Fhk

In deriving these equations, friction forces between the members of the connection are ignored as well as the withdrawal resistance of the fasteners. In EC5 the Johansen yield equations form the basis of the strength equations, however for those failure modes that involve yielding of the fastener, the equations have been modified to include for friction and withdrawal effects.

There are two types of friction effects that can arise in a connection. One will develop if the members are in contact on assembly and the other will arise when the fasteners yield and pull the members together when the fasteners deform under lateral load. The former type of friction will be eliminated if there is shrinkage of the timber or wood products when in service and because of this it is not included for in the EC5 strength equations. The latter type of friction will, however, always arise in failure modes that involve yielding of the fasteners and this has been included for in the EC5 equations relating to such modes.

Consider, for example, a single shear connection formed with a plywood gusset plate and a timber member connected by a single dowel-type fastener as shown in Figure 10.10. Assume that under the lateral shear force on the joint the fastener yields in the gusset plate and in the timber member allowing it to rotate by an angle 0, as shown, and that the coefficient of friction between the gusset plate and the timber is In addition to being subjected to bending, the fastener will be subjected to a tension force Nd due to the withdrawal effect during loading. Force Nd will have a vertical component, Nd sin 0, and a horizontal component, Nd cos 0, the latter compressing the gusset plate onto the timber and inducing an additional vertical resistive force, xNd cos 0, due to friction. The force in the fastener, Fv Rk, will equate to the sum all of the vertical forces in the connection as follows:

Fv,Rk = Nd(sin 0 + x cos 0) + Johansen's yield load for the joint(Fy Rk)

Plywood ^ gusset plate

Timber member

Dowel in single shear

Fig. 10.10. Connection with dowel-type fastener in single shear at the failure condition.

At the failure condition, Nd will be the withdrawal capacity of the fastener and in EC5 the component Nd sin 0 is taken to be Fax,Rk/4, where Fax Rk is the fastener's characteristic withdrawal capacity. Component Nd\x cos 0 is equated to a percentage of Fy k, the Johansen yield load. Taking these effects into account, the characteristic lateral load carrying capacity of a fastener, Fv Rk, in EC5 is written in the following format:

Fv>Rk = friction factor x Johansen yield load + (withdrawal capacity/4)

In EC5 the values used for the friction factor are 5% where the fastener partially yields (e.g. modes (d) and (e) in Table 10.2) and 15% where the fastener fully yields (e.g. mode (f) in Table 10.2). The use of these factors can be seen in equations (10.4)-(10.6), (10.9), (10.10) in Table 10.2 and in equations (10.12)-(10.14) and (10.18)-(10.21) in Table 10.3. In equations (10.14), (10.18) and (10.21) in Table 10.3 the numerical coefficient incorporates a factor of 1.15 for this effect.

To discriminate between the Johansen yield load and the combined withdrawal and friction forces in a connection, the latter are commonly referred to as the rope effect forces, however in EC5 reference is only made to the term Fax Rk/4 as the contribution from this effect.

As previously stated, Fax Rk is the characteristic axial withdrawal capacity of the fastener and is defined in EC5, 8.3.2, for nails and is also applicable to staples. It is defined in EC5, 8.5.2, for bolts and 8.7.2 for screws, and is discussed in 10.8. For those fasteners that are potentially susceptible to withdrawal, the minimum penetration permitted in timber is specified in the following sub-sections.

10.3.5.1 Minimum penetration when using nails (EC5, 8.3.1.2 and 8.3.2)

1. Smooth nails - the minimum pointside penetration (i.e. the penetration of the pointed end of the nail into the timber) is 8d, however at this value the pointside withdrawal capacity of the nail is taken to be zero. Where the pointside penetration is at least 12d, the full characteristic value of the withdrawal strength given in EC5, equation (8.25), can be used and between 8d and 12d the withdrawal strength should be multiplied by (tpen/4d - 2), where tpen is the pointside penetration length, as discussed in 10.8.1.

2. Other nails (as defined in EN 14592 [9]) - the minimum pointside penetration is 6d and at this value the pointside withdrawal capacity of the nail is taken to be zero. With these nails, when the pointside penetration is at least 8d the full characteristic value of the withdrawal strength can be used and between 6d and 8d the withdrawal strength should be multiplied by (tpen/2d - 3), as discussed in 10.8.1.

3. For nails in end grain special rules apply, as given in EC5, 8.3.1.2(4).

10.3.5.2 Minimum penetration when using staples (EC5, 8.4 (3))

1. The minimum pointside penetration (see dimension t2 in Figure 10.5) is 14 x the staple diameter.

10.3.5.3 Minimum penetration of screws (EC5, 8.7.2(3))

1. The minimum pointside penetration length of the threaded part of the screw must be 6 times the outer diameter of the screw, measured on the threaded part.

In EC5, 8.2.2(2), an upper limit is also set for the value of Fax>Rk/4. It is taken to be a percentage of the first term of the relevant strength equations given in Tables 10.2 and 10.3 (i.e. a percentage of the Johansen yield load (Fy k) enhanced by the friction factor associated with the rope effect) as follows:

perentage <

15% Round nails 25% Square nails 50% Other nails 100% Screws 25% Bolts 0% Dowels

The maximum percentage increase is dependent on the type of fastener being used and screws will achieve the greatest enhancement. In single shear connections when using nails, Fax Rk will be the lower of the fastener head pull-through strength (including the withdrawal strength associated with the headside penetration of the fastener) and the pointside withdrawal strength, which are discussed in 10.8.1. When dealing with bolts, the resistance provided by the washers, which is defined in EC5,10.4.3, should be taken into account.

10.3.6 Brittle failure

The EC5 strength equations in Tables 10.2 and 10.3 are only valid if there is no premature splitting or shearing of the timber resulting in a brittle-type failure. To try and eliminate the risk of such failures minimum edge, end and spacing criteria for use with dowel-type fasteners have been developed from testing programmes and the requirements for nails, staples, bolts, dowels and screws are given in Tables 10.8 and 10.9. The spacings and distances referred to in these tables are as shown in Figure 10.11. Also, to prevent splitting in timber when using nails greater than 6 mm in diameter (incorrectly stated to be 8 mm in EC5) and smooth shank screws greater than 6 mm in diameter, pre-drilling must be used and the particular requirements for these fasteners are given in Section 10 of EC5. It is to be noted that pre-drilling may also be necessary

(a) Spacing parallel and perpendicular to grain

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