## F Md fh ft

and at the final mean value condition it will be:

f Md f hf,A\ CTf,fin,t,d = 0-t,2 = I J-^ I y2,fin - — II (7.15b)

The EC5 design requirement is that the design stress be less than or equal to the design tensile strength, /t,o,d, i.e.:

• where the functions are as described previously and the rest of the factors are as follows:

• /t,0,d is the design compressive strength of the flange material parallel to the grain.

• /t,0,k is the characteristic tensile strength of the flange material parallel to the grain. Strength information for timber and LVL is given in Chapter 1 and for glulam in Chapter 6.

• kh is the size effect modification factor for members under tension. It is discussed in Chapter 2 and given in Table 2.11. The largest cross-sectional dimension of the member should be used to evaluate the factor. When dealing with LVL, it is defined as kt, and is associated with the length of the member.

7.3.1.1.2 Bending, shear and buckling stresses in the web

Although the primary function of the web is to support the shear stresses in the section, because it is subjected to compressive and tensile stresses due to bending it must also be able to withstand these stresses. With proprietary beams, the web material is normally bonded to form a continuous section, however where this is not possible web splice plates will be required to transfer the stress resultants at the junction positions.

Further, the web must be checked to confirm that it will not buckle due to shear stresses and that the glued joints between the web and the flanges will be able to transfer the horizontal shear stresses in the section. If concentrated vertical loads have to be supported by the beam, web stiffeners may be required to prevent axial web buckling, but no design guidance is given in EC5 for this condition.

(a) Bending stresses.

The maximum stresses due to bending in the web will arise at the extreme fibre locations at y1 or y2 from the neutral axis as shown in Figure 7.3c.

The maximum design bending stress in the web on the compression side of the section will be at y1:

at the instantaneous condition it will be:

and at the final mean value condition it will be:

The maximum design bending stress in the web on the tension side of the section will be at y2:

at the instantaneous condition it will be:

and at the final mean value condition it will be:

f Md \fE mean, w (1 + ^kdef.fA ffw,fi,,t,d = Ot,3 = --^.fin --TTm-^ (7.19b)

As the web has been transformed to flange material in the equivalent section, to obtain any stress in that element the calculated stress must be multiplied by the appropriate modular ratio for the deformation state as shown in equations (7.18) and (7.19).

The design requirement in Section 9 of EC5 is that the design compressive bending stress from equation (7.18) must be less than or equal to the design compressive bending strength of the web material, /c,w,d, and the design tensile bending stress from equation (7.19) must be less than or equal to the design tensile bending strength of the web material, /t,w,d, i.e.

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