails, to be concentrated on two sure the perfect pin joint.

anks to automated cutting, connections using bolts and y precisely in the workshop in order to ease the erection eloped in Switzerland was improved in order to drill the wood simultaneously. Thus, the connection play is mistant effect, as well as the random distribution amongstpia building at the International Exposition of Lisbon, acity of multiple dowel connection [5].

ade of several layers of crossing planks connected withews

eep the connection simple and reduce steels components.on against fire and water, and most of all, is morere constrained to compression forces, or small tension andote this type of connection, it represents a judicious choice,red when tension forces, shear forces or moments become too

S CONNECTIONS

r and expanded the possibilities of wood construction with Since the appearance of the smooth steel nail, a large variety the toothed steel plate. The size, shape, material and spacing to its simplicity of use and its small size, the nail still has oneectors because it has no play with the wood in which it has

ls associated with pre-drilled steel plates fixed between the

int with all the forces concentrated on a unique axle.n the inside faces of the side beams.

(fig.10). The forces are first transferred from the wood to theteel elements and transferred through the central axle which

dowels are now more reliable (fig.11). The holes are drilledof prefabricated elements on site. The multiple dowel system wood and the multiple steel plates introduced in slits insideinimal, but it is necessary to consider the partial moment a large number of connectors. Since the construction of theit is clear that the Eurocode 5 regulation overestimated the

Figur

The Blarger fittingplay isadaptecapaciof the

This tethe hig

Todaymore findinfloors

It is mhighwglulam

e 11 – Swimming centre of Sete – Hidden steel plaspatial geometry. The pin joint at the top o

between the tim

VD type connectors [6] have been developed to transfthan that of a traditional bolt (fig.12). This technique system on a single high resistance bolt. The risk of a negligible. The anchor bolt is inside the wood, andd to transfer large tension and bending moment foty of the wood is not larger than for traditional bolteplay in the connection, as well as the elimination of th

Figure 12 – Hall of Martigues – BVD Connecto

chnique shows that it is possible to transfer large tenh resistance of steel to concentrate the forces while in

’s technology on connections is particularly orientedefficient structural systems leads to the association og suitable ways to connect them together. As an exhave developed several connecting systems that are n

ost likely to believe that wood structures of the futuray bridge that has just been opened to traffic. The , 60 t of steel and 60 t of concrete (fig.13).

tes connected with steel dowels were used for this complexf the column was necessary in order to avoid interferenceber and the concrete structures

er, in a single anchor connector, a tension force 10 to 20 times uses the injection of a high resistance resin, combined with a

random distribution of forces is very small and the connection thus invisible and protected. These connectors are especiallyrces. Even if the efficiency compared to the load carrying

d connections, this system take advantage of the minimisatione reduction factor for a large number of connectors.

rs

siojec

tof vamow

e wstru

were used to anchor a tension force of 2000 kN

n and moment forces with an efficient connection that usesting the interface wood-steel to eliminate the gaps.

wards composite structures. Indeed, the search for alwaysarious types of materials, and highlights the importance ofple, researchers in the field of wood–concrete composite available in the market.

ill be made with composite systems. This next project is acture, crossing a span of 30 m, is composed of 60 m3 of

Figure 13 – Bridge of Fayette – Axonometry of a wood-steel-concrete structure supporting 300kN trucks.

Each material is used according to its specific qualification to improve the resistance / dead weight ratio of the structurethat had to be studied as a composite system. For example, wood and concrete are perfect partners to realise a light andefficient floor. It was, however, logical to only connect them to the 7 m long secondary girders since the effect ofdifferences in thermal expansion coefficients will affect the behaviour of longer spans. In the longitudinal direction of 30m, the concrete was better connected to the steel beam used for the lower cord of the main trusses (fig.14).

Figure 14 –Bridge of Fayette – The composite deck is made of a wood-concrete transversal carrying system [7]

The main trusses were constructed with a composite wood-steel structure. The large tension forces were taken throughsteel rods, while the compression forces were transferred through steel gusset in direct contact at the extremity of thetimber diagonals. This composite system allowed the designers to achieve lightness, efficiency, and economy.

5. CONCLUSION

These examples show how today’s wood engineer can choose among a large variety of connection systems. It is,however, crucial to study carefully the choice of the carrying system in order to adapt the structure to the material andtherefore to make the most of the real performance of a connection.

It is also important to notice how the continual search for more efficient structures leads to the development of compositestructures where each material is integrated according to its specific performance. This tendency requires more researchon connecting system compatible to the differential behaviour of the various materials.

6. BIBLIOGRAPHICAL REFERENCES

[1] Wood Joints in Classical Japanese Architecture, Kajima Institute Publishing Co Ltd, Japan, 1991[2] European Carpentry Companionship, France[3] Les assemblages dans la construction en bois, Le Govic, CTBA, France, 1991[4] Sequence Bois, CNDB, France, April 2000[5] The Pavilion of Utopia, Proceedings of the 5th WCTE, Montreux, Switzerland, August 17-20, 1998, p 1-80[6] Bauen mit Holz, Bruderverlag, Germany, May 1995[7] Holzbau Magazine, Germany, April 2000