Transformed second moment of area 11c ef = 1ef f + 11c ef w of the section for the final deformation condition, 11c.ef 11c ef = 9.83 x 107 mm4

5 ( FSLS.G + FSLS.Q) ■ L4 ,, , , / FSLS.G + FSLS.q\ r2 1 + kdef.

Allowable net final deflection at Mfin.sLs.aii =--Mfin.sLs.aii = 16 mm mid-span at the SLS, m^l^u 250

(adopting Le/250 from EC5 Table 7.2)

The allowable deflection exceeds the actual; therefore OK

Example 7.5.2 A stressed skin panel is used as a flat roof member spanning between two supports 4.5 m apart. The panel is 187 mm deep and its cross-section at an I-shaped beam position is shown in Figure E7.5.2. The clear distance between webs is 585 mm and the panel is glued between the flanges and the web. Including for the self-weight of the structure, the characteristic permanent loading per web is 0.35 kN/m and the characteristic variable short-duration loading is 1.10 kN/m. The timber used for the web is class C22 to BS EN 338:2003 and the flanges are both OSB/3 to BS EN 300:1997, 16 mm and 11 mm thick on the top and bottom faces, respectively, with the faces aligned parallel to the direction of span. The structure functions in service class 2 conditions.

Show that the section will comply with the rules in EC5 at the ULS.

All dimensions are in mm

1. Panel geometric properties

Effective span of the panel, Le Clear distance between webs, bf Web:

Width of the web, bw Clear height between the flanges, hw Area of the web, Aw

Flanges: Top flange thickness, htf Bottom flange thickness, hbf Beam depth, h Effective flange width of an I-beam section of the panel ( (EC5, 9.1.2))

In compression, b1c ef (Table 7.2 (EC5, Table 9.1)) b1cef = min(0.15

b1cef = 400 mm i.e the value is based on plate buckling criteria

Check on the buckling length of the compression flange - based on plate buckling criteria ( (EC5, Clause 9.1.2(5)))

In tension, b1t ef (Table 7.2 (EC5, Table 9.1)) Check on the clear flange width due to the geometric constraint of the section - ratio must not exceed 1:

The design sizes of the flanges will be In compression, bc.ef (Table 7.2 (EC5, Table 9.1))

In tension, bt.ef Effective flange width in compression, bef.c

Effective flange width in tension, bef.t (equation (7.33); EC5, equation (9.12))

b 1t.ef

Le bf

= 1.15 Fails the value is less than 1; therefore OK

bc.ef = b 1c.ef bt.ef = bf bef.c = bc.ef + bw bef.t = bt.ef + bw b1cef = 400 mm bt.ef = 585 mm befc = 447 mm bef.t = 632mm

2. Material strength properties

Table 1.3, strength class C22 (BS EN 338:2003, Table 1)

Characteristic bending strength, fm.k Characteristic shear strength, fv k Mean modulus of elasticity parallel to the grain, E0.mean

Table 1.17, OSB/3 to BS EN 300: 1997 Characteristic compression strength, f0SB.c.0.k

Characteristic tensile strength, f0SB.t.0.k

Characteristic rolling shear strength, foSB.r.k

Mean modulus of elasticity,

In the following analysis, the transformed section is based on the use of the flange material throughout the section.

Note: As the mean E value of the timber is greater than the mean E value of the OSB, there will only be a need to check the stresses in the flange at the instantaneous condition and the web at the final condition.

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

Table 2.2 (UKNA to BS EN 1990:2002, Table NA.A1.1) Factor for quasi-permanent value f2 = 0-0 of variable action, f2 (roof condition)

Table 2.6 (UKNA to EC5, Table NA.3) Material factor for solid timber, ym Ym = 1-3 Material factor for OSB, Yosb.m Yosb.m = 1-2

4. Actions

Characteristic permanent action on the structure, Gk

Characteristic variable (imposed) short-term action on the structure,


Design load due to the critical load combination, Fd

(Table 2.8, equation (c) using the unfavourable condition variable action)

Design moment due to the critical load combination, Md

Design shear force due to the critical load combination, Vd

5. Modification factors

Factor for permanent duration action and service class 2, kmod-perm (Table 2.4 (EC5, Table 3.1))

Factor for short-duration action and service class 2, kmod-short (Table 2.4 (EC5, Table 3.1))

Load sharing factor - ksys

(2.3.7 (EC5,6.6)) (n.b.1.1 can be used if required)

Depth factor for solid timber - take as 1, as the depth is greater than 150 mm, kh

0 0

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