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Fig. 7.6. Effective flange width when using plywood flanges (comparing the theoretical solution (solid line) with the EC5 criteria in Table 7.2 (dashed line)).

Fig. 7.7. Thin webbed section with a flange on the top and bottom face.

When subjected to design conditions, the stresses in the sections are derived assuming that elastic theory applies and because of the creep effect, as with thin webbed beams, the stresses must be derived at both the instantaneous and the final mean value conditions.

7.3.2.4 Stresses in flanges

A typical glued thin flanged internal I-beam with flanges on the top and bottom faces and subjected to a moment is shown in Figure 7.7.

As the flanges are thin, the stress in each flange due to bending is effectively an axial stress and the design value is taken to be the average value across the flange thickness. With proprietary beams, the flange material is normally bonded to form a continuous section using scarf or finger joints. Where this is not possible, flange splice plates will need to be designed to transfer the stress resultants at the junction position.

The maximum compressive and tensile stress under a design moment Md will arise at yi and y2, respectively, from the neutral axis as shown in Figure 7.7b. When the areas of the compression and tension flanges are not the same, yi and y2 will have different values. Also, the position of the neutral axis will not be the same at the instantaneous and the final mean value conditions, consequently the respective values of yi and y2 will be different for each condition. Following the approach used for glued thin webbed beams, these are referred to in the text as y^mst and y2,inst for the instantaneous state and yijfin and y2jfin for the final mean value condition.

(a) Stress in the compression flange.

The mean design compressive stress in the compression flange will be as follows:

• At the instantaneous condition:

• At the final mean value condition:

The design requirement in EC5, 9.1.2(7), is that the mean flange design compressive stress must be less than or equal to the design compressive strength, i.e.:

fff,inst,c,max,d and CTf,fin,max,d < /c,d (7.37)

Km where the functions are as previously described and /c,k is the characteristic compressive strength (/c,0,k or /c,9o,k, as appropriate) of the flange material. Strength information for timber and the commonly used wood-based structural products is given in Chapter 1. ksys is the system factor and for this situation will be 1.0.

(b) Stress in the tension flange.

The mean design tensile stress in the tension flange will be as follows:

• At the instantaneous condition:

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