O

Figure 5.8 Effect of normal force (here n = N/fcdAc) on slenderness limit, depending on the interpretation of the 10 %

criterion:

1. < 10 % increase of bending moments for a given normal force

2. < 10 % reduction of load capacity for a constant eccentricity (ro = 0,3, 9ef = 0, M01/M02 = 1)

Depending on which of the two basic criteria is chosen, see chapter 2, increasing the axial force will either decrease (1) or increase (2) the slenderness limit as shown in figure 3-2. Criterion 1 will be more severe for high axial loads, when there is little room for bending moments.

Criterion 2, on the other hand, will allow very high slenderness ratios for high axial loads.

In earlier drafts, including the October 2001 "final draft", a slenderness limit independent of the axial force was chosen as a compromise between the two basic criteria; for ro = 0,1 the criterion was then identical to expression (4.62) in the ENV.

In a comment to the "final draft" it was pointed out that a limit independent of the axial force could be much on the unsafe side in certain cases (see next chapter). Therefore, a new model was developed. The following is quoted from 5.8.3.1:

(1) As an alternative to 5.8.2 (6), second order effects may be ignored if the slenderness X is below a certain value Xlim. The following may be used:

where:

X slenderness ratio as defined in 5.8.3.2 A = 1 / (1+0,2^ef) B = (1+ 2ro) / n

0 0

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