Structural Timber Design Examples

As stated in 4.3, Chapter 4, 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 6.7.1 A series of glulam beams 115 mm wide by 560 mm deep with an effective span of 10.5 m is to be used in the construction of the roof of an exhibition hall. The roof comprises exterior tongued and grooved solid softwood decking exposed on the underside and covered on the top with insulation and a weather protective roof covering. The decking will provide full lateral support to the beam but load sharing between the beams is assumed not to apply. The beams are glulam strength class GL 24h to BS 1194:1999 and will function in service class 1 conditions. The bearing length at the end of each beam is 155 mm. Assume that the limiting value for vertical deflection at the instantaneous condition is span/300 and at the net final condition is span/250, for the loading condition given below:

(a) determine the required pre-camber in each beam;

(b) confirm that the beams will comply with the design rules in EC5.

Beam loading:

Characteristic permanent vertical load on each beam 1.4 kN/m

Characteristic short-term variable vertical load on each beam 2.5 kN/m

z

1. Glulam Beam geometric properties

Breadth of each beam, b

Depth of each beam, h Effective span of each beam, lc Bearing length at each end of a beam, £b

Section modulus of each beam about the y-y axis, Wy

2. Glulam properties

Table 6.2, homogeneous grade GL 24h

Characteristic bending strength, /m.g.k

Characteristic shear strength, /v.g k

Characteristic bearing strength, /c.90.g.k

Mean modulus of elasticity parallel to grain, Eo . g. mean

Mean shear modulus, G0.g.mean

Mean density of each beam (based on the ratio of pm/pk obtained from BS EN 338:2003)

b = 115 mm h = 560 mm I = 10.5m lb = 155 mm b • h2

fm.g.k = 24 N/mm2 fv.g.k = 2.7 N/mm2 fc.90.g.k = 2.7 N/mm2 Lo.g.mean = 11.6kN/mm2

3. Partial safety factors

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

Permanent actions, yg Yg = 1.35

Variable actions, yq Yq = 1.5

Table 2.2 (UKNA to BS EN 1990:2002, Table NA.A1.1)

Factor for quasi-permanent value of = 0.0

variable action, f2

Table 2.6 (UKNA to EC5, Table NA.3) Material factor for glulam at ULS, ym

4. Actions

Self-weight of a beam, Gk.seifwt

Design action from the self-weight of a beam, fkselfwt

Characteristic permanent action on a beam, Gk>p

Characteristic variable (imposed) short-term action on a beam, Qk.p

Design action from permanent action, short-term action and self-weight for the critical load case at the ULS, Fd p (Table 2.8, equation (c) using the unfavourable condition variable action)

5. Modification factors

Factor for short-duration loading and service class 1, kmod.short

Size factor for depth less than 600 mm, kh,

Fd.p = YG ■ Gk.p + YQ ■ Qk.p + YG ■ Gk.selfwt

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