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 timbertotimber 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 predrilling 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 mediumterm 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
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 Crosssectional area of each of the side As = h ■ t\ members, As
Crosssectional 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 = 135
Variable actions yq Yq = 15
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 mediumduration loading and service class 2, kmod.med
KM.connection 13
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 mediumterm duration variable action:
Nails are driven without predrilling 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
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