## Huu

where Emean = 1 Y."=1 Ei and n represents the number of members in the section.

This is an approximation that is deemed to be acceptable, and, after substituting in equation (8.8), the effective slenderness ratio of each element in the built-up column becomes:

A.ef = (the combination of equations (C.3)

In accordance with the approach given in Chapter 5 for the design of a single member axially loaded column, the relative slenderness ratio of member i in the built-up section, A.rel i, will be:

n V Ei where /c,o,k,; is the characteristic compressive strength of member i loaded parallel to the grain. In EC5 the buckling curves from which the instability factor for each axis of bending is derived are strength related, based on the fifth percentile value of the modulus of elasticity, E0.05ji, and for built-up sections the requirement is that the fifth percentile value is used, such that equation (8.11) becomes:

The kc stability factors (kcy and kcz), referred to in Chapter 5 for the design of single member columns, can now be derived for each member in the section in accordance with the procedures in 5.3.1.

Because of the rotation of the member under axial loading, a shear force component, Vd, will be generated along the interface between the members of the built-up column. Where the built-up section is connected using fasteners, the fasteners must be able to withstand this force and if glue is used the shear stress induced at the member interfaces must be checked.

Where fasteners are used, the shear force, Fi, to be taken by each fastener along the interface will be a function ofVd, and is obtained from equations (B.10) and (C.5) in EC5 as follows:

where Vd is obtained from:

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