Wxk

0.4Fm uinst u

0.4Fm

(a) Typical nailed/screwed connection (b) Typical bolted connection

Legend:

Fmax is the maximum load taken by the connection; ulnst is the instantaneous slip at the SLS.

Fig. 10.22. Typical instantaneous load-slip behaviour of a metal dowel fastener connections.

type being used, and typical load-slip curves for a nailed or screwed connection and for a bolted connection are shown in Figure 10.22.

With bolts, because of the tolerance required to enable the bolt to be fitted and the bedding in process of the bolt onto the surface of the pre-drilled hole when subjected to the lateral load, there is an immediate slip when loaded and this is shown in Figure 10.22b.

Timber has a relatively low stiffness to strength ratio resulting in comparatively flexible structural systems, and although the significance of ensuring that joint strength criteria are fully achieved in a design, for most structures failure by non-compliance with stiffness criteria is likely to be the more common reason for problems arising during the design life of a timber structure.

The stiffness of a fastener is defined as the ratio of its lateral load per shear plane divided by its slip, and knowing this relationship the slip under any load can be obtained. In EC5 this stiffness property is referred to as the slip modulus. When timber design codes were based on a permissible stress design approach, stiffness criteria were given at the working load condition and in most codes the slip limit for joints was set at 0.15 inches, nominally 0.4 mm. In EC5, different values of stiffness are given for the SLS, Kser (and the ULS, Ku), but no limit is set for joint slip. It is left to the designer to decide on the value that will be acceptable for the structure being designed.

In EC5 the instantaneous slip modulus for design at the SLS, Kser, is taken to be the secant modulus of the load-displacement curve at a load level of approximately 40% of the maximum load able to be taken by the fastener [11], and is shown for a nailed connection in Figure 10.22a. It is seen that the use of a straight-line relationship up to this limit will give a reasonably good approximation to the actual load-slip behaviour of the fastener.

From the results of many tests on joints, the instantaneous slip at approximately 40% of the maximum load has been determined by various researchers. Also, adopting failure modes that entail full yielding of the fasteners and the timber/wood product (generally referred to as type 3 failure modes, e.g. modes (f) and (k) in Table 10.2), which are the most common failure modes in an efficiently designed connection, the joint strength can be evaluated. Multiplying the result by 0.4, the SLS strength is

Table 10.13 Values for Kser for fasteners (in N/mm) in timber-to-timber and wood-based panel-to-timber connections*

Type of fastener used

Serviceability limit state slip modulus Kser ser

Nails

Bolts with or without clearance^

Dowels

Staples Screws

Without pre-drilling With pre-drilling p¿5d 0'8/30 PÍ5d/23 p¿5d 0'8/80 PÍ5d/23 PÍ5d/23 Plm5d/23

t Where there is a clearance allowance for the bolt, this should be added to the connection slip.

obtained. From these data, the slip modulus per shear plane per fastener under service load for different metal dowel type fasteners has been derived and the relationships are given in Table 10.13.

Kser is based on the diameter of the fastener, d (in mm), and where the same timber or wood-based product is used for all of the joint members, it is based on the mean density, pm (in kg/m3) of the material. Where the connection involves members of different densities, pm1 and pm2, the pm to be used in the expressions in Table 10.13 will be:

The instantaneous slip in a connection,u¡nst, is a summation of the slip in the respective members forming the connection, and for the single shear timber-to-timber (or wood-based product) connection in Figure 10.23a, the instantaneous slip will be as shown in Figure 10.23b. There will be slip in member 1 (uinst1) and in member 2

(Uinst2) and Uinst will be:

Where the members have the same properties and, say member 2 is used, u inst = u inst2 + uinst2 = 2uinst2 (10.73)

If one of the members is steel, for the same applied load the slip in the steel member will be effectively zero while the slip in member 2 will remain as before and for this situation:

For the steel-to-timber connection, the instantaneous slip will be half the value of the timber-to-timber connection and consequently its stiffness will theoretically be twice the slip modulus of the timber-to-timber connection, i.e. 2 x Kser. This is an approximation to the real behaviour as it ignores the effect of clearance between the fastener and the steel, rotation of the fastener in the steel member and yielding of the steel member where it is in contact with the fastener, and will result in an overestimate of the stiffness. Although EC5, 7.1(3), states that the slip modulus for steel-to-timber and

Slip in member 1

= uinst1

Slip in member 2

= uinst2

(a) Connection

(b) Loaded timber/timber

(or wood product) connection

Slip in member 1

= uinst1

Slip in member 2

= uinst2

Slip in member 1 = 0

0 0

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