(6) For effective widths at serviceability limit states p should be determined as follows:

Alternative 1: Use expressions (4. la) and (4. lb) butj-eplace the ultimate limit states plate slenderness Xp by the serviceability limit states plate slenderness Xp ser given by:

where:

yb acom,Ed,ser iS l^g6^ compressive stress in the relevant element (calculated on the basis of the effective cross-section) under the serviceability limit state loading.

- Alternative 2: Use expressions (4.4a) and (44b) but replace the reduced plate slenderness Xp red by the serviceability limit states plate slenderness Xp ser from expression (4.5).

(7) In determining the effective width of a flange element subject to stress gradient, the stress ratio ^ used in tables 4.1 and 4.2 may be based on the properties of the gross cross-section.

(8) In determining the effective width of a web element the stress ratio used in table 4.1 may be obtained using the effective area of the compression flange but the gross area of the web.

(9) Optionally the effective section properties may be refined by repeating (7) and (8) iteratively, but using the effective cross-section already found in place of the gross cross-section.

(10) In the case of webs of trapezoidal profiled sheets under stress gradient, the simplified method given in 4.3.4 may be used.

4.3 Plane elements with edge or intermediate stiffeners 4.3.1 General

(1)P The design of compression elements with edge or intermediate stiffeners shall be based on the assumption that the stiffener behaves as a compression member with continuous partial restraint, with a spring stiffness that depends on the boundary conditions and the flexural stiffness of the adjacent plane elements.

(2) The spring stiffness of a stiffener should be determined by applying a unit load per unit length u as illustrated in figure 4.1. The spring stiffness K per unit length may be determined from:

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