Choosing structural steel may seem easy, but fabricators or engineers will need to evaluate grades on a bill of materials. Choosing the wrong grade can mean the failure of a section on an inspection, the failure of a weld under processing, and a halt on the project while waiting for new material. For businesses purchasing RSJs, UBs, UCs, or other various steel components for a commercial project, knowing the difference between structural steel S275 and S355 is one of the most important specifications you will need to understand.
S275 and S355 are non-alloy structural steels defined by BS EN 10025-2 standard covering all hot-rolled products for use in construction throughout Europe and the UK. With respect to steel designations, the number following the letter indicates minimum yield strength measured in megapascals. S275 will yield at a strength of 275 MPa whereas S355 yields at 355 MPa. A difference in yield strength of roughly 30 per cent is significant. It determines how much load a section can carry before permanently going into a deformed state, and this is why structural engineers will specify S275 or S355 structural steels based on the design load, span, and safety factors for a particular project.
S275 is sufficient for most general fabrication work. Balconies, light industrial, agricultural, and smaller commercial structures often use S275 sections. Loads for these structures are typically in S275’s range. S275 also tends to be less expensive and easier to purchase. S355 gains an advantage in situations involving longer spans, heavier point loads, or place of smallest, lightest steel section, for example in multi-storey frames, bridges, cranes, and heavy plant. Since S355 is stronger, engineers sometimes specify a smaller, lighter beam than allowed in S275. This can significantly help reduce the costs for beam transport and simplify on site beam handling.
There are also differences in how the two grades behave in fabrication. The use of S355 requires slightly more control in welding as hydrogen cracking is a greater concern in the welding of all high strength steels. For example, if the wrong preheating temperature and welding consumables are used for the parent material, S355 is susceptible to cracking. Fabricators should always refer to the welding procedure specification for S355, as assumptions of the S275 line of welding settings will not be adequate.
Incorrectly identifying a grade does more than affect performance; it’s also a compliance issue. Structural steelwork used in the construction of commercial and industrial buildings is subject to Building Regulations and, for larger or more complex structures, the Construction (Design and Management) Regulations. The correct grade information must be captured and can be traced back to a mill certificate. This is why using a stockholder who can supply certified material is more prudent than assuming any RSJ stocked will be adequate. If your project requires S355 and the wrong grade is unknowingly substituted, the ordering party will be held liable.
When ordering materials for your projects, always take the time to review the structural drawing or engineer’s specification. Do not make assumptions based on section size. Always supply your stockholder with the drawing and request the grade and certification be noted on the delivery documentation. If you are unsure of the grade for a particular project, our team can review the specifications and provide you with the requested certified stock, whether it’s S275 sections for a simple frame or S355 for a more rigorous structural project.