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b h w r.t the modular ratio. In the calculation the mean value of modulus of elasticity of each material, £mean, should be used. The approach also applies to axial stress conditions.

Because the flange width to span ratio in these sections tends to be relatively small, the shear lag effect in the flanges can be ignored and the full flange width is used in the strength calculations.

With these sections, because their lateral stiffness is low, when used in floor construction and subjected to vibrations above the fundamental frequency of the floor, adverse vibration effects will arise. To reduce this effect and also to provide support against lateral and torsional instability, blocking or strutting between the sections, as shown in Chapter 4, is commonly fitted at intervals along the member lengths.

7.3.1.1 Strength analysis of glued thin webbed beams

The strength analysis is undertaken at the ultimate limit states (ULS) and the elastic stress distribution in typical thin webbed sections when subjected to bending is shown in Figure 7.3.

Due to the different creep behaviour of the materials in the composite section, the bending stresses within the section will vary with duration of load and moisture content and this effect has to be taken into account in the analysis. The stress distribution is determined at the instantaneous condition and for the condition arising from creep behaviour, as stated in 2.3.4.2(b), the requirement in EC5,2.2.2(1)P, is that the analysis be undertaken using final mean values of stiffness adjusted to the load component causing the largest stress in relation to strength.

In accordance with the requirements of EC5, 2.2.2(1)P, the value of the design stiffness property at these conditions will be as follows:

(a) At the instantaneous condition,

(b) At the condition associated with the final mean value of stiffness,

mean mean

where the functions are as follows:

• Ed ULS is the design value of the modulus of elasticity at the ULS.

• Gd,ULS is the design value of the shear modulus at the ULS.

• Emean is the mean value of the modulus of elasticity of the element.

• Gmean is the mean value of the shear modulus of the element.

• kdef is the deformation factor for timber and wood-based products and, for connections, it will be as defined in 2.3.2. Values for kdef for timber and some wood-related products are given in Table 2.10.

• f 2 is the factor for the quasi-permanent value of the action (see Table 2.2) causing the largest stress in relation to the strength. If this is a permanent action, a value of 1 should be used.

Based on the section profiles shown in Figure 7.3, taking the flange material as the selected material for the equivalent section and assuming:

(i) the same material is used in each flange and has a mean value modulus of elasticity of £mean,f with a deformation factor kdef,f, and

(ii) a mean value of modulus of elasticity of £mean,w with a deformation factor kdef,w for the web, the equivalent cross-sectional area, Aef, and second moment of area, Ief, of the transformed section will be as follows:

(a) At the instantaneous condition:

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