N

Gross cross-section Effective cross-section

Figure 5.1: Effective cross-section under compression

5.4 Bending moment 5.4.1 General

(1)P The moment resistance of a cross-section for bending about a principal axis shall be obtained from the following:

- if the effective section modulus

- if the effective section modulus

/ is the yield strength as defined in 3.1.1(6)P.

Weff is less than the gross elastic section modulus Wei:

Weff is equal to the gross elastic section modulus WQi :

(2)P The effective section modulus Weff shall be based on an effective cross-section that is subject only to bending moment about the relevant principal axis, with a maximum stress ffmaXtEd eclualt0 fy\>hu\ > allowing for the effects of local buckling as specified in Section 4. Where shear lag is relevant, allowance shall also be made for its effects as specified in 5.4.3.

(3) The stress ratio = a2 / ox used to determine the effective portions of the web may be obtained by using the effective area of the compression flange but the gross area of the web, see figure 5.2.

(4)P If yielding occurs first at the compression edge of the cross-section, unless the conditions given in 5.4.2(5)P are met the value of Weff shall be based on a linear distribution of stress across the cross-section.

(5)P For biaxial bending the following criterion shall be satisfied: M,,o„/McyiRd + MzSd/MczRd < 1 ...(5.4)

is the applied bending moment about the major axis;

is the applied bending moment about the minor axis;

is the resistance of the cross-section if subject only to moment about the y - y axis;

is the resistance of the cross-section if subject only to moment about the z - z axis.

where:

My.Sd

MZ,Sd

McyM M:z,Rd

rn rn

WTM1

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