4. Effective section properties
4.1. General
In BS EN 1993-1-1: 2005, effective section properties are required for the design of members with class 4 cross-sections. In this publication effective section properties are given for sections subject to compression only. Effective section properties depend on the grade of steel used and are given for rolled I and H sections and angles in S275, S355 and S460. Channels are not class 4 and therefore no effective section properties are provided.
4.2. Effective section properties of members subject to compression
The effective cross-section properties of class 4 cross-sections are based on the effective widths of the compression parts.
The effective cross-sectional area Aeff of class 4 sections in compression is calculated in accordance with BS EN 1993-1-1, §6.2.2.5 and BS EN 1993-1-5:2006, §4.3 and §4.4.
The effective section properties tables list the sections that can be Class 4 and the identifier 'W', 'F' or 'WF' indicates whether the section is Class 4 due to the web, the flange or both. In rectangular hollow sections subject to bending about the major axis, the flanges are the short sides and the webs are the long sides.
The effective area of the section is calculated from:
| For I and H sections: | A eff = A - 4 tf (1 - ρf ) cf - tw (1 - ρw) cw |
| For parallel flange channels: | A eff = A - 2 tf (1 - ρf ) cf - tw (1 - ρw ) cw |
| For equal angles: | A eff = A - 2 t (1 - ρ) h |
| For unequal angles: | A eff = A - t (1 - ρ)(h + b) |
Expressions for the reduction factors ρf, ρw and ρ are given in BS EN 1993-1-5, §4.4.
The ratio of effective area to gross area (Aeff / A) is also given in the tables to provide a guide as to how much of the section is effective. Note that for some sections although BS EN 1993-1-1 classifies them as Class 4, the effective area according to BS EN 1993-1-5 is equal to the gross area.
4.3. Effective section properties of members subject to pure bending
The effective cross-section properties of Class 4 cross-sections are based on the effective widths of the compression parts. The effective cross-sectional properties for Class 4 sections in bending has been calculated in accordance with BS EN 1993-1-1, §6.2.2.5 and BS EN 1993-1-5:2006 §4.3 and §4.4.
Effective section properties for rolled I and H sections in pure bending are not provided although effective properties have been used, where appropriate, to derive the bending resistance tables.
Equations for the effective section properties are not shown here because the process for determining these properties requires iteration. Also the equations are dependent on the classification status of each component part.
For cross-sections with a Class 3 web and Class 1 or 2 flanges, an effective plastic modulus Wpl,eff can be calculated, following the recommendations given in BS EN 1993-1-1, §6.2.2.4 (1). This clause is applicable to open and hollow sections.