Steel sections must be classified before a design can be carried out. This is because steel sections are highly subjected to large compressive stresses and could possibly buckle if the plates comprising the steel section are very slender.
This usually impedes the resistance of steel sections against applied forces causing a section to fail before the full strength of the section is realized. This phenomenon is known as local buckling and the point at which a steel section fails due to local buckling is often taken as the member capacity.
The influence of local buckling on the resistance of a steel section is very complex. However, a simplified approach has been developed that is based on the classification of steel cross-sections into categories depending on the dimensions of steel sections. This approach has been adopted in almost all codes of practice for steel structures.
Classification of Steel Cross-section to Eurocode 3
Clause 5.5.2(1) of BS-EN 1993-1-1 classifies steel sections into four categories. This is based on their ability to form plastic hinges as well as their rotation capacities.
- Class 1 cross-sections are those which can form a plastic hinge with the rotation capacity required from the plastic analysis without reduction of the resistance.
- Class 2 cross-sections are those that can develop their plastic moment resistance but have limited rotation capacity
- Class 3 cross-sections are those in which the stress in the extreme compression fibre of the steel member, assuming an elastic distribution of stresses, can reach the yield strength. However, local buckling is liable to prevent the development of the plastic resistance moment.
- Class 4 cross-sections are those in which local buckling will occur before the attainment of yield stress in one or more parts of the cross-section.
The classification of a cross-section depends on the width to thickness ratio, c/t of the compressed parts, the applied forces and the steel grade. Compressed parts include every part of a cross-section which is either totally or partially in compression under the applied loads.
The limiting values of c/t are given in BS-EN 1993-1-1. This has been simplified to Table 5.2 (sheet 1 of 2) and Table 5.2 (sheet 2 of 2) by The Steel Construction Institute (SCI) using the expression given in the Eurocode. In these tables, the various columns refer to the different types of stress distributions in each part of the cross-section (webs or flanges)
The various compressed parts in a cross section (such as a web or flange) can, in general, be in different classes. In general, a cross section is classified according to the highest (least favorable) class of its compressed parts. For example if the flanges of a UB section is class 1 while it web is class 2, then the whole section is classified as class 2 and designed as such.
For I or H cross-sections and rectangular hollow sections, two types of compressed parts are defined: internal compressed parts (classified according to Table 5.2 (sheet 1 of 2) and outstand flanges (classified according to Table 5.2 (sheet 2 of 2); angles and tubular sections are classified according to Table 5.2 (sheet 2 of 2). A cross-section which fails to satisfy the limits for class 3 is taken as class 4.
Classify the S275 Universal Steel beam section 610 × 229 × 140 according to BS-EN 1993-1-1 assuming the beam is subjected to: (a) Bending, (b) Compression.
Properties: h= 617.2mm, b=230.2mm, d=547.6mm, tw=13.1mm, tf=22.1mm, r= 12.7mm.
Since the flanges and the web are both class 1 in compression and bending the section can be designed as class one.
The section should be designed as class 4.
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