as it is the ratio UK designers are currently more familiar with and it also has a more direct relevance to the design process. The value of the instability factor for solid timber members of strength classes C14-C50 in accordance with BS EN 338:2003 for a range of slenderness ratios up to 180 is given in Table 5.3.

In Table 5.3 the lowest value of slenderness ratio given for each strength class is the value below which buckling will not arise and at which kc,y and kc,z will always be taken as unity.

Using equations (5.3) to evaluate the relative slenderness ratio, A.rel, of the member about the y-y and z-z axes and taking account of the above, the EC5 procedure for the design of axially loaded elements is as follows.

(a) Where both Xrel>y and Xrel>z are <0.3.

Under this condition, the member will not buckle and the design requirement will be that the design stress does not exceed the design strength, i.e.


CTCjo,d is the design compressive stress parallel to the grain and:

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