Z Ei

where

;0 coefficient depending on number of storeys, distribution of vertical load etc. LEI total bending stiffness of bracing members; to account for cracking in a simplified way

LEI is based on 0,4-EcdIc; and for uncracked section 0,8 may be used instead of 0,4 L total height

Note Fv,bb is a nominal buckling load, calculated for a secant stiffness representing the relevant ULS conditions (including lateral loading). It is not a load for which "pure" buckling (without eccentricities or lateral loading) would occur.

The coefficient 0,4 (or 0,8) for estimating the stiffness (see H.1.2 (3)) can be compared to 0,3/(1+^ef) in expression (5.26). Expression (5.26) is valid for isolated members, where all the vertical load considered acts on the member itself. Then there effects not only of cracking, but also of non-linearity in compression are considered. The last effect can be strong, particularly in cases where the section is uncracked, usually associated with high vertical load. For the same reason, a higher stiffness value for uncracked section is not given in 5.8.7.2. In a structure, on the other hand, most of the vertical load is normally on the braced units, which means that there is less effect of compression non-linearity on the bracing units, in which case a particular value for uncracked section (0,8) is justified1. A further difference is that the bending moment normally has a more favourable distribution in a bracing unit than in isolated members, which gives less overall effect of cracking. These circumstances together justify the use of 0,4/0,8 instead of 0,3/(1+^ef). Creep is not included in the criterion for neglecting second order effects in structures (as it is for isolated members). The reason is that for global second order effects in structures, the dominating first order effect is wind. In this circumstance, there is little effect of creep, and consequently, the effective creep ratio according to 5.8.4 will be low.

The coefficient E,0 in expression (3-6) depends on various parameters. For constant stiffness, equal load increment per storey and rigid moment restraint at the base, ^0 will depend on the number of storeys and (to some extent) on the distribution of vertical load between braced and bracing members according to Figure 5.16 (the buckling load has been calculated numerically by Vianello's method, and £0 has then been evaluated according to expression (3-6)).

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