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5.8.3.1.6 Discussion

On the whole, the new model gives good agreement with the 10%-criterion (alt. 1), and the main parameters are taken well into account. There is a slight overestimation of the slenderness limit for n = 1,0 and ^ef = 1, table 5.5, also for ro = 0,1 and n = 0,8, table 5.6. However, the overestimates are small compared to the old model (see 3.1.4) and in both cases the values are conservative compared to 10%-criterion alt. 2.

A complete verification of the new model is given in Appendix 1. A more sophisticated model could of course give even better agreement, e.g. by also including the concrete grade, but the present model is considered to be good enough. Concrete grade is indirectly taken into account in the n value.

The importance of considering creep in the slenderness limit is further substantiated in 4.3.3.

5.8.3.2 Slenderness and effective length of isolated members

C5.8.3.2 Effective length

New expressions (5.15) and (5.16) for the effective length of isolated members in frames, were introduced in the second draft. Derived to give accurate estimation, based on the definition of effective length in 5.8.1, they replace figure 4.27 in the ENV as well as expressions (5.22) and (5.23) in draft 1 of the prEN, December 1999.

The expressions in draft 1 were taken from UK proposals, included in comments on the ENV and on earlier EN drafts. It was found that they are very conservative in some cases, giving up to 40 % overestimation of the effective length for braced members and on the unsafe side in other cases, giving up to 20 % underestimation of the effective length for unbraced members.

It has been claimed that the conservativeness was deliberate, in order to cover certain unfavourable non-linear effects. However, the effective length is by definition based on linear behaviour, and the present models are aimed at giving an accurate estimation according to this, without including some hidden allowance for possible unfavourable effects. Such effects are instead explicitly addressed in 5.8.3.2 (5) and in 5.8.7.2 (4). The new expressions also avoid unsafe estimations, as in the case of unbraced members with the previous expressions.

Figure 5.12a and b show comparison between an accurate numerical calculation of the effective length and estimations according to draft 1 (a) and final draft (b) respectively.

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