The steel grade is one of the most fundamental design decisions in any structural steel project — yet it's often specified by habit rather than engineering rationale. Choosing S355 over S235 can reduce steel tonnage by 30–40%, while choosing the wrong grade can create weldability problems or cost penalties. This guide covers the most commonly used grades in European and American practice.

European Grades — EN 10025

European structural steels are designated by an 'S' (structural) followed by a number indicating the minimum yield strength in MPa at t ≤ 16 mm. Sub-designations (J0, J2, K2, etc.) indicate impact toughness requirements at different temperatures.

Gradefy (MPa)
t ≤ 16mm		fy (MPa)
16 < t ≤ 40mm		fu (MPa)Elongation (%)Equiv. ASTM
S235235225360–510≥ 26A36
S275275265410–560≥ 23A572 Gr.42
S355355345470–630≥ 22A572 Gr.50
S420420400520–680≥ 19A913 Gr.60
S460460440550–720≥ 17A913 Gr.65
Note on thickness: Yield strength reduces with increasing plate thickness. Always use the value corresponding to the actual element thickness — not the headline grade number. EN 10025 provides stepped reductions at t = 16, 40, 63, 80, 100, 150 mm.

American Grades — ASTM

American practice uses ASTM designations. The most common structural grades are:

GradeFy (MPa / ksi)Fu (MPa / ksi)Notes
A36250 / 36400–550 / 58–80Legacy grade, very common, good weldability
A572 Gr.50345 / 50450 / 65Most common AISC grade; similar to S355
A572 Gr.60415 / 60520 / 75High-strength; check weldability
A913 Gr.65450 / 65550 / 80Quenched & tempered; seismic use
A992345 / 50450 / 65Specifically for W-shapes; seismic-grade

S235 — The Starting Point

S235 is the lowest-strength common structural grade and was previously the default in many European markets. Today it is rarely specified for primary structure because S355 offers 51% more yield strength at a steel price premium of only 3–5%. S235 remains useful for secondary elements (handrails, platforms, non-structural cladding rails) where fabrication cost dominates and strength is not critical.

  • Excellent weldability — no preheating required up to 25 mm thickness
  • Good cold-forming characteristics
  • Low carbon equivalent (CE ≈ 0.35) — very low risk of hydrogen-induced cracking

S275 — A Middle Ground

S275 is common in the UK market (historically the standard grade for rolled sections) and is still widely available. It sits between S235 and S355 and is useful when S355 gives significant overcapacity but S235 is too weak. In many modern designs it has been replaced by S355 for primary structure.

S355 — The Structural Workhorse

S355 is the most widely used structural grade in European design and is the default for most primary structural elements — columns, beams, trusses, and connections. The benefits are:

  • 51% higher yield strength than S235 at a modest cost premium
  • Significant material savings: a structure in S355 typically uses 25–35% less steel by weight than equivalent S235
  • Available in all sections, plates, hollow sections, and structural profiles
  • Carbon equivalent CE ≈ 0.43 — preheating generally not needed below 30–40 mm
  • J2 sub-grade gives –20°C impact toughness, suitable for most building applications
S355 is the go-to grade for virtually all primary structural steel in European construction. Unless there is a specific reason to deviate, start here.

S420 and S460 — High-Strength Steels

High-strength steels offer further weight savings and are increasingly used in tall buildings, long-span bridges, and offshore structures. However, they come with important caveats:

  • Weldability: higher carbon equivalent (CE ≈ 0.47–0.53) means preheating is often mandatory, increasing fabrication cost
  • Ductility: elongation at fracture is lower; seismic design rules may restrict use
  • Buckling: high strength does not improve elastic buckling resistance (E is the same). Slender elements in compression see no benefit
  • Availability: not all section sizes are available in S420/S460. Confirm stock with your steelwork contractor
  • Connection design: design rules for friction-grip bolts and welds differ at higher grades
⚠ Do not specify S420 or S460 without checking fabricator capability and confirming availability for the sections required. The cost saving from reduced steel tonnage can easily be consumed by higher fabrication costs.

A36 vs S355 — Are They Interchangeable?

A36 (fy = 250 MPa / 36 ksi) and S355 (fy = 355 MPa) are not interchangeable without redesign. A36 has a 30% lower yield strength than S355. Using A36 sections in place of S355 without recalculation will result in an unsafe structure. However, A36 is roughly equivalent to European S235 in strength terms.

For international projects using American sections (W-shapes, etc.) to Eurocode design rules, A992 (fy = 345 MPa / fu = 450 MPa) is the nearest equivalent to S355 in both strength and ductility requirements.

Sub-Grade Selection (Impact Toughness)

For any structural grade, sub-grades define Charpy impact toughness at a specified temperature:

Sub-gradeTest temp (°C)Charpy energy (J)Typical application
JR+2027Unexposed internal steelwork only
J0027General building, sheltered external
J2−2027External structure, most building applications
K2−2040Fracture-critical members, cold climates

For most building structures in temperate climates, J2 is the appropriate sub-grade for primary members. Structures in cold climates or with fracture-critical connections may require K2 or the special 'M' (thermomechanical) sub-designations.

References: EN 10025-2, IS 2062:2011, ASTM A572/A992. For reference only — verify against current standards.