Sls

Joints made with dowels are easy to fabricate. They are inserted into pre-drilled holes having a diameter not greater than the dowel diameter. With a bolted connection, the diameter of the pre-drilled hole in the timber must not be more than 1 mm greater than the bolt diameter. Where steel plates are used in the connection the tolerance on the hole diameter in the steel plate (i.e. the difference between the pre-drilled hole size in the plate and the bolt diameter) will influence the characteristic strength of the connection and this is discussed in Section 10.3.

Some examples of connections formed with metal dowel type fasteners are shown in Figure 10.4.

10.2 DESIGN CONSIDERATIONS

Metal dowel type connections have to satisfy the relevant design rules and requirements of EC5 and the limit states associated with the main design effects are given in Table 10.1. The strength conditions relate to failure situations and are therefore ultimate limit states (ULS) and the displacement condition relates to serviceability limit states (SLS). No displacement limit is given in EC5 for connections, however it is a requirement that the movement at connections in a structure must be taken into account when determining the instantaneous and final displacements of the structure. Also, where connection movement will affect the stiffness distribution in a structure, to be able to determine the stress resultant distribution at the ULS the effect of this movement on stiffness properties has to be included for.

10.3 FAILURE THEORY AND STRENGTH EQUATIONS FOR LATERALLY LOADED CONNECTIONS FORMED USING METAL DOWEL FASTENERS

When subjected to lateral loading, a connection formed using metal dowel fasteners may fail in a brittle or a ductile mode and the design rules in EC5 have been developed to try and ensure that failure will be in a ductile rather than a brittle manner.

The minimum spacings, edge and end distances given in EC5 when using these fasteners have been derived to prevent splitting failure when the connection is being formed and when it is subjected to lateral load. Also, for any member in a connection subjected to a design force at an angle to the grain, a procedure is given in EC5 to ensure that the splitting capacity of the member will exceed the component of the design tension force in the member perpendicular to the grain.

'Dowel' is the generic term used for a fastener that transfers load between connected members by a combination of flexure and shear in the dowel and shear and bearing (referred to as embedment) in the timber. The ductile failure theory used for connections formed with dowels is that the fastener and the timber or wood-based material being

(c) Bolted timber-to-timber connection

(d) Bolted timber-to-timber and bolted steel hanger support

(d) Bolted timber-to-timber and bolted steel hanger support

(f) Fin-plate connection with nearly hidden dowels

(e) Fin-plate bolted connection (steel plates in slots cut in timber)

(f) Fin-plate connection with nearly hidden dowels

(e) Fin-plate bolted connection (steel plates in slots cut in timber)

(g) Dowelled connection (circular profile) (photo courtesy of Axis Timber Ltd, a member of Glued Laminated Timber Association, UK)

(h) Bolted steel-to-timber connection (photo courtesy of Donaldson & McConnell Midlands Ltd, a member of Glued Laminated Timber Association, UK)

Fig. 10.4. Examples of connections formed using metal dowel type fasteners.

Dowel bending strength

Timber or wood based material embedment strength

Dowel rotation

Embedment

Fig. 10.5. Strength/strain relationships used for dowel connections.

connected will behave as essentially rigid plastic materials in accordance with the strength-displacement relationships shown in Figure 10.5.

This assumption considerably simplifies the analysis and using such relationships, based on the possible alternative ductile failure modes that can occur in a connection, Johansen [3] derived the strength equations for connections formed using metal dowel type fasteners in timber. When using such fasteners, the possible failure modes that can arise in timber-to-timber and wood panel to timber connections are shown in Table 10.2 and, for timber-to-steel connections, in Table 10.3. The associated connection strength equations are dependent on the geometry of the connection, the embedment strength of the timber or wood-based material, the bending strength of the fastener and on the basis that the fastener will not withdraw from the connection. In the case of timber-to-steel connections, the strength of the steel plates must also be shown to exceed the connection strength and this should be carried out in accordance with the requirements of BS EN 1993-1-1 [4] and BS EN 1993-1-8 [5].

Further, in regard to timber-to-steel connections, where the steel plate thickness is less than or equal to 0.5 x the dowel diameter (d), in EC5 the plate is classified as a thin plate and when it is equal to or greater than d and the tolerance allowance for the dowel hole is less than 0.1d, it is classified as a thick plate. Strength equations have been derived for connections using each type of plate, and for those formed using steel plates with a thickness between these limits, the strength is obtained by linear interpolation between the limiting values based on thin and thick plate arrangements.

Since Johansen's equations were derived they have been slightly modified and added to by other researchers to enhance the connection strength, and the formulae now used in EC5 for metal dowel type fasteners are given in Tables 10.2 and 10.3 for the relevant failure modes that can occur. The equations given for double shear connections only apply to symmetrical assemblies and if non-symmetrical arrangements are used, new equations have to be developed or approximate solutions can be used.

Connections can be formed with fasteners in single or double shear and examples of each type are shown in Figure 10.6. In the single shear connection, there is one shear plane per fastener and in the double shear connection there are two shear planes per fastener. It is important to note that the equations given in Tables 10.2 and 10.3 refer to the characteristic load-carrying capacity of a fastener per shear plane.

For connections in single shear, the characteristic load-carrying capacity per shear plane per fastener, Fv Rk, will be the minimum value equation for the relevant single shear cases given in Tables 10.2 and 10.3. Because there is only one shear plane this value will also equate to the load-carrying capacity per fastener in the connection and

Table 10.2 Characteristic load-carrying capacity per fastener per shear plane for timber-timber and timber-wood based connections*

Connections in single shear

Failure modes

Characteristic load-carrying capacity per fastener per shear plane, iyRk, is the minimum value from the mode failure equations: (EC5, equations (8.6))

Fv.Rk = /h,l,k ■ h ' d Fv,Rk = /h,2,k • h ■ d

0 0

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