Structural steel is a basic construction material, made from specific grades of steel and manufactured in a variety of industry standard cross-sectional shapes (or ‘sections’). Structural steel grades are engineered with specific chemical compositions and mechanical properties formulated for specific applications.
In Europe, Structural Steel must adhere to the European Standard EN 10025, governed by the European Committee for Iron and Steel Standardization (ECISS), a subset of CEN (European Committee for Standardization).
There are numerous examples of European grades of structural steel such as S195, S235, S275, S355, S420, S460 etc. However, for the purposes of this article, the focus will be on the chemical composition, mechanical properties, and applications of S235, S275, and S355. These are three common structural steel grades used in all manner of construction projects across the EU.
Inline 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” “JO”.
Where:
- S – denotes the fact that it is structural steel
- 235 – related to the minimum yield strength of the steel (tested at a thickness of 16 mm)
- J2/K2/JR/JO – material toughness in relation to the Charpy impact or ‘V’notch test methodology
- W – Weathering steel (atmospheric corrosion resistant)
- Z – Structural steel with improved strength perpendicular to the surface
- C – Cold-formed
Based on the manufacturing process, chemical composition and applicable application, further letters and classifications might be used to reference particular grades/products of structural steel.
The EU Standard classifications are integrally not a universal standard and thus a number of corresponding grades with the same mechanical and chemical properties may be used in other parts of the world. For example, structural steels fabricated for the US market must be specified in compliance with the American Society for Testing and Materials (ASTM) International guidelines and will be referenced with an ‘A’ and then the relevant Grade such as A36, A53 etc.
US Equivalent Grades
| EU |
US |
| S235 |
A283C |
| S275 |
A570Gr40 |
| S355 |
A572Gr50 |
In most countries, structural steel is regulated and must match a minimum specific criterion for chemical composition, shape, size, strength etc.
Chemical Composition of Structural Steels - S235, S275 and S355
The chemical composition of structural steel is very important and highly regulated. It is a central factor which defines the mechanical properties of the steel material. In the following table, one can see the maximum % levels of certain regulated elements present in European structural steel grades S235, S275 and S355.
| Grade |
C% |
Mn% |
P% |
S% |
Si% |
| S235 |
0.22 max |
1.60 max |
0.05 max |
0.05 max |
0.05 max |
| S275 |
0.25 max |
1.60 max |
0.04 max |
0.05 max |
0.05 max |
| S355 |
0.23 max |
1.60 max |
0.05 max |
0.05 max |
0.05 max |
The chemical composition of structural steel is extremely important to the engineer and will change with specific grades based on their intended use. For instance; S355K2W is a structural steel that has been hardened (K2) and has been designed with a chemical composition to endure increased weathering (W). Thus, this grade of structural steel will have a marginally different chemical composition to the standard S355 grade.
Mechanical Properties of Structural Steel - S235, S275, S355
The mechanical properties of structural steel are central to its classification and hence, application. Although, chemical composition is a governing factor of the mechanical properties of steel, it is also very important to understand the minimum standards for the mechanical properties (performance characteristics) such as tensile strength and yield strength.
Yield Strength
The structural steel’s yield strength measures the minimum force needed to create a permanent deformation in the steel. The naming convention used in European Standard EN10025 relates to the minimum yield strength of the steel grade tested at 16 mm thick.
| Structural Steel Grade at 16 mm |
Minimum Yield Strength at nominal thickness 16 mm |
| ksi |
N/mm2 (MPa) |
| S235 |
33 000 ksi |
235 N/mm2 |
| S275 |
36 000 ksi |
275 N/mm2 |
| S355 |
50 000 ksi |
355 N/mm2 |
Tensile Strength
The tensile strength of structural steel relates to the point at which permanent deformation takes place when the material is stretched or pulled laterally along its length.
| Structural Steel Grade |
Tensile Strength MPa at Nom thickness between 3 mm and 16 mm |
| S235 |
360 – 510 MPa |
| S275 |
370 – 530 MPa |
| S355 |
470 – 630 MPa |
Typical Structural Steel ‘Sections’/Cross-Sectional Shapes
Structural steel is supplied in many grades but is typically sold pre-formed with a defined cross-sectional shape, engineered for specific applications. For instance, it is common to find structural steel sold in: I-Beams, Z-beams, Box Lintels, hollow structural section (HSS), L-shaped, steel plate etc.
Depending on the preferred application, an engineer will specify a grade of steel (frequently to meet minimum strength, maximum weight, and/or weathering requirements) and the sectional shape, relative to the preferred location and anticipated 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 varied. They are particularly beneficial because they offer the unique combination of good welding properties with assured strengths. Structural steel is a very adaptable product and is frequently favored by the engineer trying to maximize strength or structure while reducing its weight.
It is a fact that the construction sector is the biggest consumer of structural steel, where it is used for all manner of purposes and used at diverse scales. Whether a small box lintel is used to hold 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 the application.
- High rise buildings/skyscrapers
- Factories
- Houses
- Shopping malls
- Offices
- Road barriers
- Train tracks
- Bridges
Sources and Further Reading
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