Timber Tension Member Design

As stated in 4.3, in order to verify the ultimate and serviceability limit states, each design effect has to be checked and for each effect the largest value caused by the relevant combination of actions must be used.

However, to ensure attention is primarily focussed on the EC5 design rules for the timber or wood product being used, only the design load case producing the largest design effect has generally been given or evaluated in the following examples.

Example 10.13.1 A timber-to-timber tension splice connection functioning in service class 2 conditions is required to connect two 50 mm by 150 mm timber members as shown in Figure E10.13.1(a). Two 36 mm by 150mm timber side members will be used, connected by 3.35 mm diameter smooth round nails 65 mm long without pre-drilling and driven from both sides to overlap in the central member. The nails have a tensile strength of 600 N/mm2. The joint is subjected to a characteristic permanent action of 2.5 kN and a characteristic medium-term variable action of 3.5 kN and all timber is strength class C22 to BS EN 338:2003.

Determine the number of nails and the nailing pattern required to comply with the rules in EC5.

36 mm

36 mm

36 mm Side members

150 mm t

150 mm

Nailing zone

1. Geometric properties

Thickness of the side members, ti t\ = 36 mm

Thickness of the inner members, t2 t2 = 50 mm

Width of the members, h h = 150 mm Cross-sectional area of each of the side As = h ■ t\ members, As

Cross-sectional area of the inner Ain = h ■ t2 member, A;n

Nail head diameter, dh dh = 2.25 d

Nail pointside penetration, tpomt

Maximum nail pointside penetration, t2 - tpomt > 4d (Figure 10.6c (EC5, 8.3.1.1.(7)))

Minimum penetration of the nail in the inner timber member, tpoint.pen

point

Both maximum and minimum nail penetration criteria met

2. Timber and nail properties

Table 1.3, strength class C22 (BS EN 338:2003(E), Tablel)

Characteristic tension strength /t.0.k = 13 N/mm2 parallel to the grain, /t 0 k

Characteristic density of the timber, pk

Tensile strength of each nail, /u

3. Partial safety factors

Table 2.8 (UKNA to BS EN 1990:2002, Table NA.A1.2(B)) for the ULS

Permanent actions, yg Yg = 1-35

Variable actions yq Yq = 1-5

Material factor for timber, km , Material factor for connections,

KM. connection

4. Actions

Characteristic permanent action,

Characteristic variable action, Qk

Design tensile action for the critical load combination, Fd (Table 2.8, equation (c) using the unfavourable condition variable action)

5. Modification factors Factor for medium-duration loading and service class 2, kmod.med

KM.connection 1-3

Tension member width factor for solid timber, kh (Table 2.11 (EC5, 3.2))

6. Tension strength of the timber

The critical design load case at the ULS will be due to the combination of permanent and unfavourable medium-term duration variable action:

Nails are driven without pre-drilling and are less than 6 mm in diameter. In accordance with 5.3.3 (EC5, 5.2(3)), the loss of area effect can be ignored.

Design tensile stress parallel a 1t.0.d = to the grain in the side members, a 1t.0.d

Design tensile stress parallel a2t.0.d = to the grain in the inner member, a2t.0.d

Design tensile strength parallel /t.0.d = to grain, /t.0.d

Ain kmod.

Design strength exceeds design stress in members; therefore OK

7. Embedment strength of timber

Characteristic embedment strength of timber parallel to the grain, /h.k

The equation incorporates dimension factors for Mathcad:

Characteristic embedment strength of the headside timber, /h.Lk

Characteristic embedment strength of the pointside timber, /h.2.k

8. Yield moment of a nail

Characteristic yield moment of a nail, My.Rk (Table 10.5 (EC5, equation (8.14)).

fh.1.k = fh.k fh.2.k = fh.k fhk = 19.4 N/mm2 fh.1.k = 19.4 N/mm2

(The equation incorporates dimension factors for Mathcad.) 9. Withdrawal resistance

Nail pointside penetration/nail -d- = 8.66 diameter:

Nail headside penetration/nail diameter — = 10.75 (10.8.1 (EC5, 8.3.2(7))) d

Characteristic pointside withdrawal strength, /paxk (10.8.1 (EC5, 8.3.2(6)&(7)), (The equation incorporates dimensional factors for Mathcad.)

Pointside penetration factor, Dp

^point

+1 0

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