Structural steel is a standard construction material made from specific grades of steel and formed in a range of industry-standard cross-sectional shapes (or ‘Sections’). Structural steel grades are designed with specific chemical compositions and mechanical properties formulated for particular applications.
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In Europe, structural steel must comply with the European Standard EN 10025, which is governed by the European Committee for Iron and Steel Standardization (ECISS), a subset of the European Committee for Standardization (CEN).
There are many examples of European grades of structural steel – for example, S195, S235, S275, S355, S420, and S460. For the purposes of this article, we will focus on the chemical composition, mechanical properties, and applications of S235, S275, and S355, which are three common structural steel grades used in all manner of construction projects across the EU.
In line with the European Standard classifications, structural steels must be referenced using standard symbols including but not limited to S, 235, J2, K2, C, Z, W, JR, and JO, where:
- ‘S’ denotes the fact that it is structural steel;
- ‘235’ which relates to the minimum yield strength of the steel (tested at a thickness of 16mm);
- ‘J2’, ‘K2’, ‘JR’, and ‘JO’ all demonstrate the material toughness in relation to the Charpy impact or ‘V’ notch test methodology;
- ‘W’ is weathering steel (atmospheric corrosion-resistant);
- ‘Z’ represents structural steel with improved strength perpendicular to the surface, and
- ‘C’ is cold-formed.
Depending on the manufacturing process, chemical composition and relevant application, further letters and classifications might be used to reference particular grades or products of structural steel.
The EU standard classifications are not a global standard and therefore a number of corresponding grades with the same chemical and mechanical properties may be used in other parts of the world. For example, structural steels fabricated for the US market must be specified in accordance with the American Society for Testing and Materials (ASTM). International guidelines are referenced with an ‘A’ and then the relevant grade, for example, A36 or A53.
EU and US Equivalent Grades
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EU
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US
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S235
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A283C
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S275
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A570Gr40
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S355
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A572Gr50
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In most countries, structural steel is regulated and must meet a minimum specific criterion for shape, size, chemical composition and strength.
The chemical composition of structural steel is extremely important and highly regulated. It is a fundamental factor which defines the mechanical properties of the steel. In the following table, you can see the maximum percentage levels of certain regulated elements present in European structural steel grades S235, S275, and S355.
Chemical Composition of Structural Steels - S235, S275 and S355
The Chemical composition of Structural Steel is extremely important and highly regulated. It is a fundamental factor which defines the Mechanical properties of the steel material. In the following table you can see the Max % levels of certain regulated elements present in European Structural steel grades S235, S275 and S355.
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EU Grade
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C%
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Mn%
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P%
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S%
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Si%
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S235
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0.22 max
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1.60 max
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0.05 max
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0.05 max
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0.05 max
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S275
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0.25 max
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1.60 max
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0.04 max
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0.05 max
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0.05 max
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S355
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0.23 max
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1.60 max
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0.05 max
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0.05 max
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0.05 max
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The chemical composition of structural steel is incredibly important to the engineer and will change with specific grades depending on their intended use. For example, S355K2W is a structural steel that has been hardened, denoted by K2, and has been designed with a chemical composition to withstand increased weathering - W. Therefore, this grade of structural steel will have a slightly different chemical composition to the standard S355 grade.
Mechanical Properties of Structural Steel - S235, S275, S355
The mechanical properties of structural steel are fundamental to its classification and application. Even though chemical composition is a dominant factor in determining the mechanical properties of steel, it is also very important to understand the minimum standards for the mechanical properties or performance characteristics, such as yield strength and tensile strength, which are described in more detail below.
Yield Strength
The yield strength of structural steel measures the minimum force required to create a permanent deformation in the steel. The naming convention used in European Standard EN10025 refers to the minimum yield strength of the steel grade tested at 16mm thick.
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Structural Steel Grade at 16mm
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Minimum Yield Strength at nominal thickness 16mm
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psi
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N/mm2 (MPa)
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S235
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33 000
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235 N/mm2
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S275
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36 000
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275 N/mm2
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S355
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50 000
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355 N/mm2
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Tensile Strength
The Tensile Strength of structural steel relates to the point at which permanent deformation occurs when the material is pulled or stretched laterally along its length.
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Structural Steel Grade
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Tensile Strength MPa at Nom thickness between 3mm and 16mm
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S235
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360 – 510 MPa
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S275
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370 – 530 MPa
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S355
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470 – 630 MPa
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Typical Structural Steel ‘Sections’ / Cross-Sectional Shapes
Structural steel comes in many grades but is normally sold pre-formed with a defined cross-sectional shape, designed for specific applications. For example, it is common to find structural steel sold in I-beams, Z-beams, box lintels, hollow structural section (HSS), L-shaped and steel plate.
Depending on the desired application, an engineer will specify a grade of steel -usually to meet minimum strength, maximum weight and possibly weathering requirements, as well as the sectional shape - relative to the desired location and expected load to be carried or job to be performed.
Applications of Structural Steel
Structural steels are used in many ways and their application can be diverse. They are particularly useful because they offer the unique combination of good welding properties with guaranteed strengths. Structural steel is an extremely adaptable product and is often favored by engineers trying to maximize strength or ‘S’ structure while minimizing its weight.
It will come as no surprise that the construction industry is the biggest consumer of structural steel, where it is used for a number of purposes. Whether a small box lintel is used to carry the load of a structural wall in a residential property or a vast I-beam is bolted in place to hold the road surface on a bridge, structural steel can be specified, designed and fabricated for any type of job.
This information has been sourced, reviewed and adapted from materials provided by Masteel UK Ltd.
For more information on this source, please visit Masteel UK Ltd.
This article was updated on 7th August, 2019.