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* Incorporating the requirements of NA.2.3 of the UKNA to EC5 and EC0.

* Incorporating the requirements of NA.2.3 of the UKNA to EC5 and EC0.

In EC5, 2.4.1(2)P, it is stated that the design member stiffness property, Ed or Gd, is obtained from the respective mean property value as follows:

Km where Emean is the mean value of the modulus of elasticity of the timber or wood product, and Gmean is the mean value of the shear modulus of the timber or wood product.

However, equations (2.6) and (2.7) will only apply when undertaking a first-order linear elastic analysis at the instantaneous loading condition (i.e. when the load is immediately applied) at the SLS, at which state km = 1, or when undertaking a second-order linear elastic analysis at the ULS, for which condition km will be obtained from Table 2.6. The value of the member stiffness property to be used for other conditions will be different and the values to be used for the alternative situations that will arise in timber design are discussed in 2.3.4.

When dealing with connections, at the instantaneous loading condition the design stiffness at the SLS is obtained using the slip modulus, Kser, given in EC5, Table 7.1, for the type of fastener being used and at the ULS the design stiffness is Ku, which, as stated in EC5, 2.2.2(2), is equal to |Kser. As above, the value of connection stiffness at other conditions will be different and the requirements are also discussed in 2.3.4.

At the failure condition, the design value of the modulus of elasticity and the shear modulus used in strength calculations are the characteristic values E0 05 and G0 05 respectively.

2.2.21 Factors applied to a design strength at the ULS

Having derived the design value of a property, Xd, in accordance with the requirements outlined in 2.2.20, further adjustment may be necessary to take account of other factors that can affect the property, e.g. the effect of instability, system strength, etc. This is taken into account by the application of modification factors and a summary of the factors referred to in EC5 that are most likely to arise in design, the function they fulfil, and the property or strength ratio to which they will apply are given in Table 2.7.

2.2.22 Design values of geometrical data (EC0, 6.3.4)

The design values of geometrical data are the sizes used in the design of the structure and its elements, including, where relevant, the maximum deviation from straightness allowed in Section 10 of EC5. The design value is defined as ad = anom (2.8)

where ad is the nominal reference dimension, and anom is the dimension used in the design drawings and documentation.

Table 2.7 Commonly used modification factors in EC5 that affect design values

Factor symbol

Function of factor

Property or stress Relevant item(s) ratio affected in EC5

kcrit km Allows for stress redistribution when a section is subjected to bending about both the y—y and z—z axes and is stressed beyond the elastic limit. It also accounts for variation in material strength across the member section. kvol Adjusts the design tensile strength perpendicular to the grain for glued-laminated timber and LVL when the stressed volume in the apex zone of double tapered, curved or pitched cambered beams exceeds the reference volume stated in EC5. kdis Adjusts the design tensile stress strength perpendicular to the grain for glued-laminated timber and LVL in the apex zone of double tapered, curved or pitched cambered beams. Takes account of the effect of lateral torsional instability, reducing the design bending strength of a member bent about the strong y—y axis when the associated relative slenderness ratio for bending is > 0.75. Takes account of the effect of axial instability, reducing the design compression strength of a member when subjected to axial compression and the relative slenderness ratio about the y—y (for kcy) and/or z—z (for kcz) axis is > 0. 3. kc 90 Increases the compressive strength of the timber or wood-based structural product perpendicular to the grain by accepting an increase in strain at the failure condition. kv Takes account of the effect of a notch on the shear strength of a member. ksys Increases member strength properties when several similar members, assemblies or components are equally spaced and connected by a continuous load distribution system that is capable of transferring load between the member and those adjacent.

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