Effective section properties
In BS EN 1993-1-1: 2005, effective section properties are required for the design of members with Class 4 cross sections. In this website, effective section properties are given for sections subject to compression only and bending only. Effective section properties depend on the grade of steel used and are given for rolled I sections and angles in S275 and S355. Channels are not Class 4 and therefore no effective section properties are provided. For hot-finished hollow sections, effective section properties are given for S355 and S420. For cold-formed hollow sections, effective section properties are only given for S355.
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 ‘W, F’ 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 UB and UC: |  |
For rectangular hollow sections and square hollow sections: |  |
For parallel flange channels: |  |
For equal angles: |  |
For unequal angles: |  |
For circular hollow sections: | Effective areas are not tabulated for circular hollow sections in this website. BS EN 1993-1-1 6.2.2.5(5) refers the reader to BS EN 1993-1-6. |
For elliptical hollow sections: | Effective areas are not tabulated in this website, but may be calculated from: Compressive resistance of hot-rolled elliptical hollow sections [14] |
where:
De | is the equivalent diameter =  |
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 although BS EN 1993-1-1 classifies some sections as Class 4, their effective area according to BS EN 1993-1-5 is equal to the gross area.
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 have 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.
Cross section properties are given for the effective second moment of area Ieff and the effective elastic section modulus Wel,eff. The identifier 'W' or 'F' indicates whether the web or the flange controls the section Class 4 classification.
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 the range of sections covered by this website, only a selection of the hollow sections become Class 4 when subject to bending alone.
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 sections (UB, UC and channels) and hollow sections.
For the range of sections covered by this website, only a limited number of the hollow sections can be used with an effective plastic modulus Wpl,eff , when subject to bending alone.