Heavy Wall Carbon steel square tube 120x120x6mm S355J2H Greenhouse Frame Structural Tube

S355J2H is the high strength hollow section grade, and at 120 x 120 x 6 mm it is being used for real structure: greenhouse portals with wide spans, industrial shed columns, crane runway supports and heavy machine frames. Yield is 355 MPa minimum, which is 29 percent more than S275 at the same section, so a lighter wall or a smaller profile can often carry the same load and save steel weight overall.

The J2 part of the designation is the impact requirement: 27 joules minimum at minus 20 degrees Celsius. That matters for structures in northern markets, where a frame that is fine in summer can become brittle in a January cold snap if it was bought on yield strength alone. The test result is reported per heat on the mill certificate, not declared as a general property.

• 355 MPa Minimum Yield: High strength grade that lets the designer reduce wall thickness or profile size against an S275 section at equal load.
• J2 Cold Temperature Impact: 27 J minimum at minus 20 degrees Celsius, certified per heat, which is the requirement most cold climate projects miss.
• Heavy Wall for Bolted Connections: 6 mm wall gives full thread engagement for M16 bolts and allows welded end plates without local reinforcement.
• Greenhouse and Span Geometry: 120 mm face depth provides the bending stiffness needed for wide clear spans without intermediate columns.
• Controlled Corner Radius: Outside radius at 12 mm keeps the section predictable in bolted clamp connections and gutter profiles.
• Full Traceability: Heat number on every bundle, matching a 3.1 certificate with tensile, impact and chemical results.

Wide span greenhouse portals are the application we see most: a 120 x 120 x 6 mm S355J2H portal will clear a 12 to 15 m span without a centre column, which is what multi span horticultural units need for machinery access. It is also used for industrial shed columns, crane runway beams at light duty, heavy racking frames, machine bases and bridge type gantry legs. For outdoor service in wet climates we recommend hot dip galvanizing after fabrication rather than a paint system.

Heavy sections are packed in bundles up to 2 tons with timber dunnage between layers, three steel straps, waterproof paper and PVC sleeve, and capped ends. Because the bundle weight is high, we build to a forklift friendly footprint and mark the centre of gravity on the bundle tag. Certificates to EN 10204 3.1 with Charpy results at minus 20 degrees Celsius are issued per heat, and third party inspection is available before loading.

One design note: S355J2H welds differently from S275, and a procedure qualified on S275 does not automatically cover it. Carbon equivalent is higher and heat input has to be controlled on the 6 mm wall. Ask for the CEV value from the mill certificate before your fabricator writes the WPS.